Synthesis of Pd-Rh core-frame concave nanocubes and their conversion to Rh cubic nanoframes by selective etching of the Pd cores

Article abstract of DOI:10.1002/anie.201206044

The art of addition and subtraction: By confining the nucleation and growth of Rh atoms only to the corners and edges of Pd cubic seeds, Pd-Rh core-frame nanocrystals were obtained with concave side faces. The Pd cores were then selectively removed by oxidative etching to generate Rh cubic nanoframes with a highly open structure (see picture). Copyright

Full text of DOI:10.1002/anie.201206044

Angewandte
Chemie
DOI: 10.1002/anie.201206044
Bimetallic Nanocrystals
Synthesis of Pd-Rh Core–Frame Concave Nanocubes and Their
Conversion to Rh Cubic Nanoframes by Selective Etching of the Pd
Cores**
Shuifen Xie, Ning Lu, Zhaoxiong Xie, Jinguo Wang, Moon J. Kim, and Younan Xia*
Controlling the shape, morphology, and/or structure of nano-
crystals has been a subject of intensive research because it
allows the properties of nanocrystals to be tailored, thus
enhancing their applications in catalysis, electronics, photon-
ics, sensing, and biomedical research.[1–5] For noble metals,
they tend to form polyhedrons with a solid structure and
enclosed by a convex surface consisting of low-index facets,
such as {111}, {100}, and {110} in different proportions.
Therefore, the most commonly observed shapes are octahe-
drons, cuboctahedrons, and cubes, with different degrees of
truncation at corners and edges. Herein we report a facile
synthesis of Pd-Rh bimetallic nanocubes with a novel core–
frame structure and concave side faces by combining kinetic
control with surface capping, and their subsequent conversion
to Rh cubic nanoframes by selective removal of the Pd cores
by wet etching.
cubes with pores in the walls), and nanoframes (hollow cubes
only with ridges and no side faces).[2–4] Such nanocrystals have
been reported for a number of metals, including Au, Ag, Pd,
Pt, and Rh,[3–5] although most of them were made of a single
metal. There are only a few reports on bimetallic systems,
including core–shell concave nanocrystals in the composition
of Pt@Rh, Au@Pd, and Pd@Au.[5e,6,7] Owing to the formation
of a core–shell structure, only those atoms situated on the
outer surface of such a bimetallic nanocrystal are available for
catalyzing chemical reactions. In comparison, it will be
a significant advantage to formulate a bimetallic nanocrystal
into a core–frame structure, in which the atoms of both the
core and frame portions will be accessible to reactants.
Seed-mediated growth has been widely used by many
groups for generating bimetallic nanocrystals,[6–8] but most of
the products are limited to the core–shell structure. It remains
a major challenge to achieve site-selective deposition of the
second metal on the seed and thus generate an incomplete
shell. Most recently, our group demonstrated that kinetic
control is a simple and versatile means for achieving site-
selective overgrowth.[7,9] By controlling the rate at which Ag
atoms are formed in a solution, for example, we could confine
the nucleation and growth of Ag to one, three, or six of the
{100} side faces of a Pd cubic seed.[9] As a result, Pd-Ag
bimetallic nanocrystals with both Pd and Ag being exposed on
the surface were obtained in high yields. Herein we demon-
strate another type of site-specific growth in which Rh atoms
are only allowed to nucleate and grow at the corners and
edges of Pd cubic seeds to generate Pd-Rh nanocubes with
a core–frame structure and concave side faces. The spatial
confinement was achieved through a combination of kinetic
control and selective capping of Pd {100} facets by Brꢀ ions.
As Rh is highly resistant to oxidative corrosion,[10] the Pd-Rh
core–frame nanocubes were subsequently converted into Rh
cubic nanoframes with a highly open structure by selective
etching of the Pd cores.
Nanocrystals with hollow structures and/or concave faces
have recently started to receive great interest owing to the
presence of high-index facets on their surfaces, as well as
unique optical and catalytic properties. Notable examples
include nanocubes with concave side faces, nanocages (hollow
[*] S. Xie, Prof. Y. Xia
The Wallace H. Coulter Department of Biomedical Engineering,
Georgia Institute of Technology and Emory University, School of
Chemistry and Biochemistry and School of Chemical and Biomo-
lecular Engineering, Georgia Institute of Technology
Atlanta, GA 30332 (USA)
E-mail: younan.xia@bme.gatech.edu
S. Xie, Prof. Z. Xie
State Key Laboratory for Physical Chemistry of Solid Surfaces and
Department of Chemistry College of Chemistry and Chemical
Engineering, Xiamen University
Xiamen 361005 (P. R. China)
Dr. N. Lu, Dr. J. Wang, Prof. M. J. Kim
Department of Materials Science and Engineering, University of
Texas at Dallas
Richardson, TX 75080 (USA)
Figure 1 shows a summary of all the major steps involved
in the formation of Pd-Rh bimetallic nanocubes with a core–
frame structure and concave side faces, and their subsequent
conversion into Rh cubic nanoframes. Firstly, uniform Pd
nanocubes of 18 nm in edge length (Supporting Information,
Figure S1) were prepared by reducing Na2PdCl4 with l-
ascorbic acid (AA) in an aqueous solution with Brꢀ ions
serving as a capping agent for the Pd(100) surface.[11] The as-
obtained Pd nanocubes were slightly truncated at all corners
and edges, with the surface being mainly covered by the Brꢀ-
capped {100} facets. These Pd cubes were then used as seeds
for the overgrowth of Rh atoms in the presence of Brꢀ ions.
Prof. M. J. Kim
Department of Nanobio Materials and Electronics, World Class
University, Gwangju Institute of Science and Technology
Gwangju 500-712 (Korea)
[**] This work was supported in part by a grant from the NSF (DMR-
1215034) and start-up funds from Georgia Institute of Technology.
As a visiting student from Xiamen University, S.X. was also partially
supported by the China Scholarship Council (CSC). N.L., J.W., and
M.J.K. were supported by the World Class University Program from
MEST through NRF (R31-10026).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206044.
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to Pd-Rh concave nanocubes with a core–frame structure. In
the initial stage of the synthesis, the shape and size of the
obtained Pd-Rh nanocrystals (Figure 2a) seemed to be almost
identical to those of the initial Pd nanocubes when a relatively
small amount (1.0 mL) of Na3RhCl6 was added. However,
a close examination by high-resolution high-angle annular
dark-field scanning transmission electron microscopy
(HAADF-STEM) imaging (Figure 2b and c) clearly shows
that the corners of the Pd nanocubes were slightly protruded.
There are distinct atomic steps extending from the corner to
the edge, suggesting that the overgrowth of Rh atoms started
from the corners of a Pd seed and then extended to the edges.
With the addition of more Na3RhCl6 solution, the resultant
Rh atoms were continuously deposited onto the corners and
edges. As shown by the TEM images in Figure 2d–f, the
extent of concavity for the Pd-Rh core–frame nanocubes and
the average edge length both increased as more Na3RhCl6
solution was added. After 6.0 mL of the Na3RhCl6 solution
had been pumped into the reaction solution, the edge length
of the final products reached about 24 nm. Because most of
Figure 1. The three major steps involved in the synthesis of Pd-Rh
core–frame nanocubes with concave faces and Rh cubic nanoframes.
1) Selective nucleation of Rh at the corners and edges of the Pd
nanocubes owing to the capping of {100} side faces by Brꢀ ions;
2) formation of Pd-Rh bimetallic core–frame concave nanocubes as the
growth of Rh on the corners and edges of the Pd nanocubes is
continued; and 3) formation of Rh cubic nanoframes by selectively
etching away the Pd cores.
The Rh precursor was introduced at a relatively
slow rate using a syringe pump to avoid self-
nucleation and to achieve kinetically con-
trolled growth. The adsorbed Brꢀ ions pre-
vented the newly formed Rh atoms from
nucleating on the {100} side faces, so the Rh
atoms could only nucleate at the corners and
edges of a Pd cubic seed. With the deposition
of Rh atoms at all corners and edges, we
obtained Pd-Rh bimetallic nanocubes with
a core–frame structure and concave faces, the
surface of which was covered by Pd {100} and
Rh {110} facets. In the last step, the Pd cubic
cores were selectively dissolved from the Pd-
Rh core–frame nanocubes using an aqueous
etchant based on the FeIII/Brꢀ pair, generating
Rh cubic nanoframes with a unique open
structure.
The synthesis of the Pd-Rh core–frame
Figure 2. TEM images of Pd-Rh core–frame, concave nanocubes prepared using the
standard procedure after different volumes of Na3RhCl6 solution in EG (2.5 mgmLꢀ1
had been added into the reacting system: a) 1.0, d) 2.0, e) 4.0, and f) 6.0 mL.
b),c) High-resolution HAADF-STEM images of a single Pd-Rh nanocube shown in (a)
and the region marked in (b). The scale bar in the first inset is 10 nm, and applies to all
other insets.
nanocubes was conducted in a solution of
ethylene glycol (EG) containing poly(vinyl
pyrrolidone) (PVP), AA, KBr, and the 18 nm
Pd cubic seeds at 1408C. The Na3RhCl6
solution (in EG) was introduced at a slow
rate of 4.0 mLhꢀ1 by the use of a syringe pump
(see the Supporting Information for details).
)
Our previous studies on the preparation of Rh concave
nanocubes and multipods have shown that the reduction of
RhIII was extremely fast under the experimental conditions.[5e]
As a result, the overall formation rate of Rh atoms was
determined by the pumping rate, and the concentration of Rh
atoms in the reaction system could thus be maintained at
a very low level to achieve kinetically controlled growth.
Under this condition, self-nucleation of Rh could be avoided,
providing opportunities for the generated Rh atoms to
nucleate and grow on the Pd seeds only. Figure 2 shows
TEM images of the products obtained at different stages of
a synthesis, clearly detailing the evolution from Pd nanocubes
the Pd-Rh core–frame concave nanocubes were projecting
along the < 100 > direction on the copper gird, the corners
appeared darker than the edges. This also explains why the
out-extending corners gave the final products a pod-like
appearance. When the synthesis was conducted in the absence
of additional Brꢀ ions, the resultant Rh atoms also nucleated
and grew on the side faces of a Pd seed in addition to the
corners and edges (Supporting Information, Figure S2),
demonstrating the blocking effect enabled by the Brꢀ ions
adsorbed on the Pd(100) surface.
The structure and composition of the bimetallic Pd-Rh
nanocrystals in Figure 2 f were further characterized by
2
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scanning electron microscope (SEM), HAADF-STEM, and
energy dispersive X-ray (EDX) analyses (both line scanning
and mapping). As shown in Figure 3a and b, most of the Pd-
Rh nanocrystals still had a cubic shape, together with a core–
extending Rh portion was dominated by Rh {110} facets. As
the lattice mismatch between Pd and Rh is only 1.5%, it was
impossible to differentiate the elemental compositions from
lattice spacing. We applied EDX analyses to further deter-
mine the distributions of Pd and Rh in the core–frame
concave nanocube. The EDX mapping (Figure 3e) clearly
shows a color difference between the core (Pd, red) and the
frame (Rh, green). Line scan profiles along corner-to-corner
and edge-to-edge directions of a single Pd-Rh concave
nanocube (Figure 3 f and g) further confirm that the out-
extending corners and edges were dominated by Rh, while the
cubic core was essentially made of pure Pd.
As demonstrated in previous studies, both Pd and Rh
nanocrystals play important roles in the area of catalysis, such
as Suzuki coupling, CO oxidation, and hydrogenation reac-
tions.[12,13] Theoretically, for nanocrystals of fcc metals, the
surface free energies of low-index facets increase in the order
of g{111} < g{100} < g{110}. As the catalytic activities can be
enhanced with the increase of surface energies, it should be
expected that these bimetallic core–frame concave nanocubes
with both Pd {100} and Rh {110} facets simultaneously
exposed on the surface could provide some unique features
for catalytic reactions. On the other hand, it is worth noting
that there are differences in chemical stability and reactivity
between Pd and Rh. Specifically, Rh is highly resistant to
oxidative corrosion and can hardly be dissolved by aqua
regia.[10] On the contrary, Pd is much more susceptible to
chemical oxidation. Based on this difference in reactivity, we
could selectively remove the Pd cubic cores from the Pd-Rh
core–frame nanocubes to generate Rh nanoframes by chem-
ical etching.
The etching was conducted in an aqueous solution at
1008C by using an etchant based on the FeIII/Brꢀ pair, with the
addition of HCl to avoid the formation of insoluble Fe(OH)2
(see the Supporting Information). As shown by the TEM
image in Figure 4a, most of the Pd cubic cores were
selectively removed, leaving behind Rh cubic nanoframes
with a unique open structure. This result further confirms that
the Pd-Rh concave nanocubes had a core–frame structure.
Figure 4b and c shows TEM images of the Rh cubic nano-
frames projected along h100i and h110i directions, respec-
tively. The Rh ridges in the cubic nanoframe were only about
3 nm in thickness. Figure 4d shows a 3D atomic model of the
nanoframe and its projections along different zone axes.
Figure S4 shows a series of TEM images of Rh nanoframes
that were recorded from the same area of a copper grid at
different tilt angles, further confirming the open, frame-like
structure. Figure S5 shows an EDX spectrum of the cubic
nanoframes, indicating that they were consisted of pure Rh.
When Pd-Rh core–frame nanocubes with less amounts of Rh
deposited on the corners and edges were subjected to etching,
the as-obtained Rh frames tended to be broken as the Rh
ridges were too thin to provide enough mechanical strength
(Supporting Information, Figure S6). The cubic nanoframes
of Rh were thermally stabile up to 5008C as the frame
structure was well preserved after the sample had been heated
at this temperature for 1 h (Supporting Information, Fig-
ure S7).
Figure 3. Structural and compositional characterizations of the Pd-Rh
core–frame, concave nanocubes synthesized using the standard proce-
dure: a) SEM images; b),c) HAADF-STEM images; d) high-resolution
HAADF-STEM image of the region marked in (c); e) HAADF-STEM
image together with the EDX mapping of an individual Pd-Rh nano-
cube (red=Pd, green=Rh); and f),g) line scan profiles recorded from
a single Pd-Rh nanocube along the face diagonal and edge directions,
&
*
respectively. Rh black , Pd red . The scale bar in the inset of (a) is
10 nm.
frame structure and concave faces. The side face of each Pd-
Rh nanocube could be described as a “picture” in Pd encased
by a “frame” made of Rh (see the inset in Figure 3a). From
the high-resolution HAADF-STEM image (Figure 3c) of an
individual Pd-Rh concave nanocube, which was projected
along h100i zone axes, it can be clearly seen that, apart from
the corners, Rh also grew on the edges of a Pd cubic seed to
generate a perfect core–frame structure. A set of TEM images
of Pd-Rh concave nanocubes were recorded (Supporting
Information, Figure S3) from the same area of a copper grid
at different tilting angles, further confirming the concave,
core–frame structure. Figure 3d shows a HRTEM image, and
the lattice fringe spacing of 1.35 and 1.92 ꢀ marked on
a corner and an edge of a concave nanocube can be indexed to
the {220} and {200} reflections, respectively, of fcc Rh.
Interestingly, the side surfaces of the out-extending edges
were parallel to the {220} planes, forming an projection angle
of 458 with the {100} planes. This result suggested that the out-
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open structure. Two key factors are important for the selective
etching: 1) the formation of a core–frame structure, in which
the surface of Pd core is fully exposed; and 2) the difference in
resistance to oxidative corrosion between Rh and Pd. We
believe this strategy, based on site-specific overgrowth and
selective etching, can be extended to other systems to
generate nanocrystals with other novel structures and proper-
ties.
Received: July 27, 2012
Published online: && &&, &&&&
Keywords: bimetallic nanocrystals · concave nanocubes ·
.
etching · palladium · rhodium
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D
Pd þ 2 Fe3þ þ 4 Brꢀ PdBr 2ꢀ þ 2 Fe2þ
ð1Þ
!
4
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In summary, we have demonstrated the synthesis of Pd-
Rh bimetallic nanocubes with a core–frame structure and
concave faces by confined overgrowth of Rh atoms at the
corners and edges of Pd cubic seeds. The site-specific
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manipulation of the formation rate of Rh atoms with a syringe
pump and selective blocking of Pd {100} facets by Brꢀ ions.
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Communications
Bimetallic Nanocrystals
The art of addition and subtraction: By
confining the nucleation and growth of
Rh atoms only to the corners and edges of
Pd cubic seeds, Pd-Rh core–frame nano-
crystals were obtained with concave side
faces. The Pd cores were then selectively
removed by oxidative etching to generate
Rh cubic nanoframes with a highly open
structure (see picture).
S. Xie, N. Lu, Z. Xie, J. Wang, M. J. Kim,
Y. Xia*
&&&& — &&&&
Synthesis of Pd-Rh Core–Frame Concave
Nanocubes and Their Conversion to Rh
Cubic Nanoframes by Selective Etching of
the Pd Cores
6
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