Light Intensity Dependence of Photochemical Charge Separation in the BiVO₄/Ru-SrTiO₃:Rh Direct Contact Tandem Photocatalyst for Overall Water Splitting

The energy conversion efficiency of tandem photocatalysts for the overall water splitting reaction (OWS) is currently limited by our understanding of carrier separation and recombination in such systems. What is the effect of the solid–solid and solid–liquid interfaces on the carrier dynamics, and how do the photovoltage and catalytic activity depend on the light intensity? In order to address these issues, we report here on the light intensity-dependent water splitting activity and open circuit potential (OCP) measurements for a core–shell tandem made from bismuth vanadate (BiVO₄) microparticles and ruthenium-loaded rhodium-doped strontium titanate (Ru-SrTiO₃:Rh) nanoparticles. The measurements identify three operational regimes of the tandem: a threshold intensity of 8–14 mW cm–² below which no OWS occurs, a regime of strongly increasing apparent quantum efficiency (AQE) (17.7–70.2 mW cm–²), and a regime of nearly constant AQE (>171 mW cm–²). Open circuit potential measurements of the separate BiVO₄ and Ru-SrTiO₃:Rh absorbers in dilute H₂SO₄ solution (pH 3.5) confirm photoanodic behavior for BiVO₄ and photocathodic behavior for Ru-SrTiO₃:Rh and provide the light intensity dependent quasi-Fermi energies for the majority carriers in each material. The data allows modeling of the charge transfer dynamics in the tandem. In the dark, the materials form a weak p/n-junction which causes minority carrier recombination at the tandem contact and impedes the function of the photocatalyst under low light flux. At higher light intensity, charge separation of the tandem is increasingly controlled by minority carrier transfer at the solid/liquid contacts. As a result, majority carriers can flow to the SrTiO₃:Rh/BiVO₄ interface and recombine there and help equilibrate the majority carrier Fermi levels of both absorbers to a common value. The resulting shift of the band edges of both absorbers improves the rectifying character of the solid–liquid contacts and is the basis for the increase of the AQE from 0 to 1.11% (400 nm). Above 171 mW cm–², the AQE of the tandem remains nearly constant and becomes limited by intrinsic lattice and interfacial recombination of the two absorbers, by the low absorption coefficient of SrTiO₃:Rh, and by the slow water oxidation kinetics of the BiVO₄ surface. Finally, under very strong illumination, the H₂/O₂ back reaction becomes rate-limiting. These insights will be useful for the optimization of OWS tandem photocatalysts, especially under light limiting conditions.