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 tuning of such surface stoichiometry, while a comprehensive understanding at 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 maximizes the PEC performance toward the oxygen evolution reaction (OER). 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 usually 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 NiFeO_x improves, especially, the charge separation in surface V-modified BiVO₄. Results suggest that the V-modified 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 by which the surface termination governs the PEC response.