Nanocrystals composed of alternating shells of Pd and Pt can be obtained by sequentially adding different precursors

Article abstract of DOI:10.1021/ja204447k

This paper describes a layer-by-layer epitaxial approach to the synthesis of multishelled nanocrystals composed of alternating shells of Pd and Pt by starting with seeds made of Pd or Pt nanocrystals. The synthesis was conducted by sequentially adding PtCl₄^2- and PdCl₄ ^2- salt precursors into a system containing either Pd or Pt seeds (in the shape of cuboctahedrons, octahedrons, plates, or cubes) together with a weak reducing agent such as citric acid (CA). The slow reduction kinetics associated with CA played an important role in the epitaxial growth of one metal on the other, resulting in the formation of Pd-Pt multishelled nanocrystals. Owing to the capping effect of CA for {111} facets of Pd and Pt, the multishelled nanocrystals tended to be enclosed by {111} facets in the form of octahedrons or thin plates, depending on the shapes of the Pd or Pt seeds: octahedrons for cuboctahedral, cubic, or octahedral seeds, and plates for platelike seeds.

Full text of DOI:10.1021/ja204447k

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Nanocrystals Composed of Alternating Shells of Pd and Pt Can Be
Obtained by Sequentially Adding Different Precursors
Hui Zhang,†,‡ Mingshang Jin,† Jinguo Wang,§ Moon J. Kim,§ Deren Yang,‡ and Younan Xia*,†
†Department of Biomedical Engineering, Washington University, St. Louis, Missouri 63130, United States
‡State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou,
Zhejiang 310027, People’s Republic of China
§Department of Materials Science, University of Texas at Dallas, Richardson, Texas 75083, United States
S Supporting Information
b
should be a smaller bonding energy between shell atoms than
ABSTRACT: This paper describes a layer-by-layer epitaxial
approach to the synthesis of multishelled nanocrystals
composed of alternating shells of Pd and Pt by starting with
seeds made of Pd or Pt nanocrystals. The synthesis was
conducted by sequentially adding PtCl42À and PdCl42À salt
precursors into a system containing either Pd or Pt seeds (in
the shape of cuboctahedrons, octahedrons, plates, or cubes)
together with a weak reducing agent such as citric acid (CA).
The slow reduction kinetics associated with CA played an
important role in the epitaxial growth of one metal on the
other, resulting in the formation of PdÀPt multishelled
nanocrystals. Owing to the capping effect of CA for {111}
facets of Pd and Pt, the multishelled nanocrystals tended to
be enclosed by {111} facets in the form of octahedrons or
thin plates, depending on the shapes of the Pd or Pt seeds:
octahedrons for cuboctahedral, cubic, or octahedral seeds,
and plates for platelike seeds.
between core and shell atoms. In these regards, it seems to be
impossible to grow multishelled nanocrystals comprising alter-
nating layers of two different noble metals (marked by A and B)
because the bonding energy of A to B is always larger than that of
either A to A or B to B, together with a higher electronegativity
for either A than B or B than A.
For a system involving Pd and Pt, PdÀPt bimetallic nano-
dendrites were usually obtained when seeds made of Pd nano-
crystals were used with a relatively strong reducing agent such as
ascorbic acid (AA) due to the following order in bonding
energies: EPtÀPt (307 kJ/mol) > EPtÀPd (191 kJ/mol) > EPdÀPd
(136 kJ/mol).16 In parallel, we have also successfully achieved
epitaxial overgrowth of Pt shells on well-defined Pd nanocrystals
by using citric acid (CA) as both reducing and capping
agents.17,18 This reduction offers a potential route to the synthesis
of PdÀPt multishelled nanocrystals. We believe that the slow
reduction kinetics of a Pt or Pd precursor by CA is instrumental
to the epitaxial growth of a Pt shell on a Pd seed, and vice versa.19
In this communication, we report the first demonstration of
heteroepitaxial growth of PdÀPt (or PtÀPd, depending on the
starting seed) multishelled nanocrystals by sequentially adding Pt
and Pd salt precursors into an aqueous solution containing Pd or
Pt seeds with CA as both capping and reducing agents. We chose
Pd and Pt as our focus because nanocrystals made of these two
metals have been of great interest, owing to their widespread
applications in catalysis and electrocatalysis.20À22 In addition, the
multishelled nanocrystals composed of alternating Pd and Pt
layers are of particular interest for possible new features arising
from the effective coupling between adjacent layers.
Figure 1 schematically shows how multishelled nanocrystals
are obtained by alternating the deposition of Pt and Pd shells on
a seed of Pd cuboctahedron. The key to the success of this
synthesis is the use of CA as both reducing and capping agents. As
a relatively weak reducing agent, CA can generate Pt and Pd
atoms at the right pace to ensure layered, epitaxial growth
between these two metals. In addition, the lack of a galvanic
replacement between Pd and a Pt salt precursor in the absence of
BrÀ ions, as discussed in our previous report,23 and the negligible
lattice mismatch between Pd and Pt (only 0.77%) are of critical
importance to the formation of PdÀPt multishelled nanocrystals
in high quality. As shown in Figure 1, the epitaxial growth of a Pt
ver the past decade, coreÀshell nanocrystals consisting of
O
two different metals have received great interest because of
their unique properties originating from the electronic coupling
between the metals and a wide variety of promising applications
in plasmonics, catalysis, and hydrogen storage.1À7 To this end,
bimetallic coreÀshell nanocrystals with well-defined shapes have
been synthesized from noble metals such as Au, Ag, Pd, and Pt
through heteroepitaxial growth of one metal on the surface of
seeds made of another metal.8À11 For example, Yang and co-
workers have demonstrated the synthesis of PtÀPd coreÀshell
nanocrystals in the shape of cube, cuboctahedron, and octahedron
using seed-mediated epitaxial growth.12 Han and co-workers
reported the synthesis of AuÀPd coreÀshell octahedrons
through a coreduction method with cetyltrimethylammonium
chloride (CTAC) as both the reducing agent and stabilizer.13 Xie
and co-workers described the formation of rectangular AuÀPd
coreÀshell nanorods enclosed by {100} facets through the
growth of a Pd shell on the {110} and {100} facets of Au
nanorods.14 As proposed by Tian and co-workers,15 hetero-
epitaxial synthesis of bimetallic coreÀshell nanocrystals in a
solution phase is limited by a set of specific conditions: (i) there
must be a small (typically <5%) mismatch in lattice constants
between the core and shell metals; (ii) the metal for shell must
have a lower electronegativity than the metal for core; (iii) there
Received: May 15, 2011
Published: June 16, 2011
r
2011 American Chemical Society
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Figure 1. Layer-by-layer epitaxial growth of PdÀPt multishelled nano-
crystals with a Pd cuboctahedron serving as the seed. The success of this
synthesis relies on the use of citric acid as both the reductant and capping
agents to ensure a conformal growth.
Figure 3. Morphological, structural, and compositional characterizations
of the triple-shelled nanocrystals in Figure 2D, which were prepared
using the standard procedure: (A, B) STEM images, (C) line-scan
EDX spectrum of an individual nanocrystal as indicated in (B), and (D)
high-resolution TEM image. Fourier transform (FT) pattern of an
individual triple-shelled nanocrystal shown in (D) recorded along the
Æ110æ zone axis.
the preferential adsorption of CA on the {111} facets.24 The
difference in size between cuboctahedrons and octahedrons
suggests the epitaxial growth of Pt shells with a thickness of
1À2 nm on the {111} facets of Pd seeds, which is similar to what
we reported for the growth of Pt shells on Pd plates.18 In the next
step, 24 mg of Na2PdCl4 was added into the reaction system for
epitaxial growth of Pd shells on the Pd@Pt coreÀshell octahe-
drons (Figure 2C). From this image, it is clear that the octahedral
shape of the Pd@Pt coreÀshell nanocrystals formed in the first
step was essentially retained during the formation of Pd shells
while the size was increased to 16 nm. As shown by a magnified
TEM image in the inset of Figure 2C, the nanocrystal comprised
three regions with different contrasts in the sequence (starting
from the center) of bright, dark, and bright, corresponding to the
Pd seed, Pt shell, and Pd shell, respectively. The Pd shell was
about 2 nm in thickness, as obtained by comparing the size of
Pd@Pt@Pd octahedrons with that of Pd@Pt octahedrons. In the
third step, another new shell of Pt was deposited, as shown in
Figure 2D, on the surface of the Pd@Pt@Pd concentric nano-
crystals by adding 21 mg of K2PtCl4 into the same reaction
system. In this case, the octahedrons were slightly truncated at
corners with an average size of 20 nm. The magnified TEM image
(inset of Figure 2D) shows an alternating distribution of four
regions with different contrasts, implying the formation of
Pd@Pt@Pd@Pt triple-shelled nanocrystals. We believe that
the method presented here can be further extended to generate
coreÀshell nanocrystals with more Pd and Pt shells by repeating
the aforementioned deposition procedures. In addition to the use
of Pd cuboctahedrons as seeds for the alternating deposition of Pt
and Pd shells, Pd octahedrons (Figure S1, SI) and plates (Figure
S2, SI) have also been employed to synthesize PdÀPt multi-
shelled nanocrystals with other different shapes.
Figure 2. (A) TEM image of Pd cuboctahedrons of 9 nm in size that
served as the seeds in the growth steps. (BÀD) TEM images of (B)
Pd@Pt, (C) Pd@Pt@Pd, and (D) Pd@Pt@Pd@Pt nanocrystals pre-
pared using the standard procedure. (Insets) TEM images of individual
nanocrystals at a higher magnification. Scale bars are 10 nm.
shell on a Pd cuboctahedron is achieved by reducing K2PtCl4
with CA, leading to the formation of a Pd@Pt coreÀshell
nanocrystal with an octahedral shape. The selective adsorption
of CA on {111} facets of Pt is responsible for the morphological
transition from a cuboctahedron to an octahedron due to a faster
growth along the Æ100æ direction. In the following steps, Pd and
Pt are sequentially deposited on the {111} facets to generate an
octahedral nanocrystal with an alternating distribution for the Pt
and Pd shells. Experimental details for the syntheses of Pd or Pt
seeds and PdÀPt multishelled nanocrystals can be found in the
Supporting Information (SI).
Figure 2 shows TEM images of the products obtained at
different stages of heteroepitaxial growth involving sequential
addition of Pt and Pd salt precursors at 90 ꢀC in the presence of
CA. The Pd cuboctahedrons with an average size of 9 nm
(Figure 2A) were prepared, using a procedure previously re-
ported by our group,19 and then were employed as seeds to
induce/direct the epitaxial growth of Pt. After the reaction had
proceeded for 9 h in an aqueous solution containing 300 mg of
CA, 14 mg of K2PtCl4, and 1 mL of the suspension of Pd
cuboctahedrons, Pd@Pt octahedrons with an average size of
12 nm were obtained (Figure 2B). A morphological transition
from cuboctahedrons to octahedrons suggests a faster growth
rate along Æ100æ direction than Æ111æ direction for the Pt due to
We further used high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM), energy
dispersive X-ray (EDX), X-ray diffraction (XRD), and high-
resolution transmission electron microscopy (HRTEM) to char-
acterize the morphology, structure, and composition of the
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Figure 5. (A) TEM image of Pt cubes 5 nm in edge length that served as
the seeds for the growth steps. (BÀD) TEM images of (B) Pt@Pd, (C)
Pt@Pd@Pt, (D) Pt@Pd@Pt@Pd nanocrystals prepared using the
standard procedure, except for the use of the Pt cubes as the seeds
and the addition of Pd and Pt precursors in a sequence of 24 mg
Na2PdCl4, 14 mg K2PtCl4, and 24 mg Na2PdCl4. (Insets) TEM images
of individual nanocrystals at a higher magnification. Scale bars are 10 nm.
Figure 4. TEM images of Pd@Pt@Pd double-shelled nanocrystals with
the Pd layers being different thicknesses by adding different amounts of
Na2PdCl4: (A) 2.4, (B) 4.8, (C) 9.6, and (D) 38.4 mg. (Insets) TEM
images of individual nanocrystals at a higher magnification. Scale bars
are 10 nm.
Pd@Pt@Pd@Pt coreÀshell nanocrystals. The HAADF-STEM
image in Figure 3A clearly shows the triple-shelled structure of
Pd@Pt@Pd@Pt octahedrons with some truncation at corners.
The compositional distribution of each element in the triple-
shelled octahedron was revealed by EDX line-scan analysis.
Figure 3C shows a line-scan EDX spectrum of elemental Pd
and Pt that was recorded through the center of an individual
nanocrystal (marked by an arrow in Figure 3B). As can be seen,
the Pd trace has three peaks, while the Pt trace has four peaks
(Figure 3C). Careful examination further indicates that the
position of the valley and peak in the Pt trace matches with the
peak and valley peaks in the Pd trace, suggesting the formation of
shells with alternating compositions of Pd and Pt. The XRD
pattern further reveals that the triple-shelled nanocrystals are
composed of Pd and Pt with a face-centered cubic structure
(Figure S3, SI). Figure 3D shows a typical HRTEM image of an
individual triple-shelled octahedron. The HRTEM image shows
well-resolved, continuous fringes in the same orientation from
the Pd core to the Pt and Pd shells, indicating that the multi-
shelled nanocrystal is a single crystal due to an epitaxial relation-
ship between Pd and Pt, as confirmed by the corresponding
Fourier transform (FT) pattern (inset of Figure 3D). The lattice
spacing between neighboring fringes in the Pd and Pt regions are
2.27 and 1.96 Å, respectively. These values are in good agreement
with the distances between two adjacent {111} and {200} planes
of face-centered cubic (fcc) Pd and Pt. We believe that the same
crystal structure and negligible lattice mismatching between Pd
and Pt (0.77%) are responsible for the formation of single-crystal
PdÀPt multishelled nanocrystals.
K2PtCl4 amount from 1.8 to 7.2 mg. We found that 14 mg of
K2PtCl4 was the maximum amount for generating Pd@Pt
coreÀshell nanocrystals in high purity and quality. Further
increase of the K2PtCl4 amount (e.g., to 28 mg as shown in
Figure S4D in SI) resulted in the formation of Pt nanocrystals
(marked by arrows) due to homogeneous nucleation. In addi-
tion, we could control the size of Pd@Pt@Pd double-shelled
nanocrystals in the second step by varying the amount of
Na2PdCl4. As shown by the TEM images in Figure 4, the size
of the Pd@Pt@Pd nanocrystals was increased from 14 to 18 nm
when the amount of Na2PdCl4 was increased from 2.4 to 38.4 mg.
It seems to be difficult to control the size of multishelled nano-
crystals over a broad range by varying the amounts of salt
precursors starting from the seeds with {111} facets on the
surface. The strong binding effect of CA to the {111} facets of Pd
and Pt was responsible for the size limitation of a nanocrystal
enclosed by {111} facets.
In order to further clarify the heteroepitaxial growth between
Pd and Pt, we also used Pt nanocubes as the seeds for the
synthesis of PtÀPd multishelled nanocrystals. Figure 5 shows
TEM images of the products obtained in different stages.
Figure 5A shows a TEM image of Pt nanocubes with an average
size of 5 nm. After reacting for 9 h in a mixture containing the Pt
nanocubes, Na2PdCl4, and CA, the nanocrystals became octa-
hedrons with an average size of 20 nm (Figure 5B). The magnified
TEM image (inset of Figure 5B) clearly shows that there was a
single cubic Pt seed in the center of each coreÀshell octahedron.
The Pt@Pd coreÀshell octahedron was formed via epitaxial
growth of a Pd shell along Æ100æ direction on the Pt cube in the
presence of CA, which was similar to the result of a previous
report.12 In comparison with the epitaxial growth of Pd on {111}
facets of Pd@Pt seeds (Figure 2C), the overgrowth of Pd on
{100} facets of Pt seeds proceeded extensively, resulting in the
enlarged size and morphological transition from cubes to
The size and shape of PdÀPt multishelled nanocrystals could
be readily controlled by varying the amounts of K2PtCl4 and/or
Na2PdCl4. Figure S4 in SI shows TEM images of Pd@Pt
coreÀshell nanocrystals obtained by adding different amounts
of K2PtCl4 in the first step. It is clear that there was a mor-
phological transition from cuboctahedrons to octahedrons, to-
gether with a size increase from 9 to 11 nm with increasing of the
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facets. In the following step, Pt and Pd shells were formed
through conformal overgrowth on the {111} facets of Pt@Pd
octahedrons in the presence of CA, respectively, as shown in C
and D ofFigure 5. The magnified TEM images (insets of Figure 5,
C and D) revealed that only a thin shell of Pt and Pd were
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alternating the salt precursors added to a system containing Pd
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capping agents). The slow reduction kinetics associated with CA
played an important role in promoting layer-by-layer epitaxial
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positions and distribution patterns for various applications.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, TEM
b
images, XRD pattern, and CV curves of PdÀPt multishelled
nanocrystals. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
xia@biomed.wustl.edu
’ ACKNOWLEDGMENT
This work was supported in part by a DOE subcontract from
the University of Delaware (DE-FG02-03 ER15468), a research
grant from the NSF (DMR-0804088), and startup funds from
Washington University in St. Louis. As a visiting scholar from
Zhejiang University, H.Z. was also partially supported by the
“New Star Program” of Zhejiang University. J.W. and M.K. were
supported by a grant from CNMT (2010K000336) under
the 21st Frontier R&D Program of the MEST, Korea. Part of
the research was performed at the Nano Research Facility, a
member of the National Nanotechnology Infrastructure Network
(NNIN), which is supported by the NSF (ECS-0335765).
’ REFERENCES
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dx.doi.org/10.1021/ja204447k |J. Am. Chem. Soc. 2011, 133, 10422–10425