Cationic Defect Engineering for Controlling the Infrared Absorption of Hexagonal Cesium Tungsten Bronze Nanoparticles

Cesium tungsten bronzes (Cs 0.32 WO 3 ) have attracted much attention as a near-infrared absorbing material. We report the successful synthesis of highly crystalline and high purity Cs 0.32 WO 3 nanoparticles through a spray pyrolysis route. Careful analyses disclosed the presence of cationic defects, that is, a tungsten deficiency and insufficient Cs doping in the Cs 0.32 WO 3 nanoparticles. These cationic defects can be controlled by facile heat treatment in a mildly reducing atmosphere. In particular, we clarify that the tungsten deficiency is a key factor among the cationic defects to obtain high near-infrared absorption properties. Furthermore, this study clearly demonstrates the precise tunability of the optical properties by means of the lattice constants of the Cs 0.32 WO 3 crystal. The realized range of lattice constants is significantly wider than those previously reported. These findings should contribute to the engineering of Cs 0.32 WO 3 structure and properties.