Water-Assisted Formation of Amine-Bridged Polymeric Carbon Nitride: A Structural Insight into the Photocatalytic Performance for H2 Evolution under Visible Light.

Polymeric carbon nitride (C₃N₄) is a promising metal-free photocatalyst for visible-light-driven H₂ production because of its suitable band structure, stability, and earth-abundant composition.[1] However, the relationship between its intraplanar structure and photocatalytic activity is still unclear, especially the role of hydrogen bonds between tri-s-triazine units. Here, a simple water-assisted thermal polycondensation method was used to control hydrogen-bond-related structures and amine-bridged linkages in polymeric C₃N₄. Melamine was treated with water and condensed under humidified air to prepare CN-H samples, while CN-A samples were synthesized without water.
The CN-H materials showed higher photocatalytic H₂ evolution activity under visible light than the corresponding CN-A samples. Among them, CN650-H showed the best performance, indicating that water assistance and high condensation temperature effectively enhance hydrogen production. Structural analyses revealed that water-assisted synthesis reduced hydrogen bonds and increased amine bridges connecting tri-s-triazine units. These changes produced larger in-plane domains, higher crystallinity, and a more ordered C₃N₄ network.
Photophysical and electrochemical analyses showed that CN650-H had improved visible-light absorption, longer-lived charge carriers, higher photocurrent response, and lower interfacial charge-transfer resistance. These results indicate that amine bridges provide efficient pathways for intraplanar charge migration, whereas excessive hydrogen bonding restricts charge movement and accelerates recombination. Thus, water-assisted formation of amine-bridged C₃N₄ promotes electron transfer to Pt co-catalyst sites, resulting in enhanced H₂ evolution. This work clarifies the structural effects of hydrogen bonds and amine bridges in polymeric carbon nitride and suggests a strategy for designing efficient metal-free photocatalysts. [1] Jang et al. Appl. Catal. B: Environ. 2022, 310, 12131