Crystalline Intramolecular Ternary Carbon Nitride Homojunction forPhotocatalytic Hydrogen Evolution

The construction of intramolecular homojunction in thecarbon nitride framework is one of the advantageous methods to improvephotocatalytic performance. Based on the molecular self-assembly strategyto form the intramolecular homojunction, carbon nitride showed enhancedphotocatalytic activity. However, the disordered structure of the pristine g-C3N4obtained using the traditional approach inhibits the dissociation andmigration of photocarriers. Here, we adopt a method to prepare thecrystalline C3N4(HCCN) with functional group modifications to formternary intramolecular homojunction by multistep thermal polymerization.The band structure indicates that cyano and cyanamide-based molecularunits in HCCN samples have different highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels, which constitute aspecial ternary homojunction through the arrangement of energy levels andform an internal electronfield in the molecule. The analysis of photoelectron-hole pairs and photocatalytic performance confirmsthat the ternary structure accelerates the interface charge transfer and reduces reverse charge recombination. Moreover, the compactcrystalline structure of HCCN samples greatly improves the dissociation photogenerated charges. Femtosecond transient absorptionspectra explain the energy trajectory of charge carrier kinetics of HCCN and confirm that the crystal semiconductor modified byfunctional groups can promote the dissociation of excitons to free charges. This research provides an idea to obtain crystalline C3N4of ternary intramolecular homojunctions