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

TaN, as a fascinating photoanode for solar hydrogen production, is expected to split water without any bias, because its band edge potentials straddle HO redox potentials. Unfortunately, TaN 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 (reversible hydrogen electrode). In this study, density functional theory calculations show that the band edge potentials of TaN have a shift of approximately -0.42 eV relative to vacuum level when exposed to water. The increased ratio of dissociated water at TaN-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 TaN, as high as 0.6-0.9 V.