Raman spectroscopic evidence of the bronze-like recharging behavior for conducting films deposited from isopolytungstates

In situ Raman spectroscopy (spectral regions 200-1700 cm−1 and 2800-3800 cm−1) at various potentials is applied to study the oxidation states of tungsten in oxotungstate films electrodeposited on Pt support from metastable Na2WO4 + 0.5 M H2SO4 solution. Comparison of in situ and ex situ spectra confirms that the films under study consist of the hydrated tungsten bronze (HxWO3, or HxWxvW(1−x)VIO3) and undergo quasi-reversible dehydration in air. Cyclic voltammetry (CV) and scanning tunnelling microscopy (STM) are applied to demonstrate the difference of freshly deposited and aged films. The pronounced changes of electrochemical properties and morphology found by these means are not accompanied by the changes in potential-dependent spectral behavior. The most pronounced effects of potential on Raman intensities are observed in the region of 0.05-0.60 V (reversible hydrogen electrode, RHE) and correspond to the decrease in WV content (i.e. x decrease) with potential. The behavior of bands at 2900-3800 cm−1 confirms that this process is accompanied by deprotonation. There is no any WV in the films at E > 0.60 V, which region corresponds to the thermodynamically stable hydrated WO3. The experimental coulometric data are considered in the framework of the regular solution model and are applied to quantitative determination of potential-x dependence. The latter corresponds to reduction of one tungsten unity in [W10O32]4− isopoly anion, which predominates in the deposition solution and, according to polarographic data, gives the main contribution to tungsten bronze deposition.