Understanding the Role of Vanadium Vacancies in BiVO₄for Efficient Photoelectrochemical Water Oxidation

The understanding of recombination of photogenerated electron/hole pairs at defect sites is a key enabler to develop bismuth vanadate (BiVO4) photoanodes at scale and low cost for photoelectrochemical water splitting. Here, we report a systematic investigation of the impact of vanadium vacancies on the efficiency of BiVO4 photoanodes for water photooxidation. X-ray photoelectron and photoluminescence spectroscopies reveal that the surfaces of nanostructured BiVO4 photoanodes obtained by high-temperature synthesis, here used as the model system, suffer from vanadium deficiency and display an increased recombination rate of photoexcited electron/hole pairs. Our simulation indicates that these vanadium vacancies (VV) create a new sub-band gap level in the proximity of the Fermi level of BiVO4. These levels act as recombination centers, explaining the subpar onset potentials and photocurrent densities for water photooxidation observed with these vanadium-deficient BiVO4 photoanodes. We show that once the VV are eliminated, by a facile post-treatment of the BiVO4 photoanodes, the photoluminescence lifetimes of the photogenerated carriers are significantly prolonged and the number of catalytically accessible sites is increased. As a result, the photocurrent during water oxidation is increased twofold, achieving 2 mA cm-2 against the standard hydrogen electrode in a 1 M potassium borate buffer electrolyte. These findings provide insights into the critical role played by the vanadium vacancies on the optoelectronic properties of BiVO4 and a scalable approach for its effective fabrication on large-scale surfaces.