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

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.