Effect of thermal program on structure–activity relationship of g-C3N4 prepared by urea pyrolysis and its application for controllable production of g-C3N4

Graphitic carbon nitride (g-C3N4) can be easily prepared by urea pyrolysis; however, its properties are greatly affected by the thermal program used in the pyrolysis process, especially the heating rate, owing to the complexity of the pyrolysis reaction. In this work, the effects of three main parameters—pyrolysis temperature, heating time, and heating rate—on the structure and performance of g-C3N4 are systematically investigated, and the results are used to guide the controllable production of g-C3N4 to obtain suitable properties for application as a photocatalyst, adsorbent, or lubricant. The results show that urea pyrolysis follows the same reaction path regardless of the parameters. However, the polymerization reaction rate significantly affects the structure, morphology, and surface composition of g-C3N4, indicating a clear structure–activity relationship. Rapid heating produces g-C3N4 with loose thin layers, a larger surface area, more amino defects, and more conjugated structure; these characteristics promote adsorption and photocatalysis. Slow heating produces g-C3N4 with a higher polymerization degree, more complete layer structure, and tighter layer spacing, which is suitable for lubrication. A high pyrolysis temperature and long heating period further enhance the performance of g-C3N4. This study provides a reference for large-scale controllable production of g-C3N4 by urea pyrolysis.