Engineering\nDual Carbon and Nitrogen Vacancies in\ng‑C<sub>3</sub>N<sub>4</sub> for Enhanced Photodegradation\nof Tetracycline Hydrochloride

Ultrathin g-C<sub>3</sub>N<sub>4</sub> nanosheets with\nspecific\nnitrogen vacancies and combined carbon and nitrogen dual vacancies\nwere created by annealing g-C<sub>3</sub>N<sub>4</sub> under different\natmospheric conditions. The nanosheets with dual vacancies showed\nsignificant improvements in the photodegradation of tetracycline hydrochloride\n(TC-HCl) compared with both the pristine g-C<sub>3</sub>N<sub>4</sub> and its nitrogen-deficient version. Various techniques, such as\nRaman spectroscopy, electron spin resonance (ESR), X-ray photoelectron\nspectroscopy, wavelength-dependent studies, electrochemical methods,\nand photoluminescence measurements, were used to identify vacancy\ndefects, revealing that performance enhancement was particularly notable\nunder visible light. Density functional theory calculations indicated\nthat dual vacancies introduced a shallow defect state above the valence\nband, enhancing visible light absorption and reducing electron–hole\npair recombination. Conversely, nitrogen vacancies alone formed a\ndeep defect state, which extended light absorption but potentially\ntrapped photoelectrons, limiting their contribution to photoreactions.\nRadical-scavenging experiments and ESR spin-trap spectra identified\nthe superoxide radical (·O<sub>2</sub><sup>–</sup>) as\nthe primary reactive oxygen species responsible for TC-HCl degradation.\nA comprehensive degradation pathway for TC-HCl was proposed using\nliquid chromatography–mass spectrometry data. This research\nhighlights a strategic approach to boost TC-HCl photodegradation by\nengineering the vacancies in g-C<sub>3</sub>N<sub>4</sub>.