Unraveling the Presence and Positions of Nitrogen Defects in Defective g-C3N4 for Improved Organic Photocatalytic Degradation: Insights from Experiments and Theoretical Calculations

Abstract

In this work, nitrogen-defective g-C3N4 with different nitrogen defect densities is synthesized for ciprofloxacin photocatalytic degradation. Compared with pristine g-C3N4, g-C3N4 etched with NaBH4 for 1 h exhibits an approximately ten-fold increase in the rate constant of ciprofloxacin (CIP) degradation. The combined experimental analysis and theoretical calculations reveal that nitrogen defects can be incorporated into g-C3N4 in all nitrogen sites and that C─N═C is the most susceptible site. By incorporating nitrogen defects to induce defect states between the conduction band (CB) and valence band (VB), the electronic and band structures are tuned. The induced defect states can be downshifted to approach the valance band, reaching increased nitrogen defect density within optimum ranges to accommodate excited electrons to narrow the bandgap, extend the light absorption capability, and enhance the charge carrier separation and transfer efficiency. The g-C3N4 etched by NaBH4 for 2 h with over-introduced nitrogen defects exhibits a declined performance due to a deteriorated structure, and the over-downshifted defect states turn out to be a new recombination center for charge carriers.