Crystalline carbon nitrides for photocatalysis

Photocatalysis is considered as an effective approach to address energy and environmental issues. Carbon nitride (CN) is a promising metal-free semiconductor photocatalyst because of its unique properties such as tunable electronic band structure, facile/cheap synthesis and high chemical stability. However, the pristine CN prepared by the traditional thermal polymerization method is usually an amorphous or semi-crystalline conjugated bulk with a high density of structural defects, resulting in its moderate photocatalytic activity. Increasing the crystallinity of CN is an effective strategy to enhance its photocatalytic activity, and a few methods have been proposed, including high-temperature and high-pressure treatment, ionothermal method, solvothermal synthesis and microwave-assisted thermal polymerization. This review summarizes recent advances in the preparation of crystalline carbon nitrides (CCNs) and the design of CCNs-based photocatalysts in terms of nanostructure design, molecular structure engineering and construction of CCNs-based heterojunctions. In addition, their applications in a range of photocatalysis fields such as water splitting, carbon dioxide reduction, degradation of pollutants, organic synthesis and H₂O₂ production are reviewed. Finally, the concluding remarks are presented as well as challenges and prospects for future development of CCNs-based photocatalysts.