Visualizing Ultrafast\nPhotogenerated Electron and\nHole Separation in Facet-Engineered Bismuth Vanadate Crystals

Photogenerated charge separation is pivotal for effecting\nefficient\nphotocatalytic reactions. Understanding this process with spatiotemporal\nresolution is vital for devising highly efficient photocatalysts.\nHere, we employed pump–probe transient reflection microscopy\nto directly observe the temporal and spatial evolution of photogenerated\nelectrons and holes on the surface of facet-engineered bismuth vanadate\n(BiVO<sub>4</sub>) crystals. The findings suggest that the anisotropic\nbuilt-in field of BiVO<sub>4</sub> crystals propels the separation\nof photogenerated electrons and holes toward different facets through\na two-step process across varying time scales. Photogenerated electrons\nand holes undergo ultrafast separation within ∼6 ps, with electrons\ntransforming into localized small polarons toward the {010} facets\nof truncated BiVO<sub>4</sub> octahedral crystals. However, the photogenerated\nholes prolong their separation up to ∼2000 ps in a drift–diffusion\nmanner before ultimately accumulating on the {120} facets. This work\nprovides a comprehensive visualization of spatiotemporal charge separation\nat the nano/microscale on semiconductor photocatalysts, which is beneficial\nfor understanding the photocatalysis mechanism.