The Role of Surface States on Reduced TiO2@BiVO4 Photoanodes: Enhanced Water Oxidation Performance through Improved Charge Transfer

Abstract

The efficient transfer of photogenerated carriers and improved stability against corrosion are essential to maximize the performance of photoanodes. Herein, a reduced catalytic layer formed on a TiO2 protected BiVO4 (R-TiO2@BiVO4) photoanode has been prepared for progress on both fronts. Specifically, R-TiO2@BiVO4 photoanodes at pH 8 displayed a high photocurrent of 2.1 mA cm–2 at 1.23 VRHE and a more negative onset potential of 234 mVRHE compared to pristine BiVO4. We here discovered two surface states on BiVO4 photoanodes through photoelectrochemical impedance studies. In contrast, only one of them, located at higher potential, appeared on oxygen-vacancy-rich R-TiO2@BiVO4 photoanodes. For BiVO4 photoanodes, the first surface state (SS1) is located near the onset potential (∼0.45 VRHE), while the second surface state (SS2) sits near the water oxidation potential (∼1.05 VRHE). However, SS1 at lower energetics, which originated from water oxidation intermediates with slow kinetics, is passivated in R-TiO2@BiVO4 photoanodes. In contrast, the hole densities in SS2 at higher energetics were greatly enhanced in R-TiO2@BiVO4 photoanodes, due to the increased accumulation of intermediates with fast water oxidation kinetics. Therefore, SS2 is proposed as a reaction center, which is related to the amount and occupancy of oxygen vacancies. Additionally, surface recombination centers in BiVO4 photoanodes are passivated by TiO2, which prevents electron trapping into the irreversible surface conversion of VO2+ to VO2+. These observations provide fundamental understanding of the role of surface states and of the function of oxygen vacancies in BiVO4 photoanodes. Our study offers detailed insight into key strategies for optimal photoelectrochemical performance through surface property tuning.