Temperature-dependent structural evolution and electrochromic properties of peroxopolytungstic acid

Temperature-dependent in situ W L-3-edge X-ray absorption spectroscopy has been performed at 20-500 degreesC to investigate the geometric environment and electronic structure of tungsten upon the transformation of electrochro mic peroxopolytungstic acid to tungsten trioxide. The temperature-dependent variation of the coordination number and bond distance of the nearest (W-O ) sphere reveals that the peroxopolytungstate changes into crystalline tung sten oxide through the gradual removal of peroxo species, zeolitic water, h ydroxyl groups, and terminal W-O bonds, which is also confirmed by thermal analysis, powder XRD, and FT-IR spectroscopy. It is especially emphasized t hat the intermediate structure of peroxopolytungstate at 120-200 degreesC i s characterized by a substantial number of oxygen vacancies together with a partially formed oxide lattice. A thin film of peroxopolytungstic acid has been prepared on a transparent conducting substrate and post-annealed at 1 00-300 degreesC. From cyclic voltammetry and optical density measurements i n a LiClO4-propylene carbonate electrolyte, it was found that the voltammet ric exchange charges are maximized for films annealed at 150-200 degreesC, whereas the voltage response of coloration/bleaching is depressed by the in crease in annealing temperature. Considering both the electrochromic kineti cs and redox capacity, the optimum annealing temperature for the formation of WO3 film was determined to be 120-200 degreesC. Such results could be we ll explained in terms of the structural evolution of peroxopolytungstic aci d on the basis of X-ray absorption spectroscopic analysis.