One-step synthesis of monodisperse palladium nanosphere and their catalytic activity for Suzuki coupling reactions

Article abstract of DOI:10.1016/j.inoche.2011.06.006

This paper reports a facile synthesis for monodisperse palladium nanosphere with the diameter of 200 nm in the mixture solution of dimethyl sulfoxide (DMSO) and ethanol without using any other capping agent. It was found that the ratio of DMSO to ethanol is a key parameter in size and shape control. The as-prepared product in this report exhibit prominent catalytic activities in Suzuki coupling reactions and can be reused at least five times without loss of catalytic activity.

Full text of DOI:10.1016/j.inoche.2011.06.006

Inorganic Chemistry Communications 14 (2011) 1574–1578
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
One-step synthesis of monodisperse palladium nanosphere and their catalytic
activity for Suzuki coupling reactions
⁎
⁎
Lian-meng Wang, Shuai He, Zhi-min Cui , Lin Guo
School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing, 100191, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper reports a facile synthesis for monodisperse palladium nanosphere with the diameter of 200 nm in
the mixture solution of dimethyl sulfoxide (DMSO) and ethanol without using any other capping agent. It was
found that the ratio of DMSO to ethanol is a key parameter in size and shape control. The as-prepared product
in this report exhibit prominent catalytic activities in Suzuki coupling reactions and can be reused at least five
times without loss of catalytic activity.
Received 18 April 2011
Accepted 7 June 2011
Available online 22 June 2011
Keywords:
Palladium
© 2011 Elsevier B.V. All rights reserved.
Nanosphere
Catalysis
Suzuki reaction
Palladium nanomaterials have wide applications in many areas. For
example, they can serve as the hydrogen sensors and hydrogen storage
materials [1–3], excellent electrocatalysts in fuel cells [4–7]. Palladium is
also widely used as catalyst for low-temperature reduction of automobile
pollutants [8], and for petroleum cracking [9]. Among their extensive
applications, many applications of palladium nanomaterials are related
to its catalytic properties for C\C coupling reaction, such as Suzuki, Heck,
Stille, Negishi, Corriu-Kumada, Hiyama, and Sonogashira coupling
reactions [10–13].
The physical and chemical properties of nanomaterials are closely
related to their size and shape. In recent years, much effort has been
devoted to synthesize palladium nanoparticles with specific shapes. For
example, polycol reduction [14], seed-mediation growth [15], hydro-
thermal reaction [16], pyrogenation of precursor and electrical chemical
deposited [17,18], etc. Many novel properties and potential applications
would emerge frommonodisperse palladiumnanostructures withsmall
dimensions [19]. Therefore, the synthesis of uniform or monodisperse
palladium nanostructures has been intensively pursued for their
technological and fundamental scientific importance [17,20–23].
However, most of the current methods were focused on the crystallined
palladium nanoparticles limited to diameters below 10 nm. In addition,
the fine control of the size and shape of palladium nanoparticle via facile
synthetic method is still a challenge for researchers due to the existence
of the high cohesive energy between the palladium nanocluster.
Obtaining monodisperse palladium nanostructure with a diameter
larger than 100 nm is still difficult [24,25]. Moreover, palladium
nanostructures without capping agent on them are also desirable to
reduce the influence of surfactant in applications.
Herein, we present a facile and effective route for controllable
synthesis of monodisperse palladium nanosphere with the diameter of
200 nm in the mixture solution of DMSO and ethanol without using any
capping agent. The as-prepared monodisperse palladium nanosphere
shows satisfying catalytic activity toward Suzuki coupling reactions.
Moreover, the monodisperse palladium nanosphere can be easily
recovered from reaction mixture and can be reused without loss of
catalytic activity. With these superior properties, the monodisperse
palladium nanosphere would be promising candidates for applications
in catalytic industry.
All reagents and solvents for the synthesis and analysis were
commercially available and used as received. X-ray diffraction (XRD)
pattern of palladium nanosphere was carried out on a Rigaku,
Dmax2200 diffractometer equipped with a Cu Ka radiation source
(λ=1.54180 Å) for the structural determination. Microstructural
analyses were performed by using Hitachi S4800 cold field emission
scanning electron microscope (CFESEM) and a high resolution trans-
mission electron microscopy (HRTEM) (JEOL 2010F, with an energy
dispersive X-ray system).
In a typical synthesis of palladium nanosphere, 0.0224 g palladium
(II) acetate (J & K chemica) was first put into the bottom of three-neck
flask (equipped with a reflux condenser and a magnetic teflon-coated
stirring bar), and 13 ml of DMSO (Beijing chemical factory) was added
by intensive stirring to obtain a homogeneous solution at room
temperature. Then 17 ml of ethanol was added to the solution, the
reaction temperature was risen to 70 °C immediately and was kept for
1 h to make the reaction completely. The product was then collected by
centrifugation athigh speed (8000 rpm), and washed severaltimes with
⁎
Corresponding authors. Tel./fax: +86 10 82338162.
E-mail address: guolin@buaa.edu.cn (L. Guo).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.06.006

L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1574–1578
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ethanol. Eventually, the precipitate product was redispersed in ethanol
for further analysis.
nanoparticles) is single crystalline with lattice spacing of 0.224 nm,
which could be assigned to the (111) planes of fcc structured
palladium. In addition, HRTEM also indicates that the size of small
nanoparticles is about 5 nm and they are combined with each other
randomly to form the palladium nanosphere. The selected area
electron diffraction (SAED) of the palladium nanosphere further
confirms that it is crystallined. The set of rings can be indexed as the
fcc structured palladium combined with each other randomly to form
the final shape. The energy dispersive spectrometer (EDS) spectrum
in Fig. 3e shows that the nanosphere only contain Pd element (Cu
element in the EDS spectrum came from the copper grid), and it also
well agrees with the XRD result in Fig. 1 (no impure phases).
For Suzuki cross coupling reaction, 15 mg palladium nanosphere
catalyst, 0.5 mmol iodobenzene, 1 mmol phenylboronic acid, 1 mmol
K2CO3, and 0.5 mmol pentamethylbenzene (as internal standard for
high-performance liquid chromatography analysis) were added to
10 ml ethanol under stirring. The reaction was carried out at reflux (ca.
78 °C) for definite time. Then the mixture was separated quickly by
centrifugation, and the liquid was analyzed. The cycling performance of
the Pd catalyst for Suzuki cross-coupling reaction was carried out in the
same condition as mentioned above except that the catalyst was
recovered by centrifugation and washed with ethanol for 5 times for the
next cycle.
In a series of experiments, in order to reveal the formation process
of the palladium nanosphere, samples were collected at different
time intervals from the reaction mixture after the reaction temper-
ature reached 70 °C. An interesting morphology evolution process
was observed clearly. As shown in Fig. 4a, the sample was randomly
dispersed small nanoparticles with an average size of 3 nm (the
magnified image on the top right corner) in the first 5 min. At 8 min
(Fig. 4b), small palladium nanoparticles began to self-assemble into
nanosphere with the diameter of 50 nm. In this process, the
coexistence of small nanoparticles and nanosphere was observed
simultaneously. At 11 min (Fig. 4c), the palladium nanosphere grew
to big-size one with the diameter of about 160 nm. The small
nanoparticles self-assembled process almost completely and no
small nanoparticles were observed from the TEM image. At 14 min
(Fig. 4d), the monodisperse palladium nanosphere with 200 nm in
size was formed. According to the above analysis of the TEM image at
different intervals, the formation of the Pd nanosphere can be
described as follows: (1) At the initial time interval after the
temperature reached 70 °C, the color of the synthesis mixture
changed from light yellow to red gradually. At this stage, palladium
ions were reduced by ethanol to give free Pd clusters in the solution.
(2) With further reaction, the color of the mixture changed from red
to dark brown, indicating the concentration of palladium clusters has
reached the critical point and a homogeneous nucleation occurred
after the reduction of Pd ion to give the primary Pd nanoparticles.
(3) Because of the powerful reduction ability of ethanol, the reaction
was quickly dominated by thermodynamic growth. (4) At the same
time, the self-assembled process appeared in the solution, and the
small nanosphere grows to bigger one by depleting the palladium
nanoparticles as the time prolonged. This process was similar to the
reports from X. Bao group [26]. However, the size of palladium
nanomaterials prepared by their method can only reach 50 nm.
The ratio of DMSO to ethanol plays an important role in the
formation of Pd nanosphere. A series of specific ratios were set to
control the morphology of palladium nanostructure. At the ratio of
5:25 (Fig. 5a), nearly monodisperse palladium nanosphere was
presented, and the size of the nanosphere was about 160 nm. At the
ratio of 13:17 (Fig. 5b, typical reaction), the palladium nanosphere
was uniform with the size of about 200 nm. However, at the ratio of
18:12 (Fig. 5c), the monodispersity of palladium nanosphere became
worse, and the conglutination of palladium nanosphere was
observed clearly. When the ratio of DMSO to ethanol was enlarged
to 29:1(Fig. 5d), smaller palladium nanospheres with the diameter of
100 nm linked with each other to form nanochains. The correspond-
ing nanochain structure was clearly demonstrated by TEM image
(Fig. S1). In short, the experiment result demonstrated that the ratio
of DMSO to ethanol was key factor for tuning the size and
morphologies of palladium nanosphere.
In this study, palladium nanosphere was obtained by one-step
reaction in the mixture solution of DMSO and ethanol at 70 °C without
using any capping agent. The yield of palladium nanosphere was
nearly 96%. The X-ray diffraction (XRD) pattern of the as-synthesized
palladium nanosphere was shown in Fig. 1. The position of all
diffraction peaks matched well with the standard face centered cubic
(fcc) palladium crystal structure (JCPDS, card no. 05-0681). No impure
phases can be observed from the XRD pattern. In addition, the
diffraction peak was broadened due to small size of nanoparticles,
which further indicates that the palladium nanosphere was assem-
bled by small palladium nanoparticles.
The size and shape of the products were examined by scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM). Fig. 2a shows a typical large-scale SEM image of the as-produced
palladium nanosphere. The products are of spherical morphology and
have very narrow diameter distributions. The amplified image of
spherical palladium nanoparticles was shown in Fig. 2b. The surface of
palladium nanosphere is smooth and the average diameter of the
palladium nanosphere is measured to be 200 nm. The size distribution
histograms were inserted in Fig. 2a, the corresponding standard
deviation for palladium nanosphere diameter is calculated to be about
4.9%, which is below 5% and indicates that the as-produced palladium
nanosphere is monodisperse.
Fig. 3a shows a low-magnification TEM image of the palladium
nanosphere. It further confirms that the average diameter of solid
nanosphere is about 200 nm and the nanosphere has well monodis-
perse. Fig. 3b is the magnified TEM image of a single nanosphere, it
should be noted that the surface of the nanosphere is not as smooth
as the SEM image looks like, and large numbers of undulate fringe
construct the final spherical morphology. The selected position of
Fig. 3b (red frame) was enlarged in Fig. 3d and it shows that the
nanosphere is composed of small particles. The corresponding high-
resolution TEM image reveals that the primary building block (small
The palladium-catalyzed Suzuki cross-coupling reaction of aryl-
boronic acid and aryl halide is an effective synthetic route for biaryls
[15,27,28]. Herein, Suzuki coupling reaction of iodobenzene and
phenylboronic acid is adopted as a model reaction to measure the
catalytic ability of the palladium nanosphere catalyst in ethanol
solution under reflux condition. Fig. 6 shows that the palladium
Fig. 1. XRD pattern of the as-synthesized monodisperse palladium nanosphere.

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L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1574–1578
Fig. 2. (a–b) SEM images of the as-synthesized nanosphere with different magnifications, and the size distribution histograms (inset).
nanosphere is highly active for Suzuki reaction. After 40 min of
reaction time, nearly 100% conversion of iodobenzene was achieved.
This high activity was comparable with the hollow palladium
nanosphere under similar reaction condition reported by T. Hyeon
group [28]. The small palladium nanoparticles that build up to
nanosphere might be responsible for this high catalytic activity.
Another advantage of the palladium nanosphere catalyst is that it can
be recycled and reused at least five times without losing its catalytic
activity (Fig. 6). TEM image of the collected palladium nanosphere
catalyst after 5 reaction cycles shows that the nanospheres maintain
their spherical structure in spite of weakly aggregation (Fig. S2). This
result indicates palladium nanosphere prepared in our facile method
would be a promising candidate for applications in catalytic industry.
In summary, we have successfully developed a facile method to
synthesize monodisperse palladium nanosphere without employing
any capping reagent in the mixture solution of DMSO and ethanol.
High yield, low temperature, no surfactant as well as rapid reaction to
prepare monodisperse palladium nanosphere thereby provide new
Fig. 3. (a–b) TEM images of the as-synthesized nanosphere with different magnifications; (c) the corresponding SAED of single nanosphere; (d) the HRTEM of rough surface selected
from the red area in b; (e) the EDS of monodisperse nanosphere.

L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1574–1578
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Fig. 4. TEM images of the products at different stages of formation: (a) 5 min; (b) 8 min; (c) 11 min; (d) 14 min.
opportunities in a myriad of applications. In addition, the as-prepared
palladium nanosphere in this report exhibit good catalytic activities in
Suzuki coupling reactions and can be reused at least five times
without loss of catalytic activity. The morphology of the recycled
palladium nanosphere from the reaction solution remained almost an
unchanged benefit from the big size. Accordingly, monodisperse
Fig. 5. SEM images of the as-synthesized product with different ratios of DMSO to ethanol (a):5:25 (b):13:17 (c):18:12 (d): 29:1.

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