Electrochromic properties of epitaxial WO^sub 3^ thin films grown on sapphire substrates

Electrochromic properties are reported for monoclinic (001) WO3 epitaxial films grown on r-plane sapphire substrates. By applying a positive bias in 10 mM aqueous solution of sulfuric acid, an immediate increase in optical reflectance in a near-infrared region and a subsequent increase in optical absorbance in a visible to near-infrared region were generated by forming a hydrate and a tungsten bronze, respectively. The tungsten bronze disappeared after applying a negative bias in the solution, while the hydrate remained and required an annealing in air to disappear. These transitions were reversibly and repeatedly observed despite the restriction by the substrate.