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One-pot synthesis of large scale graphene nanosheets from graphite–liquidcrystal composite via thermal treatment†

Afsaneh Safavi,* Maryam Tohidi, Farzaneh Aghakhani Mahyari and Hamidreza Shahbaazi‡

Received 12th August 2011, Accepted 28th November 2011

DOI: 10.1039/c2jm13929d

An easy and cost effective route for mass production of graphene nanosheets (GNSs) is an essential

requirement for design of different sensors, conductive composites and future nanoelectronic devices.

Scalable and large area GNSs were synthesized by a thermal treatment of a graphite–ionic liquid crystal

composite as a starting material. This composite was heated in a furnace with a flow of argon gas at 700�C for 1 h. Intercalation of ionic liquid crystals between graphite layers, their decomposition and

evolution of gases assist in exfoliation of graphite and separation of layers. The proposed method

extends the scope for production of high-quality, high-yield, unoxidized and defects free GNSs for

a wide range of applications. The ability to produce bulk GNSs from a graphitic precursor with an easy

and relatively low-cost approach can propel us to real-world applications of GNSs.

Introduction

Graphene has changed its status from being an unexpected

newcomer to a glamorous star in different fields of science and

technology.1 Graphene is expected to be comprised of a single

layer, but there is considerable interest in investigating bi-layer

and few-layer graphenes as well. Synthesis and characterization

of graphenes pose challenges, but there has been considerable

progress in the past few years or so.2

Mechanical exfoliation of graphene from graphite led to the

discovery of graphene nanosheets (GNSs).2,3 Its low productivity

makes it unsuitable for large-scale utilization.4,5 Chemical vapor

deposition (CVD) and epitaxial growth have been used for

synthesis of large area GNSs.2,6,7 However, it is usually necessary

to transfer the sample to other substrates in order to make useful

devices.8 Epitaxial growth of graphene films by vacuum graphi-

tization is another approach to obtain graphene.2,4,7,9 However,

this method suffers from the need for ultrahigh vacuum condi-

tions which is expensive.8,10

Conversion of nanodiamond, arc evaporation of graphite in

the presence of H2/He mixtures and microwave plasma-enhanced

CVD are also used for production of graphene which need very

expensive reagents and/or equipment.2 These methods produce

some quantities of other unwanted carbonaceous materials.11–13

Department of Chemistry, College of Sciences, Shiraz University, Shiraz,71454, Iran. E-mail: safavi@chem.susc.ac.ir; Fax: +98-711-2286008;Tel: +98-711-6137351

† Electronic supplementary information (ESI) available: Optical imagesof GOx and GNSs suspension in water, flake-like graphite–ILCcomposite and GNSs. SEM and TEM images of different ratios ofgraphite–ILC composite before and after thermal treatment. Ramanspectra of bi-layers GNS. See DOI: 10.1039/c2jm13929d

‡ Present address: Chemistry Department, University of Calgary, 2500Uni. Dr. NW, Calgary AB, Canada T2N1N4.

This journal is ª The Royal Society of Chemistry 2012

Bulk production of graphene with chemical or thermal

reduction of graphene oxide (GOx) has also been reported.4,7,14,15

Although this low cost method is efficient for the production of

single-layer graphene, this method has lengthy experimental

times and utilizes highly dangerous and toxic oxidizing and

reducing reagents. In fact, GOx is an insulator rather than

a semi-metal and is different from graphene.16 Although the

functional groups can be removed by reduction, this leaves

a significant number of defects, which disrupt the electronic

properties.17 Recently, exfoliation of natural graphite in various

solvents by sonication has been reported.18–20 This method

provides sufficient solvent–graphene interaction to balance the

energy cost for expansion of graphite layers. Although direct

liquid-phase exfoliation offers several advantages such as

simplicity and low cost, the resulting colloidal suspensions of

graphene are at low concentrations with small flakes.20,21 Also,

the dispersed GNSs tend to aggregate with drying.22 Sometimes,

after drying, residues of solvents remain, which prevent the

aggregation of graphene layers, but deleteriously affects its

potential applicability.18,20

In other methods, graphite is partially exfoliated by reactions

involving the intercalant,23 or through thermal shock following

acid treatment of natural or expandable graphite.18,24,25 These

methods are time-consuming.

Electrochemical exfoliation of graphite is another approach for

synthesis ofGNSs.26Recently, a solution routewas introduced for

the high-yield exfoliation of graphite into few-layer GNSs.27 This

method requires high voltage and is multi step with a long soni-

cation time that results in small flakes of GNSs.27 Therefore,

a facile, simple and direct approach to produce large-scale and

defects free GNSs with large area remains a great challenge.

Ionic liquids (ILs) are used for preparation of GNSs in several

approaches such as electrochemical methods28 and exfoliation of

J. Mater. Chem., 2012, 22, 3825–3831 | 3825

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natural graphite by sonication because of their unique proper-

ties.19,20 Liquid crystals (LCs) are considered as the fourth state

of matter.29 Ionic liquid crystals (ILCs) can be considered as

materials that combine the properties of LCs and ILs.30 The ILC,

1,10-didodecyl-4,40-bipyridinium bis(triflimide), known as viol-

ogen is a thermotropic ILC (Scheme 1). Recently, we made use of

this ILC as a binder to fabricate new carbon composite elec-

trodes.31 Here, we report a one-pot, large-scale synthesis of high-

quality GNSs using thermal treatment of a graphite–ILC

composite.

Experimental methods

Chemicals

Graphite powder (particle size <100 mm, purity 99.9%) was

purchased from Fluka (Product Number 332461). 1,10-Dido-

decyl-4,40-bipyridinium bis(triflimide) was synthesized as

described elsewhere.31,32

Sample preparation

The graphite–ILC composite was prepared easily by hand-mix-

ing, in a mortar, the graphite powder and ILC with graphite/ILC

ratio of 30/70 (w/w). The uniform graphite–ILC composite was

prepared with successive mixing and heating to 100 �C. The

viscosity of ILC reduces with increasing temperature. Therefore,

repetitive mixing and heating of the composite can be effective in

increasing the homogeneity of the composite. The samples were

heated in a tube furnace (Azar Furnace-Iran, equipped with gas

flow lines) using argon (Ar) flow. The temperature programming

for GNSs preparation was heating up to 700 �C with a ramp of

3 �C min�1 and then remaining at 700 �C for 60 min. Graphite

oxide was synthesized from graphite by the Hummers method.33

The as-synthesized graphite oxide was suspended in water to give

GOx.4 Brown dispersion of GOx in water can be observed with

respect to black GNSs dispersion (ESI, Fig. S1†). It should be

mentioned that the dispersed GNSs are not stable more than

several minutes due to their defect-free (no oxygenated func-

tional groups) structures.

Scheme 1 Schematic presentation of GNSs synthesis process. Intercalation of

assist in exfoliation of graphite and separation of layers.

3826 | J. Mater. Chem., 2012, 22, 3825–3831

Characterization techniques

Scanning electron microscope (SEM) images were obtained by

Philips XL30 and Hitachi S-4160 field emission SEM (FESEM)

at accelerating voltages of 25 and 30 kV, respectively. High

resolution transmission electron microscope (HRTEM) and

TEM images were taken with electronic microscope (Tecnai F20-

200KV) and a Leo 912 AB, respectively. Atomic force micro-

scope (AFM) images were taken with Agilent 5500. Raman

spectroscopy was performed on a Jobin-Yvon LabRAM H800

with He-Ne laser of 633 nm as excitation source, spot size: 1 mm.

Spectra were taken on an extended range (100–4200 cm�1).

For Raman analysis, individual flakes were deposited from

dispersed GNSs in chlorobenzene onto a SiO2/Si substrate using

spin coating method. The 2D peaks of Raman spectra were fitted

by Lorentzian fitting. X-Ray photoemission spectroscopy (XPS)

measurements were conducted with an XR3E2 (VG Microtech)

twin anode X-ray source using Al-Ka ¼ 1486.6 eV. X-Ray

diffraction (XRD) patterns were obtained by using a D8

ADVANCE type (BRUKER-Germany) with Cu-Ka radiation

(l¼ 0.1542 nm). Powder XRD patterns were taken in 0.02� stepsat 1 s per step. FT-IR spectra were obtained by a Shimadzu FT-

IR 8300 spectrometer.

Results and discussion

Thermotropic ILC, 1,10-didodecyl-4,40-bipyridinium bis(tri-

flimide), has been used for preparation of the graphite–ILC

composite with different ratios (graphite/ILC: 70/30, 50/50 and

30/70 (w/w)) as the starting material for the synthesis of GNSs

(Scheme 1). This ILC compound has a smecticX (Smx) LC phase

at room temperature in which the molecules are arranged in

layers.31,32 The Sm phase of this ILC compound is stable up to its

decomposition temperature at 356 �C. Thus, it exhibits the widestrange of LC phase among all of the viologen salts.32 Interestingly,

the mixture of this ILC with graphite showed a layered appear-

ance and flake structure (ESI, Fig. S2A†). It is understood that

the inter-graphene layers in graphite can be intercalated by

various molecular species or ions.23,24,34,35 The graphite–ILC

composite was heated in a tube furnace under a flow of Ar.

ILCs between graphite layers, their decomposition and evolution of gases

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Different temperatures and exposure times were examined to

obtain optimum conditions (heating up to 700 �C with a ramp of

3 �C min�1 and then remaining for 60 min) for GNSs synthesis.

With increasing the temperature, the reduction in viscosity of

ILC occurred. Prior to its decomposition temperature (356 �C),this bipyridinium ILC with Sm phase are arranged in layered

structures and can penetrate within the layers in graphite. It is

believed that similar to what has been reported for ILs,36 LCs

and ILCs with unsaturated groups can have p–p stacking with

aromatic carbon structures. After 356 �C up to 700 �C, ILCdecomposition occurred. As suggested previously, decomposi-

tion of ILs with fluorine-containing anions such as BF4, PF6 and

NTf2, results in formation of hydrogen fluoride (HF), alkenes,

haloalkanes, phosphorus-containing compounds, CF3H or

SO2.37 The evolution of gases due to decomposition of ILC can

result in exfoliation of graphite and assists in separation of

graphite layers (Scheme 1).23 It is noteworthy to mention that the

flow of Ar gas during the thermal treatment process prevents the

oxidation of graphite and the final product, GNSs. As will be

discussed later, the results of some characterization techniques

will support this dramatic advantage. Thus, the proposed

method results in large scale production of unoxidized GNSs

(ESI, Fig. S2B†).

The GNSs produced herein have been characterized by

a variety of microscopic and other physical techniques including

SEM, FESEM, TEM, HRTEM, AFM, XPS, XRD, FT-IR and

Raman spectroscopy.

SEM images of the graphite–ILC composite before and after

optimum thermal treatment were investigated. SEM observation

of this composite before thermal treatment shows the formation

of flake-like structures (ESI, Fig. S3†). For GNSs synthesis,

different ratios of graphite/ILC (w/w) such as 70/30, 50/50 and

30/70 (w/w) were examined (Fig. 1, ESI, Fig. S4 and S5†). The

SEM images of the synthesized GNSs in the ratio of 30/70 (w/w)

graphite/ILC are shown in Fig. 1. It is crucial to ascertain the

exfoliation state of the products after thermal treatment. It was

found that with decreasing the ratio of graphite/ILC up to 30/70,

Fig. 1 FESEM images of the producedGNSs at different magnification ((b) a

(h) are images of the GNSs with some generated hexagonal holes in their struc

composite with a ratio of 30/70 (w/w).

This journal is ª The Royal Society of Chemistry 2012

the exfoliation of graphite to graphene layers was increased. This

can be due to increasing the intercalation of ILC into graphite

and evolution of gases.23 Further decrease in this ratio (30/70 of

graphite/ILC) had no significant effect on the synthesized

product.

As it can be seen in Fig. 1g and 1h, some hexagonal holes are

generated in GNSs with a graphite/ILC ratio of 30/70, which are

ascribed to evolution of gases. From the SEM images, it can be

seen that GNSs were exfoliated into very thin layers. The high

magnification SEM image (Fig. 1b) demonstrates thin and folded

platelets transparent to the electrons. As reported previously,

scrolling is intrinsic to GNSs due to thin thickness.38 As a control

experiment, graphite powder without ILC was treated with the

same thermal conditions (blank sample). We examined the state

of the natural graphite powder and blank sample compared to

synthesized GNSs using SEM images (ESI, Fig. S6†). No

significant alteration was observed between the blank sample and

natural graphite powder (ESI, Fig. S6 and S7†). In contrast,

a considerable difference was distinguished between synthesized

GNSs and the blank sample.

The obtained GNSs were analyzed using TEM and HRTEM

by drop casting from the dispersion onto grids (Fig. 2 and ESI,

Fig. S8†). Fig. 2 shows the TEM images of the synthesized GNSs

in the ratios of 30/70 and 50/50 (w/w) graphite/ILC. The fully

exfoliated GNSs are transparent and exhibit a very stable nature

under the electron beam. Some of the GNSs tend to fold and

crumple slightly and thus become thermodynamically more

stable.38 Thinner GNSs were observed in the ratio of 30/70 (w/w)

graphite/ILC (Fig. 2 and ESI, Fig. S8†). A more definitive

identification of graphene can be made by electron diffraction

patterns.18 The inset of Fig. 2a shows the electron diffraction

pattern of the GNSwhich confirms its crystalline nature. Also for

comparison, the TEM image of the original graphite–ILC

composite was investigated (ESI, Fig. S9†). A significant differ-

ence was observed in the transparency of the graphite–ILC

composite and GNSs due to the diversity of their thicknesses.

The representative images from these techniques indicate that the

nd (d) are higher magnification of (a) and (c) images, respectively). (g) and

tures. Synthesis conditions: optimum thermal treatment of graphite–ILC

J. Mater. Chem., 2012, 22, 3825–3831 | 3827

Fig. 2 HRTEM (a)–(b) and TEM (c)–(d) images of the produced GNSs

with thermal treatment of the graphite–ILC composite with ratios of (a)–

(b) 30/70 (w/w) and (c)–(d) 50/50 (w/w). Inset: electron diffraction pattern

of GNS.

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lateral sizes of GNSs are typically a few micrometres. By

analyzing a large number of TEM images, paying close attention

to the sheet edges, it was possible to generate sheet thickness

statistics as shown in Fig. 3. From these data we can estimate the

fraction of bi-layer and multi-layer GNSs (number of layers/total

number of observed sheets). On the basis of statistical sampling it

was estimated that 15%, 30%, 37% and 18% of the GNSs

comprised 2–3, 4, 5–10 and >10 layers of graphene, respectively

(see the histogram in Fig. 3d).

According to SEM and TEM results, the ratio of 30/70 (w/w)

graphite/ILC was selected for further analysis and

characterizations.

AFM was used to examine the thickness of the GNSs. Fig. 4

displays a typical tapping-mode AFM image of GNSs, exhibiting

sheet heights of �1–1.8 nm. This reveals characteristics of bi-

layer and multi-layer GNSs.39

Fig. 3 High magnification TEM images showing edges of the GNSs

consisting of (a) 2 and 8, (b) 4 and 5, and (c) 8 layers. (d) Histogram of the

number of sheets as a function of the number of layers per sheets.

3828 | J. Mater. Chem., 2012, 22, 3825–3831

Raman spectroscopy is a powerful nondestructive tool to

characterize carbonaceous materials, particularly for dis-

tinguishing ordered and disordered crystal structures of carbon.5

The typical features for carbon in Raman spectra are three peaks

that are observed at 1350 cm�1 (D band), 1580 cm�1 (G band)

and 2700 cm�1 (2D band).40 The G and D bands are attributed to

the first-order scattering of the E2g vibrational mode in graphite

sheets and structural defects (disorder-induced modes), respec-

tively.41,42 The second-order scattering is the 2D band which is

the overtone of the D band.42 Also, the so-called D0 peak can be

seen around �1620 cm�1 in defected graphite.18,43 Generally,

a perfect graphite crystal does not exhibit the D band. But, in

some methods, during the chemical or physical processing from

pristine graphite to GNSs, they may undergo structural changes

through the rearrangement of the carbon atoms in the basal

plane.44 For example, both oxidation and covalent functionali-

zation usually introduce defects in graphite and GNSs.1,3,7

According to previous works, the ratio between the intensity of

the D and G bands (ID/IG) could be used to qualitatively char-

acterize the extent of defects in the carbon structures.45,46 This

ratio is increased with increasing the defects and disorders.42

Decreasing the size of the crystalline grain can enhance this

ratio18,41,47 due to the limitation of the laser focal point size.48

Also broadening of the G band indicates the presence of defects

and disorders.44,46,49 So, the high degree of crystallinity of the

GNSs is confirmed by a weak or negligible D band and a strong

G band with a small D0 shoulder.50

For Raman analysis, individual flakes were deposited onto

a SiO2/Si substrate. Optical microscopy was used to identify the

position of single sheets, but because of its difficulty for identi-

fication, SEM was carried out first (Fig. 5a and 5c). Fig. 5b and

5d show Raman spectra for some marked sheets in the SEM

images. The G and 2D bands are clearly visible in all cases. These

spectra have small or non-existent D andD0 bands, indicating theabsence of defects. As shown in Fig. 5b and 5d, IG [ ID, so the

structural defects do not exist and the lateral size of GNSs is

Fig. 4 AFM image of GNSs with the corresponding AFM height image.

This journal is ª The Royal Society of Chemistry 2012

Fig. 5 (a) and (c) Optical microscopic images of some individual depositedGNSs. (b) and (d) Raman spectra of sheets 1 with bi- andmulti-layer marked

in (a) and (c), respectively. Insets: (a) and (c) corresponding SEM images of GNSs, (b) and (d) magnified SEM images of sheets 1.

Fig. 6 Deconvolution of the 2D band of Raman spectra for sheet 1 in

Fig. 5a to Lorentzian curves.

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large. These experiments confirm that high-quality and unoxi-

dized GNSs with large lateral size have been synthesized in this

case. The shape of the 2D band can be used to estimate the

number of layers in GNSs.17,18,40 Deconvolution of the 2D band

for sheet 1 in Fig. 5a shows that it is composed of four compo-

nents (Lorentzian peaks). This is a characteristic feature of the

Raman spectra of bi-layer graphene as previously reported

(Fig. 6).12 Also, sheet 2 in Fig. 5a shows the same result with bi-

layer characteristics (ESI, Fig. S10†). The shape of the 2D band

in the Raman spectrum of sheet 1 in Fig. 5c shows multi-layer

features. The results for sheets 2 and 3 in Fig. 5c were similar to

sheet 1 (data not shown).

To monitor the composition of GNSs, we employed XPS.

Fig. 7a shows the XPS spectrum of GNSs that contains only two

peaks of C1s and O1s, which are also apparent in natural

graphite (not shown). The XPS spectrum also shows that there is

no evidence of other peaks such as N1s, F1s and S2p. This

indicates that thermal treatment completely removed ILC from

synthesized GNSs. So, this method can overcome the problem of

many methods in which the residue of solvents remains between

the layers, especially for high boiling point solvents.18 The high

resolution C1s XPS of GNSs manifests two peaks corresponding

to the C–C bond (284.2 eV) of sp2 carbon in the basal plane of

This journal is ª The Royal Society of Chemistry 2012

GNSs and C–O (286.4 eV) (Fig. 7). As can be seen in Fig. 7b,

a very good fit is obtained considering only these two peaks. The

main fitted line represents graphitic carbon (C–C). The

remainder (286.4 eV) can be very well fitted considering only

J. Mater. Chem., 2012, 22, 3825–3831 | 3829

Fig. 7 (a) XPS spectrum and (b) high resolution C1s XPS spectrum of GNSs. In (b) the background has been corrected for clarity. (c) XRD patterns of

(1) ILC, (2) graphite, (3) graphite–ILC, (4) synthesized GNSs and (5) GOx. (d) FT-IR spectra of (1) graphite, (2) synthesized GNSs and (3) GOx. Inset in

part c shows the 2q range of 25.0–27.0 for XRD patterns of 2 and 4.

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a very low amount of C–O from the graphite starting material.

This result shows the absence of oxidization typically associated

with GOx.44

XRD technique was also used to analyze the crystallinity and

quality of the GNSs.12 As reported in the literature, for the

pristine graphite sample, the (002) peak appears at 26.5�, indi-cating an interlayer spacing of 0.336 nm.51,52 Some methods

introduce oxygenated functional groups on carbon sheets. With

increasing the oxidation process, the intensity of the (002)

diffraction line (d-space 0.336 nm at 26.5�) gradually weakens

and finally disappears and a new diffraction peak (d-space 0.69

nm at 12.8�) appears.51–53 So, we investigated the XRD data of

bulk GNSs and GOx (synthesized according to the Hummers

method)33 to validate the formation of unoxidized GNSs and the

complete removal of ILC from the composite after thermal

treatment (Fig. 7c). Fig. 7c shows XRD patterns for ILC, natural

graphite, graphite–ILC composite, bulk GNSs and GOx. The

XRD spectrum of ILC reveals some diffraction peaks in the

range of 5� to 20� (Fig. 7c). For the graphite–ILC composite

before thermal treatment, the diffraction peaks of ILC can be

observed besides the graphite. For the GNSs obtained by

thermal treatment of the graphite–ILC composite, the ILC peaks

have completely disappeared and several distinct peaks,

3830 | J. Mater. Chem., 2012, 22, 3825–3831

corresponding to known values of graphite, can be identified. It

is interesting to note that complete removal of ILC resolves the

main problem of the remaining residual reagents between the

layers in some methods such as direct liquid-phase exfolia-

tion.18,20 Comparison of the XRD patterns of natural graphite

and bulk GNSs indicates that the sp2 structure of carbon is

preserved after thermal treatment (Fig. 7c).28 The inset in Fig. 7c

shows that the (002) peak of graphene decreased by several

orders of magnitude compared to graphite.28 It is interesting that

no peak was observed at 12.8� corresponding to the oxidized

product (GOx). This shows that GNSs have a high crystalline

quality and the unoxidized product is produced by the proposed

method. This high degree of crystallinity of the GNSs was

previously confirmed by the electron diffraction pattern and

Raman spectroscopy. The results obtained from XRD show that

there were no impurities of other carbonaceous materials such as

amorphous carbon, carbides, diamond and CNTs in the

obtained GNSs.

Graphite, GNSs and GOx were also characterized by FT-IR

in the mid-infrared range, and the results are shown in Fig. 7d.

These compounds show the band at 1620 cm�1 attributed to the

aromatic C]C.44 Oxygenated functional groups were observed

in the IR spectrum of GOx (Fig. 7d). In the spectrum of GNSs,

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no vibrational band is observed corresponding to the presence

of oxygenated functional groups. The absence of these vibra-

tions is indicative of the successful synthesis of unoxidized

GNSs.

Conclusions

We have proposed a scalable method to produce high-quality,

high-yield, unoxidized and defects free GNSs. Intercalation of

ILC between graphite layers, their decomposition and evolution

of gases assist in exfoliation of graphite and separation of layers.

This method enables the use of low-cost material and techniques

to produce GNSs in large quantities. This approach is direct,

facile and will not result in contamination of the GNSs with

residual reagents due to the complete removal of ILC. The

synthesized GNSs have high purity and the demonstrated

method does not produce other carbonaceous materials such as

diamond, CNTs, unwanted carbides and amorphous carbon.

Dry synthesis of GNSs has a dramatic advantage compared to

the liquid-phase exfoliation method with a drawback of final

product aggregation after drying. Large area production of

GNSs is a characteristic feature that promises making device-

sized graphene in the near future. The synthesized GNSs can

have potential applications in the design of different sensors and

conductive composites.

Acknowledgements

The authors wish to express their gratitude to Shiraz University

Research Council for the support of this work. The authors

also wish to thank Mr N. Maleki for his interest and support of

this work, Prof. L. C. Chen for taking Raman spectra and her

useful comments, Dr E. Goharshadi for taking some TEM

images and Dr E. Farjami for her kind assistance during this

work.

Notes and references

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