According the formula Cs_xWO₃ (0 <x <0.33) of cesium tungsten bronze, the tungsten is in the reduced state in Cs_xWO₃  system, and low-valence tungsten ions increases with the increase of cesium ions,, which shows a reducing atmosphere is required during the Cs_xWO₃  synthesis and the reducing atmosphere for reducing the is required more with the increasing incorporation of cesium ions. In this experiment, use citric acid and ethanol as the reducing atmosphere, carboxyl or hydroxyl group of citric acid and ethanol can be oxidized into the carbon dioxide or carboxyl at high temperature and pressure, and W6 ⁺ is reduced to W4 ⁺ or W5 ⁺, Cs ions doped into the tungsten bronze structure to get Cs_xWO₃ .

Figure 3 exhibits the properties of NIR absorption with different annealing temperature. These results indicated that the NIR absorption properties sensitively depended on the annealing temperature of Cs_0.33WO₃ particle. As above mentioned, it means that the effective absorption of NIR range for Cs_0.33WO₃ composite films increase with increasing content of hexagonal tungsten bronze structure of Cs_0.33WO₃ . This is consistent well with the XRD profiles of the samples shown in XRD patterns of Cs_xWO₃  particles synthesized in different annealing temperature.

 

Figure 4 shows the properties of NIR absorption of Cs_xWO₃  with annealing at 800 ℃. As the atomic ratio (from 0.33 to 1.00) of Cs_xWO₃  increased, only NIR transmittance of Cs_0.33WO₃ and Cs_0.55WO₃  significantly was decreased while the other sample is not changes the NIR transmittances. As above mentioned, this result implied that only Cs_0.33WO₃  was formed the typical hexagonal tungsten bronze structure with superior absorption of NIR wavelength.