Unraveling the dual defect sites in graphite carbon nitride for ultra-high photocatalytic H2O2 evolution

Abstract

Defect engineering modified graphite carbon nitride (g-C₃N₄) has been widely used in various photocatalytic systems due to the enhanced catalytic activity of multiple defect sites (such as vacancies or functional groups). However, the key mechanism of action in each defect site in the corresponding photocatalytic surface reactions is still unclear. Here, the –CN groups and N vacancies were sequentially introduced into g-C₃N₄ (Nv–CN–CN) for photocatalytic production of high-value and multifunctional H₂O₂, and the effect of dual defect sites on the overall photocatalytic conversion process was systematically analyzed. The modification of the dual defect sites forms an electron-rich structure and leads to a more localized charge density distribution, which not only enhances the light absorption and carrier separation capabilities, but also significantly improves the selectivity and activity of H₂O₂ generation. Importantly, detailed experimental characterizations and theoretical calculations clearly revealed the key role of each defect site in the photocatalytic H₂O₂ surface reaction mechanism: the N vacancies can effectively adsorb and activate O₂, and the –CN groups facilitate the adsorption of H⁺, which synergistically promote H₂O₂ generation. The Nv–CN–CN reached a H₂O₂ generation rate of 3093 μmol g⁻¹ h⁻¹ and achieved an apparent quantum efficiency of 36.2% at 400 nm, significantly surpassing the previously reported g-C₃N₄-based photocatalysts. Meanwhile, a solar-to-chemical conversion efficiency of 0.23% was achieved in pure water. Constructing defects and understanding their crucial role provides significant insights into the rational use of defect engineering to design and synthesize highly active catalytic materials for energy conversion and environmental remediation.