Shaping Pd nanocatalysts through the control of reaction sequence

Article abstract of DOI:10.1039/b920523c

Cubic, multi-armed, and dendritic Pd nanoparticles with efficient electrocatalytic activities toward formic acid oxidation can be selectively prepared with high yields and good uniformity by just changing the injection sequence of reductant and surfactant

Full text of DOI:10.1039/b920523c

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Shaping Pd nanocatalysts through the control of reaction sequencew
Young Wook Lee,ab Minjung Kimab and Sang Woo Han*ab
Received (in Cambridge, UK) 5th October 2009, Accepted 15th December 2009
First published as an Advance Article on the web 14th January 2010
DOI: 10.1039/b920523c
Cubic, multi-armed, and dendritic Pd nanoparticles with efficient
electrocatalytic activities toward formic acid oxidation can be
selectively prepared with high yields and good uniformity by just
changing the injection sequence of reductant and surfactant.
agent. Fig. 1 shows typical scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) images
of the prepared Pd nanoparticles. As shown in Fig. 1,
variations of the injection order of ascorbic acid and CTAB
to the aqueous solution of K PdCl induce distinct morpho-
2
4
Pd nanoparticles appear to be of specific interest due to their
ability to catalyze a wide range of heterogeneous reactions,
logy changes of Pd nanoparticles. When CTAB was injected
prior to ascorbic acid, Pd nanocubes with an average edge
length of 21 Æ 2.5 nm were synthesized, whereas injection of
ascorbic acid prior to CTAB resulted in the formation
of sea urchin-like Pd nanodendrites with an average size of
54 Æ 7.6 nm. On the other hand, simultaneous addition of
CTAB and ascorbic acid into the reaction solution yielded
multi-armed nanoparticles with an average particle size of
18 Æ 4.5 nm which closely resemble the multi-octahedral Pt
1
–3
especially for C–C coupling reactions.
reported that Pd nanoparticles exhibit efficient electrocatalytic
It has also been
4
–7
activities in fuel cell applications
and are expected as a
8
,9
promising candidate for hydrogen storage materials. In this
regard, synthesis of Pd nanoparticles with controlled morpho-
logies is one of the most critical issues because all of these
applications need a fine control of the particle size and shape.
During the last decade, a large effort has been devoted to the
shape-controlled synthesis of the Pd nanoparticles by using
a number of synthetic strategies such as polyol processes,
aqueous phase synthesis methods, and template-assisted
nanocrystals synthesized by reducing H
2 6
PtCl with poly(vinyl
pyrrolidone) in the presence of a trace amount of iron
1
7
species. In the X-ray diffraction (XRD) patterns recorded
on three different shapes, five peaks were observed for each
pattern which can be indexed to the (111), (200), (220), (311),
and (222) reflections of a face-centered cubic (fcc) structure of
metallic Pd (JCPDS, card no. 05-0681), showing the crystalline
nature of the prepared particles (Fig. 2). The intensity ratio
between the (200) and the (111) diffractions (I(200)/I(111)) of
0.79 for a Pd nanocube sample is much larger than the
3
,10–15
syntheses.
In the synthesis of nanoparticles, a variety
of reaction parameters including type and concentration of
metal precursor, surfactant/stabilizer, reducing agent, and
foreign ion as well as solvent and temperature have been
varied. Through the variation of these parameters, the
reduction kinetics of metal precursors and relative growth
rate on particular crystallographic surfaces during the crystal
growth can be modulated, resulting in the formation of
1
1
conventional bulk intensity ratio (0.42), indicating that the
faces of the Pd nanocubes are primarily composed of {100}
planes, tend to preferentially lay on the supporting substrates
against their flat {100} facets. The I(200)/I(111) values for the
multi-armed and dendritic nanoparticles are 0.44 and 0.40,
respectively, which are comparable to the bulk value, showing
that {111} planes are dominant for these samples. High-
resolution TEM (HRTEM) images of the nanoparticles also
showed well-resolved continuous fringes with specific periods,
which were consistent with the {200} or {111} lattice spacing
of fcc Pd (Fig. S1, ESIw).
1
6
nanoparticles with various shapes.
Although the previous reports have shown the successful
preparation of particles with well-defined morphologies by
choosing appropriate combinations of reaction parameters,
the influence of the reaction sequence on the particle growth
has been rarely investigated. Here we demonstrate that cubic,
multi-armed, and sea urchin-like dendritic Pd nanoparticles
could be selectively prepared with high yields and good
uniformity by just changing the injection sequence of
reductant and surfactant under otherwise identical experi-
mental conditions (Scheme 1). To the best of our knowledge,
this is the first systematic study on the effect of injection
sequence of each component in the synthesis of nanoparticles
on the shape of resultant particles.
The formation of different shaped nanoparticles according
to the injection order of reactant and surfactant can be
Pd nanoparticles were prepared from the aqueous solution
2 4
of K PdCl with ascorbic acid as a reductant in the presence of
cetyltrimethylammonium bromide (CTAB) as a stabilizing
a
Department of Chemistry and KI for the NanoCentury, KAIST,
Daejeon 305-701 Korea. E-mail: sangwoohan@kaist.ac.kr
Environmental Biotechnology National Core Research Center,
b
Gyeongsang National University, Jinju 660-701, Korea
w Electronic supplementary information (ESI) available: Details for
synthesis and characterization, and additional experimental data. See
DOI: 10.1039/b920523c
Scheme 1 Cubic, multi-armed, and dendritic Pd nanoparticles can be
selectively prepared by changing the injection sequence of reductant
and surfactant under otherwise identical experimental conditions.
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1535–1537 | 1535

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2À
metal precursors (PdCl
4
) and corresponding fast nucleation
and growth of particles. It has been reported that high
concentrations of reduced metal atoms is crucial for the
1
9–22
formation of branched nanoparticles.
Owing to the fast
reduction rate of metal ions, there will be a large amount of
seeds with smaller sizes at the initial stage of reaction, and
subsequent fast, kinetically controlled growth of nanoparticles
resulting in the growth of dendritic particles. Although there
are a number of previous studies on the synthesis of metal
2
3
dendirtes,
facile synthesis of such the well-developed
dendritic Pd nanoparticles in aqueous solution has rarely been
2
4
reported. When CTAB and ascorbic acid were injected
simultaneously to the reaction solution, the reduction rate
is in the intermediate region, resulting in the formation of
multi-armed nanoparticles.
The suggested mechanism for the injection order-dependent
formation of multiple-shaped Pd nanoparticles was further
confirmed by following control experiments. When CTAB was
replaced by cetyltrimethylammonium chloride (CTAC), Pd
nanodendrites were exclusively formed regardless of the
injection order of surfactant and reductant (Fig. 3). This
indicates that the effect of ligand exchange was completely
suppressed by using CTAC as a surfactant, thus the fast
reduction kinetics of metal precursors was maintained whether
the reductant was introduced prior to the surfactant or
vice versa. Meanwhile, substitution of K PdCl with K PdBr
Fig. 1 SEM (left panels) and TEM (right panels) images of the
a) cubic, (b) multi-armed, and (c) dendritic Pd nanoparticles.
(
2
4
2
4
as a Pd precursor resulted in the formation of cubic Pd
nanoparticles regardless of the injection order (Fig. 3). These
experimental results clearly show that changing the order of
injection of reaction components can induce the modulation of
growth kinetics and thus produce metal nanoparticles with
different shapes.
Pd nanoparticles have been known to have efficient electro-
catalytic activity toward the oxidation of formic acid with
6
,7
higher resistance to CO than Pt catalysts, which make them
promising catalysts for fuel cell applications. On the basis of
this fact, the electrocatalytic activities of the different shaped
Pd nanoparticles toward formic acid oxidation were investi-
gated. Fig. 4a shows the cyclic voltammograms of formic acid
oxidation at Pd nanoparticles-modified electrodes in 0.1 M
Fig. 2 XRD patterns of the (a) cubic, (b) multi-armed, and (c) dendritic
Pd nanoparticles.
4
HClO solution containing 0.5 M formic acid at a scan rate of
ascribed to the modulation of reduction kinetics at the early
2
stage of the reaction. It has been reported that Pd binds Br
+
À
À
2À
more strongly than Cl ; the stability constant of PdBr
4
is
4
2À 15,18
about 10 times higher than that of PdCl
4
.
In this
42À
is readily transformed to PdBr in the
2À
regard, PdCl
4
presence of an excess of alkyltrimethylammonium bromide
by ligand exchange reaction. In our experimental condition,
2
4
À
the reduction rate of PdBr
was much slower than that of
2
À
PdCl4 (Fig. S2, ESIw). Taken together, the reduction rate is
adjusted to be in the low or medium region when the CTAB is
injected prior to ascorbic acid, resulting in the formation of
1
1
seeds with a cubic shape. Since bromide ion can promote the
formation of {100} and {110} facets through altering the order
of surface free energies by chemisorptions onto the surface of
1
2
Pd seeds, further growth of nanocrystals via atomic addition
to the surface of seeds generates Pd nanocubes. The formation
of Pd nanodendrites when the ascorbic acid was injected prior
to the CTAB can thus be the result of the fast reduction rate of
Fig. 3 TEM images of the Pd nanoparticles synthesized when (a,b)
CTAB was replaced by CTAC and (c,d) K PdCl was replaced by
K PdBr . Surfactant was injected prior to ascorbic acid in a and c, and
2
4
2
4
ascorbic acid was introduced prior to surfactant in b and d.
1
536 | Chem. Commun., 2010, 46, 1535–1537
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that the present work might open a new door toward
fabrication of novel nanostructures.
This work was supported by Basic Science Research
Program (Grant Nos. KRF-2008-313-C00415, R15-2003-
0
12-01001-0, R11-2008-052-02003), Nano R&D Program
(
Grant No. 2009-0082640), and Pioneer Research Center
Program (Grant No. 2009-0082813) through the National
Research Foundation of Korea (NRF) funded by the Korean
government (MEST).
Fig. 4 (a) Cyclic voltammograms for formic acid oxidation on
different Pd nanoparticles in 0.1 M HClO solution containing 0.5 M
4
À1
formic acid at a scan rate of 50 mV s . (b) Chronoamperometric
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Chem. Commun., 2010, 46, 1535–1537 | 1537