Preparation Of G-C₃N₄ By Supramolecular Copolymerization With Boosted Photocatalytic Solar-to-Hydrogen Efficiency

As a novel semiconductor catalyst material,g-C₃N₄ has a wide range of applications in the field of photocatalytic decomposition of water pollutants,reduction of CO₂ and antibacterial disinfection,due to its chemical stability and thermal stability and suitable forbidden band visible light response.However,small specific surface area of g-C₃N₄,low visible light utilization rate of solar energy,high photo-carriers combination restrict greatly its practical application.In this thesis,g-C₃N₄ with different morphological structures was prepared by copolymerization of different solvents and precursor monomers,forming a supramolecular structure after thermal polycondensation reaction.Various structural characterizations were performed.Photocatalytic property for H₂ production of g-C₃N₄ was tested and corresponding photocatalytic mechanism was analyzed.（1）Using trithiocyanuric acid and melamine as precursors in different polar solvents,the modified g-C₃N₄ was prepared by pyrocondensation polymerization.As a comparison,the bulk g-C₃N₄ was obtained by direct pyrocondensation polymerization of the melamine monomer.SEM and TEM tests of samples show that the composition or chemical structure of the g-C₃N₄ resulted from different polar solvents was significantly different.With photocurrent impedance spectroscopy and fluorescence spectroscopy,the mechanism of g-C₃N₄ for photocatalytic hydrogen production was analyzed by the results shows that the flake-like g-C₃N₄ sample（TCN-E）formed by ethanol solvent copolymerization can increase the photogenerated electrons transfer rate and inhibit the recombination of holes and electrons,thus has best photocatalytic efficiency.（2）Three precursor monomers,trimeric thiocyanate,melamine and 2,4-diamino-6-phenyl-1,3,5-triazine in different molar ratios,were mixed in ethanol solvent,and supermolecules with different structures were formed by a monomeric asymmetric self-assembly copolymerization method.After that,the modified g-C₃N₄ was prepared by the pyrocondensation polymerization.The effect of the amount of monomer 2,4-diamino-6-phenyl-1,3,5-triazine on photocatalytic activity of g-C₃N₄ was investigated.The catalytic mechanism was analyzed in detail by a series of characterizations,such as BET,XPS,PL,and photoelectrochemistry.The results show that the g-C₃N₄ sample（TCNP-10）prepared at a content of 10%monomer 2,4-diamino-6-phenyl-1,3,5-triazine has the highest electron-hole separation efficiency,smallest composite ratio and more reactive sites on the surface,mainly due to an appropriate amount of phenyl functional groups leading to an increase in photoelectron electron-hole separation and migration rate,which suppresses the recombination of electron and hole.The reaction kinetics of electron holes in the photocatalytic process were analyzed by ESR and transient absorption spectroscopy to further confirm the photocatalytic reaction mechanism of TCNP-10 samples.