Synthesis of Cs_0.3WO₃ with visible transparency and near-infrared absorption from commercial WO₃

In recent years, hexagonal cesium tungsten bronze, M_xWO₃ (M ​= ​Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺; 0 < x ​< ​1), nanoparticles have attracted much attention for its specific visible transparency and infrared absorption property. It is usually synthesized by wet-chemistry methods, most of which use homogeneous solution systems, also use WO₃ colloid systems. The difference between the two routes is the increase in the size of the solute particles in the wet reaction systems. If the size of the solute continues to increase, e.g., using a larger size of commercial WO₃ as a raw material. Could tungsten bronze still be synthesized even under this heterogeneous condition? In this study, a series of reaction systems were conducted, and a single-phase Cs_0.3WO₃ powder with hexagonal structure and without NH₄⁺ doping was hydrothermally synthesized using CsCl, N₂H₄·H₂O and commercial WO₃ at 200 ​°C. Then again, Cs_0.3WO₃ was also synthesized by using CsCl, NH₄OH, C₃H₆O and commercial WO₃ to study the mechanism. These suggest that Cs_0.3WO₃ is not formed in-situ on WO₃ solid, but a dissolution-recombination route related with the nucleation, growth, and agglomeration. UV–Vis–NIR and FT-IR measurements show that Cs_0.3WO₃ is transparent in visible region of 380 − 780 ​nm and absorptive in infrared region of 780 − 10000 ​nm.