Crystalline Intramolecular Ternary Carbon Nitride Homojunction for Photocatalytic Hydrogen Evolution

The construction of intramolecular homojunction in the carbon nitride framework is one of the advantageous methods to improve photocatalytic performance. Based on the molecular self-assembly strategy to form the intramolecular homojunction, carbon nitride showed enhanced photocatalytic activity. However, the disordered structure of the pristine g-C3N4 obtained using the traditional approach inhibits the dissociation and migration of photocarriers. Here, we adopt a method to prepare the crystalline C3N4 (HCCN) with functional group modifications to form ternary intramolecular homojunction by multistep thermal polymerization. The band structure indicates that cyano and cyanamide-based molecular units in HCCN samples have different highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels, which constitute a special ternary homojunction through the arrangement of energy levels and form an internal electron field in the molecule. The analysis of photoelectron–hole pairs and photocatalytic performance confirms that the ternary structure accelerates the interface charge transfer and reduces reverse charge recombination. Moreover, the compact crystalline structure of HCCN samples greatly improves the dissociation photogenerated charges. Femtosecond transient absorption spectra explain the energy trajectory of charge carrier kinetics of HCCN and confirm that the crystal semiconductor modified by functional groups can promote the dissociation of excitons to free charges. This research provides an idea to obtain crystalline C3N4 of ternary intramolecular homojunctions.