Preparation of Monodispersed Cs0.33WO3 Nanocrystals by Mist Chemical Vapor Deposition for Near-Infrared Shielding Application

In this study, single-phase Cs0.33WO3 nanocrystals were synthesized by a novel mist chemical vapor deposition method. As prepared, Cs0.33WO3 nanocrystals exhibited a microsphere-like appearance constructed with angular crystal grains with an average size of about 30–40 nm. Characterization by X-ray photoelectron spectroscopy indicated that Cs0.33WO3 nanocrystals consisted of mixed chemical valence states of tungsten ions W6+ and W5+, inducing many free electrons, which could scatter and absorb near-infrared (NIR) photons by plasmon resonance. These Cs0.33WO3 microspheres consisted of a loose structure that could be crushed to nanoscale particles and was easily applied for producing long-term stable ink after milling. Herein, a Cs0.33WO3/polymer composite was successfully fabricated via the ultrasonic spray coating method using mixed Cs0.33WO3 ink and polyurethane acrylate solution. The composite coatings exhibited excellent IR shielding properties. Remarkably, only 0.9 mg cm−2 Cs0.33WO3 could shield more than 70% of NIR, while still maintaining the visible light transmittance higher than 75%. Actual measurement results indicate that it has really good heat insulation properties and shows good prospect in heat insulation window applications.


Introduction
In modern buildings, a large part of heat exchange is contributed by heat radiation from the glass windows or glass walls, i.e., a significantly large amount of energy is consumed by the window glass. The development of efficient transparent heat-shielding (THS) glass coating technology is quite necessary for modern energy-saving buildings [1][2][3]. Among the currently used THS materials, cesium-doped tungsten bronzes (Cs 0.33 WO 3 ) have attracted significant research interest because of their great heat-shielding ability in near-infrared (NIR) range (780-2500 nm) with high visible transparency [3][4][5][6][7]; e.g., only 1 mg cm −2 Cs 0.33 WO 3 could shield more than 75% of heat radiations. Moreover, owing to its simple composition and good solubility of the elements, it could be synthesized easily via many solution methods or solid-state synthesis methods [8][9][10].
Recently, both the synthesis and nanosizing of Cs 0.33 WO 3 have achieved notable progress, indicating that Cs 0.33 WO 3 -based THS coating could easily be fabricated cost-effectively in the very near future. The solution method including the hydrothermal or solvothermal technique is popular for its simple and facile process with a low reaction temperature [11][12][13][14]. Notably, the synthesis and nanosizing

Preparation of Cs 0.33 WO Nanoparticles and Ink
The mist-CVD apparatus consists of an ultrasonic transducer, a tube furnace, and pipe fittings, as shown in Figure 1a. The purpose of the ultrasonic transducer is to generate an ultrasonic wave, which can continuously convert the solution into mist. Some absorbent papers were used to prevent the condensation of water vapor on the tube wall. Briefly, Cs 0.33 WO 3 nanoparticles (NPs) were synthesized using the following procedure: ammonium metatungstate (29.56 g, 10 mmol) and cesium hydroxide hydrate (6.45 g, 38.4 mmol) were thoroughly dissolved in deionized water (200 mL). The solution was then atomized to mist at a rate of 5 mL min −1 . N 2 was used as the carrier gas, which brought the mist to the tube furnace. The mist was then dried rapidly and transformed to white powder at a temperature of 180 • C, as shown in Figure 1b. Then, it was transferred to a ceramic boat and heated at 550 • C in a reducing atmosphere containing 95% N 2 and 5% H 2 for 3 h. After naturally cooling it down Nanomaterials 2020, 10, 2295 3 of 9 to room temperature, dark blue powder (~8.5 g) was collected, as shown in Figure 1c. Cs 0.33 WO 3 ink was prepared after wet grinding in a laboratory ball mill for 24 h at a speed of 800 rpm. Before milling, NPs (1.5 g) were mixed with a solution containing methoxy ethanol (30 mL) and KH-602 coupling agent (1.5 mL). Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 9 and heated at 550 °C in a reducing atmosphere containing 95% N2 and 5% H2 for 3 h. After naturally cooling it down to room temperature, dark blue powder (~8.5 g) was collected, as shown in Figure  1c. Cs0.33WO3 ink was prepared after wet grinding in a laboratory ball mill for 24 h at a speed of 800 rpm. Before milling, NPs (1.5 g) were mixed with a solution containing methoxy ethanol (30 mL) and KH-602 coupling agent (1.5 mL).

Preparation of Cs0.33WO3/Polymer Nanocomposites Coatings
Soda-lime glasses were used as substrates for coating fabrication. All the substrates were cleaned with alkaline detergent (RM10-07, Rigorous Co., Ltd., Shenzhen, China) and were consecutively immersed in an ultrasonic bath with de-ionized water before drying with nitrogen flux. Cs0.33WO3/polymer nanocomposites coating was fabricated utilizing an ultrasonic spray deposition apparatus, which was previously used in our study to prepare large-scale transparent conductive films and photovoltaic devices [21,22]. The spray solution consisted of dilute Cs0.33WO3 ink and PUA prepolymer. During the coating process, an optimized flow rate for carrier gas (N2) and solution spraying rate were held constant at 20 L min −1 and 0.15 mL min −1 , respectively. The wet coating was then pre-dried in vacuum at room temperature for 2 h. Finally, the solid Cs0.33WO3/polymer nanocomposites coating was formatted in a UV box, in which the PUA in nanocomposites polymerized completely.

Material Characterization and Device Testing
The crystal structure and composition were characterized via X-ray diffraction (XRD, AXS D8 Advance, Bruker, Karlsruhe, Germany) and energy-dispersive X-ray spectroscopy (EDS, Quantax 400, Bruker, Karlsruhe, Germany). The NPs and coating morphologies were characterized via scanning electron microscopy (SEM, Merlin, Zeiss, Oberkochen, Germany). The chemical composition and electronic structure of Cs0.33WO3 NPs were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, ThermoFisher, Waltham, USA). The core-level XPS multi-peaks were fitted using the Gauss multi-peak fitting method. During the fitting for W4f7/2 and W4f5/2, the interval between them was fixed as 2.1 eV. Optical transmittance spectra of the coating were obtained using a UV-Vis-IR spectrophotometer (Cary 5000, Agilent, Santa Clara, USA). The thickness of the coatings was measured using a stylus profile meter (Alpha-Step D-100, KLA-Tencor, Milpitas, USA).

Preparation of Cs 0.33 WO 3 /Polymer Nanocomposites Coatings
Soda-lime glasses were used as substrates for coating fabrication. All the substrates were cleaned with alkaline detergent (RM10-07, Rigorous Co., Ltd., Shenzhen, China) and were consecutively immersed in an ultrasonic bath with de-ionized water before drying with nitrogen flux. Cs 0.33 WO 3 /polymer nanocomposites coating was fabricated utilizing an ultrasonic spray deposition apparatus, which was previously used in our study to prepare large-scale transparent conductive films and photovoltaic devices [21,22]. The spray solution consisted of dilute Cs 0.33 WO 3 ink and PUA prepolymer. During the coating process, an optimized flow rate for carrier gas (N 2 ) and solution spraying rate were held constant at 20 L min −1 and 0.15 mL min −1 , respectively. The wet coating was then pre-dried in vacuum at room temperature for 2 h. Finally, the solid Cs 0.33 WO 3 /polymer nanocomposites coating was formatted in a UV box, in which the PUA in nanocomposites polymerized completely.

Material Characterization and Device Testing
The crystal structure and composition were characterized via X-ray diffraction (XRD, AXS D8 Advance, Bruker, Karlsruhe, Germany) and energy-dispersive X-ray spectroscopy (EDS, Quantax 400, Bruker, Karlsruhe, Germany). The NPs and coating morphologies were characterized via scanning electron microscopy (SEM, Merlin, Zeiss, Oberkochen, Germany). The chemical composition and electronic structure of Cs 0.33 WO 3 NPs were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, ThermoFisher, Waltham, USA). The core-level XPS multi-peaks were fitted using the Gauss multi-peak fitting method. During the fitting for W4f7/2 and W4f5/2, the interval between them was fixed as 2.1 eV. Optical transmittance spectra of the coating were obtained using a UV-Vis-IR spectrophotometer (Cary 5000, Agilent, Santa Clara, CA, USA). The thickness of the coatings was measured using a stylus profile meter (Alpha-Step D-100, KLA-Tencor, MI, USA).

Results and Discussions
In the mist-CVD method, the size of droplets of mist was very small (less than 20 µm), the drying and pyrolysis processes were very rapid, and the crystallization was hardly limited by space. Thus, monodispersed Cs 0.33 WO 3 could be easily precipitated from mist droplets. Figure 2a,b shows the SEM micrographs of mist-CVD prepared Cs 0.33 WO 3 before and after annealing. Both samples consist of microspheres with different diameters (approximately 2-5 µm). The sample before annealing exhibits a very smooth surface, reflecting its amorphous appearance. By contrast, the surface of annealed microspheres shows a rough appearance. A magnified image shows that the surface of annealed microspheres is constructed with angular crystal grains with an average size of about 30-40 nm.
Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 9 In the mist-CVD method, the size of droplets of mist was very small (less than 20 μm), the drying and pyrolysis processes were very rapid, and the crystallization was hardly limited by space. Thus, monodispersed Cs0.33WO3 could be easily precipitated from mist droplets. Figure 2a,b shows the SEM micrographs of mist-CVD prepared Cs0.33WO3 before and after annealing. Both samples consist of microspheres with different diameters (approximately 2-5 μm). The sample before annealing exhibits a very smooth surface, reflecting its amorphous appearance. By contrast, the surface of annealed microspheres shows a rough appearance. A magnified image shows that the surface of annealed microspheres is constructed with angular crystal grains with an average size of about 30-40 nm. In order to further study the inner structure of Cs0.33WO3, one microsphere was cut open via a focused ion beam (FIB). FIB-cut cross shows that the particles are compact solid balls piled up with crystal grains both inside and on the surface. The EDS mapping technique was employed to analyze the components by investigating the element distribution. Figure 3a demonstrates that the Cs, W, O signals are evenly distributed, indicating the presence of homogeneous chemical composition in asprepared product. The XRD pattern shows the exact crystal structure and composition information. The broad XRD pattern peak at 27.8° corresponding to Cs0.33WO3 (200) is observed for the sample before annealing, indicating its amorphous structure, which is in good agreement with SEM observation. By contrast, the sample annealed at 550 °C in Ar/H2 atmosphere shows good crystallinity, which exhibits hexagonal cesium tungsten oxide structure well matched with the standard PDF (No.83-1334), and no obvious impurity peaks can be observed in the pattern, indicating the as-prepared product consists of single-phase Cs0.33WO3 nanocrystals.  In order to further study the inner structure of Cs 0.33 WO 3 , one microsphere was cut open via a focused ion beam (FIB). FIB-cut cross shows that the particles are compact solid balls piled up with crystal grains both inside and on the surface. The EDS mapping technique was employed to analyze the components by investigating the element distribution. Figure 3a demonstrates that the Cs, W, O signals are evenly distributed, indicating the presence of homogeneous chemical composition in as-prepared product. The XRD pattern shows the exact crystal structure and composition information. The broad XRD pattern peak at 27.8 • corresponding to Cs 0.33 WO 3 (200) is observed for the sample before annealing, indicating its amorphous structure, which is in good agreement with SEM observation. By contrast, the sample annealed at 550 • C in Ar/H 2 atmosphere shows good crystallinity, which exhibits hexagonal cesium tungsten oxide structure well matched with the standard PDF (No.83-1334), and no obvious impurity peaks can be observed in the pattern, indicating the as-prepared product consists of single-phase Cs 0.33 WO 3 nanocrystals. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 9 In the mist-CVD method, the size of droplets of mist was very small (less than 20 μm), the drying and pyrolysis processes were very rapid, and the crystallization was hardly limited by space. Thus, monodispersed Cs0.33WO3 could be easily precipitated from mist droplets. Figure 2a,b shows the SEM micrographs of mist-CVD prepared Cs0.33WO3 before and after annealing. Both samples consist of microspheres with different diameters (approximately 2-5 μm). The sample before annealing exhibits a very smooth surface, reflecting its amorphous appearance. By contrast, the surface of annealed microspheres shows a rough appearance. A magnified image shows that the surface of annealed microspheres is constructed with angular crystal grains with an average size of about 30-40 nm. In order to further study the inner structure of Cs0.33WO3, one microsphere was cut open via a focused ion beam (FIB). FIB-cut cross shows that the particles are compact solid balls piled up with crystal grains both inside and on the surface. The EDS mapping technique was employed to analyze the components by investigating the element distribution. Figure 3a demonstrates that the Cs, W, O signals are evenly distributed, indicating the presence of homogeneous chemical composition in asprepared product. The XRD pattern shows the exact crystal structure and composition information. The broad XRD pattern peak at 27.8° corresponding to Cs0.33WO3 (200) is observed for the sample before annealing, indicating its amorphous structure, which is in good agreement with SEM observation. By contrast, the sample annealed at 550 °C in Ar/H2 atmosphere shows good crystallinity, which exhibits hexagonal cesium tungsten oxide structure well matched with the standard PDF (No.83-1334), and no obvious impurity peaks can be observed in the pattern, indicating the as-prepared product consists of single-phase Cs0.33WO3 nanocrystals.   XPS spectra of Cs 0.33 WO 3 nanocrystals before and after annealing in reducing atmosphere were obtained to further access compositional information, and the results are shown in Figure 4a-d. Clearly, there is no obvious difference in the XPS survey (Figure 4a) for samples before and after annealing, indicating no significant change in the composition of Cs 0.33 WO 3 during the annealing process. However, some variation could be found in core-level spectra. This shows that the main XPS peaks (including Cs3d, W4f, and O1s) of annealed sample monotonously shift to lower binding energies probably due to further pyrolysis and increasing crystallinity [23,24]. Core-level XPS of the unannealed sample shows two spin-orbit doublets, W4f 7/2 and W4f 5/2 , peaked at 35.8 and 37.9 eV, respectively. After annealing, the core-level XPS spectrum of W4f appears as a multi-peak feature. The curve can be fitted as two groups of spin-orbit doublets with W4f 7/2 and W4f 5/2 . The interval between them is fixed as 2.1 eV during the fitting. The peaks at 34.7 and 36.8 eV are ascribed to W4f 7/2 and W4f 5/2 of W 6+ . Their full widths at half maximum (FWHMs) are 1.29 and 1.67 eV, respectively. The other two peaks at 33.1 and 35.2 eV are attributed to W 5+ because of the lower Coulomb force. Their FWHMs are 0.99 and 0.82 eV, respectively. and W4f5/2 of W 6+ . Their full widths at half maximum (FWHMs) are 1.29 and 1.67 eV, respectively. The other two peaks at 33.1 and 35.2 eV are attributed to W 5+ because of the lower Coulomb force. Their FWHMs are 0.99 and 0.82 eV, respectively.
It is well known that the strong NIR shielding ability of cesium tungsten bronze stems from localized surface plasmon resonance induced by a high density of free electrons [25,26]. When it was annealed in reducing gas (H2), the solid-gas reaction could be elaborated as the following.

H . WO ≜ . WO Vö 2 H O
During the annealing process, a small part of the lattice oxygen could be extracted by H2, resulting in much oxygen vacancy and bringing a quantity of free electrons. Simultaneously, many W 6+ in Cs0.33WO3 would be reduced to W 5+ because of the loss of lattice oxygen. Thus, the increase of W 5+ is a substantial piece of evidence, illustrating that many free electrons may be introduced, and they could enhance the NIR shielding property. It was thus suggested that the unannealed samples contained only W 6+ , some of which were reduced to W 5+ after being annealed in the reduced atmosphere, and simultaneously brought many free electrons, resulting in significant enhancement of NIR shielding [4,6,11]. The O1s XPS spectra exhibit asymmetric line shapes (Figure 4d). The peak with lower binding energy (528.2 eV) and a FWHM of 1.27 eV corresponds to lattice oxygen (LO) in the stoichiometric WO3 phase (O atoms binding with W 6+ ). The second peak, at 530.8 eV (FWHM ~1.95 eV), may originate from surface O-H states or non-lattice oxygen (NLO) in the nonstoichiometric phase (O atoms binding with W 5+ ), which is in agreement with W4f XPS peaks [27,28].  It is well known that the strong NIR shielding ability of cesium tungsten bronze stems from localized surface plasmon resonance induced by a high density of free electrons [25,26]. When it was annealed in reducing gas (H 2 ), the solid-gas reaction could be elaborated as the following.
x H 2 + Cs 0.33 WO 3 Cs 0.33 WO 3−x + x Vo + 2x e + x H 2 O During the annealing process, a small part of the lattice oxygen could be extracted by H 2 , resulting in much oxygen vacancy and bringing a quantity of free electrons. Simultaneously, many W 6+ in Cs 0.33 WO 3 would be reduced to W 5+ because of the loss of lattice oxygen. Thus, the increase of W 5+ is a substantial piece of evidence, illustrating that many free electrons may be introduced, and they could enhance the NIR shielding property. It was thus suggested that the unannealed samples contained only W 6+ , some of which were reduced to W 5+ after being annealed in the reduced atmosphere, and simultaneously brought many free electrons, resulting in significant enhancement of NIR shielding [4,6,11]. The O1s XPS spectra exhibit asymmetric line shapes (Figure 4d). The peak with lower binding energy (528.2 eV) and a FWHM of 1.27 eV corresponds to lattice oxygen (LO) in the stoichiometric WO 3 phase (O atoms binding with W 6+ ). The second peak, at 530.8 eV (FWHM 1.95 eV), may originate from surface O-H states or non-lattice oxygen (NLO) in the nonstoichiometric phase (O atoms binding with W 5+ ), which is in agreement with W4f XPS peaks [27,28].
To realize application in NIR shielding, Cs 0.33 WO 3 /PUA composite coatings were prepared via the ultrasonic spray-coating technology we previously used, and its sketch is shown in Figure 5a. Assisted by the automatic X-Y table, herein, a larger-scale coating with uniform thickness was prepared. For convenience, the Cs 0.33 WO 3 /PUA composite coatings were prepared with a consistent thickness, in which the concentration of Cs 0.33 WO 3 was varied. The spray solution consisted of PUA prepolymer, ethoxyethanol, and Cs 0.33 WO 3 ink. The ink was prepared by wet grinding using annealed Cs 0.33 WO 3 with methoxyethanol and KH-602 coupling agent with the solid content of approximately 5%. Figure 5b shows a photograph of ink, exhibiting very long-term dispersion stability (as long as several months). In order to study the particles in the ink, a drop of ink was deposited on glass and dried for investigation. SEM micrographs (Figure 5c) show that this ink consisted of nanocrystals with an average size of 30-40 nm. This size is consistent with the size of crystals shown in Figure 2c, indicating that the Cs 0.33 WO 3 microspheres were loose and could be crushed to nanoscale particles after milling. By using the spray solution, Cs 0.33 WO 3 nanocrystals could be well dispersed in PUA prepolymer after UV curing. In order to study the NIR shielding property, a 5-µm-thick Cs 0.33 WO 3 /PUA composite coating was prepared in a soda-lime glass with the variation in the concentration of Cs 0.33 WO 3 from 0.3 to 1.5 g m −2 , and their photographs are shown in Figure 5d. Figure 5e illustrates their transmittance spectra in the range of 300-2800 nm. The difference between soda-lime glass and pure PUA (5-µm-thick) coated glass is very small. Either of them exhibits a high transmittance (~90%) in the visible and NIR regions, showing both glass and PUA have no obvious NIR shielding ability. The 5-µm-thick composite layer with only a small amount of Cs 0.33 WO 3 (0.3 g/m 2 ) on glass shows an obvious NIR shielding property, i.e., it is highly transparent (~83%) in the visible range, while a much lower transmittance (66%) in NIR regions is witnessed. When the concentration of Cs 0.33 WO 3 was increased from 0.3 to 0.9 mg m −2 , the visible transmittance decreased to 74% and NIR transmittance sharply decreased to below 30%. When the concentration was increased to 1.5 mg m −2 , the NIR transmittance was below 17%, while the visible transmittance was still higher than 65%, indicating its great IR shielding property and showing a good application prospect in energy-saving buildings.
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 9 To realize application in NIR shielding, Cs0.33WO3/PUA composite coatings were prepared via the ultrasonic spray-coating technology we previously used, and its sketch is shown in Figure 5a. Assisted by the automatic X-Y table, herein, a larger-scale coating with uniform thickness was prepared. For convenience, the Cs0.33WO3/PUA composite coatings were prepared with a consistent thickness, in which the concentration of Cs0.33WO3 was varied. The spray solution consisted of PUA prepolymer, ethoxyethanol, and Cs0.33WO3 ink. The ink was prepared by wet grinding using annealed Cs0.33WO3 with methoxyethanol and KH-602 coupling agent with the solid content of approximately 5%. Figure 5b shows a photograph of ink, exhibiting very long-term dispersion stability (as long as several months). In order to study the particles in the ink, a drop of ink was deposited on glass and dried for investigation. SEM micrographs (Figure 5c) show that this ink consisted of nanocrystals with an average size of 30-40 nm. This size is consistent with the size of crystals shown in Figure 2c, indicating that the Cs0.33WO3 microspheres were loose and could be crushed to nanoscale particles after milling. By using the spray solution, Cs0.33WO3 nanocrystals could be well dispersed in PUA prepolymer after UV curing. In order to study the NIR shielding property, a 5-μm-thick Cs0.33WO3/PUA composite coating was prepared in a soda-lime glass with the variation in the concentration of Cs0.33WO3 from 0.3 to 1.5 g m −2 , and their photographs are shown in Figure 5d. Figure  5e illustrates their transmittance spectra in the range of 300-2800 nm. The difference between sodalime glass and pure PUA (5-μm-thick) coated glass is very small. Either of them exhibits a high transmittance (~90%) in the visible and NIR regions, showing both glass and PUA have no obvious NIR shielding ability. The 5-μm-thick composite layer with only a small amount of Cs0.33WO3 (0.3 g/m 2 ) on glass shows an obvious NIR shielding property, i.e., it is highly transparent (~83%) in the visible range, while a much lower transmittance (66%) in NIR regions is witnessed. When the concentration of Cs0.33WO3 was increased from 0.3 to 0.9 mg m −2 , the visible transmittance decreased to 74% and NIR transmittance sharply decreased to below 30%. When the concentration was increased to 1.5 mg m −2 , the NIR transmittance was below 17%, while the visible transmittance was still higher than 65%, indicating its great IR shielding property and showing a good application prospect in energy-saving buildings. To demonstrate the potential application of Cs0.33WO3/PUA composite coatings, a homemade model (Figure 6a) including two individual independent rooms that were, respectively, installed with bare glass and Cs0.33WO3/PUA composite coatings-coated glass was used to carry out the simulation To demonstrate the potential application of Cs 0.33 WO 3 /PUA composite coatings, a homemade model (Figure 6a) including two individual independent rooms that were, respectively, installed with bare glass and Cs 0.33 WO 3 /PUA composite coatings-coated glass was used to carry out the simulation experiment of exposure to irradiation. Cs 0.33 WO 3 nanocrystals were well dispersed in PUA; therefore, most of the IR photons could be scattered and absorbed after reflection or refraction several times by plasmon resonance of free electrons [11,29,30], as shown in Figure 6b. Figure 6c shows the time-temperature curve for the window using glass with different concentrations of Cs 0.33 WO 3 radiated by 100 W heat lamps. It was found that the temperature of the room using the Cs 0.33 WO 3 /PUA composite coatings-coated window increased much slower than that using a bare soda-lime glass. When a bare soda-lime glass was used, after 150 s, room temperature reached up to 44.4 • C, while the temperature using 0.3, 0.6, 0.9, and 1.5 mg m −2 Cs 0.33 WO 3 was only 39.3, 36.6, 33.2, and 30.8 • C, respectively, showing its great heat insulation properties. Figure 6d shows the time-temperature curve of cooling to ambient temperature after the room temperature reached 50 • C. The temperature of the room with the Cs 0.33 WO 3 /PUA composite-coated window also decreased more slowly than that using bare glass. After 150 s, room temperature rapidly decreased to 34.2 • C, while the temperature using 0.3, 0.6, 0.9, and 1.5 mg m −2 Cs 0.33 WO 3 remained at 35.9, 37.4, 38.4, and 39.8 • C, respectively, indicating its really good heat insulation properties and good application prospect in heat insulation windows.
Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 9 experiment of exposure to irradiation. Cs0.33WO3 nanocrystals were well dispersed in PUA; therefore, most of the IR photons could be scattered and absorbed after reflection or refraction several times by plasmon resonance of free electrons [11,29,30], as shown in Figure 6b. Figure 6c shows the timetemperature curve for the window using glass with different concentrations of Cs0.33WO3 radiated by 100 W heat lamps. It was found that the temperature of the room using the Cs0.33WO3/PUA composite coatings-coated window increased much slower than that using a bare soda-lime glass. When a bare soda-lime glass was used, after 150 s, room temperature reached up to 44.4 °C, while the temperature using 0.3, 0.6, 0.9, and 1.5 mg m −2 Cs0.33WO3 was only 39.3, 36.6, 33.2, and 30.8 °C, respectively, showing its great heat insulation properties. Figure 6d shows the time-temperature curve of cooling to ambient temperature after the room temperature reached 50 °C. The temperature of the room with the Cs0.33WO3/PUA composite-coated window also decreased more slowly than that using bare glass. After 150 s, room temperature rapidly decreased to 34.2 °C, while the temperature using 0.3, 0.6, 0.9, and 1.5 mg m −2 Cs0.33WO3 remained at 35.9, 37.4, 38.4, and 39.8 °C, respectively, indicating its really good heat insulation properties and good application prospect in heat insulation windows.

Conclusions
Herein, a single-phase Cs0.33WO3 was successfully prepared via the novel mist chemical vapor deposition method. Thermal annealing in H2 and Ar mixed atmosphere could partly reduce W 6+ to W 5+ , as well as induce many free electrons, which could scatter and absorb near-infrared photons by plasmon resonance. The Cs0.33WO3 nanocrystals were easily prepared as long-term stable ink, which was used for the fabrication of Cs0.33WO3/PUA composite coatings via spray deposition. This composite coating exhibited a near-infrared shielding and thermal insulation performance with highly visible light transmittance. A small amount of Cs0.33WO3 could shield the most near-infrared, e.g., 0.9 mg cm −2 Cs0.33WO3 could shield more than 70% of near-infrared, while keeping the visible transmittance still higher than 75%. A practical test using a homemade model exhibited its good

Conclusions
Herein, a single-phase Cs 0.33 WO 3 was successfully prepared via the novel mist chemical vapor deposition method. Thermal annealing in H 2 and Ar mixed atmosphere could partly reduce W 6+ to W 5+ , as well as induce many free electrons, which could scatter and absorb near-infrared photons by plasmon resonance. The Cs 0.33 WO 3 nanocrystals were easily prepared as long-term stable ink, which was used for the fabrication of Cs 0.33 WO 3 /PUA composite coatings via spray deposition. This composite coating exhibited a near-infrared shielding and thermal insulation performance with highly visible