Enhanced photoelectrochemical water oxidation over a surface-hydroxylated BiVO4 photoanode: advantageous charge separation and water dissociation

Abstract

Bismuth vanadate (BiVO₄) is one of the most effective photoanode materials for photoelectrochemical water splitting, but its reaction rate is greatly limited by the poor separation efficiency of photo-generated charges and sluggish kinetics. In this work, a surface hydroxylation strategy was carried out to overcome the above-mentioned shortcomings. A simple post-synthetic NaOH immersion method was used to successfully hydroxylate the BiVO₄ photoanode surface without affecting its bulk properties. The modified surface hydroxyl group not only promotes the photo-generated charge separation efficiency of hydroxylated BiVO₄ but also tailors its local electronic structure to form high-valence Bi^(3+x)+ states. The Bi^(3+x)+ species serving as the active site is advantageous for water dissociation, thus decreasing the free energy barrier of the potential-determining step for the boosted overall water oxidation kinetics. As a result, the best hydroxylated BiVO₄ photoanode achieves a higher photocurrent density of 1.14 mA cm⁻² at 1.23 V vs. RHE under visible light irradiation, which is 4.75 times that of bare BiVO₄ (0.24 mA cm⁻²), and moreover, a cathodic shift of 190 mV at 1 mA cm⁻² is obtained. Our work affords a practical method with strong operability to improve the intrinsic photoelectrochemical performance of photoanodes by the efficient surface hydroxylation strategy.