The Cs _xWO₃ powders were prepared according to our previous studies ^{[21] and [22]}. Firstly, a precursor solution with appropriate citric acid concentration and n(Cs): n(W) = 0.3:1 was prepared by using sodium tungstate and cesium carbonate as raw materials. Then the precursor solution was transferred into a Teflon-lined autoclave and heated at 190 °C for 72 h, and finally pure Cs _xWO₃ powders could be obtained after being washed with water and ethanol three times, respectively and dried at 80 °C overnight. In this work, Cs _xWO₃ powders were prepared in the precursor solution with the concentration of citric acid being 0.55 mol/L and 1.1 mol/L, respectively.

Then the as-prepared Cs _xWO₃ powders with 1.1 mol/L citric acid were annealed in the N₂ atmosphere in a tubular furnace at 500–800 °C for 30–120 min with the flow of N₂ at 400 mL/min.

The phase composition of the samples was determined by X-ray diffraction analysis (XRD, D/max-3B, Japan). The morphology of the samples was observed by scanning electron microscopy (SEM, JEOL JSM-6460LV, Japan).

The diffuse reflectance spectra (DRS) of Cs _xWO₃ powders were characterized by using a Lambda 35 ultraviolet visible (UV–Vis) spectrophotometer at 300–1100 nm range with BaCl₂ as background. In order to evaluate the NIR shielding properties, the Cs _xWO₃ powders were dispersed in the polyvinyl alcohol (PVA) solution with the concentration of 0.1 g/mL. Then the dispersion solution was coated on the glass plate for two layers. The transmittance spectra of the glass plate coated with Cs _xWO₃ film were measured by PerKin Elmer Lambda 950 UV–Vis-NIR spectrophotometer at 300–2500 nm range.

The heat insulation of Cs _xWO₃ film was evaluated by using an insulated box (10 cm × 5 cm × 10.5 cm) made from foam board. There was a rectangular window (5 cm × 10 cm) on the top of the box. The bottom of the box was covered with a black iron plate. Red water thermometer was contacted closely with the black iron plate. Put the glass coated with Cs _xWO₃ film on the window of the insulated box and record the interior temperature when the box was irradiated by a 250 W infrared lamp. Then the curves of temperature increment dependence on irradiation time could be obtained. The heat insulation of Cs _xWO₃ film glass was evaluated by comparison with that of the blank glass.

Fig. 1 shows the XRD patterns of Cs _xWO₃ powders prepared by hydrothermal reactions in the precursor solution with different concentrations of citric acid. It is found that the diffraction patterns of Cs _xWO₃ powders prepared from the precursor solution with different concentrations of citric acid agree well with that of hexagonal cesium tungsten bronze Cs_0.32WO₃ (JCPDS 83-1334). It is obvious that the diffraction peaks become stronger with increasing citric acid content, which indicates that higher concentration of citric acid is favorable for obtaining higher crystallinity of Cs_0.32WO₃.

Fig. 1.

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	Fig. 1. XRD patterns of Cs xWO3 powders prepared by hydrothermal reaction from precursor solutions with different concentrations of citric acid: (a) 0.55 mol/L, (b) 1.10 mol/L.

As is known that the valence state of tungsten (W) in the tungsten oxide (WO₃) is hexavalent (+6), while the valence state of tungsten in the cesium tungsten bronze (Cs _xWO₃) should be lower than the hexavalent (+6). Thus reduction atmosphere in the reaction system is necessary for the synthesis of Cs _xWO₃. In this study, citric acid, an organic carboxylic acid containing one hydroxyl and three carboxyl groups, was chosen as reducing agent to provide the reduction atmosphere.

Fig. 2 shows the UV–Vis absorption spectra of Cs _xWO₃ powders prepared by hydrothermal reaction with different concentrations of citric acid. The as-prepared Cs _xWO₃ powders have a high absorption in the NIR region, and the Cs _xWO₃ powders prepared from the precursor solution with 1.1 mol/L citric acid exhibit higher absorption in the NIR region than Cs _xWO₃ prepared with 0.55 mol/L citric acid. It was reported that Cs _xWO₃ particles have higher NIR light absorption property, which is the main factor for achieving better NIR shielding ability ^[20].

Fig. 2.

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	Fig. 2. UV–Vis absorption spectra of Cs xWO3 powders prepared by hydrothermal reaction with different concentrations of citric acid: (a) 0.55 mol/L, (b) 1.10 mol/L.

Fig. 3 shows the XRD patterns of Cs _xWO₃ powder after N₂ annealing at different temperatures for 2 h. The diffraction peaks of (102) crystal plane become more obvious and stronger with the increase of N₂ annealing temperature, which indicates that the Cs_0.32WO₃ crystals can grow more completely with the increase of N₂ annealing temperature. However, after N₂ annealing at 800 and 900 °C, some tiny diffraction peaks also appear. This result indicates that too high annealing temperature may lead to the formation of other impurity crystal phase.

Fig. 3.

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	Fig. 3. XRD patterns of Cs xWO3 powders after N2 annealing at different temperatures for 2 h.

Fig. 4 shows the SEM images of Cs _xWO₃ powders after N₂ annealing at different temperatures. The Cs _xWO₃ particle size becomes bigger with the increase of N₂ annealing temperature. Particularly, the 800 °C-annealed samples exhibit micrometer-sized crystalline grains.

Fig. 4.

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	Fig. 4. SEM images of Cs xWO3 powders after N2 annealing at different temperatures: (a) no annealing, (b) 500 °C for 2 h, (c) 700 °C for 2 h, (d) 800 °C for 30 min.

Fig. 5 shows the transmittance spectra of Cs _xWO₃ synthesized from the precursor solution with 1.10 mol/L citric acid and followed by N₂ annealing at different temperatures for 2 h. In comparison with the Cs _xWO₃ products without annealing, the 500 °C-annealed Cs _xWO₃ products exhibit the best visible light transmittance and the highest NIR shielding ability, whereas the Cs _xWO₃ products annealed at 600 and 700 °C exhibit higher NIR shielding ability but lower transmittance in the visible light. In addition, the NIR shielding properties decrease again when the N₂ annealing temperature is higher than 800 °C. The Cs _xWO₃ products after N₂ annealing at 800 °C exhibit lower NIR shielding ability in spite of its excellent transparency in the visible light. Thus it can be concluded that the Cs _xWO₃ particles after N₂ annealing at 500 °C exhibit the best visible light transmittance and the highest NIR shielding properties.

Fig. 5.

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	Fig. 5. Transmittance spectra of Cs0.3WO3 films, in which the Cs xWO3 powders were annealed in the N2 atmosphere at different temperatures for 2 h.

In addition, it is found that a new inflexion appears in the curves at wavelength 330–380 nm after N₂ annealing at 600–800 °C, which indicates that the microstructure of Cs _xWO₃ may have been changed and re-oxidation of Cs _xWO₃ may be possible by N₂ annealing at higher temperature. Moreover, too high annealing temperature may lead to the quick growth of the Cs _xWO₃ crystals and thus the NIR absorption ability decreases again. It is reasonable that the particle size and surface morphology of the Cs _xWO₃ particles have effects on the visible light transparency and NIR shielding properties.

The elemental valence of the Cs _xWO₃ before and after N₂ annealing was investigated by X-ray photoelectron spectroscopy (XPS). Fig. 6(a) shows the W_{4 f} XPS profiles of Cs _xWO₃ before and after N₂ annealing at different temperatures. It is obvious that the two peaks in the W_{4 f} show a shift to lower binding energies after N₂ annealing, which indicates that the chemical state of W has been changed due to the partial reduction of W⁶⁺ to W⁵⁺. In order to further explain the chemical state of W in Cs _xWO₃, the W_{4 f} XPS spectra of Cs _xWO₃ after N₂ annealing at 700 °C is shown in Fig. 6(b). The W_{4 f} core-level spectrum can be fitted to two spin-orbit doublets, which are associated with the two different oxidation states of W atoms. As was reported by Guo et al. ^[23], the main peaks, with W_{4 f5/2} at 37.9 eV and W_{4 f7/2} at 35.8 eV, are attributed to the W atoms in the 6⁺ oxidation state, and the second doublet with a lower binding energy at 34.6 and 36.7 eV is characteristic of W atoms in the 5⁺ oxidation state. It is suggested that the non-stoichiometric cesium tungsten bronzes Cs _xWO₃ are reduced compounds, which can also be expressed as the formula Cs _xW _x⁵⁺W_{1– x}⁶⁺O₃, while the ideal oxidized stoichiometric cesium tungsten oxides should be expressed as a formula of Cs _xWO_{3+ x/2}. According to the energy band and the free electron theory, the visible light transparency and NIR shielding properties of Cs _xWO₃ should be related with the plasmon resonance of free electrons, the interband transition and small polarons. Whereas, the concentration of free carrier and oxygen vacancies have great effects on the plasmon resonance frequency of free electrons and the band gap energy, which will directly affect the starting wavelength of the NIR shielding and visible light transparency properties. So, the visible light transparency and NIR shielding properties of Cs _xWO₃ could be further improved by adjusting and controlling the concentration of free carriers in its microstructure.

Fig. 6.

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	Fig. 6. W4 f XPS profiles of the as-synthesized Cs xWO3 after N2 annealing at: (a) different temperatures, (b) 700 °C for 2 h.

Fig. 7 shows the heat insulation curves of Cs _xWO₃ films, which were prepared from the dispersions of Cs _xWO₃ powders after N₂ annealing at different temperatures. The heating rates of glass coated with Cs _xWO₃ products all are obviously lower than that of the blank glass, which indicates that the Cs _xWO₃ films have excellent heat insulation performance. In particular, the 500 °C-annealed Cs _xWO₃ products in the N₂ atmosphere exhibit the best heat insulation, and the highest heat insulation temperature difference between the blank glass and the Cs _xWO₃ film glass could reach 21 °C. Obviously, the excellent heat insulation of the 500 °C-annealed Cs _xWO₃ products is closely associated with its excellent NIR shielding properties.

Fig. 7.

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	Fig. 7. Heat insulation curves of Cs xWO3 films, in which the Cs xWO3 powders was synthesized with 1.10 mol/L citric acid and followed by annealing at different temperatures for 2 h in the N2 atmosphere.