Optimization of Combined Hydrothermal and Thermal Synthesis Conditions of g-C₃N₄ for Enhanced Photocatalytic Methane Production

Developing efficient technologies for converting solar energy into chemical fuels, known as artificial photosynthesis, is crucial for achieving carbon neutrality and addressing global energy crises. Photocatalytic methane (CH₄) production from organic waste represents a highly promising approach to generating clean energy without relying on fossil fuels. Graphitic carbon nitride (g-C₃N₄) has garnered significant attention as a photocatalyst due to its excellent chemical stability and abundance. However, bulk g-C₃N₄ suffers from a low specific surface area and a rapid recombination of photogenerated electrons and holes, limiting its practical application.

To overcome these limitations and realize efficient solar-to-chemical energy conversion, this work focuses on optimizing the synthesis conditions of g-C₃N₄ through a combination of preliminary hydrothermal treatment and subsequent thermal decomposition. Using melamine as the starting material, synthetic parameters specifically the solvent volume during hydrothermal process and the final calcination temperature were systematically investigated to enhance the specific surface area and control morphology. The optimal conditions were determined to be a solvent volume of 20 mL and a calcination temperature of 550 ℃. The structural, morphological, and optical properties were comprehensively evaluated using XRD, SEM, BET, PL, DRS, and electrochemical measurements.

The photocatalytic methane production was conducted from an aqueous acetic acid solution under visible light irradiation (450 nm) for 6 hours. Cu was applied as a co-catalyst to enhance the photocatalytic activity, and the evolved methane gas was analyzed using a GC-FID system. Under the optimized conditions, the modified catalyst achieved a quantitative methane production rate of 44.1 µmol g^-1 h^-1, which was more than seven times higher than pure g-C₃N₄. Characterization results revealed that the synergistic optimization successfully expanded the active surface area and altered the morphology, significantly suppressing electron-hole recombination. These findings provide a simple yet effective strategy for designing high-performance g-C₃N₄ photocatalysts for sustainable solar fuel production.