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A covalent organic framework (COF) named TpPa-1 was co-modified by chlorine grafting and ZnIn2S4integration for concurrent photocatalytic H2O2generation and wastewater purification under visible light. The introduction of electron-withdrawing chlorine groups on TpPa-1 decreases the density of adjacent electron clouds, which promotes the charge carrier separation and the intimate combination with ZnIn2S4by Cl–Zn/Cl–In bonds, providing an appreciably enhanced photocatalytic activity. Specifically, in pure water, the optimal ZnIn2S4/TpPa-Cl2sample can achieve a H2O2yield of 766 μmol L−1, which is 1.5, 2, 7.8 and 5 times those of ZnIn2S4/TpPa-1, TpPa-Cl2, ZnIn2S4and TpPa-1, respectively. In the tetracycline solution, 95% of tetracycline was degraded and, concomitantly, H2O2with a yield of 846 μmol L−1was generated. It is found that H2O2generation undergoes a two-step O2reduction with ·O2−as the intermediate, while ·O2−, holes and ·OH are the active species in tetracycline degradation. Likewise, other common pollutants like o-nitrophenol, acid golden yellow and acid red in wastewater could also be effectively degraded along with the appreciable production of H2O2, which realizes the so-called wastewater valorization. This study could advance COF-based photocatalysts and provide a novel insight into wastewater treatment.

Defect engineering has recently received much attention as an effective approach for improving photocatalytic performances. Herein, UiO-66-NH2with missing ligand defects (DUiO-66-NH2) wrapped by ZnIn2S4was investigated for photocatalytic H2O2production under visible light. The defects in DUiO-66-NH2enable efficient charge separation and O2capture, which are the key factors in accelerating H2O2production. Specifically, the O2adsorption capacity over ZnIn2S4/DUiO-66-NH2was improved by a factor of 3.1 as compared to its counterpart without defects. As a result, in ambient air and pure water without any sacrificial agents, the H2O2yield of ZnIn2S4/DUiO-66-NH2was 340 μmol L−1, which was significantly higher than that of ZnIn2S4/UiO-66-NH2(150 μmol L−1). The main H2O2formation pathway over ZnIn2S4/DUiO-66-NH2is an indirect oxygen reduction reaction with ·O2−as the intermediate, and the defects in UiO-66-NH2could act as active sites to adsorb and activate O2to produce ·O2−. However, the H2O2generation over ZnIn2S4/UiO-66-NH2undergoes both indirect and direct oxygen reduction reactions. This work could provide new insights and inspire further research into defect engineering of MOFs and photocatalytic H2O2synthesis.