N,N-Bis(2-hydroxyethyl)-2-Aminoethanesulfonic Acid-Assisted Liquid-phase Growth of Au@Pd coreshell nanoparticles with high catalytic activity

Article abstract of DOI:10.1246/cl.150600

Au@Pd coreshell nanoparticles were successfully synthesized via sequential reduction of Au(III) and Pd(II) ions with N,N-bis(2-hydroxyethyl)-2-Aminoethanesulfonic acid (BES) at room temperature. Their size and morphology could be changed by varying the BES concentration and pH value of the reaction system. The Au@Pd coreshell nanoparticles exhibited significantly higher catalytic activity for various Suzuki reactions than monometallic Pd or Au nanoparticles. The size-dependent catalytic activity was also observed, i.e., the Au@Pd coreshell nanoparticles of <10nm showed higher activity.

Full text of DOI:10.1246/cl.150600

Received: June 23, 2015 | Accepted: July 26, 2015 | Web Released: October 5, 2015
CL-150600
N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic Acid-assisted Liquid-phase Growth
of Au@Pd Core­Shell Nanoparticles with High Catalytic Activity
Wei Zhang, Huiping Zhao, Zhong Lu, Fengxi Chen,* and Rong Chen*
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, P. R. China
(
E-mail: rchenhku@wit.edu.cn, fxchen@wit.edu.cn)
Au@Pd core­shell nanoparticles were successfully synthe-
sized via sequential reduction of Au(III) and Pd(II) ions with
N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) at
room temperature. Their size and morphology could be changed
by varying the BES concentration and pH value of the reaction
system. The Au@Pd core­shell nanoparticles exhibited signifi-
cantly higher catalytic activity for various Suzuki reactions
than monometallic Pd or Au nanoparticles. The size-dependent
catalytic activity was also observed, i.e., the Au@Pd core­shell
nanoparticles of <10 nm showed higher activity.
solution (2 M). The whole mixture was vigorously stirred at
room temperature for another 12 h to get a dark-brown colloid
containing the Au@Pd nanoparticles. The Au@Pd nanoparticles
were collected by centrifugation and washed with ethanol and
deionized water five times before characterization.
Sample S1 was characterized by TEM, HRTEM, and
STEM-EDX, which were taken on a Philip Tecnai G20 Electron
Microscope at an accelerating voltage of 200 kV. TEM images in
Figures 1A­1C showed that sample S1 were mainly spherical
nanoparticles of 15­20 nm (average size: ca. 17 nm). The dark
core and the light shell were discernable for individual nano-
particles, indicating the formation of the core­shell nano-
structure. HRTEM image (Figure 1D) revealed lattice fringes
of Au(111) planes in the core and Pd(111) planes in the shell
(d111 = 0.237 nm for face-centered cubic Au in JCPDS #04-
0784 and d111 = 0.224 nm for face-centered cubic Pd in JCPDS
#46-1043).
STEM-EDX also confirmed the core­shell structure of S1.
As shown in Figures 2B and 2C, the Au X-ray signal originates
from the center of spherical nanoparticles while the Pd signal
forms a sheath around spherical nanoparticles. This effect is best
illustrated in the color map that overlays the Au and Pd X-ray
signals (Figure 2D). All this evidence support that the Au@Pd
nanoparticles have been successfully prepared.
Au­Pd bimetallic nanoparticles with different microstruc-
tures (e.g., alloy or core­shell) have been reported to possess
superior catalytic performance as compared to their monome-
tallic counterparts for a variety of reactions (e.g., deoxygenation
1
2,3
of octanoic acid, Ullmann coupling of aryl chlorides in water,
4
5
6
oxidation of CO and alcohols, and direct synthesis of H O ).
2
2
Among them, the Au­Pd core­shell nanoparticles with Pd atoms
concentrated on the surface (denoted as Au@Pd) are more
important for reactions that cannot be catalyzed by Au nano-
particles.
The Au­Pd bimetallic nanoparticles have been generally
synthesized via chemical reduction of the corresponding metal
salts with reducing agents (e.g., H2, hydrazine, alcohol, ascorbic
It has been well demonstrated with HEPES in previous work
that hydroxy and tertiary amine groups can reduce noble metal
salts to form nanoparticles (HEPES is another Good’s buffer,
acid, CTAC, or graphene oxide) in the literature.7
­11
In the
present work, a facile liquid-phase growth route is applied to
prepare the Au@Pd nanoparticles at room temperature in BES
aqueous solution. BES is a Good’s buffer generally used for
research in biology and biochemistry (Scheme 1). It acts as both
a mild reductant and a stabilizer during the synthesis. To the best
of our knowledge, this is the first report for the use of BES to
grow the Au@Pd nanoparticles from aqueous solution.
A typical procedure for making the Au@Pd nanoparticles
(sample S1) was as follows: Primary Au nanoparticles were first
prepared by adding H[AuCl4] solution (125 ¯L, 50 mM) into a
round-bottom flask containing 5 mL of BES solution (200 mM,
pH 7.5) and 16 mg of poly(vinylpyrrolidone) (average Mw:
55000). After adjusting the pH value to 3 by HCl solution (1 M),
the mixed solution was stirred at room temperature for 5 h to get
a purple-red Au colloid. PdCl2 solution (1 mL, 25 mM) was
then added, followed by adjusting the pH value to 3 with NaOH
HO
HO
3
SO H
N
N
HO
3
SO H
N
BES
HEPES
Figure 1. (A­C) TEM images of sample S1; (D) HRTEM
image at the edge of the single nanoparticle in (C).
Scheme 1. Chemical structures of two Good’s buffers.
Chem. Lett. 2015, 44, 1371–1373 | doi:10.1246/cl.150600
© 2015 The Chemical Society of Japan | 1371

shapes. Interestingly, many regular tetrahedral nanoparticles of
ca. 12 nm were formed in S4 (Figures S1e and S1f). These
results indicate that the BES concentration exerts great influence
on the size and morphology of the Au@Pd nanoparticles via
changing the nucleation and growth rate of nanoparticles.
At lower pH value (e.g., pH 2 for S5), the resulting Au@Pd
nanoparticles were mostly spherical with narrow size distribution
of 15­20 nm (Figure S2a). However, at higher pH value (e.g.,
pH 5 for S6), non-uniform spherical Au@Pd nanoparticles with
broad size distribution of 10­40 nm were obtained (Figure S2b).
It has been reported that the pH value of the solution significantly
1
7­20
affects the size and shape of Au and Pd nanostructures.
This
work also reveals that the Au@Pd nanoparticles could be tuned
by controlling the pH value of the reaction system.
Suzuki coupling reaction between 4-iodophenyl methyl
ether with phenylboronic acid was chosen to evaluate the
catalytic activity of the resulting Au@Pd nanoparticles. The
reaction was generally carried out as follows: Phenylboric acid
(
(
1.5 mmol), 4-iodophenyl methyl ether (1 mmol), K2CO3
2 mmol), tetrabutylammonium bromide (TBAB, 1 mmol), and
the Au@Pd nanoparticles (0.005 mmol of Pd) were added into
mL of ethanol/H2O (1:2 by volume) solution. The reaction
Figure 2. (A) TEM image of sample S1; (B) Au mapping; (C)
Pd mapping; (D) Overlapped image of Au and Pd mapping.
9
mixture was refluxed at 86 °C for 3­5 min and then cooled down
to room temperature. The product was extracted twice with
diethyl ether and dried with anhydrous Na2SO4. After being
concentrated in a rotary evaporator, the product was purified
by column chromatography performed on silica gel (200­300
mesh) with petroleum ether as eluent to get the isolated yield and
identified by comparing the H NMR spectra with standard data,
which were recorded on a Bruker-Avance II 400 MHz NMR
spectrometer.
As shown in Figure 3, the Au@Pd nanoparticles (S1 and
S2) exhibited significantly higher catalytic activity than mono-
metallic Au or Pd nanoparticles with similar sizes (TEM images
of Au and Pd nanoparticles shown in Figure S3). The isolated
yields of the coupling product on S1, S2, Pd and Au nano-
catalysts were respectively 66%, 83%, 15%, and negligible.
The different yields on S1 and S2 also implied size-dependent
catalytic activity of the Au@Pd core­shell nanoparticles for
Suzuki coupling reaction.
representing
4-(2-hydroxyethyl)-1-piperazineethanesulfonic
1
2­17
acid).
BES has the same functional groups as HEPES (i.e.,
sulfonic acid, tertiary amine, and hydroxy groups as shown
in Scheme 1). Therefore, the Au(III) and Pd(II) salts were
sequentially reduced with BES as a mild reductant to generate
the Au@Pd nanoparticles through epitaxial growth of the Pd
shell on primary Au seeds in our BES-containing synthetic
system. In addition, BES can interact with Au or Pd metals via
functional groups (e.g., sulfonate and amine groups), which help
stabilize the resulting nanoparticles.
The size and shape of the Au@Pd nanoparticles could be
changed by tuning the BES concentration and pH value of the
reaction system (samples S2­S6 in Table 1). When the concen-
tration of BES was 100 mM (S2), uniform spherical Au@Pd
nanoparticles of 4­11 nm (average size: ca. 8 nm) were obtained
1
(Figures S1a and S1b in Supporting Information). When the
BES concentration was increased to 300 mM (S3), the obtained
Au@Pd nanoparticles still preserved the spherical nanostructure
but with a broader particle size distribution of 7­20 nm (average
size: ca. 10 nm) (Figures S1c and S1d). However, further
increasing the BES concentration to 400 mM significantly
changed the morphology of the resulting Au@Pd nanoparticles
The Au@Pd core­shell nanoparticles (S1) were further
tested to catalyze other Suzuki coupling reactions. As shown in
Table 2, both aryl bromides (Entries 1­3) and iodides (Entries 4
B(OH)2
+
I
OCH3
OCH3
(
S4), which consist of a mixture of particles with a variety of
Table 1. Effects of the BES concentration and pH value on the
synthesis of Au@Pd core­shell nanoparticlesa
BES
mM
Size distribution
(average size)/nm
Sample
pH
Morphology
/
S1
S2
S3
S4
S5
S6
200
100
300
400
200
200
3
3
3
3
2
5
15­20 (17)
4­11 (8)
7­20 (10)
7­20 (14)
15­20 (17)
10­40 (35)
spherical
spherical
spherical
polyhedral
spherical
polyhedral
Figure 3. Isolated yields for Suzuki coupling between 4-
iodophenyl methyl ether and phenylboronic acid on different
nanocatalysts.
aThe general synthetic procedure described for S1 in the main
text was followed except the above specific parameters.
1372 | Chem. Lett. 2015, 44, 1371–1373 | doi:10.1246/cl.150600
© 2015 The Chemical Society of Japan

Table 2. Suzuki coupling of phenylboronic acid with various
aryl halides
Supporting Information is available electronically on J-STAGE.
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synthesis. The Au@Pd nanoparticles exhibited significantly
higher catalytic activity for Suzuki coupling reaction than
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Chem. Lett. 2015, 44, 1371–1373 | doi:10.1246/cl.150600
© 2015 The Chemical Society of Japan | 1373