Synthesis of monodisperse Pt nanocubes and their enhanced catalysis for oxygen reduction

Article abstract of DOI:10.1021/ja070440r

Monodisperse 8 nm Pt nanocubes are synthesized by reducing Pt(acac)₂ in the presence of oleic acid, oleylamine, and a trace amount of Fe(CO)₅· Self-assembly of these nanocubes results in a (100) textured array. The nanocubes show an enhanced catalysis toward oxygen reduction, and their specific activity is over twice as high as that from the commercial Pt nanoparticles. Copyright

Full text of DOI:10.1021/ja070440r

Published on Web 05/15/2007
Synthesis of Monodisperse Pt Nanocubes and Their Enhanced Catalysis for
Oxygen Reduction
†
‡
†
†
,†
Chao Wang, Hideo Daimon, Youngmin Lee, Jaemin Kim, and Shouheng Sun*
Department of Chemistry and DiVision of Engineering, Brown UniVersity, ProVidence, Rhode Island 02912,
and Technology & DeVelopment DiVision, Hitachi Maxell, Ltd., 6-20-1 Kinunodai, Tsukubamirai,
Ibaraki 300-2496, Japan
Received January 20, 2007; E-mail: ssun@brown.edu
Synthesis of platinum (Pt) nanoparticles with controlled sizes
and shapes is one of the most attractive goals in developing highly
1
active Pt catalysts for fine chemical synthesis. Well-dispersed Pt
nanoparticles are also an important catalyst for fuel cell reactions:
they catalyze hydrogen (or alcohol) oxidation at an anode and
2
oxygen reduction at a cathode. Pt nanoparticles are commonly
3
4
prepared by reducing a Pt salt with hydrogen, an alcohol, sodium
5
6
borohydride, or hydrazine. The syntheses have also achieved
3
,4b,5b
4c,7
partial control of nanoparticle shape in cube,
multipod, and
8
one-dimensional nanostructure, among which Pt nanocubes are of
particular interest in catalysis due to the presence of (100) dominant
faces on the particle surface. Studies on oxygen reduction in acid
electrolytes show that Pt (100) planes are more active than the (111)
9
ones. Using Pt nanocubes as catalyst, one would expect that less
Pt is needed in the fuel cell reactionsa goal that has long been
sought for commercialization of the fuel cell technology.10 Despite
the synthetic progresses made in the past, a reliable process for
producing monodisperse Pt nanocubes with controlled sizes is still
not available to date.
Here we report a simple high-temperature organic phase synthesis
of monodisperse Pt nanocubes and study their catalysis for oxygen
reduction in a fuel cell reaction condition. We recently reported
the synthesis of monodisperse FePt nanocubes by simultaneous
Figure 1. (A) TEM image of the 8 nm Pt nanocubes; (B) HRTEM image
of a single Pt nanocube; (C) XRD pattern of the 8 nm Pt nanocube array;
and (D) XRD pattern of the self-assembled 8 nm Pt nanocubes showing
(100) texture.
of the {100} planes at 0.196 nm in the face-centered cubic (fcc) Pt
crystal. Energy dispersive spectroscopic (EDS) analysis on the
nanocube assembly shows no evidence of Fe in the nanocubes
decomposition of iron pentacarbonyl, Fe(CO)
5
, and reduction of
platinum acetylacetonate, Pt(acac) , in the presence of oleic acid
2
and oleylamine.11 Oleic acid and oleylamine were used not only
for nanoparticle stabilization but also for nanoparticle shape control.
Sequential addition of oleic acid and oleylamine facilitated the
formation of FePt nanocubes. Our further synthesis revealed that,
regardless of the addition sequence of oleic acid and oleylamine,
nearly monodisperse Pt nanocubes could be separated as long as a
(Figure S2), indicating that the trace amount of Fe in the synthesis
does not have measurable effect on the particle composition. The
crystal structure of the Pt nanocubes was characterized by X-ray
diffraction (XRD). Figure 1C shows the XRD pattern of the fcc
structured 8 nm Pt nanocubes. Slow evaporation of the hexane
dispersion (30 min) of the 8 nm Pt nanocubes on a silicon substrate
led to a textured assembly, as shown in Figure 1D. The strong (200)
peak in the diffraction pattern indicates that the Pt nanocubes align
flat on the substrate. This further proves that the nanocubes have
a very narrow shape distribution.
5
trace amount of Fe(CO) was present during the reaction. Self-
assembly of these nanocubes gave a (100) textured array, indicating
a very narrow shape distribution in cubic Pt nanoparticles.
Electrochemical studies showed that the Pt nanocubes catalyzed
oxygen reduction in a 1.5 M H SO aqueous solution with specific
2 4
activity over 2-fold higher than that from the commercial Pt catalyst.
Monodisperse 8 nm Pt nanocubes were synthesized by dissolving
The Pt nanocubes were synthesized by high-temperature (200
°
C) reduction of Pt(acac)
Fe(CO) was the key to achieve this shape control. Without Fe-
CO) , Pt nanoparticles were still prepared, but the shape of the
particles was much less controlled (Figure S3). In the presence of
Fe(CO) , the cubes were obtained by controlling the heating rate
2
. The presence of the trace amount of
5
Pt(acac)
temperature and heating the solution at 120 and 200 °C in the
presence of Fe(CO)
solution in ODE.12 Figure 1A shows the typical
2
, oleic acid, and oleylamine in 1-octadecene (ODE) at room
(
5
5
5
transmission electron microscopic (TEM) image of the as-
synthesized 8 nm nanocubes (the lower-resolution TEM image is
also given in Figure S1). They are nearly 100% in cubic (or cube-
like) shape. High-resolution TEM (HRTEM) image of a single Pt
nanocube (Figure 1B) shows the Pt lattice fringes with the inter-
fringe distance measured to be 0.19 nm, close to the lattice spacing
at 3-5 °C per minute to 200 °C and heating at this temperature
for 30 min.12 TEM image of the reaction intermediate separated
5
from the mixture 15 min after the addition of Fe(CO) shows the
presence of ∼3 nm nanoparticles with polyhedral shapes (Figure
S4). This suggests that Pt nuclei produced in the presence of trace
5
amount of Fe(CO) have polyhedral shapes, and nanocubes evolve
from the growth of these polyhedral Pt nuclei. The shape of the Pt
particles was also controlled by the presence of oleylamine and
†
Brown University.
Hitachi Maxell, Ltd.
‡
6974
9
J. AM. CHEM. SOC. 2007, 129, 6974-6975
10.1021/ja070440r CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
the Pt (100) face.14 This suggests that the (100) face is dominant
in the 8 nm Pt nanocube assembly on the carbon paper. The specific
2
catalytic activity (in mA/cm ) was calculated by dividing the raw
electrocatalytic current over the surface area of the catalyst. Figure
2
2
C shows the specific activity for oxygen reduction (mA/cm ) of
the commercial Pt catalyst and the Pt nanocubes synthesized in
this work. For potentials ranging from 0.9 to 0.8 V (typical potential
range for oxygen reduction reaction), the activity of the Pt
nanocubes is over 2 times as high as that of the commercial catalyst,
indicating that the nanocubes indeed offer an enhancement in
oxygen reduction. Note that the shape of the Pt nanocubes after
electrochemical experiments was unchanged.
We have reported that the monodisperse Pt nanocubes can be
synthesized by the reduction of Pt(acac)
acid and oleylamine and a trace amount of Fe(CO)
2
in the presence of oleic
. Self-assembly
5
of these nanocubes results in a (100) textured array. The nanocubes
on a carbon paper show an enhanced catalysis toward oxygen
reduction, and their specific activity is over 2 times as high as that
from the commercial Pt catalyst. Detailed studies on size- and shape-
dependent catalysis of these Pt nanoparticles for oxygen reduction
at a cathode and methanol oxidation at an anode are underway.
Figure 2. (A) HRSEM image of a specific carbon fiber in the carbon paper
showing the presence of 8 nm Pt nanocubes. (B) Cyclic voltammograms
of the commercial Pt catalyst and the 8 nm Pt nanocubes with a scanning
rate of 5 mV/s in 1.5 M H2SO4 under constant nitrogen bubbling. (C)
Specific activity of oxygen reduction for (b) the commercial Pt catalyst
and (0) the 8 nm Pt nanocubes.
Acknowledgment. The work was supported by NSF/DMR
0606264 and a scholarship from Hitachi Maxell, Ltd.
Supporting Information Available: Pt nanoparticle synthesis and
characterization. This material is available free of charge via the Internet
at http://pubs.acs.org.
oleic acid. If oleylamine was used alone, spherical Pt nanoparticles
were obtained (Figure S5). Unlike the preparation of FePt nanocubes
in which the addition sequence of oleic acid and oleylamine is
important,11 the synthesis of Pt nanocubes under the current reaction
conditions does not need the sequential addition of these surfactants.
This suggests that the presence of oleic acid, a weaker surfactant
to Pt than oleylamine, facilitates the growth of the particles into
cubes. Note that oleic acid alone cannot stabilize the Pt nanoparticles
wellsmost of them form aggregates, but those suspended in hexane
show cubic shape (Figure S6) even though the particles are
polydispersed.
The catalytic activity of the 8 nm Pt nanocubes toward oxygen
reduction was evaluated in an electrochemical measurement
system12 and compared with that of the commercial spherical Pt
nanoparticle catalyst with a mean diameter of 3 nm (TEC10E50E
from Tanaka Noble Metal Ltd., Japan). The catalysts were deposited
on carbon paper (TORAY) (Figure S7) that had been sonicated in
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