Study On The Performance And Mechanism Of Carbon Nitride Photocatalytic Hydrogen Evolution Regulated By Nitrogen And Oxygen Defect

With global warming and climate change,the use of photocatalysis to convert solar energy into chemical energy has become an important means to cope with environmental pollution and the energy crisis.Among them,graphitic carbon nitride（g-C₃N₄）has received much attention in recent years as a visible light-responsive photocatalyst with the advantages of good catalytic efficiency,low cost,stability,and easy preparation.The carbon nitride material itself has some inherent disadvantages,such as low visible light absorption,fewer active sites for interfacial reaction,rapid compounding of photogenerated carrier pairs,weak electromobility,low quantum yield,low specific surface area,low electrical conductivity,etc.In this work,this thesis adjusts the defect concentration in the triazine ring skeleton by controlling the ambient atmosphere of the precursor during high-temperature polymerization,and the mechanism of these defect formation and the influence mechanism on the photocatalytic performance of g-C₃N₄ are investigated in depth by analyzing the crystal structure,optical properties and photocatalytic performance of the samples.The details are as follows:（1）The g-C₃N₄ precursor will inevitably introduce O doping in the g-C₃N₄ triazine ring skeleton during the high-temperature polymerization process,so we used argon-hydrogen mixtures containing different ratios of hydrogen to control the amount of O introduction.The crystal structure of the material was analyzed by XRD,XPS,and IR spectroscopy,and it was determined that the doping of O element in the triazine ring skeleton was formed by replacing the N atom at the N_2C position to form O replacement atoms.The O replacement atom causes lattice distortion and internal electric field,which promotes carrier separation and inhibits carrier complexation by attracting electrons,and promotes light absorption ability by shortening the forbidden bandwidth of the system by forming forbidden band intermediates near the conduction band.However,too many O replacement atoms will destroy the crystal structure of the material.After testing,the photocatalytic hydrogen production performance of the hydrogen-reduced OCN-10 is 3.3 times higher than that of the unreduced OCN,and it has good cycling stability.（2）By high-temperature thermal polycondensation technique,g-C₃N₄ with C/N structure can be prepare.However,this process generates a large amount of ammonia gas,and this ammonia gas will adversely affect the triazine ring structure of g-C₃N₄.We control the amount of ammonia by adding ammonium acetate.Ammonium acetate and melamine react in the roasting process to release a large amount of ammonia.The ammonia will generate free radicals at high temperature and etch the N atom at the N_3C position,thus generating N_3C vacancies in the triazine ring skeleton of g-C₃N₄.The N_3C vacancy reduces the crystal density due to the absence of the N atom,thereby making the（100）crystal plane spacing larger and effectively improving the carrier concentration while suppressing the separation of photo-generated carriers by capturing electrons and rapidly transferring to the semiconductor surface.After the photocatalytic activity test,the photocatalytic hydrogen production activity of NCN-6 with the introduction of N_3C is 4.11 times higher than that of NCN without the addition of ammonium acetate,and the material still has good performance after four 6-hour cyclic photocatalytic hydrogen production experiments,which proves that the material has good cyclic stability.