Supercritical CH₃OH-Triggered Isotype Heterojunction and Groups in g‑C₃N₄ for Enhanced Photocatalytic H₂ Evolution

The structure tuning of bulk graphitic carbon nitride (g-C₃N₄) is a critical way to promote the charge carriers dynamics for enhancing photocatalytic H₂-evolution activity. Exploring feasible post-treatment strategies can lead to effective structure tuning, but it still remains a great challenge. Herein, a supercritical CH₃OH (ScMeOH) post-treatment strategy (250–300 °C, 8.1–11.8 MPa) is developed for the structure tuning of bulk g-C₃N₄. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. During the ScMeOH post-treatment process, CH₃OH molecules diffused into the interlayers of g-C₃N₄ and subsequently participated in N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C₃N₄ into carbon nitride with a poly­(heptazine imide)-like structure (Q-PHI) as well as abundant methyl and hydroxyl groups. The modified g-C₃N₄ showed enhanced photocatalytic activity with an H₂-evolution rate 7.2 times that of pristine g-C₃N₄, which was attributed to the synergistic effects of the g-C₃N₄/Q-PHI isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C₃N₄ and serves as a case for the application of supercritical fluid technology in photocatalyst synthesis.