Structure Regulation Of Graphite Carbon Nitride And Photocatalytic Water Splitting To Hydrogen Performance

Graphite carbon nitride（g-C₃N₄）,a two-dimensional layered and metal-free semiconductor,has attracted the widespread attention due to its suitable band structure,easy synthesis,non-toxic,and visible light absorption.Unfortunately,the practical application of the pristine g-C₃N₄ is greatly restricted in view of the rapid recombination of photogenerated charge carrier,limited visible-light absorption range and low surface areas.Aiming to the above problems,based on the structure of g-C₃N₄,the modified g-C₃N₄ were prepared via defect regulation and surface modification.A series of characterization and photocatalytic water splitting to hydrogen performance tests of the prepared modified g-C₃N₄ were carried out,and the reasons and mechanism of the enhanced activity was discussed in depth.The specific results and details are as follows:g-C₃N₄ with nitrogen defect was successfully prepared by a simple and green thermal polymerization method of one-step calcined fumaric acid and urea.By regulating the amount of fumaric acid in the precursor,the nitrogen defects concentrations located at N–（C）₃ was regulated,achieving that the band structure was continuously regulated.It was speculated that the reason for the formation of N–（C）₃defects may be that fumaric acid reacted with urea to form the intermediate and this intermediate was unstable and decomposed to prevent the formation of N–（C）₃ sites and lead to nitrogen defects.The introduction of nitrogen defects effectively optimized band structure of g-C₃N₄,broaden visible-light response range,enhanced the driving force of hydrogen evolution and promoted the effective separation of charge carriers.The photocatalytic hydrogen evolution rate of of optimized nitrogen defective g-C₃N₄ was94.1μmol·h^-1,which was about 2.64 times that of unmodified g-C₃N₄.Nitrogen defective g-C₃N₄ located at N–（C）₃ possessed more electrons for hydrogen evolution and had stronger water molecule adsorption capacity,thus enhancing mass transfer,demonstrated by DFT theoretical calculation.The potassium-doped g-C₃N₄ with surface hydroxyl groups was prepared by introducing KCl and ammonium oxalate（OA）during the thermal polymerization process of melamine.The K⁺doped into the interlayer of g-C₃N₄ opened the tri-s-triazine ring to form the cyanide group,while the hydroxyl groups reacted with the unsaturated sp² hybrid carbon atoms to form the surface hydroxyl groups.Meanwhile,the decomposition of OA produced gas during the thermal polymerization process and promoted the introduction of hydroxyl groups,resulting in the loose surface.The introduction of K⁺,cyano groups and surface hydroxyl groups optimized cooperatively the band structure of g-C₃N₄ and promoted the visible-light absorption,made it have stronger reducing ability and effectively promoted the separation of photogenerated carriers.The photocatalytic hydrogen evolution rate of the optimized potassium-doped g-C₃N₄ with surface hydroxyl groups was 69.7μmol·h^-1,which was 8.1 times that of the original g-C₃N₄.