Oxygen doping through oxidation causes the main active substance in g-C3N4 photocatalysis to change from holes to singlet oxygen.

Abstract Improving the photocatalytic activity of graphitic carbon nitride (g-C3N4) for organic pollutant removal from water and in-depth study of the effect of oxygen doping on its photocatalytic mechanism are deserving of research attention. Oxygen-doped g-C3N4 with a suitable degree of oxidation was prepared by oxidation using concentrated acid–ultrasound double oxidation. Oxygen doping by oxidation changed the physical and chemical properties of g-C3N4, and improved its rhodamine B photocatalytic degradation efficiency. The physical and chemical properties of g-C3N4 were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and zeta potential analysis, among other methods. The photocatalytic mechanism was also studied in depth using photoluminescence, electrochemical impedance spectroscopy, transient photocurrent measurements, diffuse reflectance spectroscopy, and Mott–Schottky plots. An appropriate degree of oxidation introduced pit-like defects rich in oxygen-containing functional groups onto the g-C3N4 surface, which improves its photocatalytic performance. Furthermore, in the photocatalytic process of oxidized g-C3N4, oxygen doping was confirmed to change the main active substance of g-C3N4 photocatalysis from holes to singlet oxygen. This study provides an in-depth explanation of the photocatalytic mechanism of g-C3N4 doped with oxygen, which provides guidance and reference for the design of photocatalysts for organic pollutant removal from water and the analysis of photocatalytic mechanisms for water treatment.