Preparation and electrochromism of pyrochlore-type tungsten oxide film

Pyrochlore-type WO₃ powder was synthesized via hydrothermal method using aqueous sodium tungstate solution and oxalic acid as raw materials. The as-prepared powder was made into a soliquoid, from which films were made by dip coating process with indium–tin oxide (ITO). The obtained films were characterized by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), chronoamperometry (CA) and ultraviolet–visible (UV–Vis) absorption. Results show that the crystal of the pyrochlore-type WO₃ powder is perfect. When the calcination temperature rises from room temperature to 500 °C, the pyrochlore-type structure first becomes deformed, then it is destroyed and turns into amorphous phase, finally it will completely convert to WO₃ with a monoclinic structure. Electrochemical and optical tests demonstrate that the film calcined at 300 °C exhibits the best electrochromic performance and has a coloration efficiency of up to 68.5 cm²·C⁻¹ at 884 nm.