Understanding the role of oxidative etching in the polyol synthesis of Pd nanoparticles with uniform shape and size

Article abstract of DOI:10.1021/ja0513741

Uniform cubooctahedral nanoparticles of Pd were synthesized using a modified polyol process. Like the silver system, the Pd nanoparticles underwent dissolution due to oxidative etching by Cl-/O2. In the early stage of etching, the twinned particles in a sample were selectively removed to leave behind uniform, single-crystal cubooctahedra. If the etching was allowed to continue, the single-crystal cubooctahedra were also attacked and partially dissolved. Understanding the role of oxidative etching is critical to the achievement of both uniform shape and size. Copyright

Full text of DOI:10.1021/ja0513741

Published on Web 05/03/2005
Understanding the Role of Oxidative Etching in the Polyol Synthesis of Pd
Nanoparticles with Uniform Shape and Size
†
†
‡
,†
Yujie Xiong, Jingyi Chen, Benjamin Wiley, and Younan Xia*
Departments of Chemistry and Chemical Engineering, UniVersity of Washington, Seattle, Washington 98195-1700
Shaul Aloni and Yadong Yin
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received March 4, 2005; E-mail: xia@chem.washington.edu
Palladium nanoparticles serve as the primary catalyst for low-
1
temperature reduction of pollutants emitted from automobiles and
2
organic reactions, such as Suzuki, Heck, and Stille coupling. By
tailoring the size and/or shape, one can, in principle, enhance their
3
catalytic performance in a range of applications. To date, shape-
controlled synthesis has been achieved for many metals and alloys,
4
such as Co, Ag, Au, Pt, and FePt. Palladium nanoparticles of
various morphologies have also been prepared in the presence of
5
6
surfactants, with the mediation of RNAs, through the thermal
7
decomposition of a Pd-surfactant complex, and via the use of a
coordinating ligand. However, a more thorough understanding of
8
the possible chemical reactions involved in the formation of Pd
nanoparticles is still required in order to gain a better control over
their shape, crystallinity, and yield. Here, we demonstrate that Pd
cubooctahedral nanoparticles could be prepared with high yields
and good uniformity using a modified polyol process, in which
2
-
[PdCl
4
]
was reduced by ethylene glycol (EG) at 110 °C in the
9
presence of poly(vinyl pyrrolidone) (PVP). In particular, it was
found that oxidative etching of Pd nanoparticles by air might lead
to the removal of twinned particles in the early stage and the
dissolution of single-crystal cubooctahedra in the late stage. The
key to high yields of uniform cubooctahedral nanoparticles is to
utilize the former and eliminate the latter.
Figure 1. TEM images of Pd nanoparticles prepared in air at (A) t ) 5
min; (B) t ) 3 h; (C) t ) 5 h; and (D) t ) 7 h 40 min. Twinned particles
are indicated by tw. The inset of (A) shows the magnified image of a 5-fold
twinned nanoparticle.
Figure 1 shows TEM images of Pd nanoparticles sampled at
different stages from a reaction performed in air. At t ) 5 min
(
Figure 1A and Figure S1A), the sample mainly contained cubooc-
tahedra of 4-8 nm in size and 10% multiply twinned particles
MTPs). A magnified image of the 5-fold MTP is shown in the
(
inset. As the reaction proceeded to t ) 1 h, all the twinned particles
disappeared while the average size of the cubooctahedra grew to 8
nm. In the following 2 h, there was no significant change for both
size and shape. Figure 1B (also see Figure S1B) shows a typical
image of the sample obtained at t ) 3 h. During the next 2 h, the
cubooctahedra were slowly dissolved until they reached an average
diameter of 3 nm (Figure 1C). Beyond this point, the Pd particles
began to grow again until they reached an average size of 10 nm
by t ) 7 h 40 min (Figure 1D). By analyzing the images, the size
distribution of nanoparticles was found to be broader than those
sampled at t ) 3 h (Figure S2).
The cubooctahedral structure was supported by high-resolution
and dark-field TEM studies (Figure 2). The fringes in the HRTEM
image are separated by 2.0 Å, which agrees with the {200} lattice
spacing of face-centered cubic Pd. The dark-field image (upper right
inset) recorded by a {200} reflection beam unambiguously il-
lustrates both the single crystallinity of the nanoparticle and the
formation of {100} facets on its surface. The fringe orientation in
Figure 2. HRTEM image of a Pd cubooctahedron prepared in air at t )
3
h. The insets show the dark-field TEM image of a cubooctahedron using
the {200} reflection beam (upper right) and the geometrical model of the
cubooctahedron (lower left).
the HRTEM image implies that the nanoparticle is bound by 8
{
111} facets and 6 {100} facets. As supported by the superior
contrast in the dark-field TEM image, it can be concluded that the
single-crystal nanoparticle has a cubooctahedral shape, similar to
the model shown in the inset of Figure 2 (lower left). The powder
X-ray diffraction (PXRD) and electron diffraction (ED) pattern
†
Department of Chemistry.
Department of Chemical Engineering.
‡
7332
9
J. AM. CHEM. SOC. 2005, 127, 7332-7333
10.1021/ja0513741 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
cubooctahedra would start to dissolve slowly. This experiment not
-
only proves the role of oxidative etching by Cl /O
2
in the
dissolution of Pd nanoparticles but also demonstrates that selective
etching can be used to purify and thus control the shape of
nanoparticles.
In summary, uniform cubooctahedral nanoparticles of Pd were
synthesized via a modified polyol process. Understanding the role
of oxidative etching is critical to the achievement of both uniform
shape and size. By controlling the etching process, the twinned
particles in a sample could be selectively removed to leave behind
uniform, single-crystal cubooctahedra. Although nanoparticles of
other metals may be similarly susceptible to oxidative etching, this
factor has been largely unexplored. We believe that introduction
of an oxidant into the synthesis of metal nanoparticles may provide
a versatile tool to control their nucleation and growth into well-
defined shapes.
Figure 3. UV-vis spectra of solutions taken at different reaction stages.
The inset depicts the time dependence of the absorbance at ∼245 nm, which
2
-
is directly proportional to the concentration of the [PdCl4] species.
(Figure S3) also confirms the phase purity and crystallinity of the
Pd cubooctahedral nanoparticles.
To appreciate the oxidative etching process, we analyzed the
UV-vis spectra of the solution at different stages of the reaction
Acknowledgment. This work was supported in part by a
DARPA-DURINT subcontract from Harvard University and a
fellowship from the David and Lucile Packard Foundation. Y.X.
is a Camille Dreyfus Teacher Scholar (2002). J.C. and B.W. thank
the Center for Nanotechnology at the UW for a Nanotech and an
IGERT fellowship (funded by NSF, DGE-9987620), respectively.
(
Figure 3). The absorption peaks at 245 and 327 nm can both be
2-
4
attributed to [PdCl ] , while the peak at 212 nm corresponds to
2-
4
PVP (Figure S4). The change in the concentration of [PdCl ]
could be followed by plotting the change in absorbance at ∼245
Supporting Information Available: Experimental procedure; ED
and PXRD patterns of the Pd cubooctahedral nanoparticles produced
in air at t ) 3 h; UV-vis spectra of PVP and Na PdCl in EG; TEM
2 4
images of Pd nanoparticles synthesized under Ar; and geometrical
illustration of twinned decahedral and icosahedral particles. This
material is available free of charge via the Internet at http://pubs.acs.org.
-
nm against time (the inset). Similar to the case of Ag, the Cl /O
2
pair was responsible for the oxidative etching.10 Also, the higher
reactivity of twinned Pd particles caused them to be preferentially
etched in the early stage of a synthesis. They are more reactive
because of the necessarily higher density of defects on their
surfaces.1
1,12
Since the single-crystal cubooctahedra have fewer
defects, they are relatively more stable and oxidative etching of
their surfaces occurred more slowly. As a result, the twinned
particles were completely dissolved by t ) 1 h, and the size and
shape of cubooctahedra remained largely unchanged between t )
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