Amorphous Tungstate Precursor Route to Nanostructured Tungsten Oxide Film with Electrochromic Property

Electrochromic tungsten oxide (WO₃) films on ITO glass were fabricated by spin-coating with a tungsten peroxy acid solution, which was prepared by adding an equivolume mixture of hydrogen peroxide and glacial acetic acid to tungsten metal powder. The structural evolution of the tungstate precursor upon heat treatment was studied by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) analyses, which indicated that the as-synthesized tungstate transformed into nanocrystalline WO₃ upon heating. It is, therefore, quite clear that as-synthesized tungstate can be a good precursor for electrochromic WO₃ films. A series of WO₃ thin films were prepared on ITO glass by spin-coating with different concentrations of tungsten peroxy acid solution and then post-annealing at various temperatures. Depending on the concentration of the tungstate coating solution (200∼500 mg mL⁻¹ and the annealing temperature (100∼300 °C), the thickness and WO₃ content as well as the electrochromic properties of WO₃ films can be controlled. As a result, the optimum fabrication conditions were determined to be a tungstate solution concentration of 300∼400 mg mL⁻¹ and a post-annealing temperature of 200 °C. Finally, an inorganic–inorganic hybrid electrochromic device (ECD) composed of optimized WO₃ and Prussian Blue (PB) with desirable coloration efficiency was successfully developed.