Palladium concave nanocubes with high-index facets and their enhanced catalytic properties

Article abstract of DOI:10.1002/anie.201103002

Curvy cubes: Palladium concave nanocubes enclosed by high-index {730} facets were obtained in high purity by controlling the overgrowth of Pd cubic seeds (see scheme). The concave nanocubes showed a much higher catalytic activity than the conventional Pd

Full text of DOI:10.1002/anie.201103002

Communications
DOI: 10.1002/anie.201103002
Nanoparticles
Palladium Concave Nanocubes with High-Index Facets and Their
Enhanced Catalytic Properties**
Mingshang Jin, Hui Zhang, Zhaoxiong Xie, and Younan Xia*
[
5b]
Palladium nanocrystals have been actively explored in recent
years due to their unique properties and applications related
hyde as a reducing agent. Mirkin and co-workers demon-
strated the synthesis of Au concave nanocubes enclosed by 24
high-index {730} facets by seeded growth with cetyltrimethyl-
[1]
to catalysis. Like other systems, the catalytic activity of Pd
nanocrystals is strongly dependent on both size and shape,
with the shape playing a more important role in determining
the selectivity. Over the past decade, Pd nanocrystals have
been prepared in a variety of different shapes, with notable
examples including cube, octahedron, decahedron, icosahe-
dron, plate, rod, and bar that are typically enclosed by low-
[
5c]
ammonium chloride (CTAC) as a capping agent. Compared
to the use of a special etching, reducing, or capping agent,
kinetic control may represent a simpler and more powerful
route to the facile synthesis of noble-metal nanocrystals with a
concave structure on the surface. In general, a kinetically
controlled synthesis can be achieved by controlling a set of
reaction parameters, including reagent concentration, tem-
perature, injection rate, addition of ionic species. For exam-
ple, we have demonstrated the synthesis of Pt tetrahedra and
octahedra with a concave surface using an overgrowth
approach by varying the amount of a coordination ligand
for the Pt ions. Most recently, we reported a facile route to
the synthesis of rhodium, platinum, or platinum–rhodium
concave nanocubes by simply manipulating the reaction
kinetics with a syringe pump to alter the injection rate of a
salt precursor. Skrabalak and co-workers demonstrated the
synthesis of Au–Pd nanocubes with a concave structure by
[
2]
index facets such as {111}, {100}, and {110}. In comparison,
synthesis of Pd nanocrystals enclosed by high-index facets and
thus enhanced catalytic activities (arising from high densities
of atomic steps and kinks) have not succeeded until recently.
To this end, Sun and co-workers demonstrated an electro-
chemical approach to the synthesis of Pd tetrahexahedra
[
6]
(
greater than 60 nm in size) with high-index facets by applying
a square-wave potential to a polycrystalline sample of Pd
[
3]
powders supported on an electrode. Wang et al. reported
the synthesis of gold–palladium core–shell nanocrystals with
high-index facets by depositing Pd layers on Au tetrahexahe-
[
7]
[4]
[8]
dra or trisoctahedra by chemical reduction.
using a seed-mediated co-reduction method. Despite these
Parallel to the aforementioned convex systems, nano-
crystals with a concave structure on the surface have also
demonstrations, we still lack a simple and reliable method for
the direct synthesis of Pd concave nanocubes with compact
dimensions and in high purity, together with a potential for
high-volume production.
[
5]
received great attention for generating high-index facets. To
this end, we demonstrated the synthesis of Pd nanocubes with
[5a]
cavities on the surface through a wet etching process.
Herein, we report a simple route based on seeded growth
to the synthesis of Pd concave nanocubes bounded by high-
index {730} facets, where Pd nanocubes were used as seeds for
the reduction of a Pd precursor in an aqueous solution. The
reduction kinetics were controlled by manipulating the
concentrations of reagents, including Na PdCl , KBr, and
Zheng and co-workers reported a solvothermal method for
the synthesis of tetrahedral and trigonal bipyramidal Pd
nanocrystals with cavities in the interiors by using formalde-
2
4
[
*] M. Jin, Dr. H. Zhang, Prof. Y. Xia
Department of Biomedical Engineering, Washington University
Saint Louis, MO 63130 (USA)
ascorbic acid (AA). In general, reducing the concentrations of
Na PdCl and KBr or increasing the concentration of AA was
2
4
found to be beneficial to the formation of Pd concave
nanocubes. Owing to their high-index facets, the Pd concave
nanocubes exhibited substantially enhanced catalytic activity
relative to the conventional nanocubes enclosed by {100}
facets towards electro-oxidation of formic acid and Suzuki
coupling reaction.
E-mail: xia@biomed.wustl.edu
M. Jin, Prof. Z. Xie
State Key Laboratory for Physical Chemistry of Solid Surfaces and
Department of Chemistry, Xiamen University (People’s Republic of
China)
Dr. H. Zhang
State Key Laboratory of Silicon Materials and Department of
Materials Science and Engineering, Zhejiang University (People’s
Republic of China)
In a typical synthesis, an aqueous solution of Na PdCl was
2
4
injected (with a pipette) into a mixture containing poly(vinyl
pyrrolidone) (PVP), KBr, AA, and Pd seeds under magnetic
stirring. The Pd seeds were nanocubes with an average side
[
**] This work was supported in part by the NSF (DMR-0804088) and
startup funds from Washington University in St. Louis. As a visiting
student from Xiamen University, M.J. was also partially supported
by the China Scholarship Council (CSC). Part of the work was
performed at the Nano Research Facility (NRF), a member of the
National Nanotechnology Infrastructure Network (NNIN), which is
supported by the NSF under award no. ECS-0335765.
length of 18 nm (Figure S1, Supporting Information), which
[9]
were prepared using a procedure reported by our group.
A
closer examination indicates that a small portion of the cubes
were somewhat elongated along one of the axes to form
rectangular bars with aspect ratios (length to width) slightly
larger than one. Since both the cubes and bars were enclosed
by {100} facets, they are collectively called “nanocubes” for
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103002.
7850
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Angew. Chem. Int. Ed. 2011, 50, 7850 –7854

Figure 1. a) SEM and b) TEM images of Pd concave nanocubes that
were prepared using the standard procedure. c) TEM images of a Pd
concave nanocube recorded at different tilting angles.
Figure 2. a) HRTEM image and b) the corresponding Fourier transform
FT) pattern of a Pd concave nanocube recorded along the [001] zone
(
axis. c) HRTEM image of the corner region as marked by the white box
in (a), where the white and black colors represented {420} and {310}
facets, respectively. d) Schematic illustration of the formation of a Pd
concave nanocube from a cubic seed by preferential overgrowth at the
corners and edges.
simplicity. Figure 1 shows SEM and TEM images of Pd
concave nanocubes prepared using the standard procedure.
From the SEM image (Figure 1a), it is clear that each face of
the nanocube in the product was excavated by a square
pyramid in the center. As shown by the TEM image in
Figure 1b, the cubic nanocrystal exhibited a darker contrast in
the center than at the edges, thus confirming the formation of
a concave structure on the surface. The TEM images obtained
from a nanocube at different tilting angles (Figure 1c) also
support the formation of a concave rather than flat surface on
each side face. Additional SEM images in Figure S2 in the
Supporting Information demonstrate that the Pd concave
nanocubes could be obtained in high purity (> 90%) and
relatively large quantity. In addition to the transition from a
flat to a concave surface, the average edge length was found to
increase from 18 to 37 nm during the overgrowth process.
For a concave nanocube enclosed by high-index facets, the
Miller indices can be derived from the projection angles along
as well as some slight growth on the {100} facets along the
h100i directions (Figure 2d).
To elucidate the mechanism involved in the formation of
Pd concave nanocubes, we conducted a set of experiments
using the standard procedure, except that different concen-
trations were used for the reagents, including Na PdCl , KBr,
2
4
and AA. The results shown in Figure 3 indicate that the
formation of Pd concave nanocubes was highly sensitive to
the concentrations of these reagents. For Na PdCl and KBr,
2
4
lowering their concentrations (Figure 3a,c) favored prefer-
ential overgrowth at corners and edges of the Pd cubic seeds
and thus formation of concave nanocubes. In contrast, Pd
nanocubes with flat faces were obtained when the concen-
trations of Na PdCl (Figure 3b) and KBr (Figure 3d) were
[
10]
a selected crystallographic axis.
Table S1 (Supporting
Information) gives the angles for different types of high-
index facets when they are projected along the [001] direction.
Figure 2a shows a high-resolution (HR) TEM image of an
individual Pd concave nanocube viewed along the h100i zone
axis, as confirmed by the corresponding Fourier transform
2
4
increased relative to those used in the standard procedure.
When the concentration of Na PdCl4 was increased (Fig-
2
ure 3b) or the concentration of KBr was reduced (Figure 3c),
there was a tendency to form small Pd nanocubes through
homogeneous nucleation. In contrast, the concentration of
AA showed a different trend in influencing the formation of
Pd concave nanocubes. In this case, increasing the concen-
tration of AA (Figure 3 f) favored the formation of concave
nanocubes as opposed to the case of lowering its concen-
tration (Figure 3e). Taken together, the growth habit of Pd
cubic seeds could be readily controlled by varying the
concentrations of Na PdCl , KBr, and AA to manipulate
(
FT) pattern (Figure 2b). Obviously, eight side faces of a
concave cube can be projected edge-on from this orientation.
By comparing the measured projection angles (Figure 2a)
with the calculated ones (Table S1 in the Supporting Infor-
mation), these eight edge-on faces could be indexed as the
{
730} planes. The Miller indices of a high-index facet could
also be determined from the arrangement of atoms on the
[10]
edge-on projected face. The edge-on projected face of a Pd
concave nanocube in Figure 2c shows a series of alternating
2
4
the reduction kinetics.
{
420} (white color) and {310} (black color) sub-facets, result-
For a kinetically controlled synthesis, the diffusion rate
ing in an overall profile indexed as the {730} planes. Figure S3
in the Supporting Information shows an atomic model of {730}
planes with {100} terraces and {110} steps. On the basis of this
structural analysis, the Pd concave nanocube was supposed to
form by preferential overgrowth at corners and edges of a
cubic seed along the h111i and h110i directions, respectively,
(denoted by V ) of precursor from solution to the surface and
1
the growth rate (denoted by V ) at which atoms are generated
2
and added to the surface of a growing seed plays a critical role
in determining the growth habit of a seed, as previously
[
11]
discussed by Chernov. When V is much larger than V , the
1
2
concentration of precursor is identical in regions very close to
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7851

Communications
high concentrations of AA, V will be much smaller than V ,
1
2
and the new Pd atoms will be preferentially added to the
corners and edges of a Pd cubic seed, thus resulting in the
formation of a concave nanocube. This observation is differ-
ent from the result recently reported for the synthesis of Pt
[
7b]
concave nanocubes in the presence of KBr. The difference
can be attributed to the use of different reducing agents. For
the Pt system, Pt atoms were immediately formed in the
solution phase by reduction of the Pt precursor with a strong
reducing agent such as NaBH , and then added to the surface
4
of a growing Pt seed. In the case of Pd, the Na PdCl precursor
2
4
had to diffuse to the surface (or at least to the vicinity of the
surface) of a growing Pd seed and then be reduced into Pd
atoms owing to the use of a relatively weak reducing agent
AA. As a result, the dependency of growth behavior on the
concentration of reducing agent showed opposite trends in
these two systems.
Palladium nanocubes of other different sizes were also
employed as the seeds to grow Pd concave nanocubes.
Figure S4 (Supporting Information) shows TEM images of
Pd nanocubes of 10 and 6 nm in edge length that were used as
seeds, and their corresponding Pd concave nanocubes. The
results indicate that Pd concave nanocubes with sizes of 27
and 15 nm could still be obtained by preferential overgrowth
at corners and edges of the seeds. The results also suggest that
the size of the Pd concave nanocubes could be controlled over
a broad range by simply using Pd nanocubes with different
sizes as the seeds. For the 15 nm Pd concave nanocubes
obtained from the 6 nm seeds, the purity and uniformity still
need to be improved by optimizing the concentrations for the
reagents. In general, cubic seeds of large sizes will be
beneficial to the formation of high-quality concave nano-
cubes, as the concentration gradient on a large cube will be
greater than that on a small one.
Figure 3. TEM images of Pd nanocrystals that were prepared using the
standard procedure, except the variation in concentration for the
reagent listed below: a) 7.2 and b) 59.0 mg Na PdCl ; c) 30 and
2
4
d) 1200 mg KBr; and e) 30 and f) 120 mg ascorbic acid.
the surface of a seed and far away from the seed owing to fast
diffusion. An atom derived from a precursor molecule can be
added to different sites on a seed with equal probability, and
hence preferential overgrowth is not supported under this
condition. When V is much smaller than V , however, there
The Pd concave nanocubes 37 nm in size with {730} facets
exposed on the surface were then evaluated as catalysts for
electro-oxidation of formic acid. We benchmarked the
electrocatalytic activity of these concave nanocubes against
the conventional Pd nanocubes enclosed by {100} facets and
with an average size of 37 nm. According to previous work,
the catalytic activity of metal catalysts could be affected by
1
2
will be a gradient for the precursor concentration by which
the concentration decreases when moving from the bulk
solution towards the surface of a seed. As such, the atom
generated on the surface of a seed will be preferentially added
to the site with the highest reactivity owing to an insufficient
supply of atoms. Herein, the concentrations of Na PdCl ,
[
13]
different capping agents. However, since both the conven-
tional and concave nanocubes were synthesized using similar
methods (except the use of reagents at different concentra-
tions), their surfaces should be capped by similar chemical
2
4
KBr, and AA were adjusted to manipulate V and V and thus
1
2
the growth habit of Pd cubic seeds. Specifically, reducing the
ꢀ
concentration of Na PdCl will slow down V , and reducing
groups, PVP and Br ions. As such, we could directly compare
2
4
1
the concentration of KBr will speed up V owing to a fast
their activities to single out the influence of atomic structure
on the surface. In the first step, the electrochemically active
surface area (ECSA) of the Pd nanocrystals was determined
from the charges associated with the stripping of a Cu
monolayer underpotentially deposited (UPD) on their sur-
face, as shown in Figure S5 in the Supporting Information.
Figure 4 shows cyclic voltammograms normalized against the
ECSAs of these two catalysts for electro-oxidation of formic
acid at room temperature in a 150 mL solution containing
2
2
ꢀ
2ꢀ [12]
reduction rate for [PdCl4] relative to [PdBr ] . On the
4
other hand, increasing the concentration of AA (the reducing
agent) will always increase V . Taken together, preferential
2
overgrowth on the seeds is expected when the concentrations
of Na PdCl and KBr are reduced or the concentration of AA
[
14]
2
4
is increased.
For a cubic seed, the reactivity of different sites is
supposed to decrease in the order of corner, edge, and side
face. This difference in reactivity can be further amplified by
0.1m HClO and 2m HCOOH. It is clear that the Pd concave
4
ꢀ
the presence of Br ions that can selectively adsorb onto the
nanocubes exhibited higher electro-catalytic activity than the
conventional nanocubes, with a 1.9 times increase for the peak
current. We believe that the high-index {730} facets with a
{
100} facets and thus slow down the growth rate of a side face.
As a result, at low concentrations of Na PdCl and KBr or
2
4
7852
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ꢀ
1
1
1.3 s , which was 3.5 times higher than that of the conven-
ꢀ
1
tional Pd nanocubes (3.2 s ). Significantly, the Pd nano-
crystals in both catalysts maintained their morphologies
during the catalytic reaction, as revealed by TEM imaging
(Figure S8 in the Supporting Information).
In summary, we have demonstrated a facile synthesis of
Pd concave nanocubes enclosed by high-index {730} facets
through preferential overgrowth on Pd cubic seeds. The key
was to induce preferential overgrowth at corners and edges of
a cubic seed by lowering the concentrations of Na PdCl and
2
4
KBr or increasing the concentration of AA. Otherwise, the Pd
cubic seeds would grow into larger cubes enclosed by {100}
facets when the overgrowth took no preference along differ-
ent crystallographic directions. When compared with the
conventional Pd nanocubes enclosed by low-index {100}
facets, the Pd concave nanocubes exhibited substantially
enhanced catalytic activities towards electro-oxidation of
formic acid and in the Suzuki coupling reaction. This
approach based on kinetically controlled overgrowth can
potentially be extended to other noble metals to generate
nanocrystals with high-index facets and thus high activities
towards various types of reactions.
Figure 4. Cyclic voltammograms of the Pd nanocubes enclosed by
concave or flat faces, which were recorded at room temperature in a
solution containing 2m HCOOH and 0.1m HClO at a sweep rate of
4
ꢀ
1
5
0 mVs . The current density was normalized against the correspond-
ing ECSA. RHE=reversible hydrogen electrode.
higher density of low-coordinate atomic steps and kinks were
responsible for the enhanced activity for formic acid oxida-
tion. Besides this high surface activity generated from high-
index facets, the mass activity of Pd concave nanocubes can be
further enhanced by reducing the size of the nanocrystals (see
Figure S6 in the Supporting Information).
Received: May 1, 2011
Published online: July 5, 2011
Keywords: crystal growth · high-index facets · kinetic control ·
The Pd concave nanocubes 37 nm in size were also tested
as catalysts for the Suzuki coupling reaction, which was
performed using a procedure similar to what was described in
.
nanoparticles · palladium
[
4]
literature. In the reaction (Figure S7 in the Supporting
Information), phenylboronic acid is coupled with iodoben-
zene to form biphenyl over the Pd catalyst. In the absence of a
Pd catalyst, no biphenyl was detected in the reaction,
indicating that the Pd-containing nanocrystals were indispen-
sable for the coupling reaction. For each catalyst, the coupling
reaction was carried out three times under the same
conditions. Table 1 compares the catalytic properties of the
Pd nanocubes with a concave or flat surface for Suzuki
coupling reaction. For the Pd concave nanocubes, 99%
iodobenzene was converted into biphenyl after 20 min. In
comparison, a conversion yield of 38% was obtained for the
conventional nanocubes. When recycled one time, the con-
version yield for the Pd concave nanocubes was still as high as
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Palladium catalyst
Yield
%]
Number of
surface atoms
TOF
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[
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99ꢁ1
96
1.35ꢀ10
1.35ꢀ10
1.79ꢀ10
1.79ꢀ10
11.3ꢁ0.1
10.8
recycled concave cubes
conventional cubes
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Angew. Chem. Int. Ed. 2011, 50, 7850 –7854