Synthesis and characterization of fivefold twinned nanorods and right bipyramids of palladium

Article abstract of DOI:10.1016/j.cplett.2007.04.074

This Letter describes the first synthesis of fivefold twinned nanorods and right bipyramids of palladium in an aqueous solution, with ascorbic acid as a reducing agent and in the presence of bromide. Like the silver system, these two types of nanostructures are derived from multiple and single twinned seeds, respectively. The ascorbic acid, bromide, and reaction temperature all play important roles in the synthesis.

Full text of DOI:10.1016/j.cplett.2007.04.074

Chemical Physics Letters 440 (2007) 273–278
www.elsevier.com/locate/cplett
Synthesis and characterization of fivefold twinned nanorods and right
bipyramids of palladium
a
a
b
a,
*
Yujie Xiong , Honggang Cai , Yadong Yin , Younan Xia
a
Department of Chemistry, University of Washington, Seattle, WA 98195, USA
Department of Chemistry, University of California, Riverside, CA 92521, USA
b
Received 9 March 2007; in final form 18 April 2007
Available online 27 April 2007
Abstract
This Letter describes the first synthesis of fivefold twinned nanorods and right bipyramids of palladium in an aqueous solution, with
ascorbic acid as a reducing agent and in the presence of bromide. Like the silver system, these two types of nanostructures are derived
from multiple and single twinned seeds, respectively. The ascorbic acid, bromide, and reaction temperature all play important roles in the
synthesis.
Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction
shown in previous studies for both silver and gold, fivefold
twinned nanorods or nanowires could be grown much
longer than the single-crystal ones [6,7]. Here, we demon-
strate that this trend can also be extended to the palladium
system.
A central theme of nanotechnology research is to syn-
thesize nanostructures with well-defined sizes and shapes
[1]. One-dimensional (1D) nanostructures of palladium
with controllable sizes represent a class of particularly
interesting nanostructures to synthesize and investigate
because they are promising building blocks for fabricating
nanoelectronic devices, including hydrogen sensors and
field-effect transistors (FETs) based on carbon nanotubes
[2–4]. However, it remains a grand challenge to grow 1D
nanostructures of palladium with high crystallinity because
the face-centered cubic (fcc) structure of this metal pro-
vides no intrinsic driving force for anisotropic growth.
Recently, we demonstrated a polyol process for synthesiz-
ing single-crystal nanorods of palladium, where the aniso-
tropic growth was mainly induced by localized oxidative
etching [5]. These nanorods are still too short (ca. 30 nm)
to be directly incorporated into device fabrication because
the gaps between contacts fabricated by nanolithographic
techniques are typically on the 50–100 nm scale [5]. As
Like both silver and gold, palladium atoms can also be
directed to form highly anisotropic nanostructures assem-
bled in a solution phase by controlling the twin defects in
the seeds [8,9]. However, the twinned seed is highly suscep-
tible to oxidative etching because it contains many twin
defects on its surface [10]. Since oxygen binds more
strongly to palladium surface than silver or gold, twinned
seeds of palladium are more difficult to form when the syn-
thesis is performed in air [10]. Another obstacle to form
twinned rods is the fact that fivefold twinned seeds are only
thermodynamically favored primarily at relatively small
sizes [11]. As the size of seeds increases, the twinned struc-
tures could evolve into single-crystal cubooctahedrons [11].
In this paper, we demonstrate that the reduction could be
greatly slowed down by using ascorbic acid with a mild
reducing power so the twinned seeds could be maintained
at relatively small sizes (<8 nm) for a long period. At the
same time, we introduce bromide into the reaction system,
which is capable of chemisorbing onto the surface of palla-
dium seeds to promote the formation of {100} facets. As a
*
Corresponding author. Fax: +1 206 685 8665.
E-mail address: xia@chem.washington.edu (Y. Xia).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.04.074

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Y. Xiong et al. / Chemical Physics Letters 440 (2007) 273–278
result, the multiple and single twinned seeds could grow
into pentagonal nanorods and right bipyramids through
atomic addition (i.e., the deposition of fresh palladium
atoms on seeds). In this Letter, it was found that the oxida-
tive etching can be partially eliminated to save twinned
seeds by lowering the reaction temperature.
uct was collected by centrifugation and washed with
acetone (once) and ethanol (three times) to remove excess
PVP. The samples were then characterized by scanning
electron microscopy (SEM), transmission electron micros-
copy (TEM), and high-resolution TEM.
2.2. Separation of palladium nanorods
2. Experimental
The palladium nanorods were separated from particles
by filtering through a membrane. In this case, the as-syn-
thesized sample was dispersed in water and then filtered
through a circular inorganic membrane with a pore size
of 100 nm (Anodisc 13, Catalog NO. 6809-7013, Whatman
International, Maidstone, England). The sample shown in
Fig. S1 represents a typical example of palladium nanorods
obtained using this filtration method.
2.1. Synthesis of palladium nanorods and right bipyramids
In a typical synthesis, 0.105 g of poly(vinyl pyrrolidone)
(PVP, M.W. = 55,000, Aldrich, 856568-100 g) and 0.023 g
of L-(+)-ascorbic acid (C6H8O6, J. T. Baker, B581-05-
100g) were dissolved in 8.0 mL of water hosted in a 25-
mL, 3-neck flask (equipped with a reflux condenser and a
Teflon-coated magnetic stirring bar) and then heated to
80 °C in air under magnetic stirring. Meanwhile, 0.056 g
of sodium palladium(II) chloride (Na2PdCl4, Aldrich,
379808-1g) and 0.600 g of potassium bromide (KBr,
Fisher, P205-100g) were dissolved in 3.0 mL of water and
rapidly added into the flask. The reaction mixture was con-
tinued with heating at 80 °C in air for 3 h before the prod-
2.3. Electron microscopy characterization
The sample for TEM or SEM study was 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. After dry-
Fig. 1. (a, b) TEM and (c) SEM images of a sample prepared in an aqueous solution containing 12 mM ascorbic acid and 0.45 M KBr. The syntheses was
performed at 80 °C, with the molar ratio of the repeating unit of PVP to Na2PdCl4 being 5. (d) High-resolution TEM image taken from the end of a
fivefold twinned nanorod. The inset of (b) shows the blow-up TEM images of a single fivefold twinned nanorod. In (b), a typical right bipyramid (RB),
multiple twinned particle (MTP) and nanobar is marked, respectively.

Y. Xiong et al. / Chemical Physics Letters 440 (2007) 273–278
275
ing, the TEM grid or silicon wafer was 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 electron microscopy character-
ization. TEM images were captured using a Philips
CM100 transmission electron microscope operated at
100 kV, equipped with a Gatan Model 689 digital slow
scan camera. SEM images were taken on a FEI field-emis-
sion microscope (Sirion XL) operated at an accelerating
voltage of 20 kV. High-resolution TEM images were taken
on a JEOL 2010 LaB6 high-resolution transmission elec-
tron microscope operated at 200 kV.
morphology of the final product [8,9]. In the current syn-
thesis, palladium ions were reduced by ascorbic acid to
form palladium atoms, which subsequently aggregated to
generate nuclei. At small sizes, the twin defects – a single
atomic layer in the form of a mirror (111) plane – could
be formed or removed from the nuclei depending on the
variation in free energy [12]. Once the nuclei have grown
past a certain size, they will evolve into seeds with a sin-
gle-crystal, single twinned, or multiple twinned structure.
These seeds would further grow into a cube, right bipyra-
mid, and fivefold twinned rod, respectively. Alternatively
multiple twinned seeds could remain as MTPs during the
growth process to enable a greater surface coverage by
{111} facets, which are lowest in energy. Note that the
nanobars were also derived from the single-crystal seeds,
where the slightly anisotropic growth was induced by local-
ized oxidative etching [5]. A summary of these growth
pathways is shown in Fig. 2. From this perspective, both
the crystallinity of the seed and the ratio of {100} to
{111} facets on its surface play a vital role in forming a
fivefold twinned nanorod.
3. Results and discussion
The synthesis was typically conducted at 80 °C in an
aqueous solution containing ascorbic acid, KBr, and
PVP, with the addition of Na2PdCl4 as a precursor to pal-
ladium atoms. The PVP served as a colloidal stabilizer,
while the ascorbic acid acted as a reducing agent. Similar
to a recent demonstration [5], the bromide was mainly used
to promote the formation of {100} facets. Fig. 1a–c shows
TEM and SEM images of a typical product which con-
tained 30% fivefold twinned nanorods of ca. 8 nm in diam-
eter and lengths up to 150 nm, 17% 10-nm multiple
twinned particles (MTPs, including both icosahedrons
and decahedrons), 18% single twinned right bipyramids
(i.e., structures with two right tetrahedrons symmetrically
placed base-to-base) of ca.10 nm in edge length (at the
base), and 35% single-crystal nanocubes and nanobars
(i.e., anisotropic structures with a square cross-section
and aspect ratios of 1–1.2). It is worth pointing out that
the nanorods can be separated from other structures by fil-
tering through a membrane with a pore size of 100 nm,
thanks to their relatively long lengths and high yields (see
Fig. S1).
The reduction rate is critical to the formation of twinned
nanostructures. Theoretical analysis has suggested that the
extra energy caused by twins can be readily compensated
by maximizing {111} surface coverage when the seeds
are relatively small in size [13]. As the seeds grow bigger,
the low surface energy of {111} surface cannot remedy
the excessive strain energy, resulting in their evolution into
The structure of these palladium nanorods was further
studied using high-resolution TEM. Previous studies on sil-
ver and gold nanorods indicate that each fivefold twinned
nanorod is composed of five single-crystal subunits con-
nected by {111} twin planes, so that each nanorod has five
equivalently flat side {100} surfaces [6,7]. Such a nanorod
should preferentially lie on a flat surface against one of its
side faces rather than one of its edges. No matter which
side face is in contact with the TEM grid during sample
preparation, a twin boundary always appears in the middle
of the nanorod (see the inset of Fig. 1b). The high-resolu-
tion TEM image shown in Fig. 1d, recorded from the
end of a palladium nanorod, exhibits a (111) twin plane
oriented parallel to its longitudinal axis. Each side of this
twinned nanorod is a piece of single crystal with well-
resolved interference fringe spacing. It is worth emphasiz-
ing that the twin boundary is straight and continuous along
the entire longitudinal axis of each nanorod.
Fig. 2. Schematic illustrating the formation of Pd nanostructures with
various shapes observed in the present work. The green and gray colors
represent the {100} and {111} facets, respectively. Twins are delineated in
the figure with red lines. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
During the past several years, it has been demonstrated
that the structure of seeds involved in the synthesis of metal
nanostructures plays an important role in determining the

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Y. Xiong et al. / Chemical Physics Letters 440 (2007) 273–278
Fig. 3. TEM images of four as-obtained samples illustrating the variation in morphology when different experimental conditions were involved: (a) 48 mM
ascorbic acid and 0.45 M KBr; (b) 6 mM ascorbic acid and 0.45 M KBr; (c) 12 mM ascorbic acid and 0.22 M KBr; and (d) 12 mM ascorbic acid and
0.75 M KBr. All the syntheses were performed at 80 °C, with the molar ratio of the repeating unit of PVP to Na2PdCl4 being 5.
single-crystal cubooctahedrons [13]. When the reduction of
palladium precursor and the generation of palladium
atoms become substantially slow, the twinned seeds can
be kept at small sizes for a long period due to the slow
addition of atoms. In the present case, the mild reducing
power of ascorbic acid ensures a slow reduction so there
is enough time for twinned seeds to grow into twinned
nanorods through atomic addition. The small diameter of
resultant nanorods (ca. 8 nm) indicates that the twinned
seeds were not able to grow laterally to evolve into sin-
gle-crystal nanostructures. This argument is also supported
by the experimental data from a faster reduction. Fig. 3a
shows a TEM image of the product obtained from a syn-
thesis in which more ascorbic acid was added. Due to a fas-
ter reduction, the product did not contain any fivefold
twinned nanostructures. However, if the reduction was
too slow, only a small portion of the twinned seeds could
grow into nanorods (see Fig. 3b) due to the limited avail-
ability of palladium atoms in the solution.
larger MTPs. The major difference between MTPs and five-
fold twinned nanorods or right bipyramids lies on whether
the side {100} surface can be expressed. Only when the
side {100} surfaces can be stabilized, the twinned seeds will
favor growth into fivefold twinned nanorods and right
bipyramids. Most recently, we have demonstrated that
addition of bromide can chemisorb onto the surface of pal-
ladium seeds and alter the order of surface free energies for
different facets so the formation of {100} surface can be
greatly promoted to generate nanocubes and nanobars
[5]. In the present Letter, bromide also plays an important
role in promoting the {100} facets of resultant nanostruc-
tures. As a result, the surface of most products (e.g., five-
fold twinned nanorods, right bipyramids, nanocubes, and
nanobars) in the current synthesis is bounded by {100}
facets.
In addition to the promotion of {100} facets via surface
adsorption, bromide also contributed to the slow reduc-
tion. Because the overall stability constant of PdBr24À is
nearly 104 times higher than that of PdCl24À, almost all
chloride in the PdCl24À ions can be substituted by bromide
to form complex ions PdBr24À at [BrÀ]/[ClÀ] = 7.5 [14]. The
formation of PdBr24À is supported by the color change from
The high surface coverage of {100} facets is another key
requirement for generating fivefold twinned nanorods and
right bipyramids. Besides the possibility of forming nano-
rods, the multiple twinned seeds can also grow to become

Y. Xiong et al. / Chemical Physics Letters 440 (2007) 273–278
277
Fig. 5. (a) TEM and (b) SEM images of right bipyramids and nanobars
prepared at 100 °C in an aqueous solution containing 48 mM ascorbic acid
and 0.45 M KBr. The molar ratio of the repeating unit of PVP to
Na2PdCl4 was 5. The inset of (a) shows the blow-up TEM image of a
single right bipyramid.
Fig. 4. Influence of reaction temperature on the morphology of resultant
Pd nanostructures. The TEM images were taken from samples prepared at
(a) 90 °C and (b) 70 °C. Other experimental parameters were kept the
same as in Fig. 1, in an aqueous solution of 12 mM ascorbic acid and
0.45 M KBr, with the molar ratio of the repeating unit of PVP to
Na2PdCl4 being 5.
this synthesis, as the oxidative etching was enhanced at
high temperatures. On the other extreme, the product that
was synthesized at low temperatures, such as 70 °C, con-
tained more fivefold twinned nanorods and MTPs (see
Fig. 4b). However, the growth rate at a low temperature
was so slow that the resultant nanorods became signifi-
cantly shorter than the normal synthesis.
As compared with the multiple twinned seeds, single
twinned seeds are more resistant to oxidative etching. To
support this point, we increased the temperature from
80 °C to 100 °C and while other experimental conditions
were kept the same as in Fig. 3a (where we only obtained
right bipyramids, nanocubes, and nanobars). As a result,
we could still obtain 30% right bipyramids in the product
(see Fig. 5). Our recent Letter on right bipyramids of silver
suggested that each right bipyramid is composed of two
right tetrahedrons bisected by one single (111) twin plane
[17]. In the current Letter, the shape of the palladium right
bipyramids is well resolved based on the SEM image in
Fig. 5b, albeit we did not observe the twin planes in the
bipyramids under high-resolution TEM due to their unsuit-
able orientations.
brown to red when the Na2PdCl4 was mixed with KBr in
water. According to the Nernst equation, the potential of
palladium precursor is greatly reduced due to the forma-
tion of a more stable complex PdBr24À [15]. We believe that
this ligand replacement can significantly reduce the reduc-
tion rate. To decipher the role of bromide, we also per-
formed two syntheses by altering the concentration of
KBr in the synthesis, and the results are summarized in
Fig. 3c and d. As expected, synthesis at a low concentration
of bromide only produced single-crystal nanocubes and
nanobars due to the fast reduction, whereas the use of a
high concentration of bromide resulted in the formation
of twinned nanorods with a higher yield and longer length.
It is worth noting that low temperatures helps form high
yields of twinned structures. It is generally thought that the
oxidative etching could be partially eliminated at low tem-
peratures such as below 80 °C [16]. Fig. 4a shows a TEM
image of the product obtained at 90 °C while other exper-
imental conditions were kept the same as in Fig. 1. The
yield of multiple twinned structures was extremely low in

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Y. Xiong et al. / Chemical Physics Letters 440 (2007) 273–278
4. Summary
Appendix A. Supplementary material
We have demonstrated that fivefold twinned nano-
rods and right bipyramids of palladium could be synthe-
sized in an aqueous medium with ascorbic acid as the
reducing agent. The mild reducing power of ascorbic acid
ensures a slow reduction to favor the formation of
single and multiple twinned seeds. Another key component
in the synthesis is bromide ions which can promote the for-
mation of {100} facets as the side surfaces of both nano-
rods and right bipyramids. Our observations also indicate
that the formation of twinned structures is sensitive to reac-
tion temperature, which is related to oxidative etching.
Because the nanorods had a yield of 30% and lengths up
to 150 nm, they could be readily separated from other
nanostructures by filtration with a membrane. This chemi-
cal synthesis of Pd nanorods represents the first step
toward fabrication of nanoscale electronic and sensing
devices.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.cplett.2007.
04.074.
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This work was supported in part by ACS (PRF-44353-
AC10) and NSF (DMR-0451788), as well as a fellowship
from the David and Lucile Packard Foundation. Y.X. is
a Camille Dreyfus Teacher Scholar (2002–2007). This work
used the Nanotech User Facility (NTUF), a member of the
National Nanotechnology Infrastructure Network (NNIN)
funded by the NSF. We thank the Molecular Foundry at
the Lawrence Berkeley National Laboratory for HRTEM
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