The Study On Structural Control Of Carbon Nitride And Its Photocatalytic Performance

g-C₃N₄ is a kind of organic polymer semiconductor with a graphene-like two-dimensional layered structure.It has the advantages of simple preparation,high stability,non-toxicity and pollution-free,and has become a research hotspot in recent years.However,the photocatalytic hydrogen production activity of g-C₃N₄ is seriously restricted due to its high carrier recombination,low visible light utilization and poor crystallinity.Therefore,it is particularly important to study the synthesis method,modification and reaction mechanism of g-C₃N₄.The molten salt method is one of the new methods to improve the electronic structure and optical properties of g-C₃N₄ in recent years.Compared with traditional methods,the molten salt method has many advantages,such as high crystallinity,large specific surface area,low carrier recombination rate,and wide optical absorption range.In this paper,the effects of some parameters on the microstructures and photocatalytic properties of g-C₃N₄ prepared by the molten salt method were investigated.The main contents and conclusions are as follows:?1?g-C₃N₄ was prepared by the molten salt method in static air with urea as raw material,KCl/LiCl,NaCl/KCl and KBr/LiBr as eutectic molten salt.The results showed that g-C₃N₄ with high catalytic activity could still be prepared in static air atmosphere,which simplified the preparation process and reduced the preparation cost.In addition,halide combinations with different ion sizes as eutectic molten salts have different effects on the structure and photocatalytic activity of g-C3N4.The free pore volume formed after melting of eutectic molten salt KCl/LiCl is conducive to the secondary growth of g-C₃N₄,and the incorporation of K and Cl ions in the molten salt is a benefit to carrier migration and separation.Therefore,the photocatalytic activity of the synthesized g-C₃N₄ catalyst is 11 times higher than that of the synthesized g-C₃N₄ from KCl/NaCl and KBr/LiBr molten salt.?2?On the basis of part?1?,g-C₃N₄ with different stacking distance and crystallinity was synthesized by adjusting the pre-sintering time of g-C₃N₄ prepared by KCl/LiCl molten salt method.It was found that the photocatalytic activity of the samples increased first and then decreased with the compression of g-C₃N₄ interlayer spacing from 0.319 nm to 0.314 nm and the increase of crystallinity of the samples.The maximum photocatalytic activity was 382.41?mol h^-11 when the preheating time was 8 h and the interlayer spacing was 0.315 nm.Because of the compression of stacking space between g-C₃N₄ layers,the charge transfer rate and carrier separation rate are increased.When the interlayer spacing is 0.315 nm,the exciton separation efficiency is the best.?3?The removal of molten salt residues by boiling water will cause some g-C₃N₄ to exfoliation with thermal solvents and redshift of absorption cutoff edge of g-C₃N₄.This is because the cyanogen group on the surface of g-C₃N₄ is easily connected with hydroxyl?.OH?in water by hydrogen bond,so some of the nanorod-like g-C₃N₄ can be uniformly dispersed in water.Moreover,K and Cl,which enter the g-C₃N₄ structure in the form of ions during the molten salt process,are transferred to the solution by ion exchange due to a longtime stirring in the water bath at 80?by boiling water removal method,which results in the decrease of photocatalytic activity.