Boosted Water Oxidation Activity and Kinetics on BiVO₄ Photoanodes with Multihigh-Index Crystal Facets

The crystal facet of the BiVO₄ photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO₄ polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi₂O₃ template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO₄ film with a {121} monohigh-index facet, the BiVO₄ film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm² was achieved on the multihigh-index facet BiVO₄ photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na₂SO₄, which is about 200% improved over the normal BiVO₄ photoanode (0.61 mA/cm² at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO₄ photoanode (0.22 V vs RHE) relative to the normal BiVO₄ photoanode (0.52 V vs RHE) in 0.1 M Na₂SO₄. Although the UV-vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO₄ film were boosted obviously. Theory calculations reveal that the adsorption of H₂O molecules on BiVO₄{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO₄ is changed to the step of two protons reacting with •O to form •OOH species (•O + H₂O_(l) + 2H⁺ + 2e^- → •OOH + 3H⁺ + 3e^-), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H₂O to •OH (2H₂O_(l) + • → •OH + H₂O_(l) + H⁺ + e^-). In addition, the overpotential of water oxidation limiting step on BiVO₄{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.