Direct exfoliation of natural graphite into micrometre size few layers

graphene sheets using ionic liquidsw

Xiqing Wang,a Pasquale F. Fulvio,a Gary A. Baker,a Gabriel M. Veith,b

Raymond R. Unocic,bShannon M. Mahurin,

aMiaofang Chi

band Sheng Dai*

a

Received 7th April 2010, Accepted 28th April 2010

First published as an Advance Article on the web 19th May 2010

DOI: 10.1039/c0cc00799d

Stable high-concentration suspensions (up to 0.95 mg mL�1) of

non-oxidized few layer graphene (FLG), five or less sheets, with

micrometre-long edges were obtained via direct exfoliation of

natural graphite flakes in ionic liquids, such as 1-butyl-3-methyl-

imidazolium bis(trifluoro-methane-sulfonyl)imide ([Bmim]-[Tf2N]),

by tip ultrasonication.

Graphene is a nanometre-thick two-dimensional (2D) material

composed by hexagonal carbon lattice with delocalized pelectrons. The unique electronic, thermal, and mechanical

properties of graphene have brought great interest to this

material.1–4 The properties of graphene sheets can be greatly

affected by the number of layers, their stacking sequence,

lateral area, and the degree of surface reduction or oxidation.

Following early attempts by mechanical exfoliation of

highly oriented pyrolitic graphite (HOPG),1 many research

groups are seeking high-throughput processing routes for

producing graphene.5–7 Recent efforts in this subject include

thermal expansion of graphite oxide8 and solution processable

exfoliation of graphite oxide.9 The obtained graphene oxide

(GO) sheets have been subsequently stabilized by surface

charges,6,10 surfactants,11 or ionic liquids12 followed by reduction

with hydrazine solution or by thermal treatments in hydrogen-

rich atmospheres.7,13 Despite their capability for large scale

processing, both approaches require chemical oxidation of

graphite by the Hummers method using potassium permanganate

and sulfuric acid. Clearly, these methods are lengthy and

utilize highly toxic oxidizing and reducing reagents. In

addition, the chemical oxidation and covalent functionalization

of graphene significantly affects its conductivity due to local

disruptions of the aromatic system within the basal planes.

The electronic conductivity of reduced graphene is only

partially restored after several reduction steps.

As an alternative way, exfoliation of natural graphite flakes

into graphene in various solvents by sonication has been

reported.14–17 This method represents a simple and direct

processing to produce graphene sheets free of defects or

oxidation that other approaches suffer. The successful exfoliation

relies on the proper choice of special solvents, such as

N-methylpyrrolidone, which exhibit a surface energy matching

to that of graphene and thus are capable of providing sufficient

solvent–graphene interaction to balance the energy cost

for expansion of graphite layers. Another recent example

of stabilization of graphene directly exfoliated from graphite

utilizes perfluorinated aromatic solvents, such as octa-

fluorotoluene (C6F5CF3), which is beneficial from the charge

transfer through p–p stacking from the electron-rich graphene

sheets to the electron-deficient aromatic molecules containing

strong electron-drawing fluorine atoms.18 Although direct

liquid-phase exfoliation offers several advantages, the resulting

colloidal suspensions of graphene are still at low concentrations.

Therefore alternative liquid-phase processes, capable of producing

a reasonably high concentration of stable graphene suspension,

are highly desirable. The key parameters for such a process are

the properties of solvents used.

Ionic liquids (ILs) are a kind of semiorganic salts whose

melting point is below 100 1C.19,20 ILs exhibit several intrinsic

properties distinguishable from organic solvents, such as

extremely low vapor pressures, good thermal stability and

nonflamability.21 Most importantly, ILs have surface tensions22

closely matching the surface energy of graphite, which is a key

prerequisite of solvents for direct exfoliation of graphite.14 In

addition, the basic structural attribute of ILs is their ionicity, a

unique feature favorable for stabilization of exfoliated graphene

via Coulombic interaction through image charges.23,24 Such

advantages over most solvents14,18 make ILs the ideal systems

for synthesis of graphene.

In this communication, we demonstrate direct exfoliation of

graphite flakes under ultrasonic conditions into a dispersion

of graphenes in ILs, such as 1-butyl-3-methyl-imidazolium

bis(trifluoromethanesulfonyl)imide ([Bmim][Tf2N], see its

molecular structure in Fig. 1). This method affords a stable

suspension of graphene sheets with high concentrations, up to

0.95 mg mL�1. Scanning transmission electron microscopy

(STEM) analysis indicates that the dispersion contains sheets

with micron-sized edges and exclusively o5 layers. The material

was also characterized by X-ray photoelectron spectroscopy

(XPS) and Raman spectroscopy.

Briefly, [Bmim][Tf2N] was synthesized according to a

procedure reported by Burrel et al.25 Natural graphite flakes

(Aldrich, 20 mg) were dispersed in 10 mL of [Bmim][Tf2N] and

the mixture was subjected to tip ultrasonication for a total of

60 min using 5–10 min cycles (SONICS, 750W, 80% amplitude).

The resulting dispersion was centrifuged at 10 000 rpm for

20 min and the supernatant containing graphene sheets in IL

was collected and retained for use. The amount of un-exfoliated

or thick graphite flakes was measured quantitatively by

a Chemical Sciences Division, Oak Ridge National Laboratory,Oak Ridge, Tennessee 37831, USA. E-mail: dais@ornl.gov;Fax: +1-865-576-5235; Tel: +1-865-576-7307

bMaterials Science and Technology Division, Oak Ridge NationalLaboratory, Oak Ridge, Tennessee 37831, USA

w Electronic supplementary information (ESI) available: Experimentaland characterization details, Fig. S1–S4. See DOI: 10.1039/c0cc00799d

This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 4487–4489 | 4487

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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vacuum filtration of the sediment obtained by centrifugation

through a pre-weighted filter, followed by washing with ethanol

and drying. Given the mass of natural graphite flakes used

(20 mg), the concentration of suspended graphene in IL was

calculated to be 0.95 mg mL�1.

As shown in Fig. 1, ultrasonication of graphite flakes in

[Bmin][Tf2N] afforded a dark black dispersion even after

centrifugation. Such a dispersion was very stable with small

levels of sedimentation and aggregation, as monitored by

UV-Vis absorbance over a period of 100 h (Fig. S1 in ESIw).Upon exposure to a laser beam, the Tyndall scattering effect

was observed for a diluted dispersion.

Analysis of XPS data collected for graphite indicates that its

surface contained oxygen functional groups (10 at.% oxygen),

with 66% of these consisting of CQO moieties and 34% of

C–O species (Fig. 2A and Fig. S2A in ESIw). The O 1s XPS

spectrum of [Bmim][Tf2N] shows only a doubly-bonded

oxygen containing moiety consistent with the SQO species

in the ionic liquid (Fig. S2Bw). The suspension was subjected

to centrifugation and washing to prepare a specimen for

XPS characterization, the survey scan of graphene showed

characteristics of C, O, N, F, and S elements (at% C = 83.3;

O = 9.5; N = 4.3; F = 2.4; S = 0.5), suggesting the presence

of [Bmim][Tf2N] associated with the graphene, either by strong

non-covalent interactions or by covalent functionalization.

Analysis of O 1s narrow-scan spectrum of graphene reveals

an increase in concentration of the CQO and SQO moieties

(75% from 66%, Fig. S2C) due to the presence of CQO

groups from the graphite starting material and SQO groups

from the IL. Although the C 1s XPS spectrum of graphene

(Fig. 2A) shows an increase in spectral intensity corresponding

to C–O, C–N species (B286.6 eV) from the [Bmim][Tf2N]

when compared to that of the raw graphite sample, it suggests

low levels of oxidation of exfoliated graphene. Both

[Bmim][Tf2N] and graphene exhibit the same N species

(Fig. 2B). The broader N1s peak for graphene may be due

to low IL concentration in the final material, to prolonged

sample exposure to X-rays during analysis, or to strong

interactions between the imidazolium cation and the graphene

sheets.26

Previous TEM studies14,15 indicate that the distinguishable

edges of the exfoliated graphenes make it possible to measure

the layers of graphenes. Using such an edge-counting method,

we have identified o5 layers for most exfoliated graphene

sheets by carefully analyzing their STEM and bright field

TEM images (Fig. 3 and Fig. S3w). Shown in Fig. 3A and B

are a monolayer with folded edges and a bilayer graphene,

respectively. The exfoliated graphene sheets have micrometre

long edges, whereas a few smaller graphene fragments are also

present. Such fragments may result from the irregular crystalline

flakes of natural graphite or from the breaking of large flakes

into smaller species by ultrasonication. The atomic ordering of

carbon atoms is confirmed by BF-TEM and by selected area

electron diffraction (SAED) from a region of a few-layer

graphene (FLG) that is oriented along the [0001] zone axis

as shown in Fig. 3C and D, respectively. Although the defect

density was extremely low, dislocations and stacking faults

were also observed in a small number of FLG sheets that were

analyzed under other diffraction contrast BF TEM imaging

conditions (Fig. S3 in ESIw). Smaller graphite particles and

multi-layered structures formed after partial restacking of

FLG domains are also present in the final suspensions. We

believe that the monodispersity (e.g., thickness and lateral size)

of the graphene sheets can be improved by centrifugal proces-

sing techniques under specific conditions.27

The successful exfoliation of graphite into graphene was

further verified by Raman spectra. The spectra of two

representative regions of the graphene sample as well as that

Fig. 1 Images of the dispersion of graphite in [Bmim][Tf2N] before

(left) and after (middle) ultrasonication and the Tyndall effect of a

diluted graphene suspension using a laser pointer (right).Fig. 2 XPS C 1s (A) and N 1s (B) spectra.

Fig. 3 STEM images of dispersed graphene under the SE (A) and TE

(B) mode, respectively. The copper grid (A) was found to be sand-

wiched by a few layer graphene (bottom) and a monolayer graphene

(top) with folded edges, as highlighted in a red rectangle. The arrows in

(B) indicate the edges of a bilayer graphene. Bright field TEM

image (C) of multi-layer graphene with respective SAED pattern (D)

collected for dark area in (C).

4488 | Chem. Commun., 2010, 46, 4487–4489 This journal is �c The Royal Society of Chemistry 2010

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of natural graphite are shown in Fig. 4. Despite the decrease in

the intensity ratio of IG/ID (B6 from B19) for graphene, it

reveals low levels of defects induced by exfoliation, consistent

with the findings from XPS analyses. Deconvolution of the 2D

band (Fig. S4w) for graphene shows that this is composed of

several distinct superimposed bands, in contrast to two bands

(2D1 and 2D2) for graphite. In addition, the shape of the 2D

band of the IL-assisted exfoliated graphene resembles that

previously reported for thin flakes consisting of o5 mono-

layers,3,14,28 indicating the characteristic of few layers for the

former. Furthermore, the D0 band29 usually reported for

disordered graphitic lattices such as those introduced by hetero-

atoms, was detected for graphene samples at B1615 cm�1.

A similar weak D0 band was also found by deconvolution of

the G band for graphite (1617 cm�1).29 Hence, the D0 band in

graphene may result from pre-existing defects in graphite or

may support the findings from XPS analysis, that graphene

was stabilized by strong non-covalent interactions with

[Bmim][Tf2N] or by covalent functionalization.

In addition to [Bmim][Tf2N], some other ILs containing

non-aromatic cations, such as 1-butyl-1-methylpyrrolidinium

bis(trifluoromethanesulfonyl)-imide [C4mpy][Tf2N], have also

been tried to exfoliate graphite and give similar results. These

findings indicate that the stabilization of exfoliated graphene

by ILs possibly occurs via p–p interactions between the

graphene layers and aromatic IL cation and/or strong charge

polarization of graphene sheets by IL.23,30

In summary, few layer graphene has been prepared by the

direct exfoliation of graphite using ILs. The suitable surface

tensions and ionic feature of ILs facilitate the exfoliation of

graphite and the stabilization of graphene, affording a high

concentration of suspension. This simple method also prevents

high levels of oxidation, according to XPS and Raman studies.

Microscopy research was supported in part by Oak Ridge

National Laboratory (ORNL) Shared Research Equipment

(SHaRE) User Facility, which is sponsored by the Scientific

User Facilities Division, Office of Basic Energy Sciences, U.S.

Department of Energy (DOE). X.W., G.A.B., and S.M.M.

were supported by the Division of Chemical Sciences,

Geosciences, and Biosciences, and G.M.V. was supported by

the Division of Materials Sciences and Engineering, Office of

Basic Energy Sciences, U.S. DOE. P.F.F., R.R.U., and S.D.

were supported as part of the Fluid Interface Reactions,

Structures and Transport (FIRST) Center, an Energy Frontier

Research Center funded by the U.S. DOE, Office of

Science, Office of Basic Energy Sciences under contract

DE-AC05-OR22725 with ORNL, managed and operated by

UT-Battelle, LLC.

Notes and references

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Fig. 4 Raman spectra of natural graphite flakes (a) and two distinct

regions (b and c) of graphene film.

This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 4487–4489 | 4489

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