Study On The Preparation And Photocatalytic Mechanism Of Highly Efficient Graphitic Carbon Nitride Composite Photocatalyst Materials For Hydrogen Production Via Photolysis Of Water

Photocatalytic energy conversion technology has been recognized as one of the ideal solutions to the deteriorating global energy and environmental problems.Graphite phase carbon nitride（g-C₃N₄）is considered a highly efficient photocatalyst material for photocatalytic hydrogen production（PHE）due to its suitable band structure,good stability,environmental friendliness,and low production cost.In this paper,g-C₃N₄ based photocatalyst was taken as the research object.In order to solve the key problems such as low specific surface area of g-C₃N₄,less exposure of active site,high recombination rate of photogenerated carriers and narrow absorption range of visible light,g-C₃N₄ based photocatalyst with high PHE efficiency was successfully prepared by optimizing the preparation process and selecting nitrogen rich precursors;At the same time,various modern analytical and testing methods were used to study the effects of preparation processes and the selection of nitrogen rich precursor materials on the morphology,structure,composition,specific surface area,optical properties,and PHE efficiency of g-C₃N₄ products.The corresponding photocatalytic mechanism was revealed through theoretical calculatio products.The corresponding photocatalytic mechanism was revealed through theoretical calculations.High specific surface area porous g-C₃N₄ was prepared in bulk using cheap urea and water as raw materials and steam assisted thermal polymerization process instead of traditional thermal polymerization process.The research results indicate that the product VT-CN prepared by steam assisted thermal polymerization process is a typical g-C₃N₄,in the form of curled and wrinkled nanosheets,with a uniform and obvious porous structure.Compared to the g-C₃N₄（TT-CN）prepared by traditional thermal polymerization process,its specific surface area and pore volume have been significantly improved,reaching 60.72 m² g^-1 and 0.31 cm³ g^-1,respectively.At the same time,the band gap of VT-CN is 2.58 e V,which has a high photocurrent response.The resistance of charge carrier transfer at the interface is small,and the diffusion mobility of electrons is higher;The experimental results of photocatalytic hydrogen production show that compared to TT-CN,the photocatalytic hydrogen production rate of VT-CN has also been significantly improved,with an average hydrogen production rate of about 0.44 mmol h^-1 g^-1 under visible light;The average hydrogen production rate under simulated sunlight reaches approximately 2.85 mmol h^-1 g^-1.Based on the steam assisted thermal polymerization strategy,a new method of salt spray assisted thermal polymerization has been developed to prepare graphite phase carbon nitride photocatalyst materials.By optimizing the mass ratio between Na Cl and KCl,as well as the mass ratio of mixed salt and urea,the structure and composition of the product are regulated.When the mass ratio of urea,potassium chloride,and sodium chloride is 4:1:3,an interlayer heterojunction carbon nitride photocatalyst(SF-CN_1-3)composed of a triazine donor heptazine receptor（D-A）was obtained due to the synergistic effect of salt spray oxidation and chemical etching.Due to the formation of heterojunction,the band gap of the material is about 2.54 e V,the band gap is narrowed,and the absorption of visible light is greatly enhanced,showing an increased specific surface area to accommodate more active site,shortening the diffusion path of reactants and photogenerated charge carriers;At the same time,due to the presence of an internal electric field caused by the triazine heptazine II heterojunction,effective interface transfer and spatial separation of electron holes are achieved,resulting in stronger diffusion mobility of charge carriers.The geometry and electronic structure of SF-CN_1-3 were calculated theoretically,which proved the formation of type II heterojunction between triazine C₃N₄ and heptazine C₃N₄;The best active site of PHE is the N atom at the heptazine structure position d,and its Gibbs free energy is-3.94 e V.Compared with TT-CN and VT-CN,the photocatalytic test of SF-CN_1-3 showed an excellent visible light PHE rate of 18.13mmol h^-1 g^-1,up to 259.00 times that of TT-CN.Optimization of g-C₃N₄ morphology and photocatalytic hydrogen production performance based on hydrothermal control of precursor morphology.A three-dimensional porous graphite phase nitride carbon nanotube（TCN₃）was synthesized by controlling the mass ratio of citric acid to melamine and the hydrothermal reaction time of the intermediate.It was found that when the mass ratio of melamine to citric acid is 1:1 and the hydrothermal reaction time is 24 hours,a hexagonal prism shaped intermediate with regular shape can be obtained.After heating and annealing,the final product is a three-dimensional porous g-C₃N₄ microtube with efficient PHE performance.The diameter of g-C₃N₄ microtubes is approximately 5μm;The specific surface area is 74.16 m² g^-1;Lengths ranging from tens to hundreds of micrometers;Its band gap is about 1.93 e V,and the band gap is significantly narrowed.The average hydrogen production rate of three-dimensional porous g-C₃N₄ microtubes under visible light is about 4.74 mmol h^-1 g^-1,which is significantly higher than that of TT-CN and VT-CN.