Engineering Dual Carbon and Nitrogen Vacancies in g‑C3N4 for Enhanced Photodegradation of Tetracycline Hydrochloride

Ultrathin g-C3N4 nanosheets with specific nitrogen vacancies and combined carbon and nitrogen dual vacancies were created by annealing g-C3N4 under different atmospheric conditions. The nanosheets with dual vacancies showed significant improvements in the photodegradation of tetracycline hydrochloride (TC-HCl) compared with both the pristine g-C3N4 and its nitrogen-deficient version. Various techniques, such as Raman spectroscopy, electron spin resonance (ESR), X-ray photoelectron spectroscopy, wavelength-dependent studies, electrochemical methods, and photoluminescence measurements, were used to identify vacancy defects, revealing that performance enhancement was particularly notable under visible light. Density functional theory calculations indicated that dual vacancies introduced a shallow defect state above the valence band, enhancing visible light absorption and reducing electron–hole pair recombination. Conversely, nitrogen vacancies alone formed a deep defect state, which extended light absorption but potentially trapped photoelectrons, limiting their contribution to photoreactions. Radical-scavenging experiments and ESR spin-trap spectra identified the superoxide radical (·O2 –) as the primary reactive oxygen species responsible for TC-HCl degradation. A comprehensive degradation pathway for TC-HCl was proposed using liquid chromatography–mass spectrometry data. This research highlights a strategic approach to boost TC-HCl photodegradation by engineering the vacancies in g-C3N4.