Photoinduced Surface Oxidation of GaN Nanowires Facilitates Hydrogen Evolution

Gallium nitride nanowires (GaN NWs) have shown great potential in applications to photocatalysis, including photocatalytic hydrogen evolution for solar energy storage. Previous studies have shown that GaN NWs can undergo self-improvement under light irradiation, which is attributed to surface oxidation, forming gallium oxynitride (GaON). However, the exact oxidation pathways and the effect of surface oxidation on catalytic performance remain to be understood at the molecular level. In this study, we combine computational modeling at the density functional theory (DFT) level with linear sweep voltammetry and chronoamperometric measurements to investigate the photoinduced surface oxidation of GaN NWs. We find that the oxidation of GaN to GaON is competitive with water oxidation. The oxidized surface shows almost no change in its water oxidation capabilities, although the potential required for hydrogen evolution is significantly reduced. Oxidation of the surface also leads to changes in the electronic structure, shifting the valence band edge states toward the surface adsorbed hydroxide, making hole localization there more likely. Calculations are consistent with the observation of shifts in the onset potential for photoelectrochemical hydrogen evolution toward more positive potentials over an extended 18 h period. The reported findings on the mechanism of photoinduced surface oxidation of GaN NWs and the effect of surface oxidation on hydrogen evolution provide valuable insights for the development of more efficient GaN NW-based photocatalytic surfaces for hydrogen evolution.