Investigating the charge transfer mechanism of ZnSe QD/COF S-scheme photocatalyst for H2O2 production by using femtosecond transient absorption spectroscopy

Hydrogen peroxide (H₂O₂) has gained widespread attention as a versatile oxidant and a mild disinfectant. Here, an electrostatic self-assembly method is applied to couple ZnSe quantum dots (QDs) with a flower-like covalent organic framework (COF) to form a step-scheme (S-scheme) photocatalyst for H₂O₂ production. The as-prepared S-scheme photocatalyst exhibits a broad light absorption range with an edge at 810 nm owing to the synergistic effect between the ZnSe QDs and COF. The S-scheme charge-carrier transfer mechanism is validated by performing Fermi level calculations and in-situ X-ray photoelectron and femtosecond transient absorption spectroscopies. Photoluminescence, time-resolved photoluminescence, photocurrent response, electrochemical impedance spectroscopy, and electron paramagnetic resonance results show that the S-scheme heterojunction not only promotes charge carrier separation but also boosts the redox ability, resulting in enhanced photocatalytic performance. Remarkably, a 10%-ZnSe QD/COF has excellent photocatalytic H₂O₂-production activity, and the optimal S-scheme composite with ethanol as the hole scavenger yields a H₂O₂-production rate of 1895 mol g^–1 h^–1. This study presents an example of a high-performance organic/inorganic S-scheme photocatalyst for H₂O₂ production.