Thickness-tuned WO₃ underlayers in all-vacuum BiVO₄/WO₃ photoanodes enable long-lived holes

Engineering metal oxide heterojunctions that balance light harvesting with carrier transport remains a persistent challenge for BiVO₄/WO₃ photoanodes. Here we report an all-vacuum route to fabricate uniform, pinhole-free BiVO₄/WO₃ bilayers. WO₃ underlayers with thicknesses of 90–400 nm are deposited via reactive sputtering, followed by monoclinic BiVO₄ formed via vanadium intercalation of sputtered Bi₂O₃ at 450 °C. An optimum WO₃ thickness of 260 nm delivers the highest photoelectrochemical (PEC) water oxidation performance, yielding a photocurrent of 2.4 mA cm−2 at 1.23 V_RHE (AM 1.5G, pH 7). The bare BiVO₄/WO₃ bilayer exhibits long-lived holes (τ = 1.0 s), and Co–Pi surface loading further boosts the photocurrent to 3.9 mA cm−2 while extending τ to 6.5 s. The incident photon-to-current efficiency (IPCE) increases from 18% (405 nm) for BiVO₄/WO₃ to 32% after Co–Pi modification, consistent with a type-II band alignment that promotes electron extraction into WO₃ and efficient hole transfer at the BiVO₄/Co–Pi interface. Mott–Schottky and impedance analyses corroborate reduced charge-transfer resistance and a cathodic flat-band shift, while continuous illumination tests show stable operation in neutral electrolyte. By isolating the effect of WO₃ thickness in flat, compact bilayers, this work clarifies the trade-off between optical absorption and charge transport governing BiVO₄/WO₃ photoanodes and demonstrates a scalable, all-vacuum route towards large-area, highly efficient BiVO₄ photoanodes for PEC water splitting.