Charge carrier dynamics of surface back electron/hole recombination in BiVO4 and TiO2 photoanodes

Abstract

This work employs photoinduced absorption spectroscopy, transient photocurrent and transient absorption spectroscopy to investigate surface recombination in BiVO₄ and TiO₂ photoanodes. The kinetic competition between surface recombination and water oxidation is the key limitation in the large onset potential in BiVO₄. The recombination is identified via electron trapping/detrapping in vanadium oxygen vacancies. Arrhenius analyses suggest that the bias-independent 100 meV activation energy for surface recombination (trapping) is smaller than 200 meV for detrapping to generate photocurrent. In contrast, TiO₂ is modestly affected by surface recombination. The faster water oxidation kinetics in TiO₂ is key to suppressing surface recombination for early onset potential. Such surface recombination in BiVO₄ is attributed to the slow water oxidation resulting in surface hole accumulation, opposite to TiO₂ with faster water oxidation kinetics. These results emphasize the importance of enhancing water oxidation kinetics compared to surface electric field for efficient suppression of surface recombination.