Spontaneous Oxygen Vacancy Ionization Enhances Water Oxidation on BiVO$_4$

The influence of surface oxygen vacancies on the oxygen evolution reaction on bismuth vanadate is studied using hybrid density functional theory. Our findings reveal the thermodynamic instability of the neutral unionized defect (${\rm V}_{\rm O}^0$), leading to spontaneous ionization into ${\rm V}_{\rm O}^{2+}$. By investigating the oxygen evolution reaction mechanism on both stoichiometric and oxygen-deficient surfaces, we find that surface oxygen vacancies reduce the reaction's thermodynamic overpotential, but only when the defects are ionized. Moreover, the reaction pathway involves the formation of surface-bound peroxide and superoxide ions as intermediates. Our work provides insight into the nature of surface oxygen vacancies and shines new light on how they may enhance the photoelectrochemical properties of semiconductors.