Infrared Spectroscopic Study of the Potential Change at Cocatalyst Particles on Oxynitride Photocatalysts for Water Splitting by Visible Light Irradiation

Visible light-induced changes in Pt/(Ga1–x Zn x )(N1–x O x ), Pt/TaON, and Pt/LaTiO2N powder photocatalysts were investigated using infrared spectroscopy with adsorbed CO probe molecules on Pt cocatalysts. Among these photocatalyst materials, only the (Ga1–x Zn x )(N1–x O x ) photocatalyst has achieved overall water splitting with visible-light irradiation, although all of these materials have suitable band gaps for the reaction. For Pt/(Ga1–x Zn x )(N1–x O x ) and Pt/TaON, the C–O stretching vibrations of adsorbed CO on Pt shifted to a lower frequency under visible light irradiation. However, no frequency shift was observed for Pt/LaTiO2N. This lower-frequency shift indicates a negative electronic potential shift of the Pt cocatalysts, which results in hydrogen evolution. Differences in the light intensity dependence of the potential shifts of these photocatalysts were observed, and the involvement of a different recombination mechanism of photoexcited carriers in these photocatalysts will be discussed. These results clarify some of the reasons for the activities of various photocatalytic materials for overall water splitting.