Abstract
Temperature-dependent in situ W L3-edge X-ray
absorption spectroscopy has been performed at 20–500 °C to
investigate the geometric environment and electronic structure of tungsten
upon the transformation of electrochromic 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 tungsten oxide through the gradual
removal of peroxo species, zeolitic water, hydroxyl groups, and terminal WO
bonds, which is also confirmed by thermal analysis, powder XRD, and FT-IR
spectroscopy. It is especially emphasized that the intermediate structure
of peroxopolytungstate at 120–200 °C is 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 100–300 °C.
From cyclic voltammetry and optical density measurements in a LiClO4–propylene
carbonate electrolyte, it was found that the voltammetric exchange charges
are maximized for films annealed at 150–200 °C, whereas the
voltage response of coloration/bleaching is depressed by the increase
in annealing temperature. Considering both the electrochromic kinetics and
redox capacity, the optimum annealing temperature for the formation of WO3
film was determined to be 120–200 °C. Such results could
be well explained in terms of the structural evolution of peroxopolytungstic
acid on the basis of X-ray absorption spectroscopic analysis.