Effect of oxidation with HNO₃ on the photocatalytic properties of g-C₃N₄

Full text: The search for new efficient photocatalysts, aligned with the concept of sustainability, has aroused the interest of many scholars due to two major problems faced by humanity, the energy deficit and environmental pollution. In this context, there is graphitic carbon nitride (g-C₃N₄), a semiconductor material without toxicity, of low cost, practicality of preparation and structure of bands properly positioned for photooxidation and reduction of H₂O molecules. However, despite these advantages, g-C₃N₄ has some limitations, such as its low surface area and high rate of recombination. In this work, we investigated the effect of oxidation of g-C₃N₄ with HNO₃ in a photocatalysis reaction. The g-C₃N₄ was synthesized by the polycondensation of melamine and the material obtained was subjected to acid treatment with HNO₃ in concentrations of 1, 3 and 5M. All samples were characterized by different techniques, including DRX, FTIR, SEM. Photocatalytic activity was assessed by photodiscolouring the indigo carmine dye (20 mg/L) in the presence of 15 mg of g-C₃N₄ and 25 μL of H₂O₂ under visible light irradiation (White light bulb, 12 W). The g-C3N4 -3M sample showed the best rate of light discoloration, 66%, while the sample, without acid treatment, of 28%, thus, based on the characterizations performed, we believe that functional groups oxygenated with intermediate NO_x, C-OH and/or N-OH, may be responsible for the H₂O₂ reduction reaction forming •OH resulting in improved photocatalytic efficiency. The results presented showed that the photocatalytic properties of g-C₃N₄ have been improved, however, a rigorous study is needed regarding the reaction conditions to obtain a material with a high concentration of these intermediate NO_x functional groups. (author)