Study On Preparation Of Graphene And Conductive Behavior

Graphene is a two-dimensional atomically thick crystal with carbon atoms arranged in a honeycomb lattice.It has attracted a great amount of interest for this material combines excellent mechanical,optical and electrical properties.Many physical methods have been developed to produce single-layer or few-layer graphene,such as chemical vapor deposition(CVD),epitaxial growth,axial cut method of carbon nanotubes and liquid-phase exfoliation.While the first three methods cannot realize scale-up production since their preparation processes are subtle and complex.Some chemical methods have also been used to prepare graphene,including chemical reduction of graphite oxide,thermal reduction of graphene oxide introduced by microwaves,thermal reduction of colloidal dispersion of graphite oxide and electrochemical method.However,the first three approaches all involve the process of oxidation of graphite,which leads to many oxygen functional groups on the surface of graphene causing obvious defects.Therefore,we demonstrate the methods of liquid-phase exfoliation and electrochemical method can prepare graphene with high quality and free-defect.By the liquid-phase exfoliation method,expanded graphite,which was obtained by high temperature process from pristine expandable graphite in microwave,was exfoliated in co-solvent of N,N-Dimethyl Formamide(DMF)and water by sonication.This method avoided both oxidation and reduction process,and gained graphene dispersion of high quality.Then,the as-prepared graphene was characterized by field scanning electron microscope(SEM),transmission electron microscopy(TEM),ultraviolet-visible spectra(UV-Vis),fourier transform infrared spectra(FTIR),and Raman spectroscopy,and the results showed that the graphene prepared by the liquid-phase exfoliation method is of few structural defects and low oxidation.In this process,the optimal ratio of DMF to water to exfoliate graphene is 9:1 by volume.Too high or too low sonication power is not appropriate to produce large-scale graphene,and 20%power for 6.5 hour fits the best in this process.Exfoliation by ultrasonic processing in bath is more difficult to achieve graphene.In addition,the analysis of mechanism demonstrates that the dispersion of the nanofillers and the solvents obeys the principle of similar miscibility.The solvent with a surface tension close to 35?40 mJ·m-2 has high wettability on expanded graphene and is easy to realize exfoliation of expanded graphene.Moreover,good solvents are characterized by a solubility parameter between 11?14(cal/cm3)1/2.One-step electrochemical approach was employed to produce graphene dispersion with perfect structure by exfoliation of graphite in electrolyte.UV-Vis,FTIR,SEM,TEM and Raman were used to characterize morphology and structure of the as-prepared graphene.The as-prepared graphene are of high quality.The concentration of graphene increases when the sonication time prolongs in this preparation process.However,sonication time has a slight effect on the dispersion concentration after 80min.Four-probe method was employed to measure the conductivity of the graphene.The room-temperature(300 K)conductivity of expanded graphite,graphene prepared by liquid-phase exfoliation method and graphene prepared by electrochemical method is 6.1×10,7.1×10,and 6.7×104 S/m,respectively.These data are all much higher than conductivity value of graphene prepared by chemical reduction of graphite oxide.The result of measurements indicates that the resistance of graphene gradually decreases as the temperature increases.However,the decrease rates are different in different temperature ranges.Based on these experiment results,we propose a model which applies to relationship between DC conductivity and temperature(T)of semi-conductor.For the graphene prepared by liquid-phase exfoliation,the dependence relationship of conductivity on temperature is divided into four temperature ranges:At the range of 390 K-450 K,carrier transport behavior is well described by TA model;at the range of 340 K-390 K,carrier transport behavior is described by temperature dependence relationship in band-tail states,at the range of 275 K-315 K,carrier transport behavior is explained by short-range hopping;while at the range of 140 K?270 K,carrier transport behavior is well explained by two-dimensional variable range hopping.However,for graphene prepared by electrochemical method,at low temperature range of 100 K?120 K,the dependence of conductivity(?)on the temperature can be expressed using the relationship of the??exp[-(T0/T)1/3],and carrier transport behavior is explained by two-dimensional variable range hopping;whereas,at high temperature range of 200K?500K,the dependence relationship is consistent with the express of ??exp[-?E/k0T],and thus carrier transport behavior is well described by thermal activation(TA)model.