Excited Electron‐Rich Bi(3–x)+ Sites: A Quantum Well‐Like Structure for Highly Promoted Selective Photocatalytic CO2 Reduction Performance

Most of the current research on the photocatalytic mechanism of semiconductors is still on the simulation and evaluation of ground‐state active sites. Insights into photogenerated electron transition paths and excited‐state active sites during photocatalysis are still insufficient. Herein, combining femtosecond time‐resolved transient absorption spectroscopy, in situ Fourier‐transform infrared spectroscopy, synchronous illumination X‐ray photoelectron spectroscopy, and theoretical calculation results rationally reveal that in complex bimetallic oxyhalides the ultrathin rich oxygen vacancies (ROV) PbBiO2Cl (PBOC) double unit cell (DUC) layers facilitate migration and separation of photogenerated electrons from the bulk to Bi sites near the surface oxygen vacancies (OVs), then form the excited electron‐rich Bi(3–x)+ sites like quantum well structure. The excited Bi(3–x)+ sites act as wells for photogenerated electrons leading to lower energy barrier in the rate determining step for the formation of *CO from *COOH intermediate. Without photosensitizers and sacrificial agents, ROV DUC PBOC exhibit high CO generation rate (16.02 µmol h–1 g–1) that is 18 times higher than that of bulk PBOC. In situ characterization combined with theoretical calculation provides effective insight into the photocatalytic mechanism of photoexcited semiconductor materials. In the photoexcited state, the Bi atoms near the oxygen vacancies constitute quantum‐well‐like sites and photo‐generated electrons accumulate to form Bi(3–x)+ sites, which boost separation of photo‐generated electrons and decrease the energy barrier in the rate determining step from *COOH to *CO. Thus, the rich oxygen vacancies PbBiO2Cl double unit cell layers exhibit enhanced photocatalytic CO2 to CO conversion performance.