Crystal Phase Engineering of Carbon Nitride: Advances in Regulation Strategies and Photocatalytic Applications

Crystal phase engineering emerges as a pivotal strategy to overcome the inherent limitations (e.g., structural defects, rapid charge recombination, and insufficient light absorption), plaguing conventional graphitic carbon nitride (g-C₃N₄) synthesized by thermal polymerization for photocatalysis. This review systematically explores this innovative approach, contrasting the distinct properties and photocatalytic advantages of graphite, crystalline (CCN), and amorphous (ACN) carbon nitride phases. CCN leverages an extended π-conjugated system for superior charge transport and enhanced light absorption, while ACN exploits abundant vacancies to reduce reaction barriers, narrow the bandgap, and improve light harvesting. We critically assess their performance in diverse applications (H₂O₂ production, H₂ evolution, CO₂ reduction, pollutant degradation, biomass oxidation), emphasizing the decisive role of the engineered crystal phase in optimizing activity. Despite progress, challenges in efficiency, stability, and complex-condition applicability persist. Future research must prioritize refining synthesis for precise phase control, enhancing operational stability, and elucidating underlying reaction mechanisms to advance practical environmental and energy applications.