Letter
pubs.acs.org/NanoLett
Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen
Reduction Activity by Tuning the Alloy Particle Surface Composition
Chunhua Cui,† Lin Gan,† Hui-Hui Li,‡ Shu-Hong Yu,‡ Marc Heggen,§ and Peter Strasser*,†
†
The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division,
Technical University Berlin, Berlin 10623, Germany
‡
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and
Technology of China, Hefei 230026, P. R. China
§
Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
S Supporting Information
*
ABSTRACT: We demonstrate how shape selectivity and
optimized surface composition result in exceptional oxygen
reduction activity of octahedral PtNi nanoparticles (NPs). The
alloy octahedra were obtained by utilizing a facile, completely
surfactant-free solvothermal synthesis. We show that the choice
of precursor ligands controls the shape, while the reaction time
tunes the surface Pt:Ni composition. The 9.5 nm sized PtNi
octahedra reached a 10-fold surface area-specific (∼3.14 mA/
cm2Pt) as well as an unprecedented 10-fold Pt mass based
(∼1.45 A/mgPt) activity gain over the state-of-art Pt electrocatalyst, approaching the theoretically predicted limits.
KEYWORDS: Surface composition, PtNi octahedra, oxygen reduction reaction, ligand control
T
9−10× loss in activity when going from extended surfaces to
carbon-supported alloy nanoparticles,23 an activity gain of about
10× is realistically expected for a carbon-supported Pt−Ni
octahedron compared to a state-of-art carbon-supported
spherical Pt nanoparticle catalyst.
Surfactant-directed synthesis of well-defined shape-selective
Pt−Ni catalysts was recently reported by Wu et al. and Zhang et
al. using a careful choice of preparation techniques involving
distinctly different surfactants, reducing agents, and solvents.10−13 The octahedral Pt−Ni particles achieved a 4−7×
improvement in terms of the Pt surface area-specific activity but
displayed a mere ∼4× improvement in Pt mass activity over Pt
owing to residual capping molecules.10−12 Also, in all of these
studies, a fundamental understanding of how success or failure to
produce shape selective particles depends on the applied reaction
conditions has remained poorly understood. In particular, while
basic Wulff-type particle shapes (octahedra, cubes) could be
reproducibly produced and identified through atomic scale
microscopy, their precise surface compositions on individual
facets, key for their catalytic reactivity, remained unexplored and
uncontrolled.
More recently, Snyder et al. presented a size-dependent
dealloying study on nonshape selective Pt−Ni NPs and showed
the formation of porous Pt−Ni NPs above a critical diameter of
he sluggish kinetics of the oxygen reduction reaction
(ORR) on costly platinum cathode electrocatalysts
represents a major obstacle to a more widespread use of the
polymer electrolyte membrane fuel cell (PEMFC). Recent
rational design of geometric and electronic properties of
extended alloy catalyst surfaces have resulted in significant
improvements of the ORR activity.1−8 However, improving the
ORR activity further in practical nanoscale alloy catalysts is still a
great challenge.9 One of the most promising strategies is the
development of shape and composition-controlled Pt-based
alloy nanoparticle (NP) catalysts.10−14 These NP catalysts with
controlled shapes, that is, controlled exposed crystal facets, and
composition profiles hold the promise of providing the same
ideal surface electronic structure as extended surfaces, thereby
realizing their full activity advantage.15−17
Stamenkovic and co-workers reported a very active single
crystal Pt3Ni(111) surface, which performed 10 times (10×)
higher in ORR activity than a Pt(111) surface and 90× higher
than a commercial NP Pt/C catalyst.18 The enhancement was
attributed to the low coverage of hydroxyl species induced by the
specific electronic structure associated with an oscillatory nearsurface compositional Pt and Ni profile across the 2−4
outermost layers of the (111) surface.18−20 This was direct
experimental evidence that the near-surface composition and its
atomic arrangement are key factors to improve the ORR
activity.21,22 Stamenkovic’s report on Pt3Ni(111) triggered a
quest for shape-selective octahedral Pt alloy NPs, which would
exhibit only the active (111) facets; if successful, this could make
the 90× catalytic activity gain a reality. Considering the typical
© 2012 American Chemical Society
Received: September 3, 2012
Revised: October 3, 2012
Published: October 12, 2012
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Figure 1. (a) XRD patterns of PtNi octahedral NPs treated with different reaction times: (i) 16 h, (ii) 28 h, and (iii) 42 h. (b) Near surface compositions
of Pt (red bar) and Ni (green bar) measured by XPS (sampling depth of ∼2 nm); the red dotted line shows bulk Pt/Ni compositions measured by ICP
and EDX.
∼15 nm.24 Pt−Ni NPs ranging in particle size from 5 to 20 nm
demonstrate specific and mass ORR activity improvement
factors of 3−6× and 4−5×, respectively, due to their different
residual Ni contents and surface morphologies after dealloying.20,24,25 Carpenter et al. demonstrated 10× specific ORR
activity improvement for 12−15 nm shape-selective octahedral
Pt−Ni NPs, and their ORR-tested octahedra became porous,26
which is consistent with Snyder et al.’s conclusions. Despite their
favorable specific activity, however, the Pt mass activity was only
4−6× over Pt. In most of these PtNi studies, imperfections in
geometry and near-surface composition of the NPs were held
responsible for the lower than expected ORR activities.
Given our understanding of the electrocatalysis of octahedral
Pt−Ni nanoparticles outlined above, it is clear that a better finetuning of the near-surface composition of Pt−Ni octahedral
particles could have a great potential to further perfecting the
ORR activity gain over Pt. However, robust and facile synthetic
strategies to control the near-surface composition in octahedral
NPs have remained elusive to date and represent a critical unmet
need in fundamental fuel cell alloy electrocatalysis.
Here, we present a robust, facile, and surfactant-free
solvothermal synthesis of shape and size-selective octahedral
PtNi NPs. The shape selective NPs show an exceptional ORR
made possible by their carefully tuned alloy particle surface
composition. We show that the choice of precursor ligands
controls the shape selectivity, while we can use the reaction time
to tune the surface Pt:Ni composition and thus optimize the
ORR activity. We explain our findings in terms of a simple
nucleation/growth model. At a surface composition of about 40
at. % Pt, 9.5 nm-sized PtNi octahedra reached a 10-fold surface
area-specific (∼3.14 mA/cm2Pt) as well as an unprecedented 10fold Pt-mass based (∼1.45 A/mgPt) activity gain at 900 mV/RHE
and 5 mV/s anodic sweep rate over the state-of-art commercial
carbon-supported Pt electrocatalysts.
We have utilized a simple, surfactant-free, low-temperature
(120 °C) solvothermal synthesis to prepare unsupported sizeand shape-selective octahedral NPs. Figure S1 of the Supporting
Information shows the color change of the solvent from green to
black at 120 °C after 16 h indicating the formation of the PtNi
octahedral NPs. Figure 1a reports the X-ray diffraction (XRD)
patterns reflecting the bulk alloy phase structure of the PtNi
octahedral NPs after three different reaction times, 16 h, 28 h,
and 42 h (denoted as 16-PtNi, 28-PtNi, and 42-PtNi,
respectively). The bulk composition of the three alloy NPs was
determined as Pt:Ni = 46:54 by inductively coupled plasma mass
spectrometry (ICP-MS) and energy-dispersive X-ray spectra
(EDX) regardless of reaction time (Figure S2). This is consistent
with the three basically overlapping pattern profiles indexed to a
face-centered-cubic (fcc) phase. The octahedral morphology was
observed by transmission electron microscopy (TEM) in Figure
S3.
Following earlier reports on solvothermal techniques at higher
temperatures (200 °C),26 the dimethylfomamide (DMF) solvent
acts as a complexing agent, solvent, and reducing agent.
However, in this study, high heating rates (10 °C/min) and a
lower reaction temperature (120 °C) were utilized. A high
heating rate resulted in a short induction time and high
nucleation rates generating a large number of small seeds. Unlike
previous reports,24,26 the low reaction temperature favors slow
seed growth, keeping our particles small.24 We note that pure Pt
and pure Ni precursors could not be reduced at this low
temperature,26,27 suggesting a possible role of the exothermic
heat of mixing during PtNi alloy seed formation.28−31 The initial
seeds catalyzed the codeposition of Pt and Ni. Moreover, a lower
reaction temperature could favor the nanocrystal faceting during
growth in a colloidal solution.32
To obtain further insight in the formation mechanism of
octahedral alloy nanoparticles, we interrogated the influence of
the metal precursor ligands on the alloy particle shape selectivity.
Use of Ni(acac)2 and Pt(acac)2 reproducibly resulted in
octahedral nanoparticles. When Ni(acac)2 was replaced with
Ni acetate, keeping all other synthesis conditions constant,
uniform spherical alloy nanoparticles with ∼5 nm diameter were
obtained (Figure S4a). On the other hand, when the Pt(acac)2
precursor was replaced with K2PtCl6, particle aggregates with
smaller mean size, limited shape-selectivity, and wider size
distribution were observed (Figure S4b). These results suggested
that it is not the interaction of DMF with the (111) facets
alone,33 which determines the formation of octahedra; the
precursor ligands, such as acetyl acetonate, had a critical influence
on the particle shape and size through a modification of the
thermodynamic metal redox potential, possibly coupled to a
modified kinetic metal ion reduction (metal atom production)
rate. Both factors can have profound effects on the shapeselective growth.
We then studied how the reaction time affected the octahedral
surface composition of Pt and Ni using X-ray photoelectron
spectroscopy (XPS) (see Supporting Information). Based on the
estimated sampling depth of ∼2 nm, the measured data is
referred to as “near-surface” rather than “surface” composition.34
It must be noted that the measured surface composition is an
average value because the PtNi NPs are loaded on carbon and
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Table 1. Comparison of the ECSA, ORR Mass, and Specific
Activitiesa
reaction
timeb
particle sizec
(nm)
ECSA
(m2/gPt)
mass activity
(A/mgPt)
specific activity
(mA/cm2Pt)
16
28
42
9.0 ± 1.1
9.2 ± 0.9
9.5 ± 0.8
24.1
36.7
50.0
0.56 ± 0.065
1.02 ± 0.070
1.45 ± 0.120
2.35 ± 0.28
2.77 ± 0.20
3.14 ± 0.24
a
All activities at 0.9 V/RHE in 0.1 M HClO4, 1600 rpm, 5 mV/s.
Three independent synthesis/electrochemical tests. bReaction temperature was kept at 120 °C. cThe particle size is estimated by longest
length over two opposite ends.
every part of the particles has the same chance to be exposed to
the X-ray. Our XPS data are shown in Figure 1b; it evidences that
the reaction time directly controls the near-surface composition
of the resulting octahedral particles without affecting their size or
shape. Raising the reaction time from 16 to 42 h, the near-surface
Pt at. % increased from 30 to ∼41 at. % at identical shape, size,
and bulk composition.
To explain this, we emphasize that our experimental XPS data
provide direct evidence that the initial particle seeds catalyze
higher deposition rates for Pt than for Ni, consistent with their
relative electrochemical deposition potentials.35,36 This induces a
compositional gradient near the surface of the octahedra28−31,37
(see Figure 1b). As the dissolved precursors deplete, the
octahedra reach their time-stable final bulk composition, shape,
and size. At this point, the reaction time acts like an in situ
annealing process, mainly smoothing out the near-surface
compositional gradient by metallic interdiffusion.38,39
To monitor the particle size and shape changes with reaction
time, we performed transmission electron microscopy (TEM).
The average particle size is ∼9.0 nm for 16-PtNi, ∼9.2 nm for 28PtNi and ∼9.5 nm for 42-PtNi (see Table 1, Figure S3 and Figure
2a−c) suggesting that there is no size penalty with increasing
reaction time. Owing to the uncompleted ripening process for
16-PtNi material, the standard deviation of its particle size
distribution is larger than those of 28-PtNi and 42-PtNi. After 42
h reaction time, small particles have disappeared, and this is why
the particle size distribution became more uniform and the
deviation decreased to ±0.8 nm (Figure S3). High-resolution
TEM analysis in Figure 2c shows that the corresponding dspacing for the (111) planes is 0.216 nm, which are indexed to
the octahedral PtNi NPs terminated with {111} facets. To turn
the unsupported NPs into a practical electrocatalyst, the NPs
were supported on a high surface area carbon material. The TEM
image in Figure 2d evidence a fairly uniform distribution of the
PtNi NPs on the commercial carbon support. As shown in Figure
2e and f, the octahedral morphology and the atomic-scale
compositional distributions of Pt and Ni across an octahedron
were measured by probe corrected scanning transmission
electron microscopy complemented with electron energy loss
spectroscopy (STEM/EELS).
To evaluate the electrocatalytic ORR activities of the
octahedral NPs, the octahedral PtNi/C NPs were loaded on a
glassy carbon rotating disk electrode (RDE). Because these
catalysts were synthesized in pure DMF solvent and no other
surfactants were used in the synthesis, any additional surfactantremoving step is not needed,26 a great practical advantage over
the polyol process applied previously to prepare Pt−Ni
octahedra. The electrochemical active surface areas (ECSAs),
evaluated using CO stripping,40 were 24.1, 36.7, and 50.0 m2/gPt
for the 16-PtNi, 28-PtNi, and 42-PtNi catalysts, respectively (see
Figure 2. (a and b) TEM and (c) HRTEM images of octahedral PtNi
NPs after 42 h. (d) TEM image of octahedral PtNi NPs supported on
commercial carbon (Vulcan XC-72). (e) Cs-corrected HAADF-STEM
image of a selected 42-PtNi octahedron. (f) STEM-EELS line scans
across the octahedron (inset). Intensities are normalized by elemental
scattering cross sections.
Table 1 and Figure S5 of the Supporting Information). The
estimated CO stripping charge is somewhat larger but very close
to the estimated 2× Hupd charge, and calculated QCO/2QH is
within the region of 1.04−1.12 (Figure S5). The increased value
of ECSA for 42-PtNi is consistent with the observed higher Pt
surface concentration.
The effect of the initial near-surface alloy composition on the
ORR activity was studied in O2-saturated 0.1 M HClO4 solution
at room temperature (see Figure 3 and Table 1). Upon
increasing the reaction time from 16 to 42 h associated with
the increase in near surface Pt at. % from 30 to 41 at. %, the Pt
mass activity increased by a factor of 3× at 0.9 VRHE. The
actually observed value at 16 h was 0.56 A/mgPt, rising to
impressive 1.45 A/mgPt for the 42 h catalyst at quasi-stationary 5
mV/s scans. The Pt- surface area- specific activity increased by
about 50% from 2.35 mA/cm2Pt to impressive 3.14 mA/cm2Pt.
For comparison, the state-of-art Pt/C catalyst exhibited 0.15 A/
mgPt and 0.23 mA/cm2Pt. We emphasize that these ORR
activities represent previously unachieved consistent 10×
increases in both specific and Pt mass activity compared to a
state-of-art commercial Pt/C electrocatalyst measured under
identical conditions. The 42-PtNi sample even performs superior
to extended polycrystalline Pt electrodes (∼1.2 mA/cm2Pt).
From the changes in mass activity and the 1.5× increase in
specific activity, we conclude that the number of catalytically
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Figure 3. ORR activity of the octahedral PtNi NPs with controlled surface alloy composition. (a) ORR polarization curves. Inset shows the cyclic
voltammograms of the catalysts in N2-saturated electrolyte. (b) Mass and (c) specific ORR activities. Insets show the activities of the reference
polycrystalline Pt and commercial Pt/C catalysts.
relationships uncovered here provide ample room for new
rational pathways to more active bimetallic alloy particles.
active Pt surface sites roughly doubles. The remarkable 10×
activity gains are likely a result of an improved Pt/Ni ratio in the
second and third layers.18,41 EDX analysis of the active ORRtested 42-PtNi electrocatalyst revealed that its final bulk
composition changed to about Pt75Ni25 during the electrochemical dealloying process, thus very close to the ideal bulk
composition of the highly active extended (111) surface.
However, this specific activity is still far short of that for
extended Pt3Ni(111) crystals. This could be attributed to defects
and vacancies in the particle surface after mild dealloying in acidic
electrolytes. Moreover, TEM analysis showed that the particles
maintained an octahedral shape. Being below the critical size of
15 nm,24 the ORR-tested ∼9.5 nm 42-PtNi catalyst did not show
any indication of porosity, in line with earlier findings for Pt−Co,
Pt−Cu, and Pt−Ni NPs.24,42 Preliminary data on the stability of
the PtNi octahedra indicate that thermal postsynthesis annealing
is very detrimental to the octahedral shape and ORR activity.
Unannealed PtNi octahedra remained morphologically stable
even after tens of voltage cycles within the oxygen reduction
reaction potential range. The long-term stability of the octahedra
under fuel cell conditions requires further scrutiny.
In conclusion, we have synthesized octahedral PtNi NPs with
10× ORR activity gains in both Pt specific and Pt mass activity
over a state-of-art Pt/C electrocatalyst. We attribute this high
activity to the octahedral shape and favorable surface
composition of the final electrocatalyst. To achieve this, we
have used a facile surfactant-free solvothermal method and
showed that the reaction time correlates with the near-surface Pt
atomic composition of the octahedra without affecting their size
or shape. Longer reaction times led to higher near-surface Pt-toNi atomic ratios, which resulted in higher intrinsic activity. The
reaction time effect was rationalized based on a nucleation/
growth model that explained the near surface composition
gradient of Pt and Ni. The synthesis−structure−activity
■
ASSOCIATED CONTENT
S Supporting Information
*
Experimental details, EDX spectra, TEM images, and CO
stripping of PtNi octahedra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Dr. Frederick T. Wagner for valuable discussions. This
work was supported by U.S. DOE EERE award DE-EE0000458
via subcontract through General Motors. P.S. acknowledges
financial support through the cluster of excellence in catalysis
(UniCat).
■
REFERENCES
(1) Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C. F.;
Liu, Z. C.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson,
A. Lattice-strain control of the activity in dealloyed core-shell fuel cell
catalysts. Nat. Chem. 2010, 2, 454−460.
(2) Koh, S.; Strasser, P. Electrocatalysis on bimetallic surfaces:
modifying catalytic reactivity for oxygen reduction by voltammetric
surface dealloying. J. Am. Chem. Soc. 2007, 129, 12624−12625.
(3) Cui, C. H.; Li, H. H.; Yu, S. H. Large scale restructuring of porous
Pt-Ni nanoparticle tubes for methanol oxidation: A highly reactive,
stable, and restorable fuel cell catalyst. Chem. Sci. 2011, 2, 1611−1614.
(4) Liu, G. C.-K.; Stevens, D. A.; Burns, J. C.; Sanderson, R. J.;
Vernstrom, G.; Atanasoski, R. T.; Debe, M. K.; Dahn, J. R. Oxygen
5888
dx.doi.org/10.1021/nl3032795 | Nano Lett. 2012, 12, 5885−5889
Nano Letters
Letter
Reduction Activity of Dealloyed Pt1−xNix Catalysts. J. Electrochem. Soc.
2011, 158, B919−B926.
(5) Yang, R. Z.; Leisch, J.; Strasser, P.; Toney, M. F. Structure of
Dealloyed PtCu3 Thin Films and Catalytic Activity for Oxygen
Reduction. Chem. Mater. 2010, 22, 4712−4720.
(6) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.;
Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F.
Platinum monolayer fuel cell electrocatalysts. Top. Catal. 2007, 46,
249−262.
(7) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.;
Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Trends in
electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces.
Nat. Mater. 2007, 6, 241−247.
(8) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R.
Controlling the Catalytic Activity of Platinum-Monolayer Electrocatalysts for Oxygen Reduction with Different Substrates. Angew. Chem.,
Int. Ed. 2005, 44, 2132−2135.
(9) Wang, C.; Chi, M.; Li, D.; van der Vliet, D.; Wang, G.; Lin, Q.;
Mitchell, J. F.; More, K. L.; Markovic, N. M.; Stamenkovic, V. R.
Synthesis of Homogeneous Pt-Bimetallic Nanoparticles as Highly
Efficient Electrocatalysts. ACS Catal. 2011, 1, 1355−1359.
(10) Zhang, J.; Yang, H.; Fang, J.; Zou, S. Synthesis and Oxygen
Reduction Activity of Shape-Controlled Pt3Ni Nanopolyhedra. Nano
Lett. 2010, 10, 638.
(11) Wu, J. B.; Gross, A.; Yang, H. Shape and Composition-Controlled
Platinum Alloy Nanocrystals Using Carbon Monoxide as Reducing
Agent. Nano Lett. 2011, 11, 798−802.
(12) Wu, J.; Zhang, J.; Peng, Z.; Yang, S.; Wagner, F. T.; Yang, H.
Truncated Octahedral Pt3Ni Oxygen Reduction Reaction Electrocatalysts. J. Am. Chem. Soc. 2010, 132, 4984−4985.
(13) Wu, Y.; Cai, S.; Wang, D.; He, W.; Li, Y. Syntheses of WaterSoluble Octahedral, Truncated Octahedral, and Cubic Pt−Ni Nanocrystals and Their Structure−Activity Study in Model Hydrogenation
Reactions. J. Am. Chem. Soc. 2012, 134, 8975−8981.
(14) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. H. A
general approach to the size- and shape-controlled synthesis of platinum
nanoparticles and their catalytic reduction of oxygen. Angew. Chem., Int.
Ed. 2008, 47, 3588−3591.
(15) Wagner, F. T.; Lakshmanan, B.; Mathias, M. F. Electrochemistry
and the Future of the Automobile. J. Phys. Chem. Lett. 2010, 1, 2204−
2219.
(16) Gasteiger, H. A.; Markovic, N. M. Just a Dream-or Future Reality?
Science 2009, 324, 48−49.
(17) Choi, S.-I.; Choi, R.; Han, S. W.; Park, J. T. Shape-Controlled
Synthesis of Pt3Co Nanocrystals with High Electrocatalytic Activity
toward Oxygen Reduction. Chem.−Eur. J. 2011, 17, 12280−12284.
(18) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P.
N.; Lucas, C. A.; Markovic, N. M. Improved oxygen reduction activity on
Pt3Ni(111) via increased surface site availability. Science 2007, 315,
493−497.
(19) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.;
Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the
activity of electrocatalysts for oxygen reduction by tuning the surface
electronic structure. Angew. Chem., Int. Ed. 2006, 45, 2897−2901.
(20) Wang, C.; Chi, M.; Li, D.; Strmcnik, D.; van der Vliet, D.; Wang,
G.; Komanicky, V.; Chang, K.-C.; Paulikas, A. P.; Tripkovic, D.; Pearson,
J.; More, K. L.; Markovic, N. M.; Stamenkovic, V. R. Design and
synthesis of bimetallic electrocatalyst with multilayered Pt-Skin surfaces.
J. Am. Chem. Soc. 2011, 133, 14396−14403.
(21) Cui, C. H.; Li, H. H.; Liu, X. J.; Gao, M. R.; Yu, S. H. Surface
Composition and Lattice Ordering-Controlled Activity and Durability
of CuPt Electrocatalysts for Oxygen Reduction Reaction. ACS Catal.
2012, 2, 916−924.
(22) Srivastava, R.; Mani, P.; Hahn, N.; Strasser, P. Efficient Oxygen
Reduction Fuel Cell Electrocatalysis on Voltammetrically Dealloyed PtCu-Co Nanoparticles. Angew. Chem., Int. Ed. 2007, 46, 8988−8991.
(23) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity
benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen
reduction catalysts for PEMFCs. Appl. Catal., B 2005, 56, 9−35.
(24) Snyder, J.; McCue, I.; Livi, K.; Erlebacher, J. Structure/
Processing/Properties Relationships in Nanoporous Nanoparticles As
Applied to Catalysis of the Cathodic Oxygen Reduction Reaction. J. Am.
Chem. Soc. 2012, 134, 8633−8645.
(25) Wang, C.; Chi, M.; Wang, G.; van der Vliet, D.; Li, D.; More, K.;
Wang, H.-H.; Schlueter, J. A.; Markovic, N. M.; Stamenkovic, V. R.
Correlation between surface chemistry and electrocatalytic properties of
monodisperse PtxNi1‑x nanoparticles. Adv. Funct. Mater. 2011, 21, 147−
152.
(26) Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Atwan, M. H.;
Tessema, M. M. Solvothermal synthesis of platinum alloy nanoparticles
for oxygen reduction electrocatalysis. J. Am. Chem. Soc. 2012, 134,
8535−8542.
(27) Huang, X.; Zhang, H.; Guo, C.; Zhou, Z.; Zheng, N. Simplifying
the Creation of Hollow Metallic Nanostructures: One-Pot Synthesis of
Hollow Palladium/Platinum Single-Crystalline Nanocubes. Angew.
Chem. 2009, 121, 4902−4906.
(28) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chemistry
and properties of nanocrystals of different shapes. Chem. Rev. 2005, 105,
1025−1102.
(29) Pastoriza-Santos, I.; Liz-Marzán, L. M. N,N-Dimethylformamide
as a Reaction Medium for Metal Nanoparticle Synthesis. Adv. Funct.
Mater. 2009, 19, 679−688.
(30) Toshima, N.; Yonezawa, T. Bimetallic Nanoparticles - Novel
Materials for Chemical and Physical Applications. New J. Chem. 1998,
22, 1179−1201.
(31) Wang, D.; Peng, Q.; Li, Y. Nanocrystalline intermetallics and
alloys. Nano Res. 2010, 3, 574−580.
(32) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski,
A.; Festin, Ö .; Svedlindh, P.; Haase, M.; Weller, H. Study of Nucleation
and Growth in the Organometallic Synthesis of Magnetic Alloy
Nanocrystals: The Role of Nucleation Rate in Size Control of CoPt3
Nanocrystals. J. Am. Chem. Soc. 2003, 125, 9090−9101.
(33) Shen, Z. R.; Yamada, M.; Miyake, M. Preparation of singlecrystalline platinum nanowires with small diameters under mild
conditions. Chem. Commun. 2007, 245−247.
(34) Tao, F.; Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Renzas, J. R.;
Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. ReactionDriven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles.
Science 2008, 322, 932−934.
(35) Lee, Y. W.; Kim, M.; Kim, Z. H.; Han, S. W. One-Step Synthesis of
Au@Pd Core−Shell Nanooctahedron. J. Am. Chem. Soc. 2009, 131,
17036−17037.
(36) Peng, Z. M.; Yang, H. Designer platinum nanoparticles: Control
of shape, composition in alloy, nanostructure and electrocatalytic
property. Nano Today 2009, 4, 143−164.
(37) Xu, J.; Zhao, T.; Liang, Z.; Zhu, L. Facile Preparation of AuPt
Alloy Nanoparticles from Organometallic Complex Precursor. Chem.
Mater. 2008, 20, 1688−1690.
(38) Chen, W.; Yu, R.; Li, L.; Wang, A.; Peng, Q.; Li, Y. A Seed-Based
Diffusion Route to Monodisperse Intermetallic CuAu Nanocrystals.
Angew. Chem., Int. Ed. 2010, 49, 2917−2921.
(39) Tompkins, H. G.; Pinnel, M. R. Relative rates of nickel diffusion
and copper diffusion through gold. J. Appl. Phys. 1977, 48, 3144−3146.
(40) van der Vliet, D. F.; Wang, C.; Li, D.; Paulikas, A. P.; Greeley, J.;
Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.;
Stamenkovic, V. R. Unique Electrochemical Adsorption Properties of
Pt-Skin Surfaces. Angew. Chem., Int. Ed. 2012, 51, 3139−3142.
(41) Strasser, P. Dealloyed Core-Shell Fuel Cell Electrocatalysts. Rev.
Chem. Eng. 2009, 25, 255−295.
(42) Oezaslan, M.; Heggen, M.; Strasser, P. Size-Dependent
Morphology of Dealloyed Bimetallic Catalysts: Linking the Nano to
the Macro Scale. J. Am. Chem. Soc. 2012, 134, 514−524.
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NOTE ADDED AFTER ASAP PUBLICATION
This paper was published ASAP on October 16, 2012. The
caption of Figure 2 and the Supporting Information file have
been updated. The revised version posted on October 19, 2012.
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dx.doi.org/10.1021/nl3032795 | Nano Lett. 2012, 12, 5885−5889