Estimation of the composition parameter of electrochemically colored amorphous hydrogen tungsten oxide films

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Abstract
The electrical and optical steady state observed in electrochemical coloration has been studied using asymmetric cells consisting of evaporated amorphous tungsten oxide films with 350–6000 Å thickness. The counter electrode used is indium wire, steel wire, or antimony‐tin oxide film, and the electrolyte is a 1‐N H2SO4 aqueous solution containing 10 vol % glycerol. The current and optical transmittance of the cells decrease with increasing time during coloration, and simultaneously reach a steady state. The optical density (λ=0.5 μm) in the steady state is proportional to the thickness of the tungsten oxide film, and the absorption coefficient at λ=0.5 μm of the colored oxide film in the state is approximately 9.0×104 cm−1. The effective charges which contribute to the coloration of films calculated from the charge injected until the electro‐optical steady state were found to be 1.03–1.20×103 C/cm3. Assuming that the evaporated tungsten oxide films used have a distorted ReO3 structure, and that a hydrogen tungsten bronze HxWO3 is formed by coloration, the composition parameter x calculated from the average value of the effective charge, is 0.36, which is comparable with that of hydrogen tungsten bronze H0.33WO3 obtained for the colored crystalline WO3 films.