Electronic Structure, Optoelectronic Properties, and Photoelectrochemical Characteristics of γ-Cu₃V₂O₈ Thin Films

Thin films of n-type γ-Cu₃V₂O₈ are prepared with high phase purity via reactive co-sputtering deposition. Complementary X-ray spectroscopic methods are used to reveal that the valence band maximum consists of O 2p states, while the conduction band minimum is primarily composed of Cu 3d states. Therefore, γ-Cu₃V₂O₈ is classified as a charge transfer insulator, in which the 1.80 eV indirect band gap corresponds to the O 2p → Cu 3d transition. Through photoelectrochemical measurements, the surface of γ-Cu₃V₂O₈ photoanodes is found to display intrinsic activity for catalyzing water oxidation that is stable with time. The combination of a small optical band gap, suitable valence band energy, and excellent photoelectrochemical stability suggests that γ-Cu₃V₂O₈ could be a promising photoanode material. However, it is found that the charge extraction efficiency from these semiconductor photoanodes is strongly limited by a short (20-40 nm) hole diffusion length. Characterization of the electronic structure and transport properties of γ-Cu₃V₂O₈ photoanodes suggests strategies for improving energy conversion efficiency and provides fundamental insights that can be used for understanding and evaluating function in a broader class of emerging ternary metal oxides.