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

Abstract

The synergistic effect of coupling a photooxidation catalyst, tantalum oxynitride, and manganese oxide (MnO_x) as electrooxidation catalyst in multi-component photoanodes for water oxidation is explored in near-neutral solutions under visible light irradiation (λ > 400 nm). The anodes have been formed by using different electrochemical methods to deposit MnO_x 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 MnO_x 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 MnO_x/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 MnO_x 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 MnO_x/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 MnO_x material, and this structural degradation is mirrored in a slow deterioration of the photocatalyst performance.