Preparation And Hydrogen Production Performance Of Crystalline Graphitic Carbon Nitride Based Photocatalyst

It is the energy crisis and environmental problems that the whole world is facing together.So far,photocatalytic technologies using renewable solar energy have been extensively studied to solve these global problems.Photocatalysts are a crucial foundation for bringing the technology to industrial applications.Graphite carbon nitride has attracted widespread attention in the scientific community due to its good physical and chemical stability,non-toxicity,and ability to absorb visible light.However,the photocatalytic activity of the pristine g-C₃N₄ is limited by some defect problems,such as low light absorption efficiency,high photogenerated electron-hole recombination rate,and insufficient surface reaction at the liquid-solid interface,which hinders its practical application.Therefore,researchers at home and abroad solved these problems through a series of modification strategies.Among them,improving the crystallinity of g-C₃N₄ has been proved to be an effective strategy to improve the efficiency of carrier separation and transport,which has attracted widespread attention.In this paper,carbon nitride materials with high crystallinity were successfully synthesized by simple ionic heating and supramolecular preassembly,respectively.The research contents and main findings are as follows:1.Preparation and photocatalytic properties of crystalline graphite carbon nitride Co doped with Na⁺,K⁺In this study,Na⁺and K⁺co doped Na K-CN-600 was prepared by a simple one-step ionic thermal method.The influence of Na⁺and K⁺doping on the structure of carbon nitride was studied.It was found that Na⁺and K⁺could synergistically improve the photocatalytic performance of carbon materials.when co-doped.Through the study of different reaction temperatures,it was found that under the condition of 550℃,Na⁺could not be mixed into carbon nitride,and would destroy the structure of g-C₃N₄,thus reducing the photocatalytic performance of carbon nitride.It is also found that at 600℃,Na⁺can be doped with carbon nitride and the co-doping of Na⁺and K⁺can be realized.By HRTEM test,we found that the sample NAK-CN-600 showed high crystallinity.The hydrogen production rate of NAK-CN-600 can reach 1593 mol/（g·h）,which is more than38 times that of PCN.It is found that the addition of K⁺will introduce the mesoporous structure and make the（002）surface more closely arranged.However,the addition of Na⁺can not only enhance the order degree of（100）surface,but also introduce some new crystal surfaces,which are beneficial to improve the photocatalytic performance.In addition,compared with the single doping,the co-doping of Na⁺and K⁺is not only more conducive to the condensation polymerization of Na K-CN-600,so as to improve the crystallinity,but also to further reduce the band gap.Therefore,the hydrogen production efficiency of NAK-CN-600 is nearly 1 times higher than that of Na-CN-600 and K-CN-600,respectively.2.The morphology of g-C₃N₄ is regulated by heptazine supramolecular preassembly for efficient hydrogen productionThe MC supramolecules with the best morphology were screened by Melem and cyameluric acid in different proportions and different solvents,and then the MC-CN-N nanomaterials were further prepared by using MC supramolecules.The study of morphology showed that MC-CN-N inherited the hollow tube structure of MC supramolecular and generated some nanosheet structures,both of which showed clear lattice fringes.The hydrogen production rate of MC-CN-N can reach 2955μmol/（g·h）,58 times that of PCN.It was found that the hollow tube structure of MC-CN-N introduced the mesoporous structure,resulting in a larger specific surface area（73.845 m2/g）and higher crystallinity.Its mesopore and larger specific surface area provide more active sites for photocatalytic hydrogen production.The low layer and high crystal structure can improve the light absorption properties and accelerate the transmission of photogenerated carriers to the material surface.These factors together enhance the photocatalytic activity of MC-CN-N.By comparing MC-CN-N and MC-CN,it can be concluded that nitrogen atmosphere is beneficial to protect the micromorphology of the sample.