BiVO₄ Film Coupling with CoAl₂O₄ Nanoparticles for Photoelectrochemical Water Splitting Utilizing Broad Solar Spectrum through p–n Heterojunction, Photothermal, and Cocatalytic Synergism

Water oxidation is an endothermic and kinetics-sluggish reaction; the research of photoanodes with photothermal and cocatalytic properties is of great significance. Herein, BiVO₄/CoAl₂O₄ film photoanodes were studied for solar water splitting through coupling spinel p-type CoAl₂O₄ nanoparticles on n-type BiVO₄ films. Compared to the BiVO₄ photoanode, better performance was observed on the BiVO₄/CoAl₂O₄ photoanode during water oxidation. A photocurrent of 3.47 mA/cm² was produced on the BiVO₄/CoAl₂O₄ photoanode at 1.23 V vs RHE, which is two-fold to the BiVO₄ photoanode (1.70 mA/cm²). Additionally, the BiVO₄/CoAl₂O₄ photoanodes showed an acceptable stability for water oxidation. The BiVO₄/CoAl₂O₄ photoanode being of higher water oxidation performance could be attributed to the presence of p-n heterojunction, cocatalytic, and photothermal effects. In specific, under the excitation of λ < 520 nm light, the holes produced in/on BiVO₄ can be transferred to CoAl₂O₄ owing to the p-n heterojunctions of BiVO₄/CoAl₂O₄. Meanwhile, the temperature on the BiVO₄/CoAl₂O₄ photoanode rises quickly up to ∼53 °C under AM 1.5 G irradiation due to the photothermal property of CoAl₂O₄ through capturing the 520 < λ < 720 nm light. The temperature rising on the BiVO₄/CoAl₂O₄ photoanode improves the cocatalytic activity of CoAl₂O₄ and modifies the wettability of BiVO₄/CoAl₂O₄ for effective water oxidation.