Accelerating water oxidation on BiVO4 photoanodes via surface modification with Co dopants

Abstract

Despite the vast investigations on improving the photoelectrochemical performance of BiVO₄ for water splitting, charge recombination in the near-surface region remains a challenge. In this study, we showed that the diffusion of Co²⁺ ions into the BiVO₄ subsurface boosted the water oxidation activity and charge injection efficiency remarkably. The increase in the concentration of oxygen vacancies upon the incorporation of cobalt ions was shown by electron paramagnetic resonance (EPR) spectroscopy and confirmed by density functional theory (DFT) calculations. DFT calculations revealed that vanadium sites in the subsurface region were the most favorable sites for substitution with cobalt ions. Charge localization at surface oxygen vacancies was found less favorable in the presence of cobalt in the subsurface layer, eliminating surface recombination. This resulted in 4.25 times larger charge injection efficiency and 6.2 times higher photocurrent density at the potential of ∼0.6 V, as compared to pristine BiVO₄. This enhancement was significantly larger as compared to CoO_x-loaded BiVO₄, indicating that the suppressed recombination at the surface and improved charge transfer kinetics obtained solely by CoO_x deposition are not sufficient for enhanced activity of BiVO₄. A longer charge carrier lifetime obtained upon cobalt incorporation was observed by transient absorption spectroscopy and verified the reduced rate of recombination.