Fabrication of structural defects and carboxyl groups on graphitic carbon nitride with enhanced visible light photocatalytic activity

Structural defects and surface functional group defects engineering in graphitic carbon nitride (g-C3N4) have great significance for optimizing its electron structure and photocatalytic activity. In this study, g-C3N4 was prepared via a solvothermal method without high-temperature calcination, and then carboxyl group defects-containing g-C3N4 was synthesized through post-modification. FTIR and XPS verified the presence of highly favorable carboxyl groups. Morphological and photoelectrical experiments verified that adding branched chains with carboxyl groups onto g-C3N4 effectively increased the specific surface area and pore volume of g-C3N4, improved the absorption of visible light and enhanced the separation and migration of photogenerated carriers. As a result, the modified g-C3N4 (CN-COOH) had a high bisphenol A (BPA) photocatalytic degradation of 99.6% at 60 min and a high kinetics constant of 0.067 min−1, which was 13.4 times that of the original g-C3N4 under visible light irradiation. In addition, CN-COOH showed high photocatalytic stability with photocatalytic activity decreased by only 1.5% after 5 cycles. Active species capture experiments have demonstrated that •O2− and holes are the main active substances in the BPA photocatalytic process. These findings open a new avenue for precise carboxyl modification of g-C3N4 via an environmentally-friendly post-modification approach and provide the mechanism of carboxyl defects of g-C3N4 photocatalysts on prompting photogenerated charge separation.