Progress in Polymer Science 35 (2010) 1350–1375Contents lists available at ScienceDirectProgress in PolymerSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /p p o l y s ciRecent advances in graphene based polymer compositesTapas Kuilla a ,Sambhu Bhadra b ,Dahu Yao a ,Nam Hoon Kim c ,Saswata Bose d ,Joong Hee Lee a ,d ,∗a BIN Fusion Research Team,Department of Polymer &Nano Engineering,Chonbuk National University,Jeonju,Jeonbuk 561-756,Republic of Koreab School of Polymers and High Performance Materials,The University of Southern Mississippi,118College Drive #10076,Hattiesburg,MS 39406-0001,USAc Department of Hydrogen and Fuel Cell Engineering,Chonbuk National University,Jeonju,Jeonbuk 561-756,Republic of Korea dDepartment of BIN Fusion Technology,Chonbuk National University,Jeonju,Jeonbuk 561-756,Republic of Koreaa r t i c l e i n f o Article history:Received 8March 2010Received in revised form 13July 2010Accepted 21July 2010Available online 27 July 2010Keywords:GrapheneGraphene modification Polymer composites Mechanical properties Electrical conductivitya b s t r a c tThis paper reviews recent advances in the modification of graphene and the fabrication of graphene-based polymer nanocomposites.Recently,graphene has attracted both academic and industrial interest because it can produce a dramatic improvement in properties at very low filler content.The modification of graphene/graphene oxide and the utilization of these materials in the fabrication of nanocomposites with different polymer matrixes have been explored.Different organic polymers have been used to fabricate graphene filled polymer nanocomposites by a range of methods.In the case of modified graphene-based polymer nanocomposites,the percolation threshold can be achieved at a very lower filler loading.Herein,the structure,preparation and properties of polymer/graphene nanocomposites are discussed in general along with detailed examples drawn from the scientific literature.© 2010 Elsevier Ltd. All rights reserved.Contents 1.Introduction ........................................................................................................................13512.Graphene............................................................................................................................13522.1.Discovery of graphene (1352)Abbreviations:AFM,atomic force microscopy;CNT,carbon nanotubes;CNF,carbon nanofiber;CCl 4,carbon tetrachloride;CTEs,coefficient of thermalexpansions;CVD,chemical vapor deposition;S-CCG,chemically converted graphene;CHCl 3,chloroform;CMG,chemically modified graphene;Cryo-TEM,cryogenic transmission electron microscopy;DCM,dichloro methane;DMSO,dimethyl sulfoxide;DSC,differential scanning calorimetry;DMA,dynamic mechanical analysis;DMTA,dynamic mechanical thermal analysis;EELS,electron energy loss spectroscopy;EMI,electromagnetic interference; ,elec-trical conductivity;ESEM,environmental scanning electron microscopy;xGnP,exfoliated graphite nanoplatelet;FLG,few layer graphene;GNS,graphene nanosheets;EVA,ethylene vinyl acetate;EG,expanded graphite;FG,foliated graphite;FT-IR,fourier transform infra red;GPa,gigapascal;T g ,glass transition temperature;GO,graphite oxide;HMPA,hexamethylphosphoramide;HDPE,high density polyethylene;IR,infra red;GNP IL ,ionic-liquid-functionalized graphene;KMG,KOH treated GO;LDH,layered double hydroxide;LM,light microscopy;LLDPE,linier low density polyethylene;e //,loss factor;MPa,mega-pascal;MMA,methyl methacrylate;NMP,N-methylpyrrolidone;DMF,N,N -dimethylformamide;DMAc,N,N -dimethylacetamide;ODA,octadecylamine;PETI,phenylethynyl-terminated polyimide;PANI,polyaniline;PPS,poly(phenylene sulfide);PNIPAAm,poly(N-isopropylacrylamide);PSS,poly(sodium 4-styrenesulfonate);PVA,poly(vinyl alcohol);PS,polystyrene;PMMA,poly(methyl methacrylate);PI,polyimide;PET,poly(ethylene terepthalate);PE-g-MA,polyethylene grafted maleic anhydride;PP,polypropylene;PVC,poly(vinyl chloride);PA6,polyamide;SIS,poly(styrene-b -isoprene-b -styrene);PS,polystyrene;PU,polyurethane;PVDF,poly(vinylidene fluoride);PEDOT,poly(3,4-ethyldioxythiophene);PB −,pyrenebutyrate;PBA,pyrene butyric acid;SEM,scanning electron microscopy;SiO 2,silicaSiCsilicon carbide;SWNTs,single wall carbon nanotubes;SDS,sodium dodecyl sulfate;SDBS,sodium dodecyl benzene sulfonate;E /or G /,storage modulus;SSSP,solid-state shear pulverization;SPANI,sulfonated polyaniline;THF,tetrahydrofuran;TMEDA,tetramethylethylenediamine;TCNQ,7,7,8,8-tetracyanoquinodimethane;TGA,thermogravimetric analysis;TPU,thermoplastic polyurethane;XRD,X-ray diffraction.∗Corresponding author at:BIN Fusion Research Team,Department of Polymer &Nano Engineering,Chonbuk National University,Jeonju,Jeonbuk 561-756,Republic of Korea.Tel.:+82632702342;fax:+82632702341.E-mail address:jhl@chonbuk.ac.kr (J.H.Lee).0079-6700/$–see front matter © 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.progpolymsci.2010.07.005T.Kuila et al./Progress in Polymer Science35 (2010) 1350–137513512.2.Synthesis and structural features of graphene (1352)2.3.Surface modification of graphene (1352)2.3.1.Chemical modification of graphene (1353)2.3.2.Electrochemical modification of graphene (1356)2.3.3.–interaction (1357)3.Polymer/graphene nanocomposites (1358)3.1.Preparation methods of polymer/graphene nanocomposites (1358)3.1.1.In situ intercalative polymerization (1358)3.1.2.Solution intercalation (1359)3.1.3.Melt intercalation (1359)3.2.Graphenefilled different polymer composites (1360)3.2.1.Epoxy/graphene nanocomposites (1360)3.2.2.Polystyrene/graphene nanocomposites (1360)3.2.3.Polyaniline/graphene nanocomposites (1362)3.2.4.Nafion/graphene nanocomposites (1364)3.2.5.PVA/graphene nanocomposites (1364)3.2.6.PU/graphene nanocomposites (1365)3.2.7.PVDF/graphene nanocomposites (1366)3.2.8.Poly(3,4-ethyldioxythiophene)/graphene nanocomposites (1368)3.2.9.Polyethylene terephthalate/graphene nanocomposites (1368)3.2.10.Polycarbonate/graphene nanocomposites (1369)4.Conclusions (1369)Acknowledgements (1370)References (1371)1.IntroductionThefield of nanoscience has blossomed over the last twenty years,and the importance for nanotechnology will increase as miniaturization becomes more important in areas,such as computing,sensors,biomedical and many other applications.Advancements in these disciplines depend largely on the ability to synthesize nanoparti-cles of various materials,sizes and shapes,as well as to assemble them efficiently into complex architectures [1].Currently,nanomaterials have an enormous range of applications owing to their structural features.However, material scientists are examining materials with improved physicochemical properties that are dimensionally more suitable in thefield of nanoscience and technology.In this regard,the discovery of graphene and graphene-based polymer nanocomposites is an important addition in the area of nanoscience,playing a key role in modern science and technology[2].The discovery of polymer nanocomposites by the Toyota research group[3]has opened a new dimension in thefield of materials science.In particular,the use of inorganic nanomaterials asfillers in the preparation of polymer/inorganic composites has attracted increasing interest owing to their unique properties and numerous potential applications in the automotive,aerospace,con-struction and electronic industries[4–11].Thus far,the majority of research has focused on polymer nanocom-posites based on layered materials of a natural origin,such as a montmorillonite type of layered silicate compounds or synthetic clay(layered double hydroxide)[5–17]. However,the electrical and thermal conductivity of clay minerals are quite poor[18–20].In order to overcome these shortcomings,carbon-based nanofillers,such as carbon black,EG,CNT,and CNF have been introduced to the preparation of polymer nanocomposites[21–47].Among these,CNTs have proven to be very effective as conductivefillers[22,33–39].The only drawback of CNTs as a nanofiller is their higher production cost[48]. Therefore,the mass production of CNT based functional composite materials is very difficult.As Nicholas A.Kotov wrote in his review in Nature[49]“When carbonfibers just won’t do,but nanotubes are too expensive,where can a cost-conscious materials scientist go tofind a practical conductive composite?The answer could lie with graphene sheets”.Graphene is considered a two-dimensional carbon nanofiller with a one-atom-thick planar sheet of sp2bonded carbon atoms that are densely packed in a honeycomb crystal lattice.It is regarded as the“thinnest material in the universe”with tremendous application potential[50,51].Graphene is predicted to have remarkable properties,such as high thermal con-ductivity,superior mechanical properties and excellent electronic transport properties[52–56].These intrinsic properties of graphene have generated enormous interest for its possible implementation in a myriad of devices [57].These include future generations of high speed and radio frequency logic devices,thermally and electrically conducting reinforced nanocomposites,ultra-thin carbon films,electronic circuits,sensors,and transparent and flexible electrodes for displays and solar cells[57–71]. Graphene,as a nanofiller,may be preferred over other conventional nanofillers(Na-MMT,LDH,CNT,CNF,EG,etc.) owing to high surface area,aspect ratio,tensile strength (TS),thermal conductivity and electrical conductivity, EMI shielding ability,flexibility,transparency,and low CTE[52–56,72].Table1gives a comparative chart on the mechanical,thermal and electrical properties of graphene with CNT,steel,plastic,rubber andfiber.The tensile strength of graphene is similar or slightly higher than CNT, but much higher than steel,Kevlar,HDPE and natural rub-ber.The thermal conductivity of graphene is higher than1352T.Kuila et al./Progress in Polymer Science35 (2010) 1350–1375Table1Properties of graphene,CNT,nano sized steel,and polymers.Materials Tensile strength Thermal conductivity(W/mk)at room temperature Electrical conductivity(S/m)ReferencesGraphene130±10GPa(4.84±0.44)×103to(5.30±0.48)×1037200[73–84] CNT60–150GPa35003000–4000[35,85–88] Nano sized steel1769MPa5–6 1.35×106[89,90] Plastic(HDPE)18–20MPa0.46–0.52Insulator[91–93] Rubber(natural rubber)20–300.13–0.142Insulator[94,95] Fiber(Kevlar)3620MPa0.04Insulator[96,97]all these materials.The electrical conductivity of graphene is also higher than these materials except for steel[73–97].The superior properties of graphene compared to polymers are also reflected in polymer/graphene nanocom-posites.Polymer/graphene nanocomposites show superior mechanical,thermal,gas barrier,electrical andflame retardant properties compared to the neat polymer [2,44–47,98–101].It was also reported that the improve-ment in mechanical and electrical properties of graphene based polymer nanocomposites are much better in com-parison to that of clay or other carbonfiller-based polymer nanocomposites[2,98–101].Although CNTs show compa-rable mechanical properties compared to graphene,still graphene is a better nanofiller than CNT in certain aspects, such as thermal and electrical conductivity(Table1) [73–97].However,the improvement in the physicochem-ical properties of the nanocomposites depends on the distribution of graphene layers in the polymer matrix as well as interfacial bonding between the graphene layers and polymer matrix.Interfacial bonding between graphene and the host polymer dictates thefinal properties of the graphene reinforced polymer nanocomposite.Pristine graphene is not compatible with organic polymers and does not form homogeneous composites.In contrast,graphene oxide sheets are heavily oxygenated graphene(bearing hydroxyl,epoxide,diols,ketones and carboxyls functional groups)that can alter the van der Waals interactions sig-nificantly and be more compatible with organic polymers [68,102–106].There are also some additional carbonyl and carboxyl groups located at the edge of the sheets,which makes graphene oxide sheets strongly hydrophilic,allow-ing them to readily swell and disperse in water[107,108]. For this reason,GO has attracted considerable attention as a nanofiller for polymer nanocomposites.However,GO sheets can only be dispersed in aqueous media,which are incompatible with most organic polymers.In addition, unlike graphene,graphene oxide is electrically insulating, which makes it unsuitable for the synthesis of conducting nanocomposites.The surface modification of graphene is an essential step for obtaining a molecular level dispersion of individual graphene in a polymer matrix.The electri-cal conductivity of the resulting nanocomposites can be increased by the chemical reduction of GO,presumably by restoring the graphitic network of sp2bonds[2].2.Graphene2.1.Discovery of grapheneGraphene is the basic structural unit of some car-bon allotropes,including graphite,carbon nanotubes and fullerenes(Fig.1).It is believed to be composed of ben-zene rings stripped of their hydrogen atoms.The rolling up of graphene along a given direction can produce a car-bon nanotube.A zero-dimensional fullerene can also be obtained by wrapping-up graphene[109,110].In1940,it was established theoretically that graphene is the building block of graphite[111].In2004,Geim and co-workers at Manchester University successfully identified single lay-ers of graphene and other2-D crystals[112]in a simple tabletop experiment,which were previously considered to be thermodynamically unstable and could not exist under ambient conditions[113,114].The promising mechani-cal,electrical,optical,thermal and magnetic properties of graphene have“led to the creation of a new and exciting ‘laboratory’for the study of fundamental science.”2.2.Synthesis and structural features of grapheneGraphene can be prepared using four different meth-ods[84].Thefirst is chemical vapor deposition(CVD)and epitaxial growth,such as the decomposition of ethylene on nickel surfaces[115,116].The second is the microme-chanical exfoliation of graphite,which is also known as the‘scotch tape’or‘peel-off’method[112,117].The third method is epitaxial growth on electrically insulating sur-faces,such as SiC,and the fourth is the solution-based reduction of graphene oxide[84,118].Graphene is a single layer of carbon atoms packed densely in a honeycomb crystal lattice.The carbon-carbon bond(sp2)length in graphene is approximately0.142nm [119–121].Depending on the research group,the graphene layer thickness ranges from0.35to1nm relative to the SiO2substrate[122].Novoselov et al.measured platelet thicknesses of1–1.6nm[112].2.3.Surface modification of graphenePristine graphene materials are unsuitable for interca-lation by large species,such as polymer chains,because graphene as a bulk material has a pronounced tendency to agglomerate in a polymer matrix[106,123].It is likely that oxidation followed by chemical functionalization will facilitate the dispersion and stabilize graphene to prevent agglomeration[123,124].The functional groups attached to graphene can be small molecules[105,125–127]or polymer chains[128–130].The chemical functionalization of graphene is a particularly attractive target because it can improve the solubility and processability as well as enhance the interactions with organic polymers[131,132]. Considerable work has been carried out on the amination, esterification[104,132],isocyanate modification[133],andT.Kuila et al./Progress in Polymer Science 35 (2010) 1350–13751353Fig.1.Graphene (top left)is a honeycomb lattice of carbon atoms.Graphite (top right)can be viewed as a stack of graphene layers.Carbon nanotubes are rolled-up cylinders of graphene (bottom left).Fullerenes (C60)are molecules consisting of wrapped graphene through the introduction of pentagons on the hexagonal lattice (Reproduced with permission from Neto et al.[110]).polymer wrapping [128–130]as routes for the functional-ization of graphene.The electrochemical modification of graphene using ionic liquids has also been reported [48].The general approaches for preparing organo-modified graphene are as follows.2.3.1.Chemical modification of grapheneThis method is based on the modified Hummers method [134].Initially,graphite oxides are generally prepared from naturally occurring graphite.After oxidation,several chemical methods to obtain soluble graphene have been examined,including the reduction of graphite oxide (GO)in a stabilization medium [135],covalent modification by the amidation of the carboxylic groups [131,132],non-covalent functionalization of reduced graphene oxide [128–130],nucleophilic substitution to epoxy groups [105],and dia-zonium salt coupling [136].2.3.1.1.Reduction of graphite oxide (GO)in a stabilization medium.Park et al.[135]reported a simple route for the preparation of a homogeneous aqueous suspension of chemically modified graphene with good electrical conduc-tivity.In this method,the precursor graphene oxide was first dispersed in water followed by the addition of an aque-ous KOH solution (Fig.2).According to Park et al.,KOH,a strong base,can confer a large negative charge through reactions with the reactive hydroxyl,epoxy and carboxylic acid groups on the graphene oxide sheets,resulting in extensive coating of the sheets with negative charges and K +ions.The addition of hydrazine monohydrate to KOH-treated graphene oxide produces a homogeneous suspension of hKMG,which can remain stable for at least 4months.Recently,Li et al.prepared stable aqueous colloids of graphene sheets through the electrostatic stabilization of graphite [84].This discovery enabled the development of a facile approach to the large scale production of aque-ous graphene dispersions without the need for polymeric or surfactant stabilizers.2.3.1.2.Covalent modification of graphene.For the prepa-ration of organomodified graphene oxide/graphene,most studies followed a covalent modification technique.Differ-ent organic amines,alkyl lithium reagents,isocyanates,and diisocyanate compounds have been used for this purpose1354T.Kuila et al./Progress in Polymer Science35 (2010) 1350–1375Fig.2.A simple route for the preparation of a homogeneous aqueous suspension of graphene sheets from graphite stack:(a)graphite stack is oxidized to separate the individual layers of graphene oxide (GO),(b)GO is dispersed in water and treated with aqueous KOH solution to obtain KOH modified oxidized graphene (KMG)and finally (c)KMG is reduced using hydrazine to produce hydrazine reduced KOH modified graphene (hKMG)in the form of stable aqueous dispersion of individual graphene sheets.[131–133,137].These surface modifying agents reduce the hydrophilic character of graphene oxide sheets by forming amide and carbamate ester bonds with the carboxyl and hydroxyl groups,respectively [133].Niyogi et al.[132]reported an amide coupling reac-tion between the carboxyl acid groups of graphene oxides and octadecylamine (ODA).Thionyl chloride was used to activate the COOH groups in the presence of N,N -dimethylformamide (DMF)under reflux conditions.ODA-modified GO had a solubility of 0.5mg ml −1in THF,and was also soluble in CCl 4and 1,2-dichloroethane.Soluble graphene layers have also been prepared by react-ing graphite fluoride with alkyl lithium reagents [131].Vacuum-dried graphite fluoride and tetramethylethylene-diamine (TMEDA)were dissolved in hexane at 0◦C in an ice bath.An alkyl-lithium reagent was then added drop wise and stirred for 3days to produce functionalized graphene (Fig.3).IR spectral studies confirmed the covalent attach-ment of alkyl chains to the graphene layers [131].Xu et al.modified the surface of graphene via the cova-lent attachment of a porphyrin ring on the GO surfaces [138].Thionyl chloride was used to activate the carboxylic acid group in the presence of porphyrin using DMF as a sol-vent.Fig.4shows a schematic diagram for the modification of graphene via the covalent attachment of a porphyrin ring on to the GO surfaces.Organic isocyanates were added to a GO dispersion in anhydrous DMF and allowed to react with GO for a spe-cific period of time.The reaction was then quenched by adding methylene chloride,and the modified graphene was washed and dried [133].The isocyanate compound was attached to the hydroxyl and carboxyl groups of GO via the formation of carbamate and amide function-alities,respectively.Fig.5shows a schematic diagram of the formation of carbamate and amide functionali-ties.Isocyanate-modified graphene oxide readily forms a stable colloidal dispersion in all polar aprotic solvents,such as DMF,N-methylpyrrolidone (NMP),dimethyl sul-foxide (DMSO)and hexamethylphosphoramide (HMPA).The modification of GO with diisocyanate compounds was recently reported [137].The preparatory method was sim-ilar to isocyanate modification.However,there was an additional step.The diisocyanate treated graphene oxide was reduced by hydrazine monohydrate at 80◦C for 48h.The building blocks of the GO sheets were crosslinked to form lamellar porous structures via a reaction of diiso-cyanate with the carboxyl and hydroxyl groups on both sides of the sheets.2.3.1.3.Non-covalent functionalization of graphene.The reduction of a stable aqueous dispersion of exfoliated graphite oxide nanoplatelets with hydrazine hydrateT.Kuila et al./Progress in Polymer Science35 (2010) 1350–13751355Fig.3.Production of functionalized graphene layers from graphitefluoride and alkyl lithium reagent.Annealing of functionalized graphene produces 3-dimensional graphite.(Reproduced from Worsley et al.by permission of Elsevier Science Ltd.,UK[131]).resulted in the formation of aggregates[130].The precip-itated materials could not be re-dispersed,even after a prolonged ultrasonic treatment in water in the presence of surfactants,such as SDS and TRITON X-100.In order to overcome this problem,the reduction was carried out in the presence of polymer/polymeric anions,resulting in a stable polymer-grafted dispersion of graphene[130].Stable aqueous dispersions of graphitic nanoplatelets were prepared by coating reduced graphite oxide nanoplatelets with an amphiphilic polymer,poly(sodium 4-styrenesulfonate)(PSS)[130].In2008,an aqueous dispersion of graphene was prepared from expanded graphite using7,7,8,8-tetracyanoquinodimethane(TCNQ)anion as a stabilizer.This could provide a facile route for producing high quality water soluble and organic solvent soluble graphene sheets for a range of applications [139].It was suggested that TCNQ anions adsorb on the graphene sheets,which stabilizes them in water.The TCNQ-stabilized graphene sheets could be re-dispersed in water,DMF and dimethyl sulfoxide(DMSO).Xu et al.[140] reported the preparation of stable aqueous dispersions of graphene sheets using a water soluble pyrene derivative, 1-pyrenebutyrate(PB−),as a stabilizer because the pyrene moiety has been reported to have strong affinity to the basal plane of graphite via-stacking[141,142].For this, an alkaline solution of pyrene butyric acid(PBA)wasFig.4.Schematic showing the modification of graphene via the covalent attachment of a porphyrin ring on to the GO surfaces.The–NH2groups of prophyrene ring are covalently bonded to the carboxyl group of graphene by the amide linkage.Thionyl chloride was used to activate the carboxylic acid group in the presence of porphyrin in DMF.(Reproduced from from Xu et al.by permission of Wiley-VCH,Germany[138]).1356T.Kuila et al./Progress in Polymer Science35 (2010) 1350–1375Fig.5.Schematic showing the modification of GO using organic isocyanate in DMF medium.Scheme shows the reactions of isocyanate group with different functional groups of GO.Isocyanates react with the hydroxyl (left oval)and carboxyl groups (right oval)of graphene oxide sheets to form carbamate and amide functionalities,respectively (Reproduced from Stankovich et al.by permission of Elsevier Science Ltd.,UK [133]).reacted with a GO dispersion followed by reduction with hydrazine monohydrate at 80◦C for 24h.Both the modi-fied and reduced GO dispersed well in water and showed an electrical conductivity of 2×102S m −1.Homogeneous aqueous dispersions of reduced exfoliated graphite oxide in the presence of sulfonated polyaniline (SPANI)have been reported [128].The successful reduction of GO was confirmed by electrical conductivity measurements.The conductivity of the composite film was 30S m −1,which is similar to that of the reduced GO sheets reported in the literature [140].2.3.1.4.Nucleophilic substitution.Aliphatic amine-modified GO was prepared by a reaction of amine with an aqueous dispersion of GO [105].Nucleophilic substitution occurs on the epoxy groups of GO [143].For the small chain primary amine,C n H 2n +1NH 2(n =2,4,8,12),grafting was completed at room temperature.On the other hand,for long chain aliphatic amines (octadecylamine),the reaction mixture was heated under reflux for 24h.However,the later GO derivatives were dispersed more easily in organic solvents.X-ray diffraction (XRD)showed that the interlayer distance of the amine intercalated GO derivatives depends on the amine chain lengths and their orientation relative to the layers.According to Bourlinos et al.,the tilted orientation is more likely to be due to the hydrophilic nature of the layers [105].Bourlinos et al.[105]also reported amino acid modified graphene.In this case,an alkaline solution of amino acids was used to modify GO.Nucleophilic attack of the –NH 2end group on the epoxide groups of GO occurs,suggesting that the amino acid molecules adopt a flat orientation in the interlayer zone of GO.2.3.1.5.Diazonium salt coupling.Graphene (S-CCG)sheets wrapped chemically with sodium dodecyl benzene sul-fonate (SDBS)were obtained by the reduction of graphene oxide with hydrazine and functionalized by a treatment with aryl diazonium salts [136].Thefunctionalized graphene could be dispersed readily in N,N -dimethylformamide (DMF),N,N -dimethylacetamide (DMAc)and 1-methyl-2-pyrrolidinone (NMP)up to 1mg/mL with minimal sedimentation.Cryogenic trans-mission electron microscopy (Cryo-TEM)revealed the existence of individual sheets and a few multiple sheet structures of SDBS-coated graphene Thermogravimetric analysis showed that the thermal stability of functional-ized CCG was significantly higher than that of neat GO but lower than that of S-CCG.The degree of functionalization calculated from these weight losses was estimated to be ∼1functional group in 55carbon atoms.The chemical modifi-cation of epitaxial graphene by the covalent attachment of aryl groups to the basal carbon atoms was reported [125].Epitaxial graphene grown on SiC wafers was modified chemically with aryl groups via the formation of cova-lent bonds to the conjugated carbon atoms [118,125,144].The grafting of aryl groups was confirmed by FT-IR spec-troscopy.Nitrophenyl-functionalized epitaxial graphene exhibited some new bands at 1565and 1378cm −1,which correspond to the symmetric and asymmetric stretching modes of NO 2.2.3.2.Electrochemical modification of grapheneGraphite was treated electrochemically to produce a colloidal suspension of chemically modified graphene (CMG)[48].Fig.6shows a schematic diagram of the exper-imental set up.A commercial graphite electrode was used as the cathode,which was immersed in a phase-separated mixture of water and imidazolium-based ionic liquids.A constant potential of 10–20V was applied across the electrodes.After 30min electrochemical reaction,ionic liquid functionalized graphene (GNP IL )sheets originating from the graphite anode precipitated.Ultrasonication of dried GNP IL in DMF resulted in a homogeneous dispersion (1mg ml −1).The average length and width of the GNP IL were 700and 500nm,respectively,as evidenced from the TEM images.AFM image analysis revealed the thickness of GNP IL to be ∼1.1nm.。
