Block copolymer mediated synthesis of dendritic platinum nanoparticles

Article abstract of DOI:10.1021/ja902485x

(Figure Presented) Dendritic platinum nanoparticles are straightforwardly synthesized in high yield via a one-step aqueous-phase reaction mediated by Pluronic F127 block copolymer from the reduction of a platinum complex by ascorbic acid without the need

Full text of DOI:10.1021/ja902485x

Published on Web 06/17/2009
Block Copolymer Mediated Synthesis of Dendritic Platinum Nanoparticles
Liang Wang† and Yusuke Yamauchi*,†,‡
World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute
for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and PRESTO, Japan Science and
Technology Agency (JST), 5 Sanban-cho, Chiyoda, Tokyo, 102-0075, Japan
Received March 28, 2009; E-mail: Yamauchi.Yusuke@nims.go.jp
Synthesis of nanostructured Pt with controlled size and shape is
of considerable interest.1 Much effort has been focused on the tuning
of the specific structural features of Pt nanostructures to produce
Pt catalysts with high surface area, so as to achieve high catalytic
performance and utilization efficiency.2 It has been shown that the
catalytic activity of Pt nanoparticles in the catalytic reaction between
hexacyanoferrate(III) and thiosulfate ions is shape-dependent.1a
Accordingly, various strategies have been developed to synthesize
Pt nanostructures with different shapes, such as nanospheres,3
nanowires,4 mesoporous structures,5 etc. Despite the above suc-
cessful demonstrations, the synthesis of a new type of Pt nano-
structures, e.g., dendritic Pt nanoparticles (DPNs) with a well-
defined shape, is still highly desirable and technologically important.
The structures of DPNs are favorable for reducing the Pt consump-
Figure 1. Bright-field TEM image of the DPNs. The inset image is a dark-
tion, providing high surface area, and facilitating enhanced per-
formance in catalytic applications. Very few examples of the
synthesis of DPNs have been demonstrated to date6 in which
complicated synthesis is needed and the surface area of the product
is relative low. Developing a reliable and facile strategy to finely
control the nanostructure of the DPNs is an urgent topic to be
solved.
field TEM image of the DPNs.
ordinary ultrasonic bath under a 56 kHz operating frequency for
10 min. As the Pt deposition proceeded, the color of the reaction
solution was gradually changed from transparent light brownish-
yellow to brown and then to opaque black (Figure S1). The Pt
complex was completely reduced (Figure S2). The shape and size
of the typical synthesized DPNs were characterized by TEM. Figure
1 and Figure S3A revealed the presence of abundant DPNs with a
shape distribution of complete dendritic shape, demonstrating the
high yield formation of the DPNs (∼100%). The size of the DPNs
ranged narrowly from 13 to 23 nm with an average diameter of
17.4 nm (Figure S4). The dendritic nature of the nanoparticles was
more evident in the images obtained by scanning TEM using a
high-angle annular dark field (HAADF) (Figure 1 Inset and Figure
S3B), and each branch of the DPNs was seen to end with a rice-
shaped 3-3.5 nm tip. The number of branches on each DPN
differed from particle to particle, ranging from a few to over 20.
The HRTEM image of the individual DPN, shown in Figure 2,
indicated the nanoparticle was a dendritic entity with branching in
various directions. The lattice fringes were coherently extended
across over several branches. Since the domain boundaries were
clearly observable, as indicated by the arrows, the individual DPN
was not single-crystalline nature. The filtered image in the square
area showed the lattice fringes corresponded to {111} planes of
Pt, because both d spacings were 0.23 nm and the dihedral angle
was ∼70° (Figure 2). The X-ray diffraction (XRD) pattern of the
product was for the fcc Pt crystal structure (Figure S5) consistent
with the selected area electron diffraction (SAED) pattern of the
sample (Figure S3A Inset). Interestingly, the shape of the DPNs
could survive to some degree after the heat annealing process at
250 °C for 2 h without evident change of their original shape, but
with a little aggregation (Figure S6).
Herein, a rapid, one-step, and efficient route was proposed to
synthesize DPNs in high yield, which was mediated by Pluronic
F127 block copolymer (PEO100PPO65PEO100) from the reduction
of a Pt complex by ascorbic acid (AA) without the need for any
organic solvents, direct templates, or ion replacements. Since the
synthesis of Au nanoparticles mediated by block copolymer was
developed,7 the block copolymer mediated synthetic strategy has
been successfully applied to synthesizing a few metal nanoparticles,
such as Au,8 Pt,9 and Pd10 nanoparticles. It was noted the metal
nanoparticles prepared by this strategy were limited to spherical
nanoparticles. The lack of sharp corners and edges devalue the
advantages of metal nanoparticles, especially for Pt nanoparticles
in catalytic application. The rich edges and corner atoms derived
from the dendritic structure of the DPNs are highly desired for
improving the catalytic performance.1a,2,6b In this regard, it would
be more interesting if the block copolymer mediated synthesis could
produce DPNs. In this synthesis, besides acting as a protecting
agent, Pluronic F127 molecules played another important role,
structure-directing agent. To the best of our knowledge, this
approach is the first report on the facile synthesis of DPNs through
a block copolymer mediated synthetic strategy.
To prepare the DPNs, 5 mL of 20 mM K2PtCl4 aqueous solution
containing 0.794 mM Pluronic F127 were placed in a small beaker
and then 5 mL of 0.1 M AA were quickly added, giving final
K2PtCl4, Pluronic F127, and AA concentrations of 10, 0.397, and
0.05 mM, respectively. The mixture solution was put into an
After consecutive washing/centrifugation cycles with water, the
product had a nearly “clean” surface (Figure S7). The N2
adsorption-desorption isotherm of the product (Figure S8) gave a
surface area of 56 m2 g-1, which is even higher than the reported
† WPI Research Center for MANA, NIMS.
‡ PRESTO, JST.
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9152 J. AM. CHEM. SOC. 2009, 131, 9152–9153
10.1021/ja902485x CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
(Pluronic surfactants) was known to form a crown-ethers-like
conformation, similar to a cavity structure in aqueous solution.7a
The hydrophobic PPO groups in the Pluronic polymer were
favorably adsorbed onto the surface of the deposited metal
surface.7,9 The Pluronic chains adsorbed on the Pt surface during
the Pt deposition in this system could form cavities and then
facilitate the formation of the DPNs. It was worth noting that the
used Pluronic F127 concentration was lower than its critical micelle
concentration (CMC).7a,b Ill-defined DPNs were produced by using
a Pluronic F127 concentration over its CMC (Figure S11). Over
CMC, the PPO groups existed in the core of the micelles and the
hydrophilic PEO groups were exposed on the micelles. Therefore,
the PPO groups could not effectively adsorb on the Pt surface during
the Pt deposition, which was unfavorable for dendritic growth.
Under the current system, a low Pt concentration (1 mM) resulted
in irregular nanoparticles (Figure S12A) and relatively higher Pt
precursor concentrations were favorable for facilitating the produc-
tion of DPNs in high yield and high quality (Figure S12B-D).
In summary, DPNs were straightforwardly synthesized in high
yield via a one-step aqueous-phase reaction mediated by block
copolymer under mild conditions. The proposed method was unique
in its simplicity. As-prepared DPNs possessed the highest surface
area of all reported unsupported Pt materials. Traditionally, block
copolymers could be utilized as direct templates for synthesis of
silica-13 and metal-based5 mesoporous materials. The proposed
block copolymer mediated synthesis might open a new door toward
creation of novel metal-based dendritic materials. We expect that
this synthetic concept could be a new bridge between two frontline
disciplines: block copolymer systems and dendritic metal design.
Figure 2. HRTEM image of one DPN (left) and filtered image of the square
area (right). The domain boundaries are indicated by the arrows.
highest value 53 m2 g-1 for unsupported Pt materials.4 This value
is very close to the value 57 m2 g-1, calculated roughly for the
surface areas of nanowires with 3.25 nm in average diameter,
indicating that the inner edge and corner area are truly accessible
from outside. Such a high surface area was created from the
branches with 3-3.5 nm tips. The combination of high surface area
with the nanoarchitectures consisting of edges is advantageous for
catalytic applications.
For comparison, the specific surface areas of Pt black range from
11
20 to 28 m2 g-1
.
Porous Pt nanoparticles synthesized at 210 °C
using a complicated organic phase showed a surface area of 14 m2
g-1 6a Although multiarmed Pt nanostars with high catalytic activity
.
could be prepared by using tetrahedral Pt nanocrystals as seeds,
the size distribution was varied.6b Therefore, this synthetic strategy
can be proposed as a more simple, rapid, and straightforward
method for synthesis of DPNs with high surface areas and high
yield.
Acknowledgment. This work was supported by WPI, MEXT
of Japan and partially supported by a Grant-in-Aid for Scientific
Research (No. 19850031) from JSPS and the Murata Science
Foundation.
To help understand the process of shape evolution in this system,
two intermediate products harvested at two different times during
the reaction solution kept in the brown color stage (Stage B in
Figure S1), which was most likely the rapid growth period, were
observed by TEM (Figure S9). As Figure S9A displayed at the
earlier harvested time, AA reduced the Pt complex precursor to
yield initial irregular particles with sprouts in random directions
formed likely by aggregation of discrete Pt nanoparticles. As the
reaction proceeded, the particle size continued to increase and
secondary branches began to grow from the bodies and main
branches due to the continuous precursor reduction, leading to the
growth of the immature particles to a size of ∼6 nm at the later
harvested time (Figure S9B). Such growth continuously occurred
as the reaction proceeded until complete consumption of the Pt
precursor in the reaction solution. At ∼10 min, the color of the
reaction color remained stable opaque black, suggesting that the
reaction was complete, and the mature structures of the nanoparticles
were obtained (Figure 1). This type of crystal growth was previously
observed for the formation of hyperbranched Ag nanostructures.12
To understand the role of Pluronic F127 in this system,
investigations were done by replacing Pluronic F127 with different
surfactants (Figure S10). Based on these investigations, it was
known that Pluronic surfactants were critical for the formation of
the DPNs shown in Figure 1. It was reasonable to speculate about
Pluronic F127 molecules serving as a structure-directing agent in
this system. The PEO group in PEO-PPO-PEO surfactants
Supporting Information Available: Preparation process, additional
TEM and photo images, EDX, XRD, SAED, and N2 adsorption/
desorption isotherm data. This material is available free of charge via
the Internet at http://pubs.acs.org.
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