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.