Particle Size Dependence of Carrier Dynamics and Reactivity of Photocatalyst BiVO₄ Probed with Single-Particle Transient Absorption Microscopy

Heterogeneous photocatalytic water splitting under the irradiation of sunlight is an attractive method for generating hydrogen from water. While the photocatalytic mechanism has been extensively studied, most of the experimental studies have been performed with an ensemble of photocatalyst particles with various sizes, morphologies, and secondary structures. To gain a deeper understanding of the mechanism of photocatalysis, it is indispensable to clarify how the geometric structure of photocatalyst affects the kinetics of photogenerated carriers and redox reactions. In this study, the hole decay characteristics and photocatalytic activity of BiVO₄, a promising photocatalyst for oxygen evolution with visible light, have been investigated with single-particle transient absorption microscopy. Upon irradiation with 527 nm light, well-faceted nonaggregated crystallites show fast hole decay and little reactivity for Fe³⁺ reduction. In contrast, aggregated particles with grain boundaries between small primary crystallites show slower hole decay and higher reactivity for Fe³⁺ reduction than the nonaggregated crystallites. This behavior is increasingly pronounced as the secondary particle size of aggregated crystallite increases. This indicates that grain boundaries in aggregated particles do not work as recombination centers but play an important role in elongation of carrier lifetime and thus in enhancing the reactivity of photocatalyst through trap–detrap processes