Surface Segregation and Structural Features of Bimetallic Au−Pt

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Surface Segregation and Structural Features of Bimetallic Au-Pt Nanoparticles Lei Deng, Wangyu Hu,* Huiqiu Deng,* and Shifang Xiao Department of Applied Physics, Hunan UniVersity, Changsha 410082, China ReceiVed: January 8, 2010; ReVised Manuscript ReceiVed: May 6, 2010

Monte Carlo simulations were carried out to systematically investigate the effects of composition, size, and temperature on the surface segregation and structural features of Au-Pt nanoparticles in the present paper. A strong surface Au enrichment was observed in all of the nanoparticles, and the surface segregation of Au was promoted by increasing the particle sizes. It is found that the core-shell structure was preferred in the equilibrium Au-Pt nanoparticles with low Au composition, and three-shell onion-like structure was formed at high Au composition. The competitive multisite segregation was predicted in the core-shell nanoparticles in which Au atoms favor sites at the vertices, edges, and facets. The reverse temperature dependency of segregation for different surface sites has also been discussed. 1. Introduction Bimetallic nanoparticles (NPs) generally exhibit unusual physicochemical properties different from those of the bulk material or their individual constituents and have a number of fascinating potential applications in areas such as catalysis, fuel cells, and hydrogen storage.1,2 The unique physical and chemical properties of bimetallic NPs are determined by their sizes, shape, compositions, and surface structures.3,4 As one of the most promising bimetallic systems, the Au-Pt NPs have been extensively studied for a variety of catalytic applications, such as methanol oxidation and electrocatalytic reduction of oxygen.5-9 Because the atomic-scale surface structures and composition distribution of binary catalyst NPs are important factors controlling their chemical reactivity, especially catalytic activity and selectivity, it is a topic of particular interest to investigate the surface structures and compositions of Au-Pt catalyst NPs. Recently, Au-Pt NPs with different compositions, sizes, structures, and morphologies have been prepared by controlling synthesis conditions.5-9 Low-energy ion scattering (LEIS) experiments were carried out to determine the surface composition of bimetallic Au-Pt clusters on TiO2 (110).10 The similar masses of Au and Pt will result in only a small energy separation between their peaks in the LEIS spectra; thus, it is still very difficult to characterize the Au-Pt NP surfaces in detail using LEIS technique. Alternatively, atomic-scale simulations can provide detailed information and much insight into the surface chemistry of bimetallic NPs. Up to date, on the theoretical study of Au-Pt NPs, more attention has been paid to the thermodynamic stability, melting mechanisms, and evolution of the microstructure.11-21 Our previous studies have shown that the heat of formation for the Au-Pt alloy NPs is size-dependent.17 Furthermore, with increasing temperature, the mixed Au-Pt NPs tended to decompose into a core-shell structure with Au segregating to the surface, which has been extensively observed from experiments and simulations in clean, free, or supported Au-Pt NPs.11,13,14,16-21 However, the details of surface segregation and structural characteristics of NPs, especially the effects of size, composition, and temperature, are not completely * To whom correspondence should be addressed. Tel/Fax: +86-73188823971. E-mail: [email protected] (W.Y.H.); hqdeng@ gmail.com (H.Q.D.).

Figure 1. Variation of the dispersion (size) as a function of the size of perfect TO nanoparticles. The solid circles represent the nanoparticles with a magic number. The inset shows a typical 586 atom TO nanoparticle.

understood. In this paper, we have studied the surface segregation and structural features of Au-Pt bimetallic NPs using Monte Carlo (MC) simulations with the modified analytic embedded atom method (MAEAM) potentials. 2. Computational Methods In contrast to metal NPs composed of only one type of atom, the alloy NPs exhibit more complicated structures and some special physical and chemical properties as a result of an alloying effect and surface segregation.22,23 It is well-known that the metallic NPs present competitive structural motifs, such as the icosahedron, decahedron, cuboctahedron, and truncated octahedron (TO).3,4 A typically TO NP model is shown in Figure 1, in which there are 6 {100} facets, 8 {111} facets, 12 {111}/ {111} edges, 24 {111}/{100} edges, and 24 vertices. The TO morphologies are believed to be privileged for fcc NPs.24-26 Hence, we considered a series of TO NPs with “magic numbers” of atoms, for which atomic shells are complete.24-26 The morphology and the relationship between the size, atom number, and dispersion (the ratio of the number of surface atoms to the number of all atoms) of Au-Pt TO NPs are also shown in

10.1021/jp100194p  2010 American Chemical Society Published on Web 06/03/2010

Bimetallic Au-Pt Nanoparticles

Figure 2. Heat of formation of a Au-Pt bulk system as a function of Au concentration. The lines are the results of a disordered solid solution. The full signatures present the results of three possible intermetallics; the open signatures denote alloys at high temperature.33-39

Figure 1. It is seen that the larger the NPs, the smaller their dispersions. The sizes of TO NPs considered in the present paper are from about 2.5 to 6.5 nm, which corresponds to a range of a total number of atoms from 586 to 9201, and the dispersion is from about 46 to 21%. The equilibrium configurations of AuxPt1-x NPs (x ) 0.125, 0.250, 0.375, 0.500, 0.625, 0.750, 0.875) are obtained over the whole temperature range where they are in the solid state. The equilibrium configurations of NPs were predicted by minimizing their total energies using a MC simulation method based on the Metropolis algorithm.27,28 The MC method is particularly advantageous in studying the surface segregation of alloys as it can circumvent slow dynamic processes (such as diffusion) and provide averaged quantities of interest over a thermodynamic equilibrium ensemble. The MC simulation code is developed from ref 28, and the canonial (or NVT) ensemble was used here, with which not only the number of total atoms but also that of each element in alloy is kept constant.25,26 A starting TO configuration is generated in a random way according to the nominal composition of bulk alloy. The sampling schemes include random displacement and random site exchanges of atoms which simulate both the shape relaxation and surface segregation of NPs. The required quantities, such as the concentration distribution of chemical species, are obtained by averaging over 20 000 Monte Carlo steps per atom after the system achieves an equilibration state. The statistical uncertainty is typically close to 5%. In the present simulation, the MAEAM potentials17,29-32 are employed to describe the atomic interactions. These potentials provide good description for many-body interactions and have already been successfully applied for the study of bulk, surface, and cluster/NP systems. The MAEAM potential functions and these parameters are listed in ref 17, which were determined by fitting to the physical attributes such as lattice parameters, cohesive energy, vacancy formation energy, and elastic constants of Au, Pt, and Au-Pt intermetallic compounds. The heat of formation was computed as that in ref 17. The lattice parameters and elastic constants of binary alloys have also been tested with the fitted alloying parameters. The heats of formation for a disordered solid solution and three possible intermetallics AuPt3 (L12), AuPt (L10), and Au3Pt (L12) are shown in Figure 2. It is clear that the heats of formation for intermetallics are more positive than those for the corresponding disordered solid solution, which implies that the chemical order between Au and Pt is very weak. The present results of disordered solid solution

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Figure 3. Variation of surface Au composition as a function of the dispersion (size) of AuxPt1-x nanoparticles. The full signatures are the present results at 300 K; the open signatures present the results in ref 21 at 550 K.

and intermetallics are in reasonable agreement with firstprinciples calculations and experimental data.33-39 Therefore, the present Au-Pt alloying potential can be used for a wide range of components with reasonable accuracy. 3. Results and Discussion 3.1. Surface Segregation and Structural Features. At a given temperature, surface segregation can be determined by both the composition and size of alloy NPs. Figure 3 shows the effects of bulk compositions and NP sizes on the surface Au concentration in AuxPt1-x NPs (x ) 0.125, 0.250, 0.375, and 0.500) obtained with MC simulation at 300 K. The strong enrichment of Au in the topmost surface is observed in all of the considered Au-Pt NPs with the same size but different bulk Au compositions. The Pt segregation in the subsurface is also observed but not shown here. It is clear that the more the bulk Au composition, the more the surface Au segregation. Especially, the surface Au concentrations are almost 100% when the bulk Au concentration is greater than a certain value, which will form the core-shell structures and be discussed in the following. In the Au-Pt bimetallic system, Au atoms not only have larger sizes but also lower surface energies than those of Pt atoms. The segregating of Au atoms from the bulk to the surface will decrease the atomic mismatch elastic energy, and the internal stress can be released in this way. Furthermore, the Au atoms enriched in the surface are favorable for reducing the surface energy of the NPs. That is to say, the strong enrichment of Au atoms in NP surface is expected, which is also confirmed by our MC simulation. Here, six particle sizes are considered for Au-Pt NPs. For the Au surface concentration (Cs) of different NP sizes, the effect of NP sizes is very strong. Take the Au25Pt75 particles as an example, the Au Cs is about 57 at % for the particle of 2.46 nm with 586 atoms, and these are almost 100 at % for the particles of 5.74 (6266 atoms) and 6.56 nm (9201 atoms). It is clear that the Au Cs increases with the increasing NP sizes. The supported Au-Pt bimetallic NP catalysts were modeled with a coordination-dependent potential model,21 and those results are also shown in Figure 3, which are in good agreement with our results. The strong segregation of Au to the surface was confirmed by atom probe field ion microscopy studies of a bulk Au-Pt alloy containing only 4% Au, where the first surface monolayer was 99% Au.40 As it is known that the surface energy is one of the

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Figure 4. Whole configurations and cross section snapshots of 586 atom AuxPt1-x nanoparticles at T ) 300 K: (a) x ) 0.125, (b) x ) 0.250, (c) x ) 0.375, (d) x ) 0.460, (e) x ) 0.500, (f) x ) 0.625, (g) x ) 0.750, (h) x ) 0.825. Au is in yellow (light), and Pt in blue (dark).

Figure 5. Au composition in the shells of 586 atom AuxPt1-x nanoparticles at T ) 300 K. The inset shows different shells in nanoparticles; shell one denotes the surface, shell two is the subsurface, and so on.

driven forces for surface segregation, it is usually size-dependent for NPs. Lu and Jiang41 found that the surface energy ratio between different facets is size-independent and is equal to the corresponding bulk ratio. For the Au-Pt NPs, the strong surface segregation is clearly size-dependent even with the same Au content; thus, it is not only dominated by the size-dependent surface energy but also the lower dispersion of the bigger NPs than that of the smaller ones.21 The structural features of Au-Pt NPs are different from those of the ideal bulk alloys because of the Au surface segregation, and they are also governed by the sizes and compositions at a given temperature. As an example, the structural features of 586 atom AuxPt1-x NPs at 300 K are shown in Figure 4, in which different AuxPt1-x NPs were considered with x ) 0.125, 0.250, 0.375, 0.460, 0.500, 0.625, 0.750, and 0.825. There are different coordination numbers for the atoms located in the surface and inside of the fcc TO NPs, which is used to distinguish the surface and the internal shells of TO NPs in the present paper. For each configuration, the corresponding composition distribution of Au in the shells of the TO NP is shown in Figure 5. For the strong tendency of surface segregation, the Au atom will segregate to the surface if a surface site is available. It is seen clear in Figures 4a-c and 5 that a core-shell structure was formed with a pure core of Pt and a mixed shell when x was less than 0.46; at this time, the amount of Au is not sufficient to form a full monatomic overlayer. It is found that

Deng et al. Au atoms occupied the sites in the vertices and edges first and then the (100) and (111) facets. This is namely the competitive multisite segregation and will be discussed in detail in the following section. The structure and segregation or mixing in NPs may depend on the method and conditions of particle generation.3-8 The core-shell structure with Au segregation to the surface of clean, free, or supported Au-Pt NPs has been extensively observed.11,13,14,16-21 Using the density functional theory (DFT) approach, Wang and Johnson recently found that a Au shell and Pt core is preferred with a negative segregation energy (-1.77 eV) for the 55 atom Au-Pt alloy nanoparticle.42 From the thermodynamic point of view, the Pt core/Au shell structures are the most stable, where the surface energy and mixing energy are both reduced by Au segregating. Our previous simulations showed that the artificially mixed Au-Pt NPs tended to decompose into Pt core/Au shell structures with increasing temperature.17 In addition, the bimetallic clusters transformed to the most stable Pt core/Au shell structure from whatever initial structures upon heating above certain temperatures.13 Such Pt core/Au shell NPs would be sinter-resistant but also retain the catalytic activity as the pure metal Au NPs10 and have enhanced specific activity in methanol electrooxidation as compared to that with metal Pt NPs.9 A perfect core-shell structure shown in Figure 4d was obtained at x ) 0.46, where all of the Au atoms segregated to surface and all of the Pt atoms occupied the core region so that the surface energy and mixing energy were both minimized. The NP surface was covered completely with Au atoms when x > 0.46. An interesting phenomenon was observed, as shown in Figures 4e-h and 5, where Au atoms agglomerated to form a small core in the center of the NPs; a three-shell onion-like structure appeared with a surface Au layer, Pt shell, and Au core. The more the Au content in Au-Pt NPs, the larger the Au core sizes. At the same time, the Pt atoms favored the subsurface layer and had a homogeneous layer-like distribution rather than a compact aggregation, as shown in Figure 4h. As far as we know, such a three-shell onion-like structure has not been predicted in Au-Pt NPs before. The structures of 1289, 2406, 4033, 6266, and 9201 atom NPs are also similar to that of the 586 atom one. In the case of immiscible systems, such as Ag-Co and Ag-Cu clusters, the core-shell structures are privileged when all of the surfaces have been covered with a segregating element, and the thickness of the homogeneous surface layer increases with its overall composition.26 The onionlike structure at the thermodynamic equilibrium state was found in a set of mixing bimetallic clusters, such as in icosahedral clusters formed by Cu and Au, Pd and Pt, and Au and Ag.43-45 It was also found in Cu-Co NPs among immiscible systems.26 The atomic configurations of segregated Au-Pt NPs result from the balance between surface energy, lattice distortion, and binding energy. The total energy will be lowered by segregating Au to the surface in Au-Pt NPs. The calculated heats of formation for the segregated AuxPt1-x NPs (obtained with the present MC simulations) as well as the random NPs (disordered solid solutions) are shown in Figure 6. Naturally, as a result of the mixing effect, the heats of formation for both random and segregated NPs show compositional dependence as in bulk materials (shown in Figure 2). Compared to the bulk Au-Pt alloy systems, the heats of formation of random NPs are less positive due to the random mixing of Au and Pt atoms and size effects. However, for segregated NPs, one of the most prominent characteristics is that the heat of formation is negative as Au segregates, which means a better thermodynamic stability compared to random ones; therefore, the onion-like structures

Bimetallic Au-Pt Nanoparticles

Figure 6. Variation of heats of formation for random and segregated AuxPt1-x nanoparticles along with Au concentration. The lines with solid or open symbols present segregated or random configurations, respectively.

Figure 7. Variation of the surface and core Au composition as a function of temperature of Au0.25Pt0.75 nanoparticles. The lines with solid and open symbols are the results of surface composition and core composition, respectively.

for the segregated Au-Pt NPs are more favorable energetically. Another characteristic of the heat of formation is its strong dependence on both the sizes and nominal compositions compared with these of random ones. At a fixed Au atomic concentration, the heat of formation decreases with the decreasing NP size. Furthermore, it is more negative in the Pt-rich range than that in the Au-rich range. That is to say that the size and composition ranges of the thermodynamically stable region are extended widely by surface Au segregation. The temperature also has important influence on the surface segregation of NPs. Figure 7 shows the effect of temperature on the surface and core Au composition in Au0.25Pt0.75 NPs with different sizes. The obvious difference between surface and core Au compositions implies that Au is enriched in the surface even at high temperature. The surface composition of Au decreases by nearly 15% from 600 to 1400 K for 4033, 6266, and 9201 atom NPs. For smaller NPs, the segregation tendency was less affected by an increase of temperature, such as 586 and 1286 atom NPs; thus, strong Au segregation remained under temperatures below their melting points. The different temperature dependency of the segregation tendency between small and large NPs has also been found in Au-Pd systems.46 The spatial organization also depends on temperature. The Au-Pt NPs favored the core-shell or onion-like arrangements at lower

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Figure 8. Variation of the fractional composition of Au at different surface sites as a function of overall Au composition for 586 atom nanoparticles. The inset shows the different coordination numbers of the vertex, edge, (100) facet, and (111) facet sites.

temperature, and these arrangements were degraded at higher temperatures. In addition, the NPs are more close to a ball-like surface due to larger inward relaxations at the vertices and edges compared to those at the rest of the surface. As a result, the different surface sites (facet, edge, and vertex) are less distinguishable in the relaxed NPs. 3.2. Competitive Site Segregation. As mentioned above, AuxPt1-x NPs with increasing Au concentration exhibit competitive multisite segregation, which was qualitatively revealed in Figure 4. The amount of Au is not sufficient to form a full monatomic overlayer when x < 0.46; thus, Au atoms tend to populate lower coordinated sites. Figure 8 gives the quantitative description of this competition in a 586 atom Au-Pt NP at 300 K. The inset shows the different coordination number of those sites at the vertex (6 coordination), edge (7 coordination), (100) facet (8 coordination) and (111) facet (9 coordination). The coordination number is 12 for a perfect fcc bulk Au-Pt alloy. Starting with a Au content of x ) 0.125, the six-coordinated sites on the vertices were completely occupied with Au atoms, about 57% of the seven-coordinated sites on the edges were occupied by Au atoms, and 17% of the eight-coordinated sites in the (100) facet were occupied with Au atoms. However, no Au atoms appeared at the nine-coordinated sites in the (111) facet. At higher Au contents, the sites at the vertices, edges, and (100) facets were all completely occupied. Note that the nine-coordinated sites on the (111) facets continued to retain some Pt atoms when x < 0.46. The cohesive energies of the atoms within NPs and small clusters are also site- and structure-dependent.47 With the present MAEAM potentials, we obtained the cohesive energies (Ecsx) of Pt and Au atoms at different coordination surface sites (Zsx) in the Au and Pt nanoparticles. The relation of Ecsx and Zsx in the 586 atom Au and Pt NPs is shown in Figure 9. It is clear that the Ecsx decrease with the increasing of Zsx for both Au and Pt NPs, and the Ecsx of Au at different surface sites is always bigger than that of Pt in the NP surface. Recently, Liu et al.47 calculated the cohesive energies within small clusters using the theoretical model

Ecsx(N) ≈ [Zsx(N)/Z(∞)]1/2Ec(∞)

(1)

where Ecsx(N), Zxs(N), Ecsx(∞), and Zxs(∞) are the cohesive energies and coordination numbers of atoms at the surface of

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Figure 9. Ecsx(N) as a function of Zsx(N) for 586 atom Au and Pt nanoparticles. The solid squares and circles are the cohesive energies for Au and Pt at different coordinated surface sites, respectively. The solid line is determined with eq 1.

TABLE 1: Configurational Energy Difference (∆E, in eV/atom) for a Bulk Au Atom Exchanging with Surface Pt Atomsa configuration 6 f 12 7 f 12 8 f 12 9 f 12

∆E -0.530 -0.427 -0.311 -0.259

a The number 6, 7, 8, 9, and 12 present the sites at the vertex, edge, (100) facet, (111) facet, and bulk, respectively.

N-atom particles or in the bulk (infinite atoms), respectively. On the basis of eq 1, the cohesive energies of Au and Pt atoms at different surface sites are also shown in Figure 9 with solid lines, which indicate a similar relation compared with our MAEAM’s results. It can be found that cohesive energies are quite distinct at the surface sites with different coordination, which means that Au atoms will prefer the lower coordinated sites in the surface. These site-dependent cohesive energies lead to the competitive site segregation of Au-Pt NPs shown above. The competive site segregation phenomena can be further understood by the difference of configurational energies for the two components in the Au-Pt alloys. The difference (∆E) of configurational energies is defined as a bulk Au atom exchanging with a surface Pt atom21 Au ∆E ) (Ecsx (N) + EciPt(N)) - (EciAu(N) + EPt csx(N))

(2)

Au Pt Pt where Ecsx (N), EAu ci (N), Ecsx(N), and Eci (N) are the cohesive energies of Au or Pt atoms at different surface sites and interior ones in an N-atom particle, respectively. The resulting configurational energy difference ∆E is listed in Table 1. In each case, ∆E is negative, which means that Au atoms prefer to occupy the surface sites. Note also that the absolute value of ∆E decreases as the coordination number increases, which will lead to Au atoms favoring to the lower coordinated sites. In other words, Au atoms will favor those sites at vertices, edges, (100) facets, and (111) facets. The temperature has important influence on the competitive multisite segregation in the NPs with different sizes. The preferential filling of the different lower coordinated surface sites by the Au atoms in Au0.25Pt0.75 NPs at different temperatures is shown in Figure 10. The fractional compositions of Au

Figure 10. Variation of fractional composition of Au at total surface sites and low coordinated sites as a function of temperature: (a) 9201 atom, (b) 1289 atom, and (c) 586 atom nanoparticles.

show that the entropic or temperature effect is clear for different sites, and the change of Au composition at low coordinated sites in (111) facets and other surfaces is quite different. For large NPs, such as the 9201 atom one shown in Figure 10a, the surface Au contents decease quickly as the temperature increases. For small NPs with 586 and 1289 atoms, as shown in Figure 10b and c, it is interesting that the fractional composition of Au on the (111) facets increases as the temperature increases, which is opposite to that shown in Figure 10a. This trend is what one would expect in looking at the configurational energy difference ∆E listed in Tables 1 and 2 and the size-dependent rate of different low coordinated sites to the total surface sites shown in Figure 11. For small NPs, there are no Au atoms in the core,

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TABLE 2: Configurational Energy Difference (∆E, in eV/atom) for a Surface Au Atom Exchanging with Surface Pt Atomsa configuration 6f7 6f8 6f9 7f8 7f9 8f9

∆E 0.103 0.219 0.271 0.116 0.168 0.052

a The number 6, 7, 8, and 9 present the sites at the vertex, edge, (100) facet, and (111) facet, respectively.

Figure 11. Variation of the ratio of low coordinated sites (N) to total surface sites (NS) as a function of particle sizes.

as shown in Figure 7; thus, the evolution of the fractional composition of Au with temperature is determined by the exchanging of surface Au atoms with surface Pt atoms. As listed in Table 2, the ∆E is 0.052 eV/atom for the exchange of a (100) facet Au atom with a (111) facet Pt atom, which is the most energetically favorable. Furthermore, the exchange of an edge Au atom with a (100) Pt atom and that of a vertex Au atom with an edge Pt atom are also energetically favorable. It is seen in Figure 11 that the percentage of (111) facet sites of 586 atom NPs is about 56%; therefore, other low coordinated sites can supply enough Au atoms to maintain or increase the fractional composition of Au on (111) facets as the temperature increases. As a result, the tendency of segregation in 586 atom NPs is only marginally affected by an increase in temperature. For the larger NPs, there are some Au atoms in the core region, as show in Figure 7; therefore, the evolution of the fractional composition of Au with temperature is not only affected by the exchange between surface atoms but also that with bulk ones. It is clear in Figure 11 that the percentage of (111) facet sites of 9201 atom NPs is about 70%; therefore, other low coordinated sites cannot supply enough Au atoms to maintain the fractional composition of Au on (111) facets as the temperature increases. As list in Table 1, the ∆E is 0.259 eV/atom for the exchange of a bulk Pt atom with a (111) facet Au, and it is the most energetically favorable. Thus, surface Au contents of 9201 atom NPs decrease quickly as the temperature increases, as in a conventional bulk alloy. However, the fractional composition of Au for different lower coordinated sites gets close to a certain value (surface segregation of Au is still obvious even at the liquid state17) as the temperature increases due to an entropic effect. Further work is being prepared for the entropy effect on the surface segregation and will be published later.

4. Conclusions Using Monte Carlo simulations, the effects of composition, size, and temperature on the surface segregation and structural features of TO Au-Pt NPs have been studied systematically with semiempirical EAM potentials. The sizes of Au-Pt NPs used here range from 2.5 to 6.5 nm, which contain 586-9201 Au atoms, respectively. On the basis of the above discussion, it can be concluded that (1) a strong topmost surface enrichment of Au atoms and subsurface enrichment of Pt atoms are observed in all the NPs considered, regardless of the NP sizes, compositions, and temperatures. It is found that increasing the NP sizes or bulk Au concentration will lead to higher surface Au concentrations. (2) The structures of equilibrium NPs are dependent on the average Au content of Au-Pt NPs. The core-shell structure was privileged in the equilibrium Au-Pt nanoparticles with low Au composition, and a three-shell onionlike structure was formed at high Au composition. (3) There exists a competitive multisite segregation in the surface of Au-Pt NPs. Au atoms favor the sites at vertices, edges, and facets for the cohesive energies and configurational energies of exchanging Au and Pt atoms are not the same at those sites with different coordination numbers. (4) The reverse temperature dependency of segregation for different surface sites has been observed for different Au-Pt NPs. It is found that the surface Au content deceases rapidly as the temperature increases, especially in the (100) and (111) facets for large NPs. However, the Au content increases in the (111) facets for small NPs because of a different driving force, namely, a significant population of low-coordinated Au atoms and a small energetic penalty for their exchange with Pt atoms in the (111) facets. These results would help to understand the properties and applications for alloy nanoparticles. Acknowledgment. This work is financially supported by the National Nature Science Foundation of China (Nos. NSFC 50871038 and 50801024, NSFC-NSAF 10976009) and the High Performance Computing Center of Hunan University. References and Notes (1) Wu, M. L.; Lai, L. B. Colloids Surf., A 2004, 244, 149. (2) Natan, M. J.; Lyon, L. A. In Metal Nanoparticles-Synthesis, Characterization, and Applications; Feldheim, D. L.; Foss, Jr. C. A., Ed.; Marcel Dekker: New York, 2002; Chapter 8. (3) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. ReV. 2008, 108, 845. (4) Schmid, G. In Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany, 1999; Vol. 3, p 1325. (5) Luo, J.; Njoki, P. N.; Lin, Y.; Mott, D.; Wang, L. Y.; Zhong, C. J. Langmuir 2006, 22, 2892. (6) Luo, J.; Maye, M. M.; Kariuki, N. N.; Wang, L. Y.; Njoki, P.; Lin, Y.; Schadt, M.; Naslund, H. R.; Zhong, C. J. Catal. Today 2005, 99, 291. (7) Luo, J.; Njoki, P. N.; Lin, Y.; Wang, L. Y.; Zhong, C. J. Electrochem. Commun. 2006, 8, 581. (8) Maye, M. M.; Kariuki, N. N.; Luo, J.; Han, L.; Njoki, P.; Wang, L. Y.; Lin, Y.; Naslund, H. R.; Zhong, C. J. Gold Bull. 2004, 37, 217. (9) Zeng, J.; Yang, J.; Lee, J. Y.; Zhou, W. J. Phys. Chem. B 2006, 110, 24606. (10) Park, J. B.; Conner, S. F.; Chen, D. A. J. Phys. Chem. C 2008, 112, 5490. (11) Yang, Z.; Yang, X. N.; Xu, Z. J. J. Phys. Chem. C 2008, 112, 4937. (12) Yang, Z.; Yang, X. N.; Xu, Z. J.; Liu, S. Y. Phys. Chem. Chem. Phys. 2009, 11, 6249. (13) Liu, H. B.; Pal, U.; Ascencio, J. A. J. Phys. Chem. C 2008, 112, 19173. (14) Reyes-Nava, J. A.; Rodrı´guez-Lo´pez, J. L.; Pal, U. Phys. ReV. B 2009, 80, 161412. (15) Jose´-Yacama´n, M.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J. J. Vac. Sci. Tehcnol., B 2001, 19, 1091.

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