Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy

Abstract

The application of inorganic nanostructures for solar water splitting is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are less effective for carrier separation. Here we employ surface photovoltage spectroscopy to measure the internal photovoltages in single crystalline platinum/ruthenium-modified Rh-doped SrTiO₃ nanocrystals for the first time. Voltages of −0.88 V and −1.13 V are found between the absorber and the Ru and Pt cocatalysts, respectively, and a voltage of −1.48 V for a Rh:SrTiO₃ film on an Au substrate. This shows that the Pt and Ru cocatalysts not only improve the redox kinetics but also aid charge separation in the absorber. Voltages of +0.4 V, +0.6 V, and +1.2 V are found for hole injection into KI, K₄[Fe(CN)₆], and methanol, respectively, and a voltage of −0.7 V for electron injection into K₃[Fe(CN)₆]. These voltages correlate well with the photocatalytic performance of the catalyst; they are influenced by the built-in potentials of the donor-acceptor configurations, the physical separation of donors and acceptors, and the reversibility of the redox reaction. The photovoltage data also allowed the identification of a photosynthetic system for hydrogen evolution (80 μmol g⁻¹ h⁻¹) under visible light illumination (>400 nm) from 0.05 M aqueous K₄[Fe(CN)₆].