Facile synthesis of iridium nanocrystals with well-controlled facets using seed-mediated growth

Article abstract of DOI:10.1021/ja505716v

Iridium nanoparticles have only been reported with roughly spherical shapes and sizes of 1-5 nm, making it impossible to investigate their facet-dependent catalytic properties. Here we report for the first time a simple method based on seed-mediated growth for the facile synthesis of Ir nanocrystals with well-controlled facets. The essence of this approach is to coat an ultrathin conformal shell of Ir on a Pd seed with a well-defined shape at a relatively high temperature to ensure fast surface diffusion. In this way, the facets on the initial Pd seed are faithfully replicated in the resultant Pd@Ir core-shell nanocrystal. With 6 nm Pd cubes and octahedra encased by {100} and {111} facets, respectively, as the seeds, we have successfully generated Pd@Ir cubes and octahedra covered by Ir{100} and Ir{111} facets. The Pd@Ir cubes showed higher H₂ selectivity (31.8% vs 8.9%) toward the decomposition of hydrazine compared with Pd@Ir octahedra with roughly the same size.

Full text of DOI:10.1021/ja505716v

Communication
pubs.acs.org/JACS
Facile Synthesis of Iridium Nanocrystals with Well-Controlled Facets
Using Seed-Mediated Growth
†
‡
§
‡
†
§
Xiaohu Xia, Legna Figueroa-Cosme, Jing Tao, Hsin-Chieh Peng, Guangda Niu, Yimei Zhu,
,
†,‡
and Younan Xia*
†The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta,
Georgia 30332, United States
‡
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
§
Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United
States
*
S Supporting Information
achieve sustainable use, we need to maximize the performance of
their nanocrystals by carefully controlling the shape. To this end,
a variety of different shapes have been reported for Pt, Pd, and Rh
ABSTRACT: Iridium nanoparticles have only been
reported with roughly spherical shapes and sizes of 1−5
nm, making it impossible to investigate their facet-
dependent catalytic properties. Here we report for the
first time a simple method based on seed-mediated growth
for the facile synthesis of Ir nanocrystals with well-
controlled facets. The essence of this approach is to coat
an ultrathin conformal shell of Ir on a Pd seed with a well-
defined shape at a relatively high temperature to ensure
fast surface diffusion. In this way, the facets on the initial
Pd seed are faithfully replicated in the resultant Pd@Ir
core−shell nanocrystal. With 6 nm Pd cubes and octahedra
encased by {100} and {111} facets, respectively, as the
seeds, we have successfully generated Pd@Ir cubes and
octahedra covered by Ir{100} and Ir{111} facets. The
6
nanocrystals over the past few decades. However, to the best of
our knowledge, there is no literature report on Ir nanocrystals
with well-defined shapes. Most of the reported Ir nanocrystals are
irregular particles with sizes of 1−5 nm and roughly spherical
7
shapes. The difficulty in generating Ir nanocrystals with well-
controlled facets can be attributed to their small sizes. Because of
truncations at the corners and edges, the facets on the side faces
are too small to be resolved. The small sizes of Ir nanoparticles
can be ascribed to the relatively low energy barrier for
homogeneous nucleation compared with that for heterogeneous
nucleation during synthesis, as a result of which the newly formed
Ir atoms in a synthesis tend to self-nucleate and grow into small
particles rather than nucleate on the surface of preformed
particles to form larger particles.
Pd@Ir cubes showed higher H selectivity (31.8% vs 8.9%)
2
toward the decomposition of hydrazine compared with
Pd@Ir octahedra with roughly the same size.
Herein we demonstrate that nanocrystals exposing well-
defined Ir facets can be synthesized using seed-mediated growth,
by which Ir atoms resulting from the reduction of Na IrCl by
3
6
ascorbic acid (AA) and ethylene glycol (EG) were added onto
the surface of Pd seeds with well-defined shapes. The key to the
success of such a synthesis is to conduct the growth at a high
enough temperature (e.g., 200 °C), at which the surface diffusion
of the Ir adatoms is greatly accelerated. The fast diffusion of Ir
adatoms across the Pd surface ensures a layer-by-layer (LbL)
growth mode and thus the formation of a conformal coating of Ir.
As long as the Ir shell is smooth at the atomic level, the facets on
the resultant Pd@Ir core−shell nanocrystals will replicate those
on the initial Pd seeds. Therefore, nanocrystals covered by
different Ir facets can be readily obtained simply by using Pd
nanocrystals with different shapes as the seeds. Specifically, we
have successfully synthesized Pd@Ir cubes and octahedra
covered by Ir{100} and Ir{111} facets when Pd cubes and
octahedra, respectively, were used as the seeds. The facet-
dependent catalytic properties of these Ir-based nanocrystals
were investigated by employing them as catalysts for the selective
anocrystals of the Pt-group metals with a face-centered
N
cubic structure (i.e., Pt, Pd, Rh, and Ir) have received
particular interest in recent years because of their outstanding
performance in a vast variety of industrially important catalytic
1
reactions. It is well-documented that the activity and selectivity
of such nanocrystals for a structure-sensitive reaction are highly
dependent on the arrangement of atoms and thus the type of
crystallographic planes on the surface, which has a strong
2
correlation with the shape. In the oxygen reduction reaction, for
example, the catalytic activity of 6 nm Pd cubes enclosed by
100} facets with a square lattice was found to be 1 order of
{
magnitude higher than that of 6 nm Pd octahedra encased by
3
{
111} facets with a hexagonal lattice. In another example, the
hydrogenation of benzene catalyzed by 13 nm Pt cubes encased
by {100} facets generated only cyclohexene, whereas both
cyclohexane and cyclohexene were observed for 13 nm Pt
4
cuboctahedra covered by a mixture of {111} and {100} facets.
generation of H from the decomposition of hydrazine.
It is well-known that the Pt-group metals are very expensive
because of the ever-increasing demand and their extremely low
2
5
contents in the Earth’s crust (typically at a ppb level). In order to
Received: June 6, 2014
Published: July 24, 2014
reduce the loading amount of these precious metals and thus
©
2014 American Chemical Society
10878
dx.doi.org/10.1021/ja505716v | J. Am. Chem. Soc. 2014, 136, 10878−10881

Journal of the American Chemical Society
Communication
We started the synthesis with Pd@Ir core−shell cubes
enclosed by Ir{100} facets. In a standard procedure, a solution
of Na IrCl in EG was slowly introduced (2.0 mL/h using a
field scanning TEM (HAADF-STEM), and energy-dispersive X-
ray (EDX) mapping. The XPS data (Figure S4) confirm that the
shell of the Pd@Ir cubes is composed of Ir(0). The typical
HRTEM image of an individual Pd@Ir cube along the [100]
zone axis and the corresponding Fourier transform (FT) pattern
(Figure 1B) clearly show the Pd@Ir cube to be bounded by
Ir{100} facets. The HRTEM image also reveals the continuous
lattice fringes from the Pd core to the Ir shell, indicating an
epitaxial relationship between the two metals. As the lattice
mismatch between Pd and Ir is only 1.3% (3.89 vs 3.84 Å), it was
impossible to resolve the elemental compositions from the lattice
spacing, and the interfacial strain caused by lattice mismatch can
be neglected. We applied EDX analysis to examine the
distributions of Pd and Ir in each core−shell cube. The EDX
maps (Figure 1C) clearly show a color difference between the
core (green = Pd) and the shell (red = Ir), confirming that the
shell was dominated by Ir while the core was essentially made of
pure Pd.
3
6
syringe pump) into a mixture containing poly(vinylpyrrolidone)
PVP) as a stabilizer, AA as the reductant, and Pd cubes with an
edge length of 6 nm as the seeds, which had been preheated to
00 °C under magnetic stirring [see the Supporting Information
SI) for details]. The 6 nm Pd cubes encased by six {100} facets
(
2
(
−
were prepared in the presence of Br as a capping agent using a
previously reported procedure. As shown by the transmission
8
electron microscopy (TEM) images in Figure S1A in the SI, the
Pd cubic seeds were uniform in terms of both shape and size.
Figure 1A shows a typical TEM image of the Pd@Ir core−shell
We believe that fast diffusion of the Ir adatoms during growth
was responsible for the formation of a smooth Ir coating and thus
the appearance of well-defined Ir facets in the final products.
Figure 2A schematically illustrates two possible modes for the
Figure 1. (A) TEM image of Pd@Ir core−shell cubes with slight
truncations at the corners. The inset shows a TEM image taken from the
same sample at a higher magnification and a 2D schematic model. Red
arrows indicate slight truncations at the corner sites. (B) HRTEM image
of an individual Pd@Ir cube. The inset is the corresponding FT pattern.
(
(
C) HAADF-STEM image of an individual Pd@Ir cube and EDX maps
green = Pd, red = Ir).
cubes prepared using the standard procedure. A thin, conformal
Ir shell was formed on the surface of each Pd cubic seed, and the
shell can be clearly resolved due to the good contrast between Pd
and Ir. It can be seen that the cubic shape of the Pd seeds was fully
retained during the deposition of Ir. Our analyses on a large
number of particles (>50) indicated that the Pd@Ir cubes had an
average edge length of 7.5 nm, which is 1.5 nm greater than that
of the initial Pd cubic seeds. Therefore, the thickness of the Ir
shell on each Pd{100} facet was 0.75 nm on average. A closer
examination indicates that the Pd@Ir cubes had slight
truncations at the corners (red arrows in Figure 1A), whose
Figure 2. (A) Schematic illustrations of two possible modes of
deposition of Ir on Pd cubic seeds. (B, C) TEM images of Pd−Ir
bimetallic nanocrystals synthesized using the standard procedure for
preparing the Pd@Ir cubes except that (B) the reaction temperature was
decreased from 200 to 160 °C and (C) the 6 nm Pd cubic seeds were
replaced with the same amount of 18 nm cubic seeds and the volume of
Na IrCl solution injected was increased from 6 to 20 mL. The inset in
3 6
−
appearance can be attributed to partial desorption of Br ions
(B) is an HRTEM image taken from an individual particle shown in (B).
from the side faces of the Pd cubic seeds during the preheating
process because of the presence of a reductant. This was
9
growth of Ir on Pd cubic seeds. The newly formed Ir atoms are
expected to deposit onto the Br -free Pd(100) surface of a cubic
−
validated by the X-ray photoelectron spectroscopy (XPS) data
−
shown in Figure S2. Without the passivation by Br ions, the
seed because of the relatively high surface energy of Pd{100}.
Upon deposition, the Ir adatoms can migrate across the Pd
surface free energy of Pd{100} is expected to be higher than that
6
,9
10
of Pd{111}. Therefore, during the growth, Ir atoms tended to
preferentially nucleate and grow on the high-energy Pd{100}
facets in an attempt to reduce the total free energy of the system,
resulting in truncations at the corner sites. This argument was
also supported by the result that these slightly truncated Pd@Ir
cubes further grew into truncated Pd@Ir cubes with an enlarged
truncation area when additional Na IrCl was added to the
surface by surface diffusion. Only if the rate of surface diffusion
is higher than that of atom deposition can the growth proceed by
the LbL or Volmer−Weber mode, which gives rise to a smooth,
conformal coating of Ir. Otherwise, the growth proceeds by the
island or Frank−van der Merwe mode, resulting in the formation
of irregular Ir particles on the Pd surface and thus a rough Ir shell.
For the present synthesis, Na IrCl should be immediately
3
6
3
6
growth solution (Figure S3). We further characterized the
structure and composition of the Pd@Ir cubes in Figure 1A by
XPS, high-resolution TEM (HRTEM), high-angle annular dark-
reduced to Ir atoms because of the strong reducing power of
AA. Therefore, the concentration of the newly formed Ir atoms
in the reaction solution and thereby the deposition rate of Ir is
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Journal of the American Chemical Society
Communication
mainly determined by the rate of injection of the Na IrCl6
the surface. The core−shell structure was confirmed by the EDX
maps (Figure 3C; green = Pd, red = Ir). It is worth pointing out
that LbL growth in this case was still maintained even when the
reaction temperature was decreased from 200 to 160 °C (Figure
S7A), which is different from the case of Pd cubic seeds (Figure
2B). When the reaction temperature was further decreased to
120 °C, the growth was switched to the island mode (Figure
S7B). These results indicate that the diffusion rate of Ir adatoms
on the Pd(111) surface was higher than on Pd(100) surface at
the same temperature. Such a higher diffusion rate can be
ascribed to the relatively lower energy barrier for the diffusion of
3
solution, which can be tightly controlled through the use of a
1
0,11
syringe pump.
On the other hand, the rate of surface
diffusion can be conveniently varied by adjusting the reaction
1
0,11
temperature.
To validate our assumption, we first decreased
the reaction temperature from 200 to 160 °C while keeping the
Na IrCl injection rate the same as in the standard synthesis (2.0
3
6
mL/h). In this case, the rate of surface diffusion decreased, and
the Ir adatoms were expected to stay at the deposition sites,
facilitating island growth. As expected, Pd−Ir bimetallic
nanocrystals with a rough surface were produced as the final
products (Figure 2B). A similar morphology was observed when
the Na IrCl injection rate was increased from 2.0 to 8.0 mL/h
Ir adatoms (E ) on the Pd(111) surface. In general, a close-
diff
3
6
packed crystallographic plane such as (111) has a relatively
while the reaction temperature was maintained at 200 °C (Figure
S5). Besides the relatively high rate of surface diffusion, the small
size of the Pd cubic seeds (i.e., 6 nm) was also found to contribute
to the LbL growth and thus the formation of a smooth, conformal
Ir shell. A larger size for the Pd seeds would impose a longer
smoother surface and thus a lower Ediff than a loose-packed plane
12
with an open structure such as (100) or (110). For example, it
was shown that the value of Ediff for the diffusion of Rh adatoms is
0.16 eV on the Rh(111) surface and increases to 0.60 and 0.88 eV
12a
for Rh(110) and Rh(100), respectively. Another interesting
finding for the growth of Ir on Pd octahedral seeds was that
further increasing the volume of Ir precursor solution from 6 to
12 mL (Figure S8) did not result in an increase in the thickness of
the Ir shell. Instead, small Ir particles with irregular shapes due to
homogeneous nucleation and growth started to appear in the
product. This result implies that in the present system the energy
barrier for further deposition of Ir atoms on the Ir{111}-covered
octahedra is higher than that for the self-nucleation of Ir. We also
observed a similar phenomenon, known as self-termination for
10
travel distance for the Ir adatoms. As a result, the Ir atoms could
not diffuse across the entire Pd surface, leading to island growth
and thus the formation of rough Ir shell. For example, Pd−Ir
bimetallic nanocrystals with a rough surface were obtained when
the 6 nm Pd cubes were replaced by 18 nm Pd cubes as the seeds
while all of the other parameters were kept the same as in the
standard synthesis except that the volume of Na IrCl solution
3
6
injected was increased from 6 to 20 mL (Figure 2C).
In order to obtain nanocrystals covered by Ir{111} facets, we
extended the synthetic strategy from Pd cubes to Pd octahedral
seeds encased by eight Pd{111} facets. The procedure for the
growth of Ir on Pd octahedral seeds was the same as used for the
preparation of Pd@Ir cubes except for the substitution of 6 nm
Pd cubes with the same amount of 6 nm Pd octahedra. The
uniform 6 nm Pd octahedral seeds (Figure S6) were prepared
using citric acid as the capping agent for the Pd{111} facets
13
the growth of the (111) surface, on Pd and Ag octahedra.
Finally, we investigated the facet-dependent catalytic proper-
ties of the as-prepared Pd@Ir nanocrystals using selective H2
generation via the decomposition of hydrous hydrazine as a
model catalytic reaction. H generation is at the core of hydrogen
2
fuel cell technology. Hydrous hydrazine, H NNH ·H O, is one
2
2
2
of the safest and most efficient hydrogen storage materials, with a
3
14
according to a previously reported method. Figure 3A shows a
hydrogen content as high as 7.9 wt %. It is known that the
decomposition of hydrazine has a strong dependence on the type
15
of catalyst used and the reaction conditions. In general,
hydrazine can be decomposed in two ways: completely
(
H NNH → N + 2H ) and incompletely (3H NNH →
2 2 2 2 2 2
4
NH + N ). Nanoparticles based on Ir were found to be
3 2
effective in catalyzing the decomposition of hydrazine and the
16a
generation of H2. Obviously, maximizing the H selectivity for
2
Ir nanocrystals by engineering its surface is expected to be an
effective approach to reduce the loading of precious Ir and, at the
same time, more efficiently utilize the hydrogen storage material.
Nevertheless, it has been extremely difficult to study this subject
since essentially none of the reported Ir nanocrystals possess a
well-defined shape. The Pd@Ir cubes (Figure 1) and octahedra
(
Figure 3) covered by Ir{100} and Ir{111} facets, respectively,
enabled us to investigate the facet-dependent catalytic properties
of Ir nanocrystals toward the decomposition of hydrazine. For
comparison, we also prepared Pd@Ir cuboctahedra encased by a
mixture of Ir{100} and Ir{111} facets by coating Pd nanospheres
with smooth Ir shells (see Figure S9 for details). As shown in
Figure 4, all the three types of Pd@Ir core−shell nanocrystals
showed catalytic activity toward the decomposition of hydrazine,
while essentially no activities were observed for the correspond-
ing Pd seeds. The molar ratios of generated H + N to initially
added hydrazine were 1.18, 0.73, and 0.57 for the Pd@Ir cubes,
cuboctahedra, and octahedra, respectively, corresponding to H2
selectivities of 31.8%, 14.9%, and 8.9% (see the SI for details).
These results indicate that the Ir(100) surface has a much higher
Figure 3. (A) TEM image of Pd@Ir core−shell octahedra. The inset
shows a TEM image taken from the same sample at a higher
magnification and a 2D schematic model. (B) HRTEM image of an
individual Pd@Ir octahedron. The inset is the corresponding FT
pattern. (C) HAADF-STEM image of an individual Pd@Ir octahedron
together with EDX maps (green = Pd, red = Ir).
TEM image of the as-prepared Pd@Ir core−shell octahedra with
an average edge length of 7.8 nm. A thin, smooth Ir shell over the
octahedral Pd core can be clearly observed (Figure 3A inset).
The typical HRTEM image taken from an individual Pd@Ir
octahedron along the [110] zone axis and the corresponding FT
pattern (Figure 3B) indicate the exposure of Ir{111} facets on
2
2
H selectivity than the Ir(111) surface toward the decomposition
2
1
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ACKNOWLEDGMENTS
■
This work was supported in part by the NSF (DMR-1215034)
and startup funds from the Georgia Institute of Technology. Part
of the electron microscopy work at BNL was supported by the
U.S. Department of Energy, Office of Basic Energy Sciences,
Materials Sciences and Engineering Division, under Contract
DE-AC02-98CH1088.
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■
*
S
Supporting Information
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toward the decomposition of hydrazine, electron microscopy
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(
AUTHOR INFORMATION
■
(
9
(
Corresponding Author
younan.xia@bme.gatech.edu
Notes
M.; Zheng, Y.; Xie, S.; Li, N.; Lu, N.; Wang, J.; Kim, M. J.; Guo, L.; Xia, Y.
Phys. Chem. Chem. Phys. 2013, 15, 11822.
The authors declare no competing financial interest.
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