Kelvin Probe Force Microscopy Reveals Spatially Resolved Charge-Transfer Mechanism in CdS/BiOBr S-scheme Heterojunction Photocatalyst.

S-scheme heterojunctions hold great promise for photocatalysis, yet a comprehensive understanding of their charge-transfer mechanisms remains limited. While time-resolved techniques have provided valuable insights, the spatial resolution of charge transfer at the material surface remains underexplored. Here, we employ Kelvin probe force microscopy (KPFM) to investigate the charge-transfer dynamics in S-scheme heterojunctions, revealing spatially resolved details. Our findings show that upon illumination, the Fermi level (Ef) of n-type semiconductors increases, but a built-in electric field (IEF) persists within the heterojunction. Electrons accumulate on the surface of the reduction semiconductor (RS), resulting in a surface photovoltage (SPV) lower than that of the individual semiconductor, while holes accumulate on the oxidation semiconductor (OS) surface, producing an SPV higher than that of the bare material. The S-scheme heterojunction leads to a remarkable increase in charge separation, with 11 additional photogenerated electrons and 3,722 additional holes compared to the bare CdS and BiOBr. These results offer critical insights into the spatially resolved charge-transfer mechanisms of S-scheme heterojunctions.