Tailoring the surface termination of BiVO4 photoanodes using ammonium metavanadate enhances the solar water oxidation performance

Altering the surface stoichiometry of semiconductor electrodes is known to affect the photoelectrochemical (PEC) response. To date, several reports have hinted at the influence of the surface Bi:V ratio on the solar water oxidation performance of BiVO4 photoanodes, but only a handful of strategies have been reported to afford to fine-tune such surface stoichiometry while a comprehensive understanding of an atomic level of the role of the surface termination remains elusive. Herein, we report a new methodology that modulates the surface Bi:V ratio and, in turn, maximizes the PEC performance towards the oxygen evolution reaction (OER). We found that annealing in the presence of ammonium metavanadate drastically reduces the surface recombination while improving the charge separation. Detailed characterization revealed that this treatment filled the native surface vanadium vacancies, which are found to act as recombination centers, while inducing a significant increase in the density of oxygen vacancies, which reinforced the built-in electric field that drives the charge separation. Interestingly, coating with NiFeOx improves, especially, the charge separation in surface V-rich BiVO4. Results suggest that the V-rich surface termination altered the surface energetics of BiVO4 leading to an improved band alignment across the interface. Overall, these results provide a new platform to modulate the surface stoichiometry of BiVO4 thin films while shedding new light on the mechanisms whereby the surface termination governs the PEC response.