Design,Preparation,and Study On Photocatalytic Hydrogen Generation Of G-C₃N₄-based Photocatalysts

The core of photocatalytic hydrogen generation of water splitting is to develop photocatalytic materials with highly-efficient photocatalytic hydrogen generation activity.Among the reported photocatalytic materials,graphite carbon nitride（g-C₃N₄）is one of the most widely studied photocatalytic materials in recent years since it has the advantages of visible light response,energy level positions that match with water splitting,good photochemical reaction stability,as well as metal-free characteristic.At present,g-C₃N₄material,however,has a low hydrogen production efficiency,which is mainly caused by the higher recombination of photo-generated electron-hole pairs in the bulk phase and narrower absorption of visible light.In view of the above problems,combined with the published research,three aspects including reducing the Schottky barrier of the g-C₃N₄/C heterojunction,modifying the g-C₃N₄ benzene ring conjugated molecules,and constructing a g-C₃N₄/CaSO₄heterojunction in this paper will be implemented,so as to enhance the photo-generated carrier separation efficiency and broaden the scope of light absorption,thus,improving the photocatalytic hydrogen production performance of g-C₃N₄ material.The specific research contents are as follows:（1）N-doped C（NC）was prepared by microwave rapid heating,and g-C₃N₄/NC heterojunction was constructed.The microwave heating strategy was used to heat urea-glucose rapidly.Firstly,N-doped C material was prepared,and then mixed with melamine at a certain ratio.The mixture was heated at 550 ^oC to prepare g-C₃N₄/NC photocatalytic composite material.Theoretical calculation results show that N-doping can increase the electronic state in the Fermi level of C material,thereby reducing the Schottky barrier of the g-C₃N₄/NC heterojunction,which is conducive to the migration of photo-excited electrons from g-C₃N₄materials to C material.Thus,the separation efficiency of the electron-hole pairs in the g-C₃N₄material is improved.The results of photocatalytic hydrogen production confirmed that,compared with g-C₃N₄/C heterojunction,g-C₃N₄/NC composite photocatalytic material can significantly improve hydrogen production performance,and the g-C₃N₄/NC material with an optimized content had the highest hydrogen production rate of 23.0μmol·h^-1,which is 4 times higher than that of C/g-C₃N₄ material(5.94μmol·h^-1).（2）Modification of g-C₃N₄ material by incorporating benzene ring molecule.Benzene ring molecule modified g-C₃N₄ photocatalytic material was prepared by thermal polymerization of phthalic anhydride（PA）organic molecules with melamine,and the aromatic structure of organic monomer was embedded in the framework of g-C₃N₄.The introduction of PA has significantly broadened the light absorption edge of g-C₃N₄ material to 490 nm,enhancing light absorption and significantly improving photocatalytic hydrogen production performance.With the optimized PA content,the g-C₃N₄-PA had the highest hydrogen production rate of 5.65μmol·h^-1,which is 3.5 times higher than that of the original g-C₃N₄ sample(1.64μmol·h^-1).（3）CaSO₄/g-C₃N₄ composite photocatalytic material.CaSO₄ nanoparticles were obtained by treating paper scraps with a strong acid,and then mixed with melamine and heated at 550^oC to prepare g-C₃N₄/CaSO₄ composites.Electron microscopy observations show that the obtained CaSO₄ is plate-shaped nanoparticles having a large contact surface with g-C₃N₄,and increases the specific surface area of g-C₃N₄ material,which is conducive to transferring the photo-generated electrons from g-C₃N₄ to CaSO₄,enhancing the separation of photoelectron-hole pairs so as to improve the hydrogen production performance of the g-C₃N₄ material.After adding CaSO₄,the maximum hydrogen production rate of the g-C₃N₄/CaSO₄ composites reached 17.06μmol·h^-1,which is 12 times than that of single g-C₃N₄ hydrogen production rate(1.45μmol·h^-1).