AuâAg Bimetallic Nanoparticles: Surface Segregation and Atomic

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AuAg Bimetallic Nanoparticles: Surface Segregation and Atomic-Scale Structure Lei Deng,† Wangyu Hu,*,† Huiqiu Deng,*,† Shifang Xiao,† and Jianfeng Tang†,‡ † ‡

Department of Applied Physics, Hunan University, Changsha 410082, China Department of Applied Physics, Hunan Agricultural University, Changsha 410128, China ABSTRACT: Monte Carlo simulations were performed to study systematically the surface segregation behaviors and atomic-scale structural features of AuAg nanoparticles for a range of alloy compositions, particle sizes, and temperatures. Segregation of Ag to the surface was observed in all the particles considered. The surface segregation was promoted by increasing the particle sizes or Ag compositions and decreasing nanoparticles’ temperatures. It was found that the most stable mixing patterns are the onionlike structure with Ag-rich shell for small particles, and the alloyed-core/layered-shell structure for large particles. Accordingly, the calculated alloying extents based on Monte Carlo simulations are consistent with experimental EXAFS analysis, which indicates more obvious alloying features in nanoparticles with larger sizes or at higher temperatures, and more obvious segregated features in nanoparticles under the opposite conditions. The size distribution of Au ensembles on different coordinated sites was analyzed quantitatively, which presented varied composition- and temperature-dependent effects. The possible effects of size and shape distribution of surface ensembles on tuning the catalytic activity and selectivity of bimetallic nanoparticles were also discussed.

1. INTRODUCTION Bimetallic nanoparticles (NPs) have received enormous attention not only because of their chemical stability but also due to their unique optical,14 electronic,5,6 and catalytic716 properties distinct from those of the corresponding bulk materials and the monometallic NPs. Bimetallic AuAg NP is one of the most studied bimetallic systems.114,1725 Its unique physicochemical properties depend on the shape and structure of the particles, the surface segregation of the particles, and the alloying extent or atomic distribution in NPs.714,2628 Recently, alloyed (maximally mixed) or coreshell (minimally mixed) AuAg NPs have been obtained using different synthesis methods.1725 While a given NP configuration may exhibit a desired property, it is important to know the most stable structure for designing robust NP catalysts. Recent researches have also revealed that AuAg bimetallic NPs are highly active for low-temperature CO oxidation, and the catalytic activity can be tuned by the variation of Au/Ag molar ratios.713 A plausible explanation for this high activity of AuAg NPs was proposed, where a synergistic effect exists between specific Au and Ag atoms on the surface.710,13 The role of Au and Ag atoms on different coordinated sites that represent the most interesting morphological elements of NPs with respect to catalytic activity and selectivity, has not been completely understood yet. Therefore, a detailed atomistic understanding of surface morphologies and atomic structures of AuAg NPs for a range of sizes, compositions, and temperatures would facilitate a better understanding of bimetallic NP properties and their application in catalysis. r 2011 American Chemical Society

Experimentally, it is still a challenge to glean the structural and compositional information for small NPs. X-ray absorption spectroscopy and transmission electron microscopy have been used extensively in the characterization of NP catalysts. The former has a higher energy resolution; however, its achieved spatial resolution is several orders of magnitude lower than that of the latter. For AuAg NPs, due to the minor difference in the lattice constants of Ag and Au, direct lattice imaging using highresolution transmission electron microscopy is not very informative. It is thus not surprising that the experimental reports on the detailed structure features and quantitative surface enrichment in NPs are rare and scarcely found in the specific case of NPs such as the AuAg system.4 On the other hand, the computational approach has recently evolved to a powerful method to provide insight into the surface chemistry of bimetallic NPs.29 There are usually over several hundred atoms in a practical bimetallic NP catalyst and the variety of structures in bimetallic NP is much richer than that in a monometallic NP; thus, accurate first-principles methods are much computational intensive and impractical for simulating surface segregation phenomena. Alternatively, the semiempirical potentials, rendering a good description of many-body atomic interactions in metals, have been successfully applied in studying the energetic, structural properties and melting or alloying behaviors of AgAu Received: January 21, 2011 Revised: May 8, 2011 Published: May 20, 2011 11355

dx.doi.org/10.1021/jp200642d | J. Phys. Chem. C 2011, 115, 11355–11363

The Journal of Physical Chemistry C clusters/NPs.3038 The kinetic formation and alloying processes of AuAg NPs have been reported in detail.34 It is found that the particle achieves its ideal final form with a faceted shape to reduce the surface energy and is composed of a AgAu mixed core covered by a thin (one monolayer) shell enriched with Ag. However, the details of surface segregation and atomic-scale structural characteristics of AuAg NPs for a range of sizes, compositions, and temperatures are still awaiting systematic investigation. In the present paper, Monte Carlo (MC) simulation were carried out for the surface segregation and atomic-scale structure features of AuAg bimetallic NPs ranging from 586-atom (2.5 nm) to 9201-atom (7.2 nm) with a modified analytic embedded-atom method (MAEAM) potentials. These factors, such as temperature, size, and alloy composition ratio of NPs, which influence the surface structural characteristics have been investigated systematically. The effects of size and shape distribution of surface ensembles on tuning the catalytic activity and selectivity of bimetallic NPs were also discussed.

2. COMPUTATIONAL DETAILS In the present simulation, the MAEAM potentials are employed to describe atomic bonding in metallic systems. These potentials provide good description for many-body interactions and have already been successfully applied for the study of bulk, surface, and clusters/NPs of metals and alloys.3944 The MAEAM potential functions and the parameters for Au and Ag are given in refs 4244. The parameters were determined by fitting to the physical properties such as lattice parameters, cohesive energy, vacancy formation energy, and elastic constants of Au and Ag bulk materials. To determine the cross-interaction potential for a binary alloy, two adjustable parameters had been used and fitted to the lattice constant and heat of formation values available for disordered solid solution and possible intermetallics from the first-principles calculations and experimental data. Then, various physical properties of AuAg alloys can be calculated by means of the adjusting alloying parameters. Figure 1 shows the heats of formation for disordered solid solution and three possible intermetallics AuAg3 (L12), AuAg (L10), and Au3Ag (L12). The present heats of formation are consistent reasonably with the first-principles calculations and experimental data.4351 It is clear that the difference of heats of formation between the intermetallics and the corresponding disordered solid solution is very small, indicating the comparable thermodynamic stability of short-range order (SRO) and longrange order (LRO) in this system. It is theoretically suggested that LRO might exist in AuAg solid at low temperature from the viewpoint of thermodynamics, while the AuAg solid is experimentally reported to be SRO fcc, as the slow diffusion of atoms at low temperature.4553 The calculated lattice constants, 4.072 Å for possible intermetallics AuAg3 and 4.084 Å for Au3Ag (both with L12 structure), are consistent with those of firstprinciples calculations (4.068 and 4.093 Å for corresponding intermetallics).50 Therefore, the present AuAg cross interaction potential can be used for a wide range of components with reasonable accuracy. In order to probe the equilibrium structures and segregation properties of AuAg NPs, we have carried out MC simulations based on Metropolis algorithm to sample the statistical properties of NP configurations belonging to an NPT ensemble.5457 The advantage of the MC method is that the calculation of NP

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Figure 1. Heat of formation of AuAg bulk system as a function of Au concentration.

equilibrium configurations does not require the knowledge of exact diffusion pathways, which may be complex and cooperative in nature. A starting configuration was generated by initially creating a random distribution of Au and Ag atoms throughout the NP. A particular atom was then selected at random and exchanged with another one chosen randomly. The total energies of the system before and after the exchange were calculated using the MAEAM potentials. If the change of total energy ΔE is negative, the configuration is accepted. However, if it is positive, the configuration is accepted only if exp(ΔE/kBT) is greater than a random number selected from a uniform distribution.5457 The required quantities, such as the concentration distribution of chemical species, were obtained by averaging over 20 000 MC steps per atom after the system achieving equilibration state. The NPs used here were with truncated octahedron (TO) morphologies, which are believed to be privileged for fcc NPs.34,44,56 A typical TO NP model contains 6 {100} facets, 8 {111} facets, 12 {111}/{111} edges, 24 {111}/{100} edges, and 24 vertices. According to the magic numbers, a series of TO NPs were considered with 586, 1289, 2406, 4033, 6266, and 9201 atoms, ranging from 2.5 to 7.2 nm.44 The equilibrium configurations of AuxAg1x NPs (x = 0.25, 0.50, 0.75) were obtained in the whole temperature range (T = 10, 300, 600, and 900 K).

3. RESULTS AND DISCUSSION 3.1. Surface Segregation in Nanoparticles. Herein, we carried out MC simulations with the MAEAM potentials to study how the different factors such as temperature, particle size, and composition ratio would change the surface concentration of the constituent metals in AuAg NPs. We first focused on the surface layer since it is most relevant to active sites in catalysts. The segregation in AuAg NPs with different composition ratios were compared, as shown in Figure 2. Parts a, b, and c of Figure 2 represent the Au-rich Au0.75Ag0.25, equimolar Au0.50Ag0.50, and Ag-rich Au0.25Ag0.75 NPs under different temperatures, respectively. Figure 2d shows the segregation versus particle sizes at different temperatures. It can be seen that Ag is significantly enriched on the surfaces as compared to its bulk composition ratio in all cases. The extended X-ray absorption fine structure spectroscopy (EXAFS) study also showed that Ag predominantly resided on the surface of the bimetallic NPs.8,9,12 It is wellknown that the segregation driving force is important for surface 11356

dx.doi.org/10.1021/jp200642d |J. Phys. Chem. C 2011, 115, 11355–11363

The Journal of Physical Chemistry C

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Figure 2. Surface segregation of AuAg NPs: (a) Au0.75Ag0.25, (b) Au0.50Ag0.50, and (c) Au0.25Ag0.75 NPs under different temperatures, and (d) different-sized AuAg NPs at 10, 300, 600, and 900 K.

segregation phenomena. The lattice constants of Ag (4.09 Å) and Au (4.08 Å) are almost the same, and thus there will be no straininduced driving force toward segregation.4,58 The surface energy of Ag is less than that of Au, which favors surface enrichment of Ag.4,58 The surface energy is usually size-dependent for NPs, while the surface energy ratio between different facets is sizeindependent and equal to the corresponding bulk ratio.59 Therefore, the size-dependent surface energy cannot change the tendency of surface enrichment of Ag. For AuAg alloys, the charge transfer between Au (more electronegative) and Ag (less electronegative) atoms is also an important factor for the segregation of small-sized nanoclusters, which will favor AuAg mixing and thus hinder surface enrichment of Ag. Based on density functional theory calculations, Chen and Johnston found that charge transfer can even result in the reversed surface segregation of Au on 13-atom AuAg nanoclusters,30 which allows for the most AuAg heterobonds and the largest electron charge transfer from Ag to Au atoms. However, for the relatively large-sized NP or bulk materials, a moderate enrichment of Ag atoms on their surface is expected according to the above-mentioned thermodynamic driving forces, which is confirmed by EXAFS study and the present and previous simulations.8,9,12,3138 Surface Ag fraction is also affected significantly by temperature, composition ratios, and particle size. For example, as shown

in Figure 2ac, a temperature-dependent effect can be seen for all the NPs, where surface Ag fraction continually decreases as a function of temperature. Figure 2d illustrates surface Ag fraction in AuAg NPs as a function of particle size. For Au0.25Ag0.75 NPs, the size effect of surface Ag concentration is undistinguishable due to the high Ag concentration, leading to a silver-passivated surface. On the contrary, in Au0.75Ag0.25 NPs, an initial sharp increase for the surface Ag concentration with increasing size followed by an asymptotic plateau, approaching the bulk system. This sharp increase means a size-dependent effect of surface Ag composition for small NPs. It is known that the percentages of lower coordinated vertex (6 coordination) and edge (7 coordination) sites decrease with increasing particle size, while those of (100) facets (8 coordination) and (111) facets (9 coordination) increase with particle size.44 In addition, the cohesive energies are site-dependent and increase with increasing coordination number (CN).44 Hence, for small NPs, the size-dependent effect is dominated by lower coordinated vertex and edge sites, which are preferred by segregated Ag atoms. In Au0.50Ag0.50 cases, the size-dependent effect is less obvious than that of Au0.75Ag0.25 ones, resulting from their more negative heats of formation as shown in Figure 1, which favors intermixing of Au and Ag atoms and thus hinders surface Ag segregation. Such unique size-dependent properties, when considered the large number of NPs typically used for practical 11357



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Figure 4. Cross-section snapshots of 586-atom and 9201-atom NPs at T = 300 K: (a) 586-atom Au0.25Ag0.75 NP, (b) 586-atom Au0.50Ag0.50 NP, (c) 586-atom Au0.75Ag0.25 NP, (d) 9201-atom Au0.25Ag0.75 NP, (e) 9201-atom Au0.50Ag0.50 NP, and (f) 9201-atom Au0.75Ag0.25 NP. Au is in yellow (light) and Ag in gray (dark).

Figure 3. Variation of surface Ag fraction as a function of Ag concentration for (a) 586-atom NPs and (b) 9201-atom NPs. The open symbols represent experimental data.8,61

catalytic applications, will yield macroscopic effects on both catalytic activity and selectivity. Figure 3a,b shows the segregation behavior of NPs with respect to different alloy composition ratios. The diagonal line across the graph indicates no segregation scenario, while the dotted line shows the “max segregation” scenario.60 Open signatures in Figures 3b present available experimental results of the NPs which were calcined at 500 °C and then reduced by H2 at 600 °C.8,61 It is clear that the present results of 9201-atom NPs are well consistent with available experimental data.8,9,6164 All plots for 586-atom and 9201-atom NPs show that the surface Ag fraction is higher in Ag-richer or larger-sized NPs. These composition- and size-dependent segregation behaviors can be described by the degree of surface segregation. The simulated surface Ag fraction is defined as C = NAS/NS, where NAS is the number of surface Ag atoms and NS is the number of surface atoms. The maximum possible surface Ag fraction, as permitted by the number of available Ag atoms in the particle, is defined as Cmax = NA/NS (when NA < NS) or Cmax = 1 (when NA g NS), where NA is the number of Ag atoms in NP. Naturally, the degree of surface segregation can be defined as D = C/Cmax = NAS/NA (when NA < NS) or D = C (when NA g NS). This definition of degree of surface segregation can be used to understand how strong the trend of surface segregation is in the AuAg NPs. It can be seen that the degree of surface segregation equals the surface Ag fraction for 9201-atom NPs, where both the degree of

surface segregation and surface Ag fraction increase with increasing bulk Ag composition. For Au0.75Ag0.25 NPs, it is interesting that, although surface Ag fractions of 586-atom NPs are lower than those of 9201-atom NPs, their degree of surface segregation is higher. This unique size dependency of degree of surface segregation is because of the size dependency of surface-tovolume ratio in NPs. For example, the surface-to-volume ratio is 46% in 586-atom NP and 20% in 9201-atom NP,44 respectively; thus, the degree of surface segregation is 0.70 for the 586-atom NP and 0.50 for the 9201-atom one at 300 K. For the 586-atom NP, surface Ag fraction increases with increasing bulk Ag composition, but the degree of surface segregation is ordered as Au0.75Ag0.25 ≈ Au0.50Ag0.50 < Au0.25Ag0.75. This comparable degree of surface segregation of Au0.75Ag0.25 NPs and Au0.75Ag0.25 ones are also found in 1289-, 2406-, and 4033-atom NPs, which is not shown here for clarity. This definition of degree of surface segregation can be extended to other NPs systems showing different segregation behavior. For example, for AuPd and AuPt NPs, the degrees of surface segregation are calculated and found almost equal to one irrespective of size and composition, confirming that they are the systems with strong surface segregation indeed.44,60 In contrast, the present AuAg NPs is a system with moderate surface segregation, well consistent with the above analysis of thermodynamic driving force of surface segregation. 3.2. Nanoparticle Structural Features. Of interest is to obtain the local structural information and atomic distribution of elements in bimetallic NPs to get more insights into the structures, as well as their applications. Several mixing patterns have been proposed for bimetallic NPs, such as random or ordered mixed, subcluster segregated, coreshell, and onionring structures.58,65 In fact, the mixing patterns of NPs result from the competition and balance between surface segregation and alloy formation. As an example, the cross-section snapshots of 586-atom and 9201-atom NPs at 300 K are given in Figure 4. Layer by layer oscillations in concentration near the surface have been observed in Au0.25Ag0.75 and Au0.50Ag0.50 NPs, where the outmost NP layer is Ag rich while the sublayer is Au rich. This layering effect results from the low surface energy of Ag and the negative heat of formation of AuAg alloy; the former prefers 11358



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Ag-rich outermost layer, while the latter favors Au-rich sublayer and Ag-rich third layer. Such layering effect has been reported in the (100) and (111) surface layers of AuAg bulk alloy and AuAg NPs.36,66 In 586-atom NPs, the layering effect results in an onionlike structure, indicating the surface segregation is dominant here. This onionlike structure was also found in small AuAg clusters and other NP systems.36,67,68 In 9201-atom NPs, the layering effect is also observed near the surface. A constant concentration as in bulk system is observed in the core region due to mixing of Au and Ag atoms, indicating that the negative heat of formation is dominant here. The mixing of Au and Ag atoms leads to an alloyed-core/layered-shell structure34 in 9201-atom NPs. Such morphologies have also been observed in other sized AuAg NPs, depending on their alloy compositions and particle sizes. In addition, the onionlike structures are more obvious for smaller NPs. At high temperatures, the onionlike shell is degraded as more random intermixing of Au and Ag atoms. In addition, the NPs are more close to ball-like structure due to larger inward relaxations at the vertices and edges compared to the rest of the surface. As a result, the different surface sites (facet, edge, and vertex) are less distinguishable in the more relaxed NPs. Although qualitative mixing patterns of bimetallic NPs have been proposed, structural models quantitatively describing alloying extent are rather scarce. In order to know the alloying extent, we analyzed the quantitative parameters, JA and JB, on the basis of Au and Ag CN:69 JA ¼

Pobserved 100% Prandom

ð1Þ

JB ¼

Robserved 100% Rrandom

ð2Þ

∑

ð3Þ

∑

ð4Þ

Pobserved ¼ NAB = NAi Robserved ¼ NBA = NBi

where NAB is the CN of Ag around Au, NAA is the CN of Au around Au, and ∑NAi is the total CN of Au. Similarly, NB-A is the CN of Au around Ag, NBB is the CN of Ag around Ag, and ∑NBi is the total CN of Ag. Prandom and Rrandom can be taken as 0.25, 0.50, or 0.75 for perfect random bimetallic NPs if the atomic ratio of Au and Ag is 1:3, 1:1, or 3:1, respectively.69 With the help of CN extracted from MC simulation results or EXAFS analysis, it is possible to calculate the alloying extent according to eqs 14 and predict the structural model of NPs. The structural parameters JA and JB have been successfully applied to AuPd, CuPd, PdPt, FePt, and PtRu NPs.69 The CN analysis and alloying extent results for 586-atom and 9201-atom AuAg NPs are listed in Table 1, respectively. It can be seen that the results of CN and structural parameters JA and JB based on MC simulation are consistent well with available EXAFS analysis of calcined and reduced NPs.8 Note that JB > JA in all cases, which means that the surface is rich in Ag atoms. For 586-atom NPs, it is clear that JA < 100% and JB > 100%, which means Au atoms prefer Au atoms rather than Ag atoms, and Ag atoms also prefer Au atoms rather than Ag atoms, consistent with the homoatomic or heteroatomic interactions HAuAu > HAuAg > HAgAg.4,58 In this case, the NPs will adopt the Au-rich core and Ag-rich shell structure according to the structural models.69 This core/shell structure is an oversimplified

Table 1. Coordination Number and Alloying Extent for AuAg NPs (A = Au, B = Ag) 3:1 a

b

9201 586

1:1 expt

c

a

b

9201 586

1:3 expt

c

a

9201 586b

NAA

8.98 8.52 8.0 ( 1.2

6.47 6.44 5.6 ( 0.6

4.01 4.14

NAB

2.57 2.21 2.9 ( 1.0

5.27 4.72 4.6 ( 0.5

7.92 7.66

NBA NBB

7.70 6.67 7.8 ( 1.0 2.78 1.97 1.6 ( 0.7

5.27 4.72 3.6 ( 0.9 5.56 4.51 4.6 ( 0.7

2.60 2.54 8.38 7.14

NAi

11.55 10.73 10.9 ( 2.2 11.74 11.16 10.2 ( 1.1 11.92 11.80

NBi

10.48 8.64 9.4 ( 1.7

10.83 9.23 8.2 ( 1.6

10.98 9.68

Pobserved 0.22 0.21 0.27 ( 0.14 0.45 0.42 0.45 ( 0.10 0.66 0.65 Robserved 0.73 0.77 0.83 ( 0.26 0.49 0.51 0.44 ( 0.20 0.24 0.26

a

JA (%)

89

82

106 ( 58

90

85

90 ( 19

89

87

JB (%)

98

103

111 ( 34

97

102

88 ( 39

95

105

9201-atom NPs at 300 K. b 586-atom NPs at 300 K. c EXAFS data.8

onionlike structure suggested by MC simulation, but they are similar in nature to each other. For 9201-atom NPs, it can be seen that JA < 100% and JB < 100%, which appears a homophilic structure according to the structural models. It is noteworthy that, as suggested by MC simulation, the homophilic feature is not global, but partly local near the surface as shown in Figure 4, where an alloying feature can be seen in core region. For an ideal core/shell structure, the structural parameters are JA < 50% and JB < 50%,69 while for an ideal solid solution alloy, the structural parameters are JA = 100% and JB = 100%.69 Therefore, the solid solution core and layer shell result in the structural parameters JA < 100% and JB < 100% for 9201-atom NPs. It is interesting that the structural parameters of 9201-atom NPs are much closer to 100% than those of 586-atom NPs, indicating more obvious alloying feature in 9201-atom NPs and more obvious surface segregation feature in 586-atom NPs, which is consistent with above analysis of size-dependent degree of surface segregation. In addition, the structural parameters of all NPs are approaching 100% with increasing temperature, which is not shown here. It demonstrates that surface segregation is degraded and alloying extent is extended at higher temperature. Although actual mixing patterns of AuAg NPs strongly depend on the pretreatments and the conditions of synthesis or the presence of adsorbates,1725 the above-mentioned mixing patterns are theoretically predicted to be thermodynamically the most stable ones. If the synthesis conditions were suitable for formation of these stable structures, further experiments such as scanning transmission electron microscopy with the high-angle angular dark field imaging technique (HAADF-STEM) can be used to fully understand the nature of NP structural features, in conjunction with first-principles approach or atomic-scale simulations presented in the present work.4,70,71 Nevertheless, the detailed knowledge of the most stable NPs’ structures will greatly benefit the design and optimization of robust NP catalysts. It is known that intermixed AgAu NPs show a single plasmon resonance, as for the pure metals. For intermixed NPs of fixed size, experiment and theory agree that the plasmon frequency varies smoothly with composition between that of the pure Ag and pure Au NPs.58 On the other hand, the surface plasmons of the coreshell NPs are broad and complex. The differences in the degree of mixing or segregation have been attributed to the strong nonlinear evolution of the plasmon frequency of AgAu NPs with Ag content.58 Since the most 11359



Figure 5. Distribution of surface ensembles of 9201-atom Au0.25Ag0.75 NP: (a) snapshot of whole configuration at T = 300 K and (b) statistics for different types of surface Au ensembles as a function of temperature. Au is in yellow (light) and Ag in gray (dark).

thermodynamically favorable NPs presented herein have the structural features of coreshell NPs or/and intermixed NPs depending on their size and alloy composition, they are also expected to exhibit unique optical properties. 3.3. Nanoparticle Surface Characterization. Recent evidence suggests that the catalytic activity and selectivity, as well as the stability of bimetallic surface, can be governed by the creation of unique mixed metal surface sites (the so-called ensemble effect) and/or electronic structure change by metal metal interactions (ligand effect).72,73 In addition, the reactivity and selectivity of NP catalysts can be modified by the presence of low-coordination surface atoms and the stresses imposed on the outer-layer atoms, in association with the size and shape of NP catalysts.74 Because of the importance of such results in catalysis, we quantify herein the distribution of atoms on the NP surface. To quantitatively describe the distribution of surface Au atoms, the “surface ensemble” is defined in surface alloy, which is a group of surface Au or Ag atoms that are neighbors with each other and surrounded by Ag or Au atoms.7577 For example, if atoms 1 and 2 are neighbors, atoms 2 and 3 are neighbors, and atoms 2 and 4 are neighbors, then atoms 14 are part of one surface ensemble of size 4. The configurations of typical surface ensembles and statistical analysis of distribution of surface ensembles for 9201-atom NPs are shown in Figures 5, 6, and 7, respectively. It is clear that NP composition and temperature play key roles on surface ensembles of Au atoms. Specifically, for the Au0.25Ag0.75 NP as shown in Figure 5, an interesting feature is that Au monomers on (111) facets are dominant at low temperatures. At elevated temperature, Au monomers and dimers are dominant and increase with temperature,

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leading to the increase of total Au atom concentration on the surface. In the case of Au0.50Ag0.50 NP as shown in Figure 6, most of the Au monomers and dimers and all of the Au trimers and tetramers occupy the (111) facets, where large surface Au ensembles (chains or islands containing more than 4 atoms) also appear. Temperature effect of different surface Au ensembles is varied. The number of Au monomers and dimers on (111) facets decrease with temperature, while that of large Au ensembles increases with temperature. There are a few Au monomers and dimers at the edge and (100) facet sites, which also increase with temperature. For the Au0.75Ag0.25 NP as shown in Figure 7, an interesting feature is that (111) facets are dominated with large Au (or Ag) ensembles, while low-coordinated vertex, edge, and (100) facet sites are dominated by Au monomers and dimers. The number of Au dimers increases with temperature, while that of Au monomers first increases and then decreases with temperature. Although the temperature dependence of Au monomers and dimers are different, the sum of Au monomers and dimers at low-coordinated sites is roughly constant. Our statistics on the distribution of Au ensemble on the other sized AuAg NPs surface show that, even though the temperature and composition effects have also been found in those sized NPs, their trends are quite similar to that of 9201-atom NPs. This distribution of surface ensembles may exhibit interesting and unique catalytic properties. First, the enhanced activity is expected from the low-coordinated atoms as compared to the bulk alloy surface. The result with first-principles approach is that CO preferentially interacts with low-coordinated Au atoms on the surface of free Au NP, and the adsorption energy of CO decreases with the CN of Au atom.78,79 It is well-known that the Ag sites on the bulk or NP surfaces can easily adsorb and activate 11360



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NPs, it is notable that the shape distribution of Au trimers and tetramers are varied, a few of them have compact shapes and most of them are bent linear and linear. It demonstrates that these varied configurations correspond to different neighboring surface Ag atoms. For example, each compact Au trimer has 9 neighboring surface Ag atoms and each extended Au trimer has 10 neighboring surface Ag atoms. In AuAg NPs, the heteroatomic interactions are favored over homoatomic ones as mentioned above, thus the fraction of compact configurations is smaller than that of extended ones. These compact and extended configurations of surface Au ensembles provide varied catalytically active sites, which are expected to help in tuning the catalytic selectivity. To the best of our knowledge the above-discussed effects of surface morphology of AuAg NPs were not previously explicitly communicated. While direct experimental verification of our prediction of ensemble distribution is currently not available, this study revealed that surface morphology is the key factor in determining the catalytic activity and selectivity of bimetallic or multimetallic NPs. It further indicated that it is possible to tune the catalytic properties of alloy NPs by controlling experimentally the particle size and mole fraction of each component in order to adjust special surface active sites.


oxygen.713,29 Therefore, on the surface of AuAg bimetallic NP, the coadsorption of CO and O2 on the neighboring surface Au or Ag atoms makes the oxygen transfer reaction occur easily. The combination of so-called synergistic effect and low-coordinated atoms results in high activity for AuAg bimetallic NPs at low-temperature CO oxidation as suggested by recent experiments and first-principles calculations.713,29 Second, the conversion efficiency of CO oxidation can be promoted at certain composition as the specific Au and Ag surface configurations. Such as in Au0.75Ag0.25 NPs, the presence of Au monomers and dimers on vertex and edge sites provides much more active sites for CO adsorption than Au0.25Ag0.75 and Au0.50Ag0.50 NPs, where Au monomers and dimers mainly occupy (100) and (111) facets. As shown in Figures 5, 6, and 7, most of these active Au monomers and dimers have neighboring low-coordinated Ag atoms pairs. Considering the synergistic effect mentioned above, the Au0.75Ag0.25 NPs thus can provide site-specific enhancement of CO and O2 adsorption and promote specific reaction kinetics. Recent research has also confirmed that roomtemperature CO oxidation conversion efficiency was maximized at an optimized Au/Ag ratio of 3:1 or 8:1.7,12 On the other hand, specific surface ensembles may significantly enhance selectivity toward certain catalytic reactions. For example, Pd monomers surrounded by less active Au atoms were primarily responsible for the significantly enhanced selectivity toward H2O2 formation on PdAu alloy surface.80 Similarly, if single Au atoms are required to improve the selectivity then Au0.25Ag0.75 NPs are the recommended catalyst. When large Au (or Ag) ensembles or low-coordinated Au atoms are required, it will be better to choose the Au0.75Ag0.25 NPs. For Au0.50Ag0.50

4. CONCLUSIONS In this work, we have used MC simulation coupled with the MAEAM potential to study the segregation behavior and atomicscale structural features of AuAg NPs. The influences of different factors for the surface segregation and structural features have been presented and discussed. A segregation of Ag to the surface has been found in all AuAg NPs considered and the surface segregation behaviors are composition, size, and temperature dependent. The surface Ag fractions are higher in the Ag-richer or larger-sized NPs under lower temperature. The most thermodynamically favorable morphologies are the onionlike structures with Ag-rich shell for small NPs, and the layered-shell/alloyed-core structure for larger ones, which results from the competition and balance between surface segregation and alloy formation. The mixing patterns and alloying extent in bimetallic NPs have been analyzed based on the calculated structural parameters and it is found that there are more obvious alloying features in larger-sized NPs and more obvious segregated features in smaller-sized NPs. The surface segregation is degraded and alloying extent is extended at higher temperature. The distributions of Au ensembles on the NP surfaces have been obtained and it is found that the composition and temperature of NPs play a key role on surface ensembles of Au atoms. Based on the distribution of surface Au ensembles, the role of surface morphology in determining the catalytic activity and selectivity and the possibility of tuning the catalytic properties of bimetallic or multimetallic NPs by creating or destroying special active sites have also been discussed. Therefore, the detailed atomistic understanding of structural features and surface morphologies will greatly benefit the design and optimization of alloy NP catalysts. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-731-88822361. Fax: þ86-731-88822332. E-mail: [email protected] (W.H.); [email protected] (H.D.). 11361



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