Rational Interpretation of Correlated Kinetics of Mobile and Trapped Charge Carriers: Analysis of Ultrafast Carrier Dynamics in BiVO₄

Abstract

Bismuth vanadate (BiVO4) offers a unique combination of advantages, including being a stable, earth abundant, and visible-light responsive photocatalyst capable of water oxidation. One strategy that is widely employed to enhance the photocatalytic performance of BiVO4 is to improve the carrier transport, which is governed by the interplay between trapping and recombination. To further elucidate the photophysical processes, we investigate the dynamics of often ignored mobile electrons (3435 nm probe) and holes (580 nm probe) using transient absorption spectroscopy. Mobile electrons decay virtually to completion by ∼300 ps, while holes decay in significantly longer periods that far exceed 3000 ps. Furthermore, we use a theoretical model to rationalize the effect of light intensity on the distinctive decay pathways for electrons and holes by trapping and recombination. By employing a simple yet effective formula, we transform the electron decay profile to obtain a hole decay profile that agrees with the experimentally observed transient. A detailed theoretical analysis enables us to determine relevant photophysical parameters such as rate constant values for recombination and trapping, energy levels of traps, and number densities of traps. Results indicate that the electron-trapping process is efficient in BiVO4, and thus direct recombination of electrons with holes is suppressed. Although trapping lowers the electron mobility, it prolongs the lifetimes of holes, which is beneficial for the water-oxidation reaction.