Synergistic effect of nitrogen vacancy on ultrathin graphitic carbon nitride porous nanosheets for highly efficient photocatalytic H-2 evolution

Nanosheets exfoliation as one of the promising modification strategies for graphitic carbon nitride (g-C3N4) nanosheets has been extensively developed for improving the photocatalytic performance. However, it was commonly found that the enhancement of photocatalytic efficiency by effective nanosheets exfoliation hardly reached the expected satisfactory level, and the corresponding photocatalytic mechanism should be further investigated. Herein, on account of the well-designed thermal exfoliation strategy, the ultrathin and porous nanosheets with the thickness of three layers were elaborately constructed and possessed abundant N vacancies in the in-plane heptazine rings. Surprisingly, the ultrathin nanosheets exhibited superior visible-light-driven photocatalytic H-2-evolution activity, with a H-2-evolution rate (5.74 mmol h(-1) g(-1)) 28.7 times that of the pristine g-C3N4, and with an apparent quantum yield (AQY) of 14.9% (420 nm) much higher than that of the previously reported ultrathin g-C3N4 nanosheets. It was confirmed by systematical characterizations and theoretical calculation that, the ultrathin and porous features in cooperation with local separation of the highest occupied molecular orbital and lowest unoccupied molecular orbital sites by N vacancy-dominated in-plane electronic structure, synergistically strengthened the separation of photo-generated carriers. Meanwhile, the incorporation of the N vacancy-induced midgap state could bring the highly efficient excitation of photo-generated carriers, and abundant photocatalytic reaction sites could be provided by the ultrathin two-dimensional (2D) microstructure. Furthermore, the ultrathin nanosheet-induced quantum confinement effect could enlarge the bandgap and then boost the driving force for water reduction. This work developed one unique synthetic route to g-C3N4 nanosheets exfoliation, and highlighted the synergistic function of nanosheets exfoliation and defect engineering for highly efficient photocatalytic H-2 evolution, which would provide the feasible guidance for the exploitation of efficient g-C3N4 nanosheets-based photocatalytic system.