Cathodic electrocrystallization and electrochromic properties of doped rechargeable oxotungstates

We extend the technique of preparing oxotungstate thin films by cathodic electrocrystallization from metastable acidic solutions to fabricate vanadium- and molybdenum-doped films on platinum and fluorine-doped tin oxide (FTO). By means of cyclic voltammetry, we demonstrate a wider potential interval of highly reversible redox activity for these doped films, as compared to purely tungstate films. Electrochromic behavior in this region is dependent on the nature of the doping element. We characterize the microstructure of these films using scanning tunneling and atomic force microscopy. We also apply Raman and UV–vis spectroscopy to characterize the acidic deposition solutions. In tungstate–vanadate bath, we observe evidence of mixed isopolyanions. We also found some features of less stable mixed anions in the tungstate–molybdate bath, characterized phase composition of these films by means of XRD, and explain the results in the terms of parallel deposition from coexisting molecular precursors.