An amphiphilic pyrene sheet for selective functionalization of graphene

Article abstract of DOI:10.1039/c1cc12868j

We have demonstrated that the aromatic amphiphile consisting of a hydrophilic dendron and an aromatic segment with a planar conformation can selectively exfoliate graphite powder into single- and double-layer graphene sheets in aqueous solution through hydrophilic functionalization of graphene surfaces.

Full text of DOI:10.1039/c1cc12868j

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COMMUNICATION
An amphiphilic pyrene sheet for selective functionalization of graphenew
Dong-Woo Lee, Taehoon Kim and Myongsoo Lee*
Received 16th May 2011, Accepted 8th June 2011
DOI: 10.1039/c1cc12868j
We have demonstrated that the aromatic amphiphile consisting
of a hydrophilic dendron and an aromatic segment with a planar
conformation can selectively exfoliate graphite powder into
single- and double-layer graphene sheets in aqueous solution
through hydrophilic functionalization of graphene surfaces.
shape and conformation of the aromatic segments.7 These
results stimulated us to synthesize a sheet-like aromatic
amphiphile that would selectively adhere to a 2-D graphene
surface through efficient aromatic interactions.
In this communication, we present the design and synthesis
of an amphiphilic molecule based on an aromatic molecular
sheet that assembles selectively onto 2-D graphene surfaces
through nondestructive p–p stacking interactions (Fig. 1). The
amphiphilic molecule that functionalizes graphene surfaces
consists of an aromatic segment based on four pyrene units
and a laterally-grafted oligoether dendron. The aromatic
segment may adopt a 2-D sheet-like conformation on graphene
surfaces to maximize aromatic interactions with the basal
plane of graphene and the hydrophilic dendron would be
protruded away from the graphene surface to generate hydro-
philic outerlayers, consequently facilitating the dispersion
of graphene sheets in aqueous solution. The amphiphilic
molecule was synthesized in a stepwise fashion according to
the procedure reported previously.8 The resulting aromatic
amphiphile was characterized by NMR spectroscopy, elemental
One of the most significant recent highlights in the field of
carbon allotropes such as fullerene, single wall nanotubes, and
graphite is how to attain their processability without sacrificing
their intrinsic properties for practical application.1 In particular,
exfoliation of graphite into single layers, graphene sheets, has
attracted considerable attention because of the unusual
electronic properties of monolayers of the graphite lattice.2
Among the many exfoliation methods, dissolution of graphite
into certain solvents such as N-methyl pyrrolidine (NMP),
dimethylformamide (DMF), and o-dichlorobenzene (ODCB)
is particularly interesting because graphite could be directly
exfoliated into monolayer sheets, preserving their intrinsic
electrical properties.3 As an alternative strategy, surfactant
molecules in aqueous solutions have been demonstrated to
allow graphite flakes to exfoliate directly with concentrations
up to 1 mg mLÀ1.4 Another recent example for direct exfoliation
of graphite is provided by surface functionalization of
graphene sheets with aromatic carboxylic acids in aqueous
solutions.5 The aqueous dispersions of graphene sheets in
these cases originate from noncovalent functionalization of
graphene with hydrophilic carboxylic acid through aromatic
interactions between graphene surfaces and fused aromatic
units. The carboxylic acid groups positioned at the out of
plane graphene surface stabilize aqueous dispersions of the
graphene flakes. In addition, fused aromatic segments render
single-wall carbon nanotubes (SWNTs) with stable aqueous
dispersions through noncovalent functionalization of the side
walls.6 For selective dispersions of the 2-D graphene sheet out
of all graphite allotropes in aqueous solution, however,
elaborate molecular design of amphiphilic molecules is required.
Recently, we have shown that aromatic amphiphiles generate
stable aqueous dispersions of fullerene or SWNTs through
aromatic and hydrophobic interactions depending on the
Center for Supramolecular Nano-Assembly, Department of Chemistry,
Seoul National University, Seoul 151-747, Korea.
E-mail: myongslee@snu.ac.kr; Fax: +822 393 6096;
Tel: +82 2 8804340
w Electronic supplementary information (ESI) available: Detailed
synthetic and experimental procedures, 1H-NMR. See DOI: 10.1039/
c1cc12868j
Fig. 1 (a) Chemical structure of amphiphile 1. (b) Photograph of
graphene (A) and SWNT (B) in aqueous solution of 1. (c) Schematic
representation of graphene dispersion.
c
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analysis, and MALDI-TOF mass spectroscopy and was shown
to be in full agreement with the structure presented.
To investigate the capability of amphiphile 1 to exfoliate
graphite into graphene in aqueous solution, we prepared
graphene dispersions by sonicating
a mixture solution
(H2O/MeOH = 7 : 3 v/v) of graphite powder and amphiphile 1.
The resulting dispersions were centrifuged at 1300 rpm for
20 min and the supernatant containing graphene sheets was
collected, in which the concentration was measured to be
1.5 mg mLÀ1, a much higher concentration than those reported
for graphene sheets prepared by direct exfoliation.4,9 As
shown in Fig. 1b, sonication of graphite powder in amphiphile
1 solution afforded a dark black dispersion even after
centrifugation. Such a dispersion showed to be very stable
without any noticeable sedimentation and aggregation over a
period of 2 months. Remarkably, the comparative study with
SWNTs under the same conditions showed that the black
SWNT powder was not dispersed in amphiphile 1 solution
(Fig. 1b), indicating that 1 is unable to disperse SWNTs but
exfoliates selectively only 2-D graphene in aqueous solution.
Indeed, the flat conformation of the aromatic segment of 1
provides a large surface mismatch with the highly curved
SWNT to frustrate efficient p–p stacking interactions, thereby
unable to exfoliate curved carbon allotropes. This result
demonstrates that the 2-D planar conformation of the
aromatic segment in the molecular architecture is essential
for selective exfoliation of graphene through efficient p–p
stacking interactions.5
Fig. 3 (a) AFM image of 1-graphene sheet on SiO2/Si wafer after
spin-coating and annealing at 200 1C, (b) the height profile of the
AFM image, (c) TEM image from 1-graphene dispersion on a carbon
grid, (d) SEM image of 1-graphene dispersion on Si after spin-coating.
images showed that the sizes of the exfoliated graphene sheets
are 2–4 mm in diameter.
The direct exfoliation of graphite into graphene was further
evidenced by Raman scattering which is sensitive to the
electronic structure of graphene.12 Fig. 4 shows typical Raman
spectra of graphene sheets which are characterized by a
G-band at 1580 cmÀ1 and a 2D-band at 2680 cmÀ1. The most
prominent Raman feature is that the peak intensity of the
2D-band is much larger than the G-band (the intensity ratio,
G/2D = 0.3), indicating the exfoliation of graphite into single
layer and double layer sheets.12a In addition, the 2D-band
showed to be a single and symmetrical Lorentzian peak at
2680 cmÀ1, in contrast to that of HOPG that is split into two
different subpeaks, one small peak at 2678 cmÀ1 and the other
large peak at 2723 cmÀ1, which further confirms the single-
layer feature of graphene. These two Raman features demon-
strate that graphene in aqueous solution of 1 predominantly
consists of single- and double layer graphene sheets. The
electrical properties of dried thin films obtained from the
graphene dispersions were investigated by transparent resistance
measurements. The sheet resistance of the graphene film with
87% transparency is measured to be B5 kO sqÀ1 while that of
97% transparency is B34 kO sqÀ1, indicating much better
electrical properties than those reported for graphene sheets
The aromatic p–p stacking interactions between the pyrene
units of 1 and graphene were confirmed by UV absorption and
fluorescence spectra (Fig. 2). The absorption maximum of 1 in
the graphene dispersion at 325 nm showed to be red shifted
together with the strong hypochromic effect with respect to
that of the solution in the absence of graphene, indicative of
extended p-conjugation of the aromatic segment in the presence
of graphene. When the graphene dispersion was excited at
325 nm, the fluorescence intensity of 1 was significantly
suppressed because of fluorescence quenching, indicative
of strong p–p stacking interactions between the aromatic
segments and graphene surfaces.10
The dispersion of graphene by 1 in aqueous solution was
also investigated by AFM, TEM, and SEM measurements
(Fig. 3). The AFM image revealed graphene flakes composed
of a single layer sheet with a thickness of B2 nm, which is
consistent with heights for single layers reported previously.11
This value corresponds to the face-on arrangement of the
amphiphilic molecules on both sides of graphene sheets,
thereby forming a sandwich-like structure. TEM and SEM
Fig. 4 (a) Raman spectrum of the annealed (400 1C) 1-G sheet on
SiO2/Si after spin-coating and (b) the 2D single peak compared to
HOPG.
Fig. 2 (a) Absorption and (b) emission spectra of 1 and 1-G in
0.2 wt% (MeOH/H2O); lex = 340 nm.
c
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prepared in surfactant solutions and polymer solutions.4a,9a
We believe that this outstanding electrical property arises from
the exfoliation into defect-free graphene sheets, which will
provide applications of graphene for a transparent flexible
electrode.
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The results described here demonstrate that an aromatic
amphiphile based on a conformationally flexible aromatic
segment including four pyrene units readily exfoliates graphite
flakes into graphene sheets in aqueous solution with concen-
trations up to 1.5 mg mLÀ1. The graphene dispersions arise
from hydrophilic functionalization of graphene surfaces with
an aromatic amphiphile based on an oligoether dendron
through a combination of amphiphilicity and p–p stacking
interactions. On the other hand, curved carbon allotropes such
as SWNTs are unable to be dispersed in aqueous solution of 1,
demonstrating that the amphiphile selectively recognizes only
2-D graphene sheets. The graphene sheets of the film are
transparent and show a good electrical property as evidenced
by transparent resistance measurements. The most notable
feature of the aromatic amphiphile synthesized here is its
ability to selectively functionalize only 2D graphene sheets
among carbon allotropes with different shapes in aqueous
solution, thereby yielding water-soluble graphene. The selective
dispersions of graphene arise from its capability to adopt a
planar conformation on 2D graphene surfaces that provides
strong mismatch in shape for highly curved spherical and
tubular structures. We anticipate that our strategy provides
new opportunities for separation of graphene from carbon
allotropes and fabrication of graphene-based composite materials
that require aqueous environments such as cytocompatible
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We gratefully acknowledge the National Research Foundation
of Korea (NRF) grant funded by the Korean government
(MEST) (No. 2010-0019220) and U.S. Air force Office of
Scientific Research (FA 2386-10-1-4087). We acknowledge a
fellow of BK21 program from the Ministry of Education and
Human Resource Development. We thank Ji-Won Lee for
helping with Raman spectroscopy and Jung-Yeon Han for
measuring graphene film conductivity.
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c
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Chem. Commun., 2011, 47, 8259–8261 8261