Graphene Nanodiscs from Electrochemical assisted Micromechanical Exfoliation of Graphite: Morphology...

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Graphene nanodiscs from electrochemical assistedmicromechanical exfoliation of graphiteMorphology and supramolecular behaviorNeelima Mahatolowast Nazish Parveen and Moo Hwan Cholowast

School of Chemical Engineering Yeungnam University 712 749 Republic of Korea

ABSTRACT

We report on the synthesis of graphene nanodiscs by electrochemical assisted micromechanical exfoliation ofgraphite sheet High resolution electron micrographs show unique circular disc like polycrystalline graphenestructures The nanodiscs are transparent range between 20ndash50 nm in diameter consists 1ndash2 layers do notfold easily dispersed in aqueous ethanolic solution and exhibit supramolecular behavior of self assembly Thenanodiscs exhibit dual characteristics viz circular disc like structures when dispersed in aqueous ethanolicsolution and flake like structures when dehydrated The article addresses in detail the intricate intrigues of thesynthesis characterization and its supramolecular behavior

Keywords Electrochemical Exfoliation Micromechanical Exfoliation Nanodisc Supramolecular AssemblyHydrogen Bond Delocalized -Electrons

1 INTRODUCTIONGraphene is a planar single layer of sp2 hybridized car-bon atoms in hexagonal arrangement It is nonaromaticpossesses unique properties such as a high Youngrsquosmodulus (1060 GPa) outstanding electrical conductiv-ity (3000 Wmminus1Kminus1 thermal conductivity mechanicalproperties1ndash7 and has immense applications ranging fromtransistors transparent conducting electrodes optoelec-tronics sensors plasmonics solar cells supercapacitorselectronic devices lubricant additives waterproof coatingstructural materials tissue engineering biomaterial devicesand drug delivery8ndash12 It is anticipated to replace the tra-ditional semiconductor materials like Si and Ge whichare relatively expensive Besides replacing Si materials incomputer chips batteries supercapacitors hydrogen stor-age materials and drug carriers graphene is also believedto be a very useful material in developing high per-formance polymer composite There have been immense

lowastAuthors to whom correspondence should be addressedEmails mhchoynuackr neelapchemgmailcom

efforts in the direction for synthesizing scalable graphenein large scale which include micromechanical exfolia-tion of highly ordered polymeric graphite (HOPG)1314

chemical reduction of graphene oxide epitaxial growth15

thermal exfoliation16 microwave synthesis bottom upassembly17 electrostatic deposition18 chemical vapordeposition liquid phase exfoliation of graphite19ndash21 arcdischarging22 and solvothermal method Micromechani-cal method though produces high quality graphene sheetsbut is unsuitable for large scale production Solution phaseexfoliation by sonication method has also been found tosuffer from various drawbacks such as solvent incompati-bility extensive sonication times23ndash25 etc Researchers arealso focusing to synthesize scalable graphene that couldbe easily transferred to any desired (foreign surface) par-ticularly for flexible and stretchable molecular electronicsbased on polymers26ndash33 Among various methods chem-ical vapor deposition is considered as one of the mostreliable and advantageous technique for large area growthof graphene But it requires a rigid substrate which canremain stable at high temperatures (above sim900 C) as

Mater Express Vol 5 No 6 2015 471



Materials ExpressGraphene nanodiscs from electrochemical assisted micromechanical exfoliation of graphite

Mahato et al

Article

well as harsh conditions during the etching process How-ever chemical vapor deposition is capable of producingtransferable size graphene limited up to few centimeterseither because of the size limit of a suitable rigid sub-strate or the inhomogeneity of reaction temperature insidethe furnace The number of layers deposited have beenfound to vary from 2ndash8 when a Ni substrate is employedand 1ndash2 layers when a Cu substrate is used Further-more the graphene produced from Chemical vapor depo-sition is prone to defects The defects are believed tobe introduced either during deposition or etching processTill so far scalable graphene has been synthesized usingvarious techniques and in almost all the cases the out-come is typically polycrystalline films consisting numer-ous single crystalline grains83435 We report here on thesynthesis of 1ndash2 layered graphene nanodiscs by electro-chemical assisted micromechanical exfoliation of graphiteits morphology surface properties and supramolecularbehavior We also present a molecular model to explainthe supramolecular behavior Due to its behavior of selfassembly the nanodiscs are anticipated to be suitable fornanomicropatterning which can be useful in fabricatingnanoelectronic circuits and nanodevices

2 EXPERIMENTAL DETAILS21 Materials and MethodsThe required materials for synthesis of graphene nan-odiscs viz graphite sheet (10times 15times 05 cm3 concH2SO4 (98) and sodium phosphate dibasic were pro-cured from Duksan Pure Chemicals Co Ltd Korea Theexperiments were carried out in two phases viz(i) electrochemical exfoliation of graphite sheet intographene nanosheets followed by(ii) micromechanical exfoliation of the graphenenanosheets into graphene nanodiscs in excess volumes ofaqueous ethanol

Electrochemical exfoliation was carried out using thechrono method The electrochemical cell consisted agraphite sheet as anode and a platinum wire mesh ascathode both immersed in 100 ml of dilute H2SO4 (1)solution containing 3 g sodium phosphate dibasic as elec-trolyte The solution was stirred throughout the experi-ment The electrodes were connected to the respectiveterminals of the Electrochemical Workstation (VersaSTAT3 Princeton Research USA) and chronoampherometrywas run at plusmn10 volts for 5 h At the beginning the testsolution was transparent With the progress of the reactionit gradually turned dark and at the end of the experi-ment it became black After completion of the experimentthe solution was allowed to stand overnight The blackmass settled at the bottom was washed with DI water andethanol and dried The graphene quantity achieved by thismethod was found to be dependent on (a) the cathodematerial (b) cathode surface area and (c) current den-sity In this work both graphite sheet and a platinum wire

mesh were tested as the cathode material When a plat-inum wire mesh was used as cathode faster productionof graphene along with greater yield (sim4 g in 5 h) wasobserved compared to its graphite counterpart Further anincrease in the rate of cathodic reaction ie either byincreasing the surface area of the cathode or by increas-ing the number of reducible ionic species in the elec-trolyte was observed to increase the rate of exfoliationThe graphene obtained from electrochemical exfoliationwas multilayered (4ndash7 layered) This was then subjected tomicromechanical exfoliation in excess volumes of aqueousethanol To do so a small amount (sim05 mg) of graphenewas taken in an agate pestle and sim5 ml of aqueous ethanolwas added to it The consistency was grounded manuallyusing a mortar for about an hour with the additional vol-umes of ethanol as and when required untill the dispersionappeared almost transparent While preparing specimen forcharacterization the consistency was bath sonicated forabout 5ndash10 min to ensure maximum dispersion For AFManalysis a drop of the suspension was cast over a cleansilicon wafer and carefully dried For other investigationsviz SEM XRD TEM and Raman specimens were madefrom this suspension For TEM analysis a drop of the sus-pension was cast on a clean copper grid dried and exam-ined On the other hand a fine dispersion of dried powderwas sprinkled over carbon tape (fixed on copper stubs) forSEM investigation For XRD and XPS a greater amountof dried powder (05ndash1 g) was required for analysis

22 CharacterizationMicrostructure and phase analysis was carried outusing scanning electron microscopy (HITACHI-S4800)X-ray diffraction (XRD analytical X-pert PRO-MPDthe Netherland and high resolution transmission elec-tron microscopy (HRTEM TEM-2100 JEOL) operating at200 kV Raman spectrum was recorded on LabRam HR800 UV Raman microscope (Horiba Jobin-Yvon Franceusing laser light of wavelength 514 nm) AFM anal-ysis was done using a Digital Instruments AFM witha nanoscope IIIa controller (Digital instruments USA)The XPS measurements were done with Ar+ ion sputter-ing using a Thermo-VG Scientific MultiLab 2000 with amonochromatic Al K X-ray source (14866 eV) a passenergy of 200 eV and a hemispherical energy analyzerSince no critical final state surface charging effect wasobserved during the XPS measurements no binding energycorrection was performed for the obtained XPS spectra

3 RESULTS AND DISCUSSIONIn graphite each layer is arranged one above another byvan der Waals forces of attraction (cohesive van der Waalsenergy of about 59 kJ molminus1 carbon36) It was observedthat ultrasonication yields single layer filmsflakes asshown in Figure 2(b) as well as multilayered graphite par-ticles In order to get uniform single layered graphene

472 Mater Express Vol 5 2015



Materials ExpressGraphene nanodiscs from electrochemical assisted micromechanical exfoliation of graphiteMahato et al

Article

films we adopted mechanical grinding the multilayeredgraphene flakes in aqueous ethanol (89) using a pes-tle and mortar It was believed to provide shear frictionalforces gentle enough to separate the overlapping graphiticlayers rather than breaking those down into finer irreg-ular multilayered particles While grinding mechanicallythe multilayered (4ndash7) graphene flakes obtained from elec-trochemical exfoliation using a pestle and mortar the finemultilayered particles appeared as colloids in the ethano-lic suspension The latter gradually turned transparent andhomogeneous This process was repeated many times andit was confirmed that the nanodiscs are formed only whena small amount of graphene (multi layered) or graphitepowder is grounded with an excess volume of ethanolIf the amount of graphene during micromechanical exfo-liation is more the nanodiscs are not obtained Moreoverthe formation of nanodiscs did not occur by ultrasonifica-tion Ultrasonification produced smaller and finer particleswhich were multilayered flakes and not disc like struc-tures The steps of the synthesis process are illustratedin Figure 1 After micromechanical exfoliation the twoimportant observations worth noticing were(i) when a drop of the transparent solution cast over acopper grid quickly evaporated and examined under TEMthe discrete nanodisc like features appeared as shown inFigures 2(c)ndash(f)(ii) a similar attempt to perform an AFM measurementdid not bring similar results

Upon evaporation the nanodiscs agglomerated to formflake like structures as shown in the AFM image (Fig 4)It was not possible to image a single nanodisc usingAFM as dry sample is a prerequisite condition for AFM

Fig 1 Steps in the synthesis of graphene nanodiscs from electro-chemical assisted micromechanical exfoliation of graphite

imaging The most plausible reason behind this observa-tion is supramolecular association of the individual nan-odiscs to form flake like structures (discussed later in themanuscript)The SEM image shown in Figure 2(a) reveals flaky

agglomerates of the graphene nanodiscs in a flower likearrangement It is believed that huge number of stalked oragglomerated graphene nanodiscs give rise to flower petalslike structures The TEM images of multilayered graphene(4ndash5 layers) obtained after electrochemical exfoliation isshown in Figure 2(b) It appears as flake like structuresstacked on one another TEM and HRTEM images ofgraphene nanodiscs obtained after micromechanical exfo-liation of the multilayered graphene flakes (from elec-trochemical exfoliation) is shown in Figures 2(b)ndash(f)respectivelyThe images show circular structures of the dispersed

nanodiscs and its polycrystalline features The averagediameter of the nanodiscs (measured from 8 TEM images)is 222plusmn64 nm The nanodiscs are transparent and showsupramolecular behavior ie tend to arrange either insideways like flower petals (Fig 2(e)) or overlap with oneanother in a stalked manner or agglomerate (Fig 2(f))The HRTEM images reveal polycrystalline features and

each nanodisc consisting of several small randomly orderedzones or grains with well-defined lattice planes separatedby lean boundaries The lattice d-spacing are assigned andshown in Figures 2(e)ndash(f) The d-spacing viz 217 297and 199 Aring correspond to (100) (002) and (101) planesrespectively (slightly different than graphite 213 Aring (100)337 Aring (002) and 203 Aring (101)) Each disc as appar-ent in the HRTEM images is a complicated patchwork ofgrains connected by lean boundaries The polycrystallinefeature of the nanodiscs is apparently due to the fact thatthe precursor material for the synthesis of graphene iethe graphite sheet itself is polycrystalline Therefore theproduct of exfoliation also comes to be a polycrystallinematerial rather than single crystalline The carbon precur-sor is an important factor in deciding the properties of thesynthesized graphene and it has also been discovered anddiscussed by other researchers3738 The overlapping zonesof the transparent nanodiscs are shown in Figure 2(f) Inthis image the graphene nanodiscs are apparently trans-parent Furthermore these circular and polycrystalline 2Dstructures are different from quantum dots Quantum dotsare essentially single crystalline particles and the immedi-ate optical feature of colloidal quantum dots is their colorQuantum dots of different sizes emit light of different col-ors But here in this study we do not discover any colorphenomenon Hence we term these as graphene nanodiscsand not quantum dots The investigation on optical prop-erties of these nanodiscs is currently under progressIn the X-ray diffraction spectra of the electrochemi-

cally exfoliated graphene (Fig 3(a)) the highest intensitypeak corresponding to (002) plane was found to be shiftedtowards lower diffraction angles (the 2 angle is 266 in

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Fig 2 (a) SEM image of highly agglomerated graphene nanodiscs appear as flaky structures and somewhat arranged in flower like patterns (b) TEMimage of graphene films obtained after electrochemical exfoliation of graphite sheet followed by 20 min of bath sonication (c) TEM image ofdispersed graphene nanodiscs obtained from micromechanical exfoliation of the microflakes (obtained after electrochemical exfoliation) (d) Selectedarea diffraction pattern of the nanodiscs depicting nanostructured multilayers (e) HRTEM image of five nanodiscs showing the supramolecularassembly in a pattern resembling flower petals (f) HRTEM image of the nanodiscs showing polycrystalline features and overlap zones

graphite sheet and 265 in graphene nanodisc) indicat-ing expansion of the lattice during the exfoliation processThe crystalline domain size as calculated from Scherrerrsquosequation is 1116 nm which appears to be considerablylarger in graphene nanodiscs of average diameter 222 nm(crystalline domain size in graphite sheet was calculated

to be 2238 nm) Raman spectra of the both electrochem-ically exfoliated as well as micromechanically exfoliatedgraphene nanodiscs show similar features The Ramanspectra is shown in Figure 3(b) The bands at 1352 and1595 cmminus1 representing D and G-bands respectively TheD-band appearing in the range 1330ndash1360 cmminus1 is related





Article

Fig 3 (a) X-ray diffraction patterns of graphite and graphene nanodiscs (b) Raman spectrum of the micromechanically exfoliated graphene suggestsa 4ndash5 layered structure (c) XPS survey scan of the nanodiscs showing primarily carbon content with some traces of oxygen which might probablyhave come from oxidation of graphite sheet during exfoliation The spectrum however shows no traces of SiSiO2

to the breathing mode of -point phonons of A1g symme-try with the vibrations of the carbon atoms It correspondsto the disorder or defect in the structure that arises dueto the disorderliness in the multilayered graphene sheetsthat are stacked one on the other and the carbon rings donot align the directly sheet to sheet The D-band becomestaller with the more disorderliness in the graphene struc-ture The G-band at 1595 cmminus1 corresponds to the E2g

Fig 4 Atomic force microscopy of the agglomerated graphene nanodiscs (which form film like structures) on a silicon wafer surface

mode of graphite representing the in-plane bond-stretchingvibration of sp2-bonded or graphitic carbon atoms And itis highly sensitive to strain effects in sp2 system It canbe used as an indication for modification on the flat sur-face of graphene The GD intensity ratio changes whenthere is a change in the average size of the sp2 domainsRaman spectra of all kinds of sp2 carbon materials exhibita strong peak or 2D band in the range 2500ndash2700 cmminus1 in





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Fig 5 Continued





Article

Fig 5 Schematic showing (a) side view of the graphene plane with partial H-bonds between solvent molecules and delocalized -electrons(b) Solvent molecules surrounding the graphene disc Inter- as well as intra-molecular H-bonds are also shown (c) Two probable mechanism ofassembly in line assembly giving extended -electron delocalization over several discs and sidewise assembly giving rise to voiddefect makingit ideal for capacitor material (d) Supramolecular assembly occurs as the solvent evaporates and the discs move towards each other with recedingsolvent trail and give rise to flower petals like assembly and (e) the structures appear in TEM and AFM images

combination with the G-band This is assigned to a secondorder two phonon process and generally used to determinethe layers of graphene This is attributed to the varyingshape of 2D band at 2600 cmminus1 and each additional layercauses a shift of 25 cmminus12039ndash43 In this regard the elec-trochemically exfoliated graphene and the powdered formof micromechanically exfoliated graphene in the presentwork is 4ndash5 layered A relatively different result was

obtained from the AFM imaging where the graphene wasfound to be 1ndash2 layered (Fig 4) The XPS survey scanof the synthesized nanodiscs and selected scans for car-bon (and probable traces of silicon impurity) are shown inFigures 3(c) and (d) respectively The scans show the syn-thesized materials consisting of mainly carbon with sometraces of oxygen which might probably have come fromoxidation of graphite sheet during the exfoliation process





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However the spectrum shows no traces of silicon or SiO2

(SiO2 can be identified by twin peaks corresponding to Siand SiO2 at positions 100 and 104 eV respectively whichis not observed here)The average thickness of single layered agglomerates

measured from 25 surface profiles using tapping modeAFM (Fig 4) was 0522plusmn0266 nm which is typical for1ndash2 layer graphene also reported by other researchers44

There still remains an important question to beaddressed that why these nanostructured graphene flakesacquire disc shape while they could remain as irregu-lar flake like structures We address on this point in theview of supramolecular behavior (or self assembly) ofthe graphene nanodiscs It can be considered in a waythat what difference was brought upon by the method-ology adopted for synthesis and characterization It maybe explained that during micromechanical exfoliation inan aqueous ethanolic solution ie pestle movement in acircular direction causes interlayer shear frictional forceswhich in turn induces static charges between the individ-ual graphitic layers and hence renders an easy separa-tion Since -orbitals at perpendicular positions facilitatesdelocalization of the -electrons of the sp2 carbon atomsie above and below the graphene plane Therefore asolvent accessible surface may exist around the graphenemolecule as shown in Figures 5(a) (b) It is likely andpossible that the carbon atoms at the periphery of thegraphene plane are sp2 as well as sp3 hybridized asthe graphene was obtained from electrochemical exfo-liation and carbon atoms at the periphery can bondwith either oxygen or hydrogen atoms of the solventmolecules37 In this condition the solvent molecules(which include both water and ethanol molecules) sur-rounding the graphene see it as a reservoir of electronson top and bottom sides of the graphene plane The delo-calized -electrons on graphene plane attract the hydro-gen atoms (bearing partial positive charge) of water andethanol molecules around its surface via partial hydro-gen bonds thus creating a micelle structure that tries toachieve lowest surface area ie spherical or disc shapeSince graphene is planer 2D structure it acquires a discshape (3D molecules acquire spherical shape) Evidenceof delocalized -electrons forming hydrogen bonds withwater are available in literature45 The arrangement of sol-vent molecules around graphene plane formation of partialhydrogen bonds between delocalized -electrons and sol-vent hydrogens and inter-intra-molecular hydrogen bondsbetween solvent-graphene and solvent-solvent moleculesrespectively are shown in Figures 5(a) (b)The nanodiscs thus synthesized are supposed to pro-

vide better capability of forming composite with othermaterials such as polymers metalsmetal oxides etcby the virtue of its small size and unfoldability Itssurface provides ample opportunity for other species (poly-mer molecule or metalmetal oxide atoms) to sit uni-formly on the either sides and uniformly adhereattach

Nanodiscs seem to provide an edge over other graphenestructures meant for composite making viz film layeror flake like structures where chances are high that thestructure may get folded or agglomerate during reac-tion leaving some portions unreacted On the contrarythese nanodiscs however agglomerate but can easily dis-perse again on brief sonication Furthermore graphenenanodiscs are also thought to be a better material for fab-ricating nanomicroelectronic devices by controlling depo-sition and subsequent patterning As we see in Figure 5(c)the discs can assemble either linearly to give rise to anextended conjugation for enhanced electron transfer or in amanner to give rise to a defectvoid in the graphene layerSuch nanodiscs are believed to provide a tunable surfaceprofile for making nanostructured electronic devices com-posites and templates for biological applications Sincethe nanodiscs behave in a very complicated manner thereis required rigorous experimentation for unveiling its prop-erties Investigations on their optical optoelectronic andchemical properties and properties of nanostructured com-posites with different polymer molecules are in progress

4 CONCLUSIONIn summary we synthesized graphene nanodiscs fromelectrochemical assisted micromechanical exfoliation ofgraphite The nanodiscs exhibit dual characteristics vizcircular disc like structures when dispersed in aqueousethanol and flake like structures when dehydrated Thenanodiscs show supramolecular behavior of self assemblyVery slow and controlled evaporation give rise to singlelayer graphene (of thickness 0522 nm) where nanodiscsgradually move towards one another and can give rise todesirable size up to a few micrometers Such nanodics bythe virtue of small size planar structure and unfoldabil-ity are believed to provide a better surface for compositemaking with other molecules and biological applications

Acknowledgment This study was supported by 2013Yeungnam University Research Grant

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11 T Takamura K Endo L Fu Y P Wu K J Lee and T MatsumotoIdentification of nano-sized holes by TEM in the graphene layerof graphite and the high rate discharge capability of Li ion batteryanodes Eletrochim Acta 53 1055 (2007)

12 Z Liu J T Robinson X M Sun and H J Dai PEGylatednanographene oxide for delivery of water-insoluble cancer drugsJ Am Chem Soc 130 10876 (2008)

13 X Fu X Song and Y Zhang Facile preparation of graphene sheetsfrom synthetic graphite Mater Lett 70 181 (2012)

14 K S Novoselov D Jiang F Schedin T J Booth V V KhotkevichS V Morozov and A K Geim Two-dimensional atomic crystalsProceedings of the National Academy of Science of the United Statesof America 102 10451 (2005)

15 C Berger Z Song T Li X Li A Y Ogbazghi R Feng Z DaiA N Marchenkov E H Conrad P N First and W A DHeer Ultrathin epitaxial graphite 2D electron gas properties anda route toward graphene-based nanoelectronics J Phys Chem B108 19912 (2004)

16 Z Osvaiacuteth A Darabont P Nemes-Incze E Horvaiacuteth Z EHorvaiacuteth and L P Biro Graphene layers from thermal oxidation ofexfoliated graphite plates Carbon 45 3022 (2007)

17 J Wu W Pisula and K Muumlllen Graphenes as potential materialfor electronics Chem Rev 107 718 (2007)

18 A N Sidorov M M Yazdanpanah R Jalilian P J Ouseph R WCohn and G U Sumanasekera Electrostatic deposition of grapheneNanotechnol 18 135301 (2007)

19 Y Hernandez V Nicolosi M Lotya F M Blighe Z Sun S DeI T McGovern B Holland M Byrne Y K GunrsquoKo J J BolandP Niraj G Duesberg S Krishnamurthy R Goodhue J HutchisonV Scardaci A C Ferrari and J N Coleman High-yield pro-duction of graphene by liquid-phase exfoliation of graphite NatNanotechnol 3 563 (2008)

20 X L Li X R Wang L Zhang S W Lee and H J Dai Chemi-cally derived ultrasmooth graphene nanoribbon semiconductors Sci-ence 319 1229 (2008)

21 X Li G Zhang X Bai X Sun X Wang E Wang and H DaiHighly conducting graphene sheets and LangmuirndashBlodgett filmsNat Nanotechnol 3 538 (2008)

22 K S Subrahmanyam L S Panchakarla A Govindaraj and C NR Rao Simple method of preparing graphene flakes by an arc-discharge method J Phys Chem C 113 4257 (2009)

23 D Tasis K Papagelis P Spiliopoulos and C Galiotis Efficientexfoliation of graphene sheets in binary solvents Mater Lett 94 47(2013)

24 M Lotya P J King U Khan S De and J N Coleman High-concentration surfactant-stabilized graphene dispersions ACS Nano4 3155 (2010)

25 N Behabtu J R Lomeda M J Green A L HigginbothamA Sinitskii D V Kosynkin D Tsentalovich A N GParra-Vasquez J Schmidt E Kesselman Y Cohen Y Talmon J MTour and M Pasquali Spontaneous high-concentration dispersionsand liquid crystals of graphene Nat Nanotechnol 5 406 (2010)

26 Y Lee S Bae H Jang S Jang S-E Zhu S H Sim Y I SongB H Hong and J-H Ahn Wafer-scale synthesis and transfer ofgraphene films Nano Lett 10 490 (2010)

27 Y Zhou L Hu and G Gruumlner A method of printing carbon nano-tube thin films Appl Phys Lett 88 123109 (2006)

28 F N Ishikawa H-K Chang K Ryu P-C Chen A BadmaevL G D Arco G Shen and C Zhou Transparent electronics basedon transfer printed aligned carbon nanotubes on rigid and flexiblesubstrates ACS Nano 3 73 (2009)

29 M J Allen V C Tung L Gomez Z Xu L-M Chen K S NelsonC Zhou R B Kaner and Y Yang Soft transfer printing of chemi-cally converted graphene Adv Mater 21 2098 (2009)

30 A Reina H Son L Jiao B Fan M S Dresselhaus Z Liuand J Kong Transferring and identification of single- and few-layer graphene on arbitrary substrates J Phys Chem C 112 17741(2008)

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33 X Liang Z Fu and S Y Chou Graphene transistors fabricated viatransfer-printing in device active-areas on large wafer Nano Lett7 3840 (2007)

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37 M Favaro S Agnoli M Cattelan A Moretto C DuranteS Leonardi J Kunze-Liebha O Schneider A Gennaro andG Granozzi Shaping graphene oxide by electrochemistry Fromfoams to self-assembled molecular materials Carbon 77 405(2014)

38 F Liu M-H Jang H D Ha J-H Kim Y-H Cho and T SSeo Facile synthetic method for pristine graphene quantum dots andgraphene oxide quantum dots Origin of blue and green lumines-cence Adv Mater 25 3657 (2013)

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Received 20 December 2014 RevisedAccepted 1 June 2015





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well as harsh conditions during the etching process How-ever chemical vapor deposition is capable of producingtransferable size graphene limited up to few centimeterseither because of the size limit of a suitable rigid sub-strate or the inhomogeneity of reaction temperature insidethe furnace The number of layers deposited have beenfound to vary from 2ndash8 when a Ni substrate is employedand 1ndash2 layers when a Cu substrate is used Further-more the graphene produced from Chemical vapor depo-sition is prone to defects The defects are believed tobe introduced either during deposition or etching processTill so far scalable graphene has been synthesized usingvarious techniques and in almost all the cases the out-come is typically polycrystalline films consisting numer-ous single crystalline grains83435 We report here on thesynthesis of 1ndash2 layered graphene nanodiscs by electro-chemical assisted micromechanical exfoliation of graphiteits morphology surface properties and supramolecularbehavior We also present a molecular model to explainthe supramolecular behavior Due to its behavior of selfassembly the nanodiscs are anticipated to be suitable fornanomicropatterning which can be useful in fabricatingnanoelectronic circuits and nanodevices

2 EXPERIMENTAL DETAILS21 Materials and MethodsThe required materials for synthesis of graphene nan-odiscs viz graphite sheet (10times 15times 05 cm3 concH2SO4 (98) and sodium phosphate dibasic were pro-cured from Duksan Pure Chemicals Co Ltd Korea Theexperiments were carried out in two phases viz(i) electrochemical exfoliation of graphite sheet intographene nanosheets followed by(ii) micromechanical exfoliation of the graphenenanosheets into graphene nanodiscs in excess volumes ofaqueous ethanol

Electrochemical exfoliation was carried out using thechrono method The electrochemical cell consisted agraphite sheet as anode and a platinum wire mesh ascathode both immersed in 100 ml of dilute H2SO4 (1)solution containing 3 g sodium phosphate dibasic as elec-trolyte The solution was stirred throughout the experi-ment The electrodes were connected to the respectiveterminals of the Electrochemical Workstation (VersaSTAT3 Princeton Research USA) and chronoampherometrywas run at plusmn10 volts for 5 h At the beginning the testsolution was transparent With the progress of the reactionit gradually turned dark and at the end of the experi-ment it became black After completion of the experimentthe solution was allowed to stand overnight The blackmass settled at the bottom was washed with DI water andethanol and dried The graphene quantity achieved by thismethod was found to be dependent on (a) the cathodematerial (b) cathode surface area and (c) current den-sity In this work both graphite sheet and a platinum wire

mesh were tested as the cathode material When a plat-inum wire mesh was used as cathode faster productionof graphene along with greater yield (sim4 g in 5 h) wasobserved compared to its graphite counterpart Further anincrease in the rate of cathodic reaction ie either byincreasing the surface area of the cathode or by increas-ing the number of reducible ionic species in the elec-trolyte was observed to increase the rate of exfoliationThe graphene obtained from electrochemical exfoliationwas multilayered (4ndash7 layered) This was then subjected tomicromechanical exfoliation in excess volumes of aqueousethanol To do so a small amount (sim05 mg) of graphenewas taken in an agate pestle and sim5 ml of aqueous ethanolwas added to it The consistency was grounded manuallyusing a mortar for about an hour with the additional vol-umes of ethanol as and when required untill the dispersionappeared almost transparent While preparing specimen forcharacterization the consistency was bath sonicated forabout 5ndash10 min to ensure maximum dispersion For AFManalysis a drop of the suspension was cast over a cleansilicon wafer and carefully dried For other investigationsviz SEM XRD TEM and Raman specimens were madefrom this suspension For TEM analysis a drop of the sus-pension was cast on a clean copper grid dried and exam-ined On the other hand a fine dispersion of dried powderwas sprinkled over carbon tape (fixed on copper stubs) forSEM investigation For XRD and XPS a greater amountof dried powder (05ndash1 g) was required for analysis

22 CharacterizationMicrostructure and phase analysis was carried outusing scanning electron microscopy (HITACHI-S4800)X-ray diffraction (XRD analytical X-pert PRO-MPDthe Netherland and high resolution transmission elec-tron microscopy (HRTEM TEM-2100 JEOL) operating at200 kV Raman spectrum was recorded on LabRam HR800 UV Raman microscope (Horiba Jobin-Yvon Franceusing laser light of wavelength 514 nm) AFM anal-ysis was done using a Digital Instruments AFM witha nanoscope IIIa controller (Digital instruments USA)The XPS measurements were done with Ar+ ion sputter-ing using a Thermo-VG Scientific MultiLab 2000 with amonochromatic Al K X-ray source (14866 eV) a passenergy of 200 eV and a hemispherical energy analyzerSince no critical final state surface charging effect wasobserved during the XPS measurements no binding energycorrection was performed for the obtained XPS spectra

3 RESULTS AND DISCUSSIONIn graphite each layer is arranged one above another byvan der Waals forces of attraction (cohesive van der Waalsenergy of about 59 kJ molminus1 carbon36) It was observedthat ultrasonication yields single layer filmsflakes asshown in Figure 2(b) as well as multilayered graphite par-ticles In order to get uniform single layered graphene





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Graphene Nanodiscs from Electrochemical assisted Micromechanical Exfoliation of Graphite: Morphology...

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