Dispersion of graphene sheets in ionic liquid [bmim][PF6] stabilized by an ionic liquid polymer

Article abstract of DOI:10.1039/b914763b

Dispersion of graphene sheets in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was successfully achieved with the aid of a polymerized ionic liquid (PIL).

Full text of DOI:10.1039/b914763b

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Dispersion of graphene sheets in ionic liquid [bmim][PF6]
stabilized by an ionic liquid polymerw
Xiaosi Zhou, Tianbin Wu, Kunlun Ding, Baoji Hu, Minqiang Hou and Buxing Han*
Received (in Cambridge, UK) 22nd July 2009, Accepted 17th November 2009
First published as an Advance Article on the web 1st December 2009
DOI: 10.1039/b914763b
Dispersion of graphene sheets in ionic liquid 1-butyl-3-methyl-
imidazolium hexafluorophosphate was successfully achieved with
the aid of a polymerized ionic liquid (PIL).
new applications in different fields, such as being utilized as an
electrolyte in dye sensitized solar cells, and being used to
stabilize metal nanoparticles in catalysis, because both of them
have some unique properties. Dispersion of graphene sheets in
ILs is the key for exploring the applications, but this is
challenging. In this communication, we report an approach
to disperse graphene sheets stably in 1-butyl-3-methyl-
imidazolium hexafluorophosphate ([bmim][PF6], Scheme S1).
The key of this method is the use of an IL polymer
poly(1-vinyl-3-butylimidazolium chloride) (PIL, Scheme S1)
as the stabilizer, which could not only easily dissolve in
[bmim][PF6], but also interacted strongly with graphene sheets
through non-covalent p–p interactions. Furthermore, it was
demonstrated that the dispersion of a small amount of graphene
sheets in [bmim][PF6] could enhance the conductivity of the IL
considerably. To the best of our knowledge, this is the first
work to disperse of graphene sheets in an IL.
Graphene, a one atom thick and two-dimensional honey-
comb lattice, has attracted enormous attention in recent
years from both the experimental and theoretical scientific
communities because of its unique mechanical, electronic, and
thermal properties.1 This special nanostructure has great
potential applications in many technological fields, such as
nanocomposites,2 nanoelectronics,3 and biosensors.4 Different
methods, such as mechanical exfoliation,5 thermal expansion,6
epitaxial growth,7 and chemical vapour deposition8 has been
developed for preparing graphene. Recently, many attempts to
produce graphene sheets in a large quantity via chemical
reduction of dispersed graphite oxide have been reported.
For example, Xu et al. demonstrated that pyrenebutyric acid
could be used to non-covalently functionalize graphene sheets
via strong p–p interactions between pyrenyl rings and basal
planes of graphene sheets.9 Li and co-workers reported that
chemically converted graphene sheets obtained from graphite
could readily form stable aqueous colloids through electronic
stabilization in the presence of ammonia.10 More recently,
The detailed procedures to prepare the PIL-stabilized
graphene sheet (PIL-G) dispersion in the IL are described in
the Notes.z In brief, the dispersion of graphite oxide (GO) in
water was first prepared from graphite by modified Hummers
method (ESIw).16 Then, the PIL was added into the aqueous
dispersion of GO followed by reduction of GO with hydrazine
monohydrate. The resulting PIL-G aqueous dispersion was
mixed with [bmim][PF6], and the PIL-G was extracted into the
IL phase due to the excellent compatibility of the PIL and IL.
The dispersion of PIL-G in the IL was obtained after the
mixture was dried until the mass was independent of
drying time.
Mullen and Feng described a method to disperse graphene
¨
sheets in organic solvent supported by ionic interactions.11 In
addition, covalent functionalization of graphene sheets with
sulfonate, imidazolium or poly(vinyl alcohol) was also achieved,
which could be dispersed in water and some organic solvents.12
Ionic liquids (ILs), which are organic salts with a melting
point below 100 1C, have attracted much attention owing to
their unique properties, such as extremely low vapor pressure,
wide liquid temperature range, high ionic conductivity, wide
electrochemical window, good thermal stability, nonflammability,
and designable properties.13 They have been widely investigated
for organic chemical reactions,14a electrochemical applications,14b,c
and synthesis of inorganic nanomaterials.14d Recently,
imidazolium-based ILs have been used as media15a or stabilizers15b
for the dispersion of carbon nanotubes.15
Fig. 1 illustrates the photographs of the PIL-G aqueous
dispersion before and after mixing with [bmim][PF6]. Two
other amphiphilic polymers, namely poly(vinyl pyrrolidone)
(PVP, Scheme S1w) and poly(sodium 4-styrenesulfonate) (PSS,
Scheme S1w), which have been used to stabilize graphene
sheets in water,4b,17 were also utilized as stabilizers for the
dispersion of graphene sheets with the same procedures.
Obviously, all the polymers could stabilize the graphene sheets
in water (Fig. 1a–c). The mixtures separated into an aqueous
phase (top phase) and an IL-rich phase (bottom phase)
after the ceasing of shaking because the IL is hydrophobic.
Interestingly, the PIL-G entered the IL phase (Fig. 1d), while
graphene sheets stabilized by PVP and PSS remained in
aqueous phase (Fig. 1e and f) because they are not compatible
with the IL. This phenomenon shows that the PIL not only
possesses excellent stabilizing ability for graphene sheets, but
also has better solubility in the IL. Therefore, we can conclude
that the existence of PIL is essential for the dispersion of
graphene sheets in the IL, although the presence of polymeric
dispersing agents is undesirable for some applications.10,12a,18
It would be reasonable to expect that the fluids formed from
graphene sheets and ILs will open up some possibilities for
Contribution from Beijing National Laboratory for Molecular
Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: hanbx@iccas.ac.cn;
Fax: +86-10-62559373; Tel: +86-10-62562821
w Electronic supplementary information (ESI) available: Chemicals
used, synthesis of 1-butyl-3-methylimidazolium hexafluorophosphate
([bmim][PF6]), graphite oxide (GO), and poly(1-vinyl-3-butylimidazolium
chloride) (PIL), chemical structures of the IL and polymers used, and
Fig. S1–S4. See DOI: 10.1039/b914763b
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Fig. 1 Photographs (a), (b) and (c) correspond to aqueous dispersions
of the graphene sheets (0.8 mg mLꢁ1) stabilized by PIL, PVP and PSS,
respectively, and the weight ratio of the stabilizers to graphene sheets was
12.5; photographs (d), (e) and (f) correspond to (a), (b) and (c) after being
mixed with [bmim][PF6] of equal volume by shaking vigorously and then
depositing for 20 min.
The dried dispersion of PIL-G in the IL was diluted by
[bmim][PF6] to prepare the dispersions of different concentrations.
The photographs of the freshly prepared samples and those of
the samples after depositing for two months are presented in
the ESI (Fig. S1w). As expected, the color of these dispersions
became darker as the concentration of graphene sheets in the
IL increased (Fig. S1w). The photographs also show that the
color of the samples was not changed noticeably after depositing
for two months, indicating that the dispersion was very stable.
In order to get some quantitative results, we measured the
UV-vis absorbance of PIL functionalized graphene sheets in
[bmim][PF6] with different concentrations, which are shown in
Fig. 2. The absorbance of the PIL is negligible at 550 nm
(Fig. S2w), i.e., the absorbance at this wavelength in the inset
of Fig. 2 resulted from the graphene sheets in the dispersions.
Obviously, the absorbance of the graphene sheets in the IL
obeys Beer’s law. This indicates that the dispersion of the
graphene sheets in [bmim][PF6] was homogeneous and no
effects could be associated with concentration-dependent
aggregation of graphene sheets.19 Our experiments also
showed that the absorbance of the samples was not changed
noticeably after two months, in agreement with the direct
observation.
Fig. 3 (a) Tapping mode AFM image of PIL stabilized graphene
sheets. (b) Height profile along the line displayed in (a).
depositing a drop of the above diluted aqueous dispersion on a
new cleaved mica surface and dried in air. The average
thickness of a GO sheet was B0.84 nm (Fig. S3w), which
was consistent with that in the previous reports,9,20 suggesting
that the graphite oxide was completely exfoliated. After
reduction of the GO sheets with hydrazine monohydrate, some
of the PIL was adsorbed on the surface of the graphene sheets
by non-covalent p–p interactions between the imidazolium
rings of the PIL and graphene sheets. The thickness of single
PIL-G sheet was about 3.0 nm (Fig. 3), similar to that of
DNA-stabilized graphene sheets.21 The graphene sheets were
thicker than that of the theoretical thickness of a single
graphene layer (0.34 nm) due to the adsorption of the PIL
molecules. The physical adsorption of PIL along the surface of
the graphene sheets was further verified by X-ray photo-
electron spectroscopy (XPS) analysis (Fig. S4w). The Cl 2p
spectrum of PIL-G exhibited an intensive peak (Fig. S4dw),
suggesting that the PIL still existed on the surface of graphene
sheets after the pre-treatment.
We measured the conductivity of [bmim][PF6], PIL-
[bmim][PF6] solution (PIL-IL), and PIL-G-[bmim][PF6]
dispersion (PIL-G-IL) at various temperatures ranging from
293.15 to 323.15 K using the same apparatus and procedures
to determine the conductivity of different IL solutions.22 The
results are presented in Fig. 4.
The GO and PIL-G were characterized by atomic force
microscopic (AFM) technique. The samples were prepared by
The conductivity of the neat [bmim][PF6] determined in this
work agreed well with that reported by other authors in the
Fig. 2 UV-vis absorption spectra of PIL functionalized graphene
sheets dispersed in [bmim][PF6] with different concentrations: (1)
0.01 mg mLꢁ1, (2) 0.02 mg mLꢁ1, (3) 0.03 mg mLꢁ1, (4) 0.04 mg mLꢁ1
,
and (5) 0.05 mg mLꢁ1; dependence of the absorbance on concentration
at 550 nm (inset); the weight ratio of the PIL and graphene sheets was
12.5, and the reference solution used in the measurement was neat
[bmim][PF6].
Fig. 4 The conductivity of the IL, PIL-IL solution (10 mg Lꢁ1), and
PIL-G-IL solution at various temperatures; the concentrations of the
graphene and PIL in the PIL-G-IL solution were 0.8 mg mLꢁ1 and
10 mg mLꢁ1, respectively.
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whole temperature range.23 As illustrated in Fig. 4, the
conductivity of [bmim][PF6] decreased after introduction of
the PIL, which may result mainly from the increase of viscosity
of the IL after adding the PIL. Interestingly, the dispersion of
PIL-G in [bmim][PF6] resulted in considerable increase in the
conductivity of the IL, even though the content of the
graphene sheets was as low as 0.8 mg mLꢁ1. This may be
mainly attributed to the extraordinary electronic transport
property of graphene sheets, the homogeneous distribution
of graphene sheets in the ionic liquid matrix, and the p–p
interaction between the imidazolium rings of [bmim][PF6] and
graphene sheets.24
standards. The apparatus and procedures for the conductivity
measurement were the same as that reported previously.22
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In conclusion, uniform dispersion of graphene sheets in
[bmim][PF6] was successfully achieved with the aid of PIL.
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The authors are grateful to the National Natural Science
Foundation of China (20633080), Ministry of Science and
Technology of China (2009CB930802), and the Chinese
Academy of Sciences (KJCX2.YW.H16) for their financial
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Notes and references
z Preparation of aqueous dispersion of graphene sheets stabilized by the
polymers: We only describe procedures to prepare aqueous dispersion
of graphene sheets stabilized by the PIL because others were prepared
using the similar procedures. In the experiment, 2.0 mL of GO
aqueous solution (5.0 mg mLꢁ1) was diluted to 10 mL (1.0 mg mLꢁ1
)
with double distilled water, and then the exfoliation of GO aqueous
solution was achieved by sonicating the mixture in a water bath
(KQ-100, 40 kHz) for 5 min. The obtained brown dispersion was
centrifuged at 4000 rpm for 30 min to remove any unexfoliated GO.
Then, 100 mg of PIL was mixed with 10 mL of GO dispersion
(0.8 mg mLꢁ1) in a test tube and vigorously shaken for a few minutes.
The resulting homogeneous dispersion was transferred into a 25 ml
flask by pipette, followed by reduction with hydrazine monohydrate
(10 mL) at 100 1C for 1 h. Finally, a stable dispersion of graphene
sheets in aqueous solution was obtained.
Preparation of PIL-stabilized graphene sheet dispersion in
[bmim][PF6]: 2.0 mL of the stable dispersion (PIL-G) obtained above
and 2.0 mL of [bmim][PF6] were first added into a test tube, and mixed
entirely by vigorously shaking followed by deposition. The PIL-G was
extracted into the IL phase (Fig. 1d) and the water was vaporized and
the PIL-G dispersion in IL was dried under vacuum at 70 1C for 24 h.
The photos of the samples were taken by a digital camera (Olympus
C-4000). The UV-vis spectra were recorded with a TU-1901 spectro-
photometer (Beijing General Instrument Company). An X-ray photo-
electron spectroscopy (XPS) study was carried out with an
ESCALab220i-XL electron spectrometer from VG Scientific using
300 W AlKa radiation. The base pressure was about 3 ꢂ 10ꢁ9 mbar
and the binding energies were referenced to C1s line at 284.8 eV from
adventitious carbon. Atomic force microscopic (AFM) study was
performed with NanoScope IIIa (Veeco, US) operating in the tapping
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1
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HT4 and HT5) using DMF as eluent at a flow rate of 1 mL minꢁ1 at
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This journal is The Royal Society of Chemistry 2010
388 | Chem. Commun., 2010, 46, 386–388