Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited

Ghasan Fahim Huseien

doi:10.3390/applnano4020006

Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited

Huseien, Ghasan Fahim 2023-04-07 00:00:00 Review Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited Ghasan Fahim Huseien Department of the Built Environment, College of Design and Engineering, National University of Singapore, Lower Kent Ridge, Singapore 117566, Singapore; bdggfh@nus.edu.sg; Tel.: +65-83057143 Abstract: The demand of high performance and environmentally sustainable construction materials is ever-increasing in the construction industry worldwide. The rapid growth of nanotechnology and diverse nanomaterials’ accessibility has provided an impulse for the uses of smart construction components like nano-alumina, nano-silica, nano-kaolin, nano-titanium, and so forth Amongst various nanostructures, the core-shell nanoparticles (NPs) have received much interests for wide applications in the ﬁeld of phase change materials, energy storage, high performance pigments, coating agents, self-cleaning and self-healing systems, etc., due to their distinct properties. Through the ﬁne-tuning of the shells and cores of NP , various types of functional materials with tailored properties can be achieved, indicating their great potential for the construction applications. In this perception, this paper overviewed the past, present and future of core-shell NPs-based materials that are viable for the construction sectors. In addition, several other applications of the core-shell NPs in the construction industries are emphasized and discussed. Considerable beneﬁts of the core-shell NPs for pigments, phase change components, polymer composites, and self-cleaning glasses with enhanced properties are also underlined. Effect of high performance core-shell NPs type, size and content on the construction materials sustainability are highlighted. Keywords: nanoparticles; core-shell materials; pigments; polymer; phase change materials Citation: Huseien, G.F. Potential 1. Introduction Applications of Core-Shell Nanotechnology is deﬁned as the manipulation of shape and structure of materials Nanoparticles in Construction at the nanoscale that can be used to design, characterize and produce valuable structures, Industry Revisited. Appl. Nano 2023, devices, and systems. The nanoscale refers to the objects with sizes between 1 and 100 nm in 4, 75–114. https://doi.org/10.3390/ dimensions (1 nm = 1 10 m). Despite many challenges in manipulating the engineering applnano4020006 materials at such a small scale, the recent advancements of various imaging techniques Academic Editors: Sergei Vlassov, made it possible to design, manufacture, and study their behaviours at the nanoscale. Sven Oras and Edgars Butanovs Amongst all the nanoscale structures produced by the top-down or bottom-up approaches, the nanoparticles (NPs) became most interesting. These are usually produced in the form of Received: 22 December 2022 very ﬁne powders or colloidal suspensions [1–4]. Various emerging properties of these NPs Revised: 26 March 2023 mainly depend on their individual components that are appreciably different from their Accepted: 29 March 2023 Published: 7 April 2023 bulk counterparts [5,6]. The NPs are unique because of their enlarged surface area, quantum size effects, improved absorbance, uniformity, and surface functionalization. The quantum size effect of the NPs is responsible for their distinct physicochemical characteristics useful for sundry applications [3,7–10]. Copyright: © 2023 by the author. Selected as one of the ten topmost targeted applications of nanotechnology to amelio- Licensee MDPI, Basel, Switzerland. rate some of the most signiﬁcant issues in the developing nations, construction and archi- This article is an open access article tecture industries stand to be substantially enhanced by the uses of nanomaterials [11,12]. distributed under the terms and Despite their ongoing uses within these contexts [11,13,14], the future of nanotechnology in conditions of the Creative Commons these industries is predicted to further increase the application feasibilities. Among these Attribution (CC BY) license (https:// expected outcomes, improvements in the building material’ properties by making them creativecommons.org/licenses/by/ stronger, durable, and lighter is the main focus [15–17]. These enhancements are brought 4.0/). Appl. Nano 2023, 4, 75–114. https://doi.org/10.3390/applnano4020006 https://www.mdpi.com/journal/applnano Appl. Nano 2023, 4 76 by introducing novel collateral functions like self-heating, anti-fogging, and energy-saving coatings, and so on [18–21]. In addition, the key components for the maintenance of in- struments such as sensors that detect and report structural health have been developed to gain more beneﬁts of these nonmaterials [22]. Despite various advantages of these new technologies, an emphasis should be placed on the risk-assessment of their intended uses, wherein the fallout can be severe. One such recent example is the deliberate and widespread use of supposedly beneﬁcial chemical dichlorodiphenyltrichloroethane (DDT) that was released to control malaria and various water-borne diseases. However, instead proved to be carcinogenic to humans it became toxic to numerous bird species, and haz- ardous to environment [23]. This illustrates the importance of a proactive and meticulous approach for the risk assessment of new technologies, without which, devastating impacts to ecosystems and human health cannot be prevented. Buildings have remarkable rate of power consumption at 45% of global energy [24,25]. Many passive cooling methods have been used and in addition, phase change materials (PCM) are installed within these buildings for the purpose of promoting temperature moderation, stopping heat from accumulating, improved heat absorption and minimize indoor heat gain. The method in which PCM stores thermal energy is effective in improving the buildings’ aggregate heat capacity. Interest has been strong in PCMs that has high energy density to be deployed in buildings with high thermal inertia in order to save a high amount of energy. However, PCMs have their own drawbacks and the primary one being extra time required to charge/discharge energy process as well as storage performance, which happens due to poor thermal conductivity. Therefore, attention is focused on improving its thermal conductivity through the use of nanotechnology and nanomaterials. There has been a rapid development lately within the nanomaterials ﬁeld resulting in the latest technology with Nano-sized particles in improving the PCM’s thermophysical properties. PCMs have several thermal and physical qualities such as viscosity, heat capacity, super-cooling and thermal conductivities. These attributes could be signiﬁcantly improved through dispersal of thermal conductive nanoparticles including nanometal- oxide, nanocarbon and nanometals. The technologies of core-shell and nanoparticles are widely adopted to improve the materials properties and thermal performance, which is appropriate in passive-cooling within the built-environment. In the construction industry, one of the possible solutions for a sustainable future is to introduce novel technologies to improve the durability of materials and increase the life span. Presently, nanotechnology creates new possibilities to control and improve material properties for civil infrastructures. By combining various engineering, chemical, and biological approaches, the nanotechnology can be used for the sub-atomic manipulation of materials. To synthesize NPs, diverse chemical, biological, physical, and even hybrid techniques can be used. In this regard, this review discusses and explains the role of nanoscience and nanotechnology in the development of potential core-shell NPs applicable in the construction industry (Figure 1). Also, diverse potential applications of core-shell NPs -based high performance construction materials rooted from the state-of-the-art research are emphasized. Appl. Nano 2023, 4, FOR PEER REVIEW 3 Appl. Nano 2023, 4 77 Figure 1. Flow chart of core-shell nanoparticles, synthesis, efﬁciency and construction applications. Figure 1. Flow chart of core-shell nanoparticles, synthesis, efficiency and construction applications. 2. Core-Shell NPs Synthesis and Beneﬁts 2. Core-Shell NPs Synthesis and Benefits Nanotechnology encompasses various methods of synthesis (biological, engineering, Nanotechnology encompasses various methods of synthesis (biological, engineer- chemical and hybrid) to customize the atomic-scale properties of materials. To produce the ing, chemical and hybrid) to customize the atomic-scale properties of materials. To pro- core-shell NPs both top-down and bottom-up approaches are routinely utilized. Top-down duce appr thoach e core incorporates -shell NPs bot the conventional h top-downworkshops and bottowith m-up micr appr ofabrication oaches are tools routi in n addition ely utilized. to the equipment that are externally controlled that are used to mill, cut, shape and mould Top-down approach incorporates the conventional workshops with microfabrication the materials accordingly to the requirement [26,27]. The lithographic and mechanical tools in addition to the equipment that are externally controlled that are used to mill, cut, techniques are the conventional top-down approach. The lithographic techniques involve shape and mould the materials accordingly to the requirement [26,27]. The lithographic the use of electron or ion beam, UV, scan probing, optical near ﬁeld scanning and laser-beam and mechanical techniques are the conventional top-down approach. The lithographic processing. In addition, the mechanical techniques involve the machines that grind, cut techniques involve the use of electron or ion beam, UV, scan probing, optical near field and polish the materials according to the required speciﬁcations [28–31]. Conversely, the scanning and laser-beam processing. In addition, the mechanical techniques involve the bottom-up technique is used to assemble materials in the desired form from their chemical machines that grind, cut and polish the materials according to the required specifications composition down to the molecular level. Examples of typical bottom-up technique include [28chemical –31]. Con vapour versely deposition, , the bottom laser -up -induced technique assembly is used , chemical to assemsynthesis, ble materself-assembly ials in the desire , d colloidal aggregation as well as ﬁlm deposition and growth [32,33]. form from their chemical composition down to the molecular level. Examples of typical Both approaches have many advantages and disadvantages. However, the main bottom-up technique include chemical vapour deposition, laser-induced assembly, advantage of the bottom-up approach is related to its cost-effectiveness that can fabricate chemical synthesis, self-assembly, colloidal aggregation as well as film deposition and signiﬁcantly smaller particles than the top-down approach. This is because of its precision growth [32,33]. as the product is produced by assembling it down to molecular level. Thus, it is possible Both approaches have many advantages and disadvantages. However, the main to have total control and almost no energy loss in the entire production process. The advantage of the bottom-up approach is related to its cost-effectiveness that can fabricate preparation of core/shell NPs necessitates total control in order to coat the shell materials significantly smaller particles than the top-down approach. This is because of its preci- uniformly as the particles are formed. Therefore, bottom-up approach is more suitable for sion as the product is produced by assembling it down to molecular level. Thus, it is such synthesis. Hybrid approach involves the use of both of the aforementioned techniques. For instance, the core particles can be produced via the top-down approach. Conversely, the possible to have total control and almost no energy loss in the entire production process. bottom-up approach can address the uniformity of the shell thickness. It is recommended The preparation of core/shell NPs necessitates total control in order to coat the shell ma- to apply micro-emulsion for an accurate size and thickness regulation of the shell because terials uniformly as the particles are formed. Therefore, bottom-up approach is more water droplets can act as nano-reactors. More researchers have been focusing on the core- suitable for such synthesis. Hybrid approach involves the use of both of the aforemen- shell NPs due to their suitability to be used extensively in diverse ﬁelds such as electronics, tioned techniques. For instance, the core particles can be produced via the top-down ap- optics, chemistry, biomedicine, medicines and catalysis, etc. proach. Conversely, the bottom-up approach can address the uniformity of the shell The core-shell NPs have high functioning and distinct properties such as different thickness. It is recommended to apply micro-emulsion for an accurate size and thickness materials can be used for the core or shell. The core or shell can be highly customizable by regulation of the shell because water droplets can act as nano-reactors. More researchers modifying the properties through controlling the materials or the core to shell ratio [34]. Also, it is possible to modify the core particles’ reactivity and thermal stability via the have been focusing on the core-shell NPs due to their suitability to be used extensively in adjustments to the shell coating material, leading to improved stability and dispersion diverse fields such as electronics, optics, chemistry, biomedicine, medicines and catalysis, of the core particles. This indicated that each particle can possess exclusive properties etc. depending on the materials being used during the fabrication. Such technique is renowned The core-shell NPs have high functioning and distinct properties such as different materials can be used for the core or shell. The core or shell can be highly customizable by modifying the properties through controlling the materials or the core to shell ratio [34]. Also, it is possible to modify the core particles’ reactivity and thermal stability via the adjustments to the shell coating material, leading to improved stability and dispersion of the core particles. This indicated that each particle can possess exclusive properties de- Appl. Nano 2023, 4, FOR PEER REVIEW 4 pending on the materials being used during the fabrication. Such technique is renowned because through the application of appropriate materials it can customize the surface function according to the environment [35]. The benefits of coating core particles include improved function, surface modifications, stability, dispersion, core release control and significant decrease in the use of precious material. The core-shell particles, as the name suggests, contain a shell and a core wherein a shell can be produced by the same or dif- ferent materials used for the core [36–38]. Figure 2 shows various core-shell particles where the colours are used to differentiate between them wherein a core can consist of a single sphere (Figure 2a) or multiple spheres that are smaller in size (Figure 2b). Fur- thermore, a shell may be hollow with one small sphere inside, resembling the yolk-shell structure (Figure 2c) [39]. Figure 2 shows three forms of shell structure like a continuous layer (Figure 2a–c), a Appl. Nano 2023, 4 78 larger core sphere that contains many smaller spheres (Figure 2d–e) or simply a collec- tion of core spheres (Figure 2f) [40]. The intricacy of the core-shell structure can be ma- nipulated by inserting smaller spheres into the shell (Figure 2g) [41] that can also be done because through thr mult ough iple the shells application (Figure 2of h) appr [42,4opriate 3]. The materials core-shell it Ncan Ps ca customize n be madthe e usi surface ng the function physical accor or chding emica to l the appr envir oach onment includin [35 g ]. thThe e che beneﬁts mical dof epcoating osition, cor phe ys particles ical vapo include ur and wet chemistry. Generally, the synthesis of core-shell particles involves different stages. improved function, surface modiﬁcations, stability, dispersion, core release control and signiﬁcant First, the core decr pa ease rticles in the areuse syn of thpr esiz ecious ed foll material. owed by The the cor forma e-shell tion particles, of shell oas nto the the name core suggests, particle. Thi contain s mea th shell od d and epends a core on wher the ein type a shell of core can be anpr d oduced shell mby ater the ials same [41]or . The differ ment ain materials used for the core [36–38]. Figure 2 shows various core-shell particles where the purpose of producing the core-shell particles is to achieve suitable unconventional novel colours are used to differentiate between them wherein a core can consist of a single sphere materials and structures. Consequently, the materials with desirable attributes such as (Figure 2a) or multiple spheres that are smaller in size (Figure 2b). Furthermore, a shell may active particles with high stability, biocompatibility and synergy effect can be achieved be hollow with one small sphere inside, resembling the yolk-shell structure (Figure 2c) [39]. [44]. Figure 2. Schematic representation of different types of core–shell particles (a) single sphere (b) Figure 2. Schematic representation of different types of core–shell particles (a) single sphere (b) mul- multiple spheres are smaller in size) (c) yolk shell (d) large core sphere with one layer of many tiple spheres are smaller in size) (c) yolk shell (d) large core sphere with one layer of many smaller smaller spheres (e) large core sphere with two layers of many smaller spheres (f) simply a collec- spheres (e) large core sphere with two layers of many smaller spheres (f) simply a collection of core tion of core spheres (g) smaller spheres into the shell (h) multiple shells [36]. spheres (g) smaller spheres into the shell (h) multiple shells [36]. Diverse industries utilise a basic nanomaterial to synthesize core-shell NPs. The Figure 2 shows three forms of shell structure like a continuous layer (Figure 2a–c), a speed, simplicity, environmental friendliness and cost effectiveness of the method as well larger core sphere that contains many smaller spheres (Figure 2d,e) or simply a collection as products are prerequisites for the synthesis of these core-shell structures. Many of core spheres (Figure 2f) [40]. The intricacy of the core-shell structure can be manipulated methods have been established to meet the aforementioned requirements. These include by inserting smaller spheres into the shell (Figure 2g) [41] that can also be done through the electrochemical dealloying, sol-gel process, sonochemical process, microwave syn- multiple shells (Figure 2h) [42,43]. The core-shell NPs can be made using the physical or thesis, multi-step reduction, microe-mulsion, epitaxial growth, and Stöber method. Hy- chemical approach including the chemical deposition, physical vapour and wet chemistry. brid method involves the unification of more than one of the aforementioned methods. Generally, the synthesis of core-shell particles involves different stages. First, the core parti- Generally, sol-gel process is widely used to produce the core-shell NPs. Sol-gel method cles are synthesized followed by the formation of shell onto the core particle. This method for the synthesis of core-shell NPs offers an additional control during the reaction process depends on the type of core and shell materials [41]. The main purpose of producing the of solid materials. Homogenous multi-component systems especially mixed oxides can core-shell particles is to achieve suitable unconventional novel materials and structures. Consequently, the materials with desirable attributes such as active particles with high stability, biocompatibility and synergy effect can be achieved [44]. Diverse industries utilise a basic nanomaterial to synthesize core-shell NPs. The speed, simplicity, environmental friendliness and cost effectiveness of the method as well as prod- ucts are prerequisites for the synthesis of these core-shell structures. Many methods have been established to meet the aforementioned requirements. These include the electrochem- ical dealloying, sol-gel process, sonochemical process, microwave synthesis, multi-step reduction, microe-mulsion, epitaxial growth, and Stöber method. Hybrid method involves the uniﬁcation of more than one of the aforementioned methods. Generally, sol-gel pro- cess is widely used to produce the core-shell NPs. Sol-gel method for the synthesis of core-shell NPs offers an additional control during the reaction process of solid materials. Homogenous multi-component systems especially mixed oxides can easily be produced through the mixing of solutions containing molecular precursors. Essentially, this method yields solid materials (small molecular clusters), especially metal oxides like SiO and TiO . 2 2 Preparation of metal oxides using sol-gel process involves the conversion of monomer Appl. Nano 2023, 4 79 into a colloidal solution (sol). The solution is the precursor to be used for combination of network including discrete particles or network polymers. Usually, various metal alkoxides are used as precursor. Sol is produced when a chemical reaction occurs and eventually become a diphasic substance that has property similar to gel, implying both liquid and solid. The morphology of these phases may either be discrete particles or continuous polymer networks. Turning the colloids into the properties like gel necessitate the removal of large volume of liquids in the events from the volume of particle density that is signiﬁcantly low. One of the simplest ways to achieve this is to wait for an adequate time for the sedimenta- tion to occur before disposing the remaining liquid. In addition, the centrifugation can be applied to accelerate the phase separation. Sol-gel is a more common wet-chemical method that is used to synthesise core-shell NPs [45–47]. Microemulsions are a mixture of isotropic liquid composed of surfactant, oil, water and more commonly co-surfactant. It has a clear appearance and a stable thermodynamic system in the presence of salt and other ingredients in the liquid form. The oily substance may be due to complex mixtures of various types of hydrocarbons. In comparison to con- ventional emulsions, microemulsions are synthesised through different mixing components and do not need high shear conditions during the production process. The microemulsions are categorized as direct (dispersion of oil in water, o/w), reversed (dispersion of water in oil, w/o) and bycontinuous types. These microemulsions belong to the ternary systems wherein two immiscible substances (water and oil) which forms separate layers co-exists with a surfactant, resulting in a monolayer that form between the immiscible substances from the surfactant’s molecules. In the oil phase, the hydrophobic tails of the surfactant molecules would dissolve. However, in the liquid phase, the hydrophilic head groups would dissolve. Two-step microwave irradiation is the conventional method for rapid synthesis of gold and silver core-shell bimetallic NPs. In this technique, a bilayer organic barrier is developed surrounding the core. The desired capping agents are the citrate and ascorbic acid that facilitates the formation of core and shell material, developing a well-deﬁned boundary layer. The boundary layer is signiﬁcant for the synthesis process of various core-shell particles that are ultimately used to create the customised bimetallic core-shell NPs of desired morphology wherein the cores are triangular or spherical in shape. The high-pressure chemical vapour deposition method is an alternative in producing the core-shell materials including nanotubes. Nikolaev et al. [48] founded this method to produce the single-walled carbon nanotube (SWCNTs). In this process, a small amount of Fe(CO) is used to comb CO. Then, the mixture is passed to a heat reactor. El-Gendy et al. [49] used this technique to make NPs coated with various materials like Fe, Co, Ni, FeRu, CoRu, NiRu, NiPt, and CoPt. In this method, the reactor ’s temperature and pressure can be accurately controlled to tailor the core-shell NPs properties needed for the speciﬁc applications. Earlier, the metal-organic precursors called the metallocenes or metals that are rich in carbon were used. These precursors were inputted into a thermostatic sublimation chamber before releasing argon gas for pushing the vapour into the hot zone of the chamber. First, the precursor broke down the NPs within the cooling ﬁnger before turning into the gas phase within the hot zone for the supersaturation. Upon the initiation of the supersaturation process, the NPs were nucleated. Furthermore, careful adjustments can be made to the temperature and pressure/temperature within the corresponding sublimation chambers and chemical vapour deposition reactor in order to control the desired degree of supersaturation. At high pressure, the collision probability of gas atoms increases, thus reducing the rate of atoms diffusion from the original location. It is worth noting that when the diffusion rate is poor, the supersaturation does not occur. In this situation, the cooling ﬁnger contains the deposits of tiny clusters of atoms or individual atoms. Fe O with graphene shells as coating was prepared using the wet chemical tech- 2 3 nique [50]. Oleic acid and 1-octadecene were mixed in a solution before being placed in the reﬂux reactor and heated to 320 C to dissolve the iron oleate. Next, the solution was washed with ethanol and acetone to obtain the iron oxide particles. Normally, the Stöber process involves the preparation of SiO particles [51] with total control of uniformity in 2 Appl. Nano 2023, 4, FOR PEER REVIEW 6 occur. In this situation, the cooling finger contains the deposits of tiny clusters of atoms or individual atoms. Fe2O3 with graphene shells as coating was prepared using the wet chemical tech- nique [50]. Oleic acid and 1-octadecene were mixed in a solution before being placed in the reflux reactor and heated to 320 °C to dissolve the iron oleate. Next, the solution was washed with ethanol and acetone to obtain the iron oxide particles. Normally, the Stöber process involves the preparation of SiO2 particles [51] with total control of uniformity in size [52]. These particles offer numerous applications in the field of materials science and Appl. Nano 2023, 4 80 engineering. Since the discovery of this method by Werner Stöber et al. [51], it remains the most renowned wet chemistry approach for the NPs synthesis [53]. Being a sol-gel process, the chemical tetraethyl orthosilicate (TEOS) act as the precursor immersed in size [52]. These particles offer numerous applications in the ﬁeld of materials science and water. Alcoholic solution is added to form a reaction, forming new molecules that ag- engineering. Since the discovery of this method by Werner Stöber et al. [51], it remains gl the om most erate renowned to create wet large chemistry r clusterappr s. Du oach et afor l. [5 the 4] used NPs synthesis the sol-gel [53]. appr Being oach a sol-gel to make SiO2 process, the chemical tetraethyl orthosilicate (TEOS) act as the precursor immersed in water. shell as a coating agent for the Fe3O4 NPs, eventually producing the core-shell structure. Alcoholic solution is added to form a reaction, forming new molecules that agglomerate In this two-step procedure, the co-precipitation was first initiated to obtain Fe3O4 NPs. to create larger clusters. Du et al. [54] used the sol-gel approach to make SiO shell as a Next, it caused a reaction with tetramethyl ammonium hydroxide (TMAOH), forming a coating agent for the Fe O NPs, eventually producing the core-shell structure. In this 3 4 liquid solution that contained the proposed particles. In the second stage, SiO2 was pro- two-step procedure, the co-precipitation was ﬁrst initiated to obtain Fe O NPs. Next, it 3 4 duced through the hydrolyzation of TEOS in order to limit the formation of Fe 3O4. caused a reaction with tetramethyl ammonium hydroxide (TMAOH), forming a liquid Figure 3 shows the sol-gel unified annealing approach used by Li et al. [55] to pro- solution that contained the proposed particles. In the second stage, SiO was produced duce ZnSiO3/ZnO core-shell NPs. In this experiment, the reason for combining these two through the hydrolyzation of TEOS in order to limit the formation of Fe O . 3 4 methods was to produce broad band-gap core-shell NPs of Zinc Silicate-Zinc Oxide Figure 3 shows the sol-gel uniﬁed annealing approach used by Li et al. [55] to pro- duce ZnSiO /ZnO core-shell NPs. In this experiment, the reason for combining these (Zn2SiO4@ZnO). First, the reaction between Na2SiO3/ZnCl2 was initiated to form ZnSiO3, two methods was to produce broad band-gap core-shell NPs of Zinc Silicate-Zinc Oxide which in turn produced the shells with varied thickness before being used to coat ZnO (Zn SiO @ZnO). First, the reaction between Na SiO /ZnCl was initiated to form ZnSiO , 2 4 2 3 2 3 NPs. A low annealing temperature of 780 °C was set. Finally, the reaction between which in turn produced the shells with varied thickness before being used to coat ZnO NPs. amorphous ZnSiO3 and ZnO occurred, forming a crystalline Zn2SiO4 shell. Chai et al. [45] A low annealing temperature of 780 C was set. Finally, the reaction between amorphous adopted this technique to make core-shell Fe3O4@SiO2 NPs. The first step was to fabricate ZnSiO and ZnO occurred, forming a crystalline Zn SiO shell. Chai et al. [45] adopted this 3 2 4 Fe3O4 NPs via the solvothermal technique. Next, the hydrolyzation of tetraethyl ortho- technique to make core-shell Fe O @SiO NPs. The ﬁrst step was to fabricate Fe O NPs 3 4 2 3 4 silicate resulted in SiO2 that acted as the coating agent for Fe3O4 NPs. via the solvothermal technique. Next, the hydrolyzation of tetraethyl orthosilicate resulted in SiO that acted as the coating agent for Fe O NPs. 2 3 4 Figure 3. Core-shell particles synthesis using sol-gel combined annealing method [55]. Reproduced Figure 3. Core-shell particles synthesis using sol-gel combined annealing method [55]. Reproduced with permission from Li, Z, et al., Materials Chemistry and Physics; published by Elsevier, 2020. with permission from Li, Z, et al., Materials Chemistry and Physics; published by Elsevier, 2020. A two-step reduction technique was also used [56] to make epitaxial Au@Ni core-shell nanocrystals. In this process, various materials such as decahedral, octahedral, triangular A two-step reduction technique was also used [56] to make epitaxial Au@Ni and hexagonal plate-like as well as icosahedral were mixed initially. Subsequently, ethylene core-shell nanocrystals. In this process, various materials such as decahedral, octahedral, glycol (EG) was used for the reduction of HAuCl before being placed in a microwave triangular and hexagonal plate-like as well as icosahedral were mixed initially. Subse- with polyvinylpyrrolidone (PVP) that acted as a polymer surfactant to be heated. The core quently, ethylene glycol (EG) was used for the reduction of HAuCl 4 before being placed seeds were produced at this stage and subsequently the oil bath was heated to reduce in a microwave with polyvinylpyrrolidone (PVP) that acted as a polymer surfactant to be Ni(NO ) .6H O in EG in the presence of NaOH and PVP. Eventually, the Ni shells were 3 2 2 heated. The core seeds were produced at this stage and subsequently the oil bath was overgrown within the Au core seeds. Fan et al. [57] used similar technique but focused heated to reduce Ni(NO3)2.6H2O in EG in the presence of NaOH and PVP. Eventually, the on the seed-mediated growth. Herein, Au cores were made in the liquid form to achieve bimetallic core-shell nanocubes. Comprehensive assessment was made upon the hetero- Ni shells were overgrown within the Au core seeds. Fan et al. [57] used similar technique geneous core-shell formation on the four common metals like gold, silver, palladium and but focused on the seed-mediated growth. Herein, Au cores were made in the liquid form platinum. This experiment constituted the following bases: (a) the general conditions and to achieve bimetallic core-shell nanocubes. Comprehensive assessment was made upon growth modes to attain conformal epitaxial structures and (b) heterogeneous nucleation the heterogeneous core-shell formation on the four common metals like gold, silver, pal- and formation of various noble metals. In addition, three types of growth modes for the gold cores with heterogeneous metal shells were identiﬁed: conformal epitaxial growth (Au@Pd and Au@Ag nanocubes), island growth (Au@Pt nanospheres) and heterogeneous nucleation. Further ﬁndings include two metals with comparable lattice constants where the mismatch was less than 5%. These ﬁndings were consistent with other studies (Au@Ag (lattice mismatch, 0.2%), Au@Pd (4.7%), and Pt@Pd (0.85%)) [54–56]. Appl. Nano 2023, 4, FOR PEER REVIEW 7 ladium and platinum. This experiment constituted the following bases: (a) the general conditions and growth modes to attain conformal epitaxial structures and (b) heteroge- neous nucleation and formation of various noble metals. In addition, three types of growth modes for the gold cores with heterogeneous metal shells were identified: con- formal epitaxial growth (Au@Pd and Au@Ag nanocubes), island growth (Au@Pt nano- spheres) and heterogeneous nucleation. Further findings include two metals with com- parable lattice constants where the mismatch was less than 5%. These findings were con- Appl. Nano 2023, 4 81 sistent with other studies (Au@Ag (lattice mismatch, 0.2%), Au@Pd (4.7%), and Pt@Pd (0.85%)) [54–56]. Tsuji et al. [58] used one-polyol technique to make Ag@Cu core-shell NPs with a high yield. The method involved the use of bubbling Ar gas with added reagents like Tsuji et al. [58] used one-polyol technique to make Ag@Cu core-shell NPs with a AgNO3 and Cu(OAc)2 H2O. This two-step process was used to synthesize Ag@Cu parti- high yield. The method involved the use of bubbling Ar gas with added reagents like cles through AgNO3 reduction in EG. The Cu shells were developed by separating the Ag AgNO and Cu(OAc) H O. This two-step process was used to synthesize Ag@Cu particles 3 2 2 cores from AgNO3, before Cu(OAc)2.H2O was added. This procedure failed because no through AgNO reduction in EG. The Cu shells were developed by separating the Ag Cu@Ag core-shell particles were nucleated instead the Cu/Ag bi-compartmental particles cores from AgNO , before Cu(OAc) .H O was added. This procedure failed because no 3 2 2 were appeared. Later, various experimental processes were combined at different reac- Cu@Ag core-shell particles were nucleated instead the Cu/Ag bi-compartmental particles tion temperatures and heating times to produce Ag@Cu particles. It was found that the were appeared. Later, various experimental processes were combined at different reaction temperatur optimal con esdand ition heating for pro times ducin to g pr Ag@C oduce u pa Ag@Cu rticles particles. is to addIt tw was o re found agentthat s in the revoptimal erse. At condition the beginn for ing pro oducing f the pro Ag@Cu cess, 8 m particles L of 15.9 is m toM add Cu two (OAc reagents )2.H2O in wa rs everse. addedAt in the EGbeginning plus 8 mL of of the 477pr m ocess, M pol 8y(v mL inof ylpyrr 15.9o mM lidon Cu e) (OAc) (PVP, M .H WO : 5was 5,000 added monoin mer EG uplus nits).8 A mL 10of 0-m 477 L th mM ree 2 2 poly(vinylpyrr necked flask w olidone) as used f(P or VP th,e MW soluti : 55,000 on mixin monomer g. Ar wun as its). bubb A led 100-mL for 10thr mee in a necked t room ﬂask tem- was peraused ture to for com theplete solution ly rem mixing. ove oxyge Arn was from bubbled the soluti foro10 n fo mi llo nw at ed r oom by so temperatur aking in an e o to il completely remove oxygen from the solution followed by soaking in an oil bath at a bath at a temperature of 180 °C. The solution continued to bubble while the temperature temperature of 180 C. The solution continued to bubble while the temperature was raised was raised to 175 °C. Afterwards, the reagent solution was added with 2 mL of 15.7 mM to 175 C. Afterwards, the reagent solution was added with 2 mL of 15.7 mM AgNO and AgNO3 and left for 20 min at 175 °C. Finally, 7.0 mM, 212 mM and 1.7 mM of Cu left for 20 min at 175 C. Finally, 7.0 mM, 212 mM and 1.7 mM of Cu (OAc) .H O, AgNO (OAc)2.H2O, AgNO3 and PVP, respectively. Further investigation was conducted by var- 2 2 3 and PVP, respectively. Further investigation was conducted by varying the reaction time ying the reaction time on the reagent solution to determine the growth mechanism of on the reagent solution to determine the growth mechanism of Ag@Cu. Ag@Cu. Chae et al. [45] produced Fe O @SiO by the customised Stober method. The solution 3 4 2 Chae et al. [45] produced Fe3O4@SiO2 by the customised Stober method. The solution of 4 g Fe O particles was ultrasonicated and extra tetraethyl orthosilicate was added to 3 4 of 4g Fe3O4 particles was ultrasonicated and extra tetraethyl orthosilicate was added to raise the volume from 4 to 40 mL. A stable emulsion was obtained and it was further raise the volume from 4 to 40 mL. A stable emulsion was obtained and it was further in- inserted into a mixture containing 50 mL of ethanol and 12 mL of NH H O. The reaction 3 2 serted into a mixture containing 50 mL of ethanol and 12 mL of NH 3 H2O. The reaction solution was stirred at 400 rpm at room temperature for 4 h until the core-shell structured solution was stirred at 400 rpm at room temperature for 4 h until the core-shell structured Fe O @SiO NPs were separated using centrifugation. Figure 4 shows the entire process of 3 4 2 Fe3O4@SiO2 NPs were separated using centrifugation. Figure 4 shows the entire process Fe O @SiO synthesis. 3 4 2 of Fe3O4@SiO2 synthesis. Figure 4. Stober method for Fe3O4@SiO2 nanoparticles synthesis [45]. Reproduced with permission Figure 4. Stober method for Fe O @SiO nanoparticles synthesis [45]. Reproduced with permission 3 4 2 from Chae, H.S., et al., Colloid and Polymer Science; published by Elsevier, 2016. from Chae, H.S., et al., Colloid and Polymer Science; published by Elsevier, 2016. Sharma et al. [59] conducted a similar experiment and demonstrated that it is pos- Sharma et al. [59] conducted a similar experiment and demonstrated that it is possible sible to fabricate core-shell particles through the precipitation without the need of any to fabricate core-shell particles through the precipitation without the need of any surfactant. surfactant. The outcome (the concentrations of the core-shell particles) was compared The outcome (the concentrations of the core-shell particles) was compared with those obtained with thousing se obta dif infer edent usianionic ng differ and ent non-ionic anionic a surfactants. nd non-ionThe ic surf nano acta -T niO ts. The was n dev ano eloped -TiO2 in wa the s dev form eloped of shell in th using e form ﬂy ofash. shell The usin surfactants g fly ash. The wer e surf mainly actants used wer to e m str aengthen inly used the to strengthen the adhesion of the nano-titania shells to fly ash core. Yet again, different adhesion of the nano-titania shells to ﬂy ash core. Yet again, different types of surfactants wer types e used of surf to test actathe nts str wer ength e used of the to test TiOth adhesion e strengtonto h of th ﬂy e ash. TiO2Another adhesiotest n on was to fly conducted ash. An- without other test surfactant. was cond When ucted anionic without surfactant surfactant. was Wh used, en an the ionr ic esulting surfacta part nt w icles as use formed d, the had re- remarkable pigment properties and reﬂectance in the near-infrared region, indicating their sulting particles formed had remarkable pigment properties and reflectance in the suitability towards cool coating applications. A solution of 70% ethanol was added in the sequence of ﬂy ash, anionic (SDS) or non-ionic surfactant (TX-100) and ﬁnally titanium isopropoxide. Finally, the solution was stirred for two hours before being dried at 50–600 C to achieve a powder. Zhang et al. [60] made a study to produce PUA hybrid emulsion PA/PU with a ratio of 20 to 80 using semi-batch emulsion. In the experimental setup a digital thermometer, 250 mL four-neck glass ﬂask containing a reﬂux condenser, mechanical stirrer and nitrogen gas inlet were used. The pre-emulsion was prepared by dissolving 2.0 g per 100 g of acrylic and PU content into the water before gradually adding 5.0 g of MMA, 5.0 g of BA and 0.015 g of AA (0.1 5 wt% of the overall MMA and BA weight). The solution was then stirred Appl. Nano 2023, 4 82 before mixing additional 0.5 g. The main objective was to obtain 111.3 g of PU emulsion dispersion and 10% monomers from the reactor vessel. The temperature was set at 80 C while the contents were stirred. Next, 0.4 g of KPS per 100 g acrylic monomers composed of 10% was added and continuously stirred for 30 min. Subsequently, the temperature was increased by 5 C and simultaneously the leftover monomer pre-emulsion and initiator solution was ﬂown into the task for 4 h at a constant ﬂow rate. Next, the solution was left at 85 C for 0.5 h with stirring and waiting for the temperature to drop. Lastly, the pH value was maintained at the desirable range after adding NaHCO . 3. Core-Shell NPs Based Sustainable Pigments In the last decade, synthetic-coloured pigments have been launched in the market that resulted in more extensive scientiﬁc research focused on this area. Typical applica- tions of these pigments are varnishes, paints, plastics and textiles, printing inks, building materials and rubber, ceramic glazes and leather decoration [61–63]. The deﬁnition of the pigment durability is connected to its ability of resisting weathering processes and negating deteriorating when being placed in an external environment [64]. Recent studies have shown that efﬁcient energy consumption and environmental protection measures are deemed signiﬁcant [65]. To address this issue, the production of both sustainable and durable pigments has become the fundamental requirement within the construction indus- try. Myriad of methods have been applied in order to increase the pigments’ durability, and the most signiﬁcant is known as the core-shell method [3,66–68]. There has been a surge of development of various chemical synthesis techniques in recent years. Such research has found that multi-component materials possess diverse compositions and structures. These attributes signify remarkable property type and they are applicable in many different types of ﬁelds [69–72]. There is even more research being conducted on their distinctive core-shell structure. There are many advantages of the core-shell structure compared to other types of composite materials. One such advantage is their ability to generate or increase the strength of new chemical and physical capabilities, enabling maintenance on structural integrity, deter the core from breaking up to large particles and ascertaining dispersion effectively. In addition, they also provide conventional multi-functional compositions and structure with other advantages. Moreover, a synergetic effect between the shells and cores would even extend the performance further [73]. Science and technology ﬁeld have been attentive on the phenomena of materials that are derived from the core-shell properties because they can be ﬁnely customised [61,74,75]. A shell domain cloaks a core structural domain within each of the core or shell particle. Materials that possess core or shell particles include inorganic solids, metals and polymers. There is no difﬁculty in modifying characteristics such as size and structures as well as the particles’ composition in order to further customise their properties such as optical, magnetic, mechanical, thermal, electrical, catalytic and electro-optical attributes. Core or shell morphology can be applied to produce hollow spheres and minimize the costs of precious materials. Thus, the materials with the reduced core costs can be coated to precious materials [76,77]. Particles with the size of less than 0.1 m is classiﬁed as NPs and have been garnering much attention in research within the past few years. Essentially, NPs are smart materials with exclusive properties. Applications using NPs have more advantages compared to materials that have larger surface to volume ratio such as microscale, macroscale and bulk materials [78,79]. Due to the increased research on the NPs development, it is now possible to make NPs in symmetrical shape, such as spherical as well as other shapes including prism, hexagon, cube, wire, tube and rod [80–82]. Despite this achievement, the bulk of the research is still at early stage in terms of exploring the possible shapes that can be synthesised. There has been research that recently found the ease of production method for NPs that are non-spherical [83–85]. However, it must be stressed that NPs’ properties are dependent on the actual shape and size. Such properties that are dependent on particle size include temperature barrier, magnetic saturation and Appl. Nano 2023, 4 83 permanent magnetisation. Furthermore, coactivity of the nanocrystals is dependent on the shape of the particle as it has a direct inﬂuence on the surface anisotropy [86]. Rapid advancements are made in nanotechnology resulting in the founding of core- shell NPs, which is a leading functional material. This has attracted even more research conducted on various functional compositions core-shell NPs where it could be applied in many types of areas such as optics, catalysis, biomedicine, electronics and medicines [87]. Core-shell NPs possess beneﬁcial physiochemical properties that are exclusive, and this attribute has garnered a lot of researchers’ attention. The primary advantages of core-shell NPs are that it could increase protection level, encapsulation and controlled release [82,88]. The discovery of a variety of core/shell NPs leads to its applications to a variety of situations. However, the difﬁculty is to identify the individual type core/shell NPs that are applicable to the respective industries due to their multitude of types. Numerous studies on the core- shell NPs pigments are focusing on core/shell materials, production methods, distinctive properties and their applications. Herein, the main features of the core-shell NPs including their fabrication methods, inorganic materials and typical applications are emphasized. A discussion on diverse methods of production along with the classiﬁcations of the core-shell materials that are already being in use are outlined. The new fabrication methods of the core-shell NPs pigments within all research ﬁelds are emphasized. Finally, the application potential of core-shell NPs within paints designed for roads and other construction sectors are underscored. 3.1. Materials Based Shell Part Several materials such as metals and biomolecules are used to create core-shell NPs. There are two components, the central core and an alternative core, which is the shell. The attributes of the core-shell nanostructures include their high thermal and chemical stabilities, low toxicity, high levels of solubility and high level of permeability for speciﬁcally targeted cell. Such properties enable them to have a vast potential for functional applications in many sectors. Furthermore, micro-nano scale core-shell particles have attributes that are exclusive and unique to them compared to other particles. Essentially, the attributes combined the materials’ properties that are used for core and shell together along with smart properties that are formed via their materials. For the past few years, there have been an increased research interest in core-shell structures production [89]. This is particularly true within the pigment industry due to the high range of applications of core-shell materials in order to increase pigments’ durability. The core-shell materials could be made of both organic and inorganic materials. For instance, Cao et al. [90] developed hybrid pigments which consist of inorganic-organic structure using a mixture of precipitated SiO and TiO . In addition, 2 2 dye core@silica shell structure was fabricated using the mesoporous soft template synthesis approach [91]. This section below explores the possibility of using inorganic materials to produce core-shells materials with the focus on SiO and TiO . 2 2 3.2. Efﬁciency and Test Methods Generally, the obtained core-shell NPs are characterized using diverse analytical methods such as SEM, LC-MS, XPS, FTIR, XRD, TEM, BET, Ultraviolet-visible Spec- troscopy, Raman spectrum as well as Near-Infrared Reﬂectance and Photoluminescence Spectroscopy [37,38,47,92–96]. For instance, assessment of morphology, chromaticity and the structure of -Fe O @SiO fabricated pigments can be tested by SEM, TEM, FTIR, XPS 2 3 2 and XRD [88]. Figure 5a shows the XRD patterns of the pigments made of -Fe O @SiO 2 3 2 NPs, -Fe O @SiO and -Fe O . The formation of the core-shell structures results in the 2 3 2 2 3 diffraction peak of -Fe O @SiO particles appeared in the 2 range of 15 –25 , indicat- 2 3 2 ing the presence of amorphous SiO . Further calcinations at 1000 C could change the diffraction peak to around 22 . The results of reddish colour pigment indicated that the amorphous shell has entered into a cristobalite phase. In addition, the formation of the core-shell structure weakened the -Fe O diffraction peak. Figure 5b shows the FTIR 2 3 spectrum of the reddish pigments of -Fe O , -Fe O @SiO NPs, and Fe O @SiO . The 2 3 2 3 2 2 3 2 Appl. Nano 2023, 4, FOR PEER REVIEW 10 Generally, the obtained core-shell NPs are characterized using diverse analytical methods such as SEM, LC-MS, XPS, FTIR, XRD, TEM, BET, Ultraviolet-visible Spectros- copy, Raman spectrum as well as Near-Infrared Reflectance and Photoluminescence Spectroscopy [37,38,47,92–96]. For instance, assessment of morphology, chromaticity and the structure of α-Fe2O3@SiO2 fabricated pigments can be tested by SEM, TEM, FTIR, XPS and XRD [88]. Figure 5a shows the XRD patterns of the pigments made of α-Fe2O3@SiO2 NPs, -Fe2O3@SiO2 and α-Fe2O3. The formation of the core-shell structures results in the diffraction peak of α-Fe2O3@SiO2 particles appeared in the 2θ range of 15°–25°, indicating the presence of amorphous SiO2. Further calcinations at 1000 °C could change the dif- fraction peak to around 22°. The results of reddish colour pigment indicated that the amorphous shell has entered into a cristobalite phase. In addition, the formation of the core-shell structure weakened the α-Fe2O3 diffraction peak. Figure 5b shows the FTIR spectrum of the reddish pigments of α-Fe2O3, α-Fe2O3@SiO2 NPs, and Fe2O3@SiO2. The Appl. Nano 2023, 4 84 −1 hydroxyl (–OH) stretching vibration bands were probed at 3423.50, 1627.85 cm , 536.19 −1 and 466.75 cm , indicating a correlation to the O–Fe–O bands of α-Fe2O3. The band at hydroxyl (–OH) stretching vibration bands were probed at 3423.50, 1627.85 cm , 536.19 −1 1091.66 and 470 cm emerged from the covering of α-Fe2O3 in SiO2, indicating the bend- and 466.75 cm , indicating a correlation to the O–Fe–O bands of -Fe O . The band at 2 3 ing and stretching modes of O–Si–O. The FTIR results confirmed the formation of coating 1091.66 and 470 cm emerged from the covering of -Fe O in SiO , indicating the bend- 2 3 2 on the α-Fe2O3 surface. Further calcinations could enhance the O-Si-O bond strength as ing and stretching modes of O–Si–O. The FTIR results conﬁrmed the formation of coating well as improve the core and shell interactions. Figure 5c,d show the assessment results on the -Fe O surface. Further calcinations could enhance the O-Si-O bond strength as 2 3 well as improve the core and shell interactions. Figure 5c,d show the assessment results of the reddish pigments through the use of XPS. Figure 5c shows that Fe–O bonds and of the reddish pigments through the use of XPS. Figure 5c shows that Fe–O bonds and Si–O bonds are in the O1s pigment as evidenced from the high-resolution XPS spectrum. Si–O bonds are in the O1s pigment as evidenced from the high-resolution XPS spectrum. Meanwhile, a band that was observed at 103.5 eV in the Si 2p XPS spectrum is expected in Meanwhile, a band that was observed at 103.5 eV in the Si 2p XPS spectrum is expected in pure silica. pure silica. Figure 5. (a) XRD patterns and (b) FTIR spectra of different samples; high-resolution XPS spectra Figure 5. (a) XRD patterns and (b) FTIR spectra of different samples; high-resolution XPS spectra of of (c) O 1s and (d) Si 2p for -Fe O @SiO pigments calcined at 1000 C [88]. Reproduced with 2 3 2 (c) O 1s and (d) Si 2p for α-Fe2O3@SiO2 pigments calcined at 1000 °C [88]. Reproduced with per- permission from Chen, S., et al., Applied Surface Science; published by Elsevier, 2020. mission from Chen, S., et al., Applied Surface Science; published by Elsevier, 2020. Li et al. [47] conducted an analysis on the synthesized -Ce S @SiO core-shell ma- 2 3 2 terials L using i et a TEM l. [4test. 7] con The dTEM ucted images an an in aFigur lysis eo 6nshow the the syn silica thesiz shell ed being γ-Ceformed 2S3@SiO at 2 core-shell ma- various coating times. A clear layer covers the -Ce S but it is not found on the samples 2 3 terials using TEM test. The TEM images in Figure 6 show the silica shell being formed at that are not coated, which is in accordance to the SEM analysis. Figure 6b–d shows a various coating times. A clear layer covers the γ-Ce2S3 but it is not found on the samples correlation between the increasing thickness of the coating layer and increasing coating that are not coated, which is in accordance to the SEM analysis. Figure 6b–d shows a times. It was demonstrated that when the particles were coated once, twice and thrice correlation between the increasing thickness of the coating layer and increasing coating times, the thickness was increased to 70 nm, 100 nm and 140 nm, respectively. This clearly indicated that it is possible to control the coating thickness through number of coatings times. It was demonstrated that when the particles were coated once, twice and thrice being applied. Appl. Nano 2023, 4, FOR PEER REVIEW 11 Appl. Nano 2023, 4, FOR PEER REVIEW 11 times, the thickness was increased to 70 nm, 100 nm and 140 nm, respectively. This clearly indicated that it is possible to control the coating thickness through number of coatings times, the thickness was increased to 70 nm, 100 nm and 140 nm, respectively. This clearly being applied. Appl. Nano 2023, 4 85 indicated that it is possible to control the coating thickness through number of coatings being applied. Figure 6. TEM images of (a) uncoated γ-Ce2S3 and (b) once, (c) twice (d) thrice coated γ-Ce2S3@SiO2 core-shell particles [47]. Reproduced with permission from Li, Y.-M., et al., Surface and Coatings Figure 6. TEM imaT gec esh o n f ol (a og ) u yn ; c pu oab tli ed sh γ e- d Ce by 2S El 3 a se nd vie (b r,) 20 on 1c 8e, . (c) twice (d) thrice coated γ-Ce2S3@SiO2 Figure 6. TEM images of (a) uncoated -Ce S and (b) once, (c) twice (d) thrice coated -Ce S @SiO 2 3 2 3 2 core-shell particles [47]. Reproduced with permission from Li, Y.-M., et al., Surface and Coatings core-shell particles [47]. Reproduced with permission from Li, Y.-M., et al., Surface and Coatings Technology; published by Elsevier, 2018. Technology; Liu et published al. [97] by used Elsevier four , 2018. types of tests (FTIR, TEM, XRD and EDS) to assess the morphology of fabricated -Ce2S3@SiO2 samples. The first step was to assess the SiO2 Liu et al. [97] used four types of tests (FTIR, TEM, XRD and EDS) to assess the Liu et al. [97] used four types of tests (FTIR, TEM, XRD and EDS) to assess the thickness used to coat γ-Ce2S3. This was performed through the TEM test. Figure 7 shows morphology of fabricated -Ce S @SiO samples. The ﬁrst step was to assess the SiO 2 3 2 2 morphology of fabricated -Ce2S3@SiO2 samples. The first step was to assess the SiO2 different amounts of volume ratios of water/ethanol that was used for the preparation of thickness used to coat -Ce S . This was performed through the TEM test. Figure 7 shows 2 3 thickness used to coat γ-Ce2S3. This was performed through the TEM test. Figure 7 shows uncoated γ-Ce2S3 pigments and SiO2 xerogel coated γ-Ce2S3. Figure 7a presents the de- different amounts of volume ratios of water/ethanol that was used for the preparation different amounts of volume ratios of water/ethanol that was used for the preparation of posited surface with irregularly large chunks accompanied by small particles on the un- of uncoated -Ce S pigments and SiO xerogel coated -Ce S . Figure 7a presents the 2 3 2 2 3 uncoated γ-Ce2S3 pigments and SiO2 xerogel coated γ-Ce2S3. Figure 7a presents the de- coated γ-Ce2S3 pigments. The detected Zn signals within EDS spectra indicated that the deposited surface with irregularly large chunks accompanied by small particles on the posited surface with irregularly large chunks accompanied by small particles on the un- col uncoated our sta bi -Ce lity S opigments. f the unco The ated detected γ-Ce2SZn 3 pigm signals ents within can be EDS con spectra trolled indicated using Z that nO.the Another 2 3 coated γ-Ce2S3 pig a colour d m va ent nta stability s. ge Th oe f d th of ete e the cte applica uncoated d Zntio sin gn o -Ce afl s th wi S ese tpigments h in pig E m DS ent can spec s is be tra itcontr s ilo nd w olled ica H ted 2S using e th m ais t ZnO. s th ioe ns. Another Figure 7b–d 2 3 advantage of the application of these pigments is its low H S emissions. Figure 7b–d colour stability osh f th ows e u n pr co ese ated nce γ o -f Ce core 2S3 -pigm shell ents struc ca ture n be s wi con thtrol in aled ll th usi e pigm ng 2 Ze nn O. t pa Articles notherd uring coating. shows presence of core-shell structures within all the pigment particles during coating. advantage of the Sim applica ultantio eous n ly of , S th i si ese gn a pig l is m d ent etes cte isd it as s sh low own H 2in S e Fi m gur isse io6 nfs. . Thi Figu s pr re o v 7es b–d that SiO2 xerogel Simultaneously, Si signal is detected as shown in Figure 7f. This proves that SiO xerogel shows presence o m f acore de up -sho ell f th ste ruc coa ture tins g wi lay th er in th all at tis he fo pigm rmed e no t npa th rticles e γ-Ce d 2uri S3 surf ng c ao ce atin . Fi g. gure 7b,c on the made up of the coating layer that is formed on the -Ce S surface. Figure 7b,c on the other 2 3 Simultaneously, S oi th si er gn h aa l n is d d is ete sh cte owi d n as g sh tho awn t th e ina Fi pplica gure tio 6fn . Thi of s wpr ater ov/es etha thn ao t l Sv iO olum 2 xerog e ra el tio of 15/105 (48 hand is showing that the application of water/ethanol volume ratio of 15/105 (48 nm) and nm) and 20/100 (60 nm) results in a moderately uniform shell size. However, Figure 6d made up of the coating layer that is formed on the γ-Ce2S3 surface. Figure 7b,c on the 20/100 (60 nm) results in a moderately uniform shell size. However, Figure 7d shows that shows that when the ratio is adjusted to 25/95, the thickness of the shell is no longer other hand is showing that the application of water/ethanol volume ratio of 15/105 (48 when the ratio is adjusted to 25/95, the thickness of the shell is no longer uniform. The uniform. The main reason for this is that as water volume rise, it will accelerate TWOS nm) and 20/100 (60 nm) results in a moderately uniform shell size. However, Figure 6d main reason for this is that as water volume rise, it will accelerate TWOS hydrolysis. This hydrolysis. This means that during the coating process, shell thickness is no longer uni- shows that when the ratio is adjusted to 25/95, the thickness of the shell is no longer means that during the coating process, shell thickness is no longer uniform because of the f competition orm because between of the surface competand ition silica betw nuclei. een surface and silica nuclei. uniform. The main reason for this is that as water volume rise, it will accelerate TWOS hydrolysis. This means that during the coating process, shell thickness is no longer uni- form because of the competition between surface and silica nuclei. Figure 7. TEM images and EDS patterns of SiO xerogel coated -Ce S prepared with different 2 2 3 Figure 7. TEM images and EDS patterns of SiO2 xerogel coated -Ce2S3 prepared with different water water to ethanol ratio: (a) S0, (b) S1, (c) S2, (d) S3, (e) EDS spectra of S0 and (f) EDS spectra of to ethanol ratio: (a) S0, (b) S1, (c) S2, (d) S3, (e) EDS spectra of S0 and (f) EDS spectra of S2 [97]. S2 [97]. Reproduced with permission from Liu, S.-G. et al., Applied Surface Science; published by Reproduced with permission from Liu, S.-G. et al., Applied Surface Science; published by Elsevier, Elsevier, 2016. Figure 7. TEM images and EDS patterns of SiO2 xerogel coated -Ce2S3 prepared with different water to ethanol ratio: (a) S0, (b) S1, (c) S2, (d) S3, (e) EDS spectra of S0 and (f) EDS spectra of S2 [97]. Figure 8 shows the reﬂectance spectrum measured by Sadeghi-Niaraki et al. [98] for Reproduced with permission from Liu, S.-G. et al., Applied Surface Science; published by Elsevier, the as-produced Fe O @TiO with crystallite size (nm) of CT (32.2 nm), CFT2 (31.4 nm), 2 3 2 Appl. Nano 2023, 4, FOR PEER REVIEW 12 Appl. Nano 2023, 4 86 Figure 8 shows the reflectance spectrum measured by Sadeghi-Niaraki et al. [98] for the as-produced Fe2O3@TiO2 with crystallite size (nm) of CT (32.2 nm), CFT2 (31.4 nm), CFT4 (28.4 nm), and CFT5 (13.3 nm) samples. CT sample showed that the reflectivity at CFT4 (28.4 nm), and CFT5 (13.3 nm) samples. CT sample showed that the reﬂectivity at wavelengths improved which was due to the increase in the crystallinity of the emergent wavelengths improved which was due to the increase in the crystallinity of the emergent rutile phase. After the calcinations, the reflectance value increased as the sample experi- rutile phase. After the calcinations, the reﬂectance value increased as the sample experiences ences the crystallisation. Figure 8c shows that the presence of Fe2O3 produced darker the crystallisation. Figure 8c shows that the presence of Fe O produced darker hues within 2 3 hues within the samples in addition to NIR reflectance being reduced. The NIR solar re- the samples in addition to NIR reﬂectance being reduced. The NIR solar reﬂectance for the flectance for the samples was CT (76%), CFT2 (73%), CFT4 (68.8%), CFT5 (68.4%) and CF samples was CT (76%), CFT2 (73%), CFT4 (68.8%), CFT5 (68.4%) and CF (39.3%). Figure 8d (39.3%). Figure 8d shows the IR reflectance process within Fe2O3–TiO2 and Fe2O3 particles. shows the IR reﬂectance process within Fe O –TiO and Fe O particles. 2 3 2 2 3 Figure 8. Reﬂectance spectra of (a) T, FT2, FT4, FT5 and F samples, (b) CT, CFT2, CFT4, CFT5 and Figure 8. Reflectance spectra of (a) T, FT2, FT4, FT5 and F samples, (b) CT, CFT2, CFT4, CFT5 and CF samples, (c) photographs of CT, CFT2, CFT4, CFT5 and CF samples (d) proposed mechanism CF samples, (c) photographs of CT, CFT2, CFT4, CFT5 and CF samples (d) proposed mechanism of of IR reﬂectance in Fe O and Fe O @TiO composites [98]. Reproduced with permission from 2 3 2 3 2 IR reflectance in Fe2O3 and Fe2O3@TiO2 composites [98]. Reproduced with permission from Sadeghi-Niaraki, S. et al., Materials Chemistry and Physics; published by Elsevier, 2019. Sadeghi-Niaraki, S. et al., Materials Chemistry and Physics; published by Elsevier, 2019. Li et al. [99] tested the high temperature tolerance of the red pigments made from Li et al. [99] tested the high temperature tolerance of the red pigments made from Ce S @SiO -based core-shell NPs. Figure 9 shows the XRD patterns related to the - 2 3 2 Ce2S3@SiO2-based core-shell NPs. Figure 9 shows the XRD patterns related to the Ce S @c-SiO samples, where their production was subjected to various calcination tem- 2 3 2 γ-Ce2S3@c-SiO2 samples, where their production was subjected to various calcination peratures. There is an absence of the commonly found SiO diffraction peaks when the temperatures. There is an absence of the commonly found SiO2 diffraction peaks when calcination temperatures occur at the range from 1100 C to 1150 C. However, the diffrac- the calcination temperatures occur at the range from 1100 °C to 1150 °C. However, the tion peaks occurred during the -Ce S crystalline phase. This means that SiO failed to 2 3 2 diffraction peaks occurred during the γ-Ce2S3 crystalline phase. This means that SiO2 crystallize. However, c-SiO diffraction peak initiated as the temperature reached 1200 C. failed to crystallize. However, c-SiO2 diffraction peak initiated as the temperature This suggests that SiO will only crystallise within Ar gas atmosphere when temperature reached 1200 °C. This suggests that SiO2 will only crystallise within Ar gas atmosphere reaches 1200 C. c-SiO diffraction peak’s intensity remains approximately at constant level as temperature is further raised to 1250 C. Therefore, c-SiO is prone to crystallisation when temperature reaches 1200 °C. c-SiO2 diffraction peak’s intensity remains approxi- when two conditions are met; (a) it is within Ar-gas atmosphere (b) temperature to be at mately at constant level as temperature is further raised to 1250 °C. Therefore, c-SiO2 is least 1200 C. In another study, Li et al. [100] analyzed the -Ce S red pigments’ resistance prone to crystallisation when two conditions are met; a) it is2 wi 3 thin Ar-gas atmosphere b) through XRD test. temperature to be at least 1200 °C. In another study, Li et al. [100] analyzed the γ-Ce2S3 red pigments’ resistance through XRD test. Appl. Nano 2023, 4, FOR PEER REVIEW 13 Appl. Nano 2023, 4 87 Fig Figure ure 9 9. . XRD XRD pa patterns tterns of of the the γ-Ce -Ce2S S3@c @c-SiO -SiO2 sa samples mples aat t di dif fffer eren ent t si sintering ntering ttemperatur emperature es s iin n A Ar r g gas as 2 3 2 atmosphere [99]. Reproduced with permission from Li, Y. et al., Applied Surface Science; published atmosphere [99]. Reproduced with permission from Li, Y. et al., Applied Surface Science; published by Elsevier, 2020. by Elsevier, 2020. 3.3. Applications of Core-Shell Pigments 3.3. Applications of Core-Shell Pigments Pigments can serve as decoration or delineation purposes in the public street thus Pigments can serve as decoration or delineation purposes in the public street thus improving both the aesthetics and public safety. Infrastructures that are well-built and improving both the aesthetics and public safety. Infrastructures that are well-built and well-planned can motivate individuals to use them such as walking, cycling or ease of well-planned can motivate individuals to use them such as walking, cycling or ease of access for Personal Mobility Devices (PMDs). If individuals are more willing to do the access for Personal Mobility Devices (PMDs). If individuals are more willing to do the aforementioned activities, they are less likely to use their cars for short destination, which aforementioned activities, they are less likely to use their cars for short destination, which in return contribute to their overall health lifestyle. Figure 10 shows various red pigments in return contribute to their overall health lifestyle. Figure 10 shows various red pigments being applied on Singapore roads for pedestrian use. Despite an advancement in the being applied on Singapore roads for pedestrian use. Despite an advancement in the comprehension of the causes and effects of material failure, it remains a major concern comprehension of the causes and effects of material failure, it remains a major concern in in the entire construction industry. Exterior durability is typically enhanced through the the entire construction industry. Exterior durability is typically enhanced through the use use of high-performance coatings. Pigments are chosen for both the desired colour and of high-performance coatings. Pigments are chosen for both the desired colour and per- performance [101]. The paint industry would exclusively use high-quality pigments. It is formance [101]. The paint industry would exclusively use high-quality pigments. It is important for these pigments’ particles to be homogenous in size as it could have an effect important for these pigments’ particles to be homogenous in size as it could have an ef- on the paint’s attributes such as lightening capacity, hiding power, tinting strength and fect on the paint’s attributes such as lightening capacity, hiding power, tinting strength gloss. Furthermore, it is mandatory to apply nanoscale pigment particles in luminescent and gloss. Furthermore, it is mandatory to apply nanoscale pigment particles in lumi- materials for the purpose of UV-coatings and colouring. nescent materials for the purpose of UV-coatings and colouring. There is a higher desirability for coloured asphalt and red concrete in comparison to There is a higher desirability for coloured asphalt and red concrete in comparison to traditional materials as the former has better aesthetics from the viewpoint in architecture traditional materials as the former has better aesthetics from the viewpoint in architec- design [102]. The past few years have seen major development in nanomaterials and nan- ture design [102]. The past few years have seen major development in nanomaterials and otechnology. This has made synthesising core-shell NPs possible, which also contributed nanotechnology. This has made synthesising core-shell NPs possible, which also con- to developing pigments that are sustainable yet higher colour stability as well as able to tributed to developing pigments that are sustainable yet higher colour stability as well as tolerate harshness. The development of the pigments with increased durability has led to able to tolerate harshness. The development of the pigments with increased durability increase many potential applications in applying colours onto concrete and asphalt. This has led to increase many potential applications in applying colours onto concrete and leads to further development within the architect industry where they have the options asphalt. This leads to further development within the architect industry where they have to apply colours that carry more stability and higher tolerance to abrasion. Such develop- the options to apply colours that carry more stability and higher tolerance to abrasion. ments indeed could be combined with the aesthetic and decorative aspects of conventional Such developments indeed could be combined with the aesthetic and decorative aspects concrete thus forming an additional material with attractive features. of conventional concrete thus forming an additional material with attractive features. 4. Nano-Enhanced Phase Change Materials 4. Nano-Enhanced Phase Change Materials Buildings consume about 45% of global energy. Many passive cooling methods have Buildings consume about 45% of global energy. Many passive cooling methods have been used to lower the consumption rate. In addition, the phase change materials (PCM) been used to lower the consumption rate. In addition, the phase change materials (PCM) are installed within these buildings for the purpose of promoting temperature moderation, a stopping re installheat ed wi fr tom hin accumulation, these buildingsimpr for th oved e pur heat pose absorption of promoand ting minimize temperature indoor modheat era- Appl. Nano 2023, 4 88 gain. The method in which PCM stores thermal energy is effective in improving the build- ings’ aggregate heat capacity. Interest has been strong in PCMs that has high energy density to be deployed in buildings with high thermal inertia in order to save a high amount of energy. PCMs have their own drawbacks and the primary one being extra time required to charge/discharge energy process as well as storage performance, which happens due to poor thermal conductivity. Therefore, attention is focused on improving their thermal conductivity through the use of nanotechnology and nanomaterials. There has been a rapid development lately within the nanomaterials ﬁeld resulting in the latest technol- ogy with nanosized particles in improving the PCM’s thermophysical properties. PCM has several thermal and physical qualities such as viscosity, heat capacity, super-cooling and thermal conductivities. These attributes could be signiﬁcantly improved through dispersal of thermal conductive nanoparticles including nanometal-oxide, nanocarbon and nanometals. This article explores the research that have been recently conducted in the aforementioned development of nanomaterials that are being used to improve the PCMs thermal performance. This is appropriate in passive-cooling within the built-environment. The focus would be on materials’ type, method of synthetisation, and the outcome of the improvement. According to Mardiana and Riffat [103], about 30% of the total energy of any nation is consumed by the residential, institutional, commercial and industrial buildings. Ap- proximately, 60% of the energy is used in a building equipped with heating, ventilation and air-conditioning (HVAC) systems. PCM is a preferred building cooling method in comparison with other methods as it compliments green building with efﬁcient energy per- formance [104]. An effective strategy is phase change technology, where it could enhance the building’s thermal mass. This means removing heat from indoors, reduce temperature variations and disperse heat away from the building with the overall impact of increasing the comfort of the occupants. Studies have discovered that PCMs energy saving ranged from 10% to 30% from air-conditioning consumption within various climate in the United States [105]. During the summer, the energy savings could be up to 30% when PCMs are built on building walls. Microcapsules of PCM application results in the reduction of internal temperature of a building by 4 C and in a longer period of time, it stops the temperature from reaching for more than 28 C. PCMs are classiﬁed as inorganic, organic and eutectic. Types of inorganic PCMs include metal alloys, metals, and hydrated salts whereas an example of organic PCMs is hydrocarbons-based parafﬁn wax. There are disadvantages of PCM such as thermal insta- bility, corrosive property, sub-cooling, low thermal conductivity, leakage, phase segregation and many more [106]. In comparison, organic PCMs are sometimes more suitable due to their non-corrosive properties, immense latent heat capacity, congruent melting and self-nucleation, chemically inert as well as being thermally stable [107]. Dispersion of a con- trolled amount of nucleating or dispersant agents is a solution in addressing subcooling and phase segregation issue [106]. Nevertheless, PCM has an inherent low thermal conductivity, denoted by “k”. These results in low level of responsiveness during which a thermal change occurs rapidly due to charging/discharging process and its lowered storage capacity. Such issue becomes the centre of attention in research related to thermal energy storage. The k values of hydrocarbon-based PCM range from 0.1 to 0.4 W/mK. Noctadecane is a type of PCM, which possess low solid state thermal conductivity at 0.35 W/mK. It’s liquid state however is at 0.149 W/mK [108]. Rapid development of nanomaterials led to the emergence of novel application strategy at its high level of conductive ultra-small nanosized particles including metal oxides, carbon and metals. These can be used to produce nano-enhanced PCM (nePCM) with signiﬁcant micro-convection [3] and thermal conductivity [109]. Ample opportunities exist for nanomaterials potential applications in the cutting edge phase change technology. PCM has generated intense interest in the application of nanometer-scaled thermal conductors through nanoﬁbers, nanoparticles, nanosheets, nanotubes and nanofoams [104]. The thermal conductivity of PCM can be enhanced using three methods. First method involves Appl. Nano 2023, 4 89 the incorporation of PCM into porous media such as metallic foams and porous carbon, which has high thermal conductivity. Second method deals with the dispersion of high thermal conductivity metallic nanostructures or nanoparticles of Cu, Ag or Al to the PCM. Third method deals with the microencapsulation of the PCM [108]. The thermal conductivity and strength of microcapsules’ wall could be increased through nanoparticles that are made of silver [110]. An efﬁcient way of improving PCM additive is copper particles due to its high conductivity and low cost [111]. Three types of elements with thermal conductivity have extensively been studied [112]. These include carbon-based nanostructures such as graphene nanoﬂakes, nanoplatelets, carbon nanotubes CNT and nanoﬁbers; metallic oxide like TiO and MgO; metals like Al, Ag and Cu. There is a signiﬁcant improvement on heat transfer through the use of nanoparticles. The nanoparticles that can be applied to achieve this are carbon that possess various morphologies such as ceramic oxide (CuO, Al O ), metallic nitrides (AIN, SiN), 2 3 metallic carbides (SiC) and stable metals (gold Au) [105]. Nanomaterials that comprise of metals (Cu, Ag and Al), metal oxides (ZnO) and carbon (single wall SWCNT, graphene nanosheets, active carbon, carbon nanoﬁbers, expanded graphite sheets) increase PCM’s rate of heat transfer [113]. In this view, the prominent research being conducted on the development of thermal conductivity through the dispersion of three primary PCM nano- enhancers such as nanometals, nanocarbons and nano-metal oxides. 4.1. Nano-Metal Enhancer It is a common knowledge that metal is efﬁcient at heat conductivity. Silver in par- ticular is the optimal conductor of heat and electricity in comparison with other metals. Its thermal conduction value is approximately 430 W/(mK). The next two metals that are close to silver in terms of thermal conductivities are copper and gold. Gold and silver have two major disadvantages, vulnerable to oxidation and high cost. Therefore, copper, at a signiﬁcantly lower cost has the advantages in comparison. Despite this, all the three afore- mentioned metals have been extensively researched as possible solutions in addressing PCM’s thermal conductivities. Al-Shannaq [108] improved the PCM’s thermal conductivity (k) by 1168% through the use of nano-thick Ag shells. Specially microencapsulated pure PCM could be used to address leakages issues during its change of state from solid to liquid. However, the microencapsulated shell with poor conductivity value k served as a barrier to achieving a desirable level of heat transfer and energy storage. A method has been formulated to enhance the PCM’s microencapsulated k value that involved the use of a layer of metallic shell to cover the microcapsules. This was done by activating the surface with dopamine and conducting electroless plating. The k value was increased to 0.189 from 0.062 W/mK when the diameter of uncoated PCM was increased to 26.9 m from 2.4 m. While the diameter was retained at 26.9 m, a signiﬁcant increase of the thermal conductivity (about 1168%) of metal-coated PCM capsules (2.41 W/mk from 0.189) was achieved. Such improvement of the thermal conductivity is highly correlated with the size of the shell area that is coated with silver on the surface of the PCM microsphere. The rapid improvement occurs upon the formation of the thermal conduction pathways. Deng et al. [114] have made another signiﬁcant improvement (1030%) in the thermal conductivity of the PCM via the synthesis of AgNWs. First, shape stabilised phase change materials (polyethylene glycol-silver/EVM ss-CPCMs) composites were produced via the embedment of PEG-Ag nanowires into expanded vermiculite EVM. To prevent the PCM leakage as well as to improve its thermal conductivity, a technique was proposed whereby the mixing and embedding are performed mechanically. For the purpose of PCM latent energy storage, polyethylene glycol was used. Figure 10 shows the silver nanowires that served as the thermal conductivity promoter. Furthermore, the PCM leakage during the melting was addressed through a support matrix (EVM vermiculite), enabling the enhancement in the mechanical strength. Appl. Nano 2023, 4, FOR PEER REVIEW 16 Appl. Nano 2023, 4, FOR PEER REVIEW 16 Appl. Nano 2023, 4 90 Figure 10. (a) SEM photos of synthesized silver nanowires. (b) Variation between the predicted Figure 10. (a) SEM photos of synthesized silver nanowires. (b) Variation between the predicted t Fig her ure ma l 1c 0on . (a duc ) St EM ivit y ph kot vos alue of w sy it n h t h m es ea izsur ed e si d lvv ea rlues nano of wP irC es M . ( b n) an V oc aromp iatioos n ib tes etw [1 een 14] .t h Rep e pr rod educ icted ed thermal conductivity k value with measured values of PCM nanocomposites [114]. Reproduced with with permission from Deng, Y. et al., Chemical Engineering Journal; published by Elsevier, 2016. thermal conductivity k value with measured values of PCM nanocomposites [114]. Reproduced permission from Deng, Y. et al., Chemical Engineering Journal; published by Elsevier, 2016. with permission from Deng, Y. et al., Chemical Engineering Journal; published by Elsevier, 2016. Significant improvement in the k value of PEG infused silver vermiculite composites Signiﬁcant improvement in the k value of PEG infused silver vermiculite composites Significant improvement in the k value of PEG infused silver vermiculite composites was achieved using nanowires of length 5–20 μm and diameter 50–100 nm. An increase was achieved using nanowires of length 5–20 m and diameter 50–100 nm. An increase as was achieved using nanowires of length 5–20 μm and diameter 50–100 nm. An increase as much as 1130% for the k value (0.68 W/mK) was achieved compared to the neat PCM much as 1130% for the k value (0.68 W/mK) was achieved compared to the neat PCM with as much as 1130% for the k value (0.68 W/mK) was achieved compared to the neat PCM with latent heat capacity at 96.4 J/g. The vermiculite has incited supercooling to occur latent heat capacity at 96.4 J/g. The vermiculite has incited supercooling to occur where the with latent heat capacity at 96.4 J/g. The vermiculite has incited supercooling to occur where the temperature dropped by 7 °C upon the PCM for PEG–Ag/EVM ss-CPCMs. temperature dropped by 7 C upon the PCM for PEG–Ag/EVM ss-CPCMs. Such reaction is where the temperature dropped by 7 °C upon the PCM for PEG–Ag/EVM ss-CPCMs. Such reaction is similar to nonuniform impregnates for developing nucleation and pro- similar to nonuniform impregnates for developing nucleation and promoting the formation Such reaction is similar to nonuniform impregnates for developing nucleation and pro- moting the formation of PEG-crystal. Such improvements are as a result of high k values of PEG-crystal. Such improvements are as a result of high k values due to the dispersion of moting the formation of PEG-crystal. Such improvements are as a result of high k values due to the dispersion of silver nanowire and vermiculite. Zeng et al. [107] obtained about silver nanowire and vermiculite. Zeng et al. [107] obtained about 800% improvement in the due to the dispersion of silver nanowire and vermiculite. Zeng et al. [107] obtained about 800% improvement in the thermal conductivity using CuNWs. The premise of the re- thermal conductivity using CuNWs. The premise of the research is to explore the impact 800% improvement in the thermal conductivity using CuNWs. The premise of the re- search is to explore the impact of CuNWs, which is copper nanowires has upon the of CuNWs, which is copper nanowires has upon the tetradecanoyl (TD)’s k value as the search is to explore the impact of CuNWs, which is copper nanowires has upon the tetradecanoyl (TD)’s k value as the phase change material. The TD was synthesised and phase change material. The TD was synthesised and classiﬁed accordingly to the range tetradecanoyl (TD)’s k value as the phase change material. The TD was synthesised and classified accordingly to the range of weight fractions of CuNW. The ratio and diameter of weight fractions of CuNW. The ratio and diameter of free-standing copper nanowires classified accordingly to the range of weight fractions of CuNW. The ratio and diameter of free-standing copper nanowires were at 350–450 and 90–120 nm, respectively with were at 350–450 and 90–120 nm, respectively with 40–50 m in length. The CuNW can then of free-standing copper nanowires were at 350–450 and 90–120 nm, respectively with 40–50 μm in length. The CuNW can then be fabricated in bulk through simple technique be fabricated in bulk through simple technique involving chemical reduction that is water 40–50 μm in length. The CuNW can then be fabricated in bulk through simple technique involving chemical reduction that is water based at room temperature. based at room temperature. involving chemical reduction that is water based at room temperature. Figure 11 presents the SEM images of the composite results, demonstrating that Figure 11 presents the SEM images of the composite results, demonstrating that Figure 11 presents the SEM images of the composite results, demonstrating that CuNws in TD has decent dispersion and entanglement. It is worth noting that the rate of CuNws in TD has decent dispersion and entanglement. It is worth noting that the rate of CuNws in TD has decent dispersion and entanglement. It is worth noting that the rate of weight loss is lower in comparison to pristine TD due to the structural nature of CuNWs, weight loss is lower in comparison to pristine TD due to the structural nature of CuNWs, weight loss is lower in comparison to pristine TD due to the structural nature of CuNWs, which is similar to a sponge and is capable of storing the TD within the voids. When the which is similar to a sponge and is capable of storing the TD within the voids. When the which is similar to a sponge and is capable of storing the TD within the voids. When the CuNWs is increased by 58.9 wt%, the thermal conductivity increased up to nine-fold, an CuNWs is increased by 58.9 wt%, the thermal conductivity increased up to nine-fold, an CuNWs is increased by 58.9 wt%, the thermal conductivity increased up to nine-fold, an 800% enhancement. 800% enhancement. 800% enhancement. Figure 11. (a) SEM photos of synthesized CuNWs. (b) Thermal conductivity of PCM composites Figure 11. (a) SEM photos of synthesized CuNWs. (b) Thermal conductivity of PCM composites with increasing CuNWs loadings [107]. Reproduced with permission from Zeng, J.-L. et al., Solar w Fig ith ure in c 1r1 ea . ( si an ) g S EM CuN ph Ws otos loa of din sy gs nt[h 1es 07i]z . ed Rep Cu rod NWs. uced ( b w ) itTh h p er er m m al isc si on on duc from tivit Ze y n of g,P J. CM -L. e ct omp al., os Sol ita es r Energy Materials and Solar Cells; published by Elsevier, 2012. En wit er hg in y c Ma reatsi er n ia gls Cu an N d Ws Sola lo ra Ce din llg s;s pu [10 b7 lis ]. h Rep ed b rod y El uc se ed vie w r,it 20 h 12. per mission from Zeng, J.-L. et al., Solar Energy Materials and Solar Cells; published by Elsevier, 2012. Appl. Nano 2023, 4 91 Zeng et al. [115] improved the thermal conductivity by about 356% using AgNWs (380%). The experiment involved the synthesis of silver nanowires and production of silver-doped PCM nanocomposites. The inclusion of AgNWs at 45 wt% results in two to three times enhancement of thermal conductivity in graphene-doped PCM. The enthalpy is reduced by 50% and its heat storage capacity has also been reduced. In terms of size, graphene dopants are ten times smaller in comparison to doping with silver nanowires. Furthermore, the enthalpy value has also been reduced three times in comparison to AgNWs. Shah et al. [116] have increased the PCM thermal conductivity by 160% through the use of copper nanowires (CuNWs). The enhancement of thermal conductivity (more than 50%) of calcium chloride hexahydrate is achieved by adding a trace of CuNWs at 0.17 wt%. The use of nano-copper results in optimum enhancement of k value at 160%; or an increase to 0.564 W/mK of PCM composite in comparison to 0.217 W/mk of neat PCM. Just a trace of CuNWs could result in such a signiﬁcant improvement thus nanoadditives can be considered as cost efﬁcient when being applied in buildings. Moleﬁet et al. [117] showed 70% improvement in the thermal conductivity using CuNPs. The thermal conductivity of parafﬁn was increased almost linearly as the CuNPs amounts were increased. Parafﬁn wax was used as the PCM base, which was subsequently mixed with molecular-weight polyethylene at low, medium and high rate. The copper particles were mixed with parafﬁn mixture resulting in the enhancement of the base polyethylene PCM’s k value. Tang et al. [118] improved the thermal conductivity by 38.1% using CuNPs based on SiO -embedded-PEG PCM composite that is shape stable. When 2.1 wt% CuNPs were added, the k value was increased by 38.1% in comparison to neat PCM. Further addition of copper nanoadditives results in improvement on PEG/SiO hybrid PCMs. Wu et al. [119] have made 30.3% improvement on thermal conductivity through the use of CuNPs. Their results have shown a correlation where 1wt% of CuNPs could decrease the parafﬁn PCM heating by 30.3% and cooling by 28.2%. The charging time decreased by 30.3% while the discharging time was decreased by 28.2% upon the doping of nanocopper particles into the nanocomposites with 1 wt%. Melting PCM heat transfer rate is enhanced through the addition and mixture of nanoadditives (aluminium, copper and copper/carbon nanomaterials). In terms of improvement on heat transfer, nanocopper particles offer the most signiﬁcant rate amongst others. 4.2. Nano-Metal Oxide Enhancer Two examples of good heat conductors are alumina and copper, both of which are metal oxides with values from 30 to 40 W/mK. Pure metals typically are better heat conductors but they are not as chemically stable in comparison to the metal oxides. In addition, metal oxides are more cost effective and reliable in its performance. For these reasons, they are more sought after as a material to replace pure metals. Babapoor et al. [120] used various NPs types to enhance the thermal conductivity of k value. The metals with the enhancement percentage of Al O (144%), Fe O (144%), ZnO (110%) and SiO (110%) 2 3 2 3 2 were obtained. In these tests, nanomaterials of silica (~20 nm), alumina (~20 nm), iron oxide (~20 nm), and zinc oxide (>50 nm) were used. These nanomaterials were added as thermal enhancers and mixed with NPs (SDS) as well as surfactant (CTAB) to achieve enhanced PCM. The sample doped with Al O NPs showed the highest enhancement in 2 3 the thermal conductivity of 0.919 W/mK. The doping of NPs gave various enhancement (%) level depending on the concen- tration (wt%) of Al O NPs: 4 wt% (120%), 6 wt% (141.2%) and 8 wt% (144%); Fe O 2 3 2 3 NPs of 4 wt% (80%), 6 wt% (135%) and 8 wt% (144%); ZnO NPs of 4 wt% (85%), 6 wt% (100%) and 8 wt% (110%); SiO NPs of 4 wt% (78%), 6 wt% (110%) and 8 wt% (110%). The results revealed that higher level of concentration of conductive nanomaterials lead to higher k value of the nanocomposites. It was concluded that Al O and Fe O carry the 2 3 2 3 most signiﬁcant impact in terms of enhancing the thermal conductivity of parafﬁn-based PCM. Sharma et al. [121] achieved an improvement in the thermal conductivity of about 80% using TiO . The study involved the performance of palmitic acid (PA) based thermal 2 Appl. Nano 2023, 4, FOR PEER REVIEW 18 Appl. Nano 2023, 4 92 ergy storage of synthesised PCM composites that were doped with TiO2 NPs. By mixing TiO2 into neat PCM an enhancement in the k value was 12.7%, 20.6%, 46.6% and 80% for the corresponding TiO2 concentrations of 0.5 wt%, 1 wt%, 3 wt% and 5 wt%, respectively. energy storage of synthesised PCM composites that were doped with TiO NPs. By mixing The high concentration of TiO2 within the PCM resulted in curvilinear characteristic of TiO into neat PCM an enhancement in the k value was 12.7%, 20.6%, 46.6% and 80% for the ther corr maesponding l enhanceT m iO ent. concentrations Li et al. [12 of 2] 0.5 achi wt%, eved 1 wt%, 43.8 3% wt% and and 40 54wt%, % im rp espectively rovemen.t in the The high concentration of TiO within the PCM resulted in curvilinear characteristic of thermal conductivity using 2 TiO2 NPs foam and TiO2 NPs with a nanocarbon shell layer, thermal enhancement. Li et al. [122] achieved 43.8% and 404% improvement in the thermal respectively. The synthesis of porous TiO2 foams PTFs involved the use of octane as mi- conductivity using TiO NPs foam and TiO NPs with a nanocarbon shell layer, respectively. 2 2 croemulsifier and TiO2 as particle stabilizer (microemulsion technique) as shown in Fig- The synthesis of porous TiO foams PTFs involved the use of octane as microemulsiﬁer ure 12. The nanosized TiO2 measured at approximately 23 nm consisting of 20% rutile and TiO as particle stabilizer (microemulsion technique) as shown in Figure 12. The and 80% anatase. Polyacrylic acid-ammonium salt is used as the dispersing agent. It is nanosized TiO measured at approximately 23 nm consisting of 20% rutile and 80% anatase. added on the surface modifier along with a small amphiphilic molecule propyl gallate Polyacrylic acid-ammonium salt is used as the dispersing agent. It is added on the surface (C10H12O5). modiﬁer along with a small amphiphilic molecule propyl gallate (C H O ). 10 12 5 Figure 12. (a) SEM images of PTF. (b) Enhancement of thermal conductivities of PCM parafﬁn, Figure 12. (a) SEM images of PTF. (b) Enhancement of thermal conductivities of PCM paraffin, PTF/PCM, and PTFC/PCM composites by 43.8 and 404%. (Inset) TEM photo of the prepared PCM PTF/PCM, and PTFC/PCM composites by 43.8 and 404%. (Inset) TEM photo of the prepared PCM composite carbonized porous TiO foams (PTFC) particles [122]. Reproduced with permission from composite carbonized porous TiO2 foams (PTFC) particles [122]. Reproduced with permission from Li, Y. et al., Applied Energy; published by Elsevier, 2016. Li, Y. et al., Applied Energy; published by Elsevier, 2016. The 3D porous structure of PTFs contains continuously connected holes, enabling The 3D porous structure of PTFs contains continuously connected holes, enabling the full absorption of parafﬁn wax without the need of any surfactant. The structure can the full absorption of paraffin wax without the need of any surfactant. The structure can also absorb sucrose and can burn off at 1200 C, resulting in a thin carbon-based ﬁlm also absorb sucrose and can burn off at 1200 °C, resulting in a thin carbon-based film wherein the carbon nanolayer is only 2 nm thick. Both pure PTF and carbon-based PTF nanocomposites wherein the carb wer one n mo anr o ela conductive yer is only than 2 nm pur th eic paraf k. Bo ﬁn th with pure kPTF values and of ca 0.302 rbonand -based PTF 1.059 W/mK, respectively. The k value of pure PCM reached to 0.302 W/mK when 25 wt% nanocomposites were more conductive than pure paraffin with k values of 0.302 and of TiO was added. This clearly indicated that the addition of TiO can enhance the k value 2 2 1.059 W/mK, respectively. The k value of pure PCM reached to 0.302 W/mK when 25 wt% by 0.092 W/mK. TiO foam structure lined with carbon nanoﬁlm plus parafﬁn showed a k of TiO2 was added. This clearly indicated that the addition of TiO2 can enhance the k value of 1.059 W/m K, indicating an increase of 504% than pure parafﬁn. This signiﬁcant value by 0.092 W/mK. TiO2 foam structure lined with carbon nanofilm plus paraffin increase was mainly due to the carbon matrix adherence onto the TiO NPs surfaces. It showed a k value of 1.059 W/m K, indicating an increase of 504% than pure paraffin. This was afﬁrmed that the novel hybrid of TiO NPs-porous foam with inner-lining carbon significant increase was mainly due to the carbon matrix adherence onto the TiO 2 NPs nanoﬁlms is effective for the enhancement of PCM demanded in the industrial purposes. surfaces. It was affirmed that the novel hybrid of TiO2 NPs-porous foam with inner-lining Zhang et al. [123] made about 18.2% improvement in the thermal conductivity using TiO wher carbo ein n n aa novel nofilm thermal-insulating s is effective for ﬁl th m e and enhpolyvinyl-chloride ancement of PCM (PVC) dema ﬁlm nded matrix in th wer e in e dustrial incorporated. Both TiO and microencapsulated n-octadecane PCM were used to block UV purposes. Zhang et a 2l. [123] made about 18.2% improvement in the thermal conductivity and act as an additive to regulate the temperature. When TiO NPs were added at 6 wt%, using TiO2 wherein a novel thermal-insulating film and polyvinyl-chloride (PVC) film the k value of the pure micro-PCM was reached to 0.2356 W/mK from 0.1994 W/mK for matrix were incorporated. Both TiO2 and microencapsulated n-octadecane PCM were the matrix, indicating an increase by 18.2%. Such thin ﬁlm with excellent heat insulation used to block UV and act as an additive to regulate the temperature. When TiO2 NPs and thermal regulating properties were afﬁrmed to be useful for the indoor living spaces were added at 6 wt%, the k value of the pure micro-PCM was reached to 0.2356 W/mK and cars. from 0.1994 W/mK for the matrix, indicating an increase by 18.2%. Such thin film with Sahan et al. [124] achieved about 60% of thermal conductivity improvement using excellent heat insulation and thermal regulating properties were affirmed to be useful for sol-gel synthesized Fe O NPs. These Fe O NPs (diameters ranged from 40 to 70 nm) 3 4 3 4 wer the ein pr depar oor ed livusing ing spa iron cechloride s and cahydrates rs. ((FeCl 6H O, FeCl 4H O), hydrochloride and 3 2 2 2 ammonia. They were mixed with parafﬁn in two concentration levels (10 and 20 wt%). Sahan et al. [124] achieved about 60% of thermal conductivity improvement using Particle aggregation was minimised through surface capping of oleic acid. These Fe O 3 4 sol-gel synthesized Fe3O4 NPs. These Fe3O4 NPs (diameters ranged from 40 to 70 nm) were prepared using iron chloride hydrates ((FeCl3 6H2O, FeCl2 4H2O), hydrochloride and ammonia. They were mixed with paraffin in two concentration levels (10 and 20 wt%). Particle aggregation was minimised through surface capping of oleic acid. These Appl. Nano 2023, 4, FOR PEER REVIEW 19 Appl. Nano 2023, 4 93 Fe3O4 NPs were uniformly dispersed on the paraffin matrix. The results showed an im- provement in the k values by 48% and 60% for the corresponding NPs concentration of 10 wt% and 20 wt%, respectively. This showed that nanomagnetite particles doping in PCM NPs were uniformly dispersed on the parafﬁn matrix. The results showed an improvement w ina the s vk er values y effec by tiv 48% e to and wa60% rds th fore the imcorr proesp veme onding nt of NPs its concentration thermal conof ductiv 10 wt% ityand and cost. Jiang et 20 wt%, respectively. This showed that nanomagnetite particles doping in PCM was very al. [125] observed 55% improvement in the thermal conductivity of PCM using effective towards the improvement of its thermal conductivity and cost. Jiang et al. [125] nano-Al2O3. The microencapsulation of paraffin was responsible for the formation of observed 55% improvement in the thermal conductivity of PCM using nano-Al O . The mi- 2 3 poly(methylmethacrylate-co-methylacrylate) polymeric PCM microcapsules (MEPCM). croencapsulation of parafﬁn was responsible for the formation of poly(methylmethacrylate- These microcapsules were further added with alumina NPs via the emulsion polymeri- co-methylacrylate) polymeric PCM microcapsules (MEPCM). These microcapsules were further added with alumina NPs via the emulsion polymerization, causing signiﬁcant zation, causing significant enhancement of k value (Figure 13) from 0.245 W/mK to 0.38 enhancement of k value (Figure 13) from 0.245 W/mK to 0.38 W/mK (increase by 55%). W/mK (increase by 55%). There is near parity in terms of the enhancement rate and There is near parity in terms of the enhancement rate and dosage of nano-Al O , indi- 2 3 dosage of nano-Al2O3, indicating that the presence of nano-Al2O3 caused a higher thermal cating that the presence of nano-Al O caused a higher thermal conductivity increase of 2 3 conductivity increase of PCM microcapsules. PCM microcapsules. Figure 13. (a) SEM photos of PCM microcapsules with 27 wt% nano-alumina. (b) Thermal conductiv- Figure 13. (a) SEM photos of PCM microcapsules with 27 wt% nano-alumina. (b) Thermal conduc- ities of PCM with various contents of nano-alumina [125]. Reproduced with permission from Jiang, tivities of PCM with various contents of nano-alumina [125]. Reproduced with permission from X. et al., Applied Energy; published by Elsevier, 2015. Jiang, X. et al., Applied Energy; published by Elsevier, 2015. Tong et al. [126] improved the PCM’s thermal conductivity using nano-SiO . The polymeric melamine-urea-formaldehyde was used for the polymerisation of in situ PCM Tong et al. [126] improved the PCM’s thermal conductivity using nano-SiO2. The parafﬁn microcapsules before adding graphite and nano-SiO . The results revealed that polymeric melamine-urea-formaldehyde was used for the polymerisation of in situ PCM the successful rate of parafﬁn microencapsulation was at 80% wherein the PCM parafﬁn paraffin microcapsules before adding graphite and nano-SiO2. The results revealed that was able to sustain its thermophysical properties. The addition of nano-SiO could change th the e micr succe ocapsules ssful ra resistance te of para against ffin m high icroe temperatur ncapsul e, at rio einfor n w cing as a the t 8str 0% uctural wherstr ein ength the PCM paraffin of composite and high afﬁnity to water. The k value was improved signiﬁcantly during was able to sustain its thermophysical properties. The addition of nano-SiO2 could melting time after the nanomaterials were added. Ai et al. [127] enhanced the thermal change the microcapsules resistance against high temperature, reinforcing the structural conductivity of PCM using high energy planetary milling wherein ZrO nanopowder- strength of composite and high affinity to water. The k value was improved significantly based stearic acid PCM was developed. A new parameter called heat capability factor during melting time after the nanomaterials were added. Ai et al. [127] enhanced the (HCF) was explored. Chloroform was used to disperse the nano-ZrO PCM composites, providing a better alternative (than carbon tetrachloride) for the dispersion during ZrO thermal conductivity of PCM using high energy planetary milling wh2erein ZrO2 na- synthesis. The results revealed that chloroform could improve the surface morphology and nopowder-based stearic acid PCM was developed. A new parameter called heat capabil- spherodization of ZrO . The highest HCF value of 0.9 for the mean size of PCM particles ity factor (HCF) was explored. Chloroform was used to disperse the nano-ZrO2 PCM was 1.2 m. However, the HCF value was reduced to 0.3 when the mean size of PCM composites, providing a better alternative (than carbon tetrachloride) for the dispersion particles became 0.4 m. The optimum PCM particles’ size (1.2 m) gave a signiﬁcant enhancement in the heat storage capability of chloroform-treated composite ZrO -PCM during ZrO2 synthesis. The results revealed that chloroform could improve the surface particles. Song et al. [128] used MgOH NPs and made nePCMs to enhancing the ﬁre morphology and spherodization of ZrO2. The highest HCF value of 0.9 for the mean size resistance of PCM. The supporting materials used were nano-sized red phosphorus (RP), of PCM particles was 1.2 μm. However, the HCF value was reduced to 0.3 when the mean MgOH and ethylene propylenedieneter polymer plastic (EPDM). The observed increase size of PCM particles became 0.4 μm. The optimum PCM particles’ size (1.2 μm) gave a in the ﬁre resistance quality was ascribed to the magnesium hydroxide within the ﬂame significant enhancement in the heat storage capability of chloroform-treated composite retardant shape-stable PCM composite. It was argued that the ﬁre-resistant attributes of the PCM can be further improved through the reduction of NPs diameter. Consequently, larger ZrO2-PCM particles. Song et al. [128] used MgOH NPs and made nePCMs to enhancing surface to volume ratio of MgOH can produce rapid breakdown and high reactivity when the fire resistance of PCM. The supporting materials used were nano-sized red phos- phorus (RP), MgOH and ethylene propylenedieneter polymer plastic (EPDM). The ob- served increase in the fire resistance quality was ascribed to the magnesium hydroxide within the flame retardant shape-stable PCM composite. It was argued that the fire-resistant attributes of the PCM can be further improved through the reduction of NPs diameter. Consequently, larger surface to volume ratio of MgOH can produce rapid breakdown and high reactivity when subjected to the combustion process, indicating higher fire resistance quality attainment of PCM composite. Appl. Nano 2023, 4, FOR PEER REVIEW 20 Appl. Nano 2023, 4 94 4.3. Nano-Carbon Enhancer subjected to the combustion process, indicating higher ﬁre resistance quality attainment of PCM composite. Carbon has higher thermal conductivity when benchmarked against metals and metal oxides. Graphite, graphene and CNTs thermal conductivities can be up to five 4.3. Nano-Carbon Enhancer times higher than silver. Research studies have increasingly focused on the carbon na- Carbon has higher thermal conductivity when benchmarked against metals and metal nomaterials thermal conductivities due to their continuous decrease in production cost. Ji oxides. Graphite, graphene and CNTs thermal conductivities can be up to ﬁve times et al. [129] improved PCM’s thermal conductivity by 1700% using ultra-thin graphite higher than silver. Research studies have increasingly focused on the carbon nanomaterials foams (UGF). The k value was increased by 18 times after adding UGF (at approximately thermal conductivities due to their continuous decrease in production cost. Ji et al. [129] 1.2 vol%) into the PCM matrix. However, no changes in the specific heat fusion or melting improved PCM’s thermal conductivity by 1700% using ultra-thin graphite foams (UGF). temperature were observed. Graphite foams consisted of ultrathin graphite connected The k value was increased by 18 times after adding UGF (at approximately 1.2 vol%) into the PCM matrix. However, no changes in the speciﬁc heat fusion or melting temperature strips. These strips possessed a higher k value than metals and solid carbon foams, indi- were observed. Graphite foams consisted of ultrathin graphite connected strips. These cating their better heat response and thermal properties. Liang et al. [130] obtained 1300% strips possessed a higher k value than metals and solid carbon foams, indicating their better improvement in the thermal conductivity using superoleophilic graphene nanosheets heat response and thermal properties. Liang et al. [130] obtained 1300% improvement in mixed with porous nickel Ni foam. In the synthesis of polydimethylsiloxane the thermal conductivity using superoleophilic graphene nanosheets mixed with porous (PDMS-G-NF) modified graphene-covered nickel foam they used graphene nanosheets nickel Ni foam. In the synthesis of polydimethylsiloxane (PDMS-G-NF) modiﬁed graphene- layering onto the porous Ni foam surface, causing the formation of graphene-nickel foam covered nickel foam they used graphene nanosheets layering onto the porous Ni foam G-NF. Further modifications were performed on the G-NF surface support using siloxane surface, causing the formation of graphene-nickel foam G-NF. Further modiﬁcations were PDMS for the fabrication of shape-stable PCM composite. performed on the G-NF surface support using siloxane PDMS for the fabrication of shape- stable PCM composite. Chen et al. [131] achieved 500% improvement in the k value of PCM using CNT Chen et al. [131] achieved 500% improvement in the k value of PCM using CNT foam. foam. The PCM was absorbed by a permeable support matrix, a carbon nanotube net- The PCM was absorbed by a permeable support matrix, a carbon nanotube network with work with structure similar to sponge. The heat storage capacity of PCM was improved structure similar to sponge. The heat storage capacity of PCM was improved and became and became efficient for both heat and electricity conduction. In addition, the PCM efﬁcient for both heat and electricity conduction. In addition, the PCM composite could composite could absorb light energy and generate heat via electricity. Figure 14 shows the absorb light energy and generate heat via electricity. Figure 14 shows the PCM composite PCM composite consisted of paraffin filled soft-flexible CNT-based porous material. The consisted of parafﬁn ﬁlled soft-ﬂexible CNT-based porous material. The support matrix support matrix that is deformable has high rate of thermal conductivity during the so- that is deformable has high rate of thermal conductivity during the solidiﬁcation and lidification and melting processes. melting processes. Figure 14. (a) SEM photo of the interior of CNT porous foam revealing a highly sponge-like mi- Figure 14. (a) SEM photo of the interior of CNT porous foam revealing a highly sponge-like mi- crostructure. (b) Thermal conductivities of neat parafﬁn wax with 10 and 20 wt% loadings of CNT crostructure. (b) Thermal conductivities of neat paraffin wax with 10 and 20 wt% loadings of CNT foams, i.e., 80 and 90 wt% parafﬁn [131]. Reproduced with permission from Chen, L. et al., ACS foams, i.e., 80 and 90 wt% paraffin [131]. Reproduced with permission from Chen, L. et al., ACS Nano; published by American Chemical Society, 2012. Nano; published by American Chemical Society, 2012. Shi et al. [132] used exfoliated graphite nanoplatelets (xGnP) and grapheme and Shi et al. [132] used exfoliated graphite nanoplatelets (xGnP) and grapheme and improved the corresponding k values by 1000% and 100%, respectively. Such improvement improved the corresponding k values by 1000% and 100%, respectively. Such improve- resulted in the formation of parafﬁn PCM materials that are stable. Approximately 2 wt% ment resulted in the formation of paraffin PCM materials that are stable. Approximately of the graphene was added to parafﬁn and heated to around 185 C. The parafﬁn retained 2 its wt form % odespite f the gra reaching phene signiﬁcantly was added to high pamelting raffin apoint. nd hea Itted was to claimed around that 185 to ° decr C. The easeparaffin the cost, trace amount of graphene and xGnP can be doped together thereby improving retained its form despite reaching significantly high melting point. It was claimed that to both stability and heat dissipation of PCMs. xGnP-doped PCM resulted in a k value of decrease the cost, trace amount of graphene and xGnP can be doped together thereby 2.7 W/mK which was considerably higher than graphene-doped PCM (approximately improving both stability and heat dissipation of PCMs. xGnP-doped PCM resulted in a k value of 2.7 W/mK which was considerably higher than graphene-doped PCM (ap- proximately 0.5 W/mK) and neat paraffin (0.25 W/mK). Wang J. [133] achieved 305% improvement in the PCM thermal conductivity by adding carbon nanofibers (CNFs) as Appl. Nano 2023, 4, FOR PEER REVIEW 21 Appl. Nano 2023, 4 95 nanofillers into the palmitic acid (PA). The phase temperature change was approximately 0.5 W/mK) and neat parafﬁn (0.25 W/mK). Wang J. [133] achieved 305% improvement in the PCM thermal conductivity by adding carbon nanoﬁbers (CNFs) as nanoﬁllers into 62.5 °C after the addition of unwashed acid. The range of length and diameter of the the palmitic acid (PA). The phase temperature change was approximately 62.5 C after the CNFs was 200–500 nm and 5–50 μm, respectively (Figure 15). Alkali potassium hydrox- addition of unwashed acid. The range of length and diameter of the CNFs was 200–500 nm ide (KOH) was used to chemically treat CNFs, reducing the thermal boundary resistance and 5–50 m, respectively (Figure 15). Alkali potassium hydroxide (KOH) was used to of the fibre matrix. chemically treat CNFs, reducing the thermal boundary resistance of the ﬁbre matrix. Figure 15. (a) SEM image of M-CNF/PA with 1.0 wt% M-CNF. (b) Thermal conductivity enhancement Figure 15. (a) SEM image of M-CNF/PA with 1.0 wt% M-CNF. (b) Thermal conductivity enhance- with 0.2, 0.5, 1, 2 and 5 wt% of carbon nanoﬁbers (CNF) in palmatic acid (PA) [133]. Reproduced with ment with 0.2, 0.5, 1, 2 and 5 wt% of carbon nanofibers (CNF) in palmatic acid (PA) [133]. Repro- permission from Wang, J. et al., Journal of Applied Physics; published by AIP Publishing, 2011. duced with permission from Wang, J. et al., Journal of Applied Physics; published by AIP Pub- lishin Cui g, 2et 01al. 1. [134] improved the PCM’s thermal conductivity by 44% and 24% using nanoﬁllers of CNF and MWCNT, respectively. The synthesis of the composite involved carbon ﬁbres or nanotubes dispersion within both soy wax and parafﬁn (1, 2, 5 and Cui et al. [134] improved the PCM’s thermal conductivity by 44% and 24% using 10 wt%) at 60 C. This proved that the nanoﬁbers as additive can increase the parafﬁn k nanofillers of CNF and MWCNT, respectively. The synthesis of the composite involved values signiﬁcantly. The k value of pure parafﬁn and PCM composite (at CNF loadings carbon fibres or nanotubes dispersion within both soy wax and paraffin (1, 2, 5 and 10 of 10 wt%) were 0.320 W/mK and 0.450 W/mK, respectively. Wang et al. [135] improved wt%) at 60 °C. This proved that the nanofibers as additive can increase the paraffin k the thermal conductivity of PCM by 46% using multiwalled carbon nanotubes (MWCNT). The traditional ball milling method was used to synthesize the MWCNT-PCM composites values significantly. The k value of pure paraffin and PCM composite (at CNF loadings of added with KOH. This method could improve its dispersion in palmitic acid. The stability 10 wt%) were 0.320 W/mK and 0.450 W/mK, respectively. Wang et al. [135] improved the and homogeneity of PCM composites were improved by modifying the grafted OH groups thermal conductivity of PCM by 46% using multiwalled carbon nanotubes (MWCNT). into the MWCNT surfaces. The MWCNT-palmitic acid composites with 1 wt% of MWCNT The traditional ball milling method was used to synthesize the MWCNT-PCM compo- loading was shown to increase the k values by 46.0% and 38.0% on solid state at 25 C and liquid state at 65 C, respectively. sites added with KOH. This method could improve its dispersion in palmitic acid. The stability and homogeneity of PCM composites were improved by modifying the grafted 5. Nanopolymer Advanced Composites OH groups into the MWCNT surfaces. The MWCNT-palmitic acid composites with 1 THE deﬁnition of polymer nanocomposites (PNCs) is the combination of more than wt% of MWCNT loading was shown to increase the k values by 46.0% and 38.0% on solid one material. In addition, the matrix consists of a polymer with the dispersed phase st that ate possess at 25 ° aC minimum and liquid of one stadimension te at 65 °C, smaller respec than tiv100 ely. nm [136]. Many decades of observation have deduced that incorporating nanoﬁllers in small quantities within the polymer resulted in many improvements on its characteristics such as thermal, barrier, 5. Nanopolymer Advanced Composites mechanical and ﬂame-retardant properties while its processing is unaffected [137]. The THE definition of polymer nanocomposites (PNCs) is the combination of more than optimum nanocomposite design necessitates the individual nanoparticles to disperse homogeneously within a matrix polymer. The main challenge in terms of dispersion state one material. In addition, the matrix consists of a polymer with the dispersed phase that of nanoparticles is to achieve all the possible enhancements of its properties [137]. There is possess a minimum of one dimension smaller than 100 nm [136]. Many decades of ob- a potential for the nanoﬁllers’ uniform dispersion to result in signiﬁcant interfacial area servation have deduced that incorporating nanofillers in small quantities within the between the nanocomposites’ constituents [137]. There are various factors that inﬂuence polymer resulted in many improvements on its characteristics such as thermal, barrier, the reinforcing effect mainly polymer matrix properties, type and nature of nanoﬁller as well mecas hapolymer nical an and d fla ﬁller meconcentration. -retardant prOther opertie factors s wh focusing ile its p on roce thess particle ing is includes unaffected [137]. The its size, aspect ratio, orientation and distribution [138]. There have been numerous types optimum nanocomposite design necessitates the individual nanoparticles to disperse of nanoparticles being used to form the nanocomposites with various polymers including homogeneously within a matrix polymer. The main challenge in terms of dispersion state clays [138], carbon nanotubes [139], graphene [140], nanocellulose [141] and halloysite [142]. of nanoparticles is to achieve all the possible enhancements of its properties [137]. There is a potential for the nanofillers’ uniform dispersion to result in significant interfacial area between the nanocomposites’ constituents [137]. There are various factors that influence the reinforcing effect mainly polymer matrix properties, type and nature of nanofiller as well as polymer and filler concentration. Other factors focusing on the particle includes its size, aspect ratio, orientation and distribution [138]. There have been numerous types of nanoparticles being used to form the nanocomposites with various polymers including clays [138], carbon nanotubes [139], graphene [140], nanocellulose [141] and halloysite [142]. It is essential to evaluate the nanofiller dispersion within the polymer matrix. This is because there is a strong correlation between both the mechanical and thermal proper- Appl. Nano 2023, 4 96 Appl. Nano 2023, 4, FOR PEER REVIEW 22 It is essential to evaluate the nanoﬁller dispersion within the polymer matrix. This is because there is a strong correlation between both the mechanical and thermal properties with ties with the outcome of morphologies. The degree nanoparticles separation would result the outcome of morphologies. The degree nanoparticles separation would result in three in three possible morphologies outcome [143] namely intercalated nanocomposites, possible morphologies outcome [143] namely intercalated nanocomposites, conventional conventional composites (also known as microcomposites) and exfoliated nanocompo- composites (also known as microcomposites) and exfoliated nanocomposites (Figure 16). sites (Figure 16). In an event where the polymer is not intercalating between the layers of In an event where the polymer is not intercalating between the layers of the silicate, the the silicate, the outcome would be separate phases of composite where its properties are outcome would be separate phases of composite where its properties are within the same within the same range as seen in conventional composites [144]. range as seen in conventional composites [144]. Figure 16. Possible structures of polymer nanocomposites using layered nanoclays: (a) microcom- Figure 16. Possible structures of polymer nanocomposites using layered nanoclays: (a) microcom- posite, (b) intercalated nanocomposite and (c) exfoliated nanocomposite [143]. Reproduced with posite, (b) intercalated nanocomposite and (c) exfoliated nanocomposite [143]. Reproduced with permission from Alexandre, M. et al., Materials Science and Engineering: R: Reports; published by permission from Alexandre, M. et al., Materials Science and Engineering: R: Reports; published by Elsevier, 2000. Elsevier, 2000. An intercalated structure encompasses with at least one extended polymer chain, An intercalated structure encompasses with at least one extended polymer chain, where it intercalates between the silicate layers. The outcome is a consistent order of where it intercalates between the silicate layers. The outcome is a consistent order of multilayer morphology with polymer and clay layers that are intercalated. Exfoliated multilayer morphology with polymer and clay layers that are intercalated. Exfoliated structure would result in the event of complete and orderly dispersion of silicate layers structure would result in the event of complete and orderly dispersion of silicate layers within a continuous polymer matrix [143]. Exfoliated nanocomposites have a large surface within a continuous polymer matrix [143]. Exfoliated nanocomposites have a large sur- contact area between the nanoparticles and matrix. Such is one of the signiﬁcant differences face contact area between the nanoparticles and matrix. Such is one of the significant between conventional composites and nanocomposites. differences between conventional composites and nanocomposites. 5.1. Compatibilization in Polymer Nanocomposites 5.1. Compatibilization in Polymer Nanocomposites Compatibilization is of paramount importance to achieve a mixture of polymer or Compatibilization is of paramount importance to achieve a mixture of polymer or nanocomposite with the desired properties. Therefore, poor properties are attributable nanocomposite with the desired properties. Therefore, poor properties are attributable to to the chemical nature differences between the polymers or polymer matrix with the the chemi NPs [145 cal ]. na As ture pr eviously difference mentioned, s between compatibilization the polymers or pol is a ym signiﬁcant er matrix factor with in the obtaining NPs [145the ]. Adesir s prev ed io pr usl operties. y mentiDegradation oned, compashould tibilizatio ben kept is aat sign a low ifica pr nt obability factor in and obta itioccurs ning thwhen e the organomodiﬁer is decomposed and when degradation products and polymers are desired properties. Degradation should be kept at a low probability and it occurs when the interacting organomodwith ifier is each dec other omp.oAll sed of anthese d whhave en degr a signiﬁcant adation prinﬂuence oducts anupon d poly the mer pr s operties are and morphology of the material [146] (Figure 17). There are three methods of productions interacting with each other. All of these have a significant influence upon the properties for polymer nanocomposites; in situ polymerization, solution and melt blending. The and morphology of the material [146] (Figure 17). There are three methods of productions production method is chosen based on the polymeric matrix type, nanoﬁller and the ﬁnal for polymer nanocomposites; in situ polymerization, solution and melt blending. The products’ desired properties [15]. production method is chosen based on the polymeric matrix type, nanofiller and the final products’ desired properties [15]. Appl. Nano 2023, 4, FOR PEER REVIEW 23 Appl. Nano 2023, 4 97 Figure 17. Schematic of compatible PVDF/SWCNT nanocomposite production [146]. Reproduced Figure 17. Schematic of compatible PVDF/SWCNT nanocomposite production [146]. Reproduced with permission from Cho, K.Y. et al., Composites Science and Technology; published by Else- with permission from Cho, K.Y. et al., Composites Science and Technology; published by Elsevier, vier, 2018. 5.2. In-Situ Polymerization In-situ polymerization involves a correct dispersion of the nanoﬁller within the 5.2. In-Situ Polymerization monomer solution prior to the beginning of polymerization process. This is to ascer- In-situ polymerization involves a correct dispersion of the nanofiller within the tain the formation of the polymer between the NPs. There are various methods of initiating monomer solution prior to the beginning of polymerization process. This is to ascertain polymerization such as heat, utilising the correct initiator, etc [147]. Such method could the formation of the polymer between the NPs. There are various methods of initiating be used to achieve a polymer grafted NPs and high loading nanoﬁllers with the absent polymerization such as heat, utilising the correct initiator, etc [147]. Such method could of aggregation [148]. It is possible to include organic modiﬁers in order to assist the NPs be used to achieve a polymer grafted NPs and high loading nanofillers with the absent of dispersion and to be included within the polymerization [149]. Such method could be deemed as an alternative in producing nanocomposites through the use of polymers that aggregation [148]. It is possible to include organic modifiers in order to assist the NPs may be deemed unstable thermally or non-soluble [150]. There are occasions where such dispersion and to be included within the polymerization [149]. Such method could be method is applicable in solvent-free form [151]. Furthermore, such method may increase deemed as an alternative in producing nanocomposites through the use of polymers that the performance of the products [152]. Mini-emulsion polymerization is dependent on may be deemed unstable thermally or non-soluble [150]. There are occasions where such the monomer droplets being produced, which are subsequently dispersed into a solu- method is applicable in solvent-free form [151]. Furthermore, such method may increase tion within a nanoscale [153]. The advantages include particle morphology that can be the performance of the products [152]. Mini-emulsion polymerization is dependent on controlled [154], high functioning interfacial adhesion of the nanoﬁllers [155] and higher the monomer droplets being produced, which are subsequently dispersed into a solution transparency value [156]. This method could potentially [157] use higher nanoﬁllers with within a nanoscale [153]. The advantages include particle morphology that can be con- no presence of agglomeration, increased performance of the ﬁnal products, products with solvent-free form, outcome of covalent bond within the NPs functional groups and polymer trolled [154], high functioning interfacial adhesion of the nanofillers [155] and higher chains as well as utilising the thermoplastic and thermoset polymers. A major disadvantage transparency value [156]. This method could potentially [157] use higher nanofillers with of such method is the agglomeration easing [148,150]. no presence of agglomeration, increased performance of the final products, products with solvent-free form, outcome of covalent bond within the NPs functional groups and 5.3. Solution Blending polymer chains as well as utilising the thermoplastic and thermoset polymers. A major Blending is the most used method because it is simple in terms of producing polymer disadvantage of such method is the agglomeration easing [148,150]. nanocomposites. In comparison with other methods however, this method has higher difﬁculty in terms of achieving proper nanoﬁller dispersion within the polymer matrix [157]. 5.3. Solution Blending Solution blending is a system that encompasses both the polymer and nanoﬁller that can be dispersed within a suitable solvent without much difﬁculty [147]. The dispersion of Blending is the most used method because it is simple in terms of producing poly- the nanoﬁller within the polymer can be achieved through magnetic stirring, ultrasonic mer nanocomposites. In comparison with other methods however, this method has irradiation or shear mixing [148]. Figure 18 demonstrates the use of this method, wherein higher difficulty in terms of achieving proper nanofiller dispersion within the polymer the NPs are still dispersed within the polymer chains after the solvent evaporates. This matrix [157]. Solution blending is a system that encompasses both the polymer and nan- nanocomposite that has just been produced could be developed into a thin ﬁlm [157]. ofiller that can be dispersed within a suitable solvent without much difficulty [147]. The The solution blending posed a few constraints in economic and environmental terms. dispersion of the nanofiller within the polymer can be achieved through magnetic stir- Thus, there is a need for an optimum method to achieve the desired product while ad- ring, ultrasonic irradiation or shear mixing [148]. Figure 18 demonstrates the use of this dressing the constraints accordingly [158]. The advantages of solution blending include reduced gases permeability [159], simple operation and the use of conventional method for method, wherein the NPs are still dispersed within the polymer chains after the solvent nanoﬁllers of all types as well as the thermoset polymers and thermoplastic polymers [160]. evaporates. This nanocomposite that has just been produced could be developed into a The disadvantages include environmental and aggregation issues [158]. However, this thin film [157]. method is restricted to water soluble polymers [161]. Appl. Nano 2023, 4, FOR PEER REVIEW 24 Appl. Nano 2023, 4 98 Figure 18. Schematic illustration of solution blending method. Figure 18. Schematic illustration of solution blending method. 5.4. Melt Blending The solution blending posed a few constraints in economic and environmental Melt blending necessities the direct dispersion of nanoﬁllers into the molten polymer. terms. Thus, there is a need for an optimum method to achieve the desired product while When the mixing process starts in its melt state, the resulting polymer strain that is applied addressing the constraints accordingly [158]. The advantages of solution blending in- on the particles is dependent on the weight distribution and the weight of the molecules. clude reduced gases permeability [159], simple operation and the use of conventional The size of the agglomerates decreases when the shear stress level is high [157]. At the method for nanofillers of all types as well as the thermoset polymers and thermoplastic beginning, the larger agglomerates break apart to become smaller in size before being polymers [160]. The disadvantages include environmental and aggregation issues [158]. dispersed within the polymer matrix. Stronger shearing results when the polymer strain is How transferr ever, thed is m toethe thod new is re agglomerates. stricted to water Individual soluble po particles lymers ar [1e 61 formed ]. due to the breaking down. The primary element of this method is the timing and the chemical processes 5.4. Melt Blending between the NPs surface and the polymer [162]. Melt blending necessitates single and twin-screw extruders [163]. However, there are Melt blending necessities the direct dispersion of nanofillers into the molten poly- occasions where unfavourable outcome may ensue on the nanoﬁller ’s modiﬁed surface mer. When the mixing process starts in its melt state, the resulting polymer strain that is due to high temperatures thus optimisation is applied to address this issue [164]. The applied on the particles is dependent on the weight distribution and the weight of the most renowned method to address this is to use intermeshing co-rotating twin-screw molecules. The size of the agglomerates decreases when the shear stress level is high extruders. The disadvantage of such method is the difﬁculty in controlling the parameters [157]. At the beginning, the larger agglomerates break apart to become smaller in size such as interaction between the NPs, polymer and the procession conditions such as before being dispersed within the polymer matrix. Stronger shearing results when the residence time and temperature [165]. As such, it is not easy to achieve NPs that are polymer strain is transferred to the new agglomerates. Individual particles are formed evenly dispersed. Melt blending can be commercialised as it is compatible with a range due to the breaking down. The primary element of this method is the timing and the of industrial operations including extrusion and injection moulding [157]. The main chemical processes between the NPs surface and the polymer [162]. advantages of this are low cost, environmentally sustainable due to the absence of solvents, Melt blending necessitates single and twin-screw extruders [163]. However, there heat stability enhancement [166], improved mechanical properties [167] and good NPs are occasions where unfavourable outcome may ensue on the nanofiller ’s modified sur- dispersion [168]. Its disadvantage is the possibility of damage on the nanoﬁllers’ modiﬁed face due to high temperatures thus optimisation is applied to address this issue [164]. The surface as a result of the high temperature application [169]. Overall, each method has its most renowned method to address this is to use intermeshing co-rotating twin-screw own respective advantages and disadvantages and the selection should be based on the extruders. The disadvantage of such method is the difficulty in controlling the parame- conditions and underlying materials. ters such as interaction between the NPs, polymer and the procession conditions such as residence time and temperature [165]. As such, it is not easy to achieve NPs that are 5.5. Nanopolymers and Their Applications evenly dispersed. Melt blending can be commercialised as it is compatible with a range of Nanopolymers offer rife of applications including all the uses offered by the traditional industrial operations including extrusion and injection moulding [157]. The main ad- polymers. These applications include telecommunications, defence, household goods, daily vantages of this are low cost, environmentally sustainable due to the absence of solvents, services, utilities and basic utilities, etc. Further details include the plastic containers, heat stability enhancement [166], improved mechanical properties [167] and good NPs toothpaste, and so forth. Nanopolymers are favoured due to their many notable attributes dispersion [168]. Its disadvantage is the possibility of damage on the nanofillers’ modi- included high resistance to chemical, excellent tensile strength capacity to hold metals fied surface as a result of the high temperature application [169]. Overall, each method and other compounds. High conductive properties of nanopolymers enable their usage in has its own respective advantages and disadvantages and the selection should be based nano circuit fabrication. There is possibility to produce polymer with nanoparticles from on the conditions and underlying materials. many different structures where some can be self-assembled such as lamellar, lamellar- within-cylindrical, lamellar-within-spherical, spherical-within-lamellar and cylindrical- 5.5. Nanopolymers and Their Applications within-lamellar geometry. The examples of non-self-assembled structures are polymeric Nanopolymers offer rife of applications including all the uses offered by the tradi- nanocapsules, polymer brushes, nanoﬁbers, hyperbranched polymers, dendrimers and tional polymers. These applications include telecommunications, defence, household polymeric nanotubes. goods, daily services, utilities and basic utilities, etc. Further details include the plastic There is still a constant renovation in the nanotechnology due to the great demand containers, toothpaste, and so forth. Nanopolymers are favoured due to their many no- for practical applications. Nanoﬁbers made via electrospinning ﬁnd many uses within table the attributes incl environment. ude Due d high to their resistr aemarkable nce to chem length ical, exc and ellent ability tensi to le embed strength in ca other pacity media, to Appl. Nano 2023, 4 99 nanoﬁbers became one of the safest NMs. Other desirable properties include high porosities (over 80%), adjustable functionality and high surface-to-volume ratio. These characteristics are more effective than the conventional non-woven and polymeric membranes especially those use in the liquid ﬁltration and particulate separation. It is feasible to apply the nanoﬁbrous scaffolds exclusively as a cutting-edge component for the liquid separation and gas ﬁltration. Due to the advancement of electroblowing and electrospinning technology high performance nanoﬁbrous scaffolds became feasible. In this perception, it is customary to highlight various applications of nanoﬁbers for the solar energy harnessing and as membranes to remove heavy ions from the industrial wastewater and discharges. 6. Nanotechnology Based Smart Glass Materials The conventional usage of high-performance glazing systems is upon windows or building windows in order to decrease the amount of unwanted heat from the sun as well as reducing the workload to cool air from the air condition systems installed within the building. Aesthetically, glass facades are more desirable in terms of using them in commercial buildings [170–172]. Thus, careful studies are needed to estimate and evaluate the energy savings in practical terms for the high-performing glass. It is particularly vital for the research to be conducted on the different types of high-performance building glass. This is especially true for densely built cities such as Singapore where the heat from the sun is an issue for the buildings. Additionally, it is still uncertain whether these glasses are able to retain its efﬁciency in countries with four seasons. The glass’s U-value needs to be within acceptable range in order for it to be functional when subjected to various climates including the tropics. Assessment of glass performance necessitates active measurement while being sub- jected to a controlled source of radiant. Evidently, such testing environment may not account for actual weather, where many possibilities may not be duplicated within the test environment [171,173]. This may be compensated by subjecting the glasses to real weather conditions by actually installing them outdoor [174,175]. However, it is still not possible to conduct testing in large-scare and fast on-site characterization. Furthermore, the test is restricted to fabricate the glazing, indicating the impossibility to forecast possible problems during the design stage. The development of cutting-edge technique made it possible for professionals to be able to make simulation and assessment on the glass being installed in building during the design stage [174,176,177]. Over the counter and matured simulation codes that are open source in nature including Energy Plus and Radiance [174] are appro- priate for such assessment as they have undergone development spanning for more than ten years. Despite such tools, the assessment is still complex in terms of conducting such evaluation for high performance glazing description (glass with various coatings for many purposes) into the solar irradiance module through the use of the current glass models. The glass involving multiple layer or pane glazing must be classiﬁed and computed uniquely using more focused tool prior to be interfaced using a custom script. Further consideration is vital in terms of acquiring complete and comprehensive weather model to increase the accuracy of the solar heat gain that will enter into the building. With the exception of weather data from International Weather for Energy Calculations (IWEC), other data must be carefully considered when inputted for assessment using the aforementioned tools. In addition, the tools are developed with main consideration to indoor performance thus it does not account for the negative effects and other impacts of the sunlight being reﬂected off the building glass façade during the assessment for environmental risks. External use of the glass typically involves alternative glass material, which are mirror or opaque with high specularity. The use of such alternative materials means that their properties have no angle dependence [174]. Usually, the glass performance is evaluated via the active measurements by utilizing the known radiant sources. While, under the non-controlled weather there is no possibility to apply this type of setup. Though a passive measurement can be running under real weather conditions using the outdoor test chamber, however it is unsuitable for a large- Appl. Nano 2023, 4, FOR PEER REVIEW 26 exception of weather data from International Weather for Energy Calculations (IWEC), other data must be carefully considered when inputted for assessment using the afore- mentioned tools. In addition, the tools are developed with main consideration to indoor performance thus it does not account for the negative effects and other impacts of the sunlight being reflected off the building glass façade during the assessment for envi- ronmental risks. External use of the glass typically involves alternative glass material, which are mirror or opaque with high specularity. The use of such alternative materials means that their properties have no angle dependence [174]. Usually, the glass performance is evaluated via the active measurements by utilizing the known radiant sources. While, under the non-controlled weather there is no possi- bility to apply this type of setup. Though a passive measurement can be running under real weather conditions using the outdoor test chamber, however it is unsuitable for a Appl. Nano 2023, 4 100 large-scale testing and fast on-site characterization. Additionally, the mentioned test is limited to the fabricated glazing and thus unable to predict potential issues in the design stage. Advances in the simulation techniques have enabled the building professionals to scale testing and fast on-site characterization. Additionally, the mentioned test is limited evaluate the glass facade of a building at the design phase. Nonetheless, the typical sim- to the fabricated glazing and thus unable to predict potential issues in the design stage. ulation tools are unable to integrate the high performance glazing description. Using the Advances in the simulation techniques have enabled the building professionals to evaluate advanced coating technology, although the existing glass models can be tested but these the glass facade of a building at the design phase. Nonetheless, the typical simulation tools often lack local weather models that plays an important role in accurately assess the tools are unable to integrate the high performance glazing description. Using the advanced solar heat gain by the building. Nanotechnology is being applied in various disciplines coating technology, although the existing glass models can be tested but these tools often especially within construction materials due to its ability to decrease the consumption of lack local weather models that plays an important role in accurately assess the solar heat energy thus they have much potential. Glass is one of the most special construction ma- gain by the building. Nanotechnology is being applied in various disciplines especially ter within ials constr and ca ucti n on be materials treated wi due th to na its no ability technto olo decr gy, ease decre the asi consumption ng the tranof sfe ener r ofgy hea thus t through they have much potential. Glass is one of the most special construction materials and the building envelope (Figure 19). The study used Design Builder 3.1 and followed the can be treated with nanotechnology, decreasing the transfer of heat through the building Egyptian energy code requirement to assess the difference energy consumption between envelope (Figure 19). The study used Design Builder 3.1 and followed the Egyptian energy two types of glass, standard 6 mm clear glass and glass that is treated with nanotech- code requirement to assess the difference energy consumption between two types of glass, nology. The standard 6 mm clear glass that were used in glazed facades results in high standard 6 mm clear glass and glass that is treated with nanotechnology. The standard 6 thermal loads into the indoor environment of the building. This results in increasing use mm clear glass that were used in glazed facades results in high thermal loads into the indoor of energy in the building [178]. environment of the building. This results in increasing use of energy in the building [178]. Figure 19. The glass treated with nanotechnology [178]. Figure 19. The glass treated with nanotechnology [178]. Glass is a common material within various industries including transport, building Glass is a common material within various industries including transport, building and construction, solar energy with glass variety. It is also being used in microscopes, and construction, solar energy with glass variety. It is also being used in microscopes, tablet computers, furniture and many more. There are four advantages of using glass tablet computers, furniture and many more. There are four advantages of using glass within the building and construction sectors. Firstly, it allows natural lights to enter the wi building. thin the Secondly buildin,g itaﬁlters nd coout nstruc harmful tion sect rays ors. from Firs the tlysun , it a fr ll om owentering s natural the ligbuilding. hts to enter the Thirdly, it harmonises the environment and the building. Lastly, it is cost-effective due building. Secondly, it filters out harmful rays from the sun from entering the building. to its energy efﬁciency. Researchers and scientists have taken the motto of ‘necessity is Thirdly, it harmonises the environment and the building. Lastly, it is cost-effective due to the mother of invention’. Thus, during their research in improving the properties of glass, its energy efficiency. Researchers and scientists have taken the motto of ‘necessity is the they have developed a type of glass that requires minimal maintenance, also known as mother of invention’. Thus, during their research in improving the properties of glass, self-cleaning glass. Individuals that wear glasses will be glad that such glasses prevent mist from forming when they are enjoying hot drinks that are steaming or when they are cooking. In addition, anti-fogging glass is used in tablet computers thus they could be used in close proximity to swimming pools. In addition, anti-reﬂective glass is used in mobile phones or laptops thus users could still able to use these devices during broad daylight. As for self-cleaning glass, they are most suitable for windows and doors in ofﬁces and homes, where these SCGs do not need any frequent maintenance for cleaning. 6.1. Self-Cleaning Glass Glass is extensively used by diverse industries like automotive, solar cells, building and construction. SCG is a new type of glass being developed and is widely used in hard-to-reach areas in buildings because it requires minimal maintenance. SCG has either Appl. Nano 2023, 4 101 a layer of titania (TiO ) that measures 10–25 nm or is coated with silica on its surface via both bottom-up or top-down approach. The self-cleaning properties are the control of its wettability properties on its surface. The ﬁrst is for the surface to be complete dry, also known as hydrophobic surface, where a liquid droplet maintains a spherical shape on the surface of the glass. This is achieved either by forming a component of low surface energy or through surface roughness control. The surface becomes hydrophobic by applying a thin layer of SiO . The second technique involves complete wetting of the surface where the liquid forms a ﬁlm upon contact with the surface. This is known as hydrophilic and can be achieved through applying photocatalytic TiO coating. The coating uses the sunlight and water to rinse itself thus resulting in self-cleaning property. Therefore, solid surfaces undergo various reactions with dissimilar materials depending on the coating type being applied. Consideration should take place upon the various qualities as a result such as spreading, wettability, adhesion and interface. The wettability property of a solid is deﬁned by observing that contact angle (denoted by ) the moment liquid touches the surface of the solid. 6.2. Hydrophilic Coating A surface is deemed hydrophilic when the water contact angle (CA) is less than 90 . It is considered as super hydrophilic when its CA is less than 50 . As the liquid contacts such surfaces, it will spread out until it becomes a thin layer. The self-cleaning materials that made this possible are WO , ZnO, SnO , SiO , CdS, TiO and ZrO . The most extensively 3 2 2 2 2 used is TiO because it has more advantages in comparison with the others. New discovery made by Fujishima and Honda where they used TiO for photo-electrochemical splitting of water to hydrogen and oxygen while being subjected to UV radiation. This has resulted in an explosion of research to study the TiO photo-catalytic potential including self- cleaning coatings, photo-electro-catalysis, photovoltaics, photoelectrocatalytic degradation of organic compounds and advanced oxidation. TiO exhibits the following properties, high refractive index, good mechanical performance, transparent and semiconductor material with a high band gap. When TiO is within the wavelength range from 0.35 mm to 12 mm, it becomes stable chemically. Titania exists in three different crystal structures such as brookite, anatase and rutile. The highest refractive index is shown by the rutile phase (2.61–2.90), making it the centre of focus for optical applications. Rutile is also the most stable in terms of its thermodynamic properties especially when subjected to high temperatures. Despite various advantages, anatase has increased desire for lower temperature applications where it is necessary to form a ﬁlm on thermally sensitive substrates. Therefore, the desirable materials are amorphous or crystalline anatase, used to produce the self-cleaning glasses at temperatures below 400 C. Anatase can be changed to rutile in the range of 700 to 1100 C. 6.3. Anti-Reﬂective Coating Fujishima and Guiselin et al. invented the TiO thin ﬁlms and also patented the methods. This ﬁlm being transparent, photo-catalytically efﬁcient and abrasion-resistant can be used on glass surfaces. Several SCG are already being commercially used at the present time such as Hydrotecht from TOTO, Activt from Pilkington Glass, Thermotecht from Viridian and Bioclean from Saint Gobain. Additionally, self-glazing products are also being rolled out in liquid forms or white that target direct consumers. When a normal glass is applied the self-cleaning products, they would turn into SCG. Products that are available for users are produced by some companies such as Rain Racert from Rain Racer Developments, BalcoNanot from Balcony Systems Solutions and ClearShieldt from Ritec International. SCG can be installed in various locations including ofﬁces, facades and general buildings. The improvement of photo-catalytic activity and anatase coating necessitate a high refractive index due to low temperature processing. Other properties, apart from self-cleaning, are necessary for a glass that will be used on smart phones, spectacles and solar cells. These properties include anti-fogging, anti- abrasive and anti-reﬂection. Fraunhofer is the founder of anti-reﬂective (AR) coating in 1817. Appl. Nano 2023, 4 102 Since then, AR phenomenon is regarded as a destructive interference between air-coating interfaces by Fresnel and Poisson and light reﬂected due to substrate coating. One of the many methods of making AR coating is to construct a single-layer of coating that has low refractive index. Materials that have low refractive index cost more and also rare. Porous nanostructures can be used to effectively decrease the volume-averaged refractive indices of materials through controlling the porosity within the coatings. This results in anti-reﬂective coatings, and its hydrophobic and hydrophilic properties could be further enhanced by increasing surface roughness. At the same time, reﬂection is increased due to the decreased in transmittance, which occurs as a result of scattering diffusion in rough surfaces. Sample transmittance is the subsequent light intensity ratio that exits the after intensity ratio entered the sample. Therefore, the increase of transmittance results in decrease of photocatalytic activity as the light intensity decreases. At the same time, anti-reﬂective surfaces are part of the SCG. Therefore, in order to preserve the self-cleaning and anti-reﬂectivity properties, the ideal surface roughness is required. The assessment of the solid surface’s wettability necessitates the static contact angle and the dynamic sliding angle. The essential factor is therefore roughness of the surface and chemical functionalization. 6.4. Fabrication of Self-Cleaning Glass The glass becomes either hydrophilic or hydrophobic after the applications of a thin layer of TiO or SiO on its surface. There are two types of fabrication of nanomaterials 2 2 which are top-down and bottom-up. The top-down approach involves removal of materials gradually from massive structure until the required nanomaterial is formed. Lithography is an example of this method. Comparatively speaking, it is similar to using a block of wood and turning it into a doll by a carpenter. Bottom-up approach involves the use of atoms or molecules to be built gradually until the formation of the required nanomaterial or nanocoating. Comparatively, this is akin to using Lego blocks to build a house. The bottom-up approach is further divided into two types like gas and liquid phase. The gas phase method involves the plasma arc evaporation and chemical vapour deposition (CVD). The liquid phase technique deals with the sol-gel and molecular self-assembly. 6.5. SiO -TiO Coating 2 2 In addition to having self-cleaning function, other functions are also desirable includ- ing photocatalysis and anti-reﬂectivity, which are vital in products such as smart phones and solar cells. Glop et al. [179] used the sol-gel method and Liu et al. [180] used the pulse magnetron sputtering as methods of preparation for the photoactive antireﬂection coating. The TiO coating on the outer surface that results in self-cleaning feature increases the reﬂectance of plastic or glass substrate due to its relatively high refractive index (c. 2.5 for the anatase phase). Therefore, self-cleaning and anti-reﬂectivity attributes may not be compatible with the exception of rare instance where the structure and composition are modulated. Prado et al. [181] attempted to produce a coating that is multifunctional where its outer layer consists of dense/mesoporous TiO and its inner layer consist of meso- porous SiO AR layer. Multifunction coatings with self-cleaning attribute have discovered to perform 25–30% compared to photo degradation degree, which is produced via the conventional TiO coatings layer either porous or compact. Solar industry including solar power plants and solar energy producers primarily use glass. The amount of electricity generated or power for heating water depending on the intensity of the sunlight. The glass may be useful in terms of reducing loss of radiation and reﬂection. SCG’s primary property is its reﬂective index denoted by n. Production of glasses with anti-reﬂective properties for solar related use requires a low refractive index such as SiO where its n value is 1.4. Conversely, high reﬂective index such as titania where its n value is 2.0 is vital in improving the photocatalytic activity of the hydrophilic property in SCG. Helsch and Deubener [182] attempted to use the sol-gel coating technique to create a single type of glass that contains both functions. High transmittance is needed for this particular type of glass. The research has been a success where they used two layers consisting of SiO 2 Appl. Nano 2023, 4 103 and TiO to create a glass with both anti-reﬂective and photocatalytic properties. Through the sol-gel coating method, preparation was made on silica glass porous coatings xTiO . (1002x) SiO with 50 wt% of titania. The compatibility of anti-reﬂective and photocatalytic properties will then be achieved once the composition reached the ranger from x57.5_20. Porous coating also enhances the solar transmittance by 2.3% in comparison to silica glass that is not coated. These coatings with dual functions have greater degradation rate at 20-fold of the air borne contaminants in comparison to nanoporous ﬁlm of pure SiO . Nanoporous structures consist of materials that have high porosity and low density with simultaneous advantages in terms of possessing high pore volume, high surface area and larger pore size, whereby the diffusion pathways are accessible. Anti-reﬂection coating is often made of porous silica layers. Helsch et al. [183] made a discovery that there is a 5% enhancement (from 92 to 97%) of light transmission when borosilicate glass is at 550 nm, 35% porosity and 110 nm ﬁlm thickness. There are many applications for TiO ﬁlms on glass substrates including mirrors, windshields and window glasses. While being serviced, anti-reﬂective porous coatings will be subjected to severe environmental conditions including hail and salt atmosphere, sandstorms, dust particles and airborne volatile organic compounds. If the AR coating is damaged while being subjected to the aforementioned conditions, the solar transmittance will be reduced. Cathro et al. [184] dis- covered an increase on the refraction index of porous thin ﬁlms as a result of the adsorption of airborne contaminants. Pareek et al. [185] found that oil vapour contamination is also responsible for the increase of the refractive index of porous antireﬂective coatings. 6.6. Nanomaterial-Based Solar Cool Coatings The global building and construction industry is responsible for both 40% of the entire world’s energy consumption and emitting a third of the world’s greenhouse gases annually. At least 50% of the total energy consumed by this industry are for powering heating, ventilating and air conditioning (HVAC) systems. Passive cooling and solar heat insulation technologies are often being regarded as solutions in addressing the global energy crisis, in which they are being considered as reducing or even consume zero energy. Solar radiation plays the main role in terms of buildings gaining heat when they are transmitted via the envelope. The heat will be trapped and increased inside the building. Buildings therefore, are more likely to use nanomaterials-based solar cool coatings (NSCCs) in order to address the issue of excessive solar heat and energy consumption. These coatings are currently the most reliable in terms of passive cooling technologies. NSCCs are composite materials where it is made of thin-layered substrates mixed with nanosized additives, which is the primary component due to its distribution solar reduction function onto a normal coating material. Binders are made of thin-layered substrates and they are added with nanosized additives to provide a coating to the surfaces of buildings where required. NSCCs are widely used for the past few years as a solution to the high energy consump- tion in buildings. On [186], authors have conducted a market research to show that the solar coatings will have a 70% in saving energy on a global scale from 2013 to 2019. The number of patents being ﬁled is evidence to the increased attention and research being taken place to increase the use of NSCCs as well as proving that it is a pioneering technology in passive cooling. On [187], authors have revealed that there is a signiﬁcant increase by 38% between 2013 to 2015 on the number of patents being ﬁled on smart window coatings. Meanwhile, the patents for thermal barrier coatings have increased by 32% between 2011 and 2015. For several decades, the key component in transparent coatings used in solar heat reﬂection is metal. Al, Au, Ag, Cu and Pt have higher performance in terms of possessing high reﬂectivity and low absorptivity properties. Incorporating these metals into NSCCs leads to reﬂection of solar heat that would have otherwise penetrated into the indoor environment of the building. This passive cooling feature means that the indoor environment will need to use less energy consumption for cooling purposes. Signiﬁcant amount of research was conducted on solar cool coatings. The least difﬁcult in terms of implementation and usage are Au and Ag. Cher (2014) prepared nanogold (Au) ﬁlms to be applied on the glass Appl. Nano 2023, 4, FOR PEER REVIEW 30 These coatings are currently the most reliable in terms of passive cooling technologies. NSCCs are composite materials where it is made of thin-layered substrates mixed with nanosized additives, which is the primary component due to its distribution solar reduc- tion function onto a normal coating material. Binders are made of thin-layered substrates and they are added with nanosized additives to provide a coating to the surfaces of buildings where required. NSCCs are widely used for the past few years as a solution to the high energy con- sumption in buildings. On [186], authors have conducted a market research to show that the solar coatings will have a 70% in saving energy on a global scale from 2013 to 2019. The number of patents being filed is evidence to the increased attention and research being taken place to increase the use of NSCCs as well as proving that it is a pioneering technology in passive cooling. On [187], authors have revealed that there is a significant increase by 38% between 2013 to 2015 on the number of patents being filed on smart window coatings. Meanwhile, the patents for thermal barrier coatings have increased by 32% between 2011 and 2015. For several decades, the key component in transparent coatings used in solar heat reflection is metal. Al, Au, Ag, Cu and Pt have higher per- formance in terms of possessing high reflectivity and low absorptivity properties. In- corporating these metals into NSCCs leads to reflection of solar heat that would have otherwise penetrated into the indoor environment of the building. This passive cooling feature means that the indoor environment will need to use less energy consumption for cooling purposes. Significant amount of research was conducted on solar cool coatings. Appl. Nano 2023, 4 104 The least difficult in terms of implementation and usage are Au and Ag. Cher (2014) prepared nanogold (Au) films to be applied on the glass surface using aerosol-assisted CVD method wherein the deposition was performed inside a cold-walled horizontal-bed surface using aerosol-assisted CVD method wherein the deposition was performed inside a CVD reactor. The resulting nanogold layers possessed various morphologies depending cold-walled horizontal-bed CVD reactor. The resulting nanogold layers possessed various on different reaction temperatures. Observation was made by placing a layer of Au NPs morphologies depending on different reaction temperatures. Observation was made by at 500 °C (Figure 20a). Figure 20b,c shows individual Au NPs on the top plate of films placing a layer of Au NPs at 500 C (Figure 20a). Figure 20b,c shows individual Au NPs subjected to 400 °C. The results disclosed that the thermophoresis was responsible for the on the top plate of ﬁlms subjected to 400 C. The results disclosed that the thermophoresis increase of particle size, wherein the NPs formation and gold atoms aggregation occurred was responsible for the increase of particle size, wherein the NPs formation and gold atoms in the gas phase reactions prior to the deposition. aggregation occurred in the gas phase reactions prior to the deposition. Figure 20. (a–c) SEM (Scanning Electron Microscope) images of nano-Au deposited on the top plates Figure 20. (a–c) SEM (Scanning Electron Microscope) images of nano-Au deposited on the top at various temperatures. (d,e) SEM images of an Ag layer (d) with and (e) without a Ge wetting plates at various temperatures. (d,e) SEM images of an Ag layer (d) with and (e) without a Ge wet- layer. (f–i) TEM (Transmission Electron Microscope) and SEM images of Ag@SiO -SH (f,g), (h) Au@ ting layer. (f–i) TEM (Transmission Electron Microscope) and SEM images of Ag@SiO2-SH (f,g), (h) SiO -SH, (i) Pt@SiO -SH. (j–m) Schematic (j) of Au@TiO nanorods with various geometries; TEM 2 2 2 Au@ SiO2-SH, (i) Pt@SiO2-SH. (j–m) Schematic (j) of Au@TiO2 nanorods with various geometries; images of Au@TiO nanorods with (k) Janus, (l) eccentric, and (m) concentric geometries [187]. TEM images of Au@TiO2 nanorods with (k) Janus, (l) eccentric, and (m) concentric geometries Reproduced with permission from Zheng, L., et al., Solar Energy; published by Elsevier, 2019. [187]. Reproduced with permission from Zheng, L.,et al., Solar Energy; published by Elsevier, 2019. 7. Environmental Health and Safety Considerations 7. Environmental Health and Safety Considerations The inﬂuence of nanotechnology is apparent in industry and many aspects of life; The influence of nanotechnology is apparent in industry and many aspects of life; construction is no exception. Even though enhanced-quality materials equipped with inno- vative features are already being used, numerous potential applications of nanomaterials construction is no exception. Even though enhanced-quality materials equipped with still exist in the ﬁeld of construction that are yet to be capitalised upon. However, these endeavours do not come without risks. Negative outcomes and effects on the environment and human health are not outside the realm of possibility. Hence a prudent and cautious approach should be considered. There are several existing nanoparticles, such as titanium dioxide and carbon nanotubes, that could already be harmful to those individuals tasked with their direct use. Qualitative and quantitative risk assessments, occupational health and safety risk management, and adequate circumvention protocols for identiﬁed risks are not only important but are crucial to avoiding or mitigating potential disaster. Nanomaterials are so many in number and so varied, it is safe to assume that massive quantities of these materials will eventually be produced. Moreover, introduction of entirely new nanomaterials both trigger the requirement of adequate risk assessment procedures and suitable communication measures surrounding those risks. Presently, new nanomaterials are analysed in a manner similar to that used for chemicals, food, and consumer products, which is unsurprisingly both inefﬁcient and insufﬁcient. The challenges presented when characterising nanomaterials and generating a standardised processing approach become substantial bottlenecks to the process. However, there do exist several techniques; laser ablation inductively-coupled plasma mass spectrometry—that could stand to meet the needs of such processes. For example, aiding in the quantiﬁcation of nanomaterials as subsets of complex matrices. Despite existing awareness surrounding the potential risks for working with construc- tion nanomaterials, and the notion that these materials may even pose risks to end-users, Appl. Nano 2023, 4 105 hazard information remains limited [188]. Consequently, the Occupational Safety and Health Administration (OSHA) has no recourse to mitigate the unknown hazards of Nu- tritional risk screening (NRs), as it is without regulations nor enforceable exposure limits; this is regardless of the fact that nanomaterial contamination can take place at any time during the manufacturing, packaging, and transport of construction materials, their use on-site during construction, and after the work is complete during the operational phase. For example, a number of workers were shown to have been exposed to more than the recommended limit of titanium dioxide during the packaging process in a study conducted by Al-Bayati and Al-Zubaidi [189]. In a recent move to promote safe working practices, the CPWR developed a toolbox talk strongly recommending and endorsing the use of high efﬁciency particulate air (HEPA) ﬁlters when handling nanomaterials. This was in response to the discovery that construction nanomaterials can be converted into unintended forms when mass-manufactured [190], such as carbon-based nanomaterials becoming airborne when prepared as a solution. However, it is worth noting that HEPA ﬁlters were never designed to capture particles of under 300 nm in size, making it unlikely to eliminate the hazard, even though they may still serve to mitigate it. As mentioned before, a signiﬁcant impact stands to be made by the use of nanotech- nology within the construction industry, not only from a perspective of enhancing material properties, but also because a high proportion of all energy used by the world is consumed by commercial and residential buildings, in their lighting, heating, and air conditioning. Overtaken thus far by the adoption of nanotechnology within ﬁelds such as biomedical and electronics, the construction industry has been making up lost ground in their pursuit of innovation using a variety of nanomaterials in recent years. However, as alluded to pre- viously, adoption of novel technologies does not come without risks; the potential dangers to the environment and human health posed by nanomaterials should not go unconsidered. This is true even if the goal in their use is to preserve the environment, by utilising the energy-conserving functions provided by nanomaterials, their full lifespan must still be contemplated, as highlighted in a recent review by Rice University scientists. Unintended consequences could be far severe than those it was intended to prevent. Furthermore, the authors indicate that nanomaterials, especially CNTs, can be accidentally or incidentally introduced to the environment at various stages of their life cycle. Within their work at Rice, they go on to detail the importance of a holistic nanomateri- als’ lifecycle exposure proﬁling approach, stipulating without that level of meticulousness, critical impacts on ecosystem and human health cannot be avoided. They maintain that, as a result of no regulation being presently in place despite growing concerns, a number of MNMs should be regarded as ‘potential emerging pollutants’ until contradicting infor- mation surfaces, as there are many related risks to environmental and public health that are being disregarded without that regulation. Furthermore, they describe the element of unpredictability of the natural environment; once distributed into it, nanomaterials may transform in diverse chemical, biological, and physical fashions, altering their properties, effects, and ultimate fate. The potential routes along which nanomaterials can be released into the environment are many and often. From occupational exposure, when the material is ﬁrst being prepared, during any coating, moulding, incorporating, or compounding to contamination during installation, construction, maintenance, repair, renovation. Finally, to decommissioning or demolition processes, even beyond this stage, further risks arise when solid nanomaterials reach landﬁlls or get disposed of in incinerators. Delivery methods and approaches affect these risks, also: aerosolization of nanomaterials, adhesive wear, abrasion and corrosion, and manufacturing process wastewater efﬂuent outlets all have additional risks, speciﬁc to the method and altering the resultant hazard. 8. Using Nanomaterials Safely The question “how to utilise nanomaterials safely” does not ﬁnd itself wholly resolved, even though it is clear that discovering it is crucial to improving the performance of infras- Appl. Nano 2023, 4 106 tructure and buildings. These nanoscale ﬁbres and particles could already be contributing to a problem that the scientiﬁc community is as yet completely unaware of, or in the ways of which we know that they can. Thin strands carried airborne can acquire behaviour patterns akin to asbestos. Limited information is available for workers and manufacturers alike on keeping safe while handling these materials, while it is commonplace to appreciate the necessity of greater regulation. Given that estimates place up to half of all new building materials in 2025 as containing nanomaterials, this information is urgently sought. This was the motivation for the research team at Loughborough University, when they investigated where these materials are used, to what extent, number of potential risks, and how might the workers on the ‘front line’ mitigate these risks. It was funded in part by the Institution of Occupational Safety and Health (IOSH), in order to produce a framework and a measure of guidance. An additional challenge facing the generation of a set of guidelines as such, or indeed, any other form of regulation, is that the way health and safety legislation is applied in different countries. It may not be mandatory for manufacturers to specify information about the type of nanomaterial, or the approach with which it was used, resulting in largely unreliable and inconsistent labelling systems. 9. Conclusions The construction industry has witnessed an ever-increasing applications of various sus- tainable materials using the core-shell strategy and nanotechnologies. The following conclu- sions are made based on the in-depth and relevant literature overview of nanotechnology- based core-shell structures: i. A new class of hybrid and core-shell NPs can be developed due to the advent of the manipulation techniques of particle structures at the nanoscale. ii. Efﬁcient fabrication methods are now available for the large scale production of numerous types of core-shell nanostructures. These developed techniques have contributed to the fast-paced advancement of synthetic chemistry, device setup, colloid and interfacial science. iii. The pigments durability can remarkably be improved using the core-shell NPs. Fur- thermore, being a part of sustainable materials, these NPs have widespread applica- tions. The highest recommended materials for shells in the construction industries are SiO and TiO . 2 2 iv. Carbon-based nano-enhancers show higher thermal conductivity compared to metals or oxide-based materials. High surface afﬁnity between the organic structures and carbon nano-ﬁllers of PCM can enhance the uniform interpenetration and lower the particles’ scatterings at the interfacial surfaces. v. In the near future, the high-performance nano-enhanced phase change material technology will be of great demand. It is expected to be applicable in many areas particularly in the thermal storage within the sustainable and renewable energy ﬁeld. These applications include the solar energy power generation, industrial heat charg- ing/discharging processes, excess heat management and cooling of electronic devices. vi. The polymer nanocomposites have immense applications potential compared to the traditional materials. Thus, nanocomposites ﬁeld has been the popular research topic due to its several desirable features including ease of production, light weight and ﬂexibility. The most distinguishing aspect of polymer nanocomposites is their utility small ﬁllers, resulting in a signiﬁcant increase in the interfacial interactions than the conventional composites. vii. It is foreseeable that the core-shell NPs will continue to play a signiﬁcant role in the passive cooling technology, reducing the solar heat gain by the buildings and energy consumption. In the context of global climate change and fast urbanization-mediated energy deﬁciency and environmental deterioration, the development of core-shell NPs is expected to be faster mainly in two aspects like the large scale synthesis of Appl. Nano 2023, 4 107 high performance nanomaterials and cost-effective as well as time-efﬁcient coating fabrication techniques. viii. Accompanied by the standardised approaches and regulatory mandates, high-volume nanomaterials such as SiO and carbon black, play a major role for a variety of indus- trial applications. Despite still being in development phases for many applications, the appropriate analytical capacity for the characterisation of materials and their properties are still necessary and fundamental; existing reports from toxicological in- halation studies already indicate steady increases of nanomaterial toxicity, as opposed to demonstrating entirely new nano-speciﬁc effects and outcomes. 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Abstract

Review Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited Ghasan Fahim Huseien Department of the Built Environment, College of Design and Engineering, National University of Singapore, Lower Kent Ridge, Singapore 117566, Singapore; bdggfh@nus.edu.sg; Tel.: +65-83057143 Abstract: The demand of high performance and environmentally sustainable construction materials is ever-increasing in the construction industry worldwide. The rapid growth of nanotechnology and diverse nanomaterials’ accessibility has provided an impulse for the uses of smart construction components like nano-alumina, nano-silica, nano-kaolin, nano-titanium, and so forth Amongst various nanostructures, the core-shell nanoparticles (NPs) have received much interests for wide applications in the ﬁeld of phase change materials, energy storage, high performance pigments, coating agents, self-cleaning and self-healing systems, etc., due to their distinct properties. Through the ﬁne-tuning of the shells and cores of NP , various types of functional materials with tailored properties can be achieved, indicating their great potential for the construction applications. In this perception, this paper overviewed the past, present and future of core-shell NPs-based materials that are viable for the construction sectors. In addition, several other applications of the core-shell NPs in the construction industries are emphasized and discussed. Considerable beneﬁts of the core-shell NPs for pigments, phase change components, polymer composites, and self-cleaning glasses with enhanced properties are also underlined. Effect of high performance core-shell NPs type, size and content on the construction materials sustainability are highlighted. Keywords: nanoparticles; core-shell materials; pigments; polymer; phase change materials Citation: Huseien, G.F. Potential 1. Introduction Applications of Core-Shell Nanotechnology is deﬁned as the manipulation of shape and structure of materials Nanoparticles in Construction at the nanoscale that can be used to design, characterize and produce valuable structures, Industry Revisited. Appl. Nano 2023, devices, and systems. The nanoscale refers to the objects with sizes between 1 and 100 nm in 4, 75–114. https://doi.org/10.3390/ dimensions (1 nm = 1 10 m). Despite many challenges in manipulating the engineering applnano4020006 materials at such a small scale, the recent advancements of various imaging techniques Academic Editors: Sergei Vlassov, made it possible to design, manufacture, and study their behaviours at the nanoscale. Sven Oras and Edgars Butanovs Amongst all the nanoscale structures produced by the top-down or bottom-up approaches, the nanoparticles (NPs) became most interesting. These are usually produced in the form of Received: 22 December 2022 very ﬁne powders or colloidal suspensions [1–4]. Various emerging properties of these NPs Revised: 26 March 2023 mainly depend on their individual components that are appreciably different from their Accepted: 29 March 2023 Published: 7 April 2023 bulk counterparts [5,6]. The NPs are unique because of their enlarged surface area, quantum size effects, improved absorbance, uniformity, and surface functionalization. The quantum size effect of the NPs is responsible for their distinct physicochemical characteristics useful for sundry applications [3,7–10]. Copyright: © 2023 by the author. Selected as one of the ten topmost targeted applications of nanotechnology to amelio- Licensee MDPI, Basel, Switzerland. rate some of the most signiﬁcant issues in the developing nations, construction and archi- This article is an open access article tecture industries stand to be substantially enhanced by the uses of nanomaterials [11,12]. distributed under the terms and Despite their ongoing uses within these contexts [11,13,14], the future of nanotechnology in conditions of the Creative Commons these industries is predicted to further increase the application feasibilities. Among these Attribution (CC BY) license (https:// expected outcomes, improvements in the building material’ properties by making them creativecommons.org/licenses/by/ stronger, durable, and lighter is the main focus [15–17]. These enhancements are brought 4.0/). Appl. Nano 2023, 4, 75–114. https://doi.org/10.3390/applnano4020006 https://www.mdpi.com/journal/applnano Appl. Nano 2023, 4 76 by introducing novel collateral functions like self-heating, anti-fogging, and energy-saving coatings, and so on [18–21]. In addition, the key components for the maintenance of in- struments such as sensors that detect and report structural health have been developed to gain more beneﬁts of these nonmaterials [22]. Despite various advantages of these new technologies, an emphasis should be placed on the risk-assessment of their intended uses, wherein the fallout can be severe. One such recent example is the deliberate and widespread use of supposedly beneﬁcial chemical dichlorodiphenyltrichloroethane (DDT) that was released to control malaria and various water-borne diseases. However, instead proved to be carcinogenic to humans it became toxic to numerous bird species, and haz- ardous to environment [23]. This illustrates the importance of a proactive and meticulous approach for the risk assessment of new technologies, without which, devastating impacts to ecosystems and human health cannot be prevented. Buildings have remarkable rate of power consumption at 45% of global energy [24,25]. Many passive cooling methods have been used and in addition, phase change materials (PCM) are installed within these buildings for the purpose of promoting temperature moderation, stopping heat from accumulating, improved heat absorption and minimize indoor heat gain. The method in which PCM stores thermal energy is effective in improving the buildings’ aggregate heat capacity. Interest has been strong in PCMs that has high energy density to be deployed in buildings with high thermal inertia in order to save a high amount of energy. However, PCMs have their own drawbacks and the primary one being extra time required to charge/discharge energy process as well as storage performance, which happens due to poor thermal conductivity. Therefore, attention is focused on improving its thermal conductivity through the use of nanotechnology and nanomaterials. There has been a rapid development lately within the nanomaterials ﬁeld resulting in the latest technology with Nano-sized particles in improving the PCM’s thermophysical properties. PCMs have several thermal and physical qualities such as viscosity, heat capacity, super-cooling and thermal conductivities. These attributes could be signiﬁcantly improved through dispersal of thermal conductive nanoparticles including nanometal- oxide, nanocarbon and nanometals. The technologies of core-shell and nanoparticles are widely adopted to improve the materials properties and thermal performance, which is appropriate in passive-cooling within the built-environment. In the construction industry, one of the possible solutions for a sustainable future is to introduce novel technologies to improve the durability of materials and increase the life span. Presently, nanotechnology creates new possibilities to control and improve material properties for civil infrastructures. By combining various engineering, chemical, and biological approaches, the nanotechnology can be used for the sub-atomic manipulation of materials. To synthesize NPs, diverse chemical, biological, physical, and even hybrid techniques can be used. In this regard, this review discusses and explains the role of nanoscience and nanotechnology in the development of potential core-shell NPs applicable in the construction industry (Figure 1). Also, diverse potential applications of core-shell NPs -based high performance construction materials rooted from the state-of-the-art research are emphasized. Appl. Nano 2023, 4, FOR PEER REVIEW 3 Appl. Nano 2023, 4 77 Figure 1. Flow chart of core-shell nanoparticles, synthesis, efﬁciency and construction applications. Figure 1. Flow chart of core-shell nanoparticles, synthesis, efficiency and construction applications. 2. Core-Shell NPs Synthesis and Beneﬁts 2. Core-Shell NPs Synthesis and Benefits Nanotechnology encompasses various methods of synthesis (biological, engineering, Nanotechnology encompasses various methods of synthesis (biological, engineer- chemical and hybrid) to customize the atomic-scale properties of materials. To produce the ing, chemical and hybrid) to customize the atomic-scale properties of materials. To pro- core-shell NPs both top-down and bottom-up approaches are routinely utilized. Top-down duce appr thoach e core incorporates -shell NPs bot the conventional h top-downworkshops and bottowith m-up micr appr ofabrication oaches are tools routi in n addition ely utilized. to the equipment that are externally controlled that are used to mill, cut, shape and mould Top-down approach incorporates the conventional workshops with microfabrication the materials accordingly to the requirement [26,27]. The lithographic and mechanical tools in addition to the equipment that are externally controlled that are used to mill, cut, techniques are the conventional top-down approach. The lithographic techniques involve shape and mould the materials accordingly to the requirement [26,27]. The lithographic the use of electron or ion beam, UV, scan probing, optical near ﬁeld scanning and laser-beam and mechanical techniques are the conventional top-down approach. The lithographic processing. In addition, the mechanical techniques involve the machines that grind, cut techniques involve the use of electron or ion beam, UV, scan probing, optical near field and polish the materials according to the required speciﬁcations [28–31]. Conversely, the scanning and laser-beam processing. In addition, the mechanical techniques involve the bottom-up technique is used to assemble materials in the desired form from their chemical machines that grind, cut and polish the materials according to the required specifications composition down to the molecular level. Examples of typical bottom-up technique include [28chemical –31]. Con vapour versely deposition, , the bottom laser -up -induced technique assembly is used , chemical to assemsynthesis, ble materself-assembly ials in the desire , d colloidal aggregation as well as ﬁlm deposition and growth [32,33]. form from their chemical composition down to the molecular level. Examples of typical Both approaches have many advantages and disadvantages. However, the main bottom-up technique include chemical vapour deposition, laser-induced assembly, advantage of the bottom-up approach is related to its cost-effectiveness that can fabricate chemical synthesis, self-assembly, colloidal aggregation as well as film deposition and signiﬁcantly smaller particles than the top-down approach. This is because of its precision growth [32,33]. as the product is produced by assembling it down to molecular level. Thus, it is possible Both approaches have many advantages and disadvantages. However, the main to have total control and almost no energy loss in the entire production process. The advantage of the bottom-up approach is related to its cost-effectiveness that can fabricate preparation of core/shell NPs necessitates total control in order to coat the shell materials significantly smaller particles than the top-down approach. This is because of its preci- uniformly as the particles are formed. Therefore, bottom-up approach is more suitable for sion as the product is produced by assembling it down to molecular level. Thus, it is such synthesis. Hybrid approach involves the use of both of the aforementioned techniques. For instance, the core particles can be produced via the top-down approach. Conversely, the possible to have total control and almost no energy loss in the entire production process. bottom-up approach can address the uniformity of the shell thickness. It is recommended The preparation of core/shell NPs necessitates total control in order to coat the shell ma- to apply micro-emulsion for an accurate size and thickness regulation of the shell because terials uniformly as the particles are formed. Therefore, bottom-up approach is more water droplets can act as nano-reactors. More researchers have been focusing on the core- suitable for such synthesis. Hybrid approach involves the use of both of the aforemen- shell NPs due to their suitability to be used extensively in diverse ﬁelds such as electronics, tioned techniques. For instance, the core particles can be produced via the top-down ap- optics, chemistry, biomedicine, medicines and catalysis, etc. proach. Conversely, the bottom-up approach can address the uniformity of the shell The core-shell NPs have high functioning and distinct properties such as different thickness. It is recommended to apply micro-emulsion for an accurate size and thickness materials can be used for the core or shell. The core or shell can be highly customizable by regulation of the shell because water droplets can act as nano-reactors. More researchers modifying the properties through controlling the materials or the core to shell ratio [34]. Also, it is possible to modify the core particles’ reactivity and thermal stability via the have been focusing on the core-shell NPs due to their suitability to be used extensively in adjustments to the shell coating material, leading to improved stability and dispersion diverse fields such as electronics, optics, chemistry, biomedicine, medicines and catalysis, of the core particles. This indicated that each particle can possess exclusive properties etc. depending on the materials being used during the fabrication. Such technique is renowned The core-shell NPs have high functioning and distinct properties such as different materials can be used for the core or shell. The core or shell can be highly customizable by modifying the properties through controlling the materials or the core to shell ratio [34]. Also, it is possible to modify the core particles’ reactivity and thermal stability via the adjustments to the shell coating material, leading to improved stability and dispersion of the core particles. This indicated that each particle can possess exclusive properties de- Appl. Nano 2023, 4, FOR PEER REVIEW 4 pending on the materials being used during the fabrication. Such technique is renowned because through the application of appropriate materials it can customize the surface function according to the environment [35]. The benefits of coating core particles include improved function, surface modifications, stability, dispersion, core release control and significant decrease in the use of precious material. The core-shell particles, as the name suggests, contain a shell and a core wherein a shell can be produced by the same or dif- ferent materials used for the core [36–38]. Figure 2 shows various core-shell particles where the colours are used to differentiate between them wherein a core can consist of a single sphere (Figure 2a) or multiple spheres that are smaller in size (Figure 2b). Fur- thermore, a shell may be hollow with one small sphere inside, resembling the yolk-shell structure (Figure 2c) [39]. Figure 2 shows three forms of shell structure like a continuous layer (Figure 2a–c), a Appl. Nano 2023, 4 78 larger core sphere that contains many smaller spheres (Figure 2d–e) or simply a collec- tion of core spheres (Figure 2f) [40]. The intricacy of the core-shell structure can be ma- nipulated by inserting smaller spheres into the shell (Figure 2g) [41] that can also be done because through thr mult ough iple the shells application (Figure 2of h) appr [42,4opriate 3]. The materials core-shell it Ncan Ps ca customize n be madthe e usi surface ng the function physical accor or chding emica to l the appr envir oach onment includin [35 g ]. thThe e che beneﬁts mical dof epcoating osition, cor phe ys particles ical vapo include ur and wet chemistry. Generally, the synthesis of core-shell particles involves different stages. improved function, surface modiﬁcations, stability, dispersion, core release control and signiﬁcant First, the core decr pa ease rticles in the areuse syn of thpr esiz ecious ed foll material. owed by The the cor forma e-shell tion particles, of shell oas nto the the name core suggests, particle. Thi contain s mea th shell od d and epends a core on wher the ein type a shell of core can be anpr d oduced shell mby ater the ials same [41]or . The differ ment ain materials used for the core [36–38]. Figure 2 shows various core-shell particles where the purpose of producing the core-shell particles is to achieve suitable unconventional novel colours are used to differentiate between them wherein a core can consist of a single sphere materials and structures. Consequently, the materials with desirable attributes such as (Figure 2a) or multiple spheres that are smaller in size (Figure 2b). Furthermore, a shell may active particles with high stability, biocompatibility and synergy effect can be achieved be hollow with one small sphere inside, resembling the yolk-shell structure (Figure 2c) [39]. [44]. Figure 2. Schematic representation of different types of core–shell particles (a) single sphere (b) Figure 2. Schematic representation of different types of core–shell particles (a) single sphere (b) mul- multiple spheres are smaller in size) (c) yolk shell (d) large core sphere with one layer of many tiple spheres are smaller in size) (c) yolk shell (d) large core sphere with one layer of many smaller smaller spheres (e) large core sphere with two layers of many smaller spheres (f) simply a collec- spheres (e) large core sphere with two layers of many smaller spheres (f) simply a collection of core tion of core spheres (g) smaller spheres into the shell (h) multiple shells [36]. spheres (g) smaller spheres into the shell (h) multiple shells [36]. Diverse industries utilise a basic nanomaterial to synthesize core-shell NPs. The Figure 2 shows three forms of shell structure like a continuous layer (Figure 2a–c), a speed, simplicity, environmental friendliness and cost effectiveness of the method as well larger core sphere that contains many smaller spheres (Figure 2d,e) or simply a collection as products are prerequisites for the synthesis of these core-shell structures. Many of core spheres (Figure 2f) [40]. The intricacy of the core-shell structure can be manipulated methods have been established to meet the aforementioned requirements. These include by inserting smaller spheres into the shell (Figure 2g) [41] that can also be done through the electrochemical dealloying, sol-gel process, sonochemical process, microwave syn- multiple shells (Figure 2h) [42,43]. The core-shell NPs can be made using the physical or thesis, multi-step reduction, microe-mulsion, epitaxial growth, and Stöber method. Hy- chemical approach including the chemical deposition, physical vapour and wet chemistry. brid method involves the unification of more than one of the aforementioned methods. Generally, the synthesis of core-shell particles involves different stages. First, the core parti- Generally, sol-gel process is widely used to produce the core-shell NPs. Sol-gel method cles are synthesized followed by the formation of shell onto the core particle. This method for the synthesis of core-shell NPs offers an additional control during the reaction process depends on the type of core and shell materials [41]. The main purpose of producing the of solid materials. Homogenous multi-component systems especially mixed oxides can core-shell particles is to achieve suitable unconventional novel materials and structures. Consequently, the materials with desirable attributes such as active particles with high stability, biocompatibility and synergy effect can be achieved [44]. Diverse industries utilise a basic nanomaterial to synthesize core-shell NPs. The speed, simplicity, environmental friendliness and cost effectiveness of the method as well as prod- ucts are prerequisites for the synthesis of these core-shell structures. Many methods have been established to meet the aforementioned requirements. These include the electrochem- ical dealloying, sol-gel process, sonochemical process, microwave synthesis, multi-step reduction, microe-mulsion, epitaxial growth, and Stöber method. Hybrid method involves the uniﬁcation of more than one of the aforementioned methods. Generally, sol-gel pro- cess is widely used to produce the core-shell NPs. Sol-gel method for the synthesis of core-shell NPs offers an additional control during the reaction process of solid materials. Homogenous multi-component systems especially mixed oxides can easily be produced through the mixing of solutions containing molecular precursors. Essentially, this method yields solid materials (small molecular clusters), especially metal oxides like SiO and TiO . 2 2 Preparation of metal oxides using sol-gel process involves the conversion of monomer Appl. Nano 2023, 4 79 into a colloidal solution (sol). The solution is the precursor to be used for combination of network including discrete particles or network polymers. Usually, various metal alkoxides are used as precursor. Sol is produced when a chemical reaction occurs and eventually become a diphasic substance that has property similar to gel, implying both liquid and solid. The morphology of these phases may either be discrete particles or continuous polymer networks. Turning the colloids into the properties like gel necessitate the removal of large volume of liquids in the events from the volume of particle density that is signiﬁcantly low. One of the simplest ways to achieve this is to wait for an adequate time for the sedimenta- tion to occur before disposing the remaining liquid. In addition, the centrifugation can be applied to accelerate the phase separation. Sol-gel is a more common wet-chemical method that is used to synthesise core-shell NPs [45–47]. Microemulsions are a mixture of isotropic liquid composed of surfactant, oil, water and more commonly co-surfactant. It has a clear appearance and a stable thermodynamic system in the presence of salt and other ingredients in the liquid form. The oily substance may be due to complex mixtures of various types of hydrocarbons. In comparison to con- ventional emulsions, microemulsions are synthesised through different mixing components and do not need high shear conditions during the production process. The microemulsions are categorized as direct (dispersion of oil in water, o/w), reversed (dispersion of water in oil, w/o) and bycontinuous types. These microemulsions belong to the ternary systems wherein two immiscible substances (water and oil) which forms separate layers co-exists with a surfactant, resulting in a monolayer that form between the immiscible substances from the surfactant’s molecules. In the oil phase, the hydrophobic tails of the surfactant molecules would dissolve. However, in the liquid phase, the hydrophilic head groups would dissolve. Two-step microwave irradiation is the conventional method for rapid synthesis of gold and silver core-shell bimetallic NPs. In this technique, a bilayer organic barrier is developed surrounding the core. The desired capping agents are the citrate and ascorbic acid that facilitates the formation of core and shell material, developing a well-deﬁned boundary layer. The boundary layer is signiﬁcant for the synthesis process of various core-shell particles that are ultimately used to create the customised bimetallic core-shell NPs of desired morphology wherein the cores are triangular or spherical in shape. The high-pressure chemical vapour deposition method is an alternative in producing the core-shell materials including nanotubes. Nikolaev et al. [48] founded this method to produce the single-walled carbon nanotube (SWCNTs). In this process, a small amount of Fe(CO) is used to comb CO. Then, the mixture is passed to a heat reactor. El-Gendy et al. [49] used this technique to make NPs coated with various materials like Fe, Co, Ni, FeRu, CoRu, NiRu, NiPt, and CoPt. In this method, the reactor ’s temperature and pressure can be accurately controlled to tailor the core-shell NPs properties needed for the speciﬁc applications. Earlier, the metal-organic precursors called the metallocenes or metals that are rich in carbon were used. These precursors were inputted into a thermostatic sublimation chamber before releasing argon gas for pushing the vapour into the hot zone of the chamber. First, the precursor broke down the NPs within the cooling ﬁnger before turning into the gas phase within the hot zone for the supersaturation. Upon the initiation of the supersaturation process, the NPs were nucleated. Furthermore, careful adjustments can be made to the temperature and pressure/temperature within the corresponding sublimation chambers and chemical vapour deposition reactor in order to control the desired degree of supersaturation. At high pressure, the collision probability of gas atoms increases, thus reducing the rate of atoms diffusion from the original location. It is worth noting that when the diffusion rate is poor, the supersaturation does not occur. In this situation, the cooling ﬁnger contains the deposits of tiny clusters of atoms or individual atoms. Fe O with graphene shells as coating was prepared using the wet chemical tech- 2 3 nique [50]. Oleic acid and 1-octadecene were mixed in a solution before being placed in the reﬂux reactor and heated to 320 C to dissolve the iron oleate. Next, the solution was washed with ethanol and acetone to obtain the iron oxide particles. Normally, the Stöber process involves the preparation of SiO particles [51] with total control of uniformity in 2 Appl. Nano 2023, 4, FOR PEER REVIEW 6 occur. In this situation, the cooling finger contains the deposits of tiny clusters of atoms or individual atoms. Fe2O3 with graphene shells as coating was prepared using the wet chemical tech- nique [50]. Oleic acid and 1-octadecene were mixed in a solution before being placed in the reflux reactor and heated to 320 °C to dissolve the iron oleate. Next, the solution was washed with ethanol and acetone to obtain the iron oxide particles. Normally, the Stöber process involves the preparation of SiO2 particles [51] with total control of uniformity in size [52]. These particles offer numerous applications in the field of materials science and Appl. Nano 2023, 4 80 engineering. Since the discovery of this method by Werner Stöber et al. [51], it remains the most renowned wet chemistry approach for the NPs synthesis [53]. Being a sol-gel process, the chemical tetraethyl orthosilicate (TEOS) act as the precursor immersed in size [52]. These particles offer numerous applications in the ﬁeld of materials science and water. Alcoholic solution is added to form a reaction, forming new molecules that ag- engineering. Since the discovery of this method by Werner Stöber et al. [51], it remains gl the om most erate renowned to create wet large chemistry r clusterappr s. Du oach et afor l. [5 the 4] used NPs synthesis the sol-gel [53]. appr Being oach a sol-gel to make SiO2 process, the chemical tetraethyl orthosilicate (TEOS) act as the precursor immersed in water. shell as a coating agent for the Fe3O4 NPs, eventually producing the core-shell structure. Alcoholic solution is added to form a reaction, forming new molecules that agglomerate In this two-step procedure, the co-precipitation was first initiated to obtain Fe3O4 NPs. to create larger clusters. Du et al. [54] used the sol-gel approach to make SiO shell as a Next, it caused a reaction with tetramethyl ammonium hydroxide (TMAOH), forming a coating agent for the Fe O NPs, eventually producing the core-shell structure. In this 3 4 liquid solution that contained the proposed particles. In the second stage, SiO2 was pro- two-step procedure, the co-precipitation was ﬁrst initiated to obtain Fe O NPs. Next, it 3 4 duced through the hydrolyzation of TEOS in order to limit the formation of Fe 3O4. caused a reaction with tetramethyl ammonium hydroxide (TMAOH), forming a liquid Figure 3 shows the sol-gel unified annealing approach used by Li et al. [55] to pro- solution that contained the proposed particles. In the second stage, SiO was produced duce ZnSiO3/ZnO core-shell NPs. In this experiment, the reason for combining these two through the hydrolyzation of TEOS in order to limit the formation of Fe O . 3 4 methods was to produce broad band-gap core-shell NPs of Zinc Silicate-Zinc Oxide Figure 3 shows the sol-gel uniﬁed annealing approach used by Li et al. [55] to pro- duce ZnSiO /ZnO core-shell NPs. In this experiment, the reason for combining these (Zn2SiO4@ZnO). First, the reaction between Na2SiO3/ZnCl2 was initiated to form ZnSiO3, two methods was to produce broad band-gap core-shell NPs of Zinc Silicate-Zinc Oxide which in turn produced the shells with varied thickness before being used to coat ZnO (Zn SiO @ZnO). First, the reaction between Na SiO /ZnCl was initiated to form ZnSiO , 2 4 2 3 2 3 NPs. A low annealing temperature of 780 °C was set. Finally, the reaction between which in turn produced the shells with varied thickness before being used to coat ZnO NPs. amorphous ZnSiO3 and ZnO occurred, forming a crystalline Zn2SiO4 shell. Chai et al. [45] A low annealing temperature of 780 C was set. Finally, the reaction between amorphous adopted this technique to make core-shell Fe3O4@SiO2 NPs. The first step was to fabricate ZnSiO and ZnO occurred, forming a crystalline Zn SiO shell. Chai et al. [45] adopted this 3 2 4 Fe3O4 NPs via the solvothermal technique. Next, the hydrolyzation of tetraethyl ortho- technique to make core-shell Fe O @SiO NPs. The ﬁrst step was to fabricate Fe O NPs 3 4 2 3 4 silicate resulted in SiO2 that acted as the coating agent for Fe3O4 NPs. via the solvothermal technique. Next, the hydrolyzation of tetraethyl orthosilicate resulted in SiO that acted as the coating agent for Fe O NPs. 2 3 4 Figure 3. Core-shell particles synthesis using sol-gel combined annealing method [55]. Reproduced Figure 3. Core-shell particles synthesis using sol-gel combined annealing method [55]. Reproduced with permission from Li, Z, et al., Materials Chemistry and Physics; published by Elsevier, 2020. with permission from Li, Z, et al., Materials Chemistry and Physics; published by Elsevier, 2020. A two-step reduction technique was also used [56] to make epitaxial Au@Ni core-shell nanocrystals. In this process, various materials such as decahedral, octahedral, triangular A two-step reduction technique was also used [56] to make epitaxial Au@Ni and hexagonal plate-like as well as icosahedral were mixed initially. Subsequently, ethylene core-shell nanocrystals. In this process, various materials such as decahedral, octahedral, glycol (EG) was used for the reduction of HAuCl before being placed in a microwave triangular and hexagonal plate-like as well as icosahedral were mixed initially. Subse- with polyvinylpyrrolidone (PVP) that acted as a polymer surfactant to be heated. The core quently, ethylene glycol (EG) was used for the reduction of HAuCl 4 before being placed seeds were produced at this stage and subsequently the oil bath was heated to reduce in a microwave with polyvinylpyrrolidone (PVP) that acted as a polymer surfactant to be Ni(NO ) .6H O in EG in the presence of NaOH and PVP. Eventually, the Ni shells were 3 2 2 heated. The core seeds were produced at this stage and subsequently the oil bath was overgrown within the Au core seeds. Fan et al. [57] used similar technique but focused heated to reduce Ni(NO3)2.6H2O in EG in the presence of NaOH and PVP. Eventually, the on the seed-mediated growth. Herein, Au cores were made in the liquid form to achieve bimetallic core-shell nanocubes. Comprehensive assessment was made upon the hetero- Ni shells were overgrown within the Au core seeds. Fan et al. [57] used similar technique geneous core-shell formation on the four common metals like gold, silver, palladium and but focused on the seed-mediated growth. Herein, Au cores were made in the liquid form platinum. This experiment constituted the following bases: (a) the general conditions and to achieve bimetallic core-shell nanocubes. Comprehensive assessment was made upon growth modes to attain conformal epitaxial structures and (b) heterogeneous nucleation the heterogeneous core-shell formation on the four common metals like gold, silver, pal- and formation of various noble metals. In addition, three types of growth modes for the gold cores with heterogeneous metal shells were identiﬁed: conformal epitaxial growth (Au@Pd and Au@Ag nanocubes), island growth (Au@Pt nanospheres) and heterogeneous nucleation. Further ﬁndings include two metals with comparable lattice constants where the mismatch was less than 5%. These ﬁndings were consistent with other studies (Au@Ag (lattice mismatch, 0.2%), Au@Pd (4.7%), and Pt@Pd (0.85%)) [54–56]. Appl. Nano 2023, 4, FOR PEER REVIEW 7 ladium and platinum. This experiment constituted the following bases: (a) the general conditions and growth modes to attain conformal epitaxial structures and (b) heteroge- neous nucleation and formation of various noble metals. In addition, three types of growth modes for the gold cores with heterogeneous metal shells were identified: con- formal epitaxial growth (Au@Pd and Au@Ag nanocubes), island growth (Au@Pt nano- spheres) and heterogeneous nucleation. Further findings include two metals with com- parable lattice constants where the mismatch was less than 5%. These findings were con- Appl. Nano 2023, 4 81 sistent with other studies (Au@Ag (lattice mismatch, 0.2%), Au@Pd (4.7%), and Pt@Pd (0.85%)) [54–56]. Tsuji et al. [58] used one-polyol technique to make Ag@Cu core-shell NPs with a high yield. The method involved the use of bubbling Ar gas with added reagents like Tsuji et al. [58] used one-polyol technique to make Ag@Cu core-shell NPs with a AgNO3 and Cu(OAc)2 H2O. This two-step process was used to synthesize Ag@Cu parti- high yield. The method involved the use of bubbling Ar gas with added reagents like cles through AgNO3 reduction in EG. The Cu shells were developed by separating the Ag AgNO and Cu(OAc) H O. This two-step process was used to synthesize Ag@Cu particles 3 2 2 cores from AgNO3, before Cu(OAc)2.H2O was added. This procedure failed because no through AgNO reduction in EG. The Cu shells were developed by separating the Ag Cu@Ag core-shell particles were nucleated instead the Cu/Ag bi-compartmental particles cores from AgNO , before Cu(OAc) .H O was added. This procedure failed because no 3 2 2 were appeared. Later, various experimental processes were combined at different reac- Cu@Ag core-shell particles were nucleated instead the Cu/Ag bi-compartmental particles tion temperatures and heating times to produce Ag@Cu particles. It was found that the were appeared. Later, various experimental processes were combined at different reaction temperatur optimal con esdand ition heating for pro times ducin to g pr Ag@C oduce u pa Ag@Cu rticles particles. is to addIt tw was o re found agentthat s in the revoptimal erse. At condition the beginn for ing pro oducing f the pro Ag@Cu cess, 8 m particles L of 15.9 is m toM add Cu two (OAc reagents )2.H2O in wa rs everse. addedAt in the EGbeginning plus 8 mL of of the 477pr m ocess, M pol 8y(v mL inof ylpyrr 15.9o mM lidon Cu e) (OAc) (PVP, M .H WO : 5was 5,000 added monoin mer EG uplus nits).8 A mL 10of 0-m 477 L th mM ree 2 2 poly(vinylpyrr necked flask w olidone) as used f(P or VP th,e MW soluti : 55,000 on mixin monomer g. Ar wun as its). bubb A led 100-mL for 10thr mee in a necked t room ﬂask tem- was peraused ture to for com theplete solution ly rem mixing. ove oxyge Arn was from bubbled the soluti foro10 n fo mi llo nw at ed r oom by so temperatur aking in an e o to il completely remove oxygen from the solution followed by soaking in an oil bath at a bath at a temperature of 180 °C. The solution continued to bubble while the temperature temperature of 180 C. The solution continued to bubble while the temperature was raised was raised to 175 °C. Afterwards, the reagent solution was added with 2 mL of 15.7 mM to 175 C. Afterwards, the reagent solution was added with 2 mL of 15.7 mM AgNO and AgNO3 and left for 20 min at 175 °C. Finally, 7.0 mM, 212 mM and 1.7 mM of Cu left for 20 min at 175 C. Finally, 7.0 mM, 212 mM and 1.7 mM of Cu (OAc) .H O, AgNO (OAc)2.H2O, AgNO3 and PVP, respectively. Further investigation was conducted by var- 2 2 3 and PVP, respectively. Further investigation was conducted by varying the reaction time ying the reaction time on the reagent solution to determine the growth mechanism of on the reagent solution to determine the growth mechanism of Ag@Cu. Ag@Cu. Chae et al. [45] produced Fe O @SiO by the customised Stober method. The solution 3 4 2 Chae et al. [45] produced Fe3O4@SiO2 by the customised Stober method. The solution of 4 g Fe O particles was ultrasonicated and extra tetraethyl orthosilicate was added to 3 4 of 4g Fe3O4 particles was ultrasonicated and extra tetraethyl orthosilicate was added to raise the volume from 4 to 40 mL. A stable emulsion was obtained and it was further raise the volume from 4 to 40 mL. A stable emulsion was obtained and it was further in- inserted into a mixture containing 50 mL of ethanol and 12 mL of NH H O. The reaction 3 2 serted into a mixture containing 50 mL of ethanol and 12 mL of NH 3 H2O. The reaction solution was stirred at 400 rpm at room temperature for 4 h until the core-shell structured solution was stirred at 400 rpm at room temperature for 4 h until the core-shell structured Fe O @SiO NPs were separated using centrifugation. Figure 4 shows the entire process of 3 4 2 Fe3O4@SiO2 NPs were separated using centrifugation. Figure 4 shows the entire process Fe O @SiO synthesis. 3 4 2 of Fe3O4@SiO2 synthesis. Figure 4. Stober method for Fe3O4@SiO2 nanoparticles synthesis [45]. Reproduced with permission Figure 4. Stober method for Fe O @SiO nanoparticles synthesis [45]. Reproduced with permission 3 4 2 from Chae, H.S., et al., Colloid and Polymer Science; published by Elsevier, 2016. from Chae, H.S., et al., Colloid and Polymer Science; published by Elsevier, 2016. Sharma et al. [59] conducted a similar experiment and demonstrated that it is pos- Sharma et al. [59] conducted a similar experiment and demonstrated that it is possible sible to fabricate core-shell particles through the precipitation without the need of any to fabricate core-shell particles through the precipitation without the need of any surfactant. surfactant. The outcome (the concentrations of the core-shell particles) was compared The outcome (the concentrations of the core-shell particles) was compared with those obtained with thousing se obta dif infer edent usianionic ng differ and ent non-ionic anionic a surfactants. nd non-ionThe ic surf nano acta -T niO ts. The was n dev ano eloped -TiO2 in wa the s dev form eloped of shell in th using e form ﬂy ofash. shell The usin surfactants g fly ash. The wer e surf mainly actants used wer to e m str aengthen inly used the to strengthen the adhesion of the nano-titania shells to fly ash core. Yet again, different adhesion of the nano-titania shells to ﬂy ash core. Yet again, different types of surfactants wer types e used of surf to test actathe nts str wer ength e used of the to test TiOth adhesion e strengtonto h of th ﬂy e ash. TiO2Another adhesiotest n on was to fly conducted ash. An- without other test surfactant. was cond When ucted anionic without surfactant surfactant. was Wh used, en an the ionr ic esulting surfacta part nt w icles as use formed d, the had re- remarkable pigment properties and reﬂectance in the near-infrared region, indicating their sulting particles formed had remarkable pigment properties and reflectance in the suitability towards cool coating applications. A solution of 70% ethanol was added in the sequence of ﬂy ash, anionic (SDS) or non-ionic surfactant (TX-100) and ﬁnally titanium isopropoxide. Finally, the solution was stirred for two hours before being dried at 50–600 C to achieve a powder. Zhang et al. [60] made a study to produce PUA hybrid emulsion PA/PU with a ratio of 20 to 80 using semi-batch emulsion. In the experimental setup a digital thermometer, 250 mL four-neck glass ﬂask containing a reﬂux condenser, mechanical stirrer and nitrogen gas inlet were used. The pre-emulsion was prepared by dissolving 2.0 g per 100 g of acrylic and PU content into the water before gradually adding 5.0 g of MMA, 5.0 g of BA and 0.015 g of AA (0.1 5 wt% of the overall MMA and BA weight). The solution was then stirred Appl. Nano 2023, 4 82 before mixing additional 0.5 g. The main objective was to obtain 111.3 g of PU emulsion dispersion and 10% monomers from the reactor vessel. The temperature was set at 80 C while the contents were stirred. Next, 0.4 g of KPS per 100 g acrylic monomers composed of 10% was added and continuously stirred for 30 min. Subsequently, the temperature was increased by 5 C and simultaneously the leftover monomer pre-emulsion and initiator solution was ﬂown into the task for 4 h at a constant ﬂow rate. Next, the solution was left at 85 C for 0.5 h with stirring and waiting for the temperature to drop. Lastly, the pH value was maintained at the desirable range after adding NaHCO . 3. Core-Shell NPs Based Sustainable Pigments In the last decade, synthetic-coloured pigments have been launched in the market that resulted in more extensive scientiﬁc research focused on this area. Typical applica- tions of these pigments are varnishes, paints, plastics and textiles, printing inks, building materials and rubber, ceramic glazes and leather decoration [61–63]. The deﬁnition of the pigment durability is connected to its ability of resisting weathering processes and negating deteriorating when being placed in an external environment [64]. Recent studies have shown that efﬁcient energy consumption and environmental protection measures are deemed signiﬁcant [65]. To address this issue, the production of both sustainable and durable pigments has become the fundamental requirement within the construction indus- try. Myriad of methods have been applied in order to increase the pigments’ durability, and the most signiﬁcant is known as the core-shell method [3,66–68]. There has been a surge of development of various chemical synthesis techniques in recent years. Such research has found that multi-component materials possess diverse compositions and structures. These attributes signify remarkable property type and they are applicable in many different types of ﬁelds [69–72]. There is even more research being conducted on their distinctive core-shell structure. There are many advantages of the core-shell structure compared to other types of composite materials. One such advantage is their ability to generate or increase the strength of new chemical and physical capabilities, enabling maintenance on structural integrity, deter the core from breaking up to large particles and ascertaining dispersion effectively. In addition, they also provide conventional multi-functional compositions and structure with other advantages. Moreover, a synergetic effect between the shells and cores would even extend the performance further [73]. Science and technology ﬁeld have been attentive on the phenomena of materials that are derived from the core-shell properties because they can be ﬁnely customised [61,74,75]. A shell domain cloaks a core structural domain within each of the core or shell particle. Materials that possess core or shell particles include inorganic solids, metals and polymers. There is no difﬁculty in modifying characteristics such as size and structures as well as the particles’ composition in order to further customise their properties such as optical, magnetic, mechanical, thermal, electrical, catalytic and electro-optical attributes. Core or shell morphology can be applied to produce hollow spheres and minimize the costs of precious materials. Thus, the materials with the reduced core costs can be coated to precious materials [76,77]. Particles with the size of less than 0.1 m is classiﬁed as NPs and have been garnering much attention in research within the past few years. Essentially, NPs are smart materials with exclusive properties. Applications using NPs have more advantages compared to materials that have larger surface to volume ratio such as microscale, macroscale and bulk materials [78,79]. Due to the increased research on the NPs development, it is now possible to make NPs in symmetrical shape, such as spherical as well as other shapes including prism, hexagon, cube, wire, tube and rod [80–82]. Despite this achievement, the bulk of the research is still at early stage in terms of exploring the possible shapes that can be synthesised. There has been research that recently found the ease of production method for NPs that are non-spherical [83–85]. However, it must be stressed that NPs’ properties are dependent on the actual shape and size. Such properties that are dependent on particle size include temperature barrier, magnetic saturation and Appl. Nano 2023, 4 83 permanent magnetisation. Furthermore, coactivity of the nanocrystals is dependent on the shape of the particle as it has a direct inﬂuence on the surface anisotropy [86]. Rapid advancements are made in nanotechnology resulting in the founding of core- shell NPs, which is a leading functional material. This has attracted even more research conducted on various functional compositions core-shell NPs where it could be applied in many types of areas such as optics, catalysis, biomedicine, electronics and medicines [87]. Core-shell NPs possess beneﬁcial physiochemical properties that are exclusive, and this attribute has garnered a lot of researchers’ attention. The primary advantages of core-shell NPs are that it could increase protection level, encapsulation and controlled release [82,88]. The discovery of a variety of core/shell NPs leads to its applications to a variety of situations. However, the difﬁculty is to identify the individual type core/shell NPs that are applicable to the respective industries due to their multitude of types. Numerous studies on the core- shell NPs pigments are focusing on core/shell materials, production methods, distinctive properties and their applications. Herein, the main features of the core-shell NPs including their fabrication methods, inorganic materials and typical applications are emphasized. A discussion on diverse methods of production along with the classiﬁcations of the core-shell materials that are already being in use are outlined. The new fabrication methods of the core-shell NPs pigments within all research ﬁelds are emphasized. Finally, the application potential of core-shell NPs within paints designed for roads and other construction sectors are underscored. 3.1. Materials Based Shell Part Several materials such as metals and biomolecules are used to create core-shell NPs. There are two components, the central core and an alternative core, which is the shell. The attributes of the core-shell nanostructures include their high thermal and chemical stabilities, low toxicity, high levels of solubility and high level of permeability for speciﬁcally targeted cell. Such properties enable them to have a vast potential for functional applications in many sectors. Furthermore, micro-nano scale core-shell particles have attributes that are exclusive and unique to them compared to other particles. Essentially, the attributes combined the materials’ properties that are used for core and shell together along with smart properties that are formed via their materials. For the past few years, there have been an increased research interest in core-shell structures production [89]. This is particularly true within the pigment industry due to the high range of applications of core-shell materials in order to increase pigments’ durability. The core-shell materials could be made of both organic and inorganic materials. For instance, Cao et al. [90] developed hybrid pigments which consist of inorganic-organic structure using a mixture of precipitated SiO and TiO . In addition, 2 2 dye core@silica shell structure was fabricated using the mesoporous soft template synthesis approach [91]. This section below explores the possibility of using inorganic materials to produce core-shells materials with the focus on SiO and TiO . 2 2 3.2. Efﬁciency and Test Methods Generally, the obtained core-shell NPs are characterized using diverse analytical methods such as SEM, LC-MS, XPS, FTIR, XRD, TEM, BET, Ultraviolet-visible Spec- troscopy, Raman spectrum as well as Near-Infrared Reﬂectance and Photoluminescence Spectroscopy [37,38,47,92–96]. For instance, assessment of morphology, chromaticity and the structure of -Fe O @SiO fabricated pigments can be tested by SEM, TEM, FTIR, XPS 2 3 2 and XRD [88]. Figure 5a shows the XRD patterns of the pigments made of -Fe O @SiO 2 3 2 NPs, -Fe O @SiO and -Fe O . The formation of the core-shell structures results in the 2 3 2 2 3 diffraction peak of -Fe O @SiO particles appeared in the 2 range of 15 –25 , indicat- 2 3 2 ing the presence of amorphous SiO . Further calcinations at 1000 C could change the diffraction peak to around 22 . The results of reddish colour pigment indicated that the amorphous shell has entered into a cristobalite phase. In addition, the formation of the core-shell structure weakened the -Fe O diffraction peak. Figure 5b shows the FTIR 2 3 spectrum of the reddish pigments of -Fe O , -Fe O @SiO NPs, and Fe O @SiO . The 2 3 2 3 2 2 3 2 Appl. Nano 2023, 4, FOR PEER REVIEW 10 Generally, the obtained core-shell NPs are characterized using diverse analytical methods such as SEM, LC-MS, XPS, FTIR, XRD, TEM, BET, Ultraviolet-visible Spectros- copy, Raman spectrum as well as Near-Infrared Reflectance and Photoluminescence Spectroscopy [37,38,47,92–96]. For instance, assessment of morphology, chromaticity and the structure of α-Fe2O3@SiO2 fabricated pigments can be tested by SEM, TEM, FTIR, XPS and XRD [88]. Figure 5a shows the XRD patterns of the pigments made of α-Fe2O3@SiO2 NPs, -Fe2O3@SiO2 and α-Fe2O3. The formation of the core-shell structures results in the diffraction peak of α-Fe2O3@SiO2 particles appeared in the 2θ range of 15°–25°, indicating the presence of amorphous SiO2. Further calcinations at 1000 °C could change the dif- fraction peak to around 22°. The results of reddish colour pigment indicated that the amorphous shell has entered into a cristobalite phase. In addition, the formation of the core-shell structure weakened the α-Fe2O3 diffraction peak. Figure 5b shows the FTIR spectrum of the reddish pigments of α-Fe2O3, α-Fe2O3@SiO2 NPs, and Fe2O3@SiO2. The Appl. Nano 2023, 4 84 −1 hydroxyl (–OH) stretching vibration bands were probed at 3423.50, 1627.85 cm , 536.19 −1 and 466.75 cm , indicating a correlation to the O–Fe–O bands of α-Fe2O3. The band at hydroxyl (–OH) stretching vibration bands were probed at 3423.50, 1627.85 cm , 536.19 −1 1091.66 and 470 cm emerged from the covering of α-Fe2O3 in SiO2, indicating the bend- and 466.75 cm , indicating a correlation to the O–Fe–O bands of -Fe O . The band at 2 3 ing and stretching modes of O–Si–O. The FTIR results confirmed the formation of coating 1091.66 and 470 cm emerged from the covering of -Fe O in SiO , indicating the bend- 2 3 2 on the α-Fe2O3 surface. Further calcinations could enhance the O-Si-O bond strength as ing and stretching modes of O–Si–O. The FTIR results conﬁrmed the formation of coating well as improve the core and shell interactions. Figure 5c,d show the assessment results on the -Fe O surface. Further calcinations could enhance the O-Si-O bond strength as 2 3 well as improve the core and shell interactions. Figure 5c,d show the assessment results of the reddish pigments through the use of XPS. Figure 5c shows that Fe–O bonds and of the reddish pigments through the use of XPS. Figure 5c shows that Fe–O bonds and Si–O bonds are in the O1s pigment as evidenced from the high-resolution XPS spectrum. Si–O bonds are in the O1s pigment as evidenced from the high-resolution XPS spectrum. Meanwhile, a band that was observed at 103.5 eV in the Si 2p XPS spectrum is expected in Meanwhile, a band that was observed at 103.5 eV in the Si 2p XPS spectrum is expected in pure silica. pure silica. Figure 5. (a) XRD patterns and (b) FTIR spectra of different samples; high-resolution XPS spectra Figure 5. (a) XRD patterns and (b) FTIR spectra of different samples; high-resolution XPS spectra of of (c) O 1s and (d) Si 2p for -Fe O @SiO pigments calcined at 1000 C [88]. Reproduced with 2 3 2 (c) O 1s and (d) Si 2p for α-Fe2O3@SiO2 pigments calcined at 1000 °C [88]. Reproduced with per- permission from Chen, S., et al., Applied Surface Science; published by Elsevier, 2020. mission from Chen, S., et al., Applied Surface Science; published by Elsevier, 2020. Li et al. [47] conducted an analysis on the synthesized -Ce S @SiO core-shell ma- 2 3 2 terials L using i et a TEM l. [4test. 7] con The dTEM ucted images an an in aFigur lysis eo 6nshow the the syn silica thesiz shell ed being γ-Ceformed 2S3@SiO at 2 core-shell ma- various coating times. A clear layer covers the -Ce S but it is not found on the samples 2 3 terials using TEM test. The TEM images in Figure 6 show the silica shell being formed at that are not coated, which is in accordance to the SEM analysis. Figure 6b–d shows a various coating times. A clear layer covers the γ-Ce2S3 but it is not found on the samples correlation between the increasing thickness of the coating layer and increasing coating that are not coated, which is in accordance to the SEM analysis. Figure 6b–d shows a times. It was demonstrated that when the particles were coated once, twice and thrice correlation between the increasing thickness of the coating layer and increasing coating times, the thickness was increased to 70 nm, 100 nm and 140 nm, respectively. This clearly indicated that it is possible to control the coating thickness through number of coatings times. It was demonstrated that when the particles were coated once, twice and thrice being applied. Appl. Nano 2023, 4, FOR PEER REVIEW 11 Appl. Nano 2023, 4, FOR PEER REVIEW 11 times, the thickness was increased to 70 nm, 100 nm and 140 nm, respectively. This clearly indicated that it is possible to control the coating thickness through number of coatings times, the thickness was increased to 70 nm, 100 nm and 140 nm, respectively. This clearly being applied. Appl. Nano 2023, 4 85 indicated that it is possible to control the coating thickness through number of coatings being applied. Figure 6. TEM images of (a) uncoated γ-Ce2S3 and (b) once, (c) twice (d) thrice coated γ-Ce2S3@SiO2 core-shell particles [47]. Reproduced with permission from Li, Y.-M., et al., Surface and Coatings Figure 6. TEM imaT gec esh o n f ol (a og ) u yn ; c pu oab tli ed sh γ e- d Ce by 2S El 3 a se nd vie (b r,) 20 on 1c 8e, . (c) twice (d) thrice coated γ-Ce2S3@SiO2 Figure 6. TEM images of (a) uncoated -Ce S and (b) once, (c) twice (d) thrice coated -Ce S @SiO 2 3 2 3 2 core-shell particles [47]. Reproduced with permission from Li, Y.-M., et al., Surface and Coatings core-shell particles [47]. Reproduced with permission from Li, Y.-M., et al., Surface and Coatings Technology; published by Elsevier, 2018. Technology; Liu et published al. [97] by used Elsevier four , 2018. types of tests (FTIR, TEM, XRD and EDS) to assess the morphology of fabricated -Ce2S3@SiO2 samples. The first step was to assess the SiO2 Liu et al. [97] used four types of tests (FTIR, TEM, XRD and EDS) to assess the Liu et al. [97] used four types of tests (FTIR, TEM, XRD and EDS) to assess the thickness used to coat γ-Ce2S3. This was performed through the TEM test. Figure 7 shows morphology of fabricated -Ce S @SiO samples. The ﬁrst step was to assess the SiO 2 3 2 2 morphology of fabricated -Ce2S3@SiO2 samples. The first step was to assess the SiO2 different amounts of volume ratios of water/ethanol that was used for the preparation of thickness used to coat -Ce S . This was performed through the TEM test. Figure 7 shows 2 3 thickness used to coat γ-Ce2S3. This was performed through the TEM test. Figure 7 shows uncoated γ-Ce2S3 pigments and SiO2 xerogel coated γ-Ce2S3. Figure 7a presents the de- different amounts of volume ratios of water/ethanol that was used for the preparation different amounts of volume ratios of water/ethanol that was used for the preparation of posited surface with irregularly large chunks accompanied by small particles on the un- of uncoated -Ce S pigments and SiO xerogel coated -Ce S . Figure 7a presents the 2 3 2 2 3 uncoated γ-Ce2S3 pigments and SiO2 xerogel coated γ-Ce2S3. Figure 7a presents the de- coated γ-Ce2S3 pigments. The detected Zn signals within EDS spectra indicated that the deposited surface with irregularly large chunks accompanied by small particles on the posited surface with irregularly large chunks accompanied by small particles on the un- col uncoated our sta bi -Ce lity S opigments. f the unco The ated detected γ-Ce2SZn 3 pigm signals ents within can be EDS con spectra trolled indicated using Z that nO.the Another 2 3 coated γ-Ce2S3 pig a colour d m va ent nta stability s. ge Th oe f d th of ete e the cte applica uncoated d Zntio sin gn o -Ce afl s th wi S ese tpigments h in pig E m DS ent can spec s is be tra itcontr s ilo nd w olled ica H ted 2S using e th m ais t ZnO. s th ioe ns. Another Figure 7b–d 2 3 advantage of the application of these pigments is its low H S emissions. Figure 7b–d colour stability osh f th ows e u n pr co ese ated nce γ o -f Ce core 2S3 -pigm shell ents struc ca ture n be s wi con thtrol in aled ll th usi e pigm ng 2 Ze nn O. t pa Articles notherd uring coating. shows presence of core-shell structures within all the pigment particles during coating. advantage of the Sim applica ultantio eous n ly of , S th i si ese gn a pig l is m d ent etes cte isd it as s sh low own H 2in S e Fi m gur isse io6 nfs. . Thi Figu s pr re o v 7es b–d that SiO2 xerogel Simultaneously, Si signal is detected as shown in Figure 7f. This proves that SiO xerogel shows presence o m f acore de up -sho ell f th ste ruc coa ture tins g wi lay th er in th all at tis he fo pigm rmed e no t npa th rticles e γ-Ce d 2uri S3 surf ng c ao ce atin . Fi g. gure 7b,c on the made up of the coating layer that is formed on the -Ce S surface. Figure 7b,c on the other 2 3 Simultaneously, S oi th si er gn h aa l n is d d is ete sh cte owi d n as g sh tho awn t th e ina Fi pplica gure tio 6fn . Thi of s wpr ater ov/es etha thn ao t l Sv iO olum 2 xerog e ra el tio of 15/105 (48 hand is showing that the application of water/ethanol volume ratio of 15/105 (48 nm) and nm) and 20/100 (60 nm) results in a moderately uniform shell size. However, Figure 6d made up of the coating layer that is formed on the γ-Ce2S3 surface. Figure 7b,c on the 20/100 (60 nm) results in a moderately uniform shell size. However, Figure 7d shows that shows that when the ratio is adjusted to 25/95, the thickness of the shell is no longer other hand is showing that the application of water/ethanol volume ratio of 15/105 (48 when the ratio is adjusted to 25/95, the thickness of the shell is no longer uniform. The uniform. The main reason for this is that as water volume rise, it will accelerate TWOS nm) and 20/100 (60 nm) results in a moderately uniform shell size. However, Figure 6d main reason for this is that as water volume rise, it will accelerate TWOS hydrolysis. This hydrolysis. This means that during the coating process, shell thickness is no longer uni- shows that when the ratio is adjusted to 25/95, the thickness of the shell is no longer means that during the coating process, shell thickness is no longer uniform because of the f competition orm because between of the surface competand ition silica betw nuclei. een surface and silica nuclei. uniform. The main reason for this is that as water volume rise, it will accelerate TWOS hydrolysis. This means that during the coating process, shell thickness is no longer uni- form because of the competition between surface and silica nuclei. Figure 7. TEM images and EDS patterns of SiO xerogel coated -Ce S prepared with different 2 2 3 Figure 7. TEM images and EDS patterns of SiO2 xerogel coated -Ce2S3 prepared with different water water to ethanol ratio: (a) S0, (b) S1, (c) S2, (d) S3, (e) EDS spectra of S0 and (f) EDS spectra of to ethanol ratio: (a) S0, (b) S1, (c) S2, (d) S3, (e) EDS spectra of S0 and (f) EDS spectra of S2 [97]. S2 [97]. Reproduced with permission from Liu, S.-G. et al., Applied Surface Science; published by Reproduced with permission from Liu, S.-G. et al., Applied Surface Science; published by Elsevier, Elsevier, 2016. Figure 7. TEM images and EDS patterns of SiO2 xerogel coated -Ce2S3 prepared with different water to ethanol ratio: (a) S0, (b) S1, (c) S2, (d) S3, (e) EDS spectra of S0 and (f) EDS spectra of S2 [97]. Figure 8 shows the reﬂectance spectrum measured by Sadeghi-Niaraki et al. [98] for Reproduced with permission from Liu, S.-G. et al., Applied Surface Science; published by Elsevier, the as-produced Fe O @TiO with crystallite size (nm) of CT (32.2 nm), CFT2 (31.4 nm), 2 3 2 Appl. Nano 2023, 4, FOR PEER REVIEW 12 Appl. Nano 2023, 4 86 Figure 8 shows the reflectance spectrum measured by Sadeghi-Niaraki et al. [98] for the as-produced Fe2O3@TiO2 with crystallite size (nm) of CT (32.2 nm), CFT2 (31.4 nm), CFT4 (28.4 nm), and CFT5 (13.3 nm) samples. CT sample showed that the reflectivity at CFT4 (28.4 nm), and CFT5 (13.3 nm) samples. CT sample showed that the reﬂectivity at wavelengths improved which was due to the increase in the crystallinity of the emergent wavelengths improved which was due to the increase in the crystallinity of the emergent rutile phase. After the calcinations, the reflectance value increased as the sample experi- rutile phase. After the calcinations, the reﬂectance value increased as the sample experiences ences the crystallisation. Figure 8c shows that the presence of Fe2O3 produced darker the crystallisation. Figure 8c shows that the presence of Fe O produced darker hues within 2 3 hues within the samples in addition to NIR reflectance being reduced. The NIR solar re- the samples in addition to NIR reﬂectance being reduced. The NIR solar reﬂectance for the flectance for the samples was CT (76%), CFT2 (73%), CFT4 (68.8%), CFT5 (68.4%) and CF samples was CT (76%), CFT2 (73%), CFT4 (68.8%), CFT5 (68.4%) and CF (39.3%). Figure 8d (39.3%). Figure 8d shows the IR reflectance process within Fe2O3–TiO2 and Fe2O3 particles. shows the IR reﬂectance process within Fe O –TiO and Fe O particles. 2 3 2 2 3 Figure 8. Reﬂectance spectra of (a) T, FT2, FT4, FT5 and F samples, (b) CT, CFT2, CFT4, CFT5 and Figure 8. Reflectance spectra of (a) T, FT2, FT4, FT5 and F samples, (b) CT, CFT2, CFT4, CFT5 and CF samples, (c) photographs of CT, CFT2, CFT4, CFT5 and CF samples (d) proposed mechanism CF samples, (c) photographs of CT, CFT2, CFT4, CFT5 and CF samples (d) proposed mechanism of of IR reﬂectance in Fe O and Fe O @TiO composites [98]. Reproduced with permission from 2 3 2 3 2 IR reflectance in Fe2O3 and Fe2O3@TiO2 composites [98]. Reproduced with permission from Sadeghi-Niaraki, S. et al., Materials Chemistry and Physics; published by Elsevier, 2019. Sadeghi-Niaraki, S. et al., Materials Chemistry and Physics; published by Elsevier, 2019. Li et al. [99] tested the high temperature tolerance of the red pigments made from Li et al. [99] tested the high temperature tolerance of the red pigments made from Ce S @SiO -based core-shell NPs. Figure 9 shows the XRD patterns related to the - 2 3 2 Ce2S3@SiO2-based core-shell NPs. Figure 9 shows the XRD patterns related to the Ce S @c-SiO samples, where their production was subjected to various calcination tem- 2 3 2 γ-Ce2S3@c-SiO2 samples, where their production was subjected to various calcination peratures. There is an absence of the commonly found SiO diffraction peaks when the temperatures. There is an absence of the commonly found SiO2 diffraction peaks when calcination temperatures occur at the range from 1100 C to 1150 C. However, the diffrac- the calcination temperatures occur at the range from 1100 °C to 1150 °C. However, the tion peaks occurred during the -Ce S crystalline phase. This means that SiO failed to 2 3 2 diffraction peaks occurred during the γ-Ce2S3 crystalline phase. This means that SiO2 crystallize. However, c-SiO diffraction peak initiated as the temperature reached 1200 C. failed to crystallize. However, c-SiO2 diffraction peak initiated as the temperature This suggests that SiO will only crystallise within Ar gas atmosphere when temperature reached 1200 °C. This suggests that SiO2 will only crystallise within Ar gas atmosphere reaches 1200 C. c-SiO diffraction peak’s intensity remains approximately at constant level as temperature is further raised to 1250 C. Therefore, c-SiO is prone to crystallisation when temperature reaches 1200 °C. c-SiO2 diffraction peak’s intensity remains approxi- when two conditions are met; (a) it is within Ar-gas atmosphere (b) temperature to be at mately at constant level as temperature is further raised to 1250 °C. Therefore, c-SiO2 is least 1200 C. In another study, Li et al. [100] analyzed the -Ce S red pigments’ resistance prone to crystallisation when two conditions are met; a) it is2 wi 3 thin Ar-gas atmosphere b) through XRD test. temperature to be at least 1200 °C. In another study, Li et al. [100] analyzed the γ-Ce2S3 red pigments’ resistance through XRD test. Appl. Nano 2023, 4, FOR PEER REVIEW 13 Appl. Nano 2023, 4 87 Fig Figure ure 9 9. . XRD XRD pa patterns tterns of of the the γ-Ce -Ce2S S3@c @c-SiO -SiO2 sa samples mples aat t di dif fffer eren ent t si sintering ntering ttemperatur emperature es s iin n A Ar r g gas as 2 3 2 atmosphere [99]. Reproduced with permission from Li, Y. et al., Applied Surface Science; published atmosphere [99]. Reproduced with permission from Li, Y. et al., Applied Surface Science; published by Elsevier, 2020. by Elsevier, 2020. 3.3. Applications of Core-Shell Pigments 3.3. Applications of Core-Shell Pigments Pigments can serve as decoration or delineation purposes in the public street thus Pigments can serve as decoration or delineation purposes in the public street thus improving both the aesthetics and public safety. Infrastructures that are well-built and improving both the aesthetics and public safety. Infrastructures that are well-built and well-planned can motivate individuals to use them such as walking, cycling or ease of well-planned can motivate individuals to use them such as walking, cycling or ease of access for Personal Mobility Devices (PMDs). If individuals are more willing to do the access for Personal Mobility Devices (PMDs). If individuals are more willing to do the aforementioned activities, they are less likely to use their cars for short destination, which aforementioned activities, they are less likely to use their cars for short destination, which in return contribute to their overall health lifestyle. Figure 10 shows various red pigments in return contribute to their overall health lifestyle. Figure 10 shows various red pigments being applied on Singapore roads for pedestrian use. Despite an advancement in the being applied on Singapore roads for pedestrian use. Despite an advancement in the comprehension of the causes and effects of material failure, it remains a major concern comprehension of the causes and effects of material failure, it remains a major concern in in the entire construction industry. Exterior durability is typically enhanced through the the entire construction industry. Exterior durability is typically enhanced through the use use of high-performance coatings. Pigments are chosen for both the desired colour and of high-performance coatings. Pigments are chosen for both the desired colour and per- performance [101]. The paint industry would exclusively use high-quality pigments. It is formance [101]. The paint industry would exclusively use high-quality pigments. It is important for these pigments’ particles to be homogenous in size as it could have an effect important for these pigments’ particles to be homogenous in size as it could have an ef- on the paint’s attributes such as lightening capacity, hiding power, tinting strength and fect on the paint’s attributes such as lightening capacity, hiding power, tinting strength gloss. Furthermore, it is mandatory to apply nanoscale pigment particles in luminescent and gloss. Furthermore, it is mandatory to apply nanoscale pigment particles in lumi- materials for the purpose of UV-coatings and colouring. nescent materials for the purpose of UV-coatings and colouring. There is a higher desirability for coloured asphalt and red concrete in comparison to There is a higher desirability for coloured asphalt and red concrete in comparison to traditional materials as the former has better aesthetics from the viewpoint in architecture traditional materials as the former has better aesthetics from the viewpoint in architec- design [102]. The past few years have seen major development in nanomaterials and nan- ture design [102]. The past few years have seen major development in nanomaterials and otechnology. This has made synthesising core-shell NPs possible, which also contributed nanotechnology. This has made synthesising core-shell NPs possible, which also con- to developing pigments that are sustainable yet higher colour stability as well as able to tributed to developing pigments that are sustainable yet higher colour stability as well as tolerate harshness. The development of the pigments with increased durability has led to able to tolerate harshness. The development of the pigments with increased durability increase many potential applications in applying colours onto concrete and asphalt. This has led to increase many potential applications in applying colours onto concrete and leads to further development within the architect industry where they have the options asphalt. This leads to further development within the architect industry where they have to apply colours that carry more stability and higher tolerance to abrasion. Such develop- the options to apply colours that carry more stability and higher tolerance to abrasion. ments indeed could be combined with the aesthetic and decorative aspects of conventional Such developments indeed could be combined with the aesthetic and decorative aspects concrete thus forming an additional material with attractive features. of conventional concrete thus forming an additional material with attractive features. 4. Nano-Enhanced Phase Change Materials 4. Nano-Enhanced Phase Change Materials Buildings consume about 45% of global energy. Many passive cooling methods have Buildings consume about 45% of global energy. Many passive cooling methods have been used to lower the consumption rate. In addition, the phase change materials (PCM) been used to lower the consumption rate. In addition, the phase change materials (PCM) are installed within these buildings for the purpose of promoting temperature moderation, a stopping re installheat ed wi fr tom hin accumulation, these buildingsimpr for th oved e pur heat pose absorption of promoand ting minimize temperature indoor modheat era- Appl. Nano 2023, 4 88 gain. The method in which PCM stores thermal energy is effective in improving the build- ings’ aggregate heat capacity. Interest has been strong in PCMs that has high energy density to be deployed in buildings with high thermal inertia in order to save a high amount of energy. PCMs have their own drawbacks and the primary one being extra time required to charge/discharge energy process as well as storage performance, which happens due to poor thermal conductivity. Therefore, attention is focused on improving their thermal conductivity through the use of nanotechnology and nanomaterials. There has been a rapid development lately within the nanomaterials ﬁeld resulting in the latest technol- ogy with nanosized particles in improving the PCM’s thermophysical properties. PCM has several thermal and physical qualities such as viscosity, heat capacity, super-cooling and thermal conductivities. These attributes could be signiﬁcantly improved through dispersal of thermal conductive nanoparticles including nanometal-oxide, nanocarbon and nanometals. This article explores the research that have been recently conducted in the aforementioned development of nanomaterials that are being used to improve the PCMs thermal performance. This is appropriate in passive-cooling within the built-environment. The focus would be on materials’ type, method of synthetisation, and the outcome of the improvement. According to Mardiana and Riffat [103], about 30% of the total energy of any nation is consumed by the residential, institutional, commercial and industrial buildings. Ap- proximately, 60% of the energy is used in a building equipped with heating, ventilation and air-conditioning (HVAC) systems. PCM is a preferred building cooling method in comparison with other methods as it compliments green building with efﬁcient energy per- formance [104]. An effective strategy is phase change technology, where it could enhance the building’s thermal mass. This means removing heat from indoors, reduce temperature variations and disperse heat away from the building with the overall impact of increasing the comfort of the occupants. Studies have discovered that PCMs energy saving ranged from 10% to 30% from air-conditioning consumption within various climate in the United States [105]. During the summer, the energy savings could be up to 30% when PCMs are built on building walls. Microcapsules of PCM application results in the reduction of internal temperature of a building by 4 C and in a longer period of time, it stops the temperature from reaching for more than 28 C. PCMs are classiﬁed as inorganic, organic and eutectic. Types of inorganic PCMs include metal alloys, metals, and hydrated salts whereas an example of organic PCMs is hydrocarbons-based parafﬁn wax. There are disadvantages of PCM such as thermal insta- bility, corrosive property, sub-cooling, low thermal conductivity, leakage, phase segregation and many more [106]. In comparison, organic PCMs are sometimes more suitable due to their non-corrosive properties, immense latent heat capacity, congruent melting and self-nucleation, chemically inert as well as being thermally stable [107]. Dispersion of a con- trolled amount of nucleating or dispersant agents is a solution in addressing subcooling and phase segregation issue [106]. Nevertheless, PCM has an inherent low thermal conductivity, denoted by “k”. These results in low level of responsiveness during which a thermal change occurs rapidly due to charging/discharging process and its lowered storage capacity. Such issue becomes the centre of attention in research related to thermal energy storage. The k values of hydrocarbon-based PCM range from 0.1 to 0.4 W/mK. Noctadecane is a type of PCM, which possess low solid state thermal conductivity at 0.35 W/mK. It’s liquid state however is at 0.149 W/mK [108]. Rapid development of nanomaterials led to the emergence of novel application strategy at its high level of conductive ultra-small nanosized particles including metal oxides, carbon and metals. These can be used to produce nano-enhanced PCM (nePCM) with signiﬁcant micro-convection [3] and thermal conductivity [109]. Ample opportunities exist for nanomaterials potential applications in the cutting edge phase change technology. PCM has generated intense interest in the application of nanometer-scaled thermal conductors through nanoﬁbers, nanoparticles, nanosheets, nanotubes and nanofoams [104]. The thermal conductivity of PCM can be enhanced using three methods. First method involves Appl. Nano 2023, 4 89 the incorporation of PCM into porous media such as metallic foams and porous carbon, which has high thermal conductivity. Second method deals with the dispersion of high thermal conductivity metallic nanostructures or nanoparticles of Cu, Ag or Al to the PCM. Third method deals with the microencapsulation of the PCM [108]. The thermal conductivity and strength of microcapsules’ wall could be increased through nanoparticles that are made of silver [110]. An efﬁcient way of improving PCM additive is copper particles due to its high conductivity and low cost [111]. Three types of elements with thermal conductivity have extensively been studied [112]. These include carbon-based nanostructures such as graphene nanoﬂakes, nanoplatelets, carbon nanotubes CNT and nanoﬁbers; metallic oxide like TiO and MgO; metals like Al, Ag and Cu. There is a signiﬁcant improvement on heat transfer through the use of nanoparticles. The nanoparticles that can be applied to achieve this are carbon that possess various morphologies such as ceramic oxide (CuO, Al O ), metallic nitrides (AIN, SiN), 2 3 metallic carbides (SiC) and stable metals (gold Au) [105]. Nanomaterials that comprise of metals (Cu, Ag and Al), metal oxides (ZnO) and carbon (single wall SWCNT, graphene nanosheets, active carbon, carbon nanoﬁbers, expanded graphite sheets) increase PCM’s rate of heat transfer [113]. In this view, the prominent research being conducted on the development of thermal conductivity through the dispersion of three primary PCM nano- enhancers such as nanometals, nanocarbons and nano-metal oxides. 4.1. Nano-Metal Enhancer It is a common knowledge that metal is efﬁcient at heat conductivity. Silver in par- ticular is the optimal conductor of heat and electricity in comparison with other metals. Its thermal conduction value is approximately 430 W/(mK). The next two metals that are close to silver in terms of thermal conductivities are copper and gold. Gold and silver have two major disadvantages, vulnerable to oxidation and high cost. Therefore, copper, at a signiﬁcantly lower cost has the advantages in comparison. Despite this, all the three afore- mentioned metals have been extensively researched as possible solutions in addressing PCM’s thermal conductivities. Al-Shannaq [108] improved the PCM’s thermal conductivity (k) by 1168% through the use of nano-thick Ag shells. Specially microencapsulated pure PCM could be used to address leakages issues during its change of state from solid to liquid. However, the microencapsulated shell with poor conductivity value k served as a barrier to achieving a desirable level of heat transfer and energy storage. A method has been formulated to enhance the PCM’s microencapsulated k value that involved the use of a layer of metallic shell to cover the microcapsules. This was done by activating the surface with dopamine and conducting electroless plating. The k value was increased to 0.189 from 0.062 W/mK when the diameter of uncoated PCM was increased to 26.9 m from 2.4 m. While the diameter was retained at 26.9 m, a signiﬁcant increase of the thermal conductivity (about 1168%) of metal-coated PCM capsules (2.41 W/mk from 0.189) was achieved. Such improvement of the thermal conductivity is highly correlated with the size of the shell area that is coated with silver on the surface of the PCM microsphere. The rapid improvement occurs upon the formation of the thermal conduction pathways. Deng et al. [114] have made another signiﬁcant improvement (1030%) in the thermal conductivity of the PCM via the synthesis of AgNWs. First, shape stabilised phase change materials (polyethylene glycol-silver/EVM ss-CPCMs) composites were produced via the embedment of PEG-Ag nanowires into expanded vermiculite EVM. To prevent the PCM leakage as well as to improve its thermal conductivity, a technique was proposed whereby the mixing and embedding are performed mechanically. For the purpose of PCM latent energy storage, polyethylene glycol was used. Figure 10 shows the silver nanowires that served as the thermal conductivity promoter. Furthermore, the PCM leakage during the melting was addressed through a support matrix (EVM vermiculite), enabling the enhancement in the mechanical strength. Appl. Nano 2023, 4, FOR PEER REVIEW 16 Appl. Nano 2023, 4, FOR PEER REVIEW 16 Appl. Nano 2023, 4 90 Figure 10. (a) SEM photos of synthesized silver nanowires. (b) Variation between the predicted Figure 10. (a) SEM photos of synthesized silver nanowires. (b) Variation between the predicted t Fig her ure ma l 1c 0on . (a duc ) St EM ivit y ph kot vos alue of w sy it n h t h m es ea izsur ed e si d lvv ea rlues nano of wP irC es M . ( b n) an V oc aromp iatioos n ib tes etw [1 een 14] .t h Rep e pr rod educ icted ed thermal conductivity k value with measured values of PCM nanocomposites [114]. Reproduced with with permission from Deng, Y. et al., Chemical Engineering Journal; published by Elsevier, 2016. thermal conductivity k value with measured values of PCM nanocomposites [114]. Reproduced permission from Deng, Y. et al., Chemical Engineering Journal; published by Elsevier, 2016. with permission from Deng, Y. et al., Chemical Engineering Journal; published by Elsevier, 2016. Significant improvement in the k value of PEG infused silver vermiculite composites Signiﬁcant improvement in the k value of PEG infused silver vermiculite composites Significant improvement in the k value of PEG infused silver vermiculite composites was achieved using nanowires of length 5–20 μm and diameter 50–100 nm. An increase was achieved using nanowires of length 5–20 m and diameter 50–100 nm. An increase as was achieved using nanowires of length 5–20 μm and diameter 50–100 nm. An increase as much as 1130% for the k value (0.68 W/mK) was achieved compared to the neat PCM much as 1130% for the k value (0.68 W/mK) was achieved compared to the neat PCM with as much as 1130% for the k value (0.68 W/mK) was achieved compared to the neat PCM with latent heat capacity at 96.4 J/g. The vermiculite has incited supercooling to occur latent heat capacity at 96.4 J/g. The vermiculite has incited supercooling to occur where the with latent heat capacity at 96.4 J/g. The vermiculite has incited supercooling to occur where the temperature dropped by 7 °C upon the PCM for PEG–Ag/EVM ss-CPCMs. temperature dropped by 7 C upon the PCM for PEG–Ag/EVM ss-CPCMs. Such reaction is where the temperature dropped by 7 °C upon the PCM for PEG–Ag/EVM ss-CPCMs. Such reaction is similar to nonuniform impregnates for developing nucleation and pro- similar to nonuniform impregnates for developing nucleation and promoting the formation Such reaction is similar to nonuniform impregnates for developing nucleation and pro- moting the formation of PEG-crystal. Such improvements are as a result of high k values of PEG-crystal. Such improvements are as a result of high k values due to the dispersion of moting the formation of PEG-crystal. Such improvements are as a result of high k values due to the dispersion of silver nanowire and vermiculite. Zeng et al. [107] obtained about silver nanowire and vermiculite. Zeng et al. [107] obtained about 800% improvement in the due to the dispersion of silver nanowire and vermiculite. Zeng et al. [107] obtained about 800% improvement in the thermal conductivity using CuNWs. The premise of the re- thermal conductivity using CuNWs. The premise of the research is to explore the impact 800% improvement in the thermal conductivity using CuNWs. The premise of the re- search is to explore the impact of CuNWs, which is copper nanowires has upon the of CuNWs, which is copper nanowires has upon the tetradecanoyl (TD)’s k value as the search is to explore the impact of CuNWs, which is copper nanowires has upon the tetradecanoyl (TD)’s k value as the phase change material. The TD was synthesised and phase change material. The TD was synthesised and classiﬁed accordingly to the range tetradecanoyl (TD)’s k value as the phase change material. The TD was synthesised and classified accordingly to the range of weight fractions of CuNW. The ratio and diameter of weight fractions of CuNW. The ratio and diameter of free-standing copper nanowires classified accordingly to the range of weight fractions of CuNW. The ratio and diameter of free-standing copper nanowires were at 350–450 and 90–120 nm, respectively with were at 350–450 and 90–120 nm, respectively with 40–50 m in length. The CuNW can then of free-standing copper nanowires were at 350–450 and 90–120 nm, respectively with 40–50 μm in length. The CuNW can then be fabricated in bulk through simple technique be fabricated in bulk through simple technique involving chemical reduction that is water 40–50 μm in length. The CuNW can then be fabricated in bulk through simple technique involving chemical reduction that is water based at room temperature. based at room temperature. involving chemical reduction that is water based at room temperature. Figure 11 presents the SEM images of the composite results, demonstrating that Figure 11 presents the SEM images of the composite results, demonstrating that Figure 11 presents the SEM images of the composite results, demonstrating that CuNws in TD has decent dispersion and entanglement. It is worth noting that the rate of CuNws in TD has decent dispersion and entanglement. It is worth noting that the rate of CuNws in TD has decent dispersion and entanglement. It is worth noting that the rate of weight loss is lower in comparison to pristine TD due to the structural nature of CuNWs, weight loss is lower in comparison to pristine TD due to the structural nature of CuNWs, weight loss is lower in comparison to pristine TD due to the structural nature of CuNWs, which is similar to a sponge and is capable of storing the TD within the voids. When the which is similar to a sponge and is capable of storing the TD within the voids. When the which is similar to a sponge and is capable of storing the TD within the voids. When the CuNWs is increased by 58.9 wt%, the thermal conductivity increased up to nine-fold, an CuNWs is increased by 58.9 wt%, the thermal conductivity increased up to nine-fold, an CuNWs is increased by 58.9 wt%, the thermal conductivity increased up to nine-fold, an 800% enhancement. 800% enhancement. 800% enhancement. Figure 11. (a) SEM photos of synthesized CuNWs. (b) Thermal conductivity of PCM composites Figure 11. (a) SEM photos of synthesized CuNWs. (b) Thermal conductivity of PCM composites with increasing CuNWs loadings [107]. Reproduced with permission from Zeng, J.-L. et al., Solar w Fig ith ure in c 1r1 ea . ( si an ) g S EM CuN ph Ws otos loa of din sy gs nt[h 1es 07i]z . ed Rep Cu rod NWs. uced ( b w ) itTh h p er er m m al isc si on on duc from tivit Ze y n of g,P J. CM -L. e ct omp al., os Sol ita es r Energy Materials and Solar Cells; published by Elsevier, 2012. En wit er hg in y c Ma reatsi er n ia gls Cu an N d Ws Sola lo ra Ce din llg s;s pu [10 b7 lis ]. h Rep ed b rod y El uc se ed vie w r,it 20 h 12. per mission from Zeng, J.-L. et al., Solar Energy Materials and Solar Cells; published by Elsevier, 2012. Appl. Nano 2023, 4 91 Zeng et al. [115] improved the thermal conductivity by about 356% using AgNWs (380%). The experiment involved the synthesis of silver nanowires and production of silver-doped PCM nanocomposites. The inclusion of AgNWs at 45 wt% results in two to three times enhancement of thermal conductivity in graphene-doped PCM. The enthalpy is reduced by 50% and its heat storage capacity has also been reduced. In terms of size, graphene dopants are ten times smaller in comparison to doping with silver nanowires. Furthermore, the enthalpy value has also been reduced three times in comparison to AgNWs. Shah et al. [116] have increased the PCM thermal conductivity by 160% through the use of copper nanowires (CuNWs). The enhancement of thermal conductivity (more than 50%) of calcium chloride hexahydrate is achieved by adding a trace of CuNWs at 0.17 wt%. The use of nano-copper results in optimum enhancement of k value at 160%; or an increase to 0.564 W/mK of PCM composite in comparison to 0.217 W/mk of neat PCM. Just a trace of CuNWs could result in such a signiﬁcant improvement thus nanoadditives can be considered as cost efﬁcient when being applied in buildings. Moleﬁet et al. [117] showed 70% improvement in the thermal conductivity using CuNPs. The thermal conductivity of parafﬁn was increased almost linearly as the CuNPs amounts were increased. Parafﬁn wax was used as the PCM base, which was subsequently mixed with molecular-weight polyethylene at low, medium and high rate. The copper particles were mixed with parafﬁn mixture resulting in the enhancement of the base polyethylene PCM’s k value. Tang et al. [118] improved the thermal conductivity by 38.1% using CuNPs based on SiO -embedded-PEG PCM composite that is shape stable. When 2.1 wt% CuNPs were added, the k value was increased by 38.1% in comparison to neat PCM. Further addition of copper nanoadditives results in improvement on PEG/SiO hybrid PCMs. Wu et al. [119] have made 30.3% improvement on thermal conductivity through the use of CuNPs. Their results have shown a correlation where 1wt% of CuNPs could decrease the parafﬁn PCM heating by 30.3% and cooling by 28.2%. The charging time decreased by 30.3% while the discharging time was decreased by 28.2% upon the doping of nanocopper particles into the nanocomposites with 1 wt%. Melting PCM heat transfer rate is enhanced through the addition and mixture of nanoadditives (aluminium, copper and copper/carbon nanomaterials). In terms of improvement on heat transfer, nanocopper particles offer the most signiﬁcant rate amongst others. 4.2. Nano-Metal Oxide Enhancer Two examples of good heat conductors are alumina and copper, both of which are metal oxides with values from 30 to 40 W/mK. Pure metals typically are better heat conductors but they are not as chemically stable in comparison to the metal oxides. In addition, metal oxides are more cost effective and reliable in its performance. For these reasons, they are more sought after as a material to replace pure metals. Babapoor et al. [120] used various NPs types to enhance the thermal conductivity of k value. The metals with the enhancement percentage of Al O (144%), Fe O (144%), ZnO (110%) and SiO (110%) 2 3 2 3 2 were obtained. In these tests, nanomaterials of silica (~20 nm), alumina (~20 nm), iron oxide (~20 nm), and zinc oxide (>50 nm) were used. These nanomaterials were added as thermal enhancers and mixed with NPs (SDS) as well as surfactant (CTAB) to achieve enhanced PCM. The sample doped with Al O NPs showed the highest enhancement in 2 3 the thermal conductivity of 0.919 W/mK. The doping of NPs gave various enhancement (%) level depending on the concen- tration (wt%) of Al O NPs: 4 wt% (120%), 6 wt% (141.2%) and 8 wt% (144%); Fe O 2 3 2 3 NPs of 4 wt% (80%), 6 wt% (135%) and 8 wt% (144%); ZnO NPs of 4 wt% (85%), 6 wt% (100%) and 8 wt% (110%); SiO NPs of 4 wt% (78%), 6 wt% (110%) and 8 wt% (110%). The results revealed that higher level of concentration of conductive nanomaterials lead to higher k value of the nanocomposites. It was concluded that Al O and Fe O carry the 2 3 2 3 most signiﬁcant impact in terms of enhancing the thermal conductivity of parafﬁn-based PCM. Sharma et al. [121] achieved an improvement in the thermal conductivity of about 80% using TiO . The study involved the performance of palmitic acid (PA) based thermal 2 Appl. Nano 2023, 4, FOR PEER REVIEW 18 Appl. Nano 2023, 4 92 ergy storage of synthesised PCM composites that were doped with TiO2 NPs. By mixing TiO2 into neat PCM an enhancement in the k value was 12.7%, 20.6%, 46.6% and 80% for the corresponding TiO2 concentrations of 0.5 wt%, 1 wt%, 3 wt% and 5 wt%, respectively. energy storage of synthesised PCM composites that were doped with TiO NPs. By mixing The high concentration of TiO2 within the PCM resulted in curvilinear characteristic of TiO into neat PCM an enhancement in the k value was 12.7%, 20.6%, 46.6% and 80% for the ther corr maesponding l enhanceT m iO ent. concentrations Li et al. [12 of 2] 0.5 achi wt%, eved 1 wt%, 43.8 3% wt% and and 40 54wt%, % im rp espectively rovemen.t in the The high concentration of TiO within the PCM resulted in curvilinear characteristic of thermal conductivity using 2 TiO2 NPs foam and TiO2 NPs with a nanocarbon shell layer, thermal enhancement. Li et al. [122] achieved 43.8% and 404% improvement in the thermal respectively. The synthesis of porous TiO2 foams PTFs involved the use of octane as mi- conductivity using TiO NPs foam and TiO NPs with a nanocarbon shell layer, respectively. 2 2 croemulsifier and TiO2 as particle stabilizer (microemulsion technique) as shown in Fig- The synthesis of porous TiO foams PTFs involved the use of octane as microemulsiﬁer ure 12. The nanosized TiO2 measured at approximately 23 nm consisting of 20% rutile and TiO as particle stabilizer (microemulsion technique) as shown in Figure 12. The and 80% anatase. Polyacrylic acid-ammonium salt is used as the dispersing agent. It is nanosized TiO measured at approximately 23 nm consisting of 20% rutile and 80% anatase. added on the surface modifier along with a small amphiphilic molecule propyl gallate Polyacrylic acid-ammonium salt is used as the dispersing agent. It is added on the surface (C10H12O5). modiﬁer along with a small amphiphilic molecule propyl gallate (C H O ). 10 12 5 Figure 12. (a) SEM images of PTF. (b) Enhancement of thermal conductivities of PCM parafﬁn, Figure 12. (a) SEM images of PTF. (b) Enhancement of thermal conductivities of PCM paraffin, PTF/PCM, and PTFC/PCM composites by 43.8 and 404%. (Inset) TEM photo of the prepared PCM PTF/PCM, and PTFC/PCM composites by 43.8 and 404%. (Inset) TEM photo of the prepared PCM composite carbonized porous TiO foams (PTFC) particles [122]. Reproduced with permission from composite carbonized porous TiO2 foams (PTFC) particles [122]. Reproduced with permission from Li, Y. et al., Applied Energy; published by Elsevier, 2016. Li, Y. et al., Applied Energy; published by Elsevier, 2016. The 3D porous structure of PTFs contains continuously connected holes, enabling The 3D porous structure of PTFs contains continuously connected holes, enabling the full absorption of parafﬁn wax without the need of any surfactant. The structure can the full absorption of paraffin wax without the need of any surfactant. The structure can also absorb sucrose and can burn off at 1200 C, resulting in a thin carbon-based ﬁlm also absorb sucrose and can burn off at 1200 °C, resulting in a thin carbon-based film wherein the carbon nanolayer is only 2 nm thick. Both pure PTF and carbon-based PTF nanocomposites wherein the carb wer one n mo anr o ela conductive yer is only than 2 nm pur th eic paraf k. Bo ﬁn th with pure kPTF values and of ca 0.302 rbonand -based PTF 1.059 W/mK, respectively. The k value of pure PCM reached to 0.302 W/mK when 25 wt% nanocomposites were more conductive than pure paraffin with k values of 0.302 and of TiO was added. This clearly indicated that the addition of TiO can enhance the k value 2 2 1.059 W/mK, respectively. The k value of pure PCM reached to 0.302 W/mK when 25 wt% by 0.092 W/mK. TiO foam structure lined with carbon nanoﬁlm plus parafﬁn showed a k of TiO2 was added. This clearly indicated that the addition of TiO2 can enhance the k value of 1.059 W/m K, indicating an increase of 504% than pure parafﬁn. This signiﬁcant value by 0.092 W/mK. TiO2 foam structure lined with carbon nanofilm plus paraffin increase was mainly due to the carbon matrix adherence onto the TiO NPs surfaces. It showed a k value of 1.059 W/m K, indicating an increase of 504% than pure paraffin. This was afﬁrmed that the novel hybrid of TiO NPs-porous foam with inner-lining carbon significant increase was mainly due to the carbon matrix adherence onto the TiO 2 NPs nanoﬁlms is effective for the enhancement of PCM demanded in the industrial purposes. surfaces. It was affirmed that the novel hybrid of TiO2 NPs-porous foam with inner-lining Zhang et al. [123] made about 18.2% improvement in the thermal conductivity using TiO wher carbo ein n n aa novel nofilm thermal-insulating s is effective for ﬁl th m e and enhpolyvinyl-chloride ancement of PCM (PVC) dema ﬁlm nded matrix in th wer e in e dustrial incorporated. Both TiO and microencapsulated n-octadecane PCM were used to block UV purposes. Zhang et a 2l. [123] made about 18.2% improvement in the thermal conductivity and act as an additive to regulate the temperature. When TiO NPs were added at 6 wt%, using TiO2 wherein a novel thermal-insulating film and polyvinyl-chloride (PVC) film the k value of the pure micro-PCM was reached to 0.2356 W/mK from 0.1994 W/mK for matrix were incorporated. Both TiO2 and microencapsulated n-octadecane PCM were the matrix, indicating an increase by 18.2%. Such thin ﬁlm with excellent heat insulation used to block UV and act as an additive to regulate the temperature. When TiO2 NPs and thermal regulating properties were afﬁrmed to be useful for the indoor living spaces were added at 6 wt%, the k value of the pure micro-PCM was reached to 0.2356 W/mK and cars. from 0.1994 W/mK for the matrix, indicating an increase by 18.2%. Such thin film with Sahan et al. [124] achieved about 60% of thermal conductivity improvement using excellent heat insulation and thermal regulating properties were affirmed to be useful for sol-gel synthesized Fe O NPs. These Fe O NPs (diameters ranged from 40 to 70 nm) 3 4 3 4 wer the ein pr depar oor ed livusing ing spa iron cechloride s and cahydrates rs. ((FeCl 6H O, FeCl 4H O), hydrochloride and 3 2 2 2 ammonia. They were mixed with parafﬁn in two concentration levels (10 and 20 wt%). Sahan et al. [124] achieved about 60% of thermal conductivity improvement using Particle aggregation was minimised through surface capping of oleic acid. These Fe O 3 4 sol-gel synthesized Fe3O4 NPs. These Fe3O4 NPs (diameters ranged from 40 to 70 nm) were prepared using iron chloride hydrates ((FeCl3 6H2O, FeCl2 4H2O), hydrochloride and ammonia. They were mixed with paraffin in two concentration levels (10 and 20 wt%). Particle aggregation was minimised through surface capping of oleic acid. These Appl. Nano 2023, 4, FOR PEER REVIEW 19 Appl. Nano 2023, 4 93 Fe3O4 NPs were uniformly dispersed on the paraffin matrix. The results showed an im- provement in the k values by 48% and 60% for the corresponding NPs concentration of 10 wt% and 20 wt%, respectively. This showed that nanomagnetite particles doping in PCM NPs were uniformly dispersed on the parafﬁn matrix. The results showed an improvement w ina the s vk er values y effec by tiv 48% e to and wa60% rds th fore the imcorr proesp veme onding nt of NPs its concentration thermal conof ductiv 10 wt% ityand and cost. Jiang et 20 wt%, respectively. This showed that nanomagnetite particles doping in PCM was very al. [125] observed 55% improvement in the thermal conductivity of PCM using effective towards the improvement of its thermal conductivity and cost. Jiang et al. [125] nano-Al2O3. The microencapsulation of paraffin was responsible for the formation of observed 55% improvement in the thermal conductivity of PCM using nano-Al O . The mi- 2 3 poly(methylmethacrylate-co-methylacrylate) polymeric PCM microcapsules (MEPCM). croencapsulation of parafﬁn was responsible for the formation of poly(methylmethacrylate- These microcapsules were further added with alumina NPs via the emulsion polymeri- co-methylacrylate) polymeric PCM microcapsules (MEPCM). These microcapsules were further added with alumina NPs via the emulsion polymerization, causing signiﬁcant zation, causing significant enhancement of k value (Figure 13) from 0.245 W/mK to 0.38 enhancement of k value (Figure 13) from 0.245 W/mK to 0.38 W/mK (increase by 55%). W/mK (increase by 55%). There is near parity in terms of the enhancement rate and There is near parity in terms of the enhancement rate and dosage of nano-Al O , indi- 2 3 dosage of nano-Al2O3, indicating that the presence of nano-Al2O3 caused a higher thermal cating that the presence of nano-Al O caused a higher thermal conductivity increase of 2 3 conductivity increase of PCM microcapsules. PCM microcapsules. Figure 13. (a) SEM photos of PCM microcapsules with 27 wt% nano-alumina. (b) Thermal conductiv- Figure 13. (a) SEM photos of PCM microcapsules with 27 wt% nano-alumina. (b) Thermal conduc- ities of PCM with various contents of nano-alumina [125]. Reproduced with permission from Jiang, tivities of PCM with various contents of nano-alumina [125]. Reproduced with permission from X. et al., Applied Energy; published by Elsevier, 2015. Jiang, X. et al., Applied Energy; published by Elsevier, 2015. Tong et al. [126] improved the PCM’s thermal conductivity using nano-SiO . The polymeric melamine-urea-formaldehyde was used for the polymerisation of in situ PCM Tong et al. [126] improved the PCM’s thermal conductivity using nano-SiO2. The parafﬁn microcapsules before adding graphite and nano-SiO . The results revealed that polymeric melamine-urea-formaldehyde was used for the polymerisation of in situ PCM the successful rate of parafﬁn microencapsulation was at 80% wherein the PCM parafﬁn paraffin microcapsules before adding graphite and nano-SiO2. The results revealed that was able to sustain its thermophysical properties. The addition of nano-SiO could change th the e micr succe ocapsules ssful ra resistance te of para against ffin m high icroe temperatur ncapsul e, at rio einfor n w cing as a the t 8str 0% uctural wherstr ein ength the PCM paraffin of composite and high afﬁnity to water. The k value was improved signiﬁcantly during was able to sustain its thermophysical properties. The addition of nano-SiO2 could melting time after the nanomaterials were added. Ai et al. [127] enhanced the thermal change the microcapsules resistance against high temperature, reinforcing the structural conductivity of PCM using high energy planetary milling wherein ZrO nanopowder- strength of composite and high affinity to water. The k value was improved significantly based stearic acid PCM was developed. A new parameter called heat capability factor during melting time after the nanomaterials were added. Ai et al. [127] enhanced the (HCF) was explored. Chloroform was used to disperse the nano-ZrO PCM composites, providing a better alternative (than carbon tetrachloride) for the dispersion during ZrO thermal conductivity of PCM using high energy planetary milling wh2erein ZrO2 na- synthesis. The results revealed that chloroform could improve the surface morphology and nopowder-based stearic acid PCM was developed. A new parameter called heat capabil- spherodization of ZrO . The highest HCF value of 0.9 for the mean size of PCM particles ity factor (HCF) was explored. Chloroform was used to disperse the nano-ZrO2 PCM was 1.2 m. However, the HCF value was reduced to 0.3 when the mean size of PCM composites, providing a better alternative (than carbon tetrachloride) for the dispersion particles became 0.4 m. The optimum PCM particles’ size (1.2 m) gave a signiﬁcant enhancement in the heat storage capability of chloroform-treated composite ZrO -PCM during ZrO2 synthesis. The results revealed that chloroform could improve the surface particles. Song et al. [128] used MgOH NPs and made nePCMs to enhancing the ﬁre morphology and spherodization of ZrO2. The highest HCF value of 0.9 for the mean size resistance of PCM. The supporting materials used were nano-sized red phosphorus (RP), of PCM particles was 1.2 μm. However, the HCF value was reduced to 0.3 when the mean MgOH and ethylene propylenedieneter polymer plastic (EPDM). The observed increase size of PCM particles became 0.4 μm. The optimum PCM particles’ size (1.2 μm) gave a in the ﬁre resistance quality was ascribed to the magnesium hydroxide within the ﬂame significant enhancement in the heat storage capability of chloroform-treated composite retardant shape-stable PCM composite. It was argued that the ﬁre-resistant attributes of the PCM can be further improved through the reduction of NPs diameter. Consequently, larger ZrO2-PCM particles. Song et al. [128] used MgOH NPs and made nePCMs to enhancing surface to volume ratio of MgOH can produce rapid breakdown and high reactivity when the fire resistance of PCM. The supporting materials used were nano-sized red phos- phorus (RP), MgOH and ethylene propylenedieneter polymer plastic (EPDM). The ob- served increase in the fire resistance quality was ascribed to the magnesium hydroxide within the flame retardant shape-stable PCM composite. It was argued that the fire-resistant attributes of the PCM can be further improved through the reduction of NPs diameter. Consequently, larger surface to volume ratio of MgOH can produce rapid breakdown and high reactivity when subjected to the combustion process, indicating higher fire resistance quality attainment of PCM composite. Appl. Nano 2023, 4, FOR PEER REVIEW 20 Appl. Nano 2023, 4 94 4.3. Nano-Carbon Enhancer subjected to the combustion process, indicating higher ﬁre resistance quality attainment of PCM composite. Carbon has higher thermal conductivity when benchmarked against metals and metal oxides. Graphite, graphene and CNTs thermal conductivities can be up to five 4.3. Nano-Carbon Enhancer times higher than silver. Research studies have increasingly focused on the carbon na- Carbon has higher thermal conductivity when benchmarked against metals and metal nomaterials thermal conductivities due to their continuous decrease in production cost. Ji oxides. Graphite, graphene and CNTs thermal conductivities can be up to ﬁve times et al. [129] improved PCM’s thermal conductivity by 1700% using ultra-thin graphite higher than silver. Research studies have increasingly focused on the carbon nanomaterials foams (UGF). The k value was increased by 18 times after adding UGF (at approximately thermal conductivities due to their continuous decrease in production cost. Ji et al. [129] 1.2 vol%) into the PCM matrix. However, no changes in the specific heat fusion or melting improved PCM’s thermal conductivity by 1700% using ultra-thin graphite foams (UGF). temperature were observed. Graphite foams consisted of ultrathin graphite connected The k value was increased by 18 times after adding UGF (at approximately 1.2 vol%) into the PCM matrix. However, no changes in the speciﬁc heat fusion or melting temperature strips. These strips possessed a higher k value than metals and solid carbon foams, indi- were observed. Graphite foams consisted of ultrathin graphite connected strips. These cating their better heat response and thermal properties. Liang et al. [130] obtained 1300% strips possessed a higher k value than metals and solid carbon foams, indicating their better improvement in the thermal conductivity using superoleophilic graphene nanosheets heat response and thermal properties. Liang et al. [130] obtained 1300% improvement in mixed with porous nickel Ni foam. In the synthesis of polydimethylsiloxane the thermal conductivity using superoleophilic graphene nanosheets mixed with porous (PDMS-G-NF) modified graphene-covered nickel foam they used graphene nanosheets nickel Ni foam. In the synthesis of polydimethylsiloxane (PDMS-G-NF) modiﬁed graphene- layering onto the porous Ni foam surface, causing the formation of graphene-nickel foam covered nickel foam they used graphene nanosheets layering onto the porous Ni foam G-NF. Further modifications were performed on the G-NF surface support using siloxane surface, causing the formation of graphene-nickel foam G-NF. Further modiﬁcations were PDMS for the fabrication of shape-stable PCM composite. performed on the G-NF surface support using siloxane PDMS for the fabrication of shape- stable PCM composite. Chen et al. [131] achieved 500% improvement in the k value of PCM using CNT Chen et al. [131] achieved 500% improvement in the k value of PCM using CNT foam. foam. The PCM was absorbed by a permeable support matrix, a carbon nanotube net- The PCM was absorbed by a permeable support matrix, a carbon nanotube network with work with structure similar to sponge. The heat storage capacity of PCM was improved structure similar to sponge. The heat storage capacity of PCM was improved and became and became efficient for both heat and electricity conduction. In addition, the PCM efﬁcient for both heat and electricity conduction. In addition, the PCM composite could composite could absorb light energy and generate heat via electricity. Figure 14 shows the absorb light energy and generate heat via electricity. Figure 14 shows the PCM composite PCM composite consisted of paraffin filled soft-flexible CNT-based porous material. The consisted of parafﬁn ﬁlled soft-ﬂexible CNT-based porous material. The support matrix support matrix that is deformable has high rate of thermal conductivity during the so- that is deformable has high rate of thermal conductivity during the solidiﬁcation and lidification and melting processes. melting processes. Figure 14. (a) SEM photo of the interior of CNT porous foam revealing a highly sponge-like mi- Figure 14. (a) SEM photo of the interior of CNT porous foam revealing a highly sponge-like mi- crostructure. (b) Thermal conductivities of neat parafﬁn wax with 10 and 20 wt% loadings of CNT crostructure. (b) Thermal conductivities of neat paraffin wax with 10 and 20 wt% loadings of CNT foams, i.e., 80 and 90 wt% parafﬁn [131]. Reproduced with permission from Chen, L. et al., ACS foams, i.e., 80 and 90 wt% paraffin [131]. Reproduced with permission from Chen, L. et al., ACS Nano; published by American Chemical Society, 2012. Nano; published by American Chemical Society, 2012. Shi et al. [132] used exfoliated graphite nanoplatelets (xGnP) and grapheme and Shi et al. [132] used exfoliated graphite nanoplatelets (xGnP) and grapheme and improved the corresponding k values by 1000% and 100%, respectively. Such improvement improved the corresponding k values by 1000% and 100%, respectively. Such improve- resulted in the formation of parafﬁn PCM materials that are stable. Approximately 2 wt% ment resulted in the formation of paraffin PCM materials that are stable. Approximately of the graphene was added to parafﬁn and heated to around 185 C. The parafﬁn retained 2 its wt form % odespite f the gra reaching phene signiﬁcantly was added to high pamelting raffin apoint. nd hea Itted was to claimed around that 185 to ° decr C. The easeparaffin the cost, trace amount of graphene and xGnP can be doped together thereby improving retained its form despite reaching significantly high melting point. It was claimed that to both stability and heat dissipation of PCMs. xGnP-doped PCM resulted in a k value of decrease the cost, trace amount of graphene and xGnP can be doped together thereby 2.7 W/mK which was considerably higher than graphene-doped PCM (approximately improving both stability and heat dissipation of PCMs. xGnP-doped PCM resulted in a k value of 2.7 W/mK which was considerably higher than graphene-doped PCM (ap- proximately 0.5 W/mK) and neat paraffin (0.25 W/mK). Wang J. [133] achieved 305% improvement in the PCM thermal conductivity by adding carbon nanofibers (CNFs) as Appl. Nano 2023, 4, FOR PEER REVIEW 21 Appl. Nano 2023, 4 95 nanofillers into the palmitic acid (PA). The phase temperature change was approximately 0.5 W/mK) and neat parafﬁn (0.25 W/mK). Wang J. [133] achieved 305% improvement in the PCM thermal conductivity by adding carbon nanoﬁbers (CNFs) as nanoﬁllers into 62.5 °C after the addition of unwashed acid. The range of length and diameter of the the palmitic acid (PA). The phase temperature change was approximately 62.5 C after the CNFs was 200–500 nm and 5–50 μm, respectively (Figure 15). Alkali potassium hydrox- addition of unwashed acid. The range of length and diameter of the CNFs was 200–500 nm ide (KOH) was used to chemically treat CNFs, reducing the thermal boundary resistance and 5–50 m, respectively (Figure 15). Alkali potassium hydroxide (KOH) was used to of the fibre matrix. chemically treat CNFs, reducing the thermal boundary resistance of the ﬁbre matrix. Figure 15. (a) SEM image of M-CNF/PA with 1.0 wt% M-CNF. (b) Thermal conductivity enhancement Figure 15. (a) SEM image of M-CNF/PA with 1.0 wt% M-CNF. (b) Thermal conductivity enhance- with 0.2, 0.5, 1, 2 and 5 wt% of carbon nanoﬁbers (CNF) in palmatic acid (PA) [133]. Reproduced with ment with 0.2, 0.5, 1, 2 and 5 wt% of carbon nanofibers (CNF) in palmatic acid (PA) [133]. Repro- permission from Wang, J. et al., Journal of Applied Physics; published by AIP Publishing, 2011. duced with permission from Wang, J. et al., Journal of Applied Physics; published by AIP Pub- lishin Cui g, 2et 01al. 1. [134] improved the PCM’s thermal conductivity by 44% and 24% using nanoﬁllers of CNF and MWCNT, respectively. The synthesis of the composite involved carbon ﬁbres or nanotubes dispersion within both soy wax and parafﬁn (1, 2, 5 and Cui et al. [134] improved the PCM’s thermal conductivity by 44% and 24% using 10 wt%) at 60 C. This proved that the nanoﬁbers as additive can increase the parafﬁn k nanofillers of CNF and MWCNT, respectively. The synthesis of the composite involved values signiﬁcantly. The k value of pure parafﬁn and PCM composite (at CNF loadings carbon fibres or nanotubes dispersion within both soy wax and paraffin (1, 2, 5 and 10 of 10 wt%) were 0.320 W/mK and 0.450 W/mK, respectively. Wang et al. [135] improved wt%) at 60 °C. This proved that the nanofibers as additive can increase the paraffin k the thermal conductivity of PCM by 46% using multiwalled carbon nanotubes (MWCNT). The traditional ball milling method was used to synthesize the MWCNT-PCM composites values significantly. The k value of pure paraffin and PCM composite (at CNF loadings of added with KOH. This method could improve its dispersion in palmitic acid. The stability 10 wt%) were 0.320 W/mK and 0.450 W/mK, respectively. Wang et al. [135] improved the and homogeneity of PCM composites were improved by modifying the grafted OH groups thermal conductivity of PCM by 46% using multiwalled carbon nanotubes (MWCNT). into the MWCNT surfaces. The MWCNT-palmitic acid composites with 1 wt% of MWCNT The traditional ball milling method was used to synthesize the MWCNT-PCM compo- loading was shown to increase the k values by 46.0% and 38.0% on solid state at 25 C and liquid state at 65 C, respectively. sites added with KOH. This method could improve its dispersion in palmitic acid. The stability and homogeneity of PCM composites were improved by modifying the grafted 5. Nanopolymer Advanced Composites OH groups into the MWCNT surfaces. The MWCNT-palmitic acid composites with 1 THE deﬁnition of polymer nanocomposites (PNCs) is the combination of more than wt% of MWCNT loading was shown to increase the k values by 46.0% and 38.0% on solid one material. In addition, the matrix consists of a polymer with the dispersed phase st that ate possess at 25 ° aC minimum and liquid of one stadimension te at 65 °C, smaller respec than tiv100 ely. nm [136]. Many decades of observation have deduced that incorporating nanoﬁllers in small quantities within the polymer resulted in many improvements on its characteristics such as thermal, barrier, 5. Nanopolymer Advanced Composites mechanical and ﬂame-retardant properties while its processing is unaffected [137]. The THE definition of polymer nanocomposites (PNCs) is the combination of more than optimum nanocomposite design necessitates the individual nanoparticles to disperse homogeneously within a matrix polymer. The main challenge in terms of dispersion state one material. In addition, the matrix consists of a polymer with the dispersed phase that of nanoparticles is to achieve all the possible enhancements of its properties [137]. There is possess a minimum of one dimension smaller than 100 nm [136]. Many decades of ob- a potential for the nanoﬁllers’ uniform dispersion to result in signiﬁcant interfacial area servation have deduced that incorporating nanofillers in small quantities within the between the nanocomposites’ constituents [137]. There are various factors that inﬂuence polymer resulted in many improvements on its characteristics such as thermal, barrier, the reinforcing effect mainly polymer matrix properties, type and nature of nanoﬁller as well mecas hapolymer nical an and d fla ﬁller meconcentration. -retardant prOther opertie factors s wh focusing ile its p on roce thess particle ing is includes unaffected [137]. The its size, aspect ratio, orientation and distribution [138]. There have been numerous types optimum nanocomposite design necessitates the individual nanoparticles to disperse of nanoparticles being used to form the nanocomposites with various polymers including homogeneously within a matrix polymer. The main challenge in terms of dispersion state clays [138], carbon nanotubes [139], graphene [140], nanocellulose [141] and halloysite [142]. of nanoparticles is to achieve all the possible enhancements of its properties [137]. There is a potential for the nanofillers’ uniform dispersion to result in significant interfacial area between the nanocomposites’ constituents [137]. There are various factors that influence the reinforcing effect mainly polymer matrix properties, type and nature of nanofiller as well as polymer and filler concentration. Other factors focusing on the particle includes its size, aspect ratio, orientation and distribution [138]. There have been numerous types of nanoparticles being used to form the nanocomposites with various polymers including clays [138], carbon nanotubes [139], graphene [140], nanocellulose [141] and halloysite [142]. It is essential to evaluate the nanofiller dispersion within the polymer matrix. This is because there is a strong correlation between both the mechanical and thermal proper- Appl. Nano 2023, 4 96 Appl. Nano 2023, 4, FOR PEER REVIEW 22 It is essential to evaluate the nanoﬁller dispersion within the polymer matrix. This is because there is a strong correlation between both the mechanical and thermal properties with ties with the outcome of morphologies. The degree nanoparticles separation would result the outcome of morphologies. The degree nanoparticles separation would result in three in three possible morphologies outcome [143] namely intercalated nanocomposites, possible morphologies outcome [143] namely intercalated nanocomposites, conventional conventional composites (also known as microcomposites) and exfoliated nanocompo- composites (also known as microcomposites) and exfoliated nanocomposites (Figure 16). sites (Figure 16). In an event where the polymer is not intercalating between the layers of In an event where the polymer is not intercalating between the layers of the silicate, the the silicate, the outcome would be separate phases of composite where its properties are outcome would be separate phases of composite where its properties are within the same within the same range as seen in conventional composites [144]. range as seen in conventional composites [144]. Figure 16. Possible structures of polymer nanocomposites using layered nanoclays: (a) microcom- Figure 16. Possible structures of polymer nanocomposites using layered nanoclays: (a) microcom- posite, (b) intercalated nanocomposite and (c) exfoliated nanocomposite [143]. Reproduced with posite, (b) intercalated nanocomposite and (c) exfoliated nanocomposite [143]. Reproduced with permission from Alexandre, M. et al., Materials Science and Engineering: R: Reports; published by permission from Alexandre, M. et al., Materials Science and Engineering: R: Reports; published by Elsevier, 2000. Elsevier, 2000. An intercalated structure encompasses with at least one extended polymer chain, An intercalated structure encompasses with at least one extended polymer chain, where it intercalates between the silicate layers. The outcome is a consistent order of where it intercalates between the silicate layers. The outcome is a consistent order of multilayer morphology with polymer and clay layers that are intercalated. Exfoliated multilayer morphology with polymer and clay layers that are intercalated. Exfoliated structure would result in the event of complete and orderly dispersion of silicate layers structure would result in the event of complete and orderly dispersion of silicate layers within a continuous polymer matrix [143]. Exfoliated nanocomposites have a large surface within a continuous polymer matrix [143]. Exfoliated nanocomposites have a large sur- contact area between the nanoparticles and matrix. Such is one of the signiﬁcant differences face contact area between the nanoparticles and matrix. Such is one of the significant between conventional composites and nanocomposites. differences between conventional composites and nanocomposites. 5.1. Compatibilization in Polymer Nanocomposites 5.1. Compatibilization in Polymer Nanocomposites Compatibilization is of paramount importance to achieve a mixture of polymer or Compatibilization is of paramount importance to achieve a mixture of polymer or nanocomposite with the desired properties. Therefore, poor properties are attributable nanocomposite with the desired properties. Therefore, poor properties are attributable to to the chemical nature differences between the polymers or polymer matrix with the the chemi NPs [145 cal ]. na As ture pr eviously difference mentioned, s between compatibilization the polymers or pol is a ym signiﬁcant er matrix factor with in the obtaining NPs [145the ]. Adesir s prev ed io pr usl operties. y mentiDegradation oned, compashould tibilizatio ben kept is aat sign a low ifica pr nt obability factor in and obta itioccurs ning thwhen e the organomodiﬁer is decomposed and when degradation products and polymers are desired properties. Degradation should be kept at a low probability and it occurs when the interacting organomodwith ifier is each dec other omp.oAll sed of anthese d whhave en degr a signiﬁcant adation prinﬂuence oducts anupon d poly the mer pr s operties are and morphology of the material [146] (Figure 17). There are three methods of productions interacting with each other. All of these have a significant influence upon the properties for polymer nanocomposites; in situ polymerization, solution and melt blending. The and morphology of the material [146] (Figure 17). There are three methods of productions production method is chosen based on the polymeric matrix type, nanoﬁller and the ﬁnal for polymer nanocomposites; in situ polymerization, solution and melt blending. The products’ desired properties [15]. production method is chosen based on the polymeric matrix type, nanofiller and the final products’ desired properties [15]. Appl. Nano 2023, 4, FOR PEER REVIEW 23 Appl. Nano 2023, 4 97 Figure 17. Schematic of compatible PVDF/SWCNT nanocomposite production [146]. Reproduced Figure 17. Schematic of compatible PVDF/SWCNT nanocomposite production [146]. Reproduced with permission from Cho, K.Y. et al., Composites Science and Technology; published by Else- with permission from Cho, K.Y. et al., Composites Science and Technology; published by Elsevier, vier, 2018. 5.2. In-Situ Polymerization In-situ polymerization involves a correct dispersion of the nanoﬁller within the 5.2. In-Situ Polymerization monomer solution prior to the beginning of polymerization process. This is to ascer- In-situ polymerization involves a correct dispersion of the nanofiller within the tain the formation of the polymer between the NPs. There are various methods of initiating monomer solution prior to the beginning of polymerization process. This is to ascertain polymerization such as heat, utilising the correct initiator, etc [147]. Such method could the formation of the polymer between the NPs. There are various methods of initiating be used to achieve a polymer grafted NPs and high loading nanoﬁllers with the absent polymerization such as heat, utilising the correct initiator, etc [147]. Such method could of aggregation [148]. It is possible to include organic modiﬁers in order to assist the NPs be used to achieve a polymer grafted NPs and high loading nanofillers with the absent of dispersion and to be included within the polymerization [149]. Such method could be deemed as an alternative in producing nanocomposites through the use of polymers that aggregation [148]. It is possible to include organic modifiers in order to assist the NPs may be deemed unstable thermally or non-soluble [150]. There are occasions where such dispersion and to be included within the polymerization [149]. Such method could be method is applicable in solvent-free form [151]. Furthermore, such method may increase deemed as an alternative in producing nanocomposites through the use of polymers that the performance of the products [152]. Mini-emulsion polymerization is dependent on may be deemed unstable thermally or non-soluble [150]. There are occasions where such the monomer droplets being produced, which are subsequently dispersed into a solu- method is applicable in solvent-free form [151]. Furthermore, such method may increase tion within a nanoscale [153]. The advantages include particle morphology that can be the performance of the products [152]. Mini-emulsion polymerization is dependent on controlled [154], high functioning interfacial adhesion of the nanoﬁllers [155] and higher the monomer droplets being produced, which are subsequently dispersed into a solution transparency value [156]. This method could potentially [157] use higher nanoﬁllers with within a nanoscale [153]. The advantages include particle morphology that can be con- no presence of agglomeration, increased performance of the ﬁnal products, products with solvent-free form, outcome of covalent bond within the NPs functional groups and polymer trolled [154], high functioning interfacial adhesion of the nanofillers [155] and higher chains as well as utilising the thermoplastic and thermoset polymers. A major disadvantage transparency value [156]. This method could potentially [157] use higher nanofillers with of such method is the agglomeration easing [148,150]. no presence of agglomeration, increased performance of the final products, products with solvent-free form, outcome of covalent bond within the NPs functional groups and 5.3. Solution Blending polymer chains as well as utilising the thermoplastic and thermoset polymers. A major Blending is the most used method because it is simple in terms of producing polymer disadvantage of such method is the agglomeration easing [148,150]. nanocomposites. In comparison with other methods however, this method has higher difﬁculty in terms of achieving proper nanoﬁller dispersion within the polymer matrix [157]. 5.3. Solution Blending Solution blending is a system that encompasses both the polymer and nanoﬁller that can be dispersed within a suitable solvent without much difﬁculty [147]. The dispersion of Blending is the most used method because it is simple in terms of producing poly- the nanoﬁller within the polymer can be achieved through magnetic stirring, ultrasonic mer nanocomposites. In comparison with other methods however, this method has irradiation or shear mixing [148]. Figure 18 demonstrates the use of this method, wherein higher difficulty in terms of achieving proper nanofiller dispersion within the polymer the NPs are still dispersed within the polymer chains after the solvent evaporates. This matrix [157]. Solution blending is a system that encompasses both the polymer and nan- nanocomposite that has just been produced could be developed into a thin ﬁlm [157]. ofiller that can be dispersed within a suitable solvent without much difficulty [147]. The The solution blending posed a few constraints in economic and environmental terms. dispersion of the nanofiller within the polymer can be achieved through magnetic stir- Thus, there is a need for an optimum method to achieve the desired product while ad- ring, ultrasonic irradiation or shear mixing [148]. Figure 18 demonstrates the use of this dressing the constraints accordingly [158]. The advantages of solution blending include reduced gases permeability [159], simple operation and the use of conventional method for method, wherein the NPs are still dispersed within the polymer chains after the solvent nanoﬁllers of all types as well as the thermoset polymers and thermoplastic polymers [160]. evaporates. This nanocomposite that has just been produced could be developed into a The disadvantages include environmental and aggregation issues [158]. However, this thin film [157]. method is restricted to water soluble polymers [161]. Appl. Nano 2023, 4, FOR PEER REVIEW 24 Appl. Nano 2023, 4 98 Figure 18. Schematic illustration of solution blending method. Figure 18. Schematic illustration of solution blending method. 5.4. Melt Blending The solution blending posed a few constraints in economic and environmental Melt blending necessities the direct dispersion of nanoﬁllers into the molten polymer. terms. Thus, there is a need for an optimum method to achieve the desired product while When the mixing process starts in its melt state, the resulting polymer strain that is applied addressing the constraints accordingly [158]. The advantages of solution blending in- on the particles is dependent on the weight distribution and the weight of the molecules. clude reduced gases permeability [159], simple operation and the use of conventional The size of the agglomerates decreases when the shear stress level is high [157]. At the method for nanofillers of all types as well as the thermoset polymers and thermoplastic beginning, the larger agglomerates break apart to become smaller in size before being polymers [160]. The disadvantages include environmental and aggregation issues [158]. dispersed within the polymer matrix. Stronger shearing results when the polymer strain is How transferr ever, thed is m toethe thod new is re agglomerates. stricted to water Individual soluble po particles lymers ar [1e 61 formed ]. due to the breaking down. The primary element of this method is the timing and the chemical processes 5.4. Melt Blending between the NPs surface and the polymer [162]. Melt blending necessitates single and twin-screw extruders [163]. However, there are Melt blending necessities the direct dispersion of nanofillers into the molten poly- occasions where unfavourable outcome may ensue on the nanoﬁller ’s modiﬁed surface mer. When the mixing process starts in its melt state, the resulting polymer strain that is due to high temperatures thus optimisation is applied to address this issue [164]. The applied on the particles is dependent on the weight distribution and the weight of the most renowned method to address this is to use intermeshing co-rotating twin-screw molecules. The size of the agglomerates decreases when the shear stress level is high extruders. The disadvantage of such method is the difﬁculty in controlling the parameters [157]. At the beginning, the larger agglomerates break apart to become smaller in size such as interaction between the NPs, polymer and the procession conditions such as before being dispersed within the polymer matrix. Stronger shearing results when the residence time and temperature [165]. As such, it is not easy to achieve NPs that are polymer strain is transferred to the new agglomerates. Individual particles are formed evenly dispersed. Melt blending can be commercialised as it is compatible with a range due to the breaking down. The primary element of this method is the timing and the of industrial operations including extrusion and injection moulding [157]. The main chemical processes between the NPs surface and the polymer [162]. advantages of this are low cost, environmentally sustainable due to the absence of solvents, Melt blending necessitates single and twin-screw extruders [163]. However, there heat stability enhancement [166], improved mechanical properties [167] and good NPs are occasions where unfavourable outcome may ensue on the nanofiller ’s modified sur- dispersion [168]. Its disadvantage is the possibility of damage on the nanoﬁllers’ modiﬁed face due to high temperatures thus optimisation is applied to address this issue [164]. The surface as a result of the high temperature application [169]. Overall, each method has its most renowned method to address this is to use intermeshing co-rotating twin-screw own respective advantages and disadvantages and the selection should be based on the extruders. The disadvantage of such method is the difficulty in controlling the parame- conditions and underlying materials. ters such as interaction between the NPs, polymer and the procession conditions such as residence time and temperature [165]. As such, it is not easy to achieve NPs that are 5.5. Nanopolymers and Their Applications evenly dispersed. Melt blending can be commercialised as it is compatible with a range of Nanopolymers offer rife of applications including all the uses offered by the traditional industrial operations including extrusion and injection moulding [157]. The main ad- polymers. These applications include telecommunications, defence, household goods, daily vantages of this are low cost, environmentally sustainable due to the absence of solvents, services, utilities and basic utilities, etc. Further details include the plastic containers, heat stability enhancement [166], improved mechanical properties [167] and good NPs toothpaste, and so forth. Nanopolymers are favoured due to their many notable attributes dispersion [168]. Its disadvantage is the possibility of damage on the nanofillers’ modi- included high resistance to chemical, excellent tensile strength capacity to hold metals fied surface as a result of the high temperature application [169]. Overall, each method and other compounds. High conductive properties of nanopolymers enable their usage in has its own respective advantages and disadvantages and the selection should be based nano circuit fabrication. There is possibility to produce polymer with nanoparticles from on the conditions and underlying materials. many different structures where some can be self-assembled such as lamellar, lamellar- within-cylindrical, lamellar-within-spherical, spherical-within-lamellar and cylindrical- 5.5. Nanopolymers and Their Applications within-lamellar geometry. The examples of non-self-assembled structures are polymeric Nanopolymers offer rife of applications including all the uses offered by the tradi- nanocapsules, polymer brushes, nanoﬁbers, hyperbranched polymers, dendrimers and tional polymers. These applications include telecommunications, defence, household polymeric nanotubes. goods, daily services, utilities and basic utilities, etc. Further details include the plastic There is still a constant renovation in the nanotechnology due to the great demand containers, toothpaste, and so forth. Nanopolymers are favoured due to their many no- for practical applications. Nanoﬁbers made via electrospinning ﬁnd many uses within table the attributes incl environment. ude Due d high to their resistr aemarkable nce to chem length ical, exc and ellent ability tensi to le embed strength in ca other pacity media, to Appl. Nano 2023, 4 99 nanoﬁbers became one of the safest NMs. Other desirable properties include high porosities (over 80%), adjustable functionality and high surface-to-volume ratio. These characteristics are more effective than the conventional non-woven and polymeric membranes especially those use in the liquid ﬁltration and particulate separation. It is feasible to apply the nanoﬁbrous scaffolds exclusively as a cutting-edge component for the liquid separation and gas ﬁltration. Due to the advancement of electroblowing and electrospinning technology high performance nanoﬁbrous scaffolds became feasible. In this perception, it is customary to highlight various applications of nanoﬁbers for the solar energy harnessing and as membranes to remove heavy ions from the industrial wastewater and discharges. 6. Nanotechnology Based Smart Glass Materials The conventional usage of high-performance glazing systems is upon windows or building windows in order to decrease the amount of unwanted heat from the sun as well as reducing the workload to cool air from the air condition systems installed within the building. Aesthetically, glass facades are more desirable in terms of using them in commercial buildings [170–172]. Thus, careful studies are needed to estimate and evaluate the energy savings in practical terms for the high-performing glass. It is particularly vital for the research to be conducted on the different types of high-performance building glass. This is especially true for densely built cities such as Singapore where the heat from the sun is an issue for the buildings. Additionally, it is still uncertain whether these glasses are able to retain its efﬁciency in countries with four seasons. The glass’s U-value needs to be within acceptable range in order for it to be functional when subjected to various climates including the tropics. Assessment of glass performance necessitates active measurement while being sub- jected to a controlled source of radiant. Evidently, such testing environment may not account for actual weather, where many possibilities may not be duplicated within the test environment [171,173]. This may be compensated by subjecting the glasses to real weather conditions by actually installing them outdoor [174,175]. However, it is still not possible to conduct testing in large-scare and fast on-site characterization. Furthermore, the test is restricted to fabricate the glazing, indicating the impossibility to forecast possible problems during the design stage. The development of cutting-edge technique made it possible for professionals to be able to make simulation and assessment on the glass being installed in building during the design stage [174,176,177]. Over the counter and matured simulation codes that are open source in nature including Energy Plus and Radiance [174] are appro- priate for such assessment as they have undergone development spanning for more than ten years. Despite such tools, the assessment is still complex in terms of conducting such evaluation for high performance glazing description (glass with various coatings for many purposes) into the solar irradiance module through the use of the current glass models. The glass involving multiple layer or pane glazing must be classiﬁed and computed uniquely using more focused tool prior to be interfaced using a custom script. Further consideration is vital in terms of acquiring complete and comprehensive weather model to increase the accuracy of the solar heat gain that will enter into the building. With the exception of weather data from International Weather for Energy Calculations (IWEC), other data must be carefully considered when inputted for assessment using the aforementioned tools. In addition, the tools are developed with main consideration to indoor performance thus it does not account for the negative effects and other impacts of the sunlight being reﬂected off the building glass façade during the assessment for environmental risks. External use of the glass typically involves alternative glass material, which are mirror or opaque with high specularity. The use of such alternative materials means that their properties have no angle dependence [174]. Usually, the glass performance is evaluated via the active measurements by utilizing the known radiant sources. While, under the non-controlled weather there is no possibility to apply this type of setup. Though a passive measurement can be running under real weather conditions using the outdoor test chamber, however it is unsuitable for a large- Appl. Nano 2023, 4, FOR PEER REVIEW 26 exception of weather data from International Weather for Energy Calculations (IWEC), other data must be carefully considered when inputted for assessment using the afore- mentioned tools. In addition, the tools are developed with main consideration to indoor performance thus it does not account for the negative effects and other impacts of the sunlight being reflected off the building glass façade during the assessment for envi- ronmental risks. External use of the glass typically involves alternative glass material, which are mirror or opaque with high specularity. The use of such alternative materials means that their properties have no angle dependence [174]. Usually, the glass performance is evaluated via the active measurements by utilizing the known radiant sources. While, under the non-controlled weather there is no possi- bility to apply this type of setup. Though a passive measurement can be running under real weather conditions using the outdoor test chamber, however it is unsuitable for a Appl. Nano 2023, 4 100 large-scale testing and fast on-site characterization. Additionally, the mentioned test is limited to the fabricated glazing and thus unable to predict potential issues in the design stage. Advances in the simulation techniques have enabled the building professionals to scale testing and fast on-site characterization. Additionally, the mentioned test is limited evaluate the glass facade of a building at the design phase. Nonetheless, the typical sim- to the fabricated glazing and thus unable to predict potential issues in the design stage. ulation tools are unable to integrate the high performance glazing description. Using the Advances in the simulation techniques have enabled the building professionals to evaluate advanced coating technology, although the existing glass models can be tested but these the glass facade of a building at the design phase. Nonetheless, the typical simulation tools often lack local weather models that plays an important role in accurately assess the tools are unable to integrate the high performance glazing description. Using the advanced solar heat gain by the building. Nanotechnology is being applied in various disciplines coating technology, although the existing glass models can be tested but these tools often especially within construction materials due to its ability to decrease the consumption of lack local weather models that plays an important role in accurately assess the solar heat energy thus they have much potential. Glass is one of the most special construction ma- gain by the building. Nanotechnology is being applied in various disciplines especially ter within ials constr and ca ucti n on be materials treated wi due th to na its no ability technto olo decr gy, ease decre the asi consumption ng the tranof sfe ener r ofgy hea thus t through they have much potential. Glass is one of the most special construction materials and the building envelope (Figure 19). The study used Design Builder 3.1 and followed the can be treated with nanotechnology, decreasing the transfer of heat through the building Egyptian energy code requirement to assess the difference energy consumption between envelope (Figure 19). The study used Design Builder 3.1 and followed the Egyptian energy two types of glass, standard 6 mm clear glass and glass that is treated with nanotech- code requirement to assess the difference energy consumption between two types of glass, nology. The standard 6 mm clear glass that were used in glazed facades results in high standard 6 mm clear glass and glass that is treated with nanotechnology. The standard 6 thermal loads into the indoor environment of the building. This results in increasing use mm clear glass that were used in glazed facades results in high thermal loads into the indoor of energy in the building [178]. environment of the building. This results in increasing use of energy in the building [178]. Figure 19. The glass treated with nanotechnology [178]. Figure 19. The glass treated with nanotechnology [178]. Glass is a common material within various industries including transport, building Glass is a common material within various industries including transport, building and construction, solar energy with glass variety. It is also being used in microscopes, and construction, solar energy with glass variety. It is also being used in microscopes, tablet computers, furniture and many more. There are four advantages of using glass tablet computers, furniture and many more. There are four advantages of using glass within the building and construction sectors. Firstly, it allows natural lights to enter the wi building. thin the Secondly buildin,g itaﬁlters nd coout nstruc harmful tion sect rays ors. from Firs the tlysun , it a fr ll om owentering s natural the ligbuilding. hts to enter the Thirdly, it harmonises the environment and the building. Lastly, it is cost-effective due building. Secondly, it filters out harmful rays from the sun from entering the building. to its energy efﬁciency. Researchers and scientists have taken the motto of ‘necessity is Thirdly, it harmonises the environment and the building. Lastly, it is cost-effective due to the mother of invention’. Thus, during their research in improving the properties of glass, its energy efficiency. Researchers and scientists have taken the motto of ‘necessity is the they have developed a type of glass that requires minimal maintenance, also known as mother of invention’. Thus, during their research in improving the properties of glass, self-cleaning glass. Individuals that wear glasses will be glad that such glasses prevent mist from forming when they are enjoying hot drinks that are steaming or when they are cooking. In addition, anti-fogging glass is used in tablet computers thus they could be used in close proximity to swimming pools. In addition, anti-reﬂective glass is used in mobile phones or laptops thus users could still able to use these devices during broad daylight. As for self-cleaning glass, they are most suitable for windows and doors in ofﬁces and homes, where these SCGs do not need any frequent maintenance for cleaning. 6.1. Self-Cleaning Glass Glass is extensively used by diverse industries like automotive, solar cells, building and construction. SCG is a new type of glass being developed and is widely used in hard-to-reach areas in buildings because it requires minimal maintenance. SCG has either Appl. Nano 2023, 4 101 a layer of titania (TiO ) that measures 10–25 nm or is coated with silica on its surface via both bottom-up or top-down approach. The self-cleaning properties are the control of its wettability properties on its surface. The ﬁrst is for the surface to be complete dry, also known as hydrophobic surface, where a liquid droplet maintains a spherical shape on the surface of the glass. This is achieved either by forming a component of low surface energy or through surface roughness control. The surface becomes hydrophobic by applying a thin layer of SiO . The second technique involves complete wetting of the surface where the liquid forms a ﬁlm upon contact with the surface. This is known as hydrophilic and can be achieved through applying photocatalytic TiO coating. The coating uses the sunlight and water to rinse itself thus resulting in self-cleaning property. Therefore, solid surfaces undergo various reactions with dissimilar materials depending on the coating type being applied. Consideration should take place upon the various qualities as a result such as spreading, wettability, adhesion and interface. The wettability property of a solid is deﬁned by observing that contact angle (denoted by ) the moment liquid touches the surface of the solid. 6.2. Hydrophilic Coating A surface is deemed hydrophilic when the water contact angle (CA) is less than 90 . It is considered as super hydrophilic when its CA is less than 50 . As the liquid contacts such surfaces, it will spread out until it becomes a thin layer. The self-cleaning materials that made this possible are WO , ZnO, SnO , SiO , CdS, TiO and ZrO . The most extensively 3 2 2 2 2 used is TiO because it has more advantages in comparison with the others. New discovery made by Fujishima and Honda where they used TiO for photo-electrochemical splitting of water to hydrogen and oxygen while being subjected to UV radiation. This has resulted in an explosion of research to study the TiO photo-catalytic potential including self- cleaning coatings, photo-electro-catalysis, photovoltaics, photoelectrocatalytic degradation of organic compounds and advanced oxidation. TiO exhibits the following properties, high refractive index, good mechanical performance, transparent and semiconductor material with a high band gap. When TiO is within the wavelength range from 0.35 mm to 12 mm, it becomes stable chemically. Titania exists in three different crystal structures such as brookite, anatase and rutile. The highest refractive index is shown by the rutile phase (2.61–2.90), making it the centre of focus for optical applications. Rutile is also the most stable in terms of its thermodynamic properties especially when subjected to high temperatures. Despite various advantages, anatase has increased desire for lower temperature applications where it is necessary to form a ﬁlm on thermally sensitive substrates. Therefore, the desirable materials are amorphous or crystalline anatase, used to produce the self-cleaning glasses at temperatures below 400 C. Anatase can be changed to rutile in the range of 700 to 1100 C. 6.3. Anti-Reﬂective Coating Fujishima and Guiselin et al. invented the TiO thin ﬁlms and also patented the methods. This ﬁlm being transparent, photo-catalytically efﬁcient and abrasion-resistant can be used on glass surfaces. Several SCG are already being commercially used at the present time such as Hydrotecht from TOTO, Activt from Pilkington Glass, Thermotecht from Viridian and Bioclean from Saint Gobain. Additionally, self-glazing products are also being rolled out in liquid forms or white that target direct consumers. When a normal glass is applied the self-cleaning products, they would turn into SCG. Products that are available for users are produced by some companies such as Rain Racert from Rain Racer Developments, BalcoNanot from Balcony Systems Solutions and ClearShieldt from Ritec International. SCG can be installed in various locations including ofﬁces, facades and general buildings. The improvement of photo-catalytic activity and anatase coating necessitate a high refractive index due to low temperature processing. Other properties, apart from self-cleaning, are necessary for a glass that will be used on smart phones, spectacles and solar cells. These properties include anti-fogging, anti- abrasive and anti-reﬂection. Fraunhofer is the founder of anti-reﬂective (AR) coating in 1817. Appl. Nano 2023, 4 102 Since then, AR phenomenon is regarded as a destructive interference between air-coating interfaces by Fresnel and Poisson and light reﬂected due to substrate coating. One of the many methods of making AR coating is to construct a single-layer of coating that has low refractive index. Materials that have low refractive index cost more and also rare. Porous nanostructures can be used to effectively decrease the volume-averaged refractive indices of materials through controlling the porosity within the coatings. This results in anti-reﬂective coatings, and its hydrophobic and hydrophilic properties could be further enhanced by increasing surface roughness. At the same time, reﬂection is increased due to the decreased in transmittance, which occurs as a result of scattering diffusion in rough surfaces. Sample transmittance is the subsequent light intensity ratio that exits the after intensity ratio entered the sample. Therefore, the increase of transmittance results in decrease of photocatalytic activity as the light intensity decreases. At the same time, anti-reﬂective surfaces are part of the SCG. Therefore, in order to preserve the self-cleaning and anti-reﬂectivity properties, the ideal surface roughness is required. The assessment of the solid surface’s wettability necessitates the static contact angle and the dynamic sliding angle. The essential factor is therefore roughness of the surface and chemical functionalization. 6.4. Fabrication of Self-Cleaning Glass The glass becomes either hydrophilic or hydrophobic after the applications of a thin layer of TiO or SiO on its surface. There are two types of fabrication of nanomaterials 2 2 which are top-down and bottom-up. The top-down approach involves removal of materials gradually from massive structure until the required nanomaterial is formed. Lithography is an example of this method. Comparatively speaking, it is similar to using a block of wood and turning it into a doll by a carpenter. Bottom-up approach involves the use of atoms or molecules to be built gradually until the formation of the required nanomaterial or nanocoating. Comparatively, this is akin to using Lego blocks to build a house. The bottom-up approach is further divided into two types like gas and liquid phase. The gas phase method involves the plasma arc evaporation and chemical vapour deposition (CVD). The liquid phase technique deals with the sol-gel and molecular self-assembly. 6.5. SiO -TiO Coating 2 2 In addition to having self-cleaning function, other functions are also desirable includ- ing photocatalysis and anti-reﬂectivity, which are vital in products such as smart phones and solar cells. Glop et al. [179] used the sol-gel method and Liu et al. [180] used the pulse magnetron sputtering as methods of preparation for the photoactive antireﬂection coating. The TiO coating on the outer surface that results in self-cleaning feature increases the reﬂectance of plastic or glass substrate due to its relatively high refractive index (c. 2.5 for the anatase phase). Therefore, self-cleaning and anti-reﬂectivity attributes may not be compatible with the exception of rare instance where the structure and composition are modulated. Prado et al. [181] attempted to produce a coating that is multifunctional where its outer layer consists of dense/mesoporous TiO and its inner layer consist of meso- porous SiO AR layer. Multifunction coatings with self-cleaning attribute have discovered to perform 25–30% compared to photo degradation degree, which is produced via the conventional TiO coatings layer either porous or compact. Solar industry including solar power plants and solar energy producers primarily use glass. The amount of electricity generated or power for heating water depending on the intensity of the sunlight. The glass may be useful in terms of reducing loss of radiation and reﬂection. SCG’s primary property is its reﬂective index denoted by n. Production of glasses with anti-reﬂective properties for solar related use requires a low refractive index such as SiO where its n value is 1.4. Conversely, high reﬂective index such as titania where its n value is 2.0 is vital in improving the photocatalytic activity of the hydrophilic property in SCG. Helsch and Deubener [182] attempted to use the sol-gel coating technique to create a single type of glass that contains both functions. High transmittance is needed for this particular type of glass. The research has been a success where they used two layers consisting of SiO 2 Appl. Nano 2023, 4 103 and TiO to create a glass with both anti-reﬂective and photocatalytic properties. Through the sol-gel coating method, preparation was made on silica glass porous coatings xTiO . (1002x) SiO with 50 wt% of titania. The compatibility of anti-reﬂective and photocatalytic properties will then be achieved once the composition reached the ranger from x57.5_20. Porous coating also enhances the solar transmittance by 2.3% in comparison to silica glass that is not coated. These coatings with dual functions have greater degradation rate at 20-fold of the air borne contaminants in comparison to nanoporous ﬁlm of pure SiO . Nanoporous structures consist of materials that have high porosity and low density with simultaneous advantages in terms of possessing high pore volume, high surface area and larger pore size, whereby the diffusion pathways are accessible. Anti-reﬂection coating is often made of porous silica layers. Helsch et al. [183] made a discovery that there is a 5% enhancement (from 92 to 97%) of light transmission when borosilicate glass is at 550 nm, 35% porosity and 110 nm ﬁlm thickness. There are many applications for TiO ﬁlms on glass substrates including mirrors, windshields and window glasses. While being serviced, anti-reﬂective porous coatings will be subjected to severe environmental conditions including hail and salt atmosphere, sandstorms, dust particles and airborne volatile organic compounds. If the AR coating is damaged while being subjected to the aforementioned conditions, the solar transmittance will be reduced. Cathro et al. [184] dis- covered an increase on the refraction index of porous thin ﬁlms as a result of the adsorption of airborne contaminants. Pareek et al. [185] found that oil vapour contamination is also responsible for the increase of the refractive index of porous antireﬂective coatings. 6.6. Nanomaterial-Based Solar Cool Coatings The global building and construction industry is responsible for both 40% of the entire world’s energy consumption and emitting a third of the world’s greenhouse gases annually. At least 50% of the total energy consumed by this industry are for powering heating, ventilating and air conditioning (HVAC) systems. Passive cooling and solar heat insulation technologies are often being regarded as solutions in addressing the global energy crisis, in which they are being considered as reducing or even consume zero energy. Solar radiation plays the main role in terms of buildings gaining heat when they are transmitted via the envelope. The heat will be trapped and increased inside the building. Buildings therefore, are more likely to use nanomaterials-based solar cool coatings (NSCCs) in order to address the issue of excessive solar heat and energy consumption. These coatings are currently the most reliable in terms of passive cooling technologies. NSCCs are composite materials where it is made of thin-layered substrates mixed with nanosized additives, which is the primary component due to its distribution solar reduction function onto a normal coating material. Binders are made of thin-layered substrates and they are added with nanosized additives to provide a coating to the surfaces of buildings where required. NSCCs are widely used for the past few years as a solution to the high energy consump- tion in buildings. On [186], authors have conducted a market research to show that the solar coatings will have a 70% in saving energy on a global scale from 2013 to 2019. The number of patents being ﬁled is evidence to the increased attention and research being taken place to increase the use of NSCCs as well as proving that it is a pioneering technology in passive cooling. On [187], authors have revealed that there is a signiﬁcant increase by 38% between 2013 to 2015 on the number of patents being ﬁled on smart window coatings. Meanwhile, the patents for thermal barrier coatings have increased by 32% between 2011 and 2015. For several decades, the key component in transparent coatings used in solar heat reﬂection is metal. Al, Au, Ag, Cu and Pt have higher performance in terms of possessing high reﬂectivity and low absorptivity properties. Incorporating these metals into NSCCs leads to reﬂection of solar heat that would have otherwise penetrated into the indoor environment of the building. This passive cooling feature means that the indoor environment will need to use less energy consumption for cooling purposes. Signiﬁcant amount of research was conducted on solar cool coatings. The least difﬁcult in terms of implementation and usage are Au and Ag. Cher (2014) prepared nanogold (Au) ﬁlms to be applied on the glass Appl. Nano 2023, 4, FOR PEER REVIEW 30 These coatings are currently the most reliable in terms of passive cooling technologies. NSCCs are composite materials where it is made of thin-layered substrates mixed with nanosized additives, which is the primary component due to its distribution solar reduc- tion function onto a normal coating material. Binders are made of thin-layered substrates and they are added with nanosized additives to provide a coating to the surfaces of buildings where required. NSCCs are widely used for the past few years as a solution to the high energy con- sumption in buildings. On [186], authors have conducted a market research to show that the solar coatings will have a 70% in saving energy on a global scale from 2013 to 2019. The number of patents being filed is evidence to the increased attention and research being taken place to increase the use of NSCCs as well as proving that it is a pioneering technology in passive cooling. On [187], authors have revealed that there is a significant increase by 38% between 2013 to 2015 on the number of patents being filed on smart window coatings. Meanwhile, the patents for thermal barrier coatings have increased by 32% between 2011 and 2015. For several decades, the key component in transparent coatings used in solar heat reflection is metal. Al, Au, Ag, Cu and Pt have higher per- formance in terms of possessing high reflectivity and low absorptivity properties. In- corporating these metals into NSCCs leads to reflection of solar heat that would have otherwise penetrated into the indoor environment of the building. This passive cooling feature means that the indoor environment will need to use less energy consumption for cooling purposes. Significant amount of research was conducted on solar cool coatings. Appl. Nano 2023, 4 104 The least difficult in terms of implementation and usage are Au and Ag. Cher (2014) prepared nanogold (Au) films to be applied on the glass surface using aerosol-assisted CVD method wherein the deposition was performed inside a cold-walled horizontal-bed surface using aerosol-assisted CVD method wherein the deposition was performed inside a CVD reactor. The resulting nanogold layers possessed various morphologies depending cold-walled horizontal-bed CVD reactor. The resulting nanogold layers possessed various on different reaction temperatures. Observation was made by placing a layer of Au NPs morphologies depending on different reaction temperatures. Observation was made by at 500 °C (Figure 20a). Figure 20b,c shows individual Au NPs on the top plate of films placing a layer of Au NPs at 500 C (Figure 20a). Figure 20b,c shows individual Au NPs subjected to 400 °C. The results disclosed that the thermophoresis was responsible for the on the top plate of ﬁlms subjected to 400 C. The results disclosed that the thermophoresis increase of particle size, wherein the NPs formation and gold atoms aggregation occurred was responsible for the increase of particle size, wherein the NPs formation and gold atoms in the gas phase reactions prior to the deposition. aggregation occurred in the gas phase reactions prior to the deposition. Figure 20. (a–c) SEM (Scanning Electron Microscope) images of nano-Au deposited on the top plates Figure 20. (a–c) SEM (Scanning Electron Microscope) images of nano-Au deposited on the top at various temperatures. (d,e) SEM images of an Ag layer (d) with and (e) without a Ge wetting plates at various temperatures. (d,e) SEM images of an Ag layer (d) with and (e) without a Ge wet- layer. (f–i) TEM (Transmission Electron Microscope) and SEM images of Ag@SiO -SH (f,g), (h) Au@ ting layer. (f–i) TEM (Transmission Electron Microscope) and SEM images of Ag@SiO2-SH (f,g), (h) SiO -SH, (i) Pt@SiO -SH. (j–m) Schematic (j) of Au@TiO nanorods with various geometries; TEM 2 2 2 Au@ SiO2-SH, (i) Pt@SiO2-SH. (j–m) Schematic (j) of Au@TiO2 nanorods with various geometries; images of Au@TiO nanorods with (k) Janus, (l) eccentric, and (m) concentric geometries [187]. TEM images of Au@TiO2 nanorods with (k) Janus, (l) eccentric, and (m) concentric geometries Reproduced with permission from Zheng, L., et al., Solar Energy; published by Elsevier, 2019. [187]. Reproduced with permission from Zheng, L.,et al., Solar Energy; published by Elsevier, 2019. 7. Environmental Health and Safety Considerations 7. Environmental Health and Safety Considerations The inﬂuence of nanotechnology is apparent in industry and many aspects of life; The influence of nanotechnology is apparent in industry and many aspects of life; construction is no exception. Even though enhanced-quality materials equipped with inno- vative features are already being used, numerous potential applications of nanomaterials construction is no exception. Even though enhanced-quality materials equipped with still exist in the ﬁeld of construction that are yet to be capitalised upon. However, these endeavours do not come without risks. Negative outcomes and effects on the environment and human health are not outside the realm of possibility. Hence a prudent and cautious approach should be considered. There are several existing nanoparticles, such as titanium dioxide and carbon nanotubes, that could already be harmful to those individuals tasked with their direct use. Qualitative and quantitative risk assessments, occupational health and safety risk management, and adequate circumvention protocols for identiﬁed risks are not only important but are crucial to avoiding or mitigating potential disaster. Nanomaterials are so many in number and so varied, it is safe to assume that massive quantities of these materials will eventually be produced. Moreover, introduction of entirely new nanomaterials both trigger the requirement of adequate risk assessment procedures and suitable communication measures surrounding those risks. Presently, new nanomaterials are analysed in a manner similar to that used for chemicals, food, and consumer products, which is unsurprisingly both inefﬁcient and insufﬁcient. The challenges presented when characterising nanomaterials and generating a standardised processing approach become substantial bottlenecks to the process. However, there do exist several techniques; laser ablation inductively-coupled plasma mass spectrometry—that could stand to meet the needs of such processes. For example, aiding in the quantiﬁcation of nanomaterials as subsets of complex matrices. Despite existing awareness surrounding the potential risks for working with construc- tion nanomaterials, and the notion that these materials may even pose risks to end-users, Appl. Nano 2023, 4 105 hazard information remains limited [188]. Consequently, the Occupational Safety and Health Administration (OSHA) has no recourse to mitigate the unknown hazards of Nu- tritional risk screening (NRs), as it is without regulations nor enforceable exposure limits; this is regardless of the fact that nanomaterial contamination can take place at any time during the manufacturing, packaging, and transport of construction materials, their use on-site during construction, and after the work is complete during the operational phase. For example, a number of workers were shown to have been exposed to more than the recommended limit of titanium dioxide during the packaging process in a study conducted by Al-Bayati and Al-Zubaidi [189]. In a recent move to promote safe working practices, the CPWR developed a toolbox talk strongly recommending and endorsing the use of high efﬁciency particulate air (HEPA) ﬁlters when handling nanomaterials. This was in response to the discovery that construction nanomaterials can be converted into unintended forms when mass-manufactured [190], such as carbon-based nanomaterials becoming airborne when prepared as a solution. However, it is worth noting that HEPA ﬁlters were never designed to capture particles of under 300 nm in size, making it unlikely to eliminate the hazard, even though they may still serve to mitigate it. As mentioned before, a signiﬁcant impact stands to be made by the use of nanotech- nology within the construction industry, not only from a perspective of enhancing material properties, but also because a high proportion of all energy used by the world is consumed by commercial and residential buildings, in their lighting, heating, and air conditioning. Overtaken thus far by the adoption of nanotechnology within ﬁelds such as biomedical and electronics, the construction industry has been making up lost ground in their pursuit of innovation using a variety of nanomaterials in recent years. However, as alluded to pre- viously, adoption of novel technologies does not come without risks; the potential dangers to the environment and human health posed by nanomaterials should not go unconsidered. This is true even if the goal in their use is to preserve the environment, by utilising the energy-conserving functions provided by nanomaterials, their full lifespan must still be contemplated, as highlighted in a recent review by Rice University scientists. Unintended consequences could be far severe than those it was intended to prevent. Furthermore, the authors indicate that nanomaterials, especially CNTs, can be accidentally or incidentally introduced to the environment at various stages of their life cycle. Within their work at Rice, they go on to detail the importance of a holistic nanomateri- als’ lifecycle exposure proﬁling approach, stipulating without that level of meticulousness, critical impacts on ecosystem and human health cannot be avoided. They maintain that, as a result of no regulation being presently in place despite growing concerns, a number of MNMs should be regarded as ‘potential emerging pollutants’ until contradicting infor- mation surfaces, as there are many related risks to environmental and public health that are being disregarded without that regulation. Furthermore, they describe the element of unpredictability of the natural environment; once distributed into it, nanomaterials may transform in diverse chemical, biological, and physical fashions, altering their properties, effects, and ultimate fate. The potential routes along which nanomaterials can be released into the environment are many and often. From occupational exposure, when the material is ﬁrst being prepared, during any coating, moulding, incorporating, or compounding to contamination during installation, construction, maintenance, repair, renovation. Finally, to decommissioning or demolition processes, even beyond this stage, further risks arise when solid nanomaterials reach landﬁlls or get disposed of in incinerators. Delivery methods and approaches affect these risks, also: aerosolization of nanomaterials, adhesive wear, abrasion and corrosion, and manufacturing process wastewater efﬂuent outlets all have additional risks, speciﬁc to the method and altering the resultant hazard. 8. Using Nanomaterials Safely The question “how to utilise nanomaterials safely” does not ﬁnd itself wholly resolved, even though it is clear that discovering it is crucial to improving the performance of infras- Appl. Nano 2023, 4 106 tructure and buildings. These nanoscale ﬁbres and particles could already be contributing to a problem that the scientiﬁc community is as yet completely unaware of, or in the ways of which we know that they can. Thin strands carried airborne can acquire behaviour patterns akin to asbestos. Limited information is available for workers and manufacturers alike on keeping safe while handling these materials, while it is commonplace to appreciate the necessity of greater regulation. Given that estimates place up to half of all new building materials in 2025 as containing nanomaterials, this information is urgently sought. This was the motivation for the research team at Loughborough University, when they investigated where these materials are used, to what extent, number of potential risks, and how might the workers on the ‘front line’ mitigate these risks. It was funded in part by the Institution of Occupational Safety and Health (IOSH), in order to produce a framework and a measure of guidance. An additional challenge facing the generation of a set of guidelines as such, or indeed, any other form of regulation, is that the way health and safety legislation is applied in different countries. It may not be mandatory for manufacturers to specify information about the type of nanomaterial, or the approach with which it was used, resulting in largely unreliable and inconsistent labelling systems. 9. Conclusions The construction industry has witnessed an ever-increasing applications of various sus- tainable materials using the core-shell strategy and nanotechnologies. The following conclu- sions are made based on the in-depth and relevant literature overview of nanotechnology- based core-shell structures: i. A new class of hybrid and core-shell NPs can be developed due to the advent of the manipulation techniques of particle structures at the nanoscale. ii. Efﬁcient fabrication methods are now available for the large scale production of numerous types of core-shell nanostructures. These developed techniques have contributed to the fast-paced advancement of synthetic chemistry, device setup, colloid and interfacial science. iii. The pigments durability can remarkably be improved using the core-shell NPs. Fur- thermore, being a part of sustainable materials, these NPs have widespread applica- tions. The highest recommended materials for shells in the construction industries are SiO and TiO . 2 2 iv. Carbon-based nano-enhancers show higher thermal conductivity compared to metals or oxide-based materials. High surface afﬁnity between the organic structures and carbon nano-ﬁllers of PCM can enhance the uniform interpenetration and lower the particles’ scatterings at the interfacial surfaces. v. In the near future, the high-performance nano-enhanced phase change material technology will be of great demand. It is expected to be applicable in many areas particularly in the thermal storage within the sustainable and renewable energy ﬁeld. These applications include the solar energy power generation, industrial heat charg- ing/discharging processes, excess heat management and cooling of electronic devices. vi. The polymer nanocomposites have immense applications potential compared to the traditional materials. Thus, nanocomposites ﬁeld has been the popular research topic due to its several desirable features including ease of production, light weight and ﬂexibility. The most distinguishing aspect of polymer nanocomposites is their utility small ﬁllers, resulting in a signiﬁcant increase in the interfacial interactions than the conventional composites. vii. It is foreseeable that the core-shell NPs will continue to play a signiﬁcant role in the passive cooling technology, reducing the solar heat gain by the buildings and energy consumption. In the context of global climate change and fast urbanization-mediated energy deﬁciency and environmental deterioration, the development of core-shell NPs is expected to be faster mainly in two aspects like the large scale synthesis of Appl. Nano 2023, 4 107 high performance nanomaterials and cost-effective as well as time-efﬁcient coating fabrication techniques. viii. Accompanied by the standardised approaches and regulatory mandates, high-volume nanomaterials such as SiO and carbon black, play a major role for a variety of indus- trial applications. Despite still being in development phases for many applications, the appropriate analytical capacity for the characterisation of materials and their properties are still necessary and fundamental; existing reports from toxicological in- halation studies already indicate steady increases of nanomaterial toxicity, as opposed to demonstrating entirely new nano-speciﬁc effects and outcomes. 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Environ. Health Perspect. 2010, 118, 49–54. [CrossRef] [PubMed] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

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Applied Nano – Multidisciplinary Digital Publishing Institute

Published: Apr 7, 2023

Keywords: nanoparticles; core-shell materials; pigments; polymer; phase change materials

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Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited

Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited

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Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited

Potential Applications of Core-Shell Nanoparticles in Construction Industry Revisited

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