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Engineering metal oxide heterojunctions that balance light harvesting with carrier transport remains a persistent challenge for BiVO 4 /WO 3 photoanodes. Here we report an all-vacuum route to fabricate uniform, pinhole-free BiVO 4 /WO 3 bilayers. WO 3 underlayers with thicknesses of 90–400 nm are deposited via reactive sputtering, followed by monoclinic BiVO 4 formed via vanadium intercalation of sputtered Bi 2 O 3 at 450 °C. An optimum WO 3 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 BiVO 4 /WO 3 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 BiVO 4 /WO 3 to 32% after Co–Pi modification, consistent with a type-II band alignment that promotes electron extraction into WO 3 and efficient hole transfer at the BiVO 4 /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 WO 3 thickness in flat, compact bilayers, this work clarifies the trade-off between optical absorption and charge transport governing BiVO 4 /WO 3 photoanodes and demonstrates a scalable, all-vacuum route towards large-area, highly efficient BiVO 4 photoanodes for PEC water splitting.
Salih et al. (Tue,) studied this question.