Visualizing Ultrafast Photogenerated Electron and Hole Separation in Facet-Engineered Bismuth Vanadate Crystals

Photogenerated charge separation is pivotal for effecting efficient photocatalytic reactions. Understanding this process with spatiotemporal resolution is vital for devising highly efficient photocatalysts. Here, we employed pump-probe transient reflection microscopy to directly observe the temporal and spatial evolution of photogenerated electrons and holes on the surface of facet-engineered bismuth vanadate (BiVO₄) crystals. The findings suggest that the anisotropic built-in field of BiVO₄ crystals propels the separation of photogenerated electrons and holes toward different facets through a two-step process across varying time scales. Photogenerated electrons and holes undergo ultrafast separation within ∼6 ps, with electrons transforming into localized small polarons toward the {010} facets of truncated BiVO₄ octahedral crystals. However, the photogenerated holes prolong their separation up to ∼2000 ps in a drift-diffusion manner before ultimately accumulating on the {120} facets. This work provides a comprehensive visualization of spatiotemporal charge separation at the nano/microscale on semiconductor photocatalysts, which is beneficial for understanding the photocatalysis mechanism.