Experimental Study On The Synthesis Of Graphene Nanoflakes By Magnetically Rotating Non-thermal Arc Plasma

Graphene is a novel two-dimensional nanomaterial,which,due to its excellent physical,chemical and mechanical properties,has been proposed for a wide array of applications in many fields.Non-thermal plasma gas phase synthesis is a new process for the synthesis of graphene developed in recent years.In contrast to other approaches,the plasma process is a single-step,continuous and rapid technique that does not require the use of catalysts or substrates.At present,there are few reports on the synthesis of graphene by non-thermal plasma gas phase,and the process conditions for the formation of graphene are still unclear,which limits the further development of this technology.In order to solve these problems above,in this paper,a magnetically rotating non-thermal arc plasma generator is developed.Driven by the axial magnetic field,the gliding arc rotates rapidly around the cathode in a cylindrical generator so that the arc plasma can maintain a non-equilibrium state.The effect of magnetic field on arc characteristics is studied.The results show that the voltage decreases with the increase of current.As the magnetic field increases,the arc voltage,rotating speed,re-breakdown frequency,electric field in the anode region and the non-equilibrium degree in the plasma region increase.On this basis,the experimental platform for the synthesis of carbon nanomaterials is established.The graphene nanoflakes(GNFs)are prepared by cracking hydrocarbon gases(CH4,C2H2,C2H4 and C3H8).The characterization results of transmission electron microscope,Raman and BET analysis indicate that the GNFs layers range of 1?20 layers and sizes range from 50 to 200 nm.GNFs have a typical folded shape and GNF sheets are connected to each other.For this method,the GNFs yield reaches 7.7%,and the comprehensive energy consumption is about 0.4 kW·h/g.The relationship among product morphology and process parameters such as current,raw material type,buffer gas type,raw material concentration and gas flow rate is preliminarily explored in the experiment.The nucleation and growth mechanism of graphene under the condition of non-thermal arc plasma is analyzed.The results show that low nucleation precursor concentration and abundant H are necessary conditions for the formation of higher quality graphene.Carbon nano-onions(CNOs)with a particle size of 70?200 nm and a highly graphitized structure are synthesized from C3H8 in the atmosphere of Ar-H2 buffer gas.The formation conditions and mechanism of CNOs are preliminarily studied.The results show that long residence time and moderate hydrogen are helpful to the production of CNOs.The analysis suggests that CNOs are formed by GNFs through curling and closing in the plasma region.Finally,using CH4 as raw material,the effect of buffer gas type on the microstructure of carbon nanomaterials is studied in detail.The results show that in pure CH4 or Ar-CH4 atmosphere,the products are mainly the mixture of spherical particles and GNFs.When the buffer gas is He or H2,the products are GNFs with fewer defects,high crystallinity and good thermal stability.When N2 is used as a buffer gas,the products consist of GNFs,a tangled layer of graphite and nitrogen-doped nanoparticles which resemble "carbon dots".It is further confirmed that the low concentration of nucleation precursor and abundant H promote the formation of graphene,while the complex product morphology in N2 might bedue to the incorporation of nitrogen atoms.The non-thermal plasma process developed in this paper is simple to operate,easy to obtain raw materials,low comprehensive energy consumption and environmentally friendly,which provides a new sight for the large-scale synthesis of graphene and other carbon nanomaterials.