ZnIn2S4/MOF S-scheme photocatalyst for H2 production and its femtosecond transient absorption mechanism

Abstract

Photocatalytic water splitting is a popular pathway for H2 evolution, but the slow water oxidation greatly hampers the overall activity. To harness photogenerated holes in an efficient and lucrative way, the water oxidation reaction is replaced by selective oxidation of organic compounds to achieve simultaneous production of H2 and value-added chemicals. Herein, an alternative tactic is reported where an organic compound (benzylamine, BA) not only serves as the precursor for N-benzylidene-benzylamine (NBBA) production but also provides hydrogen sources for H2 evolution, achieving the goal under anhydrous conditions. This process is realized using an S-scheme photocatalyst composed of ZnIn2S4 and the UiO-66-NH2 (U6N) metal-organic framework (MOF). The S-scheme carrier transfer mechanism was validated by in-situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) and femtosecond transient absorption (fs-TA) spectroscopy. With increased carrier efficiency and reinforced redox power endowed by the S-scheme heterojunction, the composite performed better than ZnIn2S4 and MOF. The performance was further ameliorated by Pt-cocatalyst modification, achieving an H2 production rate of 5275 μmol h−1 g−1 as well as BA conversion of 94.3% with 99.3% NBBA selectivity. Mechanistic studies reveal that BA is initially oxidized to carbon-centered radicals and further to imines along with the release of protons. The imine reacts with another BA molecule to form NBBA, while the protons are reduced to H2. This work provides new insights into concurrent photocatalytic H2 production and selective organic oxidation from organic amines using S-scheme photocatalysts.