Aspects of the water splitting mechanism on (Ga1-xZnx)(N1-xOx) photocatalyst powder are discussed on the basis of the effects of cocatalyst loading, light intensity, hydrogen/deuterium isotopes, and reaction temperature on photocatalytic activity. The loading amount of Rh2-yCryO3 cocatalyst significantly affected the photocatalytic behavior with varying light intensity. When the loading amount of Rh2-yCryO3 as a H-2 evolution cocatalyst was insufficient, the reaction order for light intensity was lower than unity. This is because photoexcited electrons accumulated in the photocatalyst and recombined with photoexcited holes more frequently than they contributed to the water splitting reaction. When a sufficient amount of cocatalyst was loaded, on the other hand, accumulation of photoexcited electrons was suppressed and the water splitting rate increased monotonically with light intensity. At a light intensity equivalent to solar irradiation, AM 1.5, the water splitting rate using modified (Ga1-xZnx)(N1-xOx) remained under the light-intensity-proportional regime. The isotope effect of H2O/D2O on overall water splitting using Rh2-yCryO3/(Ga1-xZnx)(N1-xOx) was smaller than previously reported for electrochemical H-2 evolution reactions. Moreover, the apparent activation energy of the overall water splitting was as small as 8 kJ mol(-1), and independent of the reactants. These results arise from the limited number of photoexcited carriers present in the Rh2-yCryO3/(Ga1-xZnx)(N1-xOx) , which should become available for redox reactions on the surface of the photocatalyst. A kinetic model of photocatalytic water splitting proposed in this work suggests that the reaction probability of photoexcited holes for O-2 evolution versus recombination with intrinsic electrons of the photocatalyst determines the photocatalytic activity of the Rh2-yCryO3/(Ga1-xZnx)(N1-xOx).
