Graphene Quantum Dot-Modified Mn0.2Cd0.8S for Efficient Overall Photosynthesis of H2O2

Abstract

The photosynthesis of hydrogen peroxide (H₂O₂) via the selective two-electron oxygen reduction reaction (ORR) is emerging as a promising method for producing this important chemical. However, the reliance on sacrificial agents has limited the practical application of many photocatalysts. Herein, nitrogen-doped graphene quantum dots (NGQDs) are loaded onto the surface of Mn_xCd_1-xS solid solution nanowires to enable overall H₂O₂ photosynthesis in pure water under an air atmosphere. The optimized Mn_0.2Cd_0.8S/NGQDs (M_0.2NG5) composite achieves a high yield of 6885 µmol g⁻¹ h⁻¹, which is 10.4 times and 4.5 times higher than that of CdS (661 µmol g⁻¹ h⁻¹) and Mn_0.2Cd_0.8S (1522 µmol g⁻¹ h⁻¹), respectively. The NGQDs act as co-catalysts, enhancing the conductivity of the system. The strong electronegativity and polarity of the incorporated nitrogen and functional groups on the NGQDs edges enhance the ability of carbon atoms to activate O₂ into ·O₂⁻. The mechanism of H₂O₂ photosynthesis is investigated using in situ Fourier transform infrared spectroscopy, intermediate trapping experiments, and theoretical calculations. This work offers new insights into the design of non-noble metal co-catalyst-modified photocatalysts for sacrificial-agent-free H₂O₂ production.