Polyhedral bimetallic alloy nanocrystals exclusively bound by {110} facets: Au-Pd rhombic dodecahedra

Article abstract of DOI:10.1002/anie.201007220

{110}-faceted alloy nanocrystals: Rhombic dodecahedral Au-Pd alloy nanocrystals (NCs) enclosed exclusively by 12 {110} facets were prepared by a simple one-pot aqueous synthetic method. The picture shows an SEM image of the NCs (inset: ideal RD structure). The NCs exhibit higher surface-enhanced Raman scattering and electrocatalytic activities than {111}-faceted nanoparticles. Copyright

Full text of DOI:10.1002/anie.201007220

Communications
DOI: 10.1002/anie.201007220
Bimetallic Nanocrystals
Polyhedral Bimetallic Alloy Nanocrystals Exclusively Bound by {110}
Facets: Au–Pd Rhombic Dodecahedra**
Young Wook Lee, Minjung Kim, Shin Wook Kang, and Sang Woo Han*
Controlled synthesis of nanocrystals (NCs) with well-defined
morphologies is an important subject in the field of nano-
technology because intrinsic physicochemical properties of
nanostructures strongly depend on their shapes.[1] In partic-
ular, fine-tuning of the optical and electronic characteristics
of noble metal NCs by tailoring their morphologies has been
extensively studied because of their promising applications in
plasmonics, nanoelectronics, and surface-enhanced Raman
scattering (SERS)-based diagnosis.[2–4] Furthermore, the
shape of metal NCs has a profound influence on their
catalytic activity and selectivity in a variety of chemical
reactions.[5–10] This observation can be attributed to the
different types of exposed crystallographic planes (facets)
on their surfaces, which can determine the overall catalytic
properties of the NCs.
chemical properties than their monometallic counterparts
and these properties can be modulated by controlling the
shape and exposed facets of the NCs,[9,23–27] development of
efficient synthetic strategies for alloy NCs with desirable
structures is important.
One-pot synthesis of RD Au–Pd NCs was achieved by co-
reduction of Au and Pd precursors with ascorbic acid in the
presence of cetyltrimethylammonium chloride (CTAC). In a
typical synthesis, aqueous solutions of an equimolar HAuCl4/
K2PdCl4 mixture, ascorbic acid, and CTAC were mixed, and
the resultant solution was kept under ambient conditions for
about 1 h. A representative SEM image of the asprepared
product (Figure 1a) shows that the majority of the sample
Although a number of shaped NCs have been synthesized
over the past decade, most NCs of noble metals with face-
centered cubic (fcc) structure are bound by low-index {111}
and {100} surfaces owing to minimization of surface energy
during NC growth.[11] Recently, we and others have succeeded
in preparing noble metal NCs enclosed completely by {110}
facets, which have the highest surface energy among the low-
index facets, such as Au and Pd rhombic dodecahedra.[12–16]
There are also a few reports on production of metal NCs with
exposed high-index {hk0} and {hhl} (h > 1) facets.[17–22] How-
ever, compared to monometallic NCs, engineering crystalline
facets by shape-controlled synthesis of bimetallic alloy NCs
has been limited due to the complex reduction kinetics of
metal precursors and the difficulty in finding suitable
structure-directing agents. In particular, alloy NCs bound by
high-energy facets have been little explored to date. Here we
report on the synthesis of rhombic dodecahedral (RD) Au–Pd
bimetallic alloy NCs enclosed exclusively by 12 {110} facets in
high yield by a simple one-pot aqueous synthetic method.
Since bimetallic NCs often exhibit better physical and
Figure 1. a) SEM image of RD Au–Pd NCs. The inset shows an ideal
RD structure bound by 12 {110} surfaces. b) High-magnification
HRTEM image of an RD Au–Pd NC. The corresponding selected-area
FFT pattern is shown in the inset. c) XRD pattern of RD Au–Pd NCs.
The intensities and positions for pure Au (pink lines at the bottom)
and Pd (brown lines at the bottom) references were taken from the
JCPDS database. d) HAADF-STEM-EDS mapping images of the RD
Au–Pd NCs.
[*] Y. W. Lee, M. Kim, S. W. Kang, Prof. S. W. Han
Department of Chemistry and KI for the NanoCentury
KAIST, Daejeon 305-701 (Korea)
Fax: (+82)42-350-2810
E-mail: sangwoohan@kaist.ac.kr
Homepage: http://ntl.kaist.ac.kr
(>85%) consists of RD NCs with an average edge length of
48 ꢀ 7 nm. An ideal RD structure bound by 12 {110} surfaces
(inset of Figure 1a) matches well with the synthesized NCs.
The prepared NCs were stable in aqueous solution for up to
several weeks under ambient conditions. The HRTEM image
demonstrates that the exposed surfaces of the NCs are bound
by {110} facets; the d spacing of 1.40 ꢀ for adjacent lattice
fringes corresponds to that of the (220) plane of fcc Au–Pd
(Figure 1b). The selected area fast Fourier transform (FFT)
pattern displayed in the inset of Figure 1b also shows that the
[**] This work was supported by Basic Science Research Programs
(2008-0062042, 2010-0029149), Future-based Technology Develop-
ment Program (Nano Fields) (2009-0082640), and PRC Program
(2009-0082813) through the National Research Foundation (NRF)
funded by the Korean government (MEST), and was also supported
by the Industrial Core Technology Development Program by the
Ministry of Knowledge Economy (10037397).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007220.
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prepared NC is a single crystal. Figure 1c shows the XRD
pattern of the RD Au–Pd NCs. The characteristic diffraction
peaks of the fcc structure of metallic Au–Pd can be observed,
and peak positions intermediate between those of pure Au
and Pd reveal the formation of single-phase Au–Pd alloy.
Elemental mapping of Au and Pd (Figure 1d) and the
compositional line profiles on a single NC (Figure S1 in the
Supporting Information) obtained by high-angle annular
dark-field scanning TEM and energy-dispersive X-ray spec-
troscopy (HAADF-STEM-EDS) reveal that the prepared
NCs are definitely Au–Pd alloy. The Au/Pd atomic ratio of the
RD Au–Pd NCs was estimated to be 78/22 by inductively
coupled plasma atomic emission spectrometry (ICP-AES).
The higher Au/Pd ratio than in the raw solution may be
ꢁ
attributable to the higher reduction potential of AuCl4
ꢁ
[AuCl4 /Au: + 1.002 V vs. standard hydrogen electrode
(SHE)] compared to PdCl4 (PdCl42ꢁ/Pd: + 0.591 V vs.
2ꢁ
SHE).[28]
The formation process of the NCs could be monitored by
TEM measurements at different reaction times (Figure 2).
Initially, small NCs grew within about 5 min (Figure 2a).
Figure 3. a) TEM and b) HRTEM images of the Au–Pd NCs after a
reaction time of 5 min. The inset in b) shows the three-dimensional
structural model of an ideal rhombic dodecahedron. c) HAADF-STEM-
EDS mapping images of the Au–Pd NCs. d) HAADF-STEM image and
cross-sectional compositional line profiles of an Au–Pd NC. Scale bar:
20 nm.
of surfactant releases fewer Au and Pd atoms for nucleation
and thus decreases the number of seeds and results in
formation of larger NCs.[29]
Formation of RD Au–Pd NCs may result from a kineti-
cally controlled nucleation and growth process. Recently, we
found that Au@Pd core–shell NCs with {111}-faceted octahe-
ꢁ
2ꢁ
dral structure were formed when AuCl4 and PdCl4
precursors were co-reduced by CTAC in the absence of
ascorbic acid.[30] Due to the weak reducing power of CTAC, it
took about 48 h to complete the reaction even at elevated
temperature (ca. 908C). This slow reduction kinetics facili-
tated thermodynamic control over nucleation/growth of the
NCs; formation of the Au@Pd NCs was initiated by nucle-
ation of Au atoms to form an Au octahedral core bound by
thermodynamically stable {111} facets, followed by the
epitaxial growth of a Pd layer onto the Au core. On the
basis of this finding, we believe that the RD Au–Pd NCs
enclosed by the high-energy {110} facets are kinetically
trapped under our experimental conditions. On the other
hand, cationic surfactant CTAC as capping agent plays a
critical role in formation of the RD Au–Pd NCs with exposed
high-energy {110} surfaces. Reportedly, CTAC can drive the
formation of metal NCs enclosed by high-energy facets
through preferential adsorption of CTA+ on these facets
rather than on low-energy ones.[18,21] In fact, we could not
obtain RD NCs when CTAC was replaced by other commonly
used capping agents such as polyvinylpyrrolidone (PVP),
cetyltrimethylammonium bromide (CTAB), sodium dodecyl
sulfate (SDS), and tetraoctylammonium bromide (TOAB), as
shown in Figure S3 of the Supporting Information. Recently,
we reported that simultaneous reduction of HAuCl4 and
K2PdCl4 by ascorbic acid in the presence of PVP yields
popcorn-shaped Au–Pd alloy nanoparticles (NPs) enclosed
predominantly by {111} facets.[31] The production of irregular
Figure 2. TEM images of NCs collected at different reaction times.
a) 5, b) 20, c) 30, d) 40, and e) 60 min. f) Change in the edge length of
NCs during the reaction, estimated by TEM analysis.
Close inspection of the TEM images of the initially formed
NCs clearly shows that most of the NCs have RD shape with a
mean edge length of 15 ꢀ 2 nm (Figure 3a and b). Elemental
mapping (Figure 3c) and the compositional line profiles
(Figure 3d) obtained by HAADF-STEM-EDS also revealed
the alloy nature of the NCs. As the reaction proceeded, the
NCs grew in size over about 1 h (Figure 2b–f). Beyond 1 h, no
significant change in structure was observed. These findings
imply that the Au–Pd alloy NCs with RD morphology evolved
at the very early stages of the reaction and then acted as seeds
for further NC growth. Changes in the size of the NCs during
the reaction further indicate that NC size can be controlled by
means of different reaction times. The NC size could also be
modulated by varying the amount of CTAC. For example, a
twofold increase in the amount of CTAC under otherwise
identical synthetic conditions gave RD Au–Pd NCs with an
edge length of 75 nm (Figure S2 in the Supporting Informa-
tion), which may be ascribed to the fact that the larger amount
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particles on using CTAB, which has the same cation as CTAC,
suggests that the Clꢁ counteranion is also important in
formation of NCs. The Brꢁ ion has a strong binding affinity to
metal surfaces, and thus alters the binding scheme of CTA+
onto the NC surface and the relative growth rates between
different facets.[13,18,21]
Control experiments with different molar ratios of
HAuCl4 and K2PdCl4 also revealed that the Au/Pd ratio in
the precursor solution is a key to formation of RD NCs. The
NCs synthesized by using only HAuCl4 as metal precursor
have truncated octahedral and decahedral shapes, while
irregularly shaped Pd NPs were produced under identical
conditions in the absence of HAuCl4 (Figure S4 in the
Supporting Information). Thus, both Au and Pd precursors
are necessary for growth of RD NCs. In addition, reactions
with 3/1 and 1/3 molar ratios of HAuCl4/K2PdCl4 did not lead
to formation of RD NCs; instead, roughly truncated octahe-
dral NCs were produced (Figure S5 in the Supporting
Information), which are enclosed dominantly by {111} facets
(Figure S6 in the Supporting Information). These results
demonstrate that the metal precursors, HAuCl4 and K2PdCl4,
also play a vital role in the control of NC structure. This may
be due to modulation of the reduction kinetics through
variation of the relative amounts of each metal precursor;
synthesis of RD Au–Pd NCs could be achieved when an
appropriate HAuCl4/K2PdCl4 molar ratio was used. Similar
observations have also been reported. For instance, mono-
disperse Au–Cu bimetallic nanocubes were prepared by using
Cu(acetylacetonate)2 and HAuCl4 as both metal precursors
and effective structure-directing agents.[32]
The exposed {110} surfaces of the synthesized NCs might
be expected to benefit a number of applications, such as the
development of efficient SERS platforms with unprecedented
activities, because high-energy {110} surfaces are believed to
have higher SERS activity than the other low-index facets
{111} and {100}.[12,33] To investigate the SERS activity of the
RD Au–Pd NCs, we measured SERS spectra of various
analytes such as 1,4-phenylene diisocyanide (1,4-PDI),
4-aminobenzenethiol (4-ABT), benzenethiol (BT), and rho-
damine 6G (R6G) adsorbed on the NCs, and compared them
with those obtained on predominantly {111} faceted popcorn-
like Au–Pd alloy NPs with similar composition (Au/Pd atomic
ratio 69/31, Figure S7 in the Supporting Information). The
two substrates exhibited similar surface plasmon features in
the UV/Vis extinction spectra (Figure S8 in the Supporting
Information). The RDAu–Pd NCs give stronger SERS signals
than the Au–Pd NPs for all adsorbates tested (Figure 4); the
SERS intensities obtained with the RD NCs are at least two
times higher than those with the Au–Pd NPs. Since the SERS
intensity depends on the packing density of the nanostruc-
tures on the surface, all SERS spectra shown in Figure 4 were
obtained by averaging more than ten spectra recorded at
different areas on the surface. Although the exact origin of
the enhanced SERS activity of the RD NCs is not clear at
present, it can be assumed that the {110} facets of the NCs
endow them with high SERS activity. However, the stronger
SERS signals from the RD NCs compared to the NPs may
also be ascribed to the presence of sharp edges and apexes on
the surface of the RD NCs, which could serve as “hot spots”
Figure 4. SERS spectra of a) 1,4-PDI, b) 4-ABT, c) BT, and d) R6G on
RD Au–Pd NCs and popcornlike Au–Pd alloy NPs.
for SERS.[34,35] Detailed studies on the facet-dependent SERS
activity are currently underway.
The electrocatalytic activity of noble metal NCs has been
intensively studied on different exposed surfaces. To inves-
tigate the influence of surface facets on the catalytic
performance of NCs, the electrocatalytic properties of the
RD Au–Pd NCs in ethanol oxidation were examined and
compared with those of the predominantly {111} faceted Au–
Pd alloy NPs. Pd-based nanostructures are known to be
efficient electrocatalysts for ethanol oxidation in alkaline
media, and incorporation of Au into Pd catalysts further
improves catalytic activity and selectivity as well as resistance
to poisoning.[36] Figure 5a shows the cyclic voltammograms
(CVs) of a glassy carbon electrode (GCE) modified with the
RD Au–Pd NCs and Au–Pd NPs in 0.1m KOH. Typical redox
peaks associated with the oxidation/reduction of Au–Pd alloy
are observed.[37] The current densities were normalized to the
electrochemically active surface areas (ECSA).[38] The obser-
Figure 5. CVs of RD Au–Pd NCs and the popcornlike Au–Pd alloy NPs
on GCE in a) 0.1m KOH and b) 0.1m KOH+0.5m ethanol. Scan rate:
50 mVsꢁ1. Currents were normalized with respect to the ECSA.
c) Current densities and mass activities for ethanol oxidation on the
RD NCs and NPs. d) CA curves of RD NCs and NPs on GCE in 0.1m
KOH+0.5m ethanol at ꢁ0.1 V vs. Ag/AgCl.
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Model XL30 S FEG). TEM images were obtained with a Phillips
Model Tecnai F20 transmission electron microscope operating at
200 kV after placing a drop of hydrosol on carbon-coated Cu grids
(200 mesh). HRTEM and HAADF-STEM characterizations were
performed with a FEI Tecnai G2 F30 Super-Twin transmission
electron microscope operating at 300 kV. The effective electron probe
size and dwell time used in HAADF-STEM-EDS mapping experi-
ments were 1.5 nm and 200 ms per pixel, respectively. The compo-
sitions of the NCs were determined by ICP-AES (OPTIMA
3300DV). XRD patterns were obtained on a Bruker AXS D8
DISCOVER diffractometer with CuKa (0.1542 nm) radiation. For
Raman measurement, drop-cast films of NCs on Si substrates were
soaked overnight in analyte solution in ethanol (10ꢁ4 m for 1,4-PDI, 4-
ABT, and BT; 10ꢁ6 m for R6G). The substrate was removed from the
solution, washed thoroughly with ethanol, and dried under ambient
conditions. Raman spectra were obtained on a Jobin Yvon/HORIBA
LabRAM spectrometer equipped with an integral microscope
(Olympus BX 41). The 632.8 nm line of an air-cooled He/Ne laser
was used as excitation source. Raman scattering was detected with
1808 geometry by using a thermoelectrically cooled 1024 ꢁ 256-pixel
charge coupled device (CCD) detector. The laser beam (1.5 mW) was
focused onto a spot (ca. 1 mm diameter) with an objective lens (ꢁ 50,
NA = 0.50). Data acquisition times were usually 60 s. The holographic
grating (1800 grooves/mm) and the slit allowed a spectral resolution
of 1 cmꢁ1. The Raman band of an Si wafer at 520 cmꢁ1 was used to
calibrate the spectrometer. Cyclic voltammetry (CV) measurements
were carried out in a three-electrode cell by using a CH Instrument
Model 600C potentiostat. Drop-cast films of NCs on GCE (diameter:
3 mm) served as working electrodes. Before CV measurements, 4 mL
of aqueous NC solution (0.5 mgAu+Pd mLꢁ1 according to ICP-AES),
which was obtained by centrifugation, was dropped onto a GCE
(metal loading: 28 mgcmꢁ2). After the solution was dried, 4 mL of
Nafion solution (0.05%) was added, and then dried in a Dry-Keeper.
The dried working electrode was cleaned by sequentially washing
with ethanol and water, and then electrochemically cleaned by 50
potential cycles between ꢁ0.8 and 0.4 Vat a scan rate of 50 mVsꢁ1 to
remove residual stabilizing agents on the surface of the NCs. Pt wire
and Ag/AgCl (in saturated KCl) were used as the counter- and
reference electrodes, respectively. All CVs were obtained at room
temperature, and the electrolyte solutions were purged with high-
purity N2 gas for about 1 h before use.
vation of single reduction peak for each sample with the same
potential of about ꢁ0.1 V versus Ag/AgCl demonstrates that
Au and Pd atoms are homogeneously distributed on the
surfaces of the Au–Pd RD NCs and NPs, and the surface
compositions of the two samples are very similar.[37,39] Fig-
ure 5b displays the ethanol electrooxidation activities on
different catalysts in 0.1m KOH solution containing 0.5m
ethanol. Characteristic well-separated anodic peaks in the
forward and reverse potential scans associated with ethanol
oxidation are observed. Notably, the onset potential of the
RD Au–Pd NCs is more negative than that of the Au–Pd NPs
(ꢁ0.50 and ꢁ0.35 V vs. Ag/AgCl, respectively). In addition,
the ECSA-normalized current density of the RD Au–Pd NCs
is much higher than that of the Au–Pd alloy NPs. In the
forward scan, the peak current density of the RD Au–Pd NCs
is 9.78 mAcmꢁ2, which is about 6.2 times higher than that of
the Au–Pd NPs (1.58 mAcmꢁ2; Figure 5c). Moreover, the
corresponding mass activity of the RD Au–Pd NCs is
ꢁ1
346.8 mAmg
55.3 mAmg
,
whereas that of the Au–Pd NPs is
AuþPd
ꢁ1
(Figure 5c and Figure S9 in the Supporting
AuþPd
Information). These findings clearly indicate that the RDAu–
Pd NCs have markedly enhanced electrocatalytic activity for
ethanol oxidation compared to the Au–Pd NPs. Chronoam-
perometry (CA) experiments at ꢁ0.1 V versus Ag/AgCl also
reveal that the electrochemical stability of the RD Au–Pd
NCs in ethanol electro-oxidation is superior to that of the Au–
Pd NPs (Figure 5d). The enhanced electrocatalytic activity
and stability of the RD Au–Pd NCs may be attributed to their
exposed highly reactive {110} surfaces, which originate from
their dodecahedral structure.
In summary, we have developed a facile one-pot synthesis
of RD Au–Pd alloy NCs bound entirely by {110} facets.
Formation of this unique structure was achieved through a
kinetically controlled nucleation and growth process. The RD
NCs exhibit higher SERS and electrocatalytic activities than
{111}-faceted NPs. We expect that the exposed high-energy
{110} surfaces of the NCs will promote their potential optical
and catalytic applications, and the present work may be
extendable to fabrication of other multicomponent metal NCs
with desirable morphologies.
Received: November 17, 2010
Published online: March 10, 2011
Keywords: alloys · gold · nanocrystals · palladium ·
.
Raman spectroscopy
Experimental Section
HAuCl4 (Aldrich, 99.99 + %), K2PdCl4 (Aldrich, 98%), ascorbic acid
(Dae Jung Chemicals & Metals Co., 99.5%), CTAC (Aldrich,
solution in water, 25 wt%), sodium citrate dihydrate (Aldrich,
99%), CTAB (Aldrich, 95%), SDS (Fluka, 99%), TOAB (Aldrich,
98%), and PVP (MW = 55000) were used as received. Other
chemicals, unless specified, were of reagent grade, and Milli-Q
water with a resistivity greater than 18.0 MWcm was used in the
preparation of aqueous solutions.
In a typical synthesis of RD Au–Pd NCs, 1 mL of a 5 mm aqueous
solution of HAuCl4/K2PdCl4 (molar ratio 1/1) was added to an
aqueous solution of CTAC (5 mL, 50 mm), and then an aqueous
solution of ascorbic acid (100 mm, 50 mL) was added to this mixture.
The resultant reaction solution was kept under ambient conditions for
about 1 h. The resultant hydrosol was subjected to centrifugation
(6000 rpm for 5 min, twice) to remove excess reagent.
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