Synthesis of g-C₃N₄-based nanocomposites with low Au loading for efficient methylene blue degradation

In this study, nanocomposite materials were synthesized in two consecutive processes. Gold nanoparticles (AuNPs) were simultaneously deposited on the zinc oxide (ZnO) substrate and then calcined with urea to form the Au/ZnO/graphitic carbon nitride (g-C₃N₄) composite. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and high-resolution transmission electron microscopy were used to analyze the crystal structures, content, and morphology of the Au/ZnO/g-C₃N₄sample. The composite exhibited an average crystallite size of about 25-30 nm. In addition, the optical properties of the samples were investigated using UV-vis diffuse reflectance spectra (DR/UV-VIS). This revealed the characteristic surface plasmon resonance peak of the AuNPs at 543 nm, and the 0.1-Au/ZnO/g-C₃N₄nanocomposite had a smaller band gap (3.154 eV) than ZnO. Under the optimal conditions (0.1-Au/ZnO/g-C₃N₄of 0.5 g l^-1, methylene blue of 10 mg l^-1, and pH of 6), the reaction proceeds rapidly with a rate constant (k_ap) of 0.023 min^-1and the 0.1-Au/ZnO/g-C₃N₄nanocomposite had the highest photoreaction with the degradation efficiency of 94.9% in 120 min. The superiority of this material is that it has a high efficiency compared to previous studies on the combination of Au, ZnO, and g-C₃N₄, while it uses only a minimal Au content of 0.1 wt.% compared to the mass of ZnO. It is exceptionally stable and resistant to alkalis and acids. The activity of the radical scavengers was also tested, and the results indicated that •OH and •O₂^-radicals play a crucial role in the degradation process. The ability to degrade various dyes, the high reusability after 4 regenerations, and the efficiency are still maintained at approximately 90%. The electrical energy consumption of 65.83 WhL^-1shows the potential of the material for future applications.