Influence of co-existing species on charge transfer in dye-sensitized nanocrystalline oxide semiconductors in aqueous suspension for H2 evolution under visible light

Abstract

The water splitting under visible irradiation is of significant importance for the ultimate purpose of producing fuel from abundant water and the sunlight for the sustainable future society. The proper construction of an antenna for the efficient photon collection in the visible region and the reaction site for water splitting on the semiconductor surface has been desired and studied in order to design the efficient total system which enables the separation of photo-excited hole and electron, leading to the final redox reactions of water on the surface. In this paper, the dye-sensitized nanocrystalline oxide semiconductors of TiO2, SnO2, and ZnO are examined as the aqueous colloidal suspension for an application to the photocatalytic H2 evolution under visible light (>390nm) as a half-cell of water splitting system. The efficiency of H2 evolution is strongly influenced by the combination of oxide semiconductors, dye-sensitizers, and co-existing electron donor species in the suspension system. We herein perform the detailed analyses based on the photochemical and photo-electrochemical measurements to elucidate the combinational effects. The charge transfer properties are discussed in terms of the kinetics involving the electron injection and charge recombination, which reveals rather conjugated effects caused by the specific combination of the dye-sensitizers and co-existing electron donor species with oxide semiconductors than the simple mechanisms based on the thermodynamics involving the energy level of conduction band relative to LUMO of dye-sensitizers. The inversed results of H2 evolution between TiO2 and SnO2 with two kinds of electron donor species (alcohol and amine) are also analyzed in detail in terms of the charge transfer processes of dye-sensitized nanocrystalline oxide semiconductors in the suspension systems.