Efficient Degradation of Tetracycline via Coupling of Photocatalysis and Photo-Fenton Processes over a 2D/2D α-Fe2O3/g-C3N4 S-Scheme Heterojunction Catalyst

Graphitic carbon nitride (g-C3N4) has been widely used as a potential photocatalytic material for the removal of tetracycline from water. However, the poor visible light absorption ability and high recombination rate of the photogenerated charge significantly inhibit the catalytic activity of g-C3N4. Therefore, facile methods to improve the photocatalytic efficiency of g-C3N4 need to be developed. Hematite (alpha-Fe2O3), which has a good visible light absorption and corrosion resistance, is often used for photocatalysis and photo-Fenton reactions. Therefore, a two-dimension/two-dimension (2D/2D) S-scheme heterojunction constructed of g-C3N4 and alpha-Fe2O3 nanosheets could be expected to improve the degradation efficiency of tetracycline. In this study, 2D/2D Sscheme alpha-Fe2O3/g-C3N4 photo-Fenton catalysts were prepared using a hydrothermal strategy. The photo-Fenton catalytic activity of alpha-Fe2O3/g-C3N4 (alpha-Fe2O3 50% (w)) was significantly improved by the addition of a small amount of H2O2, removing 78% of tetracycline within 20 min, which was approximately 3.5 and 5.8 times the removal achieved using alpha-Fe2O3 and g-C3N4, respectively. The high catalytic activity was attributed to the synergy between the photocatalysis and Fenton reaction promoted by the continuous Fe3+/Fe2+ conversion over the 2D/2D S-scheme heterojunction. The 2D/2D S-scheme heterojunction was crucial in the fabrication of the alpha-Fe2O3/g-C3N4 photocatalyst with a large surface area, adequate active sites, and strong oxidation-reduction capability. Furthermore, the photo-Fenton reaction provided additional hydroxyl radicals for the degradation of tetracycline with the aid of H2O2. The excess reaction product (Fe3+) was reduced to Fe2+ by the photogenerated electrons from the conduction band of alpha-Fe2O3. The resulting Fe2+ could participate in the photo-Fenton reaction. The morphological structures of alpha-Fe2O3/g-C3N4 were analyzed using transmission electron microscopy to demonstrate the formation of a 2D/2D structure with face-to-face contact, and the optical properties of the composites were measured using ultraviolet-visible diffuse reflectance spectroscopy. alpha-Fe2O3/g-C3N4 possessed a significantly improved visible light absorption compared to g-C3N4. Five sequential cyclic degradation tests and X-ray diffraction (XRD) patterns obtained before and after the reaction showed that the alpha-Fe2O3/g-C3N4 composites possessed stable photo-Fenton catalytic activity and crystal structures. Transient photocurrent responses of alpha-Fe2O3/g-C3N4 demonstrated that the prepared composites exhibited a higher charge transfer efficiency compared to that of single alpha-Fe2O3 and g-C3N4. In addition, according to the photoluminescence analysis and active species trapping experiments, a possible S-scheme heterojunction charge transfer process in the photo-Fenton catalytic reaction was proposed. This study provided a promising method for the construction of a high-performance photo-Fenton catalytic system to remove antibiotics from wastewater.

Automated summary

In this study, a 2D/2D S-scheme heterojunction of α-Fe2O3/g-C3N4 was constructed and used as a photo-Fenton catalyst, significantly improving the degradation efficiency of tetracycline. The catalyst exhibited a high catalytic activity due to the synergy between photocatalysis and Fenton reaction. The study provided insights into the construction and reaction mechanism of the catalyst, offering a promising method for the development of high-performance photo-Fenton catalytic systems in wastewater treatment.