Study On Modification Of Polymeric Graphitic Carbon Nitride For Photocatalytic Evolution Of Hydrogen

Graphitic carbon nitride?g-C₃N₄?polymer is a kind of organic polymer semiconductor with the ability of visible light response.Because of its unique electronic structure and energy band gap,it is widely used in the fields of photocatalytic hydrogen evolution,photocatalytic degradation of organic pollutants and photocatalytic reduction of carbon dioxide.It is one of the effective means to solve the problem of energy and environmental pollution.However,the bulk g-C₃N₄ prepared by the direct thermal polymerization method still has disadvantages such as small specific surface area,difficult photo-generated carrier transfer,and serious photo-generated electron-hole pair recombination,which means that its photocatalytic activity is limited.In this paper,g-C₃N₄ polymers with different morphologies were prepared by hydrothermal post-treatment and supramolecular self-assembly modification.The increased specific surface area could provide more reactive sites and promoting photo-generated carriers separation could enhance the photocatalytic performance of g-C₃N₄.The bulk g-C₃N₄ precursor obtained by one-step thermal polymerization of melamine through a hydrothermal post-treatment reactor was modified to obtain microsheet,nanorod and microprism superlattice g-C₃N₄ products with porous crystal structure,larger specific surface area and higher charge separation efficiency.The results of the characterization and the photodegradation of RhB under visible light show that,g-CN-20 exhibits the best performance among all the products due to its microsheet morphology,higher photo-generated carrier separation efficiency,and larger specific surface area.In addition,the effects of the time of hydrothermal modification,the temperature of hydrothermal modification,the concentration of bulk g-C₃N₄ and ultrasonic-assisted peeling on the photocatalytic hydrogen evolution performance of g-C₃N₄ polymer were explored.The g-C3N4 polymer photocatalyst with high catalytic activity was prepared through supramolecular self-assembly by using melamine,dicyandiamide as raw materials and acetonitrile as a solvent.The effects of the ratio of the melamine to dicyandiamide,the reaction temperature and the reaction time on the prepared g-C3N4 were investigated.The characterization results showed that,when the mass ratio of 2:1,the reaction temperature of120?and the reaction time of 12h,as-prepared gCN-2-120-12 has nanospherical morphology,larger specific surface area,higher separation efficiency of photogenerated electrons and holes and high photocatalytic activity for RhB photodegradation.The morphology,structure,optical properties and photocatalytic activity of g-C3N4polymer prepared by one-step thermal polymerization?bulk g-C3N4?,hydrothermal post-treatment modification?gCN-12?and supramolecular self-assembly?pgCN?were compared.It was observed that pgCN has a disordered two-dimensional planar structure,larger specific surface area and pore volume,more surface reactive sites and higher separation efficiency of photogenerated carrier.The photocatalytic activity of pgCN with disordered planar structure is better in comparison with bulk g-C3N4 and its hydrothermal post-treatment product:the photodegradation rate of RhB is 2.4 and 1.5 times than that of bulk g-C3N4 and gCN-12,respectively.Moreover,pgCN shows excellent photocatalytic performance for photocatalytic H2 evolution reaction,which is 18.2 and 3.3 times higher than that of bulk g-C₃N₄ and gCN-12,respectively.