Platinum concave nanocubes with high-index facets and their enhanced activity for oxygen reduction reaction

Article abstract of DOI:10.1002/anie.201007859

Many facets: A simple synthetic route, which is based on reduction in aqueous solution, results in Pt concave nanocubes (see picture) enclosed by high-index facets such as {510}, {720}, and {830}. The nanocrystals exhibit electrocatalytic activity (per un

Full text of DOI:10.1002/anie.201007859

DOI: 10.1002/anie.201007859
Electrocatalysis
Platinum Concave Nanocubes with High-Index Facets and Their
Enhanced Activity for Oxygen Reduction Reaction**
Taekyung Yu, Do Youb Kim, Hui Zhang, and Younan Xia*
Platinum nanoparticles are widely used as the primary
catalysts in a myriad of industrial processes such as CO/NOx
oxidation in catalytic converters, nitric acid production,
petroleum cracking, as well as hydrogen (or alcohol) oxida-
tion and oxygen reduction reactions in fuel-cell technology.[1]
For most catalytic reactions, it has been shown that high-index
planes, which are associated with large numbers of atomic
steps, edges, and kinks hold the key to the enhancement of
catalytic performance in terms of activity and/or selectivity. A
number of protocols have been demonstrated for generating
Pt nanoparticles enclosed by high-index facets, including
those based on electrochemical reduction and heat treat-
ment.[2] For example, Sun and co-workers have reported the
synthesis of tetrahexahedral (THH) Pt nanocrystals with
high-index facets, such as {730}, {210}, and {520}, by applying a
square-wave potential to polycrystalline Pt microspheres
supported on a glassy carbon electrode.[2a] Although these
Pt nanocrystals have been shown to have high catalytic
activity, their sizes are still relatively too large and the method
of preparation is rather limited in terms of production
volume. It still remains a challenge to produce Pt nanocrystals
with high-index facets by using a simple, scalable route based
on wet chemical reduction.
Over the past several years, kinetic control has been
demonstrated as a simple and versatile approach to the shape-
controlled synthesis of noble-metal nanocrystals in the
solution phase. In general, kinetic control can be achieved
by: 1) substantially slowing down the formation rate of
atoms,[3] 2) using a weak reducing agent,[4] 3) introducing an
oxidation process,[5] and 4) taking advantage of Ostwald
ripening.[6] When the concentration of metal atoms in the
solution is low, the atoms tend to add to the edges and corners
of a seed rather than the entire surface, thus leading to the
formation of nanocrystals with thermodynamically unfavor-
able morphologies, including rods, plates, multipods, and
dendritic structures.[7]
In recent years, nanocrystals with concave rather than flat
faces have attracted attention because of their high-index
facets.[8] To this end, Zheng and co-workers have demon-
strated the synthesis of concave Pd polyhedral nanocrystals
with high electrocatalytic activity for formic acid oxidation.[8a]
Mirkin and co-workers have also reported the synthesis of
concave cubic Au nanocrystals, and demonstrated higher
chemical activity compared to octahedra enclosed by low-
index {111} facets.[8b] Herein we report the first synthesis of Pt
concave nanocubes enclosed by high-index facets including
{510}, {720}, and {830} by slowly adding an aqueous NaBH4
solution and
a mixture containing K2PtCl4, KBr, and
Na2H2P2O7 into deionized water by using two syringe
pumps. In this synthesis, the formation of a Pt pyrophosphato
complex (that is formed by mixing K2PtCl4 and Na2H2P2O7)
and the slow addition of this precursor by a syringe pump are
believed to play a key role in the formation of Pt concave
nanocubes. In this case, the seeds selectively overgrow from
corners and edges, and the Brꢀ ion serves as a capping agent
to block the growth of the h100i axis. The Pt concave
nanocubes exhibited substantially enhanced specific activity
(per unit surface area) relative to those of Pt nanocubes,
cuboctahedra, and commercial Pt/C catalysts that are
bounded by low-index facets such as {100} and {111} toward
the oxygen reduction reaction (ORR), which is the rate-
determining step in a proton-exchange membrane (PCM) fuel
cell.
[*] Dr. T. Yu,[+] D. Y. Kim,[+] Prof. H. Zhang, Prof. Y. Xia
Department of Biomedical Engineering
Washington University
Saint Louis, MO 63130 (USA)
E-mail: xia@biomed.wustl.edu
D. Y. Kim[+]
Department of Chemical and Biomolecular Engineering
(BK21 graduate program)
Korea Advanced Institute of Science and Technology (KAIST)
335 Gwahangro, Yuseong-gu, Daejeon 305-701 (Korea)
Prof. H. Zhang
State Key Laboratory of Silicon Materials and
Department of Materials Science and Engineering
Zhejiang University
Hangzhou, Zhejiang 310027 (People’s Republic of China)
[+] These authors contributed equally to this work.
In a typical synthesis, an aqueous NaBH4 solution and a
mixture containing K2PtCl4, KBr, and Na2H2P2O7 were
prepared separately and then injected simultaneously at an
injection rate of 67 mLminꢀ1 by using two syringe pumps into
deionized water maintained at 958C. The color of the solution
immediately turned from light pink to black upon the addition
of the reactant solutions, thus indicating rapid reduction of
PtCl42ꢀ into elemental Pt by NaBH4. Figure 1a shows a typical
transmission electron microscopy (TEM) image of the
product that contains Pt nanocubes with a concave structure.
[**] This work was supported in part by the NSF (DMR-0804088) and
startup funds from Washington University in St. Louis. Y.X. was also
partially supported by the World Class University (WCU) program
through the National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology (R32-20031). Part of
the research was performed at the Nano Research Facility, a
member of the National Nanotechnology Infrastructure Network
(NNIN), which is supported by the National Science Foundation
under award ECS-0335765.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007859.
Angew. Chem. Int. Ed. 2011, 50, 2773 –2777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2773

Communications
Figure 1. a) TEM and b,c) HRTEM images of Pt concave nanocubes
Figure 2. TEM images showing the course of morphological evolution
for Pt concave nanocubes. The reaction time was a) 1, b) 3, c) 5, and
d) 20 min, respectively.
synthesized by continuously adding an aqueous NaBH4 solution and a
mixture of K2PtCl4, KBr, and Na2H2P2O7 using two syringe pumps into
deionized water held at 958C. The injection rate was 67 mLminꢀ1, and
the KBr/Na2H2P2O7 ratio was 3:1. d) FT pattern of the concave nano-
cube shown in (c) recorded along the [001] zone axis.
concave structure. The observation of a few small Pt nano-
particles during the growth stage (Figure 2b–d) suggests the
possibility of homogeneous nucleation for the newly formed
Pt atoms, even though most of the Pt atoms were preferen-
tially added to the seeds.
The edge lengths of the concave nanocubes were in the range
of 15–40 nm. Figure 1b,c, shows high-resolution TEM
(HRTEM) images of a single concave nanocube projected
along the [100] axis, as confirmed by the corresponding
Fourier transform (FT) pattern (Figure 1d). The angles
between the facets of the projected concave nanocube and
the {100} facets of an ideal cube were 168, 118, 158, and 218,
which are in agreement with {720}, {510}, {720}, and {830}
facets, respectively (Figure 1c and Figure S1a in the Support-
ing Information). By measuring the angles of several concave
nanocubes, we could conclude that the surface of a Pt concave
nanocube was mainly enclosed by {720} facets, together with
some other high-index facets such as {510}, {830}, and {310}. A
similar structure was recently reported by Mirkin and co-
workers for concave nanocubes of Au, the surface of which
was enclosed by mostly {720} and some {310} planes.[8b]
To gain an insight into the morphological evolution of the
Pt concave nanocubes, aliquots of the reaction solution were
removed at various reaction times and examined by TEM.
Figure 2a–d shows typical TEM images of the products
sampled at 1, 3, 5, and 20 min, respectively. At t = 1 min, the
product contained a large number of small Pt nanocrystals
with sizes of less than 4 nm (Figure 2a). In the following
2 min, a number of truncated cubic nanocrystals (ca. 10 nm in
size) were observed, thus indicating a rapid growth of the
nanocrystals in size (Figure 2b). As the reaction proceeded to
t = 5 min, most of Pt nanocrystals exhibited a cubic shape and
a concave structure, thus showing a morphological transition
from truncated cubes to concave cubes (Figure 2c). During
the next 15 min, the concave cubes further increased in size
until they reached edge lengths of 15–40 nm (Figure 2d).
These results indicate that overgrowth of Pt occurred on the
seeds along their corners and edges, or the h111i and h110i
directions, respectively, thus leading to the formation of a
In a recent report on the synthesis of Rh nanocrystals, it
was demonstrated that manipulating the reaction kinetics
could lead to the formation of nanocrystals having anisotropic
shapes and/or concave structures.[3c] In the present synthesis,
the formation of a complex between K2PtCl4 and Na2H2P2O7
and the slow addition of this complex with a syringe pump
seem to play the most important role in slowing down the
reduction, thus promoting kinetically controlled growth of
nanocrystals rather than thermodynamically favored growth.
It has been reported that Pt ions can interact with different
types of phosphate compounds to form various PtII or PtIV
phosphato complexes.[9] As shown in the UV/Vis spectra
(Figure S2a in the Supporting Information), the absorption
peak at 215 nm disappeared after mixing with Na2H2P2O7,
thus suggesting that K2PtCl4 formed a complex with pyro-
phosphate ions.[10] The resultant Pt pyrophosphato complex
was more difficult to reduce than the initial PtII ions. On the
other hand, when aqueous solutions of Na2H2P2O7 and
K2PtCl4 were separately prepared and injected into a reaction
solution, small Pt nanocrystals with irregular shapes rather
than concave nanocubes were observed (Figure S2b). In an
effort to obtain a deep understanding of the relationship
between the reduction rate and kinetically controlled growth
of Pt concave nanocubes, we also conducted the synthesis by
rapidly adding solutions of the Pt pyrophosphato complex and
NaBH4 with two pipettes instead of syringe pumps, while all
other experimental parameters were same as those used to
prepare the samples shown in Figure 1. The product was
dominated by truncated, nanometer-sized octahedra, which is
the thermodynamically favored morphology (Figure S3). The
results indicate that a slow reduction rate is vital for kineti-
2774
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2773 –2777

cally controlled growth, which is a prerequisite for the
formation of Pt concave nanocubes.
The high-index facets of Pt have been shown to have
significantly higher ORR activities than their low-index
counterparts in acidic solutions.[12] This behavior can be
attributed to the favorable adsorption of O2 molecules onto
the stepped surfaces.[13] Cyclic voltammetry (CV) curves
recorded at room temperature in a N2-purged 0.1 molLꢀ1
HClO4 solution with a sweep rate of 50 mVsꢀ1 are shown in
Figure S4a. The specific electrochemically active surface
In addition to the reduction rate, we also systematically
investigated the effect of KBr on the morphology of Pt
nanocrystals. It has been reported that the selective chem-
isorption of Brꢀ ions on the {100} facets of Pt nanocrystals
reduces the growth rate along the h100i axis, thus facilitating
the formation of nanocubes and nanobars bounded by the
{100} facets.[11] To elucidate the role of Brꢀ ions in controlling
the morphology of Pt nanocrystals, we also conducted a set of
experiments with different KBr/Na2H2P2O7 molar ratios.
When we used only Na2H2P2O7 as a capping agent instead
of a KBr/Na2H2P2O7 mixture, the resultant Pt nanocrystals
were cuboctahedra enclosed by both {100} and {111} facets
(Figure 3a,b). By increasing the amount of KBr, the mor-
phology of Pt nanocrystals exhibited dramatic changes:
truncated cubes were formed at a small KBr/Na2H2P2O7
ratio (1:3; Figure 3c,d); cubes at an intermediate ratio of
KBr/Na2H2P2O7 (1:1; Figure 3e,f); and concave cubes at a
high KBr/Na2H2P2O7 ratio (3:1; Figure 1). These observations
demonstrate that reducing the rate of precursor reduction and
selective chemisorption of Brꢀ on {100} facets are both
indispensable for the formation of Pt concave nanocubes.
areas (ECSAs) of Pt nanocrystals were 17.1 m2 gPtꢀ1 for
ꢀ1
cuboctahedra, 5.9 m2 gPt
for cubes, and 6.2 m2 gPt
for
ꢀ1
concave nanocubes, respectively. The Pt cuboctahedra,
cubes, and concave cubes were deposited on glassy carbon
rotating disk electrodes (RDE) and used for ORR measure-
ments at room temperature in O2-saturated 0.1 molLꢀ1
HClO4 solutions, with a sweep rate of 10 mVsꢀ1 and a
rotation rate of 1600 rpm (Figure S4b). To achieve a better
understanding of the surface effect between high- and low-
index facets, we compared the specific activities (i.e., kinetic
current per unit surface area of the nanocatalyst) of these
three Pt catalysts. As illustrated in Figure 4a, the specific
activity of the Pt concave cubes was 3.1–4.1 and 1.9–2.8 times
greater than that of the Pt cubes and cuboctahedra, respec-
tively, in the potential region between 0.8 and 0.95 V, thus
demonstrating that the Pt concave cubes exhibited greatly
enhanced catalytic activity in the ORR because of its high-
index facets. It is worth pointing out that for the low-index
facets of Pt, the activity of the ORR increases in the order of
Figure 3. TEM and HRTEM images of a,b) cuboctahedra prepared
under the same conditions as in Figure 1 except that the synthesis was
conducted in the absence of KBr, c,d) truncated nanocubes prepared
under the same conditions as in Figure 1 except that the synthesis was
conducted with a low KBr/Na2H2P2O7 ratio (1:3), and e,f) nanocubes
prepared under the same conditions as in Figure 1 except that the
synthesis was conducted with an intermediate KBr/Na2H2P2O7 ratio
(1:1).
Figure 4. Comparison of the electrocatalytic properties of the Pt
~
&
*
concave cubes ( ), cubes ( ), and cuboctahedra ( ). a) Specific
activities given as kinetic current densities (jk) normalized against the
ECSA of the catalyst. b) Specific activities of these catalysts at 0.9 V
versus RHE. For all catalysts, the metal loading on the glassy carbon
electrode was 15.3 mgcmꢀ2
.
Angew. Chem. Int. Ed. 2011, 50, 2773 –2777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2775

Communications
following relation: specific ECSA = 0.5Q/(mqH), where Q is the
charge associated with Hupd adsorption or desorption, m is the mass of
loaded metal, and qH is the charge required for the adsorption of a
monolayer of hydrogen on a Pt surface.
The ORR measurements were conducted at room temperature in
0.1m HClO4 solutions under a flow of O2 (Airgas, Research grade)
using the glassy carbon RDE at a rotation rate of 1600 rpm and a
sweep rate of 10 mVsꢀ1. The kinetic current was calculated based on
the Koutecky–Levich equation:
(100) and (111) when a non-absorbing electrolyte such as
perchloric acid is used.[14] Since Pt cuboctahedra have a
mixture of {100} and {111} facets, they are supposed to have a
higher ORR activity than Pt cubes enclosed by {100} facets
(Figure 4). In comparison with the commercial Pt/C catalyst,
the Pt concave cubes exhibited a specific activity that was 3.6
times higher as compared to the 3.2 nm Pt particles on Pt/C
catalyst at 0.9 V (Figure 4b). However, the mass activity (i.e.,
kinetic current per unit Pt mass of catalyst) of Pt concave
cubes was still less than that of the Pt/C catalyst, presumably
owing to their smaller ECSAs per unit weight of Pt associated
with the relatively larger particle size (Figure S5).
1
i
1
1
ð1Þ
¼
þ
ik id
where i is the experimentally measured current, id is the diffusion-
limiting current, and ik is the kinetic current. For each catalyst, the
kinetic current was normalized to ECSA in order to obtain specific
activities.
In summary, we have demonstrated the synthesis of Pt
concave nanocubes enclosed by high-index facets including
{510}, {720}, and {830} by using a simple route based on
reduction in an aqueous solution. The key to the success of
this method is the use of a Pt pyrophosphato complex as the
precursor and by controlling the reaction rate with a syringe
pump; both facilitate selective overgrowth of seeds from
corners and edges, with Brꢀ serving as a capping agent to
block the {100} facets. The Pt concave nanocubes exhibited
substantially enhanced electrocatalytic activity per unit sur-
face area towards ORR compared with those of Pt cubes,
cuboctahedra, and commercial Pt/C catalysts that are
bounded by low-index planes such as {100} and {111}. This
work presents another example of kinetic control for
generating nanocrystals with high-index facets. We expect
that this synthetic strategy could be extended to the synthesis
of other noble-metal nanocrystals with high-index facets for
highly active catalysts.
Instrumentation: TEM studies were carried out with a FEI Tecnai
G2 Spirit microscope operated at 120 kV by drop casting the
dispersions of nanoparticles on carbon-coated copper grids. High-
resolution TEM analyses were performed using a JEOL 2100F
microscope operated at 200 kV accelerating voltage. Both CV and
ORR polarization curves were obtained with a PARSTAT 283
poteniostat (Princeton Applied Research). UV/Vis spectra were
recorded with a Cary 50 spectrometer (Varian). ICP-MS (ELAN
DRC II, Perkin Elmer) was used to determine the amount of Pt
catalyst used in each sample for electrochemical measurements.
Received: December 13, 2010
Published online: February 15, 2011
Keywords: electrocatalysis · high-index facets · nanostructures ·
.
oxygen reduction reaction · platinum
[1] a) J. Kua, W. A. Goddard, J. Am. Chem. Soc. 1999, 121, 10928;
b) J. Greeley, J. K. Nørskov, M. Mavrikakis, Annu. Rev. Phys.
Chem. 2002, 53, 319; c) X. Wang, M. Waje, Y. Yan, J. Electro-
chem. Soc. 2004, 151, A2183; d) J. K. Nørskov, J. Rossmeisl, A.
Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jꢁnsson, J.
Phys. Chem. B 2004, 108, 17886; e) X. Teng, X. Liang, S.
Maksimuk, H. Yang, Small 2006, 2, 249; f) B. Lim, X. Lu, M.
Jiang, P. H. C. Camargo, E. C. Cho, E. C. E. Lee, Y. Xia, Nano
Lett. 2008, 8, 4043; g) C. Wang, H. Daimon, T. Onodera, T. Koda,
S. Sun, Angew. Chem. 2008, 120, 3644; Angew. Chem. Int. Ed.
2008, 47, 3588.
[2] a) N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, Z. L. Wang, Science
2007, 316, 732; b) S. W. Lee, S. Chen, W. Sheng, N. Yabuuchi, Y.-
T. Kim, T. Mitani, E. Vescovo, Y. Shao-Horn, J. Am. Chem. Soc.
2009, 131, 15669; c) Z.-Y. Zhou, Z.-Z. Huang, D.-J. Chen, Q.
Wang, N. Tian, S.-G. Sun, Angew. Chem. 2010, 122, 421; Angew.
Chem. Int. Ed. 2010, 49, 411.
[3] a) P.-F. Ho, K.-M. Chi, Nanotechnology 2004, 15, 1059; b) Y.
Xiong, A. R. Siekkinen, J. Wang, Y. Yin, M. J. Kim, Y. Xia, J.
Mater. Chem. 2007, 17, 2600; c) H. Zhang, W. Li, M. Jin, J. Zeng,
T. Yu, D. Yang, Y. Xia, Nano Lett. 2011, DOI: 10.1021/nl104347j.
[4] a) I. Washio, Y. Xiong, Y. Yin, Y. Xia, Adv. Mater. 2006, 18, 1745;
b) Y. Xiong, I. Washio, J. Chen, H. Cai, Z.-Y. Li, Y. Xia,
Langmuir 2006, 22, 8563; c) B. Lim, P. H. C. Camargo, Y. Xia,
Langmuir 2008, 24, 10437.
Experimental Section
Synthesis of Pt concave nanocubes: In a typical synthesis, a Pt
pyrophosphato complex was prepared by adding K2PtCl4 (4 mg,
0.01 mmol), Na2H2P2O7 (2 mg, 0.01 mmol), and KBr (6 mg,
0.03 mmol) into deionized water (2 mL). Separately, NaBH4
(0.4 mg) was dissolved in 2 mL deionized water at room temperature.
Then, the solutions of the Pt pyrophosphato complex and NaBH4
were continuously added using two syringe pumps at an injection rate
of 67 mLminꢀ1 into 3 mL deionized water held at 958C under vigorous
magnetic stirring. The reaction mixture was left to stand at 958C in air
for 30 min and then cooled to room temperature. All chemicals were
obtained from Aldrich and used as received.
Electrochemical measurements: Electrochemical measurements
were performed using
a glassy carbon RDE connected to a
potentiostat. A hydrogen reference electrode was used, and the
counter electrode was a platinum mesh (1 cm ꢀ 1 cm) attached to a
platinum wire. To prepare the working electrode, the sample was
diluted to 0.15 mgmLꢀ1 (based on inductively coupled plasma(ICP)-
MS measurements) and 20 mL of the dispersion was transferred onto
the glassy carbon RDE with a geometric area of 0.196 cm2. The
loading amount of Pt for all catalysts was 3 mg. After evaporation of
water, the electrode was coated with 15 mL of 0.05 wt% Nafion
solution.
CV measurements were carried out in 0.1 molLꢀ1 HClO4
solutions under a flow of N2 (Airgas, ultrahigh purity) at a sweep
rate of 50 mVsꢀ1. The ECSA was estimated by measuring the charges
associated with Hupd adsorption/desorption (Q) between 0.05 and
[5] Y. Xiong, J. M. McLellan, J. Chen, Y. Yin, Z.-Y. Li, Y. Xia, J. Am.
Chem. Soc. 2005, 127, 17118.
[6] a) Y. Sun, Y. Xia, Adv. Mater. 2003, 15, 695; b) Y. Sun, B. Mayers,
Y. Xia, Nano Lett. 2003, 3, 675.
0.40 V after double-layer correction and assuming
a
value of
[7] a) J. Park, B. Koo, Y. Hwang, C. Bae, K. An, J.-G. Park, H. M.
Park, T. Hyeon, Angew. Chem. 2004, 116, 2332; Angew. Chem.
Int. Ed. 2004, 43, 2282; b) J. Park, B. Koo, K. Y. Yoon, Y. Hwang,
210 mC cmꢀ2 for the adsorption of a monolayer of hydrogen onto a
Pt surface (qH). Then, the specific ECSA was calculated based on the
2776
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2773 –2777

M. Kang, J.-G. Park, T. Hyeon, J. Am. Chem. Soc. 2005, 127,
8433; c) B. Koo, J. Park, Y. Kim, S.-H. Choi, Y.-E. Sung, T.
Hyeon, J. Phys. Chem. B 2006, 110, 24318; d) J. Zeng, J. Tao, W.
Li, J. Grant, P. Wang, Y. Zhu, Y. Xia, Chem. Asian. J. 2010, 2011,
6, 376.
[10] L. I. Elding, L. F. Oisson, J. Phys. Chem. 1978, 82, 69.
[11] a) Y. Xia, Y. Xiong, B. Lim, S. E. Skrabalak, Angew. Chem. 2009,
121, 62; Angew. Chem. Int. Ed. 2009, 48, 60; b) B. Lim, M. Jiang,
J. Tao, P. H. C. Camargo, Y. Zhu, Y. Xia, Adv. Funct. Mater. 2009,
19, 189.
[8] a) X. Huang, S. Tang, H. Zhang, Z. Zhou, N. Zheng, J. Am.
Chem. Soc. 2009, 131, 13916; b) J. Zhang, M. R. Langille, M. L.
Personick, K. Zhang, S. Li, C. A. Mirkin, J. Am. Chem. Soc. 2010,
132, 14012; c) C.-L. Lu, K. S. Prasad, H.-L. Wu, J. A. Ho, M. H.
Huang, J. Am. Chem. Soc. 2010, 132, 14546.
[12] a) M. D. Maciꢂ, J. M. Campina, E. Herrero, J. M. Feliu, J.
Electroanal. Chem. 2004, 564, 141; b) A. Kuzume, E. Herrero,
J. M. Feliu, J. Electroanal. Chem. 2007, 599, 333.
[13] a) D. W. Blakely, G. A. Somorjai, Surf. Sci. 1977, 65, 419;
´
b) N. M. Markovic, R. R. Adzic, B. D. Cahan, E. B. Yeager, J.
[9] a) R. D. Cornelius, P. A. Hart, W. W. Cleland, Inorg. Chem. 1977,
16, 2799; b) R. N. Bose, R. E. Viola, R. D. Cornelius, J. Am.
Chem. Soc. 1984, 106, 3336; c) R. J. Mishur, C. Zheng, T. M.
Gilbert, R. N. Bose, Inorg. Chem. 2008, 47, 7972.
Electroanal. Chem. 1997, 377, 249.
[14] V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross,
´
C. A. Lucas, N. M. Markovic, Science 2007, 315, 493.
Angew. Chem. Int. Ed. 2011, 50, 2773 –2777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2777