Gemini surfactant assisted synthesis of two-dimensional metal nanoparticles/graphene composites

Article abstract of DOI:10.1039/c2cc16890a

We demonstrate the synthesis of 2D metal nanoparticles (MNPs)/graphene nanocomposites using small cationic surfactants as stabilizers. 2D sandwich-like MNPs/graphene nanocomposites with a uniform distribution of MNPs can be achieved via a one-pot in situ growth and reduction protocol.

Full text of DOI:10.1039/c2cc16890a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 2119–2121
www.rsc.org/chemcomm
COMMUNICATION
Gemini surfactant assisted synthesis of two-dimensional metal
nanoparticles/graphene compositesw
a
a
a
a
a
Chunpeng Song, Dongqing Wu,* Fan Zhang, Ping Liu, Qinghua Lu* and
ab
Xinliang Feng*
Received 7th November 2011, Accepted 22nd December 2011
DOI: 10.1039/c2cc16890a
4
poly(methacrylic acid sodium salt) (PMAA), poly(diallyldimethyl
We demonstrate the synthesis of 2D metal nanoparticles
MNPs)/graphene nanocomposites using small cationic surfactants
5
ammonium chloride) (PDDA), and poly(N-vinyl-2-pyrrolidone)
(
6
(PVP) as stabilizers. However, the reproducibility of the resultant
as stabilizers. 2D sandwich-like MNPs/graphene nanocomposites
with a uniform distribution of MNPs can be achieved via a one-pot
in situ growth and reduction protocol.
2D composites is limited due to the intrinsic polydispersity of
polymers. Compared to the polymers as stabilizers, small organic
surfactants can possess prominent features such as well-defined
molecular structures, easily adjustable physical properties and
Two-dimensional (2D) nanocomposites built from assembly of
inorganic nanoparticles and graphene have aroused a great deal of
interest due to their potential applications in supercapacitors,
7
controllable supramolecular behavior in solution. Therefore, they
may not only effectively improve the dispersion of GO/RGO in
8
aqueous solution, but also give rise to controllable morphology of
1–3
sensing, and catalysis. Graphene sheets can not only serve as
a flat template for the anisotropic growth and decoration of
nanoparticles, but also prevent them from agglomeration.
In addition, the conductive graphene layer in 2D nanocomposites
can offer a platform for the fast transportation of charge carriers
when electrical conductivity is essentially required. In this respect,
chemically modified graphenes (CMGs), which usually refer to
graphene oxide (GO), reduced graphene oxide (RGO) and their
functional derivatives, are more appealing than pristine graphene
9–11
composites by tailoring with different molecular structures.
Nevertheless, the application of small organic surfactants for
directing the growth of MNPs on graphene surface has been
scarcely reported.
Herein, we demonstrate the synthesis of 2D MNPs/graphene
nanocomposites using small cationic surfactants as stabilizers.
Typically, cetyl trimethylammonium bromide (CTAB) and cationic
gemini surfactant, bis(cetyldimethylammonium) butane dibromide
(Gem), which can be regarded as the dimer of CTAB coupled with
an alkyl spacer, were investigated in this work. It turns out that
both CTAB and Gem show strong ionic interactions with GO,
whereas the GO/Gem has better dispersibility in aqueous solution.
Further, GO/Gem displays lower surface roughness than that of
GO/CTAB, suggesting that Gem can be more homogenously
functionalized on the GO surface ascribable to the improved
3
because of their easy accessibility and processability. While the
simple physical blending of CMGs and metal nanoparticles
(MNPs) or the reduction of metal precursors in the presence of
GO/RGO suffers from poor structural control of nanocomposites
2
formation, the surface modification of GO/RGO may provide
powerful means to fabricate composites with hierarchical super-
structures by taking advantage of the improved compatibility
between these components.
1
2
amphiphilic property and interface behavior of Gem. As a result,
D sandwich-like RGO/Gem/GNPs nanocomposites with a
2
GO and RGO are well-known for the negatively charged nature
due to the presence of abundant oxygen-containing groups on
their basal planes and edges. Functional polymers are often
utilized to modify the surface of GO/RGO through electrostatic
or hydrophobic interactions, and in this way to promote their
assembly with inorganic species. For instance, 2D nanocomposites
of RGO and multifarious MNPs including gold nanoparticles
uniform distribution of GNPs can be achieved via a one-pot
in situ growth and reduction protocol. In addition, graphene-based
2D nanocomposites with various MNPs such as PtNPs and PdNPs
can be prepared, suggesting the general universality of the current
approach.
GO was prepared from natural graphite flakes by using a
13
modified Hummers method, and the Gem was synthesized
(GNPs), platinum nanoparticles (PtNPs) and palladium nano-
particles (PdNPs) have been produced with assistance of
1
4
according to the literature. To examine the electrostatic inter-
action between cationic surfactant and GO, the zeta potential of
ꢀ1
GO/CTAB and GO/Gem suspension with 0.3 mg mL of GO
and at different surfactant concentrations was measured. As
displayed in Fig. 1, for both GO/Gem and GO/CTAB, with
the increasing concentration of surfactant, the increase of zeta
potential tends to be exponential. At a relatively low concen-
tration of surfactant (o1 mM), the zeta potential increases
rapidly from negative to positive, indicating that the surfactant
a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong
University, 800 Dongchuan Road, Shanghai, 200240, P. R. China.
E-mail: wudongqing@sjtu.edu.cn, qhlu@sjtu.edu.cn,
feng@mpip-mainz.mpg.de
b
Max Planck Institute for Polymer Research, Ackermannweg 10,
5
5128, Mainz, Germany
w Electronic supplementary information (ESI) available. See DOI:
0.1039/c2cc16890a
1
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2119–2121 2119

View Online
The samples were prepared by spin coating the dispersion on silica
wafer (see ESIw). The average thickness of GO is about 0.92 nm
Fig. 2d), whereas it increases to 1.44 nm for GO/Gem with a
(
slightly increased roughness (Fig. 2e). This result suggests
that Gem is adsorbed on GO in the form of a molecular layer
with uniform surface coverage (Scheme S1, ESIw). Differently,
GO/CTAB exhibits a surface roughness of tens of nanometres,
typical for a strong aggregation of CTAB on the GO surface
(
Fig. S2, ESIw). We further noted that even at a low concentration
Fig. 1 Chemical structure of Gem and CTAB used in this work (left)
and non-linear fitting of the zeta potential (right) of GO/Gem (black) and
GO/CTAB (red) suspension at different concentrations of surfactant.
of CTAB (less than 0.5 mM), the strong aggregation of CTAB on
GO was not avoidable. Therefore, different self-assembly behavior
must take place between CTAB and Gem with GO under
experimental conditions. In this regard, different surface morphol-
ogy of functionalized GO is generated and should exert profound
influence on the fabrication of 2D MNPs/graphene composites
when additional metal salts are involved during the synthesis
process (see below).
molecules can be effectively adsorbed on the GO surface, thus
modifying its charge density. It is noteworthy that at low concen-
tration (o1 mM), the zeta potential of GO/Gem increases more
rapidly than that of GO/CTAB (reflected by its large slope). This
implies a high interfacial activity of Gem and pronounced assem-
bly with GO with respect to CTAB. When the concentration of
surfactants increases to 1 mM, the zeta potential reaches its
saturated value and an obvious plateau is observed. This result
indicates that the GO surface is fully covered with cationic
molecules in both cases. In addition, the saturated zeta potential
of GO/Gem is about twice higher than that of GO/CTAB, further
emphasizing the exceptional amphiphilic property of Gem on
GO/Gem, GO/CTAB and GO were subsequently subjected to
the preparation of RGO/surfactant/GNPs respectively (Scheme S1,
ESIw). In a typical experiment, the aqueous dispersion of GO
ꢀ
1
(0.3 mg mL ) was added dropwise to the water solution of CTAB
or Gem (1 mM) with ultrasonication, which yielded a light-yellow
homogenous suspension. Afterwards, 2.7 mL of GO/Gem or
GO/CTAB suspension was added into 20 mL aqueous solution
1
2
of HAuCl (1 mM) and the mixture was stirred at 80 1C for 30 min.
Finally, 4.5 mg of sodium borohydride (NaBH ) was added and
4
solid surface. z Therefore, the Gem with two cationic units may
modify the GO surface in a more favourable way.
4
the mixture was stirred for another 120 min at the same tempera-
ture. In this way, NaBH could serve as reductant for both GO and
Fourier transform infrared spectroscopy (FTIR) was conducted
for GO/CTAB and GO/Gem in order to gain the insight of ionic
interactions between GO and the surfactants (Fig. 2a).
The presence of CQO in carboxylic acid moieties (nCQO at
4
metal precursor. Remarkably, the resultant RGO/Gem/GNPs
could be well dispersed in water, while RGO/CTAB/GNPs showed
strong aggregation and were precipitated from aqueous solution.
The morphology of the resultant composites, RGO/Gem/
GNPs and RGO/CTAB/GNPs, was inspected by means of
transmission electron microscopy (TEM). As shown in the
TEM images of RGO/Gem/GNPs at different magnifications
ꢀ1
ꢀ1
1
740 cm ), C–OH (nC–OH at 1385 cm ) and C–O–C (nC–O at
ꢀ
1
250 cm ), of GO was confirmed, and the peak at 1630 cm
ꢀ1
1
could be ascribed to the skeletal vibration of graphitic domains.
For both GO/Gem and GO/CTAB, a new peak appears at
ꢀ
1
1
410 cm , attributable to the symmetric (n
s
) vibration of the
ꢀ
+
(
Fig. 3a and b), small GNPs are uniformly distributed on the
COO group. The original NR
4
asymmetric bending peak of
ꢀ1
ꢀ1
RGO surface. The size distribution analysis (Fig. S3, ESIw) of
particles reveals that GNPs possess an average size of about
Gem (1611 cm ) shifts to 1631 cm (overlapped by the peak of
8
graphitic domain vibration) for GO/Gem. As a result, it is
1
4 nm. In contrast, the GNPs show a severe aggregation on the
surface of RGO/CTAB, with a size ranging from 5 to 30 nm
Fig. S4b, ESIw). This can be due to the two head groups of Gem
reasonable to believe that most of the carboxylic groups in GO
have been functionalized with quaternary ammonium moieties
8
(
through ionic interactions.
The surface morphology of GO, GO/CTAB and GO/Gem was
then inspected by atomic force microscopy (AFM) (Fig. 2b and c).
leading to strong interactions with nanoparticles, and the remaining
tail chains forming a confinement environment to protect the
15
nanoparticles from aggregation. For comparison, RGO/GNPs
without using surfactants were also prepared under the same
experimental conditions (Fig. S4c and d, ESIw), in which the GNPs
exhibit irregular shape and strong aggregation on certain areas of
RGO surface. By using H PtCl and K PdCl as precursors, 2D
2
6
2
4
RGO/surfactant/PtNPs and RGO/surfactant/PdNPs were prepared
via a similar procedure. TEM images of 2D RGO/Gem/PtNPs
(Fig. 3c) and RGO/Gem/PdNPs (Fig. 3d) demonstrate that the
worm-like PtNPs with an average size of about 8 nm (Fig. S3, ESIw)
and PdNPs with an average size of about 7 nm (Fig. S3, ESIw) are
uniformly deposited on RGO, whereas severe aggregation of PtNPs
(
mostly larger than 20 nm, Fig. S4e (ESIw)) and PdNPs (with an
Fig. 2 (a) FTIR spectra of GO (black), Gem (red), GO/CTAB (cyan)
and GO/Gem (blue). AFM images of GO (b) and GO/Gem (c). (d)
The cross section identified by the line in Fig. 2b showing the height of
individual GO; (e) the cross section identified by the line in Fig. 2c
showing the height of individual GO/Gem.
average size of about 16 nm, Fig. S4f (ESIw)) can be found in RGO/
CTAB/PtNPs and RGO/CTAB/PdNPs. These results further
indicate that the effective modification of GO with Gem is crucial
to achieve RGO/MNPs composites with uniform deposition of
2
120 Chem. Commun., 2012, 48, 2119–2121
This journal is c The Royal Society of Chemistry 2012

View Online
3+
2+
redox of Fe /Fe . It is clear that the CV wave current densities
of the modified GCE are higher than those of the bare GCE
(Fig. S7, ESIw). In particular, the RGO/Gem/GNPs modified
GCE shows the highest CV wave current intensity (72 mA) and it
is interesting to note that the highest current intensity (50 mA) of
RGO/CTAB/GNPs modified GCE is lower than that (59 mA) of
RGO/GNPs. This result suggests that the electrochemical perfor-
mance of 2D composites depends on the distribution of GNPs on
the surface of RGO. Apparently, the poor electrocatalytic activity
of RGO/CTAB/GNPs can be ascribed to the strong aggregation
1
8
of GNPs, which may reduce the accessible surface area of
GNPs. On the contrary, in the case of RGO/Gem/GNPs, the
uniform deposition of GNPs with smaller size leads to an effective
utilisation of the active surface of GNPs and the conductivity
of RGO.
In summary, a novel protocol to prepare 2D MNPs/graphene
composites assisted by gemini surfactants is presented. We expect
that the morphology and the distribution of the MNPs on
graphene sheets can be further controlled by employing diversified
cationic surfactants such as gemini surfactants with different
spacer and hydrophobic tails as well as bola-type surfactants.
Such work is now underway in our lab.
Fig. 3 (a and b) TEM images of RGO/Gem/GNPs at different
magnifications, (c and d) TEM images of RGO/Gem/PtNPs and
RGO/Gem/PdNPs, respectively.
This work is supported in part by the 973 Program of China
(
2012CB933404, 2012CB933803 and 2009CB930403), National
Natural Science Foundation of China (21174083, 21102091,
0925310 and 51173103), BASF, Shanghai Pujiang Program
11PJ1405400) and Shanghai Outstanding Academic Leaders
11XD1403000).
MNPs and that the present strategy can indeed be universal to
fabricate graphene based 2D composites with different MNPs.
The composites of RGO/Gem/MNPs were further analyzed
by X-ray diffraction (XRD, Fig. S5 (ESIw)) and X-ray photo-
electron spectroscopy (XPS, Fig. S6 (ESIw)), respectively.
Taking RGO/Gem/GNPs as the example, the XRD pattern
shows obvious peaks at 38.11, 44.41, 64.61, 77.51 and 81.91,
corresponding to the (111), (200), (220), (311) and (222)
diffraction planes, respectively, indicating that GNPs exist in
the form of crystalline state. Meanwhile, the crystallite size of
GNPs calculated from the Scherrer formula (see ESIw) is in
accordance with the TEM result. In addition, the absence of
diffraction peak at around 251 suggests that the typical
5
(
(
Notes and references
z It is noteworthy that the zeta potential of Gem is lower than that of
CTAB in aqueous solution (Fig. S1, ESIw).
1
2
P. V. Kamat, J. Phys. Chem. Lett., 2010, 1, 520–527.
H. Bai, C. Li and G. Q. Shi, Adv. Mater., 2011, 23, 1089–1115.
3 D. Q. Wu, F. Zhang, P. Liu and X. L. Feng, Chem.–Eur. J., 2011,
17, 10804–10812.
4 S. J. Guo, D. Wen, Y. M. Zhai, S. J. Dong and E. K. Wang,
ACS Nano, 2010, 4, 3959–3968.
1
6
p-stacking of graphene sheets has been efficiently prevented
due to the decoration of GNPs on graphene in a 2D sandwich
5
6
7
Y. X. Fang, S. J. Guo, C. Z. Zhu, Y. M. Zhai and E. K. Wang,
Langmuir, 2010, 26, 11277–11282.
Q. Wu, Y. X. Xu, Z. Y. Yao, A. R. Liu and G. Q. Shi, ACS Nano,
2010, 4, 1963–1970.
R. Oda, I. Huc and S. J. Candau, Chem. Commun., 1997, 2105–2106.
1
7
manner. This should also be responsible for the good
dispersibility of RGO/Gem/GNPs in water. The C1s XPS
pattern of GO (Fig. S6a, ESIw) indicates the presence of two
main types of carbon bonds: C–C at 284.6 eV and C–O (epoxy
and alkoxy) at 286.6 eV, which are consistent with the
literature report. For RGO/Gem/GNPs (Fig. S6b, ESIw), the
peaks associated with C–C (284.6 eV) become predominant
and the intensity of oxidized carbon species (C–O at 286.6 eV)
is greatly weakened. These features are typical for an efficient
8 Y. Y. Liang, D. Q. Wu, X. L. Feng and K. Mullen, Adv. Mater.,
¨
2009, 21, 1679–1683.
9
S. B. Yang, X. L. Feng, L. Wang, K. Tang, J. Maier and
K. Mullen, Angew. Chem., Int. Ed., 2010, 49, 4795–4799.
0 Q. Su, S. P. Pang, V. Alijani, C. Li, X. L. Feng and K. Mullen, Adv.
Mater., 2009, 21, 3191–3195.
¨
1
1
1
¨
1 Y. X. Xu, H. Bai, G. W. Lu, C. Li and G. Q. Shi, J. Am. Chem.
Soc., 2008, 130, 5856–5857.
2 F. M. Menger and J. S. Keiper, Angew. Chem., Int. Ed., 2000, 39,
4
reduction of GO to RGO. In addition, a significant Au4f
signal corresponding to the binding energy of Au (Fig. S6c, ESIw)
is shown up. This supports the conclusion that the GNPs have
been effectively assembled on the RGO.
1
906–1920.
13 W. S. Hummers and R. E. Offerman, J. Am. Chem. Soc., 1958,
0, 1339.
4 R. Zana, M. Benrraou and R. Rueff, Langmuir, 1991, 7,
072–1075.
15 L. Zhong, T. F. Hao and M. H. Liu, Langmuir, 2008, 24, 11677–11683.
8
1
The electrochemical behavior of different GNPs/graphene
composites was evaluated in potassium ferrocyanide solution.
The cyclic voltammograms (CV) of a bare glassy carbon electrode
1
1
6 Z. H. Tang, S. L. Shen, J. Zhuang and X. Wang, Angew. Chem.,
Int. Ed., 2010, 49, 4603–4607.
7 Z. L. Hu, M. Aizawa, Z. M. Wang, N. Yoshizawa and H. Hatori,
Langmuir, 2010, 26, 6681–6688.
18 M. C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.
(GCE) and RGO/CTAB/GNPs, RGO/Gem/GNPs and RGO/
1
GNPs modified GCE were compared (Fig. S7, ESIw). Each CV
cycle shows a couple of oxidation and reduction waves due to the
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2119–2121 2121