The photocatalytic activity of (Ga(1-x)Zn(x))(N(1-x)O(x)) loaded with Rh-Cr mixed-oxide (Rh(2-y)Cr(y)O3) nanoparticles for overall water splitting under visible-light irradiation (lambda > 400 nm) is investigated with respect to reaction pH and gas pressure. The photocatalytic performance of the catalyst is found to be strongly dependent on the pH of the reactant solution but largely independent of gas pressure. The present photocatalyst exhibits stable and high photocatalytic activity in an aqueous solution of pH 4.5 for 72 h. The photocatalytic performance is much lower at pH 3.0 and pH 6.2, attributable to corrosion of the cocatalyst and hydrolysis of the catalyst. The dispersion of Rh(2-y)Cr(y)O3 as a cocatalyst on the (Ga(1-x)Zn(x))(N(1-x)O(x)) surface promotes hydrogen evolution, which is considered to be the rate-determining step for overall water splitting on this catalyst.

The photocatalytic activity of (Ga1- x Zn x )(N1- x O x ), a solid solution of GaN and ZnO, for H2 and O2 evolution in the presence of methanol and silver nitrate as sacrificial reagents under visible light (λ > 420 nm) is investigated in detail. (Ga1- x Zn x )(N1- x O x ) evolves H2 from an aqueous methanol solution when loaded with nanoparticulate Rh2- y Cr y O3 as a cocatalyst, and evolves O2 from an aqueous silver nitrate solution without a cocatalyst. Structural analyses indicate that the H2 evolution activity is strongly dependent on the crystallinity and composition of the catalyst, while the rate of O2 evolution is proportionally related to the specific surface area. The activity for H2 evolution from methanol solution is of the same order as for overall water splitting, but is an order of magnitude lower than that for O2 evolution from silver nitrate solution. The results of photocatalytic reactions and photoelectrochemical measurements suggest that the rate-determining step for overall water splitting using (Ga1- x Zn x )(N1- x O x ) is the H2 evolution process.