BiOCl Atomic Layers with Electrons Enriched Active Sites Exposed for Efficient Photocatalytic CO₂ Overall Splitting

Given the limited exposure of active sites and the retarded separation of photogenerated charge carriers in those developed photocatalysts, photocatalytic CO₂ splitting into value-added chemicals has suffered from the poor activity and remained in great challenge for real application. Herein, hydrothermally synthesized BiOCl with layered structure (BOCNSs) was exfoliated into thickness reduced nanosheets (BOCNSs-w) and even atomic layers (BOCNSs-i) via ultrasonication in water and isopropanol, respectively. In comparison with the pristine BOCNSs, the exfoliated BiOCl, especially BOCNSs-i with atomically layered structure, exhibits much improved photocatalytic activity for CO₂ overall splitting to produce CO and O₂ at a stoichiometric ratio of 2:1, with CO evolution rate reaching 134.8 µmol g^-1 h^-1 under simulated solar light (1.7 suns). By surpassing the photocatalytic performances of the state-of-the-art Bi_lO_mX_n (X: Cl, Br, I) based photocatalysts, the CO evolution rate is further increased by 99 times, reaching 13.3 mmol g^-1 h^-1 under concentrated solar irradiation (34 suns). This excellent photocatalytic performance achieved over BOCNSs-i should be benefited from the shortened transfer distance and the increased built-in electric field intensity, which accelerates the migration of photogenerated charge carriers to surface. Moreover, with oxygen vacancies (V_O) introduced into the atomic layers, BOCNSs-i is exposed with the electrons enriched Bi active sites that could transfer electrons to activate CO₂ molecules for highly efficient and selective CO production, by lowering the energy barrier of rate-determining step (RDS), *OH + *CO₂^- → HCO₃^-. It is also realized that the H₂O vapor supplied during photocatalytic reaction would exchange oxygen atoms with CO₂, which could alter the reaction pathways and further reduce the energy barrier of RDS, contributing to the dramatically improved photocatalytic performance for CO₂ overall splitting to CO and O₂.