Photo-assisted electrodeposition of manganese oxide on TaON anodes: effect on water photooxidation capacity under visible light irradiation

The synergistic effect of coupling a photooxidation catalyst, tantalum oxynitride, and manganese oxide (MnOx) as electrooxidation catalyst in multi-component photoanodes for water oxidation is explored in near-neutral solutions under visible light irradiation (lambda > 400 nm). The anodes have been formed by using different electrochemical methods to deposit MnOx on screen-printed TaON films, pre-modified with TiO2 coatings (TiO2-TaON). By using SEM/EDX, ICP-MS analysis of the amount of Mn deposited and, electrochemical techniques, we demonstrate that selective deposition of fine-structured nm-sized MnOx flakes on the photocatalytically active TiO2-TaON surface is achieved when the electrodeposition is carried out under visible light irradiation, but not in the dark. The MnOx/TiO2-TaON anodes produced using the photo-assisted method exhibit improved activity and better long-term stability during water photooxidation under visible light irradiation when compared to the TiO2-TaON films modified with MnOx in dark. A 7-fold enhancement in the oxidative photocurrent densities under voltammetric and chronoamperometric conditions in 0.1 M Na2SO4 (pH = 6) is observed for the films prepared by the photo-assisted method. The Mn-loading in the best performing films is ca. 0.8 wt% and higher loadings were found to lower the photocatalytic activity. Continuous water photooxidation over MnOx/TiO2-TaON anodes in 0.1 M Na2SO4 (pH = 6) at potentials more positive than ca. 1.0 V vs. reversible hydrogen electrode coarsens the fine structure of the MnOx material, and this structural degradation is mirrored in a slow deterioration of the photocatalyst performance.