Development of template methods for the preparation of porous photocatalysts of graphite-like carbon nitride

As a metal-free photocatalyst with high catalytic performance, carbon nitride is non-toxic, harmless, and stable in the natural environment. Owing to its facile synthesis, stable physical and chemical properties, tunable structure, and suitable band gap, graphite-like carbon nitride (g-C₃N₄) plays an increasing role in the field of photocatalysis. It has attracted extensive attention in the fields of evolution of hydrogen and oxygen via water-splitting hydrolysis and in the degradation of organic pollutants. In particular, g-C₃N₄ is identified to have a high specific surface area (SSA) because of its special lamellar structure. Meanwhile, the abundant pores intrinsic in it are able to provide both transporting channels for photogenic carriers or reactive species and a large number of active sites for redox reactions. These merits endow it with high photoelectrical properties. The preparation methods of the pore structures of such catalyst include hard templates, soft templates, and non-template ones. The hard template method enables the preparation of regular pore structures but requires additional removal treatment. However, the soft templates can be decomposed during the high-temperature preparation of g-C₃N₄, which avoids the use of toxic reagents and consequently is harmless to the environment, and the template-free method does not involve any templates, which will simplify the experimental process from the aspect of sample preparation with reduced cost. In this paper, the advantages and disadvantages of various preparation methods were elaborated and compared based on the literature review in recent years. The developments and applications in the environmental and energy aspects were summarized by combining the commonly used modification methods, which provided the perceptions with respect to the development of metal-free g-C₃N₄-based photocatalysts in the future. Further, the photocatalytic mechanism was explained, and the four different precursors of g-C₃N₄ were compared. Finally, the ongoing outlook and perspectives will be covered in this review.