Molecular structures of carbon nitrides for photocatalysis

In the dynamic landscape of artificial photosynthesis research, the recent focus has both on the inorganic semiconductors and the polymeric materials. Carbon nitrides (CN), with melon as a noteworthy representative, have emerged as promising candidates for polymeric semiconductors and photocatalysts. However, the reported CNs are often misidentified and mispresented. This review emphasizes the classification of the CN family from the formation process and the characterization of these materials, shedding light on the distinctions between ionothermal and conventional synthesis via thermal condensation. Five types of CNs are first distinguished based on their 2D structures. Then, the corresponding synthesis process, characterization, and photocatalytic applications are summarized. Furthermore, the role of different ions in crystalline PTI-CN and PHI-CN structures is discussed. We conclude with a discussion on identifying the reported CNs by combining designation and the synthesis process.