Preparation And Surface Modificationg Of G-C₃N₄ Photocatalysts Based On Supramolecule Precursors

The conversion of low energy density solar energy into chemical energy by photocatalysis is an effective way to crack current energy shortage and environmental pollution problems.Graphitic carbon nitride（g-C₃N₄,CN）has attracted much attention because of its suitable energy band structure,good chemical and thermal stability,abundant raw materials and simple preparation.However,the defects of bulk CN,such as small specific surface area,insufficient visible light utilization,low carrier separation efficiency and poor photocatalytic activity,limit its development and application.In this thesis,the self-assembled supramolecular strategy,i.e.,using supramolecular complex formed by the self-assembly of melamine and cyanuric acid through hydrogen bonding as CN precursors.The preparation and modification work of CN photocatalysts was carried out by introducing C,N dual vacancies and adjusting the monomer ratio to further explore the potential of CN performance enhancement and systematically investigate the photocatalytic performance enhancement mechanism.The main research contents and results are as follows:（1）Equal proportions of melamine and cyanuric acid were self-assembled by hydrogen bonding and hydrothermal reaction to form hexagonal supramolecular precursors with good crystallinity and stable structure,and then CN microrods containing C,N dual vacancies（H-CN）were prepared by high-temperature pyrolysis of the supramolecules under hydrogen atmosphere.The degradation rate constant of H-CN to rhodamine B（RhB）under simulated sunlight irradiation reaches 0.19363 min^-1,which is 16.52 times higher than that of bulk CN.The comparative experiments show that the C,N dual vacancies are more effective than individual C or N vacancies for the enhancement of photocatalytic performance.The enhanced photocatalytic activity is mainly due to the combined effect of the high specific surface area of CN microrods and the double vacancy defects,which enhances both the optical absorption performance and carrier separation efficiency,while shifting the conduction band position upward,which is more favorable for the generation of the key active species·O₂^-.（2）The effect of the ratio of two monomers,melamine and cyanuric acid,on the hydrogen bonding self-assembly process was systematically investigated.The results show that when the ratio of the two deviates from the ideal one（1:1）,melamine and cyanuric acid perform the self-assembly reaction in an unsaturated coordination mode.In addition,the change of the monomer ratio does not affect the crystal structure and functional group composition of the supramolecule but causes the change of the microscopic morphology of the supramolecule and the decrease of the structural homogeneity.The CN preparation process was simplified,and CN photocatalysts with different structures were prepared based on the high-temperature pyrolysis reaction of supramolecular precursors with different monomer ratios.（3）When the monomer ratios deviate from 1:1,the photo-response range and carrier separation efficiency of the samples are significantly enhanced;when the deviation further expands,the ordered structure is destroyed,and the performance decreased.The M:C=1:2sample has the best RhB photodegradation performance,and the degradation constant reaches0.60416 min^-1 under simulated solar irradiation,which is 57.65 times higher compared with the bulk CN.Meanwhile,the degradation performance of the sample for methylene blue（MB）and methyl orange（MO）is also enhanced by 23.51 and 23.27 times,respectively,showing versatility and high efficiency in the degradation of organic dyes.The experimental results provide a new idea for the simple preparation of CN materials with excellent performance and also lay the foundation for the wide application of CN.