COF/In₂S₃ S-Scheme Photocatalyst with Enhanced Light Absorption and H₂O₂-Production Activity and fs-TA Investigation

Photocatalytic hydrogen peroxide (H₂O₂) synthesis from water and O₂ is an economical, eco-friendly, and sustainable route for H₂O₂ production. However, single-component photocatalysts are subjected to limited light-harvesting range, fast carrier recombination, and weak redox power. To promote photogenerated carrier separation and enhance redox abilities, an organic/inorganic S-scheme photocatalyst is fabricated by in situ growing In₂S₃ nanosheets on a covalent organic framwork (COF) substrate for efficient H₂O₂ production in pure water. Interestingly, compared to unitary COF and In₂S₃, the COF/In₂S₃ S-scheme photocatalysts exhibit significantly larger light-harvesting range and stronger visible-light absorption. Partial density of state calculation, X-ray photoelectron spectroscopy, and femtosecond transient absorption spectroscopy reveal that the coordination between In₂S₃ and COF induces the formation of mid-gap hybrid energy levels, leading to smaller energy gaps and broadened absorption. Combining electron spin resonance spectroscopy, radical-trapping experiments, and isotope labeling experiments, three pathways for H₂O₂ formation are identified. Benefited from expanded light-absorption range, enhanced carrier separation, strong redox power, and multichannel H₂O₂ formation, the optimal composite shows an impressive H₂O₂-production rate of 5713.2 µmol g^-1 h^-1 in pure water. This work exemplifies an effective strategy to ameliorate COF-based photocatalysts by building S-scheme heterojunctions and provides molecular-level insights into their impact on energy level modulation.