Thermal Stability and Shape Evolution of Tetrahexahedral Au–Pd

Article pubs.acs.org/JPCC

Thermal Stability and Shape Evolution of Tetrahexahedral Au−Pd Core−Shell Nanoparticles with High-Index Facets Rao Huang,† Yu-Hua Wen,*,† Gui-Fang Shao,‡ Zi-Zhong Zhu,† and Shi-Gang Sun§ †

Institute of Theoretical Physics and Astrophysics, Department of Physics, Xiamen University, Xiamen 361005, China Institute of Pattern Recognition and Intelligent System, Department of Automation, Xiamen University, Xiamen, 361005, China § State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China ‡

ABSTRACT: Nanosized metallic particles with high-index facets have exhibited excellent electrocatalytic activity and thus attracted intense interests over the past few years. Moreover, bimetallic particles with high-index facets could further enhance the catalytic activity by the synergy effects of high-index facets and electronic structures of the alloy. In this article, we employed atomistic simulations to investigate the thermal stability and shape evolution of tetrahexahedral Au−Pd core−shell nanoparticles respectively enclosed by {210} and {310} facets. The ground-state energy calculations indicated that the {210} faceted nanoparticles are more structurally stable than the {310} faceted ones. More importantly, it has been discovered that the former possess better thermal and shape stabilities than the latter. The Lindemann index was introduced to shed light on the melting mechanism, and the atomic distribution function was adopted to describe the diffusion tendency. For these two high-index terminated Au−Pd bimetallic nanoparticles, the core and the shell exhibit different thermal evolution as they are heated to melting, though the melting generally proceeds from the shell into the core. Beyond the overall melting, Au atoms prefer to aggregate near the surface to favor the minimization of the total energy. These results are helpful for understanding the composition, shape, and thermodynamic properties of high-index faceted nanoparticles and therefore could be of great importance to the development of bimetallic core−shell nanocatalysts with both high reactivity and excellent stability.

1. INTRODUCTION Metallic nanocatalysts have received a tremendous amount of interest in both scientific research and industrial applications in the past decade.1 Increasing efforts have been devoted to an extensive array of fundamental researches for developing metallic nanocatalysts with enhanced reactivity and utilization efficiency. Generally, these efforts can be classified into two strategies: morphology control and component design. The former route is inspired by the fact that catalytic reactions usually take place at surfaces, and the performances of catalysts thereby strongly depend on their surface structures. For facecentered cubic (fcc) metals such as Au, Pd, Pt, Ag, etc., among all crystallographic planes that terminate the nanocrystal surface and hence determine its shape, high-index facets exhibit excellent activity and selectivity for chemical reactions due to a much higher density of low-coordinated step atoms, ledges, and kinks compared with low-index facets such as {111} and {100} ones.2−6 However, in solution chemistry, high-index facets disappear easily during nanoparticle formation because the rate of crystal growth in the direction perpendicular to a high-index facet with high surface energy is much faster than © 2013 American Chemical Society

that along the normal direction of a low-index one. Therefore, it has remained challenging to synthesize high-index-faceted nanoparticles (NPs). A breakthrough progress has been achieved by Sun and co-workers, who have successfully prepared tetrahexahedral (THH) Pt NPs bounded with highindex facets such as {730}, {210}, and {520} ones by an electrochemical route.3,4 These THH Pt NPs exhibited greatly enhanced catalytic activity compared with the existing commercial Pt catalysts. Subsequently, THH Pd and Au NPs have also been prepared by electrochemical and wet-chemical methods.5,6 The second route is to synthesize bimetallic or multimetallic NPs by adding other metallic elements selectively, providing another possibility of improving the chemical activity of metallic nanocatalysts. Comparing with monometallic NPs, bimetallic (or multimetallic) ones such as Pt−Au, Pt−Pd, Pt− Rh, and Pt−Pd−Au systems have demonstrated higher catalytic activities toward oxygen reduction reactions (ORRs),7 methReceived: February 8, 2013 Revised: March 13, 2013 Published: March 14, 2013 6896

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anol oxidation reactions (MORs),8,9 and for preferential CO oxidation in hydrogen feeds (PROXs).10,11 Furthermore, the bimetallic (or multimetallic) NPs are vital for exploiting the bifunctional (or multifunctional) catalytic activity. As an example, in Pt−Au NPs, Pt can provide a site for MORs or ORRs and Au for adsorption of hydroxide groups or oxidation of CO to CO2.12,13 More recently, the aforementioned two strategies have been successfully combined, that is, to prepare bimetallic NPs bounded by high-index facets. These NPs may be expected to exhibit high catalytic activity compared to the monometallic ones due to the synergy effects of high-index facets and electronic structures of the alloy. The as-prepared THH Pd−Pt bimetallic NPs which mainly enclosed by {10 3 0} high-index facets exhibit a catalytic activity that is at least 3 times higher than the THH Pd NPs and 6 times higher than commercial Pd black catalysts for the electrooxidation of formic acid.14 Especially, it has been observed that the convex polyhedral Au−Pd core−shell (Au@Pd) NPs predominantly enclosed by {12 5 3} high-index facets exhibited a significantly better electrocatalytic activity toward ethanol oxidation than that of cubic and octahedral Au@Pd NPs bounded by low-index {100} and {111} facets.15 Moreover, the THH NPs with entirely highindex {730} facets were found to display the best electrocatalytic activity in Au@Pd NPs with tetrahexahedral, concave octahedral, and octahedral morphologies.16 All the existing results indicated that the excellent chemical activities of bimetallic NPs are highly dependent on both the surface structures of the particles and their component distribution. Thermal stability of bimetallic nanoparticle catalysts is of considerable importance for their applications. On the one hand, catalytic reactions preferentially take place on surfaces, and surface structures of NPs will thereby play key roles during the catalytic processes. However, a large number of catalytic reactions are high-temperature involved, especially the cracking of petroleum and the purification of automobile exhaust gases. In these occasions, the high-index planes are easily destroyed at elevated temperature due to their high surface energy compared with low-index ones such as {111} and {100}. Hence, the problem of particle stability becomes particularly acute for the catalytic performance of NPs. On the other hand, the sintering of metal is strongly temperature-dependent and closely related to the Tammann temperature.17 When the Tammann temperature is reached, bulk atoms start to move, leading to the enhanced atomic diffusion and resultant coalescence among particles. Therefore, the examination of thermal stability is indispensable for developing new approaches to not only stabilize the high-index facets in metal NPs but also suppress their sintering and coarsening at high temperatures. However, in spite of the continually emerging reports on the synthesis, characterization, and catalytic performance of alloyed nanocatalysts with “open-structure” surfaces, a thorough study of the thermal stabilities of these NPs with different high-index facets is still absent. The consideration of the aforementioned two aspects naturally motivates us to investigate whether these highindex faceted bimetallic NPs would remain stable at different ambient temperatures. In this article, the most extensively synthesized Au@Pd nanocatalysts18,19 were chosen to be investigated by atomistic simulations. Because of the typical significance of tetrahexahedral shape, the THH Au@Pd NPs bounded with high-index facets have been addressed. Considering that the {730} surface is periodically composed of two {210} subfacets followed by

one {310} subfacet and the {520} surface is periodically composed of one {210} subfacet followed by one {310} subfacet, {210} and {310} surfaces can be regarded as representatives of high-index planes. Therefore, THH Au@Pd NPs, enclosed with {210} and {310} high-index facets, respectively, have been examined in this work. Besides, different shell thicknesses have been considered in modeling the THH Au@Pd NPs to trace the effects of core/shell ratio. These NPs were heated to high temperatures to examine their thermal stability. Both the shape evolution and the atomic distribution at different temperatures were explored. To the best of our knowledge, this is the first report about the thermal stability and shape evolution of polyhedral core−shell NPs with highindex facets at the atomistic level. A brief description of the simulation methods is given in the following section. The calculated results, discussion, and comparison with other results are presented in the third section. The main conclusions are summarized in the fourth section.

2. SIMULATION METHODOLOGY In order to reasonably correspond to the structure and shape of those core−shell NPs experimentally observed, THH Au@Pd NPs terminated by {210} (denoted as NP1) and {310} facets (denoted as NP2) were constructed from a large cubic fcc single crystal. Both types of NPs possessed an octahedral Au core; see Figure 1 for the detailed structures. It should be noted

Figure 1. Schematic illustration of THH Au@Pd NPs enclosed by (a) {210} and (b) {310} facets. Note that cross section of shell has been presented. Coloring denotes type of atom: gold, Au atom; blue, Pd atom. Corresponding atomic arrangement of (c) {210} and (d) {310} surfaces.

that the THH NPs bounded by 24 high-index facets can be considered as a cube with each face capped by a square pyramid. Aiming at facilitating a comparison study of both highindex faceted NPs, the side length l1 of the cube was equally fixed at 12a0 (that is, about 4.66 nm, a0 = 3.8813 Å) to form close atomic numbers which were 10 831 for NP1 and 9553 for NP2. Meanwhile, for the purpose of examining the core effects, the side length l2 of the octahedron (Au core) can take values of 14a0, 12a0, 10a0, 8a0, or 6a0 (denoted as NP1(or NP2)-1, 6897

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relaxation time at each temperature, and the statistical quantities were obtained in the last 25 ps. The desired temperature and ambient pressure are maintained by NoseHoover thermostat27 and Berendsen approach,28 respectively. The equations of atomic motion are integrated by velocity Verlet algorithm29 with a femtosecond time step.

NP1(or NP2)-2, NP1(or NP2)-3, NP1(or NP2)-4, or NP1(or NP2)-5, respectively). Similarly, a series of THH Au@Pd NPs with different sizes were modeled in the calculations of structural stability. Based on our previous works,20−23 the quantum-corrected Sutton−Chen (Q-SC) type potential was adopted to describe the interatomic interactions. These potentials represent manybody interactions, and their parameters are optimized to describe the lattice parameter, cohesive energy, bulk modulus, elastic constants, phonon dispersion, vacancy formation energy, and surface energy, leading to an accurate description of many properties of metals and their alloys.24−26 The total potential energy for a system of atoms can be written as ⎡ 1 U = ∑ Ui = ∑ ε⎢ ⎢⎣ 2 i i

⎤ ∑ V (R ij) − c ρi ⎥⎥ ⎦ j≠i

3. RESULTS AND DISCUSSION 3.1. Structural Stability. The structural stability of materials is generally evaluated by the cohesive energy which equals to absolute value of the total energy at ground state (namely, the summation of the potential energy of each atom in atomistic simulations since the kinetic energies are actually zero). This criterion indicates that the structure with lower potential energy tends to be more stable. For example, the Pd and Au bulk atoms have an average energy of −3.89 and −3.81 eV,30 respectively, indicating that Pd is slightly superior in structural stability than Au. Figure 2 shows the size dependence of the energy obtained using the conjugate gradient method (CGM) scheme for the

(1)

in which V(Rij) is a pair interaction function defined by the following equation: ⎛ a ⎞n V (R ij) = ⎜⎜ ⎟⎟ ⎝ R ij ⎠

(2)

accounting for the repulsion between the i and j atomic cores; ρi is a local electron density accounting for cohesion associated with atom i defined by ⎛ a ⎞m ρi = ∑ ⎜⎜ ⎟⎟ R j ≠ i ⎝ ij ⎠

(3)

In eqs 1−3, Rij is the distance between atoms i and j; a is a length parameter scaling all spacings (leading to dimensionless V and ρ); c is a dimensionless parameter scaling the attractive terms; ε sets the overall energy scale; n and m are integer parameters such that n > m. Given the exponents (n, m), c is determined by the equilibrium lattice parameter, and ε is determined by the total cohesive energy. The model parameters for Pd and Au are listed in Table 1. In order to describe the

Figure 2. Size-dependent ground-state energies of both types of THH Au@Pd NPs with different core/shell ratios.

Table 1. Potential Parameters Used in Atomistic Simulations for Au-Core/Pd-Shell NPs26 element

n

m

ε (meV)

c

a (Å)

Pd Au

12 11

6 8

3.2864 7.8052

148.205 53.581

3.8813 4.0651

fully relaxed THH Au@Pd NPs of both types with three different core−shell ratios (l1:l2 = 6:3; 6:5; 6:7). Meanwhile, as a reference, the energies of corresponding pure Pd NPs respectively enclosed by {210} and {310} facets (i.e., l1:l2 = 6:0) were also calculated. The data points were connected by smooth curves. Here, to make a comparison of these NPs with different surface structures, a common definition of particle size, based on equivalent volume, has been introduced as follows:

atomic interaction between Pd and Au, the geometric mean was used to obtain the energy parameter ε, while the arithmetic mean was used for the remaining parameters.24,25 Upon starting the molecular dynamics (MD) simulations, all the NPs were first quasi-statically relaxed to the states of energy minimization. The total energy could be obtained by summing the energy of atoms in the NPs. After full relaxation, both types of NPs were subjected to a continuous heating. To make the simulations more reliable, we employed constant volume and temperature molecular dynamics (NVT-MD) to allow energy and box volume fluctuations, which may be critical to the resulting dynamics. The NPs underwent the heating process consisting of a series of NVT-MD simulations from 0 to 1800 K with a temperature increment of 50 K. However, a smaller step of 10 K was adopted to investigate the melting behavior more accurately when the temperature reached to around the melting point. The simulations were carried out for 200 ps of the

d=

3

N a0 4

(4)

where N is the total number of atoms in the NP and a0 = 3.8813 Å is the lattice constant of bulk Pd. Note that there are four atoms in each unit cell of an fcc lattice, and a03/4 thus stands for the equivalent volume of each atom in the particle. As seen from Figure 2, for each core/shell ratio, a clear size effect is presented: the large size leads to a low-energy configuration. This suggests that the surface effect of nanostructures serves as a dominant factor in determining their stabilities under ultrafine crystal sizes, which has been verified by many existing studies.22,24 Moreover, the curves also 6898

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temperature, namely, the melting point Tm, should be specified as the temperature at which the heat capacity reaches its maximum. It can be demonstrated that for both types of the NPs, the Tm gradually increases as the octahedral core size reduces, that is, from 1320 to 1400 K for NP1 and from 1300 to 1380 K for NP2. Besides, stemming from the difference in surface structures, the Tm of NP1 is about 20 K higher than that of NP2 for the same core size. Furthermore, comparing with the melting of monometallic NPs, a notable feature of these caloric curves for the bimetallic ones is that the potential energy exhibits different degrees of decrease beyond the phase transition. That is, the larger core size leads to a more distinct reduction. This result is closely associated with the remarkable atomic diffusion behavior after overall melting, which will be discussed later. Interestingly, the phenomenon of two-stage melting in core−shell structures such as Pt-core/Pd-shell NPs32 has not been observed here. The reason may consist in that the Au core possesses a bulk melting point of 1337 K, which is close to that of Pd shell. However, in Pt-core/Pd-shell NPs, the bulk Tm of the Pt in the core is 2045 K, which is much higher than that of the Pd in the shell, thus resulting in different melting modes. Despite the determination of melting points, it is desirable to deeply understand the thermal stabilities of these NPs and to shed light on the stability-associated phenomena. In order to detailedly examine the melting mechanism of THH Au@Pd NPs, a simple but effective measurement, the Lindemann index, was introduced to characterize the thermal evolution in the heating process. It provides a good description of the thermally driven disorder of the systems. For a system of N atoms, the local Lindemann index for the ith atom is defined as the rootmean-squared (rms) bond length fluctuation as33

demonstrated that the NPs with smaller core sizes are generally more stable, which should stem from the difference between the energies of Pd and Au atoms aforementioned. Besides, by comparing the results for the two types of NPs, we can find that the NP1 possesses slightly better stability than the NP2 when the size and core−shell ratio are both fixed. This arises partly from the fact that the NP1 contains more Pd atoms than the NP2 with the same side length and core size. 3.2. Thermal Stability. Important information concerning the thermodynamic properties, the characteristics and progress of melting during the heating process can be obtained from data records from MD simulations. The phase transition temperature from solid to liquid is usually identified by investigating the variation in the thermodynamic quantities such as potential energy and specific heat capacity. Note that the heat capacity can be induced as a function of temperature according to the equation31 Cp(T ) =

dU 3 + R gc dT 2

(5)

where U is potential energy and Rgc = 8.314 J/(mol K). Figure 3a and 3b respectively show the temperature dependence of potential energies and specific heat capacities for the THH Au@Pd NPs enclosed by {210} and {310} facets with different compositions. The solid−liquid phase transition can be clearly identified by the sharp rise of the potential energy and the abrupt peak of the heat capacity. The critical

δi =

1 N−1

∑ j≠i

⟨R ij 2⟩ − ⟨R ij⟩2 ⟨R ij⟩

(6)

and the system-averaged Lindemann index is calculated by

δ=

1 N

∑ δi i

(7)

where Rij is the distance between the ith and jth atoms. The Lindemann index was originally developed to study the melting behavior of bulk crystals. The Lindemann criterion suggests that the melting occurs when the index is in the range of 0.1− 0.15, depending on materials,34 while a smaller critical index of about 0.03 was adopted in clusters and homopolymers due to the relaxed constraint of the surface atoms.33 The temperaturedependent Lindemann indices during the heating process were respectively calculated for Au and Pd atoms in both types of NPs (see Figure 4). A similar trend can be seen from this figure: In the initial stage, the Lindemann indices for Au and Pd atoms increase linearly with rising temperature. Gradually, they begin to deviate from the linear increase and show sharp jumps around the melting points, indicating the occurrence of solid− liquid phase transition, which precisely correspond to the abrupt increase of potential energy. Beyond the overall melting, the Lindemann indices continue to increase linearly. As seen in Figure 4, the critical indices of 0.03 and 0.04 should be adopted for Au and Pd here, respectively. Although Au possesses a profoundly lower bulk melting point than Pd, the Lindemann criterion suggests that the Au core melts at higher temperature than the Pd shell in both types of NPs, thus implying that the

Figure 3. Temperature-dependent potential energies and specific heat capacities of both types of THH Au@Pd NPs with different compositions during the heating process. Note that the solid lines denote the potential energy, and the dashed lines denote the heat capacity. 6899

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with the decreasing core size and are slightly higher for NP1 than NP2 at the same core size. This corroborates the results of the potential energy and capacity heat in Figure 3. In order to visualize the melting process at atomic level, the snapshots of the cross sections for these NPs have been taken at representative temperatures during the simulation of continuous heating. The results showed that there was no essential difference on the melting mechanism between NP1 and NP2 with the same Au core size. As a representative, Figure 5 has illustrated the snapshots of NP2. Note that only snapshots of NP2−1 (Figure 5a) and NP2−5 (Figure 5b) were displayed since the other three ones with medium core/shell ratios did not present typical characteristics. In these snapshots, the concept of Lindemann atom was introduced: the atom whose Lindemann index exceeds the critical value (0.03 for Au and 0.04 for Pd) is defined as Lindemann atom; otherwise, it is noted as non-Lindemann atom. Different behaviors of the two NPs can be clearly observed by comparison of Figures 5a and 5b. At 1100 K, the original sharp corners of the Pd shell in both NPs have become obtuse, and Pd Lindemann atoms have emerged in the outermost layer. Because of the different core sizes in the two NPs, the vertexes of the Au octahedron have disappeared in NP2−1, while all the core atoms in NP2−5 were still arranged in perfect fcc order. Afterward, as the temperature was further increased, the melting began to develop from the outer layer to the interior in both NPs. However, the overall melting in NP2−1 happened at 1310 K, much lower than 1390 K for NP2−5. Moreover, the premelting of the core in NP2−1 can be clearly identified at 1300 K in the heating process. In contrast, the core with a much smaller size in NP2−5 melted simultaneously as a whole, and thereafter the core atoms diffuse into the outer layer. Besides the thermal evolution of the THH Au@Pd NPs under the heating process, as discussed above, the diffusive behavior of atoms is also an important issue that needs to be addressed because of its technological importance for application in catalysts. Generally, the diffusivity exhibits a rapid increase with elevated temperature. This trend inevitably

Figure 4. Temperature-dependent Lindemann index for Pd and Au in both types of bimetallic NPs with different compositions during the heating process. Dashed lines indicate the critical Lindemann index: 0.04 for Pd and 0.03 for Au.

melting proceeds from the surface into the interior. In addition, the critical temperatures of both the core and the shell increase

Figure 5. Snapshots of cross sections of (a) NP2−1 and (b) NP2−5 NPs taken at five representative temperatures during the heating process. Coloring denotes type of atom: green, Pd non-Lindemann atom; blue, Au non-Lindemann atom; yellow, Pd Lindemann atom; and red, Au Lindemann atom. 6900

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factor S, based on the relative position of the atoms, was proposed as follows:36

results in the component redistribution in bi/multimetallic NPs, finally altering the surface structures and thus affecting the performances of NP catalysts. Therefore, the atomic distribution of the NPs at different temperatures is of importance for exploring the catalytic properties of bi/multimetallic NPs. Furthermore, analysis of atomic distribution in the core−shell NPs is helpful for the understanding of the phase mixture or separation behaviors of bi/multimetallic NPs induced by the diffusion. Aiming to quantify the distributions of both types of toms in the NPs, the temperature-dependent atomic distribution function N(r) was calculated for Pd and Au, respectively, where N(r) dr is the number of atoms within a shell of thickness dr at r from the center of mass. Figure 6 demonstrates

S=

1 R2

1 N

N

∑ (ri 2 − R2)2 i=1

(8)

in which ri is the distance of atom i from the particle center of mass and R is the rms of ri: R=

1 N

N

∑ ri 2 i=1

(9)

According to this definition, the shape factors of spherical and cubic NPs with an fcc lattice structure, for instance, are about 0.435 and 0.515, independent of atomic number. Moreover, when the atomic number is given, the particle configuration in which more atoms are distributed far from its center of mass tends to exhibit a smaller value of S. Figure 7 demonstrates the temperature-dependent shape factors of Pd and Au during the heating process in both types of the THH Au@Pd NPs. With varying core/shell ratio, the initial value of S was fixed for the Au core (denoted as SAu) at around 0.50 due to the similar octahedral structure but different for the Pd shell (denoted as SPd) in each NP. Considering that there were more Pd atoms in NP1 located far from the center of mass than in NP2 with the same side lengths of l1 and l2, the NP1 naturally showed a smaller SPd than the NP2. Besides, for both types of NPs with fixed l1, the larger l2 led to a more “hollow” shell and hence resulted in a smaller SPd. As the NP was heated, the shape factors successively deviated from their initial values: the SPd decreased first, then increased, and ultimately stabilized at 0.45. On the contrary, the SAu distinctly increased first, then decreased, and were finally sustained at about 0.40. Since the temperature at which the shape factor reached its extremum in each NP was generally consistent with the one where the Lindemann indices accomplished the jump (see Figure 4), the variation of S from its extremum to the terminal value actually displayed the diffusion process and redistribution of atoms after the overall melting. Therefore, the fact that the terminal value of SPd was relatively greater than that of SAu indicated the tendency of Au atoms appearing near the surface. Furthermore, it can be observed from Figures 7a and 7c that with the rising temperature, the temperature where SPd began to change was 100 K lower in the NP2 than in the NP1 for the same core size. This implies that the NP1 possesses better shape stability than the NP2, in accord with the result of their thermal stabilities aforementioned. Meanwhile, the starting temperature of the variation of SPd was about 150 K lower than the one of the deviation of the corresponding Lindemann index from its linear behavior, indicating the shape transformation happened prior to the surface premelting in both types of NPs. As for the case of the Au core, it can be found from Figures 7b and 7d that the starting temperature of the variation of SAu was larger than that of SPd, and the difference between them was especially notable in the NP with the minimum core size (NP1(or NP2)−5). Additionally, before overall melting, the shape transformation of Au core experienced a narrow temperature range for small core size. These results are clearly aroused by the melting mechanism from the surface to the interior and accord with the description of Figure 5.

Figure 6. Atomic distribution functions of Pd and Au in NP2−1 at four representative temperatures during the heating process.

the N(r) of NP2−1 at four representative temperatures. As is known, the initial core−shell structure was well maintained at 300 K, which can be reflected by the two distinct peaks of Au and Pd that were relatively far apart from each other. However, at 1310 K where the overall melting took place (see Figure 5), the peak of Pd lowered slightly and was broadened into the original core area, implying the migration of Pd atoms toward the interior to a certain extent. Meanwhile, the right shift of the peak should be attributed to the thermally induced lattice expansion (see Figure 6). Correspondingly, the initial peak of Au decreased and a new peak near the surface began to rise. As the temperature continued rising, the above trend was strengthened. At 1800 K, the Au atoms were distinctly far from the center of mass and tended to preferentially aggregate near the surface, independent of their initial positions. This behavior was originated from the tendency of minimizing the total energy of Au−Pd bimetallic NPs and the fact that Au has larger atomic radius and lower surface energy than Pd (1.63 and 2.05 J/m2 for Au and Pd, respectively).24,35 A convincing evidence for this result is the decline in potential energy after the phase transition, as seen in Figure 3. The potential energy decreased more significantly with more Au atoms emerging near the surface. 3.3. Shape Evolution. The particle shape is closely associated with the surface structures. The Miller index of facets determines the shapes of polyhedra bounded by them. Therefore, the shape change is indicative of the transformation of particle’s surface. To precisely characterize the shape evolution of the THH Au@Pd NPs during heating, the shape 6901

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THH NPs as the NPs are heated to melting. After the overall melting, the enhanced atomic diffusivity makes Au atoms prefer to aggregate near the surface so as to minimize the total energy of NPs. These results suggest that the structural and thermal stabilities of THH core−shell NPs could be tunable by adjusting the surface Miller index and core/shell ratio, indicating a highly promising strategy to synthesize core− shell NPs of both excellent catalytic performance and good stability. Our findings make it possible to realize the component redistribution of bimetallic (or multimetallic) NPs by controlling the annealing process, which is desirable for improving the selectivity of catalysts for chemical reactions. This study is expected to have important implications not only to the exploitation of high-index faceted bimetallic NPs with high catalytic activity but also to the further design of multimetallic nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph (+86) 592-218-2248; Fax (+86) 592-218-9426. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21021002, 51271156, and 11204252) and the Fundamental Research Funds for the Central Universities (Grant 2012121010). We are grateful for beneficial discussions with Dr. N. Tian and Z. Y. Zhou.

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REFERENCES

(1) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 1688−1691. (2) Blakely, D. W.; Somorjai, G. A. The Stability and Structure of High Miller Index Platinum Crystal Surfaces in Vacuum and in the Presence of Adsorbed Carbon and Oxygen. Surf. Sci. 1977, 65, 419− 422. (3) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity. Science 2007, 316, 732−735. (4) Tian, N.; Zhou, Z. Y.; Sun, S. G. Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles. J. Phys. Chem. C 2008, 112, 19801− 19817. (5) Tian, N.; Zhou, Z. Y.; Yu, N. F.; Wang, L. Y.; Sun, S. G. Direct Electrodeposition of Tetrahexahedral Pd Nanocrystals with HighIndex Facets and High Catalytic Activity for Ethanol Electrooxidation. J. Am. Chem. Soc. 2010, 132, 7580−7581. (6) Ming, T.; Feng, W.; Tang, Q.; Wang, F.; Sun, L. D.; Wang, J. F.; Yan, C. H. Growth of Tetrahexahedral Gold Nanocrystals with HighIndex Facets. J. Am. Chem. Soc. 2009, 131, 16350−16351. (7) Ataee-Esfahani, H.; Wang, L.; Nemoto, Y.; Yamauchi, Y. Synthesis of Bimetallic Au@Pt Nanoparticles with Au Core and Nanostructured Pt Shell toward Highly Active Electrocatalysts. Chem. Mater. 2010, 22, 6310−6318. (8) Lee, H.; Habas, S. E.; Somorjai, G. A.; Yang, P. Localized Pd Overgrowth on Cubic Pt Nanocrystals for Enhanced Electrocatalytic Oxidation of Formic Acid. J. Am. Chem. Soc. 2008, 130, 5406−5407. (9) Lim, B.; Jiang, M. J.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. A. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324, 1302−1305. (10) Nilekar, U.; Alayoglu, S.; Eichhorn, B.; Mavrikakis, M. Preferential CO Oxidation in Hydrogen: Reactivity of Core-Shell Nanoparticles. J. Am. Chem. Soc. 2010, 132, 7418−7428.

Figure 7. Shape factors of Pd shell and Au core in NP1 (a, b) and in NP2 (c, d) during the heating process. Dashed lines indicate the initial values at low temperatures.

4. CONCLUSIONS In summary, atomistic simulations were carried out to systematically investigate the structural and thermal stabilities of two types of THH Au−Pd core−shell NPs enclosed by {210} and {310} facets, respectively. The results show that the structural stability is strongly dependent on the particle size and core/shell ratio. The THH NPs enclosed by {210} facets are slightly more stable than those bounded by {310} facets for the same side length and core/shell ratio. Furthermore, it has been revealed that the former possesses a better thermal stability than the latter. Generally, the melting proceeds from shell to core in these two high-index faceted core−shell NPs, though fairly dissimilar melting processes can be observed with the varying core sizes. Furthermore, the core and the shell exhibit different behaviors during shape evolution in both types of 6902

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(30) Kittel, C. Introduction to Solid State Physics; John Wiley & Sons: New York, 1996. (31) Qi, Y.; Cagin, T.; Johnson, W. L.; Goddard, W. A. Melting and Crystallization in Ni Nanoclusters: The Mesoscale Regime. J. Chem. Phys. 2001, 115, 385−394. (32) Huang, R.; Wen, Y. H.; Zhu, Z. Z.; Sun, S. G. Two-Stage Melting in Core-Shell Nanoparticles: An Atomic-Scale Perspective. J. Phys. Chem. C 2012, 116, 11837−11841. (33) Zhou, Y. Q.; Karplus, M.; Ball, K. D.; Berry, R. S. The Distance Fluctuation Criterion for Melting: Comparison of Square-Well and Morse Potential Models for Clusters and Homopolymers. J. Chem. Phys. 2002, 116, 2323−2329. (34) Lowen, H. Melting, Freezing and Colloidal Suspensions. Phys. Rep. 1994, 237, 249−324. (35) Mezey, L. Z.; Giber, J. Calculation of Surface Free Energies and Characteristics of Surface Segregation of Solids. Surf. Sci. 1982, 117, 220−231. (36) Wang, Z. L.; Zhong, Z. Q.; Wang, S. Y. A New Shape Factor Measure for Characterizing the Cross-Section of Profiled Fiber. Text. Res. J. 2012, 82, 454−462.

(11) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Ru-Pt Core-Shell Nanoparticles for Preferential Oxidation of Carbon Monoxide in Hydrogen. Nat. Mater. 2008, 7, 333−338. (12) Fernndez, P. H.; Rojas, S.; Ocon, P.; Fuente, J. L. G.; Fabian, J. S.; Sanza, J.; Pena, M. A.; Garcia, F. J. G.; Terreros, P.; Fierro, J. L. G. Influence of the Preparation Route of Bimetallic Pt-Au Nanoparticle Electrocatalysts for the Oxygen Reduction Reaction. J. Phys. Chem. C 2007, 111, 2913−2923. (13) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters. Science 2007, 315, 220−222. (14) Deng, Y. J.; Tian, N.; Zhou, Z. Y.; Huang, R.; Liu, Z. L.; Xiao, J.; Sun, S. G. Alloy Tetrahexahedral Pd-Pt Catalysts: Enhancing Significantly the Catalytic Activity by Synergy Effect of High-Index Facets and Electronic Structure. Chem. Sci. 2012, 3, 1157−1161. (15) Kim, D.; Lee, Y. W.; Lee, S. B.; Han, S. W. Convex Polyhedral Au@Pd Core-Shell Nanocrystals with High-Index Facets. Angew. Chem., Int. Ed. 2012, 51, 159−163. (16) Lu, C. L.; Prasad, K. S.; Wu, H. L.; Ho, J. A.; Huang, M. H. Au Nanocube-Directed Fabrication of Au-Pd Core-Shell Nanocrystals with Tetrahexahedral, Concave Octahedral, and Octahedral Structures and Their Electrocatalytic Activity. J. Am. Chem. Soc. 2010, 132, 14546−14553. (17) Sun, J. M.; Ma, D.; Zhang, H.; Liu, X. M.; Han, X. W.; Bao, X. H.; Weinberg, G.; Pfander, N.; Su, D. S. Toward Monodispersed Silver Nanoparticles with Unusual Thermal Stability. J. Am. Chem. Soc. 2006, 128, 15756−15764. (18) Yang, C. W.; Chanda, K.; Lin, P. H.; Wang, Y. N.; Liao, C. W.; Huang, M. H. Fabrication of Au-Pd Core-Shell Heterostructures with Systematic Shape Evolution Using Octahedral Nanocrystal Cores and Their Catalytic Activity. J. Am. Chem. Soc. 2011, 133, 19993−20000. (19) Zhang, L.; Zhang, J.; Kuang, Q.; Xie, S.; Jiang, Z.; Xie, Z.; Zheng, L. Cu2+-Assisted Synthesis of Hexoctahedral Au-Pd Alloy Nanocrystals with High-Index Facets. J. Am. Chem. Soc. 2011, 133, 17114−17117. (20) Huang, R.; Wen, Y. H.; Zhu, Z. Z.; Sun, S. G. Structure and Stability of Platinum Nanocrystals: From Low-Index to High-Index Facets. J. Mater. Chem. 2011, 21, 11578−11584. (21) Wen, Y. H.; Fang, H.; Zhu, Z. Z.; Sun, S. G. A Molecular Dynamics Study of Shape Transformation and Melting of Tetrahexahedral Platinum Nanoparticle. Chem. Phys. Lett. 2009, 471, 295−299. (22) Huang, R.; Wen, Y. H.; Zhu, Z. Z.; Sun, S. G. Pt-Pd Bimetallic Catalysts: Structural and Thermal Stabilities of Core-Shell and Alloyed Nanoparticles. J. Phys. Chem. C 2012, 116, 8664−8671. (23) Wen, Y. H.; Huang, R.; Li, C.; Zhu, Z. Z.; Sun, S. G. Enhanced Thermal Stability of Au@Pt Nanoparticles by Tuning Shell Thickness: Insights from Atomistic Simulations. J. Mater. Chem. 2012, 22, 7380− 7386. (24) Sankaranarayanan, S. K. R. S.; Bhethanabotla, V. R.; Joseph, B. Molecular Dynamics Simulation Study of the Melting of Pd-Pt Nanoclusters. Phys. Rev. B 2005, 71, 195415. (25) Ikeda, H.; Qi, Y.; Cagin, T.; Samwer, K.; Johnson, W. L.; Goddard, W. A. Strain Rate Induced Amorphization in Metallic Nanowires. Phys. Rev. Lett. 1999, 82, 2900−2903. (26) Cagin, T.; Kimura, Y.; Qi, Y.; Li, H.; Ikeda, H.; Johnson, W. L.; Goddard, W. A. Calculation of Mechanical, Thermodynamic and Transport Properties of Metallic Glass Formers Calculation of Mechanical, Thermodynamic and Transport Properties of Metallic Glass Formers. Mater. Res. Soc. Symp. Proc. 1999, 554, 43−48. (27) Evans, D. L.; Holian, B. L. The Nose-Hoover Thermostat. J. Chem. Phys. 1985, 83, 4069−4074. (28) Berendsen, H. J. C.; Postma, J. P. M.; Gunsteren, W. F.; van; DiNola, A.; Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684−3690. (29) Swope, W. C.; Anderson, H. C.; Berens, P. H.; Wilson, K. R. A Computer Simulation Method for the Calculation of Equilibrium Constants for the Formation of Physical Clusters of Molecules: Application to Small Water Clusters. J. Chem. Phys. 1982, 76, 637− 649. 6903

dx.doi.org/10.1021/jp401423z | J. Phys. Chem. C 2013, 117, 6896−6903