Shape Evolution of Octahedral and Triangular Platelike Silver

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DOI: 10.1021/cg900563y

Shape Evolution of Octahedral and Triangular Platelike Silver Nanocrystals from Cubic and Right Bipyramidal Seeds in DMF

2009, Vol. 9 4700–4705

Masaharu Tsuji,*,†,‡ Yoshinori Maeda,‡ Sachie Hikino,† Hisayo Kumagae,† Mika Matsunaga,† Xin-Ling Tang,‡ Ryoichi Matsuo,‡ Masatoshi Ogino,‡ and Peng Jiang§ †

Institute for Materials Chemistry and Engineering and ‡Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan, and §National Center for Nanoscience and Technology, Beijing 100080, People’s Republic of China

Received May 25, 2009; Revised Manuscript Received August 15, 2009

ABSTRACT: Octahedral and triangular platelike silver (Ag) nanocrystals were synthesized using cubic and right bipyramidal nanocrystals as seeds using a two-step preparation method. First, cubic and right-triangular bipyramidal Ag nanocrystals with {100} facets were prepared by reducing AgNO3 in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant under oil-bath heating. Subsequently, the obtained Ag seeds were added to N,N-dimethylformamide (DMF) solution containing Agþ ions for overgrowth of Ag crystals with {111} facets. Transmission electron microscope (TEM) observations show that Ag octahedra and triangular plates were formed, respectively, from Ag cubic and bipyramidal seeds. It is found that the average size of Ag octahedra is about 14% smaller than that expected from original sizes of the Ag cubes, although that of Ag triangular plates agreed with that expected from that of the Ag bipyramids. This is explainable by oxidative etching of the corner at the second step. Results show that it is possible to prepare Ag nanocrystal structures with {111} facets in DMF that is generally difficult to prepare using conventional methods. Introduction Recently, control of metallic nanostructures has been the focus of intensive research because of their shape-dependent chemical and physical properties. Among them, nanostructured Au, Ag, and Au@Ag core-shell crystals have attracted considerable attention mainly as a result of their remarkable optical properties and numerous applications in the fields such as catalysts, surface plasmonics, surface-enhanced Raman scattering, and chemical and biological sensing.1-8 Different chemical and physical properties of metallic crystals arise from different crystal surface orientation. For example, the {111}, {100}, and {110} surfaces of a face centered cubic (fcc) metal such as gold, silver, and Au@Ag have very different surface atom densities, electronic structures, and chemical reactivities. Therefore, the controllable preparation of nanocrystals with different shapes and exposed surfaces is very important and challenging. We have recently succeeded in the syntheses of Au core-Ag shell nanocrystals denoted as Au@Ag using a two-step method.6 When HAuCl4 3 4H2O was reduced in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP: Mw = 40 000) as a polymer surfactant in the first step, a mixture of single-crystal octahedral, single-twinned triangular platelike, and multiple-twinned decahedral Au nanocrystals was obtained. Although these Au nanoparticles have different shapes, a common feature is that they are mainly surrounded by {111}-type facets. When Agþ ions were reduced using these Au nanocrystals as seeds in EG solution under microwave (MW) heating in the second step, cubic, right-triangular bipyramidal, and one-dimensional (1D) rod/wire types of Ag shells with {100}-type dominant facets were formed epitaxially around octahedron, single-twinned triangular plate, and decahedron, respectively, as shown on the left side of Figure 1. *Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 09/16/2009

Recently, Gao et al.7 found that tetrahedral and decahedral Ag nanocrystals having {111} facets could be prepared in high yields when AgNO3 was reduced in N,N-dimethylformamide (DMF) solution with the assistance of PVP (Mw = 1 300 000) under the condition of conventional oil-bath heating. On the basis of TEM and SEM observation of obtained products, they proposed that decahedral Ag nanocrystals are assembled step-by-step from five tetrahedra. An important suggestion in their work is that favorable facets of prepared Ag nanocrystals can be changed to {111}-type in DMF. Although tetrahedra and decahedra having {111} facets were prepared in DMF, octahedra and single-twinned triangular plates having the same {111} facets were not formed. Combining our experiments with the findings of Gao et al.,7 we tried to synthesize Au@Ag core-shell nanocrystals having {111} Ag shells in DMF.8 As a result, we succeeded in the preparation of triangular or hexagonal platelike, octahedral, and decahedral nanocrystals surrounded by the Ag shells having the same {111} facets, as shown on the right side of Figure 1. These findings demonstrate that Ag shells with favorable facets and shapes can be controlled effectively by selecting EG or DMF as solvent. Using this technique the shape-selective preparation of Ag shells might be possible, because a good correlation exists between core and shell crystals, depending on the solvent. In this study, we applied the two-step growth method described above to the preparation of shape-controlled Ag nanocrystals. We prepared cubes and right-triangular bipyramids having {100} facets in EG in the first step. These nanocrystals were used as seeds to form octahedra and other particles having {111} facets in DMF. We examined what nanocrystals arise from these seeds and their growth mechanisms. It is generally difficult to synthesize octahedral silver nanocrystals because cubes, right-triangular bipyramids, and pentagonal rods and wires having {100} facets are generally favorable products.1,4 Consequently, an advantage of the twostep method used here is that the octahedral Ag nanocrystals r 2009 American Chemical Society

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Figure 1. Au@Ag core-shell nanocrystals produced in ethylene glycol under microwave heating (left side) and in DMF under oilbath heating (right side).

might be produced from cubes because of the duality between these two crystals, as found for Au@Ag crystals.6 The main purpose of this study is to develop a new two-step preparation method of Ag nanocrystals having {111} facets, which are generally difficult to synthesize using conventional one-step preparation methods. We demonstrate here that the Ag nanocrystals having {111} facets and definite sizes could be preferentially overgrown on the Ag crystal seeds having {100} facets using DMF under the condition of oil-bath heating. Recently, Tao et al.2c reported that self-organized Ag supercrystals including octahedra provided promising plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography. Therefore, the shape and size control syntheses of Ag polyhedral nanocrystals such as octahedra and triangular plates and the clarification of their growth mechanisms during the preparation process are very important. Experimental Section For this study, AgNO3 (99.8%), EG (99.5%), DMF (99.5%), and NaCl (99.5%) were purchased from Kishida Chemical Co. Ltd. In addition, PVP powders (average molecular weight Mw = 40 000 and =1 300 000 in terms of monomer units) were purchased, respectively, from Wako Pure Chemical Industries Ltd. and Sigma-Aldrich Corp. All these reagents were used without further purification. In our experiments, EG and DMF were used as the reductant and solvent. Preparation of the Ag nanocrystals was conducted in two separated steps. During the first step, Ag nanocrystal seeds were prepared using oil-bath heating in EG, similarly to the process reported previously.4e, In the process, 0.3 mM AgNO3, 0.3 mM NaCl, and 262 mM PVP (Mw = 40 000) were first dissolved in 19 mL of EG solvent. The resultant solution was heated from room temperature to 185 °C in an oil bath for 20 min and kept at 185 °C for 10 min. Subsequently, 1 mL of AgNO3 (0.93 M) was injected drop-by-drop into the solution at a flow rate of 1 mL/min and heated at 197 °C for 1 h. The final concentrations of AgNO3, NaCl, and PVP were, respectively, 47, 0.29, and 249 mM. After naturally cooling to room temperature, the Ag seeds were separated and obtained from EG/C2H5OH solution by centrifuging the colloidal solution at 6000 rpm for 8 min and 15 000 rpm for 30 min three times. Small amounts of Ag rods and wires involved in the solution of seeds were removed by the first short-time centrifugal separation at a slow rotational speed. At the second step, the Ag seeds were added to DMF solution (30 mL) containing 47 mM AgNO3 and 126 mM PVP (Mw =1 300 000). The solution was then heated in an oil bath at 140 °C for 3 h. Product solutions were

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Figure 2. Typical TEM images of (a) Ag seeds prepared in the AgNO3/PVP(Mw = 40 000)/EG solution and (b) Ag crystals prepared in the Ag seeds/AgNO3/PVP(Mw =1 300 000)/DMF solution under oil-bath heating for 3 h. (c)-(f) show crystal structures, TEM images, and SAED patterns of each crystal. centrifuged at 13 000 rpm for 60 min. The precipitates were collected and then redispersed in deionized water, respectively. For TEM (JEM-2010; JEOL) observation, samples were prepared by dropping colloidal solutions of the products on carbon-coated Cu grids. Absorption spectra of the product solutions were measured in the UV-visible (vis)-near infrared (NIR) region using a Shimadzu UV3600 spectrometer.

Results and Discussion Syntheses of Ag Seeds Having {100} Facets in EG. Figure 2a presents a typical TEM image of the Ag nanocrystals obtained by reducing AgNO3 in EG under the oil-bath heating. A mixture of cubes and right-triangular bipyramids was found, respectively, with their yields of 77 and 23%. Crystal structures of these Ag nanocrystals, shown in Figure 2c-1 and d-1, have been positively evidenced by selected area electron diffraction (SAED) patterns, as shown in Figure 2c-2 and d-2 and inclined-angle TEM observation, as reported previously.4c Square SAED patterns show that the incident electron beams are perpendicular to {100} facets, being consistent with the crystal structures shown in Figure 2c-1 and d-1. Syntheses of Ag Nanocrystals Having {111} Facets in DMF. Figure 2b presents typical TEM images of products obtained in DMF by the addition of AgNO3 into the Ag seed solution and heated for 3 h. The TEM images display a mixture of Ag nanocrystals of various shapes. Hexagonal, rhombic, and triangular platelike Ag nanocrystals are produced. To examine the shapes of the hexagonal particles, TEM images were observed from different angles (Figure 3). The hexagonal particles become rhombic or rectangular by changing view angles. These data imply that hexagonal, rhombic, and rectangular particles have the same structures and the hexagonal particles have no platelike structures. Figure 4 shows another TEM image, in which the side views of the triangular particles are visible. Expanded TEM images and their SAED patterns of hexagonal particles and triangular plates are shown in Figure 2e-1, f-1, e-2, and f-2. For hexagonal particles, typical hexagonal {220} reflections of FCC crystals were observed. In the case of a triangular plate, aside from similar hexagonal strong {220} reflections of FCC

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Figure 3. TEM images of octahedral Ag nanocrystals observed from different view angles. Particles circled with red lines are typical octahedral particles prepared in DMF under oil-bath heating for 3 h.

Figure 4. Particles encircled by red lines are typical TEM images of triangular twin plates prepared in DMF under oil-bath heating for 3 h. Red arrows indicate twin plates.

crystals, weak forbidden 1/3{422} reflections are found.6,8 These results imply that these two crystals have {111} facets and that the latter triangular plate has a twinned plane within the {111} planes along the perpendicular incident direction of the electron beam. On the basis of these data, it was concluded that hexagonal particles are octahedral particles, whereas triangular particles are triangular plates having a twin-plane in the center plate. In our experiments, we used EG and PVP (Mw = 40 000) for the preparation of {100} facets in the first step, whereas DMF and PVP (Mw = 1 300 000) were used for the preparation of {111} facets in the second step. We used different molecular weights of PVP in the second step. Therefore, not only DMF but also PVP (Mw = 1 300 000) might be responsible for the shape change of Ag nanocrystals. To examine the relative importance of DMF and PVP, we have prepared Ag nanocrystals using the DMF/PVP (Mw =40 000) system. Results show that favorable facets were {111} facets, indicating that DMF is a key factor that changes the favorable morphologies of Ag nanocrystals. Recently, Yang and co-workers2a succeeded in the preparation of silver cuboctahedron and octahedron in 1,5pentanediol using cubes as seeds. When metal atoms add to

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Figure 5. TEM images of Ag seeds and Ag nanocrystals prepared from Ag seeds/AgNO3/PVP/DMF solution under oil-bath heating for 1, 2, and 3 h.

the {100} faces of a nanocube, they migrate to the edges of the face, resulting in the elongation of the {111} facets in the presence of PVP. In their study, such a redox active metal salt as copper(II) chloride was added to the system. Actually, Cu(II) is known to be reduced to Cu0 in polyol solvent and Cu nanoparticles are produced.9 When Cu(II) and Agþ are reduced simultaneously in polyol, Cu/Ag alloys are produced.10 Consequently, the possibility exists that Cu is involved in the products when Cu(II) is added to polyol solvent. The present method requires no catalyst such as Cu. Therefore, an advantage of the present method is that there is no possibility of contamination with residual Cu, so that pure Ag octahedra can be prepared. Growth Mechanisms of Octahedra and Right-Triangular Bipyramids of Ag Nanocrystals. According to Wulff’s rule for favorable morphologies of metallic crystals, the possibility for the formation of the face centered cubic (FCC) nanocrystals surrounded by {111} and/or {100} facets with low surface energies becomes large in final thermodynamic equilibrium shapes.11 In principle, FCC crystals can possess various shapes covered by {111} and/or {100} facets because of possible dynamic routes during crystal nucleation and growth.12 Crystal facets having faster growth rates disappear and those having slower growth rates finally remain. The difference in growth rates along various crystal plane directions dominates possible shapes of the obtained product. Under the condition of fast oil-bath heating, where crystal shapes are determined not by thermodynamic equilibrium shapes but by dynamic ones, site-selective growth occurs because of face-selective adsorption of surfactant. Figure 2c-1 and d-1 show a noteworthy point: single-crystal cubic and single-twinned right bipyramidal Ag nanocrystals are surrounded solely by the most stable {100}-type facets. This indicates that the formation of the Ag crystals with {100} facets is more favorable because of the selective adsorption of PVP to {100}-type facets of Ag nanoparticles in EG.1h,4 To determine the growth mechanism of octahedral nanocrystals, we have observed TEM images and UV-vis-NIR spectra of Ag nanocrystals at different reaction times. Figure 5a-d shows Ag seeds and nanocrystals obtained in

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Figure 6. UV-vis-NIR absorption spectra of the Au core solution and Ag seeds prepared in EG (0 h) and Ag nanocrystals prepared from Ag seeds/AgNO3/PVP/DMF under oil-bath heating for 1, 2, and 3 h. Product solutions were diluted in EG by a factor of 10 before measurements.

DMF after heating for 1, 2, and 3 h. Although small changes in morphologies are found after heating for 1 h, great changes are observed between 1 and 2 h. Monodispersed octahedra involving a small amount of triangular plates were produced after heating for 3 h. No changes in morphologies were observed for octahedra and triangular plates after additional heating for 1 h (total heating time 4 h, not shown in Figure 5). Figure 6 shows UV-vis-NIR extinction spectra of the Ag seeds prepared in EG and the Ag nanocrystals obtained in DMF after heating for 1, 2, and 3 h. The extinction spectrum of the Ag seeds exhibits a broad surface plasmon resonance (SPR) band in the 320-1400 nm region with a peak at ∼430 nm. This main peak can be ascribed to SPR bands of cubes and a small amount of right bipyramids.1c,1e,1h After heating for 1 h in DMF, the peak at ∼410 nm band attributable to cubes and right bipyramids decreases and a broad tail band attributable to some intermediate structures of cubes and right bipyramids appears in the 500-1400 nm region. This broad SPR peak becomes strong with increasing reaction time from 1 to 3 h and a weak peak is observed at ∼700 nm. Octahedra with average edge lengths of 40 and 250-300 nm are known to give SPR peaks respectively at ∼480 and ∼800 nm,1h,2a which shows that the peak position shifts to red with increasing crystal size. The broad peak at ∼700 nm observed in this study is ascribed to the SPR band of octahedra, with an average edge length of ∼170 nm. The SPR bands of Ag triangular plates have been studied: peaks red-shift with increasing size.5,13 The peak position of the triangular plate, with an average length of ∼200 nm is expected to appear at ∼1000 nm. Consequently, the SPR bands of triangular plates are involved in broad tail bands around ∼1000 nm. Figure 7 depicts the growth mechanism of Ag nanocrystals from perfect cubes to octahedra. Cubic Ag seeds consist of six {100} facets. Overgrowth of Ag atoms on the cubic Ag crystals gives truncated octahedra having {100} and {111} facets (process 7a) and octahedra having only {111} facets are formed (process 7b). Typical TEM images of the cubes, truncated octahedra, and octahedra observed at 0, 2, and 3 h are shown to demonstrate the validity of this growth process. Additional growth, producing octahedra with a larger average size (process 7c), was not observed.

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Figure 7. Shape conversion from perfect cube to truncated-octahedron and octahedron and TEM images of Ag cube, truncatedoctahedron, and octahedron observed after heating in DMF for 0, 2, and 3 h.

Figure 8. Shape conversion from right bipyramid to truncated triangular plate, triangular plate and bitetrahedron and TEM images of Ag right bipyramid, truncated triangular plate, triangular plate observed after heating in DMF for 0, 2, and 3 h.

Figure 8 presents the possible evolution mechanism of the Ag nanocrystals from the right bipyramid to triangular plate together with typical TEM images of seed, intermediate, and product. Single-twinned right-triangular Ag seeds consist of three {100} planes on the top and bottom of the twin plane. Bihexagonal or bitruncated-triangular Ag consisting of two {111} top and bottom planes as well as six {111} and six alternative {100} side facets are produced initially (process 8a). Triangular bipyramids change to truncated triangle plates where areas of original {100} facets decrease, whereas six side {111} facets increase concomitantly with increasing heating time (process 8a). After 3 h heating, {100} facets disappear and triangular plates having six {111} facets grow (process 8b). Further growth to bitetrahedra (process 8c) and decahedra, which were observed for the synthesis of Ag nanostructures without using Ag seeds,8 could not be found. This suggests that crystal growth of the triangular plate stops when the original {100} facets disappear.

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Figure 9. Shape conversion via truncated cube to truncated-octahedron and octahedra and TEM images of Ag cube, truncatedoctahedron, and octahedron in DMF. A minor growth route via etching of projection of the cube is also shown.

To confirm crystal growth mechanisms more quantitatively, we compared average sizes of the Ag seeds and octahedral and triangular plates prepared at the second step. Figures 7c and 8c portray size changes from cubic and bipyramidal seeds to an octahedron and a triangular plate, as estimated by assuming ideal crystal growth. The edge length of an octahedron is larger than that of a cube by a factor of 2.12. On the other hand, the edge length of a triangular plate becomes twice fold of the right bipyramid, although we found no change in the height or width of the seed and product (0.82). We have measured average sizes of the seeds and products obtained after heating for 3 h. The average edge length of the octahedra (170 ( 8 nm) is larger than that of cubes (93 ( 11 nm) by a factor of 1.83. This indicates that the average size of octahedra is about 14% smaller than that expected. This is explainable by etching of each corner of the cubes (about 7%) during the crystal growth from cubes to octahedra. The Ag octahedron, whose anisotropic growth is thought to be induced by localized oxidative etching of corner {111} facets on a cubic seed, gives slightly smaller sizes of octahedron (Figure 7c). In fact, Xia and his co-workers reported that when Ag nanocubes with sharp corners are heated in EG containing a small amount of PVP, the characteristically sharp corners become truncated, generating nanocrystals with a rounded profile.1f,h When we heated cubes in DMF in the presence of PVP (Mw = 1 300 000, 126 mM) for 4 h at 147 °C, little shape change was observed. This indicates that morphology change attributable to thermal effects and uncovered PVP does not occur in our conditions and that the etching of the corners giving {111} facets takes place in the presence of AgNO3 in the second step. Figure 9 presents the crystal growth mechanism of an octahedron through truncated-cube and typical TEM images of related Ag nanocrystals. We found that the sharp corners of some cubes become truncated, in most cases generating nanocrystals with a slightly rounded profile (process 9a). Subsequently, {111} facets grow up over {100} facets. We found an exceptional case in which etching of a corner occurs after crystal growth of {111} facets (see the red circle in Figure 9b). In this case, a corner remained during growth of {111} facets and a small projection will be etched before the formation of a perfect octahedron. The average edge length of the triangular plates (390 ( 10 nm) is greater than that of right-triangular bipyramids

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(195 ( 6 nm) by a factor of 2.0. The width of the triangular plate (160 ( 5 nm) is equal to the average height of the bipyramid edge lengths (160 ( 6 nm). Therefore, the average size of the triangular plates agrees with that expected from bipyramids. Consequently, for bipyramids, the etching of corners is unimportant. As in the evolution of octahedra, the crystal growth and the etching on the corners compete in the formation of a triangular plate from a triangular bipyramid. Twin planes are known to serve active sites for crystal growth1h,5 therefore, the growth rate of {111} facets along the twin plane is expected to be much faster than the etching rate. This is a main reason why etching is not observed in the formation of single-twin triangular plates. We found here that octahedra and triangular Ag plates are produced, respectively, from cubic and right bipyramidal Ag seeds. However, opposite crystal growth was observed for the formation of Au@Ag crystals in EG, where cubes and triangular bipyramids are produced, respectively, from octahedral and triangular platelike seeds, (see left side in Figure 1). Large cubic and triangular bipyramidal Au@Ag nanocrystals were produced after {111} facets completely disappear by the addition of sufficient amount of Agþ ions at the second step.6 On the other hand, results of this study show that that the crystal growth stops immediately after {100} facets disappear and further crystal growth leading to larger crystals through processes 7c and 8c in Figures 7 and 8 is not observed. In our previous experiments for Au@Ag crystals in EG, growth rates of Ag shells are much faster than those in DMF so that larger Ag shells are formed under MW heating for only 3 min.6 However, it took about 3 h for the full overgrowth of new Ag layers in DMF. In our recent detailed experiments for Au@Ag nanocrystals in DMF using octahedral and triangular platelike Au cores, quasispherical cuboctahedra and hexagonal triangular plates having both {111} and {100} facets are formed initially; {100} facets then disappear during further crystal growth (see the Supporting Information, Figure S1).14 For the formation of such intermediates, some energy barriers probably exist. We found that further growth of larger octahedra and triangular plates was not observed after the formation of octahedra and triangular plates. This implies that the formation of such intermediates as quasi-spherical cuboctahedra and hexagonal triangular plates is unfavorable in DMF for pure Ag nanocrystals, although it can be created in the formation of Au@Ag nanocrystals in DMF. Good shape correlation exists between the seeds and products in DMF. Moreover, monodispersed Ag nanocrystals are produced using monodispersed Ag seeds because the crystal growth stops at definite stages. Therefore, the sizecontrolled syntheses of the Ag octahedra and triangular plates are possible if we use monodispersed Ag cubes and right bipyramids as seeds (e.g., preparation of monodispersed octahedra in Figure 5d). Consequently, the present two-step synthesis method, which is executed by changing the solvent and reductant, is a promising technique for the shape-controlled and size-controlled syntheses of the Ag octahedra and triangular plates surrounded solely by {111} facets. Conclusion We studied shape conversion from Ag cubes and righttriangular bipyramids having {100} facets to octahedra and triangular single-twin plates having {111} facets using a two-

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step process with EG and DMF as the reductant and solvent. After the Ag cube and bipyramid seeds were prepared in EG, octahedra and triangular plates were synthesized in DMF. The average size of the octahedra is smaller than that expected from the perfect conversion of cubes to octahedra by about 14% because of etching of corners of the cubes during crystal growth. On the other hand, the average size of the triangular single-twin plates is close to that expected from conversion of right-trianglar bipyramids to triangular plates. No further crystal growth to larger crystals occurs. Therefore, this technique is a new promising technique for preparation of a shapecontrolled and size-controlled synthesis method of octahedra and triangular platelike particles of silver, which are generally difficult to prepared using other methods. Acknowledgment. This work was supported by a Joint Project of Chemical Synthesis Core Research Institutions, Grant-in-Aid for Scientific Research on Priority Areas “unequilibrium electromagnetic heating” and Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (19033003 and 19310064), and the Kyushu Univ. GCOE program “Novel Carbon Resource Sciences.” We thank IMCE of Kyushu University for financial support to Prof. P. Jiang as a visiting professor during 2007.4 - 2007.6.

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Supporting Information Available: Growth mechanisms of Au@Ag nanocrystals in DMF and TEM images of their intermediate nanocrystals (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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