New Experimental Evidences of Pt–Pd Bimetallic ... - ACS Publications

May 24, 2012 - Posts and Telecommunications Institute of Technology, km 10 ... Ho Chi Minh City University of Technology (HUTECH),144/24 Dien Bien Phu...
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New Experimental Evidences of Pt−Pd Bimetallic Nanoparticles with Core−Shell Configuration and Highly Fine-Ordered Nanostructures by High-Resolution Electron Transmission Microscopy Viet Long Nguyen,*,†,‡,§,∥ Michitaka Ohtaki,† Takashi Matsubara,∥ Minh Thi Cao,⊥ and Masayuki Nogami∥ †

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka, 861-8580, Japan ‡ Posts and Telecommunications Institute of Technology, km 10 Nguyen Trai, Hanoi, Vietnam § Laboratory for Nanotechnology, Ho Chi Minh Vietnam National University, Linh Trung, Thu Duc, Ho Chi Minh, Vietnam ∥ Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan ⊥ Ho Chi Minh City University of Technology (HUTECH),144/24 Dien Bien Phu, Ward 25, Binh Thach, Ho Chi Minh City, Vietnam S Supporting Information *

ABSTRACT: In our facile synthesis method, poly(vinylpyrrolidone)-protected Pt and Pt−Pd bimetallic nanoparticles with controllable polyhedral core−shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol with silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the size range of 20 nm. Important evidences of core−shell configurations of the Pt−Pd core−shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core−shell Pt−Pd nanoparticles in the size range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of ∼3 nm in thickness as the atomic Pd layers grown on the Pt cores. Very interesting characteristics of surface structure of Pt nanostructures and Pt−Pd core−shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt−Pd bimetallic nanoparticles showed that the Frank−van der Merwe and Stranski−Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. It is predicted that the FM growth becomes the main favorable growth compared with the SK growth in the formation of the thin Pd shells of Pt−Pd core−shell nanoparticles. The experimental evidence of the deformations of lattice fringes and lattice-fringe patterns was found in Pt and Pt−Pd core−shell nanoparticles. The interesting renucleation and recrystallization at the attachments between the nanoparticles are revealed to form a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and promising utilization of them are proposed for next generations of various fuel cells with low cost. Finally, the products of Pt−Pd core−shell nanoparticles can be potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane fuel cell and direct methanol fuel cell using the very low Pt loading with better cost-effective design.

1. INTRODUCTION Noble metal nanoparticles (Au, Ag, Pt, Pd, Ru, and Rh), particularly precious platinum (Pt) and palladium (Pd) nanoparticles, are of extreme importance in electronics, optics, photonics, communication, catalysis, biology, and medicine because of their large surface-to-volume ratios and quantum size effects as well as morphology and structure sensitivity effects in a variety of the nanosized ranges. In addition, various kinds of their new nanostructures or nanoparticles can be synthesized via chemical method for various target applications.1,2 So far, very strong adsorption and absorption of hydrogen have been found on two kinds of metals, Pt and Pd.2−7 Therefore, Pt- or Pd-based catalysts are obviously necessary for practical applications for hydrogen fuels and © 2012 American Chemical Society

hydrogen energy industries. In fact, Pt- and Pd-based catalysts show important characteristics for hydrogen oxidation reaction (HOR), methanol oxidation reaction (MOR), and oxygen reduction reaction (ORR) for practical applications for various kinds of fuel cells.3−11 High durability and stability of Pt−Pd core−shell nanoparticles were confirmed in their testing for the electrolytes of various kinds of fuel cells.8 Nowadays, therefore, controlled synthesis and preparation methods of bimetallic core−shell nanoparticles are of vital importance. In addition, a synergistic effect was discovered to be the cause of the Received: February 5, 2012 Revised: May 13, 2012 Published: May 24, 2012 12265

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2. EXPERIMENTAL SECTION 2.1. Synthesis. 2.1.1. Chemical. Chemicals from Aldrich and Sigma-Aldrich were used in our controlled synthesis of Pt nanoparticles as well as Pt−Pd bimetallic nanoparticles with core−shell structure. They were polyvinylpyrrolidone (FW = 55 000), sodium tetrachloropalladate (II) hydrate (ACS reagent), and chloroplantinic acid hexahydrate (ACS reagent) as precursors. EG was used as both the solvent and the reducing agent. Silver nitrate (metal basis, 99.9999%) was used as a modifying agent. All of our chemicals used were of analytical grade and were used without any further purification. However, the successes of preparation processes have significantly depended on the experimental experiences. 2.1.2. Synthesis of Polyhedral Pt Nanoparticles. In a typical process of the controlled synthesis of the Pt nanoparticles, 3 mL of EG, 1.5 mL of 0.0625 M H2PtCl6, 3 mL of 0.375 M PVP, and 0.5 mL of 0.04 M AgNO3 were used. The details and steps of our process procedures were previously presented.20,25 In general, H2PtCl6 was completely reduced with EG. As a result, a dark-brown solution containing Pt nanoparticles was obtained as the final product. 2.1.3. Synthesis of Polyhedral Pt−Pd Core−Shell Nanoparticles. In a typical process, the successive reduction of Na2PdCl4 with EG after complete reduction of H2PtCl6 with EG was used for the controlled synthesis of as-prepared Pt−Pd core−shell nanoparticles. Here 3 mL of EG, 1.5 mL of 0.0625 M H2PtCl6, 3 mL of 0.375 M PVP, and 0.5 mL of 0.04 M AgNO3 were used. A dark-brown solution of the as-prepared Pt nanoparticles thus obtained was used as the Pt cores for the controlled synthesis of Pt−Pd core−shell nanoparticles. Then, 1.5 mL of 0.0625 M Na2PdCl4 and 3 mL of 0.375 M PVP were added for making the Pd shells on the as-prepared Pt cores. In this step, Na2PdCl4 was reduced with EG for the formation of the thin Pd shells on the as-prepared Pt cores. The details of our experimental steps and procedures were previously presented. By using this process, we have obtained a darkbrown solution of the as-prepared Pt−Pd core−shell nanoparticles as a product. 2.2. Material Characterization. 2.2.1. UV−vis-NIR Spectroscopy. A ∼0.5 mL portion of the reaction mixtures of our precursor solutions and as-prepared products was collected via a 0.5 mL standard pipet during the synthesis. They were investigated by UV−vis-NIR spectroscopy (an Ubest 570 UV− vis-NIR spectrotometer) for the investigations of kinetics, mechanisms, and final formation of Pt nanoparticles from the EG solution. UV−vis absorption spectra were recorded for the prepared Pt nanoparticles as well as prepared Pt−Pd core−shell nanoparticles without the centrifugation as follows. The Samples include Sample 1 (3 mL of ethanol and 30 μL of the product of the dark-brown solution of Pt nanoparticles), Sample 2 (3 mL of ethanol and 30 μL of the product of the dark-brown solution of Pt−Pd core−shell nanoparticles), Sample 3 (3 mL of ethanol and 30 μL of H2PtCl6 in EG), Sample 4 (3 mL of ethanol and 30 μL of Na2PdCl4 in EG), and Sample 5 (3 mL of ethanol and 30 μL of the mixture of H2PtCl6.6H2O and Na2PdCl4). 2.2.2. X-ray Diffraction. In X-ray diffraction (XRD) measurements, 1.5 mL of the dark-brown solution product was used for the pure Pt nanoparticles and the pure core−shell Pt−Pd bimetallic nanoparticles by removing PVP polymer and impurities using ethanol, acetone, and hexane. Then, the pure nanoparticles were homogeneously dispersed in 3 mL of

significantly enhanced catalytic activity originating from their specific core−shell configuration.12 Therefore, Pt−Pd core− shell nanoparticles can also be very attractive catalysts for Suzuki and Heck cross-coupling reactions in the chemical synthesis.13 Moreover, the Pt−Pd bimetallic nanoparticles have been studied with efforts for confirming the engineered core− shell nanostructures with the size and morphology controls of the cores as well as the shells.14,15 Through the epitaxial seeded growth, Pt−Pd nanoparticles with cubic core−shell morphology were controllably engineered in the size range of 30−50 nm using K2PtCl4, TTAB, NaBH4, H2PtCl6·6H2O, K2PdCl4, Lascorbic acid, and other experimental conditions, evidencing the core−shell nanostructures.16 In general, the nucleation and growth of the metal shells on the metal or oxide cores depend on the Frank−van der Merwe (FM) layer-by-layer and Stranski−Krastanov (SK) island-onwetting-layer growth modes.17,18 However, little has been known and confirmed for Pt−Pd core−shell nanoparticles, for example, faceted Pt−Pd core−shell nanoparticles observed by STEM tomography method.19 At present, experimental evidence of high-resolution TEM images of Pt−Pd core−shell nanoparticles are still relatively rare and less, and more convincing evidence is needed. Therefore, more experimental evidence of core−shell configurations will be of very meaningful contributions for scientific society. However, wellcontrolled syntheses of Pt- or Pd-based bimetallic nanoparticles with polyhedral morphology by chemical polyol reduction are still scarce. Furthermore, little evidence with high-resolution TEM images of the Pt−Pd core−shell nanoparticles has been reported so far despite their great promises for practical applications in various fuel cells such as polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC). It should be noted that the successful preparation processes of Pt−Pd core−shell nanoparticles by polyol method or other chemical techniques should continue in further studies in the control and optimization of nanosystems of the core− shell configurations. In this research, the controlled syntheses of Pt and Pt−Pd core−shell nanoparticles have been presented with respect to the issues of utilizing modified polyol method. The role of AgNO3 is important to the controlled syntheses of Pt and Pt− Pd core−shell nanoparticles with polyhedral morphology. High-resolution TEM images of Pt−Pd core−shell configuration are presented. Clearly, the thin Pd shells in the range of 1−3 nm thickness are epitaxially grown in solutions on the Pt cores in the formation of Pt−Pd core−shell nanoparticles with the size range of 25 nm, according to layer-to-layer mechanism and the favorable FM growth mode. The experimental evidences of high-resolution TEM images enable us to understand the homogeneous nucleation and growth of both the Pt cores and the Pd shells in the homogeneous ethylene glycol (EG) solution in our preparation procedures. The nanostructures of the Pt nanoparticles and the Pt−Pd core− shell nanoparticles showed the deformations of lattice fringes and lattice-fringe patterns. Therefore, the critical issues of high durability and stability of various core−shell catalysts with the very thin Pt or Pd shells need to be intensively studied. Our proposals of Pt- or Pd-based superlattices by self-assembly with controlling the attachment modes can lead to novel applications or discoveries in catalysis, particularly in PEMFC and DMFC. Finally, the uses of a low Pt or Pd loading in the Pt-based catalysts are some of the best economical ways for large-scale commercialization of fuel cells. 12266

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ethanol by ultrasonication. After that, the volumes of 3 mL of ethanol of the pure Pt nanoparticles or the pure Pt−Pd core− shell nanoparticles were used. They were set onto a planar glass substrate for the XRD measurements with a small area of ∼1 cm2. The ethanol containing the pure nanoparticles was very slowly poured on the glass substrate dropwisely by taking some hours. The glass substrates with the pure nanoparticles were dried at 80 °C for 6 h prior to the measurement. The XRD patterns were recorded in the range of 2θ of 5−90° by a diffractometer (X’Pert-Phillips) operating at 45 kV/40 mA with Cu Kα radiation (1.54056 Å). 2.2.3. Transmission Electron Microscopy. To analyze the asprepared Pt nanoparticles as well as the as-prepared Pt−Pd core−shell nanoparticles, we have used three kinds of transmission electron microscopes (TEMs) (JEOL-JEM2100F, JEOL-JEM-2010, JEOL-JEM-2010XII) operated at 200 kV by using CCD camera or detector. The details of the sample preparation for microanalysis by TEM methods were discussed in our previous works. 25 In addition, The specifications and spatial resolutions of JEOL-JEM-2100F are around 0.1 nm (lattice resolution) and around 0.19 nm (point resolution), and those of JEOL-JEM-2010 are around 0.14 nm (lattice resolution) and around 0.23 nm (point resolution) at 200 kV. In the good and stable operations of TEM for highresolution images, we have chosen experimental conditions of the camera length of 1000 mm and beam energy of 100 kV or 200 kV to obtain the best quality and results of high-resolution images. In HRTEM modes, the spot size and brightness were automatically set. The issues of beam shift and tilt were automatically set. In particular, the modes for the exact objection of images can be suitably selected in the diffraction mode, imaging mode or STEM mode for the best images. In most of HRTEM measurements, the operation modes of TEM were selected in diffraction mode. In addition, the patterns of selected area electron diffraction (SAED) were taken at 200 kV, and nanobeam diffraction (NBD) patterns were determined for our evaluations of the nanostructure of Pt−Pd core−shell nanoparticles.

Figure 1. UV−vis spectra of samples: (a) Na2PdCl4, (b) H2PtCl6, (c) Na2PdCl4 and H2PtCl6, (d) PVP protected Pt nanoparticles with control method using AgNO3, and (e) PVP protected Pt−Pd bimetallic nanoparticles with control method using AgNO3.

transition of the [PtCl6]2− ions in the homogeneous solution of H2PtCl6 in EG (285 nm) and complex formation of Pt nanoparticles in the homogeneous solution of EG and PVP (258 nm). The sample of H2PtCl6 exhibited the highest absorbance in comparison with that of the two other samples containing Pt nanoparticles. The growth and shape formation mechanisms of platinum nanoparticles were studied.20−22 In the case of Na2PdCl4 in EG, there is evidence of the three absorption peaks located at around 272, 327, and 443 nm because of the [PdCl4]2− complexes of Na2PdCl4 in EG (curve (a) in Figure 1). In contrast, the mixture of Na2PdCl4 and H2PtCl6 showed only the two absorption peaks located at around 272 and 443 nm, and the peak at 443 nm was very weak (curve (c) in Figure 1). However, for the as-prepared product of PVP protected Pt−Pd bimetallic nanoparticles, the UV−vis absorption spectrum showed only one strong absorption peak located at 258 nm as the same as the case of PVP-protected Pt nanoparticles. Therefore, the strong decrease in the absorption intensity in the UV−vis spectra clearly indicated the final formation of the as-prepared PVP protected Pt and Pt−Pd nanoparticles. 3.1.2. Structure of Pt and Pt−Pd Core−Shell Nanoparticles. Figure 2 shows the XRD patterns of the as-prepared Pt nanoparticles (line a) and the as-prepared Pt−Pd core−shell bimetallic nanoparticles (line b). The interesting structure of the crystalline face-centered cubic (fcc) phase was confirmed in both the as-prepared Pt nanoparticles and the Pt−Pd core− shell bimetallic nanoparticles. The diffraction peaks were characterized by the (111), (200), (220), (311), and (222) reflections peaks in comparison with the fcc phase of bulk Pt (or bulk Pd) with respect to 2θ values of around 39, 46, 67, 81, and 85°, respectively.20,21 The XRD patterns in Figure 2 look similar to their appearance, shape, and XRD peak locations at their XRD diagrams, but the diffraction intensities and widths of peaks are significantly different. The intensity of the XRD pattern of the as-prepared Pt−Pd core−shell bimetallic nanoparticles is much stronger than that of the as-prepared Pt nanoparticles. The ratios of diffraction intensity between the Pt nanoparticles and the Pt−Pd core−shell bimetallic nano-

3. RESULTS AND DISCUSSION 3.1. Formation of PVP−Pt and Pt−Pd Core−Shell Nanoparticles. 3.1.1. UV−vis Absorption Spectra of PVP−Pt and Pt−Pd Core−Shell Nanoparticles. Figure 1 shows UV− vis absorption spectra of the five samples including Sample 1, the as-prepared Pt nanoparticles, Sample 2, the prepared Pt−Pd core−shell nanoparticles, Sample 3, H2PtCl6 in EG, Sample 4, Na2PdCl4 in EG, and Sample 5, the mixture of H2PtCl6 and Na2PdCl4 in EG. The strong decrease in the peak intensity of the ligand-to-metal charge-transfer absorption at ∼300 nm is an important experimental evidence of the final formation of Pt nanoclusters and nanoparticles by the complete reduction of the [PtCl6]2− ions with EG in our experimental process. The similar evidence of complete reductions of the [PtCl6]2− and [PdCl4]2− ions with EG in the final formation of the darkbrown product solution was indicated by the strong decrease in the absorption intensity of the dark-brown product solution of the as-prepared Pt−Pd core−shell nanoparticles. The UV−vis absorption spectrum of the product of the Pt nanoparticles showed the strong absorption at ∼258 nm. This showed that the fast reduction of the [PtCl6]2− ions happened in the formation of Pt nanoparticles protected by PVP in the excess EG solvent. There were the strong absorption peaks at around 258 and 285 nm due to the ligand-to-metal charge-transfer 12267

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Figure 2. Two XRD patterns: (a) Pt nanoparticles and (b) Pt−Pd core−shell bimetallic nanoparticles.

particles are calculated as 1.758, 1.499, 1.367, 1.518, and 1.308, respectively. The important XRD evidence showed that the diffraction intensity was strongly enhanced by the core−shell Pt−Pd bimetallic nanoparticles in comparison with the Pt nanoparticles from 1.3 to 1.75. The typical XRD peaks of Pt nanoparticles were used to compare with those of the corresponding bulk Pt material. The average size of Pt nanocrystallites can be calculated by the width of the diffraction peaks according to the Debye−Scherrer equation: D = 0.9λ/(β cos θ), where β is the full width at half-maximum (fwhm) of the peak, θ is the angle of diffraction, and λ is the wavelength of the X-ray radiation. However, only five peaks assigned to a single fcc structure were observed in the limited 2θ range of 5−90° by the XRD method. Therefore, the core−shell morphology can be only exactly determined by the TEM and HRTEM methods. 3.2. Characterization of Pt and Pt−Pd Core−Shell Nanoparticles. Figures 3, 5, and 6 present TEM and HRTEM images of as-prepared Pt nanoparticles with the most characteristic polyhedral morphologies and shapes. It should be noted that the as-prepared nanoparticles were observed in the polyhedral morphology, typically such as cubes, octahedra, and tetrahedra. Therefore, the favorable nucleation and growth of Pt nanoparticles are the homogeneous modes, and the anisotropic inhomogeneous nucleation and growth are unfavorable. The size was well-controlled in the range of 20 nm. The surface of Pt nanostructures with surface defects such as kinks, atomic steps, islands, and terraces can be clearly seen in the high-resolution images as shown in Figures 3b and 5. In many works, they were considered to be the most efficient catalytic centers for the significant improvements of catalytic activity of the Pt catalysts. Therefore, we need to find a method to determine the surface concentrations of kinks, atomic steps, islands, and terraces in various Pt-based catalysts for electrocatalysis.23,46,47 Figures 4 and 7−9 show the HRTEM images of as-prepared Pt−Pd core−shell nanoparticles with the most characteristic polyhedral morphology and shape. The thin Pd shells grown over the Pd cores have led to form the core−shell configuration with the well-controlled size in the nanosized range of about 15−25 nm. The thickness or the coated shell was wellcontrolled in the range of 1−3 nm. The Pt−Pd core−shell

Figure 3. (a,b) TEM and HRTEM images of as-prepared Pt nanoparticles. (c,d) Important models of random and direct attachments or bondings of two particles for novel applications in catalysis, medicine, and biology. Scale bars: (a) 50 nm and (b) 10 nm.

nanoparticles also showed characteristic polyhedral morphology and shape, typically such as cubes, octahedra, and tetrahedra. Similarly, the dominant nucleation and growth mode of Pt−Pd core−shell nanoparticles are the homogeneous modes, and the anisotropic inhomogeneous nucleation and growth modes are negligible. Most of the as-prepared polyhedral Pt nanoparticles as well as the Pt−Pd core−shell nanoparticles including cubes, octahedra, tetrahedral, and polyhedra or truncated polyhedra exhibit the low-index facets of {111}, {110}, and {100} planes. So far, the {111}, {110}, and {100} low-index facets are some experimental evidence and observations of the most stable and durable planes of polyhedral morphology and shape observed in cyclic voltammetry (CV) experiments for specific catalytic activity, sensitivity, and selectivity of various Pt-based catalysts. On the basis of our careful observations and analyses as well as excellent experimental evidence in the reactions of the asprepared nanoparticles of both Pt nanoparticles and Pt−Pd core−shell nanoparticles, we have proposed the models of the interactions among them. There are various kinds of random and direct attachments or bondings between the nanoparticles including corner−corner, edge−edge, surface−surface, surface− corner, and surface−edge attachments and any possible kinds of various arbitrary and direct attachments. The important evidence of the combinations among the polyhedral Pt nanoparticles is seen in Figures 3, 5, and 6. It is predicted that renucleation and recrystallization easily occur at the 12268

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Figure 4. HRTEM images of as-prepared Pt−Pd core−shell nanoparticles with polyhedral morphology. Scale bars: (a−b) 10 nm. Figure 5. (a−f) Single-crystalline HRTEM images of polyhedral Pt nanoparticles with the most characteristic polyhedral morphologies. The high crystallization of the as-prepared Pt nanoparticles was confirmed. (f) Deformation of lattice fringe or lattice-fringe patterns in the local area corresponding to its color description. Scale bars: (a−f) 10 nm.

connections and boundaries between them. Therefore, the catalysts containing Pt nanoparticles or Pt−Pd bimetallic nanoparticles highly require their homogeneous distribution in the whole catalysts to show the best catalytic performance. The as-prepared Pt nanoparticles were combined to form the larger particles with the good lattice-fringe matching of the particles of the same nature, as shown in Figure 5. In particular, Figures 5a and 6 showed excellent bonding evidence of surface−surface, surface−corner, and corner−corner attachments. Our proposed models in Figure 3c−g according to our experimental evidence show extreme importance of selfassembly of metallic nanoparticles with various control methods, especially for nanoparticles, such as Pt, Au, Ag, and so on, to form the very large superlattice nanostructures (Supporting Information). We propose our ideas in making new ordered superstructures that the nanoparticles can be combined through the only corner−corner attachment and edge−edge attachment for the large superlattices in the longdistance ordered nanostructures or novel textures. Hence, we propose that self-assembly under novel and innovative chemical control methods on the basis of the controlled attachments and self-assemblies by corner−corner, edge−edge, surface−surface, surface-corner, and surface-edge as well as other improved or innovative attachments in any nanosystems or nanofluidic systems will be created or discovered in future. These extremely important issues are very crucial and attractive to scientists and researchers.

Figure 5a−f clearly shows the fine crystal nanostructures of the as-prepared Pt nanoparticles with octahedral morphology grown from homogeneous EG solution. The cubic Pt nanoparticles show the lattice fringes with the interfringe distances of around 0.196 nm assigned to the {100} planes in Figures 5e and 6a. The octahedral Pt nanoparticles with good crystal morphology show the lattice fringes of ∼0.234 nm in Figure 5f. In the case of tetrahedral Pt nanoparticles, the fringe spacing was measured as ∼0.230 nm assigned to the {111} planes in Figure 3a as that of octahedral Pt nanoparticles. The lattice fringe estimated around 0.234 nm was assigned to the {111} planes. The lattice-fringe pattern was clearly observed only in the same particle. In addition, the deformations of lattice fringes or lattice-fringe pattern were clearly observed. At the locations having the deformations of lattice fringes or lattice-fringe pattern, structural changes were confirmed. However, we suggested that the possible reasons of the deformations of lattice fringes or lattice-fringe patterns could be also explained by the phenomena of TEM measurements and techniques as well as optical phenomena generated during TEM operations. In particular, most important evidence can be clearly clarified in the nanostructural 12269

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Figure 6. (a,b) Single-crystalline HRTEM images of polyhedral Pt nanoparticles. The combination of two as-prepared Pt nanoparticles by their surface attachment 2 nm was confirmed in our various synthesis processes. Scale bars: (a) 10 and (b) 2 nm. Figure 8. (a−e) HRTEM images of Pt−Pd core−shell nanoparticles with the polyhedral morphology. Scale bars: (a,b) 2 nm, (c−f) 10 nm.

phenomena of the structural transformations and dislocations in solids.24 Figure 5e shows the very clear crystal nanostructure of cubic Pt nanoparticles. In such a particle, it was observed that the lattice-fringe pattern was very clear. In addition, the deformations of lattice fringes (f i) and lattice-fringe pattern were seen in one such particle in Figure 5f. These phenomena can be explained by two ways. The first one is due to the changes of Pt atomic structures in the internal nanoparticles, which can be assigned to the dislocations.24 The second one is due to the optical phenomena of high-resolution TEM measurements and investigations during the operations of TEM. In our measurements, we can observe only a small number of the particles at the magnification of 10 nm scale bar, from one to four nanoparticles, for the fine nanostructures or fine nanocrystals. For scientists and researchers, the biggest question is how to see and determine exactly 10 or more nanoparticles at this magnification range at the same time. They are the scientific challenges and technical barriers in nanoscience. Therefore, our modified polyol method proved that metallic and bimetallic nanoparticles can be controlled in both size and morphology for their excellent and potential applications in various kinds of research areas in catalysis, biology, and medicine in the various nanosized ranges.

Figure 7. HRTEM images of Pt−Pd core−shell nanoparticles. Deformations of lattice fringes or latticefringe patterns of one core− shell particle are clearly seen. Scale bars: (a−b) 10 nm. 12270

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shown the very thin Pd shells about from 5 ± 1 Pd monolayers about 1.34 nm to 7 ± 1 Pd monolayers about 1.87 nm, as shown Figure 7b.25 In addition, the roughness and flatness of the surface morphologies of both Pt nanoparticles and Pt−Pd core−shell nanoparticles can also be observed. A variety of polyhedral nanoparticles with respect to a same variety of lattice fringes and lattice-fringe patterns was found in our high-resolution TEM images. By XRD method, only a limited number of five diffraction peaks were observed as the hard proofs of the averaged structure of metal nanoparticles, for example, Pt nanoparticles by chemical method exhibiting an fcc crystal structure despite the variety of nanostructures, shapes, and morphologies.20−29 In TEM and HRTEM measurements, a limited number of Pt nanoparticles or Pt−Pd core−shell nanoparticles was observed in the fine nanostructures with the finely visible lattice fringes and lattice-fringe patterns. Certainly, the experimental limitations also exist for analyzing the fine nanostructures of the HRTEM images of Pt nanoparticles or Pt−Pd core−shell nanoparticles observed in various qualitative and quantitive mircoanalyses; for example, clear observation is limited up to five or six particles, and to our knowledge, no evidences have been proven for the clear observations for more than ten nanoparticles in the 10 scale-bar magnification with high resolution. In the future, a critical question to scientists is how to observe the fine nanostructures of ten or more nanoparticles at the same time. In the as-prepared products, the octahedral Pt−Pd core−shell nanoparticles with very high yields were formed by our modified polyol method. Therefore, our method can offer a simple way of producing various patterns of core− shell bimetallic nanoparticles. Recently, the issues of {hkl} high-index planes of Pt (or Pd) nanoparticles have been of special interests because of the significant enhancement of catalytic activity.26−29 The models of high-index crystal planes are also proposed. However, at present we cannot determine exactly how many low- and highindex planes of Pt-based catalysts have significant impacts on the structural and catalytic sensitivity. So far, it was reported that the catalytic durability and stability of {111}, {110}, and {100} low-index planes of the Pt-based catalysts have been confirmed.30,31 Furthermore, we should need more exact experimental evidence of high-index planes in both theoretical and experimental methods, such as XRD method, HRTEM images, and CV electrochemical measurements in the use of the Pt based catalysts or Pt nanoparticles in the size range of 10 nm.32,33 Besides, this will also lead to important issues about the exact number of low- and high-index crystal planes of Pt nanoparticles or noble metal nanoparticles and how they influence the catalytic properties. In the nanobeam method, we have just investigated 17 crystal planes of Pt nanoparticles including the specific planes (111), (200), (220), (311), (222), (400), (311), (420), (422), (333), (511), (440), (531), (442), (600), (620), and (533).34 Through high-resolution TEM method, it has been proven that the low-index {111}, {100}, and {110} planes of the Pt nanoparticles were clearly observed with respect to the surface characterization of terraces, steps, kinks, islands, and other surface defects.34−36,46,47 In electrochemical CV measurements, we demonstrated that the influence of the electrocatalytic properties of Pt catalyst in acidic environment heavily depends on various parameters such as size, shape, heat-treatment processes for the catalysts, and so

Figure 9. (a−f) HRTEM images of Pt−Pd core−shell nanoparticles with the octahedral morphology. Scale bars: (a−f) 10 nm.

Figure 6 shows a clear evidence of the combination of two Pt particles. This proved that the as-prepared Pt nanoparticles can be heavily collided at the nanoscale range. As a result, they can be attached each another. The new Pt nanostructures can be formed with a good lattice matching. Importantly, we can clearly see the fringe lattice in a local area of 10−14 nm dimension, but the remaining area outside of this observation field cannot be seen by TEM technique at the same time. Figure 7 shows a cube-like core−shell Pt−Pd nanoparticle consisting of five to seven Pd monolayers on the Pt core in the clearest visible directions. The Pt core has its size of ∼10 nm. The structural changes and atomic changes between the Pt core and the Pd shell were clearly observed at the conjunction of the core and the shell, as shown in the inset. In image analysis, the colors in the inset showed the significant structural changes in the core−shell configuration. Here we also found the deformation of lattice fringe and lattice-fringe patterns in the Pd shell in the inset (A2). In these core−shell configurations, the value of lattice-fringe spacing was estimated to be ∼0.240 nm, being exactly assigned to the lattice-fringe spacing between the {111} planes of the Pd metal shell. The morphology of the Pt core was studied in detail. It is certain that the Pt core and the Pd shell are separated. However, their clear boundaries need to be intensively investigated. The characteristics of surface Pt−Pd nanostructures with the thin Pd shells with surface defects, typically such as kinks, atomic steps, islands, and terraces, can be also clearly seen in the high-resolution images shown in Figure 4 as well as in Figures 7−9, which have 12271

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be changed to tailor the size of the thin or thick Pd atomic monolayers in the exact manners. The core−shell structure of Pt−Pd bimetallic nanoparticles showed the fine atomic arrangements in both the fine arrangements of Pt atoms in the Pt cores and the finely ordered arrangements of the Pd atoms in the Pd shells. Most of Pt−Pd core−shell nanoparticles with octahedral morphology and truncated octahedral morphology in Figure 9 also exhibited the same homogeneous nucleation and growth of the Pd shells on the Pt cores in the most favorable FM layer-bylayer modes. However, the coexistence of both FM and SK nucleation and growth modes was observed in Figures 8 and 9. To control the favorable FM nucleation and growth mode, experimental conditions (time, temperature, supply rate of precursors, mixing rate, the reaction volumes of precursor solutions for synthetic reaction, etc.) should be strictly controlled for a wide range of various products. In our present research, excellent experimental evidence of the Pt−Pd core− shell nanoparticles appears to be a desirable product of Pt−Pd core−shell nanoparticles used as good and economical catalysts in PEMFC and DMFC. One of the best ways for enabling large-scale commercialization of fuel cells using Pt catalyst is to utilize Pt metal or to use a very low Pt loading, especially in PEMFC and DMFC. Some recent works have focused on the requirements of high current density in the testing measurements in PEMFCs as a function of the Pt loadings (using a kind of commercial Pt catalyst) on the surface area per unit volume of the electrodes without concerning surface characterization of the Pt catalyst.41,42 In our research, the NBD method was used with an electron probe to determine the lateral resolution from one to tens nm for the diffraction patterns with the high symmetry. It is observed that the SAED showed the high symmetry of the diffraction patterns as for single Pt crystal.43,49,50 Figure 10 showed the evidence of HRTEM image of tetrahedral Pt−Pd core−shell nanoparticle with the very good SAED patterns. The results indicated one of the best Pt−Pd core−shell nanoparticles with the fcc single-crystal core−shell nanostructures with the good lattice matching, for example, the fcc singlecrystal structure for both Pt and Pd metals. Recently, the theoretical results of Pd−Pt core−shell nanoparticles have showed the least structurally stable structures in comparison with Pt−Pd core−shell bimetallic nanoparticles with the more stable structures in a systematic study on their structural and thermal stabilities with various core−shell and alloyed structures by using atomistic simulations.51 Therefore, we should find any ways of designing new Pt based catalysts43 or new Pt-nanostructure catalysts with a low Pt metal weight or composition. To obtain the large surface area of Pt catalysts, we can synthesize superlattices in proper chemical methods and nanostructure engineering in chemistry. At present, there are interesting exotic superlattices observed.44 Recently, upconversion nanophosphors (UCNPs) have been organized into superlattices over multiple length scales facilitating the nanocrystal characterization and enabling systematic studies of shape-directed assembly through a simple method using EG solution.45 Recently, Ag−Pd core−shell catalyst with the shell containing between 1 and 10 layers of Pd atoms has reported for the hydrogen production from formic acid decomposition at room temperature.48 Therefore, polyol method in our research is a promising chemical synthetic method for specific core− shell morphology. Therefore, the novel ideas and methods of

on. In particular, it was confirmed in the electrochemical measurements that the most stable facets for MOR and ORR reactions are the {111}, {100}, and {110} planes of active sites of Pt atoms or some other planes according to the HRTEM images of Pt and Pt-based nanoparticles, predicting that the improvements of catalysts are enhanced by an increase in the number of surface defects.37 To obtain high catalytic activity, we should optimize the processes of catalyst preparations so that the Pt-based catalysts reach the best stable and durable nanostructures as well as high performance in practical applications. On the basis of experimental evidence of high-resolution images of Pt−Pd core−shell bimetallic nanoparticles, we predicted that there was coexistence of the FM layer-by-layer and SK island-on-wetting-layer growth modes of the Pd shells on the Pt cores. The value of lattice-fringe spacing was estimated to be ∼0.240 nm exactly assigned to the lattice-fringe spacing between the {111} planes of the Pd metal shell. It is predicted that the FM growth became the favorable growth mode in the formation of the thin Pd shells of Pt−Pd core− shell nanoparticles. Here the thin Pd shell or coating can be considered as the atomic Pd monolayers. In addition, bimetallic core−shell catalysts exhibited a significant enhancement of catalytic activity due to a synergetic effect between the layers of the core, the shell, and their interfaces in the specific core−shell geometry.12,38 In addition, two main mechanisms of nucleation and growth were observed, which are the epitaxial growth of a metal shell on a metal core and the nonepitaxial growth of an oxide shell on a metal core or a metal shell on an oxide core. The shape of core−shell spheres can be controllably synthesized by this way.39 In our previous research, we proposed potential applications of various core−shell nanoparticles using a very thin shell of noble metal such as Au, Pt, Pd, and so on or their various alloys of Pt−Au, Pt−Pd, and so on. The critical issue is a very low Pt loading for its economical utilization.25,40 So far, Pt metal and Pt-based bimetallic nanoparticles have increasingly shown their high potential applications in catalysis, biology, and medicine, for example, Pt-based catalysts in PEMFC and DMFC. Figures 7−9 presented the good Pt−Pd core−shell configurations with the separation between the Pt core and the Pd shell. In addition, one Pt−Pd core−shell nanoparticle with tetrahedral morphology in Figure 8a showed nearly the same Pd shell thickness of about L = 2.2 nm (Li, i = 1, 2, 3) according to the vertical direction of {111} facets. The shell as the Pd atomic monolayers was about 9 ± 1 monolayers of Pd atoms. The near similar thickness of the Pd shells according to various vertical directions of the {111} facets of the thin Pd shells on the core Pt nanoparticles confirmed the homogeneous nucleation and growth during the synthesis. Figure 8d shows well-resolved nanostructures of Pt−Pd core−shell nanoparticles with the Pd shells (Li, i = 1,2,3,4); the thicknesses of the shells L1, L2, L3, and L4 are near the same size around 2.5 nm with atomic monolayers of 11 ± 1 or 12 ± 1. The high-resolution TEM image showed the homogeneous nucleation and growth of the Pd shells on the Pt cores in the most favorable FM layer-by-layer modes. We suggested that the as-prepared Pt−Pd core−shell nanoparticles have the thin Pd shells of less than 14 or 15 atomic Pd monolayers around 3.2 nm, that is, in the range of 20 monolayers about 4.5 nm. At present, no evidence of the formation of one atomic monolayer of the shell metal on the different metal core is observed. Therefore, the volume ratio of PVP/Pd precursor solutions can 12272

dx.doi.org/10.1021/jp303117y | J. Phys. Chem. C 2012, 116, 12265−12274

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Article

ASSOCIATED CONTENT

S Supporting Information *

Superlattice nanostructures, octahedral and cubic morphology of Pt nanoparticles, single-crystalline HRTEM images of polyhedral Pt nanoparticles, and core-shell morphology of PtPd bimetallic nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Nguyen Viet Long; Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, 6-1 Kasugakouen, Kasuga, Fukuoka, 861-8580, Japan; E-mail: [email protected]; nguyenvietlong01@ gmail.com; [email protected]. Phone/ Fax: +81-(0)92-583-8835. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the Global COE Program, Novel Carbon Resource Sciences, in Kyushu University, Japan, for the financial support in our research.

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Figure 10. (a) HRTEM image of tetrahedral Pt−Pd core−shell nanoparticle. (b,c) Patterns of selected area electron diffraction (SAED). Scale bars: (a) 2 nm, (b) 10 (1/nm).

REFERENCES

(1) Qiang, Q.; Ostafin, A. E. Metal Nanoparticles in Catalysis. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2004; pp 475− 503. (2) Polarz, S. Adv. Funct. Mater. 2011, 21, 3214. (3) Duncan, H.; Lasia, A. J. Electroanal. Chem. 2008, 621, 62. (4) Lebon, A.; García-Fuente, A.; Vega, A.; Aguilera-Granja, F. J. Phys. Chem. C 2012, 116, 126. (5) Shao, M. J. Power Sources 2011, 196, 2433. (6) Chen, M.; Wu, B.; Yang, J.; Zheng, N. Adv. Mater. 2012, 24, 862. (7) Sasaki, K.; Naohara, H.; Cai, Y.; Choi, Y. M.; Liu, P.; Vukmirovic, M. B.; Wang, J. X.; Adzic, R. R. Angew. Chem., Int. Ed. 2010, 49, 8602. (8) Nilelar, A. U.; Sasaki, K.; Farberow, C. A.; Adzic, R. R.; Mavrikakis, M. J. Am. Chem. Soc. 2011, 133, 18574. (9) 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. Top. Catal 2007, 46, 249. (10) Vukmirovic, M. B.; Bliznakov, S. T.; Sasaki, K.; Wang, J. X.; Adzic, R. R. Electrochem. Soc. Interface 2011, 20, 33. (11) Koenigsmann, C.; Santulli, A. C.; Gong, K.; Vukmirovic, M. B.; Zhou, W.; Sutter, E.; Wong, S. S.; Adzic, R. R. J. Am. Chem. Soc. 2011, 133, 9783. (12) Toshima, N.; Yan, H.; Shiraishi, Y. Chapter 3: Recent Progress in Bimetallic Nanoparticles: Their Preparation, Structures and Functions. In Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control; Corain, B., Schmid, G., Toshima, N., Eds; Elsevier BV: Amsterdam, 2004; pp 49−75. (13) Narayanan, R. Molecules 2010, 15, 2124. (14) Zhang, H.; Jin, M.; Wang, J.; Li, W.; Camargo, P. H. C.; Kim, M. J.; Yang, D.; Xie, Z.; Xia, Y. J. Am. Chem. Soc. 2011, 133, 6078. (15) Zhang, H.; Jin, M.; Wang, J.; Kim, M. J.; Yang, D.; Xia, Y. J. Am. Chem. Soc. 2011, 133, 10422. (16) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Nat. Mater. 2007, 6, 692. (17) Peng, Z.; Yang, H. Nano Today 2009, 4, 143. (18) Chen, J.; Lim, B.; Lee, E. P.; Xia, Y. Nano Today 2008, 4, 81. (19) Ercius, P.; Duden, T.; Radmilovic, V. R. Microsc. Microanal. 2010, 16, 1650. (20) Long, N. V.; Chien, N. D.; Hayakawa, T.; Hirata, H.; Lakshminarayana, G.; Nogami, M. Nanotechnology 2010, 21, 35605.

making new superlattices with the largest surface area as well as surface characterization of Pt- or Pd-based catalysts with surface-, edge-, and corner-attachments can offer potentially novel applications in catalysis in our next studies.

4. CONCLUSIONS In this research, we have proven that a modified polyol method using a structure-modifying agent as AgNO3 provides a general, good, and easy-to-apply way to synthesize the fine nanostructures of Pt nanoparticles as well as the Pt−Pd core−shell nanoparticles with size and morphology control in chemistry. The issues of polyhedral morphology and shape have been studied by high-resolution TEM techniques to show the surface characteristics as well as structural changes in the as-prepared nanoparticles. The clear deformations of the lattice fringes and lattice-fringe patterns in the as-prepared nanoparticles were found, which also showed importance of significant improvement in high-resolution TEM techniques for the HRTEM objections of the fine nanostructures of metals, alloys, and so on by which we can directly observe fine crystal nanostructures of the as-prepared nanoparticles as many as more than 10 particles at the same time in the 10 nm scale-bar range in the future. Our ideas of self-attachments of the as-prepared nanoparticles with chemical and physical control methods in structural engineering can synthesize highly ordered superlattices for practical applications in catalysis, medicine, and biology as well as various kinds of science and technology. Finally, our research results demonstrate the importance of the realization of PEMFC and DMFC technology by using novel core−shell catalysts with the very thin Pd or Pt shells as the controlled monolayers shells in the meaning of the utilization of Pt metal element in the very low Pt loading in this direction. 12273

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(21) Song, H.; Kim, F.; Connor, S.; Somorjai, S. A.; Yang, P. J. Phys. Chem. B. 2005, 109, 188. (22) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (23) Somorjai, G. A.; Contreras, A. M.; Montano, M.; Rioux, R. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10577. (24) Miyabayashi, K.; Higashimoto, M.; Shen, Z.; Miyake, M. Chem. Lett. 2011, 40, 705. (25) Long, N. V.; Asaka, T.; Matsubara, T.; Nogami, M. Acta Mater. 2011, 59, 2901. (26) Tian, N.; Zhou, Z.; Sun, S.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (27) Tian, N.; Zhou, Z.; Yu, N.; Wang, L.; Sun, S. J. Am. Chem. Soc. 2010, 132, 7580. (28) Tian, N.; Zhou, Z.; Sun, S. J. Phys. Chem. C 2008, 112, 19801. (29) Huang, X.; Zhao, Z.; Fan, J.; Tan, Y.; Zheng, N. J. Am. Chem. Soc. 2011, 133, 4718. (30) Furuya, N.; Koide, S. Surf. Sci. 1989, 220, 18. (31) Subhramannia, M.; Pillai, V. K. J. Mater. Chem. 2008, 18, 5858. (32) Bi, Y.; Lu, G. Electrochem. Commun. 2009, 11, 45. (33) Rodríguez-López, M; Solla-Gullón, J; Herrero, E; Tuño,́ P; Feliu, J M; Aldaz, A; Carrasquillo, A. J. Am. Chem. Soc. 2010, 132, 2233. (34) Long, N. V.; Ohtaki, M.; Hien, T. D.; Jalem, R.; Nogami, M. J. Nanopart. Res. 2011, 13, 5177. (35) Long, N. V.; Ohtaki, M.; Hien, T. D. Colloid Polym. Sci. 2011, 289, 1373. (36) Long, N. V.; Ohtaki, M.; Hien, T. D.; Nogami, M. Electrochim. Acta 2011, 56, 9133. (37) Lee, S. W.; Chen, S.; Suntivich, J.; Sasaki, K.; Adzic, R. R.; ShaoHorn, Y. J. Phys. Chem. Lett. 2010, 1, 1316. (38) Bai, F.; Sun, Z.; Wu, H.; Haddad, R. E.; Xiao, X.; Fan, H. Nano Lett. 2011, 11, 3759. (39) Zhang, J.; Tang, Y.; Lee, K.; Ouyang, M. Science 2010, 327, 1634. (40) Long, N. V.; Hien, T. D.; Ohtaki, M.; Nogami, M. Int. J. Hydrogen Energy 2011, 36, 8478. (41) Stevens, D. A.; Wang, S.; Sanderson, R. J.; Liu, G. C. K.; Vernstrom, G. D.; Atanasoski, R. T.; Debe, M. K.; Dahn, J. R. J. Electrochem. Soc. 2011, 158, 899. (42) Debe, M. K. J. Electrochem. Soc. 2012, 159, 54. (43) Long, N. V.; Ohtaki, M.; Uchida, M.; Jalem, R.; Hirata, H.; Chien, N. D.; Nogami, M. J. Colloid Interface Sci. 2011, 359, 339. (44) Henzie, J.; Grünwald, M.; Widmer-Cooper, A.; Geissler, P. L.; Yang, P. Nat. Mater. 2012, 11, 131. (45) Ye, X.; Collins, J. E.; Kang, Y.; Chen, J.; Chen, D. T. N.; Yodh, A. G.; Murray, C. B. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 22430. (46) Gontard, L. C.; Chang, L.; Hetherington, C. J. D.; Kirkland, A. I.; Ozkaya, D.; Dunin-Borkowski, R. E. Angew. Chem. 2007, 119, 3757. (47) Chang, L. Y.; Barnard, A. S.; Gontard, L. C.; Dunin-Borkowski, R. E. Nano Lett. 2010, 10, 3073. (48) Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Nat. Nanotechnol 2011, 6, 302. (49) Goodhew, P. J.; Humphreys, F. J.; Beanland, R. Chapter 3: Electron Diffraction. In Electron Microscopy and Analysis; Goodhew, P. J., Humphreys, F. J., Beanland, R., Eds; Taylor & Francis: New York, 2001; pp 40−65. (50) Fultz, B.; Howe, J. A. Appendix: A.6 Indexed Single Crystal Diffraction Patterns. A.7 Stereographic Projections. In Transmission Electron Microscopy, and Diffractometry of Materials; Fultz, B., Howe, J., Eds; Springer-Verlag: Berlin, 2008; pp 703−716. (51) Huang, R.; Wen, Y.; Zhu, Z.; Sun, S. J. Phys. Chem. C 2012, 116, 8664.

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on May 24, 2012. Figure 10 has been updated. The correct version was published on May 25, 2012.

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