Poly(heptazine imide): Crystalline Allotrope of Polymeric Carbon Nitrides for Solar to Chemical Energy Conversion

Polymeric carbon nitride (PCN), an emerging semiconductor photocatalyst, has drawn intense interest for solar to chemical energy conversion, owing to advantages of metal-free contents, visible-light response, and robust stability. However, the amorphous melon-based PCN formed by conventional thermal-induced polymerization exhibits moderate photocatalytic performance, that is, restricted by its high exciton binding energy. In recent years, poly(heptazine imide) (PHI), a crystalline allotrope of PCN with high crystallinity and visible-light absorption, has been widely investigated in various photocatalytic applications due to its enhanced performance. In this mini-review, we have summarized the progress in the synthesis, modification, and photocatalytic applications of PHI. We have presented the synthetic protocols, electronic structure, and structural identification of PHI. This review summarizes and provides a detailed analysis of four main strategies for modifying PHI: crystal structure, nanostructure, molecular structure, and atomic composition. Finally, the article discusses the challenges and potential future directions for optimizing and advancing the functions of state-of-the-art PHI photocatalysts. These insights are expected to offer valuable perspectives for driving the development of PHI and related conjugated polymers.