Femtosecond transient absorption spectroscopy investigation on ultrafast electron transfer in S-scheme ZnO/CdIn2S4 photocatalyst for H2O2 production and benzylamine oxidation

Photocatalytic hydrogen peroxide (H₂O₂) production is a crucial process for clean energy conversion, involving the reduction of O₂ through two electrons. However, this process is often hampered by the sluggish water oxidation involving the photogenerated holes. To address this challenge, we have constructed a dual-functional S-scheme ZnO/CdIn₂S₄ heterojunction systerm coupling the H₂O₂ generation with a value-added benzylamine (BA) oxidation reaction. In this dual-functional photocatalytic system, photogenerated electrons in CdIn₂S₄ efficiently reduce O₂ to produce H₂O₂, while photogenerated holes in ZnO selectively oxidize BA to N-benzylidenebenzylamine. Leveraging the advantages of the S-scheme heterojunction, the optimized ZnO/CdIn₂S₄ photocatalyst displays an enhanced H₂O₂ production rate (386 ​μmol·L⁻¹·h⁻¹) and BA oxidation fraction (81 ​%) than pure ZnO or CdIn₂S₄. Femtosecond transient absorption (fs-TA) spectroscopy confirm the ultrafast S-scheme electron transfer from the ZnO conduction band (CB) to the CdIn₂S₄ valence band (VB) upon photoexcitation of the ZnO/CdIn₂S₄ composite. Besides, timely depletion of VB holes in ZnO and CB electrons in CdIn₂S₄ can accelerate the interfacial electron transfer in the ZnO/CdIn₂S₄ S-scheme heterojunction. The innovative design of the ZnO/CdIn₂S₄ S-scheme photocatalyst provides new insights for developing efficient dual-functional heterojunction photocatalytic systems and introduces a novel method for studying S-scheme heterojunctions using fs-TA spectroscopy.