Optimization of g-C₃N₄ Nanostructures by CH₂ Introduction and Relay Modification for Photocatalytic Hydrogen Evolution

While extensive efforts have focused on increasing the level of photocatalytic hydrogen evolution of the g-C3N4 nanostructure, these approaches are often constrained by the excessive reliance on single-step modification methodologies, which significantly restricts the potential for performance enhancement. Herein, we propose a relay-modification strategy that begins with the occupation of the CH3-induced N defect sites in the g-C3N4 nanostructure with CH2 groups and is followed by the subsequent annealing process in ambient air. Computational modeling and material characterization suggested that the introduced CH2 groups could significantly accelerate change in charge carrier transportation within the g-C3N4, improve visible light absorption, and decrease the adsorption-free energy of hydrogen intermediates. Consequently, the g-C3N4 nanostructure enriched with CH2 groups yielded a hydrogen evolution rate of 9.0 mmol g–1 h–1, which is much higher than that of pristine g-C3N4 (2.3 mmol g–1 h–1). The subsequent relay modification, i.e., calcination treatment, yields an impressive H2 evolution rate of 14.3 mmol g–1 h–1, more than 16 times higher than that of the nonfunctionalized g-C3N4-derived sample and superior to most reported g-C3N4. Experimental characterizations showed that the remarkable hydrogen production activity could be attributed to relay-modification-induced enhanced visible light absorption and improved electron–hole pair separation.