• BiOCl atomic layers (BOCNSs-i) were prepared by exfoliating hydrothermally synthesized BiOCl via ultrasonication in isopropanol, achieving efficient photocatalytic CO2 overall splitting to CO and O2.
• BOCNSs-i exhibits a CO evolution rate of 134.8 µmol g−1 h−1 under simulated solar light (1.7 suns) and 13.3 mmol g−1 h−1 under concentrated solar irradiation (34 suns), surpassing state-of-the-art BilOmXn photocatalysts.
• The atomic-layer thickness shortens charge transfer distance and enhances built-in electric field, promoting charge carrier separation and migration to the surface.
• Oxygen vacancies introduce electrons enriched Bi active sites that lower the energy barrier of the rate-determining step, while H2O vapor exchanges oxygen atoms with CO2 to further improve photocatalytic performance.