Electrodeposited oxotungstate films: Towards the molecular nature of recharging processes

In situ Raman spectroscopy is applied to supplement voltammetric, spectroelectrochemical, and XRD data on redox transformations of electrodeposited oxotungstate films. These films undergo electrochromic transition at rather positive potentials, as compared to usual sputtered tungsten oxides. The depth of electroreduction for the films conditioned in acidic solutions under open circuit is about 0.11 e− per W atom. Coloration of the films correlates with the decrease of Raman band, corresponding to the terminal W(VI)O vibration in the hydrated phase of highly defective tungstic acid (hydrated tungsten oxide). Our data allow to state the absence of oxotungstate octahedra rearrangement in the course of reduction at positive RHE potentials, and to assume that slightly deeper reduction up to 0.15 W(V)/[W(V) + W(VI)] ratio is possible at more negative potentials. We also demonstrate that the gas phase reduction is less reversible as compared to electrochemical reduction in solution. The most possible nature of films degradation in the gas phase is their partial dehydration in the course of reduction.