Enhancement of photoelectrochemical water oxidation of surface-modified BiVO₄/TiO₂(001) via efficient cascade hole transfer

Photoelectrochemical (PEC) water oxidation has attracted much attention for sustainable solar-to-fuel development. Over the years, surface modification of metal oxides photoanode has gained much attention for enhancing PEC performance via cascade hole transfer mechanism. However, the origin of such enhancement is far from being well-understood and most of the current photoanode still suffer from low conversion efficiency and fast electron-hole recombination. Therefore, this study addresses such fundamental challenges by investigating the effect of surface modification of BiVO₄ photoanode with (001)-exposed TiO₂ nanostructures and co-deposition of Rumbpy ([Ru(bipyP)(dmb)₂]²⁺, where bipyP: 2,2′-bipyridine-4,4′-diphosphoric acid and dmb: 4,4′-dimethyl-2,2′-bipyridine)) and cobalt(II) meso -tetra(4-carboxyphenil)porphyrin (CoTCPP) co-catalyst. Based on the result from spectroscopic and electrochemical analyses, cascade hole transport from BiVO₄ to Rumbpy and CoTCPP was responsible for improving water oxidation kinetics. More importantly, modification with (001)-exposed TiO₂ was found to be able to increase photocurrent density of the photoanode and reduced the water oxidation onset potential with near-perfect Faradaic efficiency of 97%. Further investigation using density functional theory calculations revealed that such enhancement in PEC performance was originated from the formation of midgap states at the interface of BiVO₄ and (001)-exposed TiO₂. It is believed that this new midgap states was formed as a result of hybridization of V 3d –O 2p –Ti d orbitals which could provide further efficient assistance for the already existing cascade hole transfer process. © 2026 Elsevier Ltd.