Electron–Phonon Coupling Dynamics at Oxygen Evolution Sites of Visible-Light-Driven Photocatalyst: Bismuth Vanadate

Abstract

Bismuth vanadate (BiVO4) is an effective visible-light-driven photocatalyst for oxygen evolution from water. To understand the mechanism of photocatalytic oxidation of water, it is important to detect and characterize holes at the surfaces of powdered catalysts. Here, we report the transient absorption of BiVO4 in a wide time range from subpicosecond to 200 μs upon the excitation across the band gap with 400 nm femtosecond pulses. The effect of electron scavenger (Fe3+) on transient absorption decays indicates that the transitions at λ < 700 nm are mainly contributed by holes at the surfaces. While the transient absorption at λ > 700 nm rises almost instantaneously, the absorption λ < 700 nm has a slower rise component of τ ∼ 15 ps due to filling of surface traps with holes. Moreover, the rise component is modulated with strongly oscillating signals caused by coherent excitation of an external phonon mode between Bi3+ and VO43–. Thus, the transitions at λ < 700 nm associated with surface-trapped holes are strongly coupled to the external phonon mode. This study demonstrates that the time-domain spectroscopy is useful for characterizing the vibrational structure specific to the surface charge trap sites of powdered photocatalysts.