First principles studies on the redox ability of (Ga sub(1-x)Zn sub(x))N sub(1-x)O sub(x) solid solutions and thermal reactions for H sub(2) and O sub(2) production on their surfaces

The (Ga sub(1-x)Zn sub(x))N sub(1-x)O sub(x) solid solution has been emerging as an effective photocatalyst for water splitting utilizing the visible solar spectrum, regarded as a host GaN bulk doped with ZnO impurities. H sub(2) and O sub(2) production occur simultaneously and stoichiometrically on the surface of (Ga sub(1-x)Zn sub(x))N sub(1-x)O sub(x) particles. In this work, we characterize the redox ability of (Ga sub(1-x)Zn sub(x))N sub(1-x)O sub(x) and find that a solid solution with a ZnO concentration of 0.125 < x< 0.250 is optimal for water splitting. This is consistent with the experimental finding that the maximum photocatalytic activity of (Ga sub(1-x)Zn sub(x))N sub(1-x)O sub(x) is achieved at x= 0.13. The thermal reactions of water splitting are modeled on both the GaN and an idealized (Ga sub(1-x)Zn sub(x))N sub(1-x)O sub(x) (101&cmb.macr; 0) surface. The computed activation barriers allow us to gain some clues on the efficiency of water splitting on a specific photocatalyst surface. Our results suggest that the non-polar (101&cmb.macr; 0) and polar (0001) surfaces may play different roles in water splitting, i.e., the (101&cmb.macr; 0) surface is responsible for O sub(2) production, while hydroxyl groups could dissociate on the (0001) surface.