Gold nanoparticles-graphene hybrids as active catalysts for Suzuki reaction

Article abstract of DOI:10.1016/j.materresbull.2010.06.041

Graphene was successfully modified with gold nanoparticles in a facile route by reducing chloroauric acid in the presence of sodium dodecyl sulfate, which is used as both a surfactant and reducing agent. The gold nanoparticles-graphene hybrids were characterized by high-resolution transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction and energy X-ray spectroscopy. We demonstrate for the first time that the gold nanoparticles-graphene hybrids can act as efficient catalysts for the Suzuki reaction in water under aerobic conditions. The catalytic activity of gold nanoparticles-graphene hybrids was influenced by the size of the gold nanoparticles.

Full text of DOI:10.1016/j.materresbull.2010.06.041

Materials Research Bulletin 45 (2010) 1413–1418
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Gold nanoparticles–graphene hybrids as active catalysts for Suzuki reaction
Yang Li, Xiaobin Fan *, Junjie Qi, Junyi Ji, Shulan Wang, Guoliang Zhang, Fengbao Zhang
School of Chemical Engineering & Technology, Tianjin University, Weijin Road, Tianjin 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Graphene was successfully modified with gold nanoparticles in a facile route by reducing chloroauric
acid in the presence of sodium dodecyl sulfate, which is used as both a surfactant and reducing agent. The
gold nanoparticles–graphene hybrids were characterized by high-resolution transmission electron
microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray
diffraction and energy X-ray spectroscopy. We demonstrate for the first time that the gold
nanoparticles–graphene hybrids can act as efficient catalysts for the Suzuki reaction in water under
aerobic conditions. The catalytic activity of gold nanoparticles–graphene hybrids was influenced by the
size of the gold nanoparticles.
Received 10 February 2010
Received in revised form 12 June 2010
Accepted 15 June 2010
Available online 18 June 2010
Keywords:
A. Metal
A. Nanostructure
B. Chemical synthesis
D. Catalytic properties
ß 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
Previously, we found that deoxygenation of graphene oxide
(
GO) occurred under alkaline conditions, providing a green and
Graphene is a flat monolayer of carbon atoms tightly packed
cost effective way to prepare graphene on a large-scale [22]. Here,
we developed a facile route to homogeneously disperse Au
into a honeycomb lattice. It continues to attract immense interest
in fundamental physics and for its potential applications after
experimentally isolated in 2004, mostly due to the unusual
electronic properties and effects that arise from its one-atom
thickness [1]. It may be a promising alternative to carbon
nanotubes in polymer nanocomposites [2,3]. It could also be
applied in nanoelectronics as an insulating material or semicon-
ductor [1,4]. In addition, due to its low-cost, large specific surface
area and strong interactions with metal clusters, graphene is a
promising material for catalytic applications [5,6]. The dispersion
of metal nanoparticles on graphene sheets potentially provides a
new way to develop catalytic materials [7]. However, only a few
metal nanoparticles–graphene hybrids have been reported [8,9],
and fewer studies have investigated their catalytic properties.
Gold (Au) nanoparticles have attracted considerable attention
in recent years. Their catalytic applications in heterogeneous
organic reactions are particular attractive. These reactions are
traditionally carried out with homogeneous transition metal
catalysts [10–16]. The Suzuki reaction, which is predominately
catalyzed by palladium (Pd) catalysts, is a powerful and convenient
synthetic method in organic chemistry for the generation of
biaryls, conducting polymers, and liquid crystals [17–21]. Consid-
ering Au(I) is isoelectronic with Pd(0), Au nanoparticles may also
catalyze the Suzuki reaction.
nanoparticles with
a small size distribution on chemically
modified graphene in water. We also demonstrated for the first
time that Au nanoparticles (as Au–graphene hybrids) can be used
as efficient catalysts for the Suzuki reaction of aryl halides with
arylboronic acids.
2. Experimental
2.1. Synthesis of Au–graphene hybrids
Graphite oxide was prepared by Hummer method [23] and
exfoliated into GO by sonication in water [24]. The prepared GO
was heated in a sodium hydroxide (NaOH) solution to obtain a
suspension of reduced graphene. Then, 40 mL sodium dodecyl
sulfate (SDS, 0.1 M) aqueous solution was added into 15 mL of the
À1
reduced graphene suspension (ꢀ6 mg mL ) and sonicated for
4
5 min. Subsequently, chloroauric acid (HAuCl ) (0.02 M) was
added to this mixture, and it was refluxed at 100 8C with magnetic
stirring for 6 h. After cooling to room temperature, the mixture was
then washed extensively with deionized water and centrifuged
several times, in order to remove excessive surfactants.
2.2. Suzuki reaction catalyzed by Au–graphene hybrids
Iodobenzene (204 mg, 1.0 mmol) was added into a stirred
mixture of NaOH (160 mg), phenylboronic acid (146 mg,
*
Corresponding author. Fax: +86 22 27408778.
E-mail address: xiaobinfan@tju.edu.cn (X. Fan).
2
1.2 mmol) and deionized water (H O) (20 mL). This was followed
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.06.041

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Y. Li et al. / Materials Research Bulletin 45 (2010) 1413–1418
Scheme 1. Suzuki reaction using Au–graphene as the catalyst.
[(Fig._1)TD$FIG]
Fig. 1. AFM image (a) of modified graphene, deposited on freshly cleaved mica substrates, the bright ‘‘spots’’ are the Au nanoparticles deposited on the graphene, and the insert
image of cross-section analysis, as well as corresponding 3D image and (b) of Au–graphene hybrids.
[(Fig._2)TD$FIG]
Fig. 2. TEM image (a) and corresponding EDX result (b) of Au nanoparticles (average diameter of ꢀ3 nm) homogeneously deposited on graphene. The Cu peaks in EDX come
from the copper grid. Enlarged HRTEM image (c) of Au nanoparticles with a clear lattice of 0.238 nm.

Y. Li et al. / Materials Research Bul
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by the addition of Au–graphene hybrids at a mole fraction of 1%.
This reaction is illustrated in Scheme 1. The mixture was then
stirred at 100 8C in an oil bath for 4 h and extracted with ethyl
acetate (3 Â 20 mL). The combined organic extract was dried over
anhydrous sodium sulfate (Na
2 4
SO ), and the resulting mixture was
analyzed by gas chromatography (GC).
2.3. Characterization
The Au–graphene hybrids were characterized by high-resolu-
tion transmission electron microscopy (HRTEM) (Philips Tecnai G2
F20), atomic force microscopy (AFM) (CSPM 5000), energy X-ray
spectroscopy (EDX) (Philips Tecnai G2 F20), X-ray photoelectron
spectroscopy (XPS) (Perkin-Elmer, PHI 1600 spectrometer) and
Raman spectroscopy (NT-MDT NTEGRA Spectra). The catalytic
activity of the Au–graphene hybrids was measured by gas
chromatography (GC) on an Agilent 6890N GC-FID system.
3
. Results and discussion
3.1. Synthesis of Au–graphene hybrids
The purified Au–graphene hybrids show impressive stability in
water, as no precipitation is observed in the suspension for months
Fig. 2S, Supporting Information). The stability of the hybrids in
(
water can be attributed to the resulted dodecanoic acid, which is
generated from thermally decomposed SDS and acts as an
excellent stabilizer. Dodecanoic acid and the sulfonic groups not
only provide stability, but also contribute to the homogeneous and
sufficient dispersal of Au nanoparticles on graphene sheets. The
residual sulfonic groups of SDS (Fig. 3S, Supporting Information)
adsorbed on the Au–graphene hybrids should extend to the
solution and provide electrostatic repulsions, which stabilize the
suspension [25]. They also behave as active sites for the adsorption
of Au ions and as the nucleation centers for Au nanoparticles.
Additionally, the pH of the reaction solution in our experiment is
8
.5, and the alkalinity facilitates formation of smaller particles with
a faster nucleation rate [26].
An AFM image of the Au–graphene hybrids (Fig. 1) shows that
monodispersed nanoparticles are homogenously deposited on the
surface of most graphene sheets. No free nanoparticle is observed
outside the graphene sheets even after intensive sonication during
the preparation of the sample for AFM characterization. It may
demonstrate the strong interaction between the sheets and the
nanoparticles (cross-section analysis in Fig. 2S, Supporting
Information). The homogenous deposition of Au nanoparticles
on graphene is further confirmed by HRTEM (Fig. 2(a)). The
corresponding EDX result (Fig. 2(b)) shows that Au nanoparticles
are abundant in the hybrids. An enlarged image of the Au
nanoparticles clearly shows the typical lattice of crystalline gold
Fig. 3. Raman spectra of graphene oxide (a), graphene (b) and Au–graphene hybrids
c).
(
nanotubes [31,32]. This result therefore suggests a chemical
interaction or bond between the Au nanoparticles and graphene
[30,31]. As the graphene sheets in this study are obtained by
chemical reduction of GO, they likely contain many carbon
vacancies and defects [9], which may enhance the interaction
between Au nanoparticles and graphene [33,34]. Moreover, the G
À1
bands of graphene and Au–graphene hybrids (1622 cm ) are
À1
(
Fig. 2(c)). The measured regular d-spacing of about 0.238 nm in
shifted to lower frequency compared with that of GO (1629 cm ).
this image corresponds to the {1 1 1} planes of gold [27]. The XRD
pattern also confirms the formation of the Au nanoparticles
This could probably be explained by an alternating pattern of
single-double carbon bonds within sp carbon ribbons during the
2
(
Fig. 4S, Supporting Information).
reactions [35].
Raman spectra of GO, graphene and the Au–graphene hybrids
The size of the Au nanoparticles deposite on the surface of
graphene sheets can be adjusted by varying the [Au]/[graphene]
mass ratio. Because no free metal nanoparticle appeares outside
the graphene sheets in the AFM and HRTEM images obtained from
different regions of samples, the amount of reactants used in the
synthesis could be used to calculate the mass ratio of [Au]/
[graphene]. The result from this calculation is similar to that from
EDX elemental analysis. Fig. 4 shows the HRTEM images and the
associated nanoparticle size distributions. When the mass fraction
of Au nanoparticles is 8%, the average diameter of the Au
nanoparticles deposited on the graphene is 2–3 nm with a
relatively small distribution (Fig. 4(a) and (b)). In comparison,
À1
are shown in Fig. 3. The two bands at about 1358 cm
622 cm correspond to the disorder-induced D band and the in-
and
À1
1
phase vibration of the graphene lattice (G band), respectively [28].
In line with other research groups’ studies on chemically reduced
graphene [29], the I(D)/I(G) intensity ratio of graphene (I(D)/
graphene
I(G)
= 2.06) in Fig. 3(b) is slightly higher than that of GO
GO
(
I(D)/I(G) = 1.99) (Fig. 3(a). This result may reflect the reduction
in size of the graphene [29]. The I(D)/I(G) intensity ratio of Au–
Au–graphene
graphene (I(D)/I(G)
= 2.22, Fig. 3(c) is much larger than
that of graphene. Such an enhancement is also observed when
other metal nanoparticles are deposited on GO [30] and carbon

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Y. Li et al. / Materials Research Bulletin 45 (2010) 1413–1418
Fig. 4. HRTEM image (a) and size distribution (b) of Au nanoparticles (average diameter is ꢀ3 nm) on graphene, produced using a mass fraction of 8%. HRTEM image (c) and
size distribution (d) of Au nanoparticles (average diameter is 7.5 nm) on graphene, produced using a mass fraction of 21%. The scale bars in (a) and (c) are 50 nm and 0.2
respectively.
mm,
the average diameter of the Au nanoparticles increases to about
.5 nm when the mass fraction is 21% (Fig. 4(c)). This result is
Optimum conversion and selectivity are both obtained using a
7
mole fraction of 1.0% of Au–graphene catalysts in a reaction for 4 h
at 100 8C. The reactions are also carried out under similar
condition with Au nanoparticles (synthesis in Scheme 1S,
Surpporting Information) as catalysts. The conversion (55.7%)
and selectivity (27.3%) are much lower than with Au–graphene
catalysts. The graphene sheets can stabilize the supported Au
nanoparticles against aggregation, resulting in better catalytic
activity than pure Au nanoparticles.
As a comparison, the reaction is also performed with palladium
(II) acetate, which is commonly used to catalyze the Suzuki
reaction [43]. Its conversion and selectivity under the same
conditions are 79.3% and 85.6%, respectively. This result indicates
the catalytic activity of the Au–graphene hybrid is comparable to
the commercial palladium (II) acetate. Further study should be
carried out to improve the catalytic activity of the Au–graphene
hybrid.
consistent with previous studies on metallic nanoparticles, where
nanoparticle size was easily controlled by the amount of
corresponding precursors [36]. The sizes distributions of the Au
nanoparticles are further confirmed by UV–vis analysis (Fig. 7S,
Supporting Information).
As shown in the XPS spectra (Fig. 5), the binding energy (4f7/2)
of the smaller Au nanoparticles (2–3 nm) on graphene (Fig. 5(a)) is
obviously blue shifted by 0.6 eV compared with the larger Au
nanoparticles (ꢀ7.5 nm) (Fig. 5(b)). This energy shift could be
interpreted as the effect of cluster size on core level binding energy
[
37]. An similar size effect has been also reported in Au
nanoparticles on another catalyst carrier [38]. It has been
hypothesized that the size-dependent changes in electronic
structure may cause unusual catalytic properties [39].
3
.2. Suzuki reaction catalyzed by Au–graphene hybrids
The Suzuki reactions are carried out with different Au–
graphene catalyst samples. Table 1 summarizes the performance
of these Au–graphene catalysts in water at 100 8C. The results
confirm that the ꢀ7.5 nm Au nanoparticles supported on graphene
The catalytic activity of the Au–graphene hybrids is explored in
the Suzuki reaction of iodobenzene with phenylboronic acid to the
form carbon–carbon bond of biaryls (Scheme 1). These reactions
are generally carried out under an inert atmosphere in a mixture of
organic solvent and aqueous inorganic base [40]. However, recent
studies have indicated that the presence of water is required for the
Suzuki reaction [41,42]. In this study, we investigate the reaction
with Au–graphene catalyst in air with water as the solvent, to
develop a green and low-cost strategy.
Table 1
Catalytic results of different Au–graphene hybrids in the Suzuki reaction under
same reaction conditions.
a
Catalyst
Conversion (%)
Selectivity (%)
8 wt% Au–graphene (2–3 nm)
21 wt% Au–graphene (ꢀ7.5 nm)
76.5
59.8
85.8
11.2
To optimize the reaction condition, a series of experiments
with different qualities of Au–graphene catalysts have been
carried out as illustrated in Fig. 8S (Surpporting Information).
a
2
Iodobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), NaOH (4 mmol), H O
(20 mL), Au–graphene hybrids (1 mol%), 100 8C, 4 h.

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Table 3
Suzuki coupling of allyl iodide and bromobenzene with phenylboronic acid
[
D
I
L
N
]
.
a
Reactant
Conversion (%)
Selectivity (%)
Bromobenzene
Allyl iodide
54.2
40.3
26.1
26.0
a
2
Reactant (1.0 mmol), phenylboronic acid (1.2 mmol), NaOH (4 mmol), H O
(20 mL), Au–graphene hybrids (2–3 nm Au nanoparticles, 1 mol%), 100 8C, 4 h.
4. Conclusion
A simple, economical and environmentally friendly process was
developed to homogeneously deposit Au nanoparticles with
controlled sizes on graphene sheets. The catalytic activity of the
hybrids was investigated in the Suzuki reaction for the first time.
The Au–graphene catalyst showed impressive performance even
when the reaction was carried out in water under aerobic
conditions. This study suggests graphene, as an economical
substitute for carbon nanotubes, could act as a prominent support
in heterogeneous catalysis. These catalysts were easily recovered.
Furthermore, this study could be extended to other type of
graphene hybrids.
Acknowledgements
The author would like to acknowledge the support of this work
by National Natural Science Foundation of China (20776095) and
Programme of Introducing Talents of Discipline to Universities
(No: B06006).
Fig. 5. Au 4f electron region of the X-ray photoelectron spectrum of Au–graphene
hybrids produced with a mass fraction 8% (a) (average diameter of Au nanoparticles
is 2–3 nm) and the corresponding spectrum of large Au nanoparticles (about
7
3
.5 nm) on graphene (b). The binding energy of the smaller Au nanoparticles (2–
nm) on graphene shows an obvious blue shift of 0.6 eV compared with the result
for the larger Au nanoparticles.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.materresbull.2010.06.041.
(
mass fraction of 21%) are less catalytically active than ꢀ3 nm Au
nanoparticles (mass fraction of 8%). The increased catalytic activity
of the smaller nanoparticles deposited on graphene may be
attributed to more active sites on their surfaces. This result is
consistent with previous studies [44,45], which highlighted the
important role that the size of metal catalysts plays in C–C bond
forming reactions such as the Suzuki reaction. The ability to
precisely control the size of the Au nanoparticles formed on
graphene is therefore an advantage of our method.
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