The Effect of Oxygen Vacancies on Oxygen Evolution: The Case of BiVO_4

Abstract

Bismuth vanadate, BiVO4, is one of the most promising photoanode materials for the challenging oxygen evolution half-reaction in solar-driven water splitting. The material tends to be rich in oxygen vacancies, which strongly affects its photoelectrochemical properties. Experimental evidence suggests that oxygen deficiency is beneficial for the oxygen evolution reaction in the material, but the mechanism behind this enhancement is still controversial. The defects could be involved directly in the reaction if present at the surface, and the occupancy of the defect states could also play an important role. The latter is seldom considered in mechanistic studies, however.Using density functional theory, we show that the surface oxygen vacancy in bismuth vanadate is stablest when fully ionized. We investigate how this affects the oxygen evolution mechanism by mapping out the stablest reaction intermediates and compare the resulting pathway with those on the unionized oxygen-deficient surface as well as the defect-free material. The overpotentials required to drive the reaction in each case are computed to quantify whether or not vacancy formation, and subsequent ionization, improves the thermodynamics of oxygen evolution.