Precise defect engineering g-C3N4 fabrication to improve hydrogen production performance

•Vacancy and cyanamide defects modified g-C3N4 is prepared by a multi-step strategy.•The performance of NCMCN for HER is growth about 10.65 folds compared with MCN.•Mechanism of the improved performance is revealed by experiments and DFT results. Designing a reasonable defect structure in carbon nitride could effectively suppress the recombination of charges and holes. However, there was a lack of precise strategies for planning defect engineering. To address this, a multi-step strategy involving secondary thermal condensation and sodium assistance was successfully developed. This strategy synthesized -NHx vacancies and cyanamide defects co-modified g-C3N4 (NCMCN). By adjusting the bulk g-C3N4 (MCN), the two defect structures obtained enhanced the light response ability, increased the specific surface area, and reduced the band gap. These improvements were supported by experimental results and density functional theory (DFT) calculations. Consequently, NCMCN demonstrated outstanding photocatalytic performance compared to both pristine MCN and nitrogen vacancy-modified NVMCN. Notably, the Hydrogen Evolution Reaction (HER) results of NCMCN were 10.65 times higher than those of MCN.