The figure is the diffuse reflectance spectroscopy of cesium tungsten bronze (CsxWO3) powder synthesis in different hydrothermal reaction time. The figure shows, with the extension of reaction time, the absorbance of CsxWO3 gradually increases in at visible and near-infrared short-wave region; the absorbance lowest point appears at a wavelength of 450 nm, with high absorbance in the UV and short wave near infrared region, it showed good performance on UV absorption and short-wave near-infrared absorption properties. High absorption in the direction of the UV edge is because of CsxWO3 intrinsic absorption, which is mainly determined by the band gap of CsxWO3; and absorption in the visible and short wave near infrared may be related to free carrier absorption in CsxWO3 system. In general, the higher the concentration of free carriers, the more obvious the free carrier absorption. It’s reported in the literature that CsxWO3 is a low resistivity material, there are more free carriers in the system, and thus there is a strong free carrier absorption and free carrier absorption coefficient has following relationship to the free carrier concentration and the wavelength of light absorbed:

 

N is the refractive index of medium, e0 is the vacuum permittivity, the carrier effective mass, m is the magnetic permeability, μ is the wavelength of the incident light, n is the free carrier concentration. Free carrier absorption coefficient α and the square of the wavelength and irradiation free carrier concentration n is proportional, so at wavelengths of greater than 450 nm, the CsxWO3 absorbance increase with increasing wavelength, it shows strong absorption properties in the near infrared shortwave. The increasing hydrothermal reaction time contributes to a thorough reaction and the formation of complete crystals, and in the mean while, may increase the content of the cesium ions into the tungsten bronze, which improved carrier concentration of CsxWO3 system, so its absorbance in the visible and short wave near infrared region gradually increases with the reaction time.