Study On Preparation Of Graphene And Derivatives By Magnetically Rotating Non-Thermal Arc Plasma

Graphene,as a new type of two-dimensional material,has received extensive attention from scientific research and industrial circles due to its excellent physical and chemical properties,but large-scale and low-cost production of graphene is still a challenge.Plasma gas-phase synthesis of graphene is a new method for graphene synthesis developed in recent years.It has the characteristics of continuous synthesis,no catalyst,and simple operation,showing the potential of large-scale synthesis of graphene.At present,there are few studies on the plasma gas phase synthesis of graphene,the relationship between process parameters and product characteristics is still unclear,and the formation mechanism of graphene is also controversial.In this paper,a magnetically stabilized gliding arc discharge was developed,and graphene,nitrogen-doped graphene,and hydrogenated graphene were successfully prepared.The relationship between process parameters and product structure,morphology,elemental composition and other characteristics was systematically studied,and the formation process of graphene in non-thermal arc plasma was speculated by combining reaction kinetics simulation.The main research contents and results are as follows:1.A magnetically stabilized gliding arc discharge was designed,and the plasma characteristics were studied by using high-speed photography,signal acquisition card and emission spectroscopy techniques.Under the synergistic effect of Lorentz force and aerodynamic drag,the arc rotates rapidly along the central electrode,forming a disk-shaped plasma region.With the increase of the magnetic field,the arc speed and the re-breakdown frequency between the arc and the electrode increase,which is beneficial to maintain the stability of the plasma.Compared with the traditional bladetype sliding arc,the magnetic rotating non-thermal arc has a larger plasma area and stable discharge,which is beneficial to the plasma chemical process.2.Graphene nano-flakes(GNFs)were synthesized from hydrocarbon gas using a magnetically stabilized gliding arc discharge,and the properties of the products were analyzed by means of various material characterization methods.The results show that the monolithic size of the synthesized GNFs is 50-300 nm,which is composed of 1-20 graphite layers stacked,and the monolithic sheet has an obvious wrinkled structure;the monolithic sheets are connected to each other to form a branched chain structure similar to carbon black.The highest yield of synthesized GNFs can reach 15.8%,and the comprehensive energy consumption can be as low as 0.1 kWh/g,which has the prospect of large-scale production.By studying the influence of process parameters such as current,raw material type and concentration,reaction system H/C ratio and gas pressure on product characteristics,it is shown that lower carbon concentration and high H/C ratio are necessary conditions for graphene formation.(1)The lower carbon concentration helps to form a small amount of polycyclic aromatic hydrocarbons(PAHs),and the collision frequency of PAHs is low,which leads to the planar growth,and finally the two-dimensional sheet structure is formed;(2)A high H/C ratio helps to generate more H atoms.On the one hand,hydrogen atoms reduce the PAHs concentration,which facilitates the formation of 2D cores,and on the other hand,during lamella growth,the dangling bonds at the terminating edges of H atoms promote planar growth.At the same time,H atoms have the effect of etching the amorphous state,which helps to reduce the stacking of lamellae and form thinner GNFs.Furthermore,at a given H/C ratio,increasing the gas pressure compresses the plasma volume and increases the radical concentration,which increases the thickness and yield of GNFs.3,The results show that the CH4/Ar emission spectra are dominated by CH,C2 and ArI lines.At low current,electron e and Ar*play an important role in CH4 pyrolysis,e+CH4=>e+CH3+H and Ar*+CH4=Ar+CH3+H contributed 70%to CH4.Reaction H+CH4=>CH3+H2 also contributes to CH4 transformation and the contribution increases with the increase of current.The increase of concentration and current increased the concentration of C2 species,while the addition of H2 inhibited the formation of C2 by recombination with C2,C2H and C2H3 radicals.The gaseous products after CH4 transformation are mainly H2 and C2H2,of which H2 comes from the recombination reaction of H with CH4,C2H4 and other species.C2H2 comes from the reaction of H with C2H3 and other free radicals.The numerical simulation results are consistent with the experimental results,and the appropriate concentration of carbon precursors is conducive to GNFs generation.4.By introducing nitrogen sources(N2,NH3,acetonitrile,isopropylamine)into the reaction system,few-layer nitrogen-doped graphenes(N-GNFs)were synthesized,and the effect of process parameters on the properties of the product was studied.The results showed that:(1)The nitrogen doping level of the N-GNFs was 1%～4.9%,the specific surface area was up to 438m2/g,and the yield was up to 11.3%;(2)The synthesized N-GNFs were dominated by the pyrrole nitrogen configuration,and the nitrogen doping level increases with the increase of H/C ratio;(3)Compared with using N2 or NH3 as nitrogen source,using acetonitrile and isopropylamine as nitrogen-containing carbon precursors can obtain N-doping level of N-GNFs,possibly cracking out more nitrogencontaining precursors,promoted the generation of N-GNFs;(4)N-GNFs are closely related to the reaction pressure.With the increase of pressure,the synthesis rate and nitrogen doping level of N-GNFs gradually increase,but the sheet distortion will be aggravated.In addition,combined with the emission spectra and product characteristics,the formation process of N-GNFs in the plasma region is similar to that of GNFs.It is speculated that HCN acts as a nitrogen-containing precursor,which binds to the edge of the sheet and promotes the formation of pyrrolic nitrogen.A high H/C ratio facilitates HCN production,which further leads to increased nitrogen doping levels.5.By synthesizing hydrogenated graphene(HG)under low current discharge conditions,the effects of current and H/C ratio on product properties were investigated.The results show that under the current of 0.1-0.2 A,as the current decreases,the sheet size of the synthesized product decreases,the thickness becomes thicker,and the hydrogen content increases.This means that with lower energy input,graphene is more easily hydrogenated during its formation.(2)The addition of H2 has the dual effect of etching and hydrogenation in the system.Low-energy H particles lead to hydrogenation,while high-energy H particles are more inclined to etching.(3)With the increase of hydrogen content,the energy band gap gradually increases to a maximum of 3.4 eV,and the material changes from paramagnetic to ferromagnetic with high saturation magnetization.