Photoelectrochemical H₂ evolution on WO₃/BiVO₄ enabled by single-crystalline TiO₂ overlayer modulations

Tungsten oxide/bismuth vanadate (WO₃/BiVO₄) has emerged as a promising photoanode material for photoelectrochemical (PEC) water splitting owing to its facilitated charge separation state differing significantly from single phase materials. Practical implementation of WO₃/BiVO₄ is often limited by poor stability arising from the leaching of V⁵⁺ from BiVO₄ during PEC operations. Herein, we demonstrate that the synthesis of a tungsten oxide/bismuth vanadate/titanium oxide (WO₃/BiVO₄/TiO₂) heterostructure onto a fluorine-doped tin oxide-coated glass substrate through a combined simple hydrothermal-spin coating strategy will advance PEC performance while slowing water oxidation kinetics and improving photostability. We show that surface postmodification with a nanometer-thick layer of (1 0 1) monofacet-selective single-crystalline TiO₂ provides stable photocurrent density, up to 1.04 mA cm^-2 at 1.23 V (compared to a reversible hydrogen electrode in 0.5 M Na₂SO₄), with excellent quantum efficiency (45% at 460 nm) and long-term photostability (24 h). Interestingly, crystalline TiO₂ activation layers behave differently from previous TiO₂ amorphous layers, blocking surface defects while improving corrosion resistance, photostability, and the electron transfer process. These results indicate a ≈2.5 times enhancement in photoelectrocatalytic activity related to referenced WO₃/BiVO₄ photoanodes, encouraging the use of single-crystalline TiO₂ modulations to develop a range of materials for PEC/photocatalytic applications.