ARTICLE pubs.acs.org/JPCC
Infrared Spectroscopic Study of the Potential Change at Cocatalyst Particles on Oxynitride Photocatalysts for Water Splitting by Visible Light Irradiation Xuwang Lu,† Athula Bandara,†,§ Masao Katayama,† Akira Yamakata,‡ Jun Kubota,† and Kazunari Domen*,† † ‡
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan ABSTRACT: Visible light-induced changes in Pt/(Ga1xZnx)(N1xOx), 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 (Ga1xZnx)(N1xOx) photocatalyst has achieved overall water splitting with visible-light irradiation, although all of these materials have suitable band gaps for the reaction. For Pt/(Ga1xZnx)(N1xOx) and Pt/TaON, the CO 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.
1. INTRODUCTION Solar-irradiation induced hydrogen production from water is expected to play an important role in future energy systems because hydrogen is fundamentally clean and can be used in combustion engines, fuel cells, and as a base material in many chemical industries. There are several methods of solar hydrogen production, such as the electrolysis of water by solar voltaic cells, biomass reforming, and photoelectrochemical or photocatalytic water splitting. Photocatalytic water splitting is the most attractive method among these because of its simplicity and large-scale feasibility.14 While several photocatalysts for water splitting with extremely high quantum yields have been found to operate in the ultraviolet light region, only a few materials have been developed that are active in the visible-light region.1,3,4 To obtain a higher solar energy conversion efficiency, the development of photocatalysts with narrower band gaps (longer wavelengths of absorption edges) is required for the efficient utilization of visible-light, which comprises approximately half of the energy in solar irradiation. Oxynitride compounds are promising materials for water splitting, and some of them have been reported with suitable band gaps for visible light absorption. (Ga1xZnx)(N1xOx) (abbreviated GaN:ZnO hereafter) forms a solid solution in the wurtzite crystal structure with a band gap of 2.8 eV. After modification with a cocatalyst such as a mixed Cr/Rh oxide, the optimized photocatalyst GaN:ZnO can split water stoichiometrically into H2 and O2 under visible light irradiation (
