Synthesis of palladium icosahedra with twinned structure by blocking oxidative etching with citric acid or citrate ions

Article abstract of DOI:10.1002/anie.200604032

(Figure Presented) A remedy for etching: Citric acid and citrate ion were used to prevent oxidative etching from the reaction system by taking advantage of their strong binding to Pd surfaces. This method allowed Pd icosahedra (see picture) to be formed in yields as high as 80%.

Full text of DOI:10.1002/anie.200604032

Communications
DOI: 10.1002/anie.200604032
Nanostructures
Synthesis of Palladium Icosahedra with Twinned Structure by Blocking
Oxidative Etching with Citric Acid or Citrate Ions**
Yujie Xiong, Joseph M. McLellan, Yadong Yin, and Younan Xia*
Palladium is well-known for its remarkable capacity for
the major product of typical solution-phase syntheses con-
ducted in air. For example, the fast reduction of Na [PdCl ] by
[
1]
hydrogen absorption. It is also widely used as the primary
catalyst for low-temperature reduction of automobile pollu-
tants, hydrogenation and dehydrogenation reactions, and
carbon–carbon-bond-forming reactions such as Suzuki cou-
2
4
ethylene glycol produced only 10% decahedra; the major
[
8]
product (90%) was truncated cubes or octahedra. This
result can be attributed to the fact that both decahedra and
icosahedra contain many twin boundary defects on the
surface, which are the primary sites for oxygen absorption.
As a result, MTPs are highly susceptible to an oxidative
[
2]
pling. Most of these applications are related to the
adsorption of hydrogen on Pd nanoparticles. A recent
simulation revealed that Pd icosahedra can absorb a larger
[
3]
[9]
quantity of hydrogen than their cubic analogues. Another
important property of Pd nanoparticles that remains largely
environment. Our recent studies revealed that in the polyol
synthesis, the MTPs were completely dissolved as a result of
[
4]
[8]
unexplored is their surface plasmon resonance (SPR).
A
oxidative etching as the reaction was prolonged. Therefore,
potential application based on the SPR feature is surface-
enhanced Raman scattering (SERS), whereby the enhance-
ment factor is determined mainly by the strength of local
it remains a grand challenge to synthesize Pd MTPs in
relatively high yields. In comparison, the formation of Au and
Ag MTPs is much easier to accomplish. A plausible explan-
ation is that oxygen binds more strongly to the Pd surface than
[
5]
electric field. It has been reported that metal nanostructures
with sharp corners or edges are especially active substrates for
[
10]
to Au or Ag, which greatly accelerates the etching of Pd
MTPs. For the same reason, it is impossible to completely
block oxidative etching in a Pd synthesis by simply bubbling
2
SERS, and the local value of j E j could be more than 500
[
6]
times that of the applied field. For this reason, an
icosahedron with 12 corners and 30 edges is anticipated to
be a superior substrate for SERS detection. These opportu-
nities inspired us to target the synthesis of Pd icosahedra.
Pd is a face-centered cubic (fcc) noble metal, and Pd
nanoparticles may take three different shapes when fast
nucleation and growth are involved: truncated cubes or
[
8]
an inert gas through the reaction system. Herein we
demonstrate, for the first time, that the introduction of citric
acid or citrate ions can block oxidative etching and promote
the formation of MTPs, thanks to the strong binding of these
species to the Pd surface. As a result, Pd icosahedra could be
readily obtained in high yields in the presence of citric acid.
We also demonstrate that the sizes of these MTPs can be
controlled by adjusting the reaction time and temperature.
The synthesis was typically conducted in an aqueous
solution that contained citric acid and poly(vinyl pyrrolidone)
(PVP) at 908C for 26 h with Na [PdCl ] added as a precursor
[
7]
octahedra, decahedra, and icosahedra. The truncated cubes
or octahedra are single crystals, whereas both decahedra and
icosahedra are multiply twinned particles (MTPs). It has been
shown by simulation that icosahedra are favored at small
sizes, decahedra at intermediate sizes, and truncated cubes or
2
4
[
7]
octahedra at large sizes. However, MTPs have never been
to Pd atoms. The PVP served as a stabilizer, and the citric acid
acted as a reductant according to the mechanism of conven-
[
11,12]
[*] Dr. Y. Xiong, J. M. McLellan, Prof. Y. Xia
tional citrate reduction.
Figure 1a shows the SEM image
Department ofChemistry
University ofWashington
Seattle, WA 98195 (USA)
Fax: (+1)206-685-8665
of a typical product, which contained 80% icosahedra with
edge lengths of approximately 25 nm and 20% slightly
truncated octahedra (also see Figure S1 in the Supporting
Information). The inset of Figure 1a shows a high-magnifica-
tion SEM image of an icosahedron, from which one can
clearly resolve the exact shape. Each icosahedron is com-
posed of 20 tetrahedra, and, as a result, has three types of
rotational axes: twofold, threefold, and fivefold. Figure 1b
shows a TEM image of the as-synthesized Pd icosahedra. The
inset of Figure 1b shows that most of the icosahedra have a
projected hexagonal shape under TEM, which may corre-
spond to the orientation with either a twofold or a threefold
axis parallel to the electron beam. Orientation along the
fivefold axis under TEM is rare because an icosahedron
should preferentially lie on a flat surface against one of its
faces rather than a corner. Figure 1c and 1d shows typical
high-resolution TEM images of an icosahedron. It has a three-
fold axis oriented parallel to the electron beam, which is in
E-mail: xia@chem.washington.edu
Dr. Y. Yin
The Molecular Foundry
Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
[
13]
[
**] This work was supported in part by the ACS (PRF-44353-AC10), the
NSF (DMR-0451788), and the David and Lucile Packard Founda-
tion. Y. Xia is a Camille Dreyfus Teacher Scholar (2002–2007). Y.Y.
was supported by the Director, Office of Science, US Department of
Energy, under Contract DE-AC03-76SF00098. This work used the
Nanotech User Facility (NTUF) at the UW, a member ofthe National
Nanotechnology Infrastructure Network (NNIN) funded by the
NSF. We thank the Molecular Foundry at the Lawrence Berkeley
National Laboratory for HRTEM analysis.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Figure 1. Electron microscopy characterization ofPd icosahedra syn-
thesized at 908C in the presence of0.13 m citric acid with a molar
ratio ofthe repeating unit ofPVP to the Pd precursor of5:1. a) SEM
image; b) TEM image; and c, d) high-resolution TEM images. The
insets of(a) and (b) show the magni if ed SEM and TEM images,
respectively, ofa single icosahedron. The inset of(c) shows a
schematic illustration ofthe icosahedron. The reaction time was 26 h.
Figure 2. a, b) TEM images ofsamples prepared under the same
conditions as those in Figure 1 except that the reaction time was
shortened to 5 h and 21 h, respectively. c) TEM image ofthe product
synthesized under the same conditions as those in Figure 1 except
that sodium citrate was used instead ofcitric acid. d) TEM image of
the product synthesized under the same conditions as those in
Figure 1 except that the hydroxy end group ofPVP had been removed
by oxidation with AgNO3.
accordance with both experimental observation and simula-
[
13]
tion of Au icosahedron.
The inset of Figure 1c gives a
current system is worth investigating. As demonstrated in our
recent studies, formation of MTPs requires the elimination of
geometrical model that corresponds to the icosahedron
shown in the same figure. The phase purity and high
crystallinity of the Pd icosahedra is also supported by
powder X-ray diffraction (XRD). All of the peaks in a typical
XRD pattern of an as-synthesized sample (see Figure S2 in
the Supporting Information) can be assigned to fcc Pd
[
8,9b]
oxidative etching from a reaction system.
In the present
case, there are two possible factors responsible for the
absence of etching: the reducing power and the binding
ability of citric acid. To decipher the mechanism, ascorbic acid
(with a reducing power close to that of citric acid) was used as
the reducing agent. In this case, the product consisted mainly
of single-crystal nanocubes (see Figure S3 in the Supporting
Information). In contrast, the use of sodium citrate as a
reducing agent also produced MTPs (Figure 2c). As shown by
structural analysis and computational studies, both citric acid
and citrate ion bind to the Pd surface more strongly than
(
JCPDS card 05-0681). The ratio between the intensities of
the (111) and (200) peaks is higher than the value reported for
a conventional powder sample (2.66 versus 2.38), which
indicates that the diffraction from {111} planes was enhanced
for the icosahedra owing to texturing effects. This result can
be understood by noting that icosahedra preferentially lie on
a flat substrate against their flat {111} faces and thus increase
the diffraction intensity from these planes.
To elucidate the growth mechanism of the Pd icosahedra,
we monitored the evolution of their shape with TEM by
taking samples at various reaction times. Figure 2a and b
shows TEM images of Pd nanoparticles sampled at 5 hours
and 21 hours, respectively. At t = 5 h, the sample contained
mainly icosahedra with an average size of 8 nm, which
suggests that the icosahedra could be formed in a stage as
early as nucleation, rather than through the evolution of
single-crystal cubes or octahedra in the growth process. If the
reaction was allowed to proceed, the size uniformity of the
icosahedra gradually improved: nanoparticles below a certain
size slowly disappeared while relatively large icosahedra
became bigger. This observation can be attributed to the
involvement of Ostwald ripening in the growth process, by
which big particles grow at the expense of smaller ones.
Because the icosahedra were formed in the very early
stage, the exceptional stability and high yield of MTPs in the
[
14]
ascorbic acid.
We believe that this strong binding is
responsible for the absence of etching when citric acid or
citrate ion is present. The strong binding of citric acid or
citrate ion can block oxidative etching through two different
mechanisms: 1) These species can compete with oxygen
adsorption on the surface of Pd icosahedra, and thus reduce
the amount of adsorbed oxygen. 2) After binding to the
particle surface, it is easier for these species to react with and
completely exhaust the adsorbed oxygen atoms. We used
Raman spectroscopy to identify the species (i.e., citric acid or
citrate ion) adsorbed on the surface of Pd icosahedra.
However, as the number of adsorbed species on the Pd
icosahedra is quite small and the electrical field enhancement
by Pd is limited, the Raman signals from aggregated films of
Pd icosahedra were not strong enough for resolving the peaks.
To solve this problem, we deposited the Pd icosahedra on to
an aggregated film of Ag nanocubes and then recorded the
SERS spectrum (see Figure S4 in the Supporting Informa-
tion). The signals obtained from samples prepared in this way
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791

Communications
were significantly enhanced, from which it could be con-
cluded that a mixture of citric acid and citrate species were
present on the surface of the Pd icosahedra.
Besides its role as a stabilizer, PVP can also serve as a
reducing agent in the synthesis. Most recently, we demon-
strated that the ends of commercially available PVP are
terminated in hydroxy groups, which can serve as a reductant
[
15]
for the synthesis of metal nanoparticles.
To clarify this
point, we performed a synthesis by using PVP whose hydroxy
end groups had been removed in advance by oxidization with
AgNO . Figure 2d shows a typical TEM image of the sample
3
prepared in the presence of PVP that lacks the hydroxy end
group. No major morphological difference could be found
when compared with sample prepared by the normal syn-
thesis. However, synthesis in the absence of PVP produced
some MTPs with irregular shapes (see Figure S5 in the
Supporting Information). These results suggest that PVP
served mainly as a steric stabilizer to eliminate random
agglomeration and direct the particles to grow into the well-
defined icosahedral shape during the synthesis.
Figure 3. Correlation between the percentage ofMTPs and the concen-
tration ofcitric acid. The number ofMTPs was counted of r both
icosahedra and decahedra. All syntheses were performed with ethylene
glycol as the solvent and reductant at 1108C for 3 h in the presence of
citric acid at various concentrations. The molar ratio ofthe repeating
unit ofPVP (MW =55000) to the Pd precursor was 1.5:1. The insets
show TEM images ofsamples synthesized under the corresponding
conditions.
The range of temperature was also critical to the
formation of Pd icosahedra. For example, we found that
both the shape and the size of the product depended on
temperature in two different ways (see Figure S6 in the
Supporting Information). Firstly, low temperatures resulted in
the incomplete reduction of Na [PdCl ] by citric acid together
The icosahedra are presumed to exhibit SPR features
different from those of truncated cubes. Figure 4a shows the
UV/Vis extinction spectrum taken from an aqueous suspen-
sion of Pd icosahedra 25 nm in edge length. The SPR band is
red-shifted relative to that of Pd truncated cubes of similar
2
4
with slow growth rates for the Pd nanoparticles. As a result,
icosahedra synthesized at 808C were relatively small. This
dependence provides a practical way to tune the size of
icosahedra. Secondly, with the increase of temperature, the
oxidative etching was greatly enhanced and the binding of
citric acid or citrate ion to the Pd surface became weaker, thus
making it much easier to etch the MTPs. The yield of MTPs
was substantially decreased when the temperature was
increased from 90 to 1008C.
The blocking of oxidative etching with citric acid or citrate
ion seems to be a generic method for improving the yield of
MTPs. In a recent work, a typical polyol synthesis produced
9
1
0% single-crystalline, truncated cubes or octahedra and
0% MTPs after 5 minutes. When the reaction time was
[
8]
prolonged to 3 hours, all the MTPs dissolved owing to
oxidative etching. By introducing citric acid into the polyol
synthesis, the MTPs could be preserved and thus accumulated
throughout the reaction. Figure 3 shows the dependence of
the yield of MTPs on the concentration of citric acid. The
reaction time was set at 3 hours, a period long enough to
ensure the complete dissolution of MTPs in the absence of
citric acid. The yield of MTPs increased slightly to 20% when
citric acid was added to the reaction at a final concentration of
0
.13 mm. The MTPs were stable over a period of more than
one week. The effect of citric acid on blocking the oxidative
etching was also found to be applicable to the aqueous
synthesis of Ag and Au MTPs (see Figure S7 in the Support-
ing Information). Notably, sodium citrate has also been used
as a stabilizer in the preparation of twinned seeds that were
used for the growth of Ag and Au nanorods. However, it
has been ignored by the research community that the citrate
anion may actually play an important role in preserving MTPs
by completely blocking the oxidative etching process.
Figure 4. a) UV/Vis extinction spectrum ofthe as-prepared Pd icosahe-
dra shown in Figure 1. b) Raman spectra of1,4-BDT molecules
adsorbed onto 25-nm edge length icosahedra (top, see Figure 1) and
[
16]
[
4b]
nanocubes (bottom) ofPd. The spectra were normalized against the
ꢀ
1
signals ofsilicon (at 977.63 cm ).
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Angewandte
Chemie
[
4b]
size (390 nm versus 330 nm).
The SPR peaks of Pd
resolution TEM, electron diffraction (ED), scanning electron micro-
scopy (SEM), and UV/Vis spectroscopy.
nanostructures are relatively broad, mainly because the
[
17]
TEM images were captured on a Phillips 420 transmission
electron microscope operated at 120 kV. HRTEM images and ED
patterns were taken on a JEOL 2010 LaB6 high-resolution trans-
mission electron microscope operated at 200 kV. SEM images were
taken on a FEI field-emission scanning electron microscope (Sir-
ion XL) operated at an accelerating voltage of 10 kV. Samples for
TEM and SEM studies were prepared by drying a drop of the aqueous
suspension of particles on a piece of carbon-coated copper grid (Ted
Pella, Redding, CA) or silicon wafer under ambient conditions. The
TEM grid or silicon wafer was then transferred to a gravity-fed flow
cell and washed for 1 h with deionized water to remove excess PVP.
Finally, the sample was dried and stored in vacuum for TEM and SEM
characterization. All the percentages of particles herein were
calculated from 200–300 nanoparticles. Powder XRD patterns were
dielectric function of Pd has a large imaginary part. The
exceptionally wide range of wavelengths across which Pd
icosahedra strongly absorb light could potentially make them
[
18]
useful as nanoscale photothermal heating elements.
The Pd icosahedra with well-defined surfaces provide an
ideal substrate for SERS to study the compositional depend-
ence of chemical adsorption and reactions on Pd nano-
[
19]
catalysts. As an initial study of the SERS activities of these
nanoparticles, we measured the SERS spectra of 1,4-benzen-
dithiol (1,4-BDT) molecules immobilized on Pd icosahedra as
well as on 25-nm edge length truncated cubes of Pd. Well-
resolved SERS spectra (Figure 4b) could be readily obtained
for the Pd icosahedra. The magnitude of signals in the
spectrum of the icosahedra was 2.4 times that of the truncated
cubes. We recently estimated the surface enhancement factor
of the 25-nm edge length Pd cubes to be approximately 2.2 ”
recorded on a Philips 1820 diffractometer equipped with a CuKa
radiation source (l = 1.54180 Š). UV/Vis spectra were measured on
a Hewlett–Packard 8452A diode-array spectrophotometer.
The substrates for SERS studies were prepared by drop-casting
the aqueous sol (0.5 mL) onto a Si wafer and allowing it to dry
completely under vacuum. Once dried, the films were incubated in a
1 mm solution of 1,4-benzendithiol (1,4-BDT) in ethanol for 1 h. The
samples were then taken out, rinsed with copious amounts of ethanol
to remove any unadsorbed 1,4-BDT molecules, and dried in a stream
of air. Raman spectra were taken with a Renishaw inVia Raman
spectrometer attached to a Leica DMLM optical microscope using a
3
[5e]
1
0 , which means that the icosahedra would have a surface
3
enhancement factor on the order of about 5.3 ” 10 . This
improvement is most likely related to their sharp corners and
edges, as well as their more red-shifted SPR band. These
results indicate that Pd icosahedra are attractive substrates
for SERS detection of molecular species. We expect that well-
defined Pd icosahedra will advance the use of Pd in various
SERS-related applications.
In summary, citric acid and the citrate ion have been
utilized to prevent oxidative etching from a reaction system
by taking advantage of their strong binding to the Pd surface.
As a result, Pd icosahedra could be synthesized in aqueous
solutions in high yields in the presence of citric acid. The
introduction of citric acid into polyol syntheses also helped
improve the yield of MTPs. We expect that this method can be
extended to other reaction systems, although some modifica-
tions to the experimental conditions might be required. As a
potential application, these Pd icosahedra could serve as
active substrates for SERS detection. The exceptional chem-
ical sensitivity of Pd towards hydrogen should make these Pd
icosahedra especially useful for hydrogen storage and SPR-
5
5
0 ” objective (numerical aperture = 0.63) and laser excitation at
14 nm (4 mW at the sample). The Raman scattering signals were
collected on a thermoelectrically cooled (ꢀ608C) CCD detector.
For the SERS detection of species adsorbed on Pd icosahedra, the
as-synthesized Pd particles were washed with ethanol and water three
times to remove any free citric acid or citrate ion. After complete
washing, the samples were dispersed in water and then drop-cast onto
a Si wafer that was initially covered by an aggregated film of 100-nm
edge length Ag nanocubes. Once dried, the samples were charac-
terized on the same day by using the Raman system.
Received: September 29, 2006
Published online: December 19, 2006
Keywords: citrate · etching · icosahedra · nanostructures ·
.
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[
1,20]
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