Preparation And Formaldehyde Purification Performance Of G-C₃N₄-based Photocatalytic Composites

Graphite-phase carbon nitride（g-C₃N₄）is a typical semiconductor formed by stacking nonmetal nano-layers.The electronic structure of the medium forbidden band width（2.7eV）enables it to be excited by visible light.The charge generated by the excitation migrates rapidly in the layer,but the transfer resistance between the layers is large.This leads to a higher reactivity at the edge of the semiconductor compared to the inside of the layer,with considerable active sites and redox effects.In addition,g-C₃N₄ is considered to be a two-dimensional photocatalytic material with considerable potential due to its excellent thermal and chemical stability,non-toxicity,and low cost and easy production.Taking into account the excellent photocatalytic performance of the heterojunction interface,this paper adopts the hydrothermal method to achieve the purpose of obtaining a good effect of the MoS₂/g-C₃N₄ binary nano heterojunction,and to improve the photocatalytic degradation effect of g-C₃N₄ on water-phase organic pollutants and gas-phase formaldehyde.Choose the appropriate MoS₂ material for the photocatalysis field,and obtain the few layers of MoS₂ nanosheets through a unique exfoliating method.The structure and performance of typical g-C₃N₄ are optimized by element doping,molten salt adjustment and morphology control.Finally,based on the two improved materials,a 2D-2D mode heterojunction compound semiconductor material was successfully constructed.Compared with the current similar products,the obtained target heterojunction exhibits good photocatalytic performance,which depends on the improvement and cooperation of each step of the scheme.And through the control of the variables in each step of the experiment and the test and analysis of the effect of degrading rhodamine B and formaldehyde,the best preparation process is obtained.The main verdicts are as follows:（1）A simple liquid-phase ultrasonic exfoliating method was used to prepare the MoS₂ thin nano-layer,and the small molecule intercalant oleic acid and the macromolecular surfactant PVP were selected to assist the exfoliating.The effects of exfoliating time,addition amount of surfactant and solvent on the quality and concentration of the nanolayer are discussed.The quality of the MoS₂ nanolayers prepared under the conditions of exfoliating time of 10h,MoS₂,oleic acid and PVP was 1:1:1,and the solvent was DMF.By observing the surface morphology of the nanosheets,it is found that the nanosheets are uniformly distributed in the solvent,showing a thin layer of yarn.The lateral size is mostly 100-300 nm,and there are a few nanosheets with a size of about 500 nm.The thickness of the nanosheets is in the range of 3-7 nm,indicating that there are mostly 2-6 layers of few MoS₂ nanosheets.Raman and XRD also show that MoS₂ is effectively exfoliated,and it is a material formed from a few layers.The UV-Vis test showed the characteristic peaks of MoS₂ nanosheets.And the characteristic peak intensity gradually increases with time,indicating that the concentration of the nano-dispersion solution becomes higher.The dispersion stability proves that the sample is basically unchanged after 30 days and has good stability.（2）S-doped g-C₃N₄ was prepared by mixing TAA and melamine by a simple thermal polycondensation process,and selecting LiCl/KCl and NaCl/KCl as molten salts to adjust the polymerization process.The influence of doping amount of sulfur source and molten salt on g-C₃N₄ was discussed.S-doped g-C-3N₄ prepared under the conditions of TAA doping of 30% and molten salt body of NaCl/KCl showed good photocatalytic performance.The surface morphology shows that the specific surface area of the sample is larger when the S doping amount is 30%.Under the NaCl/KCl salt melting system,large-sized crystals of about 4-8 μm were produced.The structural analysis showed that S successfully replaced the N in the g-C₃N₄ lattice,and the distance between layers increased.In addition,the NaCl/KCl salt melt has a better degree of crystallinity and a more regular molecular structure.All samples show significantly improved charge carrier nonradiative recombination rate in optical properties,with the maximum band gap reduced to ¹.75 eV,and the band gap of NaCl/KCl salt melt samples is 1.90 eV.Compared with 30% sulfur-doped samples,the photocatalytic degradation efficiency and reaction rate constant of NaCl/KCl salt melt samples are significantly improved.After 2 h of reaction,the degradation efficiency of RhB was increased to 98.3%.At the same time,the degradation effect of formaldehyde gas was 42.1% after 2h.This is due to the unsaturation sites provided by the S element in the heptazine ring and the intermediate band gap electronic state produced.At the same time,the degree of polymerization and crystallinity are improved to improve the recombination of photo-generated carriers.The photocatalytic activity is greatly improved.（3）MoS₂/g-C₃N₄ composite nano-photocatalytic heterojunction was obtained by combining the prepared MoS₂ nano-dispersion with Na/K molten salt synthesized S-doped g-C₃N₄ through the hydrothermal reaction and bonding interface.The effects of temperature,the amount of the MoS₂ added and the residual surfactants in the nanosheets on the heterojunction are discussed.The MoS₂/g-C₃N₄ heterojunction prepared at a temperature of 140℃ and a MoS₂ nanolayer addition of 1% showed improved photocatalytic activity.Through the characterization of the material structure,it is proved that the interlayer bonding between MoS₂ and g-C₃N₄ is quite tight.The appearance of some small peaks belonging to MoS₂ in XRD confirms the successful recombination of the two semiconductors.Optical properties show that the existence of heterogeneous interface effectively inhibits the recombination of charge carriers,and the light absorption capacity is slightly enhanced.The best sample band gap is 1.88 eV.The photocatalytic degradation efficiency and reaction rate constant of the sample are significantly improved compared to a single semiconductor.The best sample degrades RhB concentration by 82.9% in 1 h,and obtains 77.6%degradation effect on formaldehyde gas in 2h.In addition,the investigation found that the residual surfactant did not significantly affect the photocatalytic performance of the heterojunction,and even improved the progress of the photocatalytic reaction to a certain extent.The above-mentioned improved photocatalytic efficiency is due to the existence of heterogeneous interfaces.Electrons can be quickly transferred through the interface connected by chemical bonds.At the same time,the flow direction of electrons and holes is changed,and the recombination of photo-generated carriers is effectively reduced.