J. Phys. Chem. C 2008, 112, 1645-1649
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Ni@Pt Core-Shell Nanoparticles: Synthesis, Structural and Electrochemical Properties Yumei Chen, Fan Yang, Yu Dai, Weiqi Wang, and Shengli Chen* Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China ReceiVed: October 10, 2007; In Final Form: October 30, 2007
Core-shell nanoparticles composed of a nonnoble metal core and a noble metal shell are of great significance in many areas including chemical catalysis, optical detection, and magnetic separation. Through a modification of the commonly used polyol process, Ni@Pt core-shell nanoparticles that are less than 10 nm in total size and have a very thin Pt shell can be fabricated by a sequential reduction approach. The prepared core-shell nanoparticles were characterized with TEM, XRD, molecular dynamics (MD) simulations, and electrochemical method. It was found that these core-shell particles exhibit the structural characteristics of fcc Ni nanocrystals with a slightly expanded lattice constant but the electrochemical properties of a Pt surface with a significantly shortened Pt-Pt interatomic distance than for pure Pt nanoparticles. The structural characteristics of the prepared core-shell particles revealed by the TEM, XRD, and electrochemical analyses were well verified by MD simulations of a Ni@Pt core-shell particle with a monolayer Pt shell. It is believed that the prepared Ni@Pt core-shell nanoparticles could be promising cathode catalysts in PEM fuel cells with much reduced Pt content but significantly increased catalytic activity.
1. Introduction Over the past few years, there has been a surge in research on nanocrystals with core-shell architectures owing to their superior catalytic,1 optical,2 magnetic,3 and electrical4 properties. Among core-shell nanoparticles of various combinations, those made of an inexpensive metal core and a noble metal shell have received particular interest because of the functional and economic advantages that they can provide.1,2a,5 In addition to shielding the core metal from corrosion in air and solutions, a noble metal shell could result in a dramatic modulation of the optical, magnetic, and biological responses of the core metal.2a,3a,6 Furthermore, core-shell nanoparticles with very thin noble metal shells are of great significance in chemical catalysis. Noble metals (e.g., Pt, Pd) are excellent and versatile catalysts in various reactions but occur in nature at very low levels of abundance.7,8 Arranging noble metals as thin (ideally monolayer) shells on proper nonnoble metal cores not only greatly reduces their use, but could significantly enhance their catalytic properties via the so-called strain and ligand effects of the core substrate on the supported noble metal overlayer.8,9 Although extensive studies have been conducted on the preparation of core-shell-structured nanoparticles, the fabrication and characterization of bimetallic core-shell particles with a total size of less than 10 nm and with a monolayer metal shell remain challenging tasks. Redox replacement processes, in which the outermost layer of the core is sacrificially converted into the shell material through a redox reaction, might be the most successful approach for producing well-defined bimetallic core-shell nanoparticles with very thin noble metal shells. Using this method, Cheon et al. successfully prepared a series of Co core-noble metal (i.e., Au, Pt, Pd) shell nanoparticles and characterized them by monitoring their supermagnetic properties.3 Similarly, Adzic et al. performed nearly monolayer deposition of Pt on CoM (M ) Au, Pd, or Pt) alloy nanoparticles by electrochemically displacing a predeposited UPD (underpotential-deposited) Cu monolayer.1c * Corresponding author. E-mail:
[email protected]. Phone: +862768754693. Fax: +8627-68754067.
It has been shown that PtNi alloys are promising catalysts for the oxygen reduction reaction (ORR), which is currently among the most important reactions of interest because of its critical role in polymer electrolyte membrane (PEM) fuel cells.8 In addition, superparamagnetic Ni nanoparticles exhibit excellent performance in biological separation and purification.10 Therefore, Ni@Pt core-shell nanoparticles with monolayer Pt shells are expected to be highly functional materials for magnetic and catalytic applications. In this article, we show that sub-10-nm Ni@Pt core-shell particles with an approximate monolayer Pt shell can be readily produced through a sequential reduction method based on a modified polyol process. The prepared nanoparticles exhibit the XRD patterns of Ni particles with a slightly expanded lattice constant but the electrochemical properties of compressively strained Pt surface (i.e., with a shorter Pt-Pt interatomic distance than in pure Pt particles). Such a unique structure revealed by the XRD patterns and electrochemical properties are in good agreement with the results of molecular dynamic simulations of a Ni@Pt core-shell nanoparticle. Although the Ni@Pt particles have a much reduced content of Pt, they exhibit significantly enhanced catalytic activity toward oxygen reduction as compared to pure Pt catalysts, showing their great promise in solving the problem of the high demand for precious platinum metal in the cathodes of state-of-the-art PEM fuel cells. 2. Experimental Section 2.1. Nanoparticle Preparation and Physical Characterization Procedures. To prepare nanosized Ni core particles, a mixture of NaOH (0.6 g), Ni(Ac)2 (0.0747 g), and oleic acid (0.85 g) in 50 mL of 1,2-propanediol was heated to 118 °C and stirred at 118 °C for 20 min until a clear green solution was obtained. The solution was then heated to 138 °C, at which point 10 mL of 1,2-propanediol solution containing 0.162 g of KBH4 was slowly added into the reaction flask. The green solution turned black upon the addition of KBH4 solution, indicating the formation of Ni particles. The reaction process was carried out under N2 atmosphere to prevent oxidation of
10.1021/jp709886y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008
1646 J. Phys. Chem. C, Vol. 112, No. 5, 2008 the Ni particles. After formation of the Ni particles, the colloid solution was aged for about 20 min under stirring and N2 atmosphere at 138 °C, and then 3.102 mL of 1,2-propanediol containing H2PtCl6 (0.008 g/mL) was added dropwise. The reaction solution was stirred continuously for another 30 min and then cooled to room temperature under N2 atmosphere. Finally, the black powder was isolated by centrifugation and washed 3-4 times with ethanol. The powder was then dried at 40 °C in a vacuum oven. TEM images were obtained on a JEOL JEM-2010 transmission electron microscope working at 200-kV accelerating voltage. Histograms of the nanoparticle size distribution, assuming a spherical shape, were obtained from the measurement of about 200 particles found in an arbitrarily chosen area of enlarged micrographs. High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM2010FEF ultrahigh-resolution microscope working at 200-kV accelerating voltage. The samples for TEM observation were prepared by dispersing the metal nanoparticles in ethanol at room temperature and then collecting them on a carbon-coated copper grid. Powder X-ray diffraction (XRD) patterns were obtained on a Shimadzu XRD-6000 X-ray diffractometer using a Cu KR radiation source operating at 40 kV and 30 mA. The XRD profile was recorded at a scanning rate of 4°/min. The metal nanoparticle samples for XRD were washed with ethanol 3-4 times and then dried in a vacuum oven at 40 °C. 2.2. Electrochemical Measurements. Electrochemical measurements were performed with a three-electrode configuration. The working electrodes were made by casting carbon-supported nanoparticles (Ni@Pt/C or Pt/C) as a thin film onto a glass carbon (GC) rotating disk electrode (geometrical area ) 0.196 cm2) with Nafion as a binding agent. The counter electrode was a Pt foil, and the reference electrode was a saturated calomel electrode (SCE) and was separated from the working electrode by a Luggin capillary. However, the potentials throughout this article are quoted against a reversible hydrogen electrode. To prepare Ni@Pt/C particles, a certain amount of carbon black powder was added to the Ni@Pt particle solution, and the mixture was stirred overnight. The amount of carbon black was determined so that the total metal content (Ni + Pt) in the resulting Ni@Pt/C nanoparticles was 20 wt % (Pt, 8%; Ni, 12%). The Pt/C particles used in this study were Johnson-Matthey 40% Pt/C catalyst. 2.3. Molecular Dynamics Simulations. Simulations were conducted with the LAMMPS package (MD code developed by S. Plimpton, available at http://www.cs.sandia.gov/∼sjplimp/ lammps.html.) The MEAM (modified embedded-atom method) many-body potentials for pure Ni and pure Pt included in LAMMPS package were used to describe the Ni-Ni and PtPt interactions in the Ni and Ni@Pt particles. The interaction between Pt and Ni in Ni@Pt particle was described with the Pt-Ni cross-potential developed by Baskes et al.11 The corresponding potential parameters and angular screening parameters for these MEAM interactions are included in tables in refs 11b and c. All simulations were carried out in the canonical ensemble at room temperature (300 K) using a Nose/Hoover temperature thermostat with a temperature damping parameter of 0.1 ps. The initial velocities were set to 0, and nonperiodic and shrinkwrapped boundary conditions were used in three dimensions during the simulations. MD simulations of 250000 total steps were conducted with a time step of 1 fs. To eliminate the influence of the initial configurations, we sampled the coordinates of all atoms every 10 steps in the last 50000 MD steps when the system had reached the equilibrium state. The reported
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Figure 1. TEM images of the prepared (a) Ni and (b) Ni@Pt particles. (c) Histograms of the particle size distributions of Ni and Ni@Pt particles. The dashed and solid curves are the corresponding Gaussianfitting size distribution curves for the Ni and Ni@Pt particles. (d) Schematic diagram of an fcc cuboctahedron core-shell nanoparticle with a monolyer shell.
distributions of the interatomic distances in the particles are the corresponding statistical values from the 5000 sampled data sets. 3. Results and Discussion 3.1. Fabrication and Physical Characterization of Ni@Pt Core-Shell Nanopartciles. The fabrication of nanosized Ni@Pt core-shell particles in this study relies on a few modifications of the commonly used polyol process, in which a polyol is used as both the solvent and the reductant. Because of their relatively weak reduction capability, polyols are generally not ideal reductants for producing very small particles of less-reducible nonnoble metals such as Ni, Fe, etc. Even in the presence of heterogeneous nuclei of a noble metal such as Pd, polyol reduction usually produces Ni particles of several tens of nanometers.5c,12 Nanosized particles of these metals are usually obtained by thermal decomposition of organometallic compounds that are mostly expensive and toxic. In this work, we introduced a strong reductive agent (i.e., KBH4) into the polyol solution to induce instantaneous nucleation of Ni, which is the key step for the formation of nanosized particles in solution. Using 1,2-propanediol as the polyol agent, Ni(Ac)2 as the metal precursor, and oleic acid as capping agent, we were able to obtain Ni particles with an average size of ca. 4.2-4.6 nm, as shown by the TEM images in Figure 1a and the corresponding size distribution in Figure 1c. The formation of a Pt shell on these Ni nanoparticles was achieved by choosing a reduction temperature at which Pt nucleation would not take place in homogeneous polyol solution but would occur in the presence of Ni cores. We found that the formation of Pt particles through 1,2-propanediol reduction occurs at ∼158 °C in the absence of Ni particles. However, Pt can be reduced on Ni particles by 1,2-propanediol at temperatures as low as 118 °C. We chose 138 °C for deposition of the Pt shell. To ensure the formation of an approximate monolayer Pt shell, a rational design of the Pt/Ni atomic ratio is required.
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Figure 3. XRD patterns of the prepared Ni (bottom) and Ni@Pt (top) particles.
Figure 2. HRTEM image of a Ni@Pt particle.
If we assume that the Ni particles are crystallized in a cuboctahedron shape, as suggested by an earlier thermodynamic analysis of fcc nanocrystals using the concept of localized metal bonds (i.e., considering only nearest-neighbor interatomic interactions),13 and that the Pt shell grows pseudomorphically on the Ni particles as shown in Figure 1d, the amount of Pt atoms in a monolayer shell over a Ni particle with a mean diameter of ∼4.4 nm is about 26% of the total Ni atoms.13 If we assume that the Ni core particles are sphere-shaped, the number of Pt atoms required to form a monolayer shell over a 4.4 nm Ni particle is about 20% of the total Ni atoms. The actual shape of the Ni nanoparticles might be neither regular cuboctahedrons nor spheres, but rather irregular polyhedrons between the idealized shapes. Considering the much larger atomic size of Pt than Ni, a Pt/Ni ratio of 1:5 was chosen for preparation of the Ni@Pt core-shell particles in present study. Such an amount of Pt should produce a near-monolayer Pt shell. A TEM image of the resulting Ni@Pt particles is shown in Figure 1b, and the corresponding particle size distribution is given in Figure 1c. It can be seen that the particles are fairly monodisperse, with an average diameter of ca. 5.4 nm. It is expected that the addition of a Pt monolayer shell would increase particle size by less than 0.55 nm (twice the Pt atom diameter). The size histograms of Ni and Ni@Pt particles in Figure 1c show that the particle sizes increase by about 0.8 nm after Pt shell formation. Considering that the amount of Pt added to the solution was adjusted so that approximately only one layer of Pt atoms can be formed, we can exclude the formation of a double layer of Pt. Thus, the increase of the particle size by more than 0.55 nm seems to imply that a lattice expansion of the Ni core particles takes place upon deposition of a very thin Pt shell. Of course, the size histogram in Figure 1c cannot serve as firm evidence for the lattice expansion of the particle because the size distributions were obtained from a limited number of particles and the size estimation from the TEM image might have some uncertainties because of the different contrasts of Pt and Ni particles. However, as shown by the XRD results below, the expansion of Ni core does occur after deposition of Pt shell. Figure 2 presents a high-resolution TEM (HRTEM) image of a Ni@Pt particle, which shows that the particle has an irregular polyhedral shape. Because the Pt shell is very thin, it is impossible to resolve the core-shell structure in this image. Figure 3 shows the X-ray diffraction (XRD) patterns of the fabricated particles before and after Pt shell deposition. In
addition to the characteristic diffraction peaks at 2θ values of 44.5°, 51.8°, and 76.4° corresponding to the Miller indices (111), (200), and (220), respectively, of the fcc crystal Ni, the particles without Pt shells also exhibit two diffraction peaks associated with nickle oxide at 2θ values of 34° and 60.4°. Because that the entire synthesis process was conducted under N2 atmosphere, the oxide component in the Ni particles must arise during the posttreatment processes. It can be seen that these oxide diffraction peaks are almost unseen on the XRD spectrum of particles with Pt shells, implying that the formation of the Pt shell increases the tolerance of the particle against oxidation in solutions and air. The particles with Pt shells give three XRD peaks at 2θ values of 43.3°, 49.7°, and 74°, which are very close to those of pure Ni particles, but far from the corresponding values associated with Pt crystals (39.8°, 46.3°, and 68.2°). This indicates that the Ni@Pt core-shell particles have the lattice structure of Ni nanocrystals. The absence of Pt diffraction characteristics implies the formation of an extremely thin Pt shell. In addition, the slight shift of the XRD peaks toward lower 2θ values means a lattice expansion of the Ni core in the presence of the Pt shell, which confirms the implications from the TEM observations discussed above. 3.2. Electrochemical Properties of the Ni@Pt Nanoparticles. The electrochemical properties of materials are very sensitive to their surface composition and structures. Therefore, the presence of a very thin shell over nanometer-sized particles, which is difficult to detect by most physical characterization methods, could be manifested by the electrochemical responses of the core-shell particles. To perform electrochemical measurements, the prepared particles were supported on carbon black powders with a total metal content (Ni + Pt) of 20 wt % (Pt, 8%, Ni, 12%). Commercially available 40% Pt/C catalyst particles (Johnson-Matthey, JM) were used for comparison with the present 20% Ni@Pt/C particles because they have similar total metal volumes. Figure 4 shows (a) the cyclic voltammograms (CVs) of the Ni@Pt/C particles prepared in this work and the JM Pt/C particles in argon-saturated sulfuric acid solution and (b) the specific activities (activity per unit area of catalyst surface, which is usually represented by the kinetic current density, jk) of these particles toward the oxygen reduction reaction (ORR) performed in O2-saturated sulfuric acid solution. The values of jk were obtained from the steady-state polarization curves according to the Koutecky-Levich equation14
jk )
j × jL jL - j
(1)
In eq 1, j is the measured current density at various potentials on the steady-state polarization curve, and jL is the limiting diffusion current density. The surface areas of the catalyst
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Figure 4. Comparisons between the electrochemical behaviors of Ni@Pt core-shell particles and pure Pt particles at room temperature. (a) CVs obtained in argon-saturated 0.5 M H2SO4 at a scanning rate of 50 mV‚s-1. (b) Specific activities toward the oxygen reduction reaction measured by rotating the electrodes at 1600 rpm.
Figure 5. (a) Volume evolution during MD relaxation of a cuboctahedron Ni@Pt core-shell particle. (b) Distributions of nearest-neighbor interatomic distances in the core of the Ni@Pt particle (solid line) and in a pure Ni particle (dotted line) with the same total number of atoms as the core of the Ni@Pt particle.
particle used to calculate the current densities were obtained from the charges involved in the hydrogen adsorption/desorption current peaks in Figure 4a. It can be seen from Figure 4a that these particles exhibit welldefined current peaks associated with hydrogen adsorptiondesorption processes on a Pt surface, implying a Pt nature of the particle surface. Usually, CVs of PtM alloys containing less than 50 atom % Pt do not exhibit these hydrogen peaks,15 whereas the present Ni@Pt particles contains only 17 atom % Pt. It is also seen in Figure 4a that the current peak associated with the reduction of platinum oxide in the CV obtained from the Ni@Pt particles shifts by more than 60 mV toward positive potential as compared to that of a pure Pt particle. This indicates that the desorption of the oxygenate species (e.g., OH) from the Ni@Pt particle surface is easier than that from the surface of pure Pt particles, i.e., the oxygenate species have a lower adsorption energy on the Ni@Pt particle surface. The weak adsorption of oxygenate species was also observed by Markovic and co-workers on bulk-Ni-supported Pt-skin electrodes obtained through the surface segregation of Pt atoms upon annealing of Pt3Ni alloys.8 Actually, the difference in adsorption properties between supported metal monolayers and their parent metals has been shown to be a general phenomenon due to the modification of the electronic properties of surface atoms by the underlying metal via geometric strain and ligand interactions.8,9 For a Pt monolayer on a Ni surface, a compressive strain of the Pt-Pt distance in the Pt monolayer occurs, resulting in weak interactions between the Pt surface atoms and some simple adsorbates such as H, CO, and OH.8,9 It has been shown that the adsorption of OH (or other oxygenate species) on a Pt surface can inhibit its catalytic activity toward the oxygen reduction reaction (ORR), whereas the weak adsorption of the oxygenated species would increase the surface active sites for ORR.8,16 As shown in Figure 4b, the specific activity for the ORR on the Ni@Pt particles is significantly higher than that on pure Pt particles, agreeing with
the positive shift of the oxide reduction peak in the CV of the Ni@Pt particles. Thus, the electrochemical results in Figure 4 imply that the prepared particles contain a shell of Pt with compressed Pt-Pt interatomic distances. In addition, the results shown in Figure 4b indicate that the Ni@Pt particles could be promising catalysts for the cathodes of PEM fuel cells with much reduced Pt content but significantly increased catalytic activity. PEM fuel cells hold great promise in powering electric vehicles but is currently halted in commercialization because of the high demand for Pt in the cathode. 3.3. Molecular Dynamics (MD) Simulations of a Ni@Pt Core-Shell Nanoparticle. To verify the structural characteristics of the Ni@Pt core-shell particles as revealed by the TEM, XRD, and electrochemical results, we conducted molecular dynamics (MD) simulations of a Ni@Pt core-shell nanoparticle with a monolayer Pt shell. A cuboctahedral fcc nanoparticle composed of the “magic number” of 4033 atoms, which corresponds to an effective spherical diameter of 4.4 nm, was constructed according to the lattice constant of the Ni crystal (i.e., 0.352 nm) to form the core; on this core, an atomic monolayer of Pt was coated pseudomorphically with the same lattice constant (Figure 1d). This core-shell particle served as the initial state for MD simulations and was allowed to relax at 300 K. As shown in Figure 5a, the volume of the core-shell particle increased rapidly at the beginning of the relaxation and finally reached an equilibrium value. The energy of the particle exhibited a similar variation, reaching a minimum value at the time when the volume became stable (not shown). This clearly indicates that the deposition of a Pt shell on a Ni particle would result in a lattice expansion to minimize the energy of the final core-shell particle, in agreement with the TEM and XRD results shown above. The particle expansion should be a result of the size mismatch between the Pt and Ni atoms. Pt atoms have to squeeze themselves when forming a pseudomorphic overlay on the surface of a Ni particle, which will result in a strong repulsion between neighboring Pt atoms, thus causing an
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J. Phys. Chem. C, Vol. 112, No. 5, 2008 1649 guiding the fabrication and characterization of core-shell nanoparticles with extremely thin noble metal shells and in understanding the unique properties of these nanomaterials. In addition, the prepared Ni@Pt core-shell nanoparticles could have a promising application in fuel cells as an ultralow-Pt cathode catalyst.
Figure 6. Distribution of nearest-neighbor Pt-Pt distances in the shell of a Ni@Pt particle.
expansion of the Pt enclosure. The Ni atoms underneath will expand along with the Pt shell because of the attractive interaction between Pt and Ni atoms. As a comparison, we also conducted a simulation of the pure Ni core particle without a Pt shell. In contrast to the Ni@Pt particle, the pure Ni cuboctahedral fcc nanoparticle showed a slight decrease in volume upon relaxation, implying a lattice contraction in the Ni nanoparticle as compared to the bulk Ni crystal. The lattice contraction of the pure Ni particle and the expansion of Ni core in Ni@Pt particle are further evidenced by the distributions of the nearest-neighbor metal bond distances shown in Figure 5b. It can be seen that the Ni-Ni nearest-neighbor distance distribution peaks at about 0.25 nm in the Ni core of the Ni@Pt particle, which is slightly larger than the Ni-Ni metal bond distance in bulk Ni crystal (0.249 nm). In contrast, the peak of the Ni-Ni nearest-neighbor distance distribution in a pure Ni particle containing the same number of atoms was at about 0.247 nm, slightly smaller than that in bulk Ni crystal. Figure 6 shows the distribution of nearest-neighbor interatomic distances in the Pt shell of the relaxed Ni@Pt coreshell particle. The peak value occurs at about 0.255 nm, which is larger than the value in the initial particle (0.249 nm, the nearest-neighbor metal bond distance in Ni crystal) and far less than that in pure Pt particles (0.276 nm). This implies that the expansion of the core-shell particle is a result of the expansion of the Pt shell, which occurs because of the overly strong compression of the Pt atoms arranged as an enclosure with the lattice constant of the Ni core. However, the expansion only slightly increased the Pt-Pt interatomic distances. The equilibrium Pt-Pt bond distance in the Pt shell was still much smaller than that in the pure Pt crystal, reflecting a compressively strained Pt surface. This result is in correspondence with the implications of the electrochemical results. 4. Conclusions Sub-10-nm Ni@Pt core-shell nanoparticles with nearmonolayer Pt shells can be readily fabricated by a modified polyol process. TEM images show that the prepared core-shell particles have a mean diameter of ca. 5.0-5.4 nm with a narrow size distribution. These particles exhibit structural characteristics of nearly fcc Ni nanocrystals, as indicated by their XRD patterns. Both the TEM and XRD results indicate that the deposition of the Pt shell could result in a lattice expansion of the Ni core. The electrochemical responses of these particles indicate that they have a Pt surface with a compressively strained Pt-Pt interatomic distance as compared to that in pure Pt nanoparticles. The structural characteristics revealed by the TEM images, XRD patterns, and electrochemical properties of the prepared coreshell particles are well verified by MD simulations of a Ni@Pt core-shell particle with a monolayer Pt shell. The strategies and results presented in this article would be very useful in
Acknowledgment. This work was supported by the Natural Science Foundation of China (Grants 50632050 and 20573082), the State Education Ministry of China under the program for New Century Excellent Talents in Universities of China (NCET06-0612), the program for Research Foundation of Doctoral Program (20050486025), and the State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology (Zhejiang University of Technology). References and Notes (1) (a) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.; Lee, J.; Hyeon, T. J. Am. Chem. Soc. 2004, 126, 5026. (b) Zhou, S. H.; Varughese, B. H.; Eichhorn, B.; Jackson, G.; Mcllwrath, K. Angew. Chem., Int. Ed. 2005, 44, 4539. (c) Zhang, J.; Lima, F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.; Hanson, J.; Adzic, R. R. J. Phys. Chem. B 2005, 109, 22701. (d) Jun, C. H.; ParK, Y. J.; Yeon, Y. R.; Choi, J. R.; Lee, W. R.; Ko, S. J.; Cheon, J. Chem. Commun. 2006, 1619. (e) Meada, K.; Teramura, K.; Lu, D. L.; Saito, N.; Inoue, Y.; Domen, K. Angew. Chem., Int. Ed. 2006, 45, 7806. (2) (a) Hu, J. W.; Li, J. F.; Ren, B.; Wu, D. Y.; Sun, S. G.; Tian, Z. Q. J. Phys. Chem. C 2007, 111, 1105. (b) Hu, J. W.; Zhang, Y.; Li, J. F.; Liu, Z.; Ren, B.; Sun, S. G.; Tian, Z. Q.; Lian, T. Chem. Phys. Lett. 2005, 408, 354. (c) Baer, R., Neuhauser, D.; Weiss, S. Nano Lett. 2004, 4, 85. (3) (a) Park, J. I.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5743. (b) Park, J. I.; Kim, M. G.; Jun, Y. W.; Lee, J. S.; Lee, W. R.; Cheon, J. J. Am. Chem. Soc. 2004, 126, 9072. (c) Lee, W. R.; Kim, M. G.; Choi, J. R.; Park, J. I; Ko, S. J.; Oh, S. J.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 16090. (4) Xiang, J.; Lu, W.; Hu, Y. J.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489. (5) (a) Sao-Joao, S.; Giorgio, S.; Penisson, J. M.; Chapon, C.; Bourgeois, S.; Henry, C. J. Phys. Chem. B 2005, 109, 342. (b) Pachon, L. D.; Thathagar, M. B.; Hartl, F.; Rothenberg, G. Phys. Chem. Chem. Phys. 2006, 8, 151. (c) Nagaveni, K.; Gayen, A.; Subbanna, G. N.; Hegde, M. S. J. Mater. Chem. 2002, 12, 3147. (6) (a) Kim, J. H.; Chung, H. W.; Lee, T. R. Chem. Mater. 2006, 18, 4115. (b) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (7) (a) Panagiotopoulou, P.; Kondarides, D. I. Catal. Today 2006, 112, 49. (b) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638. (8) (a) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (b) Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2002, 106, 11970. (c) Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M. J. Electroanal. Chem. 2003, 554, 191. (d) Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2006, 128, 8813-8819. (e) Shao, M. H.; Sasaki, K.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128, 3526. (9) (a) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. ReV. Lett. 2004, 93, 156801. (b) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810. (c) Karlberg, G. S. Phys. ReV. B 2006, 74, 153414. (10) Lee, I. S.; Lee, N.; Park, J.; Kim, B. H.; Yi, Y. W.; Kim, T.; Kim, T. K.; Lee, I. H.; Paik, S. R.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 10658. (11) (a) Baskes, M. I.; Angelo, J. E.; Bisson, C. L. Modell. Simul. Mater. Sci. Eng. 1994, 2, 505. (b) Wang, G. F.; Van, Hove, M. A.; Ross, P. N.; Baskes, M. I. J. Chem. Phys. 2005, 122, 024706. (c) Baskes, M. I. Phys. ReV. B 1992, 46, 2727. (d) Baskes, M. I. Mater. Chem. Phys. 1997, 50, 152. (12) Hegde, M. S.; Larcher, D.; Dupont, L.; Beaudoin, B.; Elhsisen, K. T.; Tarascon, J. M. Solid State Ionics 1997, 93, 33. (13) Hardeveld, R. V.; Montfoort, A. V. Surf. Sci. 1966, 4, 396. (14) Bard, A. J.; Faulker, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons: New York, 2001. (15) (a) Huang, Q. H.; Yang, H.; Tang, Y. W.; Lu, T. H.; Akins, D. L. Electrochem. Commun. 2006, 8, 1220. (b) Santos, L. G. R. A.; Oliveira, C. H. F.; Moraes, I. R.; Ticianelli, E. A. J. Electroanal. Chem. 2006, 596, 141. (c) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2002, 106, 4181. (16) Sanjeev, Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. J. Electrochem. Soc. 1995, 142, 1409.
