Embedded iron and nitrogen co-doped carbon quantum dots within g-C₃N₄ as an exceptional PMS photocatalytic activator for sulfamethoxazole degradation: The key role of FeN bridge

The covalent modification of graphite-phase carbon nitride (g-C₃N₄) by carbon quantum dots (CQDs) is a promising way to improve its photocatalytic performance. Although CQDs can act as charge mediator for electrons storage and transfer, it is crucial to construct efficient interfaces, which serves as active sites for tight connect with g-C₃N₄ and fast release of electrons. Herein, a novel iron and nitrogen co-doped carbon quantum dots (Fe, N-CQDs) assembled g-C₃N₄ composite (FNCCN) was constructed via FeN bridge by hydrothermal and thermal polymerization methods. The FNCNN rivaled popular nitrogen-doped carbon quantum dots (N-CQDs) combined with g-C₃N₄ composites for peroxymonosulfate (PMS) activation and degradation of sulfamethoxazole (SMX). The SMX degradation rate (0.0963 min⁻¹) is 5.38 and 3.15 times higher than that of g-C₃N₄ and N-CQDs/g-C₃N_4. The characterization and DFT calculation results suggest that Fe, N-CQDs was successfully incorporated into carbon matrix of g-C₃N₄ via FeN covalent bond, which joints the two counterparts closely and creates electron transport channels for higher electrons transfer efficiency. The FeN coordinated structure promotes PMS reduction on Fe²⁺ of FeN sites for HO and SO₄⋅- production and expedited photo carries separation through Fe³⁺/Fe²⁺ cycling. In addition, the degradation efficiency, ROS identification, EPR test and degradation pathway of SMX was determined, and the intermediate toxicity of degradation products was predicted. This work constructed a tight atomic-level connection between hydrophobic g-C₃N₄ and hydrophilic CQDs, which improved an effective strategy for accelerating electron transport and efficient photocatalytic activation of PMS.