Multistage Polymerization Design for g-C3N4 Nanosheets with Enhanced Photocatalytic Activityby Modifying the Polymerization Process of Melamine

摘要

Graphene-like g-C3N4 nanosheets (NSs) have been successfully synthesized with a modified polymerization process of melamine by cocondensation with volatile salts. Volatile ammonium salts such as urea–NH4Cl/(NH4)2SO4/(NH4)3PO4 were added with melamine to modulate the thermodynamic process during polymerization and optimize the structure formation in situ. The surface area, surface structure, and surface charge state of the obtained g-C3N4 NSs could be controlled by simply adjusting the mass ratio of the melamine/volatile ammonium salt. As a consequence, the g-C3N4 NSs exhibited much higher activity than bulk g-C3N4 for the photocatalytic degradation of target pollutants (rhodamine B, methylene blue, and methyl orange), and it also exhibited greater hydrogen evolution under visible light irradiation with an optimal melamine/volatile ammonium salt ratio. The as-prepared g-C3N₄ NSs with melamine–urea–NH₄Cl showed the highest visible light photocatalytic H₂ production activity of 1853.8 μmol·h^–1·g^–1, which is 9.4 times higher than that of bulk g-C₃N₄ from melamine. The present study reveals that the synergistic effect of the enhanced surface area, surface structure, and surface charge state is the key for the enhancementof photocatalytic degradation and hydrogen evolution, which couldbe controlled by the proposed strategy. The result is a good explanationfor the hypothesis that adding properly selected monomers can trulyregulate the polymerization process of melamine, which is beneficialfor obtaining g-C₃N₄ NSs without molecular self-assembly.Considering the inexpensive feedstocks used, a simple synthetic controllingmethod provides an opportunity for the rational design and synthesis,making it decidedly appealing for large-scale production of highlyphotocatalytic, visible-sensitizable, metal-free g-C₃N₄ photocatalysts.