Production Of Graphene Nanosheets By Supercritical CO₂ Technology Coupled With Mechanical Exfoliation

Since its discovery in 2004,graphene has attracted close attention all over the world because it has excellent mechanical,thermal,electrical and optical properties and a great promising prospect in the fileds of electronic devices,aerospace equipment,biosensors and high performance composites.How to obtain high quality,single and few-layer graphene nanosheets has become the urgent matter.In this paper,the exfoliation of graphite into few-layer graphene nanosheets by supercritical CO2 process coupled with mechanical exfoliation were first investigated and single and few-layer graphene nanosheets were obtained.The effects of operating parameters on graphene yield was detailedly studied,the optimal conditions of preparing graphene was determined,the graphene nanosheets were characterized and the possible mechanism of preparation process was explored.First,the production of graphene nanosheets with 3500 mesh graphite flakes and 1000 mesh expanded graphite by supercritical CO2 process coupled with mechanical exfoliation was developed,the effects of different graphite on the properties of graphene were studied.The results showed that the size of graphene exfoliated from 3500 mesh graphite flake was hundreds of nanometers,where the size of graphene exfoliated from 1000 mesh expanded graphite was about few microns,thus the size differ was about an order of magnitude.In addition,the exfoliating ratio of graphite flake was also very low compared with expanded graphite.Therefore,1000 mesh expanded grphite was used as raw materials in the experiment.Second,the effects of operating parameters such as pressure,stirring time,stirring speed and initial graphite on graphene yield were studied by control variate method.According to the experiment results,the optimum conditions were as follows:temperature of 40 ?,pressure of 13 MPa,stirring time of 60 minutes,stirring speed of 1000 rpm and initial graphite mass of 0.5 g.The concentration of graphene dispersion could reach 1.4 mg/mL by UV-Vis,the yield was 28%.The exfoliated graphene nanosheets were characterized,SEM and TEM showed that single and few-layer graphene sheets were obtained,and the electron diffraction pattern suggested the exfoliated graphene were well crystallized in hexagonal symmetry structure;AFM displayed that more than 88%of the graphene sheets were less than three layers,where monolayer and bilayer were the major products(altogether 71%).XRD indicated that the graphene structure was not distorted during exfoliation process and no other functional groups were introduced.Raman showed that the ID/IG ratio(0.45)of graphene had little defect,which is much smaller than those of chemically reduced GO(1.1?1.5),the shape and position of 2D peak futher proved that the graphite was exfoliated into single or few-layer graphene.The possible mechanism of production of graphene by supercritical CO2 process coupled with mechanical exfoliation was discussed and the electrical conductivity was measured by a four-point probe instrument.The results showed that the electrical conductivity of graphene film was very high,reaching 2.1 x 105 S/m.Finally,our experimental device was improved,the ultrasound was introduced in the stainless steel autoclave,magneton was replaced by mechanical stirring,jagged mechanical exfoliation device was replaced by venturi mechanical exfoliation device.Production of graphene nanosheets by ultrasonic-assisted supercritical CO2 process coupled with mechanical exfoliation were developed.The effects of ultrasonic power and ultrasonic time on graphene yield were studied by control variate method.The optimum conditions were as follows:temperature of 40 ?,pressure of 13 MPa,initial graphite mass of 0.5 g,mechanical stirring speed of 1000 rpm,ultrasonic power of 180 W and ultrasonic time of 90 min.The exfoliated graphene were characterized,SEM and TEM showed that large lateral size of single and few-layer graphene nanosheets were obtained,and the electron diffraction pattern suggested the exfoliated graphene were well crystallized in hexagonal symmetry structure,which confirmed that the crystallinity of graphene structure was not damaged during exfoliation process;Raman showed that the ID/IG ratio of graphene was 0.24,which was smaller than the craft without ultrasound,this proved that the quality of graphene by ultrasonic-assisted supercritical CO2 process coupled with mechanical exfoliation were much higher.AFM indicated the large size graphene was obtained,more than 92%of the graphene sheets were less than three layers,where monolayer and bilayer were 78%(35%of monolayer,43%of bilayer).Then the cyclic voltammetry of graphene electrode was tested by the three electrode system.The cyclic voltammetry curve showed that the specific capacitance decreased with the increase of scanning speed.And when the scanning speed was 5 mV/s,the specific capacitance was 268 F/g,which showed a good electrochemical performance.