Understanding the Poly (Triazine Imide) Crystals Formation Process: The Conversion from Heptazine to Triazine

Abstract Poly (triazine imide) (PTI) generally obtained via ionothermal synthesis features extended π‐conjugation and enhanced crystallinity. However, in‐depth investigation of the polycondensation process for PTI is an onerous task due to multiple influencing factors and limited characterization techniques. Herein, to simplify the polymerization route and exclude non‐essential factors, PTI was prepared by calcining only melamine and LiCl. This study aims to identify the pivotal role of LiCl in PTI formation, which can convert heptazine‐based intermediates into more stable triazine‐based PTI framework. Based on this discovery, we demonstrate the transformation process of the prepared samples from amorphous Bulk g‐C 3 N 4 to regular PTI, and further prove that the reaction with LiCl causes disruption of heptazine covalent organic frameworks. Additionally, the PTI exhibits higher photocatalytic water splitting performance due to efficient charge carrier mobility and separation, as well as faster reaction kinetics. This discovery deepens understanding of the polycondensation process of PTI crystals and provides insights toward the rational design of crystalline carbon nitride‐based semiconductors.