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, b Shannon M. Mahurin, a Miaofang Chi b and 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]-[Tf 2 N]), by tip ultrasonication. Graphene is a nanometre-thick two-dimensional (2D) material composed by hexagonal carbon lattice with delocalized p electrons. 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 oxide 8 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 liquids 12 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 (C 6 F 5 CF 3 ), 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 tensions 22 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 solvents 14,18 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][Tf 2 N], 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][Tf 2 N] 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][Tf 2 N] and the mixture was subjected to tip ultrasonication for a total of 60 min using 5–10 min cycles (SONICS, 750 W, 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: [email protected]; Fax: +1-865-576-5235; Tel: +1-865-576-7307 b Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA w Electronic supplementary information (ESI) available: Experimental and 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 Downloaded by University of Tennessee at Knoxville on 18 November 2010 Published on 16 June 2010 on http://pubs.rsc.org | doi:10.1039/C0CC00799D View Online
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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-
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: [email protected];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
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
1 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang,S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004,306, 666.
2 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183.3 C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam andA. Govindaraj, Angew. Chem., Int. Ed., 2009, 48, 7752.
4 D. E. Jiang, B. G. Sumpter and S. Dai, J. Chem. Phys., 2007, 126,134701.
5 S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217.6 D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat.Nanotechnol., 2008, 3, 101.
7 V. C. Tung, M. J. Allen, Y. Yang and R. B. Kaner, Nat.Nanotechnol., 2009, 4, 25.
8 M. J. McAllister, J. L. Li, D. H. Adamson, H. C. Schniepp,A. A. Abdala, J. Liu, M. Herrera-Alonso, D. L. Milius, R. Car,R. K. Prud’homme and I. A. Aksay, Chem. Mater., 2007, 19, 4396.
9 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas,A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff,Carbon, 2007, 45, 1558.
10 Y. Y. Liang, D. Q. Wu, X. L. Feng and K. Mullen, Adv. Mater.,2009, 21, 1679.
11 X. L. Li, G. Y. Zhang, X. D. Bai, X. M. Sun, X. R. Wang,E. Wang and H. J. Dai, Nat. Nanotechnol., 2008, 3, 538.
12 X. S. Zhou, T. B. Wu, K. L. Ding, B. J. Hu, M. Q. Hou andB. X. Han, Chem. Commun., 2010, 46, 386.
13 L. J. Cote, F. Kim and J. X. Huang, J. Am. Chem. Soc., 2009, 131,1043.
14 Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Y. Sun,S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun’ko,J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy,R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari andJ. N. Coleman, Nat. Nanotechnol., 2008, 3, 563.
15 M. Lotya, Y. Hernandez, P. J. King, R. J. Smith, V. Nicolosi,L. S. Karlsson, F. M. Blighe, S. De, Z. M. Wang, I. T. McGovern,G. S. Duesberg and J. N. Coleman, J. Am. Chem. Soc., 2009, 131,3611.
16 A. A. Green and M. C. Hersam, Nano Lett., 2009, 9, 4031.17 V. Georgakilas, A. B. Bourlinos, R. Zboril, T. A. Steriotis,
P. Dallas, A. K. Stubos and C. Trapalis, Chem. Commun., 2010,46, 1766.
18 A. B. Bourlinos, V. Georgakilas, R. Zboril, T. A. Steriotis andA. K. Stubos, Small, 2009, 5, 1841.
19 T. Welton, Chem. Rev., 1999, 99, 2071.20 Z. Ma, J. H. Yu and S. Dai, Adv. Mater., 2010, 22, 261.21 C. Chiappe and D. Pieraccini, J. Phys. Org. Chem., 2005, 18, 275.22 J. Restolho, J. L. Mata and B. Saramago, J. Colloid Interface Sci.,
2009, 340, 82.23 J. A. Harnisch and M. D. Porter, Analyst, 2001, 126, 1841.24 S. K. Reed, O. J. Lanning and P. A. Madden, J. Chem. Phys., 2007,
126, 084704.25 A. K. Burrell, R. E. Del Sesto, S. N. Baker, T. M. McCleskey and
G. A. Baker, Green Chem., 2007, 9, 449.26 N. Liu, F. Luo, H. X. Wu, Y. H. Liu, C. Zhang and J. Chen, Adv.
Funct. Mater., 2008, 18, 1518.27 A. A. Green and M. C. Hersam, J. Phys. Chem. Lett., 2010, 1, 544.28 A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri,
F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth andA. K. Geim, Phys. Rev. Lett., 2006, 97, 187401.
29 A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner andU. Poschl, Carbon, 2005, 43, 1731.
30 K. F. Allison, D. Borka, I. Radovic, L. Hadzievski andZ. L. Miskovic, Phys. Rev. B: Condens. Matter Mater. Phys.,2009, 80, 195405.
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