Interfacial Effects on the Band Edges of Ta3N5 Photoanodes in an Aqueous Environment: A Theoretical View

Abstract

Ta₃N₅, as a fascinating photoanode for solar hydrogen production, is expected to split water without any bias, because its band edge potentials straddle H₂O redox potentials. Unfortunately, Ta₃N₅ photoanodes can split water only when a bias of at least 0.6-0.9 V is applied. It means that they exhibit an onset potential as high as 0.6-0.9 V_RHE (reversible hydrogen electrode). In this study, density functional theory calculations show that the band edge potentials of Ta₃N₅ have a shift of approximately -0.42 eV relative to vacuum level when exposed to water. The increased ratio of dissociated water at Ta₃N₅-water interface will further make the band edge potentials shift -0.85 eV relative to vacuum level, implying the anodic shifts of the onset potential for water oxidation. The findings may reveal the mystery of the unexpectedly high onset potential of Ta₃N₅, as high as 0.6-0.9 V_RHE.