Seed-mediated synthesis of Pd-Rh bimetallic nanodendrites

Article abstract of DOI:10.1016/j.cplett.2010.06.009

This Letter describes a simple, aqueous-phase route to the synthesis of Pd-Rh bimetallic nanodendrites consisting of Rh branches anchored to a Pd nanocrystal core. Palladium nanocrystals with various shapes, including truncated octahedron, cube, octahedron, and thin plate, have all been successfully employed as seeds to grow Rh branches via the reduction of Na ₃RhCl₆ with l-ascorbic acid in an aqueous solution. The degree of Rh branching could be controlled by varying the concentration of Na₃RhCl₆ involved in a synthesis. Electron microscopy analysis revealed that growth of Rh branches proceeded via attachment of small Rh particles that had been formed via homogeneous nucleation in solution.

Full text of DOI:10.1016/j.cplett.2010.06.009

Chemical Physics Letters 494 (2010) 249–254
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Seed-mediated synthesis of Pd–Rh bimetallic nanodendrites
a
a,*
b
a
a
Hirokazu Kobayashi , Byungkwon Lim , Jinguo Wang , Pedro H.C. Camargo , Taekyung Yu ,
b
a,
**
Moon J. Kim , Younan Xia
a
Department of Biomedical Engineering, Washington University, St. Louis, MO 63130, United States
Department of Materials Science, University of Texas at Dallas, Richardson, TX 75083, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This Letter describes a simple, aqueous-phase route to the synthesis of Pd–Rh bimetallic nanodendrites
consisting of Rh branches anchored to a Pd nanocrystal core. Palladium nanocrystals with various shapes,
including truncated octahedron, cube, octahedron, and thin plate, have all been successfully employed as
Received 18 March 2010
In final form 3 June 2010
Available online 8 June 2010
seeds to grow Rh branches via the reduction of Na
3
RhCl
6
with
L-ascorbic acid in an aqueous solution. The
degree of Rh branching could be controlled by varying the concentration of Na
3
RhCl involved in a
6
synthesis. Electron microscopy analysis revealed that growth of Rh branches proceeded via attachment
of small Rh particles that had been formed via homogeneous nucleation in solution.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Rhodium is a key catalyst invaluable to many reactions, including
nanostructures. Here we describe a simple, aqueous-phase route to
the synthesis of Pd–Rh bimetallic nanodendrites consisting of Rh
branches anchored to a core of Pd nanocrystal with different
shapes, including truncated octahedron, cube, octahedron, and thin
plate. Our synthetic protocol simply involves the reduction of
hydrogenation, hydroformylation, carbonylation, and reduction of
nitrogen oxides in a catalytic converter [1–4]. Meanwhile, Pd has
been widely used to catalyze hydrogenation or dehydrogenation,
carbon–carbon bond forming reactions, petroleum cracking, and
formic acid oxidation [5–7]. Recently, a new type of bimetallic nano-
structure, so-called ‘bimetallic nanodendrite’, has been prepared by
a heterogeneous, seeded growth method, and has attracted inten-
sive attention as a highly active catalyst for chemical or electro-
chemical reactions [8–11]. For instance, Eichhorn and co-workers
prepared Au–Pt nanodendrites, which showed the enhanced CO tol-
erance in the hydrogen oxidation reaction in a proton-exchange
membrane (PEM) fuel cell [8]. Recently, our group reported the syn-
thesis of Pd–Pt nanodendrites, which displayed a substantially en-
hanced activity for the oxygen reduction and formic acid oxidation
reactions in a PEM fuel cell [9,11]. However, bimetallic nanoden-
drites prepared so far have been restricted to those consisting of
Pt branches supported on Au or Pd nanocrystal seeds. The synthesis
of bimetallic nanodendrites with a wide variety of compositions re-
mains a challenge.
3 6
Na RhCl by L-ascorbic acid in the presence of Pd nanocrystal seeds
in an aqueous solution. Using this simple approach, we were able
to produce Pd–Rh nanodendrites in high yields. We also investi-
gated the growth mechanism and found that particle attachment
played an important role in the formation of
morphology.
a branched
2
. Experimental
2.1. Preparation of Pd nanocrystal seeds with different shapes
Palladium nanocrystals used as seeds were prepared in aqueous
solutions, as reported previously [13]. Palladium nanocrystals with
a truncated octahedral shape were synthesized by heating 11 mL of
an aqueous solution containing poly(vinyl pyrrolidone) (PVP,
1
05 mg, MW = 55 000, Aldrich),
citric acid (60 mg, Fisher), and Na
00 °C in air under magnetic stirring for 3 h. Palladium nanocrys-
tals with a cubic shape were synthesized by heating 11 mL of an
aqueous solution containing PVP (105 mg), -ascorbic acid
60 mg), KBr (300 mg, Fisher), and Na PdCl (57 mg) at 80 °C in
L
-ascorbic acid (60 mg, Aldrich),
2
PdCl (57 mg, Aldrich) at
4
Both Pd and Rh are noble metals that crystallize with a face-
centered cubic (fcc) packing, and these two metals have a lattice
mismatch of only 2.3%. Recently, Pd nanocrystals have been pre-
pared in a rich variety of shapes, including truncated octahedron,
cube, octahedron, and thin plate [12–14], making them ideal can-
didate as the seeds for heterogeneous, seeded growth of bimetallic
1
L
(
2
4
air under magnetic stirring for 3 h. Palladium nanocrystals with
an octahedral shape were synthesized by heating 11 mL of an
aqueous solution containing PVP (105 mg), citric acid (60 mg),
and Na
6 h. Palladium nanoplates with a hexagonal or triangular shape
were synthesized by heating 11 mL of an aqueous solution con-
2 4
PdCl (57 mg,) at 90 °C in air under magnetic stirring for
*
Corresponding author.
2
*
* Corresponding author.
E-mail addresses: limb@seas.wustl.edu (B. Lim), xia@biomed.wustl.edu (Y. Xia).
0
009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.06.009

2
50
H. Kobayashi et al. / Chemical Physics Letters 494 (2010) 249–254
Fig. 1. (A) TEM image of truncated octahedral Pd nanocrystals used as the seeds. The inset shows an HRTEM image of a single Pd truncated octahedron. (B–D) TEM images of
Pd–Rh nanodendrites synthesized by reducing Na RhCl with -ascorbic acid in the presence of the truncated octahedral Pd seeds in an aqueous solution. The concentration of
Na RhCl was (B) 1.6, (C) 3.2, and (D) 6.4 mM, and the reaction time was 3 h.
3
6
L
3
6
Fig. 2. (A) HAADF-STEM image of a single Pd–Rh nanodendrite shown in Fig. 1D. (B) Compositional line profiles of Pd and Ph on the Pd–Rh nanodendrite recorded along the
line shown in the HAADF-STEM image.

H. Kobayashi et al. / Chemical Physics Letters 494 (2010) 249–254
251
Fig. 3. TEM images showing the morphological evolution of Pd–Rh nanodendrites. The reaction was conducted under the same conditions as those in Fig. 1D except that the
reaction time was shortened to (A) 1, (B) 1.5, (C) 2, and (D) 10 min. The inset of (A) shows a TEM image of a single Pd–Rh particle consisting of Rh bumps on the Pd core.
taining PVP (105 mg) and Na
2
PdCl
4
(57 mg) at 100 °C in air under
3. Results and discussion
magnetic stirring for 3 h. These Pd nanocrystals were washed with
deionized water three times before being used as seeds.
We first employed truncated octahedral Pd nanocrystals with
an average size of ꢀ9 nm as the seeds for the formation of Pd–Rh
bimetallic nanodendrites. A TEM image of the as-prepared Pd trun-
cated octahedrons shows that they were uniform in both shape
and size (Fig. 1A). An HRTEM image of a single Pd truncated octa-
hedron indicates that it was a piece of single crystal with its sur-
face being enclosed by both {1 1 1} and {1 0 0} facets (Fig. 1A,
inset). The Pd–Rh nanodendrites were synthesized in an aqueous
2.2. Synthesis of Pd–Rh bimetallic nanodendrites
In a typical synthesis of Pd–Rh nanodendrites, 1 mL of the aque-
ous suspension of Pd nanocrystals and 6 mL of an aqueous solution
containing PVP (35 mg) and -ascorbic acid (60 mg) were added
into a 25-mL, three-necked flask. The mixture was heated to
00 °C in air under magnetic stirring. Meanwhile, a known amount
of sodium hexachlororhodate(III) (Na RhCl , 6.3–25 mg, Aldrich)
was dissolved in 3 mL of deionized water at room temperature.
The aqueous solution of Na RhCl was then injected into the flask
L
1
3 6
solution by reducing Na RhCl with L-ascorbic acid as a reducing
3
6
agent in the presence of truncated octahedral Pd seeds. Fig. 1B–
D, shows typical TEM images of the products prepared with differ-
ent concentrations of Na RhCl . In all cases, Rh grew into branches
3
6
3
6
by pipette. The reaction mixture was heated at 100 °C in air for 3 h,
and then cooled down to room temperature.
on the Pd seeds, generating Pd–Rh bimetallic nanocrystals with an
overall dendritic morphology. The compositional line profiles
(Fig. 2) of Pd and Rh on a Pd–Rh nanodendrite shown in Fig. 1D
2.3. Characterization
confirmed its bimetallic composition consisting of a Pd core and
Rh branches. For the sample prepared at a relatively low concen-
Transmission electron microscopy (TEM) images were obtained
tration for the Na
very few (5–10) branches, and most of them remained single
branches (Fig. 1B). As the Na RhCl concentration was increased,
the number of Rh branches increased, and secondary branching
was also observed. These results demonstrate that the degree of
Rh branching can be readily controlled by varying the concentra-
3 6
RhCl precursor, each Pd–Rh nanodendrite had
with a FEI Tecnai G2 Spirit microscope operated at 120 kV by drop
casting the nanoparticle dispersions on carbon-coated copper
grids. High-resolution TEM (HRTEM), high-angle annular dark-field
scanning TEM (HAADF-STEM), and energy dispersive X-ray (EDS)
analyses were performed using a JEOL 2100F microscope operated
at an accelerating voltage of 200 kV.
3
6
3 6
tion of the Na RhCl precursor involved in a synthesis. The TEM

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H. Kobayashi et al. / Chemical Physics Letters 494 (2010) 249–254
Fig. 4. (A) HRTEM image of a single Pd–Rh nanodendrite. (B–D) Magnified HRTEM images of the boxed regions in (A). The images reveal that the Rh branches have the same
lattice orientation as the Pd core at the interfaces, but the lattice fringes are not perfectly aligned across all the branches.
analysis confirmed the absence of isolated Rh nanoparticles in the
product.
We investigated the morphological evolution of the Pd–Rh
nanodendrites by taking aliquots of the reaction solution at various
stages and then viewing them by electron microscopy. The reaction
that the Rh particles were randomly fused into the Rh bumps
nucleated on the Pd seed and the growing Rh branches (Fig. 4B
and D).
In this synthesis, the formation of small Rh particles at the early
stages of the reaction can probably be attributed to the rapid in-
crease in the degree of supersaturation for Rh atoms due to the fast
3 6
was conducted at a relatively high concentration of Na RhCl (see
Fig. 1D). Fig. 3A–D, shows typical TEM images of the Pd–Rh nano-
structures that were sampled at 1, 1.5, 2, and 10 min, respectively.
At t = 1 min, the Pd–Rh sample contained small Rh particles with
sizes less than 3 nm in great abundance in addition to some Rh-
decorated Pd seeds (Fig. 3A). This observation suggests that homo-
geneous nucleation of Rh occurred in the solution, as well as heter-
ogeneous nucleation on the surface of the Pd nanocrystal seeds.
The image in the inset of Fig. 3A clearly shows the formation of
Rh bumps via heterogeneous nucleation on the Pd seed. As the
reaction proceeded, the number of small Rh particles rapidly de-
creased while Rh branches on each Pd seed increased in both num-
ber and size (Fig. 3B and C), indicating that the Rh branches were
formed through attachment of small Rh particles. The Pd–Rh parti-
cles eventually evolved into a highly branched structure through
the digestion of essentially all the small Rh particles (Fig. 3D).
Fig. 4A displays an HRTEM image of one of the Pd–Rh nanoden-
drites shown in Fig. 1D, which shows that the surface of the Pd core
was almost covered by a number of Rh branches. The Rh branches
had an average diameter of ꢀ3 nm. The HRTEM images in Fig. 4B
and C, reveal the continuous lattice fringes from the Pd core to
the Rh branches at the interfaces, indicating that the Rh bumps
were grown epitaxially on the Pd seed (note that Pd and Rh only
have a lattice mismatch of 2.3%). For the Rh branches, however, lat-
tice fringes were not aligned across the entire branches, suggesting
3 6
reduction of Na RhCl precursor by L-ascorbic acid. It is known that
a higher degree of supersaturation can facilitate the homogeneous
nucleation in a solution. The attachment of small Rh particles may
be driven by their high energy due to a large surface area-to-vol-
ume ratio [15,16]. In previous studies of seeded growth of bimetal-
lic nanodendrites [8–10], nanocrystals were considered to grow by
heterogeneous nucleation and subsequent growth by atomic addi-
tion. Our results clearly show that both homogeneous and hetero-
geneous nucleation of Rh occurred at the very early stages of the
synthesis and the Rh branches grew via particle attachment. This
kind of nanocrystal growth has also been observed in the synthesis
of Pd–Pt nanodendrites [11].
Palladium nanocrystals with different shapes were also evalu-
ated as seeds to direct the dendritic growth of Rh branches.
Fig. 5A–C, shows TEM images of Pd nanocrystals with cubic, octa-
hedral, and plate-like morphologies, respectively. The cubic and
octahedral nanocrystals are enclosed by the {1 0 0} and the
{1 1 1} facets, respectively, while the thin plates with a triangular
or hexagonal shape are covered by the {1 1 1} facets on their top
and bottom faces. Fig. 5D and E, shows TEM images of Pd–Rh nano-
crystals prepared with the cubic and octahedral Pd nanocrystals as
the seeds, respectively. In both cases, the products exhibited a
morphology similar to that of the Pd–Rh nanodendrites obtained
from the truncated octahedral Pd seeds shown in Fig. 1D. Interest-

H. Kobayashi et al. / Chemical Physics Letters 494 (2010) 249–254
253
Fig. 5. (A–C) TEM images of Pd nanocrystals with cubic, octahedral, and plate-like morphologies, respectively. (D–F) TEM images of Pd–Rh bimetallic nanocrystals grown on
the Pd seeds shown in (A–C), respectively. In each case, the synthesis was conducted by reducing Na
solution.
3 6
RhCl with L-ascorbic acid in the presence of Pd seeds in an aqueous
ingly, the use of Pd nanoplates as seeds resulted in the formation of
an unconventional nanostructure consisting of a dense array of Rh
branches that were distributed more or less evenly over the entire
surface of a Pd nanoplate core (Fig. 5F). These results indicate that
growth of Rh branches is essentially independent of the facets ex-
pressed on the surface of the Pd nanocrystals used as the seeds, and
suggest that our approach can be considered as a general route to
the synthesis of Pd–Rh bimetallic nanodendrites.
4. Summary
We have demonstrated the synthesis of Pd–Rh bimetallic nano-
dendrites consisting of a dense array of Rh branches supported on a
Pd nanocrystal core via a simple, aqueous-phase route. We have
shown that both homogeneous and heterogeneous nucleation of
Rh occurred at the very early stages of the synthesis and the
growth of Rh branches proceeded via particle attachment. The de-

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H. Kobayashi et al. / Chemical Physics Letters 494 (2010) 249–254
gree of Rh branching could be controlled by varying the concentra-
tion of a Na RhCl precursor involved in a synthesis. Combining the
properties of both Pd and Rh, the Pd–Rh bimetallic nanodendrites
hold great potential for fundamental studies of the catalytic prop-
erties. This synthetic strategy based on seeded growth could also
be extended to other bimetallic systems such as Pt–Rh.
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This work was supported in part by the NSF (DMR-0804088)
and startup funds from Washington University. J.W. was supported
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