Insight into electrical conductivity of graphene and functionalized graphene: Role of lateral dimension of graphene sheet

lable at ScienceDirect

Materials Chemistry and Physics xxx (2015) 1e9

Contents lists avai

Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Insight into electrical conductivity of graphene and functionalizedgraphene: Role of lateral dimension of graphene sheet

Manash Jyoti Deka, Upama Baruah, Devasish Chowdhury*

Material Nanochemistry Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk,Guwahati, 781035, India

h i g h l i g h t s

* Corresponding author.E-mail address: [email protected] (D. Chowdh

http://dx.doi.org/10.1016/j.matchemphys.2015.07.0360254-0584/© 2015 Published by Elsevier B.V.

Please cite this article in press as: M.J. Deka,dimension of graphene sheet, Materials Che

g r a p h i c a l a b s t r a c t

� Exfoliated graphene were obtainedfrom SDS mediated exfoliation ofgraphite.

� The exfoliated graphene subjected tofunctionalization.

� Impedance measurements carriedout on graphene and functionalizedgraphene.

� Sheet resistance determined for gra-phene and functionalized graphene.

� Role of lateral dimension of the gra-phene sheet in a.c conductivity ofgraphene.

a r t i c l e i n f o

Article history:Received 30 March 2015Received in revised form25 May 2015Accepted 10 July 2015Available online xxx

Keywords:Nano-structuresChemical synthesisElectron microscopyFourier transform infrared spectroscopyElectrical conductivity

a b s t r a c t

Exfoliated graphene were obtained from SDS mediated exfoliation of graphite. The exfoliated graphene(GexfeSDS) were further subjected to functionalization to obtain reduced graphene (reGO), acid chloridefunctionalized graphene oxide (GOeCOCl), ester functionalized graphene oxide (GOeCOOR), and amidefunctionalized graphene oxide (GOeCONH2). The impedance measurements were done on functional-ized graphene by a simple layer-by-layer (LbL) assembly technique on silicon wafer substrate usingchitosan as cationic binder. Further, the role of lateral dimension of the graphene sheet in a.c conductivitywas also studied. It was observed that introduction of bulky groups in the graphene sheets may lead tothe band gap opening which is responsible for lower a.c conductivity in case of functionalized graphene.Moreover sheet resistance values were determined for graphene and functionalized graphene.

© 2015 Published by Elsevier B.V.

1. Introduction

Graphene, which is one or few-layers stacked sheets of sp2-hybridized carbon atoms arranged in a planar hexagonal latticedensely packed in a honeycomb two-dimensional (2D) network, isa wonderful material possessing outstanding properties like large

ury).

et al., Insight into electrical cmistry and Physics (2015), h

specific surface area, high intrinsic mobility [1], high Young'smodulus [2], high thermal conductivity [3], optical transmittance,ambipolar electric field effect [4] etc. It finds wide ranging appli-cations in solar cells [5], sensors [6e8], catalysis [9,10], energyconversion [11,12], storage devices, surface coating [13], ultra-capacitors [14,15] transparent conductive electrodes and opto-electronic devices [16]. The electrical properties of graphene can betuned by applying electric field, uniaxial strain and chemicalfunctionalization. Electrical properties especially the a.c

onductivity of graphene and functionalized graphene: Role of lateralttp://dx.doi.org/10.1016/j.matchemphys.2015.07.036

M.J. Deka et al. / Materials Chemistry and Physics xxx (2015) 1e92

conductivity has been studied on graphene, graphene oxide,reduced graphene oxide [17,18] and CNT based nanocomposites[19]. Electrochemical impedance spectroscopy has also been usedto differentiate between graphene oxide and reduced graphene[20]. The change in impedance of graphene in presence of analyteshas been used as an electrochemical sensor. For example Song et al.demonstrated electrochemical sensing platform based on Chito-san- Reduced Graphene Oxide/Concanavalin Layer for Assay ofGlucose and Urea [7]. Zhang et al. used tryptamine-functionalizedreduced graphene oxide (TRAerGO) for label-free electrochemicalimpedance DNA biosensor [6]. Kim et al. demonstrated impedancesensing platform between probe protein molecules and reducedgraphene oxide (rGO) substrate [21]. However a.c conductivitystudy on graphene upon chemical functionalization with differentfunctional groups has not been studied in detail. Moreover ACconductivity involves measuring both the electrical component(due to movement of electron) and ionic component (due tomovement of ions). So, A.C conductivity in this case will providetotal conductivity of the system. As functionalization of grapheneinvolve introducing defect and functionality which is ionized, so itis extremely important to study a.c conductivity to determine thetotal conductivity in the system.

In this work we tried to carry out a systematic study on the a.cconductivity of exfoliated graphene, graphene oxide, reduced gra-phene oxide and other functionalized graphene. Exfoliated gra-phene were obtained from SDS mediated exfoliation of graphite.The exfoliated and oxidized graphene sheets were further sub-jected to functionalization to obtain reduced graphene (reGO), acidchloride functionalized graphene oxide (GOeCOCl), ester func-tionalized graphene oxide (GOeCOOR), and amide functionalizedgraphene oxide (GOeCONH2). The impedance measurements weredone by a simple layer-by-layer (LbL) assembly technique on siliconwafer substrate using chitosan as cationic binder. Further, the roleof lateral dimension of the graphene sheet in a.c conductivity wasalso studied. Graphene can be prepared chemically with differentlateral sheet dimension. It was observed that when deposited thea.c conductivity of graphene sheet depend upon lateral dimensionof graphene sheet.

2. Experimental section

2.1. Chemicals used

Graphite nanopowder and chitosan were purchased from SiscoResearch Laboratories, Pvt. Ltd. Sodium Dodecyl Sulphate(C12H25NaO4S) (SDS), Hydrazine hydrate (H6N2O), Sulphuric Acid(H2SO4), Nitric acid (HNO3) and Hydrochloric acid (HCl) were pur-chased from Merck, India. All the chemicals were used as obtainedwithout further purification. The water used was from a Milli-Qwater purification system.

2.2. Exfoliation of graphite nanopowder using SDS

A dispersion of graphite nanopowder was prepared bydispersing 0.01 g of graphite nanopowder in 100mL of SDS solution(0.5 mg/mL). This solution was then sonicated for 30 min in a bathsonicator to exfoliate the graphene platelets. Sonication was fol-lowed by centrifugation of this solution at 2500 rpm for 30 min toseparate the lighter exfoliated platelets (top layer) from the heavierones (bottom layer). The exfoliated graphene was washed withwater for several times to remove SDS. The concentration of exfo-liated graphene dispersion was determined using calibration curve(Fig. S1, Supplementary material).

Please cite this article in press as: M.J. Deka, et al., Insight into electrical cdimension of graphene sheet, Materials Chemistry and Physics (2015), h

2.3. Exfoliation and oxidation of graphite nanopowder (GrP) usingH2SO4/HNO3

Commercially available graphite nanopowder (GrP) was exfoli-ated and oxidized in a single step using a simple chemical reaction.Here, an acid mixture i.e., a 3:1 mixture of H2SO4/HNO3, has beenused as an oxidizing agent followed by a simple sonication andcentrifugation technique at room temperature to separate theexfoliated oxidized Graphene sheets. Typically 0.01 g GrP wasdispersed in 3:1 acid mixture of concentrated H2SO4 (98wt%)/HNO3(16 M). This mixture was then sonicated for 24 h followedby dilution and centrifugation at 2500 rpm for 30 min to separatethe lighter exfoliated graphene oxide platelets. The supernatantsolution was then separated from the heavier particles andanalyzed separately. The heavier particles were collected and re-dispersed in water and the process of centrifugation and disper-sion was repeated several times. The resultant dispersion after afew cycles was then evaporated at a temperature above 100 �C toobtain the solid product. Chemical reduction of GO was also carriedout by mixing 0.005 g GO with 20 mL hydrazine hydrate solution.The mixture was then heated for 6 h at 60 �C. In the reduction step,GO is transformed into rGO by elimination of the oxygenatedfunctional groups.

2.4. Functionalization of graphene oxide (GO) to ester (GOeCOOR),amide (GOeCONH2) and acid chloride (GOeCOCl)

2.4.1. ReductionChemical reduction of GO was carried out by mixing 0.005 g GO

with 20mL (30%) hydrazine hydrate solution. Themixturewas thenrefluxed for 6 h at 60 �C. In the reduction step, GO is transformedinto reduced GO (rGO) by elimination of the oxygenated functionalgroups. Then rGO was first collected using centrifugation at2500 rpm for 15 min. Then it was washed with water several timesto remove the excess hydrazine hydrate. Finally it was dried in ovenat 60 �C and collected, re-dispersed in water for furthercharacterization.

2.4.2. EsterificationFor esterification, 0.01 g of GO was dispersed in water and then

mixed with poly(ethylene glycol) in the 1:2 ratio and to this about40 mL concentrated H2SO4 was added (which acts as catalyst) andheated for one and half hour at 150 �C. In this case the fruity smellof the product was used as a preliminary indication of the forma-tion of ester-functionalized graphene.

2.4.3. AmidificationFor further functionalization to amide, the dispersed GOeCOOR

and ethylene diamine in 1:3 ratio was mixed in presence of 50 mLpyridine and refluxed at 120 �C for 3 h to form a solid product ofGOeCONH2. Then the solid product was washed with water severaltimes to remove excess ethylene diamine and pyridine to get pureGOeCONH2.

2.4.4. Chlorination5 mg GO powder was taken in a crucible and thionyl chloride

(SOCl2)was added until the liberation of SO2 and HCl gas ceased atroom temperature which confirmed the completion of reaction.Then the solid GOeCOCl was collected for further characterizations.

2.5. Characterization

The characterization of exfoliated graphene and functionalizedgraphene were carried out by UVeVisible absorption spectropho-tometer (Shimadzu, UV-2600), Dynamic Light Scattering (Malvern


M.J. Deka et al. / Materials Chemistry and Physics xxx (2015) 1e9 3

Zetasizer Nanoseries, Nano ZS90), Fourier Transform InfraredSpectrophotometer (Nicolet 6700), Scanning Electron Microscopy(Carl Zeiss

Pigma-VP), Atomic Force Microscopy (NTEGRA PRIMA

NT-MDT). Raman analysis was performed using Laser Micro Ramansystem, Horiba Jobin Vyon, Model LabRam HR, using 514 nm laserat room temperature. All a.c impedance measurements were per-formed at room temperature under ambient conditions using animpedance analyzer HIOKI 3532-50, LCR Hi-TESTER.

200 300 4000

1

2

3

Abs

orba

nce(

a.u.

)

Wavelength(nm)

rGO GO Gexf-SDS

Fig. 1. UVeVisible spectra of GexfeSDS, GO and rGO aqueous dispersions.

3. Results and discussion

Commercially available graphite nanopowder was exfoliatedwith SDS. Graphite nanopowder was dispersed in SDS solutions in amanner similar to surfactant aided carbon nanotube dispersion[22]. We could successfully exfoliate graphite by SDS. The optimi-zation of the dispersion of exfoliated graphene was also carried outusing UVevisible spectroscopy with LamberteBeer law (A ¼ εCl,where ε is the absorption coefficient, C is the concentration and l isthe path length). Different concentrations of graphitic dispersionwere prepared to plot the calibration curve (Fig. S1, Supplementarymaterial) which could be later used to find CGr (concentration ofexfoliated graphene).

Graphene nanopowder was also exfoliated and oxidized in asingle step using a simple chemical oxidizing agent followed by asimple sonication and centrifugation technique at room tempera-ture to separate the exfoliated oxidized graphene sheets [23]. Theexfoliated and oxidized graphene sheets were further subjected tofunctionalization viz. reduction, chlorination, esterification, ami-dification to obtain reGO, GOeCOCl, GOeCOOR, GOeCONH2respectively. The schematic representation summarizing thechemical reactions done for functionalization is shown in Scheme 1.

UVevisible spectroscopy study was performed on GexfeSDS, GOand rGO aqueous dispersions. The stacked absorption spectra are

Scheme 1. The schematic representation summarizing


shown in Fig. 1. It is evident from the figure that for SDS exfoliatedgraphene there is no prominent peaks in UVevisible region, but onoxidation GO has two distinct peaks one near 250 nm and the othernear 300 nm. The peak near 250 nm can be assigned to n-s*transition of the eOH and eCOOe group arising due to oxidation ofgraphene. The other peak near 300 nm can be assigned to p-p*transition of the C]C bond. The presence of these two peaks for theoxidized sample indicates the successful incorporation of oxygen-ated functionalities on graphene sheets.

However, on reduction of GO to rGO using hydrazine hydrate(30%) solution, the peak near 300 nm disappeared and C]C (p-p*)peak is shifted towards high energy region (lower wavelength)which indicate the successful reduction of GO to rGO eliminatingoxygenated functional groups.

the chemical reactions done for functionalization.


Wavenumber(cm-1)3500 50010001500200025003000

Tran

smi

ance

3530 1690 690

36801710

34502910 1640

1090

3580

3430 17201180

2880

Gexf--SDS

GO

rGO

GO-COOR

GO-CONH2

GO-COCl

Fig. 3. Stacked FTIR spectra of Gexf-SDS, GO, rGO, GO-COOR , GO-CONH2 and GO-COCl.


The SDS mediated exfoliated graphene (GexfeSDS) and GOsheets were further characterized using Scanning Electron Micro-scopy (SEM) and Atomic Force Microscopy (AFM). Fig. 2A and Bshows the representative Scanning Electron Microscope images ofGexfeSDS and GO sheet respectively. The images show that exfoli-ated graphene has a typical lateral size of ~1.2 mm. Fig. 2C shows therepresentative Atomic Force Microscope tapping-mode image ofexfoliated graphene sheet deposited onto a silicon wafer substrate.The images show that lateral dimension of the exfoliated sheet is~200 nm. Fig. 2D shows the cross-section height and the thicknessof the sheet obtained is 2.6 nm which correlates to approximately5e6 layers of graphene sheets.

The successful functionalization of graphene sheet withdifferent functional moiety is demonstrated by FTIR spectroscopy.Fig. 3 shows the stacked FTIR spectra GexfeSDS, GO, rGO, GOeCOCl,GOeCOOR and GOeCONH2. The FTIR spectra shows that while nomajor peak was observed for exfoliated graphene but upon oxida-tion three prominent peaks appearing at 3450, 1720 and 1180 cm�1

indicates successful incorporation of hydroxyl groups (eOH), car-boxylic acid group (eCOOH) and CeO group respectively onto thegraphene sheet. It is also interesting to note that upon reduction ofGO to form rGO, all the significant peaks were reduced in intensitydue to removal of all the oxygenated functional groups from the GOsurface. However, esterification results in peaks at 3480, 1640 and1090 cm�1 which are assigned for hydroxyl (OH), carboxylate(COO�) and carbonyl (CO) groups group on the GO sheets. Emer-gence of peak at 2910 cm�1 for GOeCOOR (ester) indicates thesuccessful esterification of GO. After amide functionalization thepeak at 1710 cm�1 occurs due to carboxylic acid group. A new peakat 3680 cm�1 for GO-CONH2 (amide) can be assigned to NeHstretching vibrational band which is the characterization peak ofamide.

For acid chloride functionalized graphene there are threeprominent peaks at 3530, 1690 and 690 cm�1 which can be

Fig. 2. Representative Scanning Electron Microscope images of (A) GexfeSDS (B) GO sheetdeposited onto a silicon wafer substrate (D) Cross section measurements taken along the llayers of graphene sheets [19].


assigned to incorporation of hydroxyl(eOH), carboxylic(eCOOH)and acid chloride(eCOCl) groups on to the bare GO nanosheetsdemonstrating the successful functionalization of graphene.

Raman spectra were recorded for GexfeSDS, GO, GOeCOCl,GOeCOOR and GOeCONH2. Fig. 4A shows the stacked Ramanspectra of exfoliated graphene and functionalized graphene sheets.The summary of the characteristic peaks D, G and 2D is tabulated inTable 1. The band located near 1575e1590 cm�l is due to thegraphitic structure (sp2 hybridized), whereas the peak near1365e1390 cm�1 originated from disordered structure in the

(C) Representative Atomic Force Microscope tapping-mode image of GexfeSDS sheetine, indicating a sheet thickness of 2.6 nm, which is equivalent to approximately 5e6


GO-COOR

Raman shi (cm-1)

Gexf-SDS

GO

GO-CONH2

GO-COClIn

tens

ity

1500 2000 2500

Fig. 4. Stacked Raman spectra of Gexf-SDS, GO, GO-COOR, GO-CONH2 and GO-COCl.

Table 2Calculation of lateral dimensions from DLS.

Size (nm) <L> Lateral Dimension (nm)

200 ± 50 110 ± 12 (small)600 ± 50 655 ± 12 (medium)1000 ± 50 1269 ± 12 (large)


carbon (sp3 hybridized). The ID/IG ratio indicated the amount ofdisorder present in the nanosheets. It is evident from Table 1, uponfunctionalization the degree of disorder increased from 0.16 to 0.95indicating the existence of localized sp3 defects in the sp2 carbonnetwork structure. Moreover on functionalization, all the bandswere shifted towards higher wavenumbers with respect to exfoli-ated graphene sheet which indicates successful functionalization ofgraphene.

Further evidence for successful functionalization of graphenesheet came from zeta potential value which is tabulated in Table 2(given in Supplementary material S2). The zeta potential value forexfoliated graphene sheets was determined to be �36.30 mV.However upon oxidation it changes to �1.04 mV and furtherreduction of GO gives a value of �14.40 mV. The zeta potential ofGOeCOOR, GOeCONH2 and GOeCOCl were found tobe �1.08, �2.01 and �1.90 mV respectively.

Three different lateral dimensions <L> of exfoliated graphene,GO and r-GO were synthesized. The lateral dimension <L> for eachdispersion was calculated from DLS measurement using the for-mula [24]:

< L> ¼ ð0:07±0:03ÞaDLSð1:5±0:15Þ (1)

Table 2 shows the lateral dimensions calculated from DLSmeasurement for GexfeSDS, GO and reGO. Thus dispersions ofthree different lateral dimensions were prepared as 110 ± 12(small), 655 ± 12 (medium) and 1269 ± 12 (large).

To measure the electrical conductivity of GexfeSDS, GO, reGOand different functionalized graphene, layer by layer assembly wasprepared on silicon wafer with chitosan as cationic binder. Thestrategy used is shown in Scheme 2. Typically silicon wafer sub-strate was cut ~ 1 cm � 1 cm and subjected to acid treatment and

Table 1Calculation of ID/IG ratio of exfoliated graphene and different functionalizedgraphene.

Sample name D band (cm�1) G band (cm�1) 2D band (cm�1) ID/IG

GexfeSDS 1376 1578 2725 0.16GO 1381 1584 2735 0.29GOeCOOR 1385 1585 2740 0.89GOeCONH2 1387 1590 2735 0.91GOeCOCl 1388 1625 2738 0.95


sonication to remove the thin nanometer layer of SiO2 and exposethe eOH group. After acid treatment the wafer becomes hydro-philic and is wetted with water. Then the wafer is dipped intochitosan solution to introduce a layer of chitosan. Since chitosanhas a zeta potential value þ35 mV, it can form a stable film overnegatively charged silicon wafer through electrostatic interaction.Over the layer of chitosan graphene and other functionalized gra-phene was deposited through dip coating method. In this case toothe interaction between the chitosan and graphene and otherfunctionalized graphene was electrostatic in nature as the surfacecharge over graphene and other functionalized graphene is nega-tive felicitating interaction with positively charged chitosan.

3.1. A.c conductivity study

As discussed earlier we prepared dispersion of exfoliated gra-phene, graphene oxide and reduced graphene oxide of differentlateral dimensions for impedance measurement. The impedancemeasurements were carried out using HIOKI 3532-50, LCR Hi-TESTER. The plots were recorded in the frequency range between42 Hz and 1 MHz at 1 V which is the default voltage of the system.The measurements were carried out at room temperature andunder a relative humidity of 60%. On silicon wafer substrates(1 cm � 1 cm), graphene and functionalised graphene weredeposited using chitosan as cationic binder then it was connectedto the impedance analyzer using conductive silver paste on bothend of the silicon wafer. In general Impedance (Z) is defined as thefrequency domain ratio of the voltage to the current. In otherwords, it is the voltageecurrent ratio for a single complex expo-nential at a particular frequency. Impedance will be a complexnumber, with the same unit as resistance. For a sinusoidal currentor voltage input, the polar form of the complex impedance relatesthe amplitude and phase of the voltage and current. Impedance isinversely related to conductivity [25]. In other words decrease inimpedance means higher conductivity and vice versa.

Fig. 5 shows log impedance (Z) vs log f (frequency) of GexfeSDS,GO and rGO of three different lateral dimensions labeled as small,medium and large. It is evident from the plot as the lateraldimension is increased impedance decreases significantly. Largerlateral dimension sheets have more surface area compared tosmaller ones, thereby reducing the discontinuity in the system. Forthe case GexfeSDS, rGO and GO, the a.c conductivity follows theorder large > medium > small with respect to lateral dimensions.As the larger lateral dimension sheets showed higher conductivity,so we did the a.c conductivity studies of other functionalized gra-phene in the lateral size range designated as “large” i.e.,1269 ± 12 nm.

Fig. 6A shows the dependence of impedance with frequency ofdifferent functionalized graphene sheet. The plot was recorded forGOeCOCl, GOeCOOR, and GOeCONH2 and compared withGexfeSDS, GO, and rGO with same size dispersion of graphenesheet. The plot demonstrates that a.c conductivity rGO was deter-mined to be highest. Upon functionalization, due to the introduc-tion of bulky groups onto the graphene sheets its conductivitychanges to lower value due to structural disorder. It is evident fromthe plot that functionalization with electron donating groups like


Sonica on

Silicon wafer30 min ==

-

Conc.HCL Chitosan Solu on

Exfoliated Graphene in SDS

Dried in air

washed

Dried in air

Dipped

Dried in air

washed

Silicon

Silicon/Chitosan

Silicon/Chitosan/Graphene:func onalized graphene

LBL assembly

Procedure To Make LBL Assembly Of

Silicon,chitosan And Graphene:Func onalized

graphene

+ + +

- - -Dipped

Scheme 2. Schematic representation of layer by layer assembly (LBL) of chitosan and graphene or different functionalized graphene onto Si wafer substrate.

1.5 2.0 2.5 3.0 3.5 4.03.5

4.0

4.5

5.0

5.5

6.0

6.5

Log

Z

Log f

Si Si/CH/rGOSMALL Si/CH/rGOMEDIUM Si/CH/rGOLARGE

2.0 2.5 3.0 3.5 4.03.5

4.0

4.5

5.0

5.5

6.0

6.5

Log

Z

Log f

Si Si/CH/Gexf-SDSSMALL Si/CH/Gexf-SDSMEDIUM Si/CH/Gexf-SDSLARGE

2.0 2.5 3.0 3.5 4.04.0

4.5

5.0

5.5

6.0

6.5

Lo

g Z

Log f

Si Si/CH/GOSMALL Si/CH/GOMEDIUM Si/CH/GOLARGE

A

BC

Fig. 5. Log Z vs Log f plot of (A) GexfeSDS, (B) GO, (C) rGO.


COOR results in higher a.c conductivity in the graphene sheet thancompared to introduction of electron withdrawing groups likeCOOH, CONH2, COCl in the graphene backbone.

Fig. 6B shows the Nyquist diagram of exfoliated graphene andfunctionalized graphene. In Nyquist plots Zcosq is plotted againstZsinq where Z is the impedance and q is the phase angle. Zcosq isthe real part and Zsinq is the imaginary part and denoted by Z0 andZ00 respectively. The point of intersection of the two semicircles of


complex-impedance plot coincides with the bulk resistance (Rb) ofthe sample. The semicircles corresponding to the charge transferresistance limiting process is associated with the graphene andreduced graphene oxide mainly which observes highest a.c con-ductivity. For functionalized graphene the semicircles were foundto be expanded corresponding to lower conductivity value and theobserved conductivity order is as follows Si/CH/rGO > Si/CH/GexfeSDS > Si/CH/GOeCOOR > Si/CH/GOeNH2 > Si/CH/GO > Si/CH/


2.0 2.5 3.0 3.5 4.03.5

4.0

4.5

5.0

5.5

6.0

6.5

Log

Z

Log f

Si Si/CH/GO-COCl Si/CH/GO Si/CH/GO-CONH2 Si/CH/GO-COOR Si/CH/Gexf-SDS Si/CH/rGO

A

C

1.5 2.0 2.5 3.0 3.5 4.00

-10

-20

-30

-40

-50

-60

Phas

e(de

gree

)

Log f

Si Si/CH/GO-COCl Si/CH/GO-CONH2 Si/CH/GO Si/CH/GO-COOR Si/CH/Gexf-SDS Si/CH/rGO

B

0.0 5.0x104 1.0x1050.0

-5.0x104

-1.0x105

Z// /ohm

Z//ohm

Si Si/CH/GO-COCl Si/CH/GO Si/CH/GO-CONH2 Si/CH/GO-COOR Si/CH/Gexf-SDS Si/CH/rGO

Fig. 6. (A) Stacked Log Z vs Log f plot of exfoliated graphene and various functionalized graphene. (B) Nyquist diagram of exfoliated graphene and functionalized graphene (C) PhaseVs Log f plot of exfoliated graphene and functionalized graphene.


GOeCOCl. Fig. 6C shows the phase vs logf plot of exfoliated gra-phene and other functionalized graphene and the same trend isobserved as before.

3.2. Measurement of sheet resistance

We have also measured the sheet resistance of each assemblyusing the following relation: [26e29].

Є ¼ CD=Є0A (2)

Where,Єo is the permittivity of free space, C is capacitance, A is areaand D is the thickness of LBL assembly films, and

бAC ¼ uЄoЄ tand (3)

where, u ¼ 2pf, f is the frequency of applied signal.Then sheet resistance is calculated as [30].

Rsheet ¼ D=бAC$A (4)

Table 3 summarizes the sheet resistances calculated for gra-phene and different functionalized graphene using equation (4) forfrequency 100 and 1000 Hz. It is evident from the table that

Table 3Sheet resistance values for different graphene samples i.e., oxidized, reduce

Sample Sheet resistance at 100 Hz (

Si 3.1 � 107

Si/CH/rGO 1.0 � 105

Si/CH/GRP 1.2 � 105

Si/CH/GOeCOOR 2.6 � 106

Si/CH/GOeCONH2 2.8 � 106

Si/CH/GO 3.1 � 106

Si/CH/GOeCOCl 1.8 � 107


exfoliated graphene and reduced graphene oxide has lower sheetresistance than functionalized graphene. Among the differentfunctionalized graphene, graphene sheets with electron donatinggroupeCOOR has comparatively lower sheet resistance than that ofgraphene sheet modified with electron withdrawing groups. Thereason could be that although functionalization of graphene sheetintroduces structural distortion in the planer structure, howeverþ Igroups will ensure more electron density in the sheet than havingeI groups. The sheet resistance values are consistent with Nyquistplot. We want to add that we have recorded the impedance ofgraphene and graphene oxide at 0.5 Hz which can be taken as nearDC. Thus a.c conductivity of graphene and graphene oxide at 0.5 Hzwas determined to be 2.88 � 10�4 Scm�1 and 7.35 � 10�5 Scm�1

respectively and the corresponding sheet resistance was deter-mined to be 3.11 � 106 Ohm and 7.21 � 107 Ohm respectively.

3.3. Mechanism for difference in conductivity of graphene andfunctionalized graphene

We put forward a simple mechanism to explain the a.c con-ductivity trend in GexfeSDS, GO, rGO and other functionalizedgraphene (viz. GOeCOCl, GOeCOOR, and GOeCONH2). As

d and functionalized.

Ohm) Sheet resistance at 1000 Hz (Ohm)

3.6 � 107

4.5 � 105

5.0 � 105

2.9 � 106

3.3 � 106

3.6 � 106

2.1 � 107


Scheme 3. Schematic representation of the probable mechanism for difference in conductivity of graphene exfoliated and functionalized graphene.


exfoliated graphene and reduced graphene oxides have planar andhexagonal structures having alternate eCeCe and eC]C- bonds,delocalization of sp2 hybridised carbon electrons is possible givingzero band gap energy. On functionalization, the planar structure isdistorted and delocalization is also hindered gradually [6,17,31e33].It is already reported that on functionalization electrical conduc-tivity of MWCNT decreases due to unbalance polarization effect andphysical structure defects due to severe condition during chemicaltreatment processes [34]. Introduction of bulky groups in the gra-phene sheets may lead to the band gap opening which is respon-sible for lower conductivity in case of functionalized graphene. Aschematic representation of the probable mechanistic pathway fordifference in conductivity between graphene exfoliated anddifferent functionalized graphene is shown in Scheme 3. Moreoverit has also been demonstrated that lateral size of graphene sheetsalso have role in the electrical conductivity. Larger size distributiongraphene sheets having larger lateral dimension possess higherconductivity than the lower size distribution sheets. This is true forboth exfoliated graphene and other functionalized graphene.

4. Conclusion

In this work graphene were obtained from SDS mediated exfo-liation of graphite. The exfoliated graphene were further subjectedto functionalization to obtain reGO, GOeCOCl, GOeCOOR, andGOeCONH2. The impedance measurements were done on func-tionalized graphene by a simple layer-by-layer (LbL) assemblytechnique on silicon wafer substrate using chitosan as cationicbinder. Further, the role of lateral dimension of the graphene sheetin a.c conductivity was also studied. It was observed that intro-duction of bulky groups in the graphene sheets may lead to theband gap opening which is responsible for lower a.c conductivity incase of functionalized graphene. Moreover sheet resistance valueswere highest for GOeCOCl and smallest for reGO. Such under-standing of electrical properties of graphene and different


functionalized graphene is important as it will pave the way ofusing these materials in electronic applications.

Acknowledgement

The authors thank Central Instrumentation Facility, IIT Guwa-hati for Raman studies. The authors also thank SERB, New Delhi,Grant No. SB/S1/PC-69/2012 and BRNS, Mumbai, Grant No. 34/14/20/2014-BRNS. UB and MJD thank IASST, Guwahati and SERB, NewDelhi for fellowship.

Appendix A. Supplementary material

Supplementary material related to this article can be found athttp://dx.doi.org/10.1016/j.matchemphys.2015.07.036.

References

[1] S.V. Morozov, K.S. Novoselov, M.I. Katsnelson, F. Schedin, D.C. Elias,J.A. Jaszczak, A.K. Geim, Phys. Rev. Lett. 100 (2008) 016602e016604.

[2] J.W. Jiang, J.S. Wang, B. Li, Phys. Rev. B 80( (2009) 113405e113409.[3] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau,

Nano Lett. 8 (2008) 902e907.[4] S. Myung, J. Park, H. Lee, K.S. Kim, S. Hong, Adv. Mater. 22 (2010) 2045e2049.[5] X. Wang, L. Zhi, K. Mullen, Nano Lett. 8 (2008) 323e327.[6] Z. Zhang, L. Luo, G. Chen, Y. Ding, D. Deng, C. Fan, Biosens. Bioelectron. 60

(2014) 161e166.[7] Y. Song, H. Liu, H. Tan, F. Xu, J. Jia, L. Zhang, Z. Li, L. Wang, Anal. Chem. 86(

(2014) 1980e1987.[8] A.T. Ezhil Vilian, S.M. Chen, Y.H. Chen, M. Ajmal Ali, M.A. Fahad, A. Hemaid,

J. Colloid Interface Sci. 423 (2014) 33e40.[9] J. Huang, L. Zhang, B. Chen, N. Ji, F. Chen, Y. Zhang, Z. Zhang, Nanoscale 2

(2010) 2733e2738.[10] F. Yongsheng, S. Xiaoqiang, W. Xin, Mater. Chem. Phys. 131 (2011) 325e330.[11] K.V. Prashant, J. Phys. Chem. Lett. 2 (2011) 242e251.[12] L. Dai, Acc. Chem. Res. 46 (2013) 31e42.[13] D. Prasai, J.C. Tuberquia, R.R. Harl, G.K. Jennings, K.I. Bolotin, ACS Nano 6

(2012) 1102e1108.[14] Y. Shao, S. Zhang, H.M. Engelhard, G. Li, G. Shao, Y. Wang, J. Liu, I.A. Aksay,

Y. Lin, J. Mater. Chem. 20 (2010) 743e748.[15] C. Bora, J. Sharma, S. Dolui, J. Phys. Chem. C 118 (2014) 29688e29694.



[16] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Nat. Phot. 4 (2010) 611e622.[17] A. Bonanni, M. Pumera, Electrochem. Commun. 20 (2013) 52e54.[18] Alamusi, W.F. Yuan, Surina, Y. Li, N. Hu, H.M. Ning, Y.L. Liu, L.K. Wu, S. Atobe,

H. Fukunaga, Appl. Phys. Lett. 103 (2013) 221903.[19] Alamusi, J.M. Xue, L.K. Wu, N. Hu, J. Qiu, C. Chang, S. Atobe, H. Fukunaga,

T. Watanabe, Y.L. Liu, H.M. Ning, J.H. Li, Y. Li, Y. Zhao, Nanoscale 4 (2012)7250e7255.

[20] E. Casero, A.M. Parra-Alfambra, M.D. Petit-Domínguez, F. Pariente, E. Lorenzo,C. Alonso, Electrochem. Commun. 20 (2012) 63e66.

[21] K.S. Kim, Y.M. Um, J. Jang, W.S. Choe, P.J. Yoo, ACS Appl. Mater. Interfaces 5(2013) 3591e3598.

[22] M. Lotya, Y. Hernandez, P.J. King, R.J. Smith, V. Nicolosi, L.S. Karlsson,F.M. Blighe, S. De, Z. Wang, I.T. McGovern, G.S. Duesberg, J.N. Coleman, J. Am.Chem. Soc. 131 (2009) 3611e3620.

[23] J. Zhang, H. Zou, Q. Qing, Y. Yang, Q. Li, Z. Liu, X. Guo, Z. Du, J. Phys. Chem. B107 (2003) 3712e3718.

[24] M. Lotya, A. Rakovich, J.F. Donegan, J.N. Coleman, Nanotechnology 24 (2013)265703e265709.


[25] A. Charles, S. Matthew, Fundamentals of Electric Circuits, vol. 3, McGraw-Hill,2006, pp. 387e389.

[26] K.W. Chew, T.C. Ng, Z.H. How, Int. J. Electrochem. Sci. 8 (2013) 6354e6364.[27] P.C. Yue, F. Qing, W. Li-jun, Z. Qian, C. Meng, J. Cent. South Univ. Technol. 3

(2007) 348e352.[28] N. Jaiswal, D. Kumar, S. Upadhyay, O. Parkash, Ionics 21 (2015) 497e505.[29] A.R. Pal, B.K. Sarma, N.C. Adhikary, J. Chutia, H. Bailung, Appl. Surf. Sci. 258

(2011) 1199e1205.[30] A. Gogoi, N. S. Sarma, Ionics, http://dx.doi.org/10.1007/s11581-015-1448-5.[31] S. Gilje, S. Han, M. Wang, K.L. Wang, R.B. Kaner, Nano Lett. 7 (11) (2007)

3394e3398.[32] C. Punckt, M.A. Pope, J. Liu, Y. Lin, I.A. Aksay, Electroanalysis 22 (23) (2010)

2834e2841.[33] D.W. Boukhvalov, M.I. Katsnelson, A.I. Lichtenstein, Phys. Rev. B 77 (2008)

035427.[34] A.B. Sulong, N. Muhamad, J. Sahari, R. Ramli, B.M. Deros, J. Park, Eur. J. Sci. Res.

29 (1) (2009) 13e21.


Insight into electrical conductivity of graphene and functionalized graphene: Role of lateral dimension of graphene sheet

Documents