Study On The Surface/Interface Regulation Of Graphitic Carbon Nitride-based Photocatalysts For Enhancing Degradation Of Organic Pollutants In Water

Photocatalytic technology can realize the high-efficiency conversion between solar energy and chemical energy,making it being an effective way to prevent and control water pollution.In recent years,visible-light catalysts represented by graphitic carbon nitride(g-C₃N₄)have been extensively studied due to the advantages of simple preparation,abundant precursor reserves,and high stability.Improving the carrier separation efficiency,light absorption threshold,and surface-active site distribution of g-C₃N₄ to obtain higher visible light catalytic activities is the current research focus.In this research,the photocatalytic reaction process is optimized and the photocatalytic reaction activity is improved by adjusting the surface and interface properties of g-C₃N₄.Three types of g-C₃N₄-based visible-light catalysts are prepared,including carbon-oxygen dual doped g-C₃N₄(abbreviated as C,O-g-C₃N₄),cobalt sulfide supported g-C₃N₄(abbreviated as Co₉S₈/g-C₃N₄),and modified bismuth sulfide supported g-C₃N₄(abbreviated as FGBS/g-C₃N₄).The obtained catalysts were applied to the visible-light photocatalytic degradation of organic pollutants bisphenol A(BPA)and 2,4-dichlorophenoxyacetic acid(2,4-D)in water environment,and its catalytic activity and catalytic reaction process are explored.Firstly,based on the heteroatom doping principle to regulate the surface morphology and electronic band structure,the easy-to-obtain malonic acid and urea were used as precursors via thermal polycondensation method to obtain C,O-g-C₃N₄catalyst,which was used for visible-light photocatalytic BPA degradation.The characterization results show that the light absorption threshold of C,O-g-C₃N₄ is red-shift to 700 nm compared with pristine g-C₃N₄(425 nm).Besides,the surface structure of C,O-g-C₃N₄ is more fragmented,which can expose more reactive sites and promote carrier migration and separation.Besides,carbon and oxygen dual-doping adjust the electronic energy band structure of g-C₃N₄,and obtain lower valence band position,making two active species with strong oxidizing ability exist simultaneously in the reaction system.The 1%C,O-g-C₃N₄(the mass ratio of malonic acid to urea is about1%)with surface morphology optimization and band structure adjustment,behave 100%photocatalytic degradation efficiency of 15 mg/L BPA in 2.5 h,and there is no significant decrease in activity of the catalyst after repeated use.Secondly,in view of the characteristics of interfacial chemical bond-induced carrier migration,Co₉S₈/g-C₃N₄ was prepared by the in-situ growth method using transition metal sulfide--Co₉S₈as the loading substrate.The prepared Co₉S₈/g-C₃N₄interface has the Co-N_x as the electronic medium to induce electron migration to form a Z-scheme heterojunction,which promotes the separation of photo-generated carriers.The effective carrier migration enhances the electrons reduction and holes oxidation utilization efficiency.The interface effect and simultaneous redox effect enhance the2,4-D oxidation and Cr(VI)reduction efficiency of catalyst.In addition,15%Co₉S₈/g-C₃N₄(the mass ratio of Co₉S₈ to g-C₃N₄ is about 15%)exhibits an excellent photocatalytic and structural stability during five recycling processes.Finally,using interfacial functional groups to assist in regulating the built-in electric field,a part of g-C₃N₄ acts as a sacrificial agent to generate FGBS/g-C₃N₄ p-n heterojunction with abundant interface functional groups(-COOH,N-(C)₃,C-N-H).The interface functional group modulates the width of the space charge region at the interface of the FGBS/g-C₃N₄ p-n heterojunction,thereby improving the built-in electric field of the heterojunction and improving the efficiency of photo-generated carrier migration and separation.The functional groups modify the crystallization process of the pristine Bi₂S₃ to obtain a hollow microsphere and improve the light absorption capacity.At the same time,the functional groups adjust the electronic energy band structure of Bi₂S₃ for the generation of superoxide radicals.2FGBS/g-C₃N₄(the mass ratio of FGBS to g-C₃N₄ is about 24%)shows a higher photocatalytic activity than conventional 2Bi₂S₃/g-C₃N₄,and the photocatalytic degradation efficiency of 15 mg/L BPA is 70%in 4 h.After five cycles of repeats,the photocatalytic activity and physico-chemical structure of 2FGBS/g-C₃N₄ remained,indicating that the catalyst has the potential to be reused.