Surface Photovoltage Measurements on a Particle Tandem Photocatalyst for Overall Water Splitting

Surface photovoltage spectroscopy (SPS) is used to measure the photopotential across a Ru-SrTiO₃:Rh/BiVO₄ particle tandem overall water splitting photocatalyst. The tandem is synthesized from Ru-modified SrTiO₃:Rh nanocrystals and BiVO₄ microcrystals by electrostatic assembly followed by thermal annealing. It splits water into H₂ and O₂ with an apparent quantum efficiency of 1.29% at 435 nm and a solar to hydrogen conversion efficiency of 0.028%. According to SPS, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of -2.45 V at 435 nm (2.44 mW cm^-2), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO₄ microcrystals. These findings promote the understanding of solar energy conversion with inorganic particle photocatalysts.