The energies of the conduction and valence band edges of a material relative to the water redox potential levels are critical factors governing photocatalytic activity for overall water splitting, i.e., evolution of H2 and O2 from water. In this study, we discuss the large discrepancy in the experimentally measured conduction and valence band edge positions of a variety of transition metal oxides (TMOs) as determined in vacuum using ultraviolet photoelectron spectroscopy (UPS) and in an aqueous solution via flat band potential measurements. We speculate that the discrepancy stems from the different degree of electron transfer across the surface due to the different environment at the surface of the TMOs in vacuum and in water, with significant electron exchange occurring at the TMO/water interfaces but not at the TMO/vacuum interfaces. Accurately modeling the electronic structure, particularly at TMO/water interfaces, is a significant challenge, however. Using first-principles density functional theory (DFT) calculations on rutile TiO2 and CoO model systems, we identify the optimal approaches to accurately predict the band edge positions in vacuum and in water. We then validate the optimized schemes on other TMOs, including FeO, NiO, MnO, anatase TiO2, WO3, and V2O5, demonstrating very good agreement with experimental measurements in both vacuum and in water. Our study provides the groundwork for accurate estimation of band edge positions in TMOs using an explicit solvation method, and further, offers guidelines for using explicit solvation calculations to screen for photocatalytic water splitting capability across potential photocatalysts in an aqueous environments.

Explicit solvation prediction of band edges of transition metal oxide photocatalysts: Optimal methodology for dealing with localized d orbitals in aqueous environments

2019 Fall National Meeting

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