This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 2553–2555 2553

Cite this: Chem. Commun., 2011, 47, 2553–2555

One-pot synthesis and electrocatalytic activity of octapodal Au–Pd

nanoparticlesw

Jong Wook Hong, Young Wook Lee, Minjung Kim, Shin Wook Kang and Sang Woo Han*

Received 8th November 2010, Accepted 14th December 2010

DOI: 10.1039/c0cc04856a

Bimetallic alloy Au–Pd nanoparticles with an unprecedented

octapodal shape have been prepared by a one-pot aqueous

synthesis method. This unique structure was produced through

selective etching of {100} facets by in situ generated Br�ions.

The octapodal Au–Pd nanoparticles exhibited efficient electro-

catalytic properties toward ethanol oxidation.

During the past decade, bimetallic nanoparticles (NPs) with a

core–shell and alloy structures have received a great deal of

attention owing to their remarkable catalytic properties, which

are superior to those of monometallic NPs.1 The catalytic

activity and selectivity of NPs can be tuned by controlling their

morphology, because the exposed surfaces of the NPs have

distinct crystallographic planes (facets) that can determine

their overall catalytic properties.2–4 Accordingly, shape-

controlled synthesis of bimetallic NPs has been extensively

studied in efforts to optimize their properties. Notably, a two-

step seeding growth method has been widely used to prepare

bimetallic core–shell NPs with well-defined geometries, wherein

a second metal layer is grown over the pre-synthesized seed

NPs.5–8 Very recently, we have found that core–shell NPs with

controlled morphologies could also be synthesized through a

one-step protocol.9 On the other hand, galvanic replacement

reaction10,11 and co-reduction of two metal precursors with a

suitable reducing agent12,13 are generally employed to prepare

bimetallic alloy NPs. However, compared to the fabrication of

core–shell NPs, shape-controlled synthesis of alloy NPs is

restrained by complex reaction environments.14,15 Therefore,

development of a facile and simple strategy for the preparation

of bimetallic alloy NPs with desirable structures and exploring

their properties are important undertakings for expanding

potential applications.

Herein, we report on the one-pot aqueous synthesis of

bimetallic alloy Au–Pd NPs with an unprecedented octapodal

shape. This unique and complex nanostructure has been

obtained through a selective etching process. Among the

various bimetallic NPs fabricated to date, Au–Pd NPs are

very fascinating due to their excellent catalytic activities

for a variety of chemical reactions.16 NPs with well-defined

branches or multi-arms have also been the subject of intense

research due to interest in their unique morphologies and

prominent catalytic properties.7,8 Yang and co-workers

recently demonstrated that multi-armed NPs have outstanding

surface-enhanced Raman scattering activities.17 In this regard,

we have also investigated the electrocatalytic properties of the

synthesized NPs toward ethanol oxidation.

The one-pot synthesis of Au–Pd octapodal NPs was

achieved by co-reduction of Au and Pd precursors with

L-ascorbic acid in the presence of cetyltrimethylammonium

chloride (CTAC). In a typical synthesis, a NaAuBr4/K2PdCl4mixture in a molar ratio of 1 : 1, CTAC, and L-ascorbic acid

were mixed together with highly purified water and the

resultant solution was heated at 50 1C in an oven for 4 h.

Representative scanning electron microscopy (SEM) and

transmission electron microscopy (TEM) images of the prepared

samples are presented in Fig. 1a and b, respectively, showing

that the majority of the samples consisted of branched NPs

and each NP has eight pods (see also Fig. S1, ESIw). Elemental

Fig. 1 (a) SEM, (b) TEM, and (c) HAADF-STEM-EDS mapping

images of the Au–Pd octapodal NPs. (d) HAADF-STEM image and

cross-sectional compositional line profiles of an Au–Pd octapodal NP.

Inserted scale bar is 10 nm.

Department of Chemistry and KI for the NanoCentury, KAIST,Daejeon 305-701, Republic of Korea.E-mail: sangwoohan@kaist.ac.krw Electronic supplementary information (ESI) available: Details forsynthesis and characterization, and additional experimental data. SeeDOI: 10.1039/c0cc04856a

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2554 Chem. Commun., 2011, 47, 2553–2555 This journal is c The Royal Society of Chemistry 2011

mapping of Au and Pd (Fig. 1c) and the compositional line

profiles on a single octapodal NP (Fig. 1d and 2) obtained by

the high-angle annular dark-field scanning TEM energy-

dispersive X-ray spectroscopy (HAADF-STEM-EDS) reveal

that the prepared nanostructure is an Au–Pd alloy and Au is

more abundant than Pd at the inner region of the NPs. The

Au/Pd ratio was estimated to be 71 : 29 by using an inductively-

coupled plasma-atomic emission spectrometer (ICP-AES).

The higher Au content, especially at the inner part of the

NPs, is attributed to the higher reduction potential of AuBr4�

(0.854 V vs. standard hydrogen electrode (SHE)) as compared

to PdCl42� (0.591 V vs. SHE).18 The formation of NPs is

assumed to be initiated by predominant nucleation of Au

followed by the co-reduction of residual Au and Pd ions on

the surface of the Au-enriched seeds.19

The structural evolution of NPs was monitored by TEM

measurements, as shown in Fig. 3. Initially, octahedral Au–Pd

NPs rapidly formed, within B10 min. This was also reflected

in distinct changes in the UV-vis spectral features of the

reaction mixture (Fig. S2, ESIw). When the reaction started,

the intensities of characteristic peaks corresponding to Au(III)

and Pd(II) complexes at 230 and 285 nm, respectively, rapidly

decreased. After 5 min, the peak intensities were unchanged,

indicating that the reduction of metal precursors was

completed within 5 min. As the reaction proceeded, octahedral

Au–Pd NPs were gradually etched in the [100] direction and

transformed into truncated nanostructures with concave

features, and eventually octapodal structures with slightly rough

surface features were produced. Beyond 4 h, no significant

change in the structure was observed. HAADF-STEM-EDS

data for the initial and intermediate structures showed that all

the nanostructures were Au–Pd alloys (Fig. S3, ESIw). Theshape evolution of NPs is attributed to the selective etching of

{100} faces during the reaction by Br� ions which were

generated by the reduction of AuBr4�. It has been reported

that Br� can oxidatively etch noble metal NPs.20,21 Since the

surface energy of the {100} face is higher than that of the {111}

face and the vertices of the NPs are less capped by stabilizers

owing to a steric problem,17,22 Br� should preferentially

etch vertices in the [100] direction. This in turn leads to the

formation of truncated structures that ultimately yield

octapodal NPs (Fig. 3f). The anisotropic etching of the

{100} faces was confirmed by high-resolution TEM (HRTEM)

measurements. The d-spacing for adjacent lattice fringes in the

truncated regions was 2.00 A, which correlates well with that

of the (200) plane of Au–Pd alloy (Fig. 4). Meanwhile, the

lattice spacing of 2.30 A in the pod regions corresponds to the

(111) planes of Au–Pd alloy (Fig. S4, ESIw).23 The rough

surface morphologies of the pods may be ascribed to the

re-deposition of dissociated metals onto the stable {111}

surfaces through an Ostwald ripening process.22 This

re-deposition process also leads to increased average length

of the long axial direction of octapodal NPs as compared to

the distance between parallel opposite faces of the initial

octahedral seeds (from 39 to 66 nm).

As shown above, the presence of Br� is expected to be

crucial to the formation of octapodal NPs. To decipher the

effect of the Br� ions, similar experiments were performed,

employing HAuCl4 as an Au precursor in the presence of

different amounts of NaBr (Fig. S5, ESIw). In the absence of

NaBr, octapodal NPs were not formed, and instead polyhedral

NPs were produced. As the concentration of NaBr was

increased, pods were produced, although their structure was

not well developed. Interestingly, when the concentration of

Br� was increased to 0.2 mM, which was the same with that of

the original experiment, synthesized NPs were significantly

dissimilar from those shown in Fig. 1. This could be attributable

to the feeding rates of Br�. In the original experimental

conditions, the Br� ions gradually participated in the reaction,

as they were provided solely by the reduction of AuBr4�,

thereby facilitating the formation of octapodal NPs. However,

Fig. 2 HAADF-STEM image and cross-sectional compositional line

profiles of an Au–Pd octapodal NP along the long axis (a) and the pod

of the nanostructure (b).

Fig. 3 TEM images of nanostructures collected at different reaction

times: (a) 10 min, (b) 15 min, (c) 20 min, (d) 2 h, and (e) 4 h.

Black diagrams in insets are the projection of the red-colored three-

dimensional structures under the electron beam. (f) Scheme for the

shape evolution of NPs by the selective etching process.

Fig. 4 (a) HRTEM image of an Au–Pd octapodal NP recorded along

the [001] zone axis. (b) High-magnification HRTEM image and

(c) corresponding FFT pattern of the square region in (a).

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 2553–2555 2555

with the presence of a relatively large amount of Br� at the

initial stage of the reaction, octapodal NPs were not realized.

Further increasing the concentration of NaBr to 1 mM yielded

multiply-shaped polyhedral NPs. These results clearly indicate

that the in situ generation of Br� through the one-pot protocol

is the key factor for the formation of the octapodal Au–Pd

NPs. On the other hand, the presence of an appropriate

amount of L-ascorbic acid is also critical, because it produces

both homogeneous NP seeds and Br� by reducing the Au

precursors. Well-defined octapodal NPs were produced when

the concentration of L-ascorbic acid was higher than 0.1 mM,

which is equivalent to the amount in the original feeding

solution (Fig. S6, ESIw).To investigate the catalytic activity of Au–Pd octapodal

NPs, the electrocatalytic properties of the NPs toward ethanol

oxidation were tested and the results were compared with

those of flower-like Au–Pd alloy NPs with similar composition

and size (Au/Pd ratio = 66 : 34, Fig. S7, ESIw)19 and a

commercial Pd/C catalyst (Fig. S8, ESIw). It has been known

that Pd NPs have efficient electrocatalytic activity for the

ethanol oxidation and incorporation of Au into Pd catalysts

further improves catalytic activity and selectivity as well as the

resistance to poisoning.23 Fig. 5a shows cyclic voltammograms

(CVs) of ethanol oxidation with different catalysts in a 1 M

KOH solution containing 0.1 M ethanol. The voltammograms

show that the current density of the octapodal NPs, which is

normalized to the electrochemically active surface area

(ECSA, Fig. S9, ESIw), is the highest among the different

catalysts. The peak current density of the octapodal NPs was

2.16 mA cm�2, whereas that for the flower-like NPs and Pd/C,

respectively, was 1.10 and 0.65 mA cm�2 (Fig. 5b). Further-

more, the corresponding mass activity of the octapodal NPs

was 0.92 A mg�1Pd, which is about 2 and 4 times higher than that

of the flower-like NPs (0.49 A mg�1Pd) and the Pd/C catalyst

(0.24 A mg�1Pd), respectively (Fig. 5b). The chronoamperometric

experiments also reveal that the electrochemical stability of the

octapodal NPs for ethanol electro-oxidation is superior to that

of the other catalysts; the oxidation current on the octapodal

NPs at the end of the measurement period is much higher

than that on the other NPs (Fig. S10, ESIw). The observed

enhanced electrocatalytic activity and stability of the octapodal

NPs are attributed to the presence of a number of active sites

on their surfaces such as highly active facets, gaps between

pods, and some defect sites, which originate from their octapodal

structures with rough surface features. Previous studies on the

electro-oxidation of ethanol showed that the active oxygen

atoms readily adsorb on the active sites. They can then readily

oxidize the intermediates on the nanocatalysts, thus leading to

enhanced catalytic activities and stabilities.24 In addition, the

improved activity of the octapodal NPs may also be ascribed

to the relatively higher fraction of {100} facets on their

surfaces as compared to other particles due to their truncated

regions. Recently, Wang et al. found that Pd(100) provides the

best surface among the low-index planes for the dissociation of

ethanol molecules.25

In summary, bimetallic alloy Au–Pd nanoparticles with an

unprecedented octapodal shape have been prepared by a one-pot

aqueous synthesis method. This unique structure was produced

through selective etching of {100} facets by in situ generated Br�

ions. The octapodal Au–Pd nanoparticles exhibited efficient

electrocatalytic properties toward ethanol oxidation. The present

work involving mixed metal catalysts with controlled morpho-

logy is expected to provide a promising strategy for developing

efficient anode catalysts of fuel cells.

This work was supported by Basic Science Research Programs

(KRF-2008-313-C00415, 2008-0062042, 2010-0029149), Future-

based Technology Development Program (Nano Fields)

(2009-0082640), and PRC Program (2009-0082813) through the

National Research Foundation (NRF) funded by the Korean

government (MEST).

Notes and references

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10489.

Fig. 5 (a) CVs in 1 M KOH + 0.1 M ethanol of the Au–Pd

octapodal NPs, flower-like Au–Pd NPs, and Pd/C on glassy carbon

electrodes. Scan rate: 50 mV s�1. (b) Current densities and mass

activities for ethanol oxidation on the three different types of catalyst.

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