Au@Void@Ag Yolk–Shell Nanoclusters Visited by Molecular

Letter pubs.acs.org/JPCL

Au@Void@Ag Yolk−Shell Nanoclusters Visited by Molecular Dynamics Simulation: The Effects of Structural Factors on Thermodynamic Stability Hamed Akbarzadeh,*,† Esmat Mehrjouei,† and Amir Nasser Shamkhali‡ †

Department of Chemistry, Faculty of Basic Sciences, Hakim Sabzevari University, 96179- 76487 Sabzevar, Iran Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, 56199-11367 Ardabil, Iran

‡

S Supporting Information *

ABSTRACT: Au@void@Ag yolk−shell nanoclusters were studied by molecular dynamics simulation in order to study the effects of core and shell sizes on their thermodynamic stability and structural transformation. The results demonstrated that all of simulated nanoclusters with different core and shell sizes are unstable at temperatures lower than 350 K in such a way that Ag atoms are collapsed into the void space and fill it, which leads to creation of a more stable core−shell morphology, and at the melting point, only core−shell structures with altered thickness of the shell exist. Also, at higher temperatures, Au atoms tend to migrate toward the surface, and an increase of both the core and shell sizes leads to an increase of the thermodynamic stability. Moreover, a Au147@void@Ag252 nanocluster with the largest core and shell and minimum void space exhibited the most thermodynamic stability and highest melting point. Generally, the core and shell sizes affect the stability and thermal behavior of yolk−shell nanoclusters cooperatively.

B

energy storage devices,24 lithium-ion batteries,25 solar cells,26 fuel cells,27,28 sensors,29,30 surface-enhanced Raman scattering (SERS),31 photocatalysis,32,33 and nanocatalysis.34−41 There are experimental studies for synthesis and structural characterization of YSNs with different sizes (nanometer to micrometer), shapes (rod, ellipsoidal, sphere, cubic, octahedral),9−14,42−44 and core and shell chemical compositions including metal nanoparticles (NPs)@silica,45 metal oxid@ silica,46 metal NPs@carbon,47 metal NPs@metal oxide,48 metal NPs@polymer,49 silica@metal oxide,50 silica@carbon,51 and [email protected] Moreover, synthesis of multilayered shell YSNs53 and multiple core YSNs54 has been recently reported. It has been demonstrated that YSNs with metal cores such as Ag, Au, Pt, and Ni provide high catalytic activities for various reactions in comparison with solid nanoparticles made of pure metal with the same size.10,12 Ag−Au BNCs have received a lot of attention during the past decades because of their diverse use in various fields such as catalysis,55−58 sensors,59,60 optics,61−65 electronics,66−69 and medical applications.70 Recently, nanorattles with movable Au−Ag alloy cores inside of the Au−Ag alloy nanoshells have been successfully synthesized.71 Also, a yolk−shell nanostructure was prepared for Ag@Au nanoparticles based on the galvanic replacement reaction with a Au nanorod encapsulated inside of a silver nanocage.72 YSNs with Au cores exhibit promising catalytic activity for reduction of p-

imetallic nanoclusters (BNCs) containing two different metals are attractive materials in a variety of fields of science and technology due to their possibility for designing novel structures with specific properties. These properties can be tuned by varying the chemical composition, atomic ordering, shape, and size.1 Core−shell (CS) nanostructures, one of the common BNCs, are composite materials where an inner core is surrounded by one or more extra shells.2 In recent years, much interest has been focused on CS structures due to their superior and tunable catalytic properties, which can be tailored by adjusting the elemental composition, size of the core, thickness of the shell, and their interface.3,4 For example, bimetallic CS nanoclusters such as Au−Ag ones have exhibited higher catalytic activity in comparison with the corresponding monometallic nanoclusters, which have been widely used in various reactions.5,6 The CS structures for which the core contains one material surrounded by a shell of another material, with suitable geometries and configurations, can be used for the fabrication of new and complex nanoarchitectures such as hollow CS structures via removal of the core.7,8 A combination of CS structures and the large empty cavity of the hollow CS structure represents a unique class of CS structures with a characteristic core@void@shell configuration, which is known as rattle CS or yolk−shell nanoparticles (YSNs), and shows the merits of both structures.9−14 YSNs with unique and interesting properties, that is, low density, void space between the core and the shell, a movable core, and the ready tailor-ability and functionality of both the core and shell, can be used in many applications including drug/gene delivery and biomedical imaging,15−23 © XXXX American Chemical Society

Received: April 21, 2017 Accepted: June 15, 2017 Published: June 15, 2017 2990

DOI: 10.1021/acs.jpclett.7b00978 J. Phys. Chem. Lett. 2017, 8, 2990−2998

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The Journal of Physical Chemistry Letters nitrophenol and 2-nitroaniline, oxidation of aerobic alcohol, and CO oxidation.34−37,41 The enhanced catalytic activity of YSNs can be related to the movable core and the void space between the core and the shell, which provide a homogeneous environment for the reaction to take place and create more exposed active sites.73−76 Understanding the thermal and structural properties of YSNs is particularly important for their applications. Although there are experimental investigations on synthesis, structural, characterization, and catalytic activity of YSNs, an examination on the thermal behavior of YSNs has not been reported up to this time. The energetic stability and structural and physical properties of YSNs may be changed significantly with increasing temperature compared to the bulk material. From the point of view of catalytic application, it is essential to know the highest temperature at which the YSN presents a special stability. For this reason, the study and investigation of thermal behavior of YSNs under prolonged heating is important. It seems that YSNs are a new and novel class of largely unexplored nanomaterials that may demonstrate thermal behavior different and more complex than that of other CS nanoparticles. It is expectable that thermal properties of YSNs may be tuned by the size of the core and the separation between the core and the shell (void). A theoretical study that investigates the effect of the core and the void sizes on thermal behavior of YSNs has not been performed. In this work, we investigate the effects of core and shell sizes on the thermodynamic stability and thermal behavior of Au@ void@Ag YSNs with an Au core coated with a Ag shell by molecular dynamics (MD) simulation. Then, the obtained results will be discussed using various analysis methods of simulation data. In this study, Au@void@Ag yolk−shell nanoclusters composed of a Au inner core, a Ag outer shell, and the void inner space between the core and the shell were simulated. In fact, the Au core was encapsulated by the outer layer of the Ag. In order to investigate the effect of shell size, Au13@void@Ag92, Au13@void@Ag162, and Au13@void@Ag252YSNs with an equal number of Au atoms in the core and variable shells were selected. Also, in order to investigate the effect of the core size, Au13@void@Ag252, Au55@void@Ag252, and Au147@void@ Ag252YSNs were compared. The snapshots of equilibrium structures of these nanoclusters at T = 300 K are shown in Figure 1. It is noticeable that previous experimental studies indicated that in YSNs the core region is not symmetrically located at the center of the nanocluster.77−81 The software used in our simulation was DL_POLY 4.03.82 The equations of motion were integrated through the Verletleapfrog algorithm.83 In order to study the thermal behavior, the whole YSNs were heated in an NVT ensemble from 1 to 1200 K in 10 K steps. The temperature of the systems was controlled using a Nosé−Hoover thermostat84,85 with a relaxation time of 0.1 ps. At each temperature, the MD simulation time was equal to 10 ns, and also, the cutoff radius for all of the simulations was taken to be 15 Å. Au−Au, Ag−Ag, and Ag−Au interactions were simulated by many-body quantum Sutton−Chen (QSC) potentials.86,87 There are several previous studies that reported using QSC potentials for metal−metal interactions with appropriate results.88−91 It is possible for the results of melting points to be under- or overestimated. However, it is noticeable that the results are compared to each other, but concise values of the melting points are not the aim of this work. The QSC parameters for

Figure 1. Snapshots of the atomic arrangement of Au@void@Ag YSNs with variable shells and variable cores including (a) Au13@ void@Ag92, (b) Au13@void@Ag162, (c) Au13@void@Ag252, (d) Au55@void@Ag252, and (e) Au147@void@Ag252 and (f) a schematic illustration of the mentioned YSNs. Coloring denotes the type of atom: violet, Ag atom; yellow, Au atom.

Ag−Ag, Au−Au, and Au−Ag interactions are given in Table S1 of the Supporting Information. For Au−Ag interactions, the geometric mean was used to calculate the energy parameter ε, while the arithmetic mean was used for the remaining parameters (a, n, m, and c). (I) Ef fect of Shell Size. We have presented the configurational energies per atom for the Au@void@Ag YSNs with variable shells including Au13@void@Ag92, Au13@void@Ag162, and Au13@void@Ag252 at different temperatures in the heating process in Figure 2. According to this figure, we can identify a

Figure 2. Configurational energies and isochoric heat capacities per atom versus temperature for Au@void@Ag YSNs with variable shells.

simple jump in every curve for each nanocluster, which indicates the temperature range of the phase transition. In order to specify the melting points with more accuracy, the isochoric heat capacities (Cv) per atom for the nanoclusters at different temperatures in the heating process are illustrated in Figure 2. According to this figure, the first-order phase transition can be identified. It can be observed that the temperature corresponding to the maximum peak value in the heat capacity is in good agreement with a simple jump of the caloric curve.92,93 It can be observed from Figure 2 that for all of the simulated 2991

DOI: 10.1021/acs.jpclett.7b00978 J. Phys. Chem. Lett. 2017, 8, 2990−2998

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where NAu and εcoh Au are the number and the bulk cohesive energy of Au atoms, respectively. The similar meaning is also Au−Ag is the calculated total energy of stands for NAg and εcoh Ag . Etotal the nanocluster, and N= NAu+ NAg is its total number of atoms. Therefore, a lower Δ value exhibits more thermodynamic stability of the nanocluster. The calculated Δ values versus temperature for Au13@void@Ag92, Au13@void@Ag162, and Au13@void@Ag252 nanoclusters are illustrated in Figure S4 of the Supporting Information. On the basis of Δ values, the stability order of mentioned nanoclusters is found to be Au13@ void@Ag92 < Au13@void@Ag162 < Au13@void@Ag252, which is in good agreement with data of the configurational energy and heat capacity. Also, the changes of the numbers of Au−Au, Au−Ag, and Ag−Ag bonds for Au@void@Ag YSNs with different shell sizes throughout the heating process are shown in Figure S3 of the Supporting Information. It can be observed that for all of the simulated nanoclusters Au−Au bonds at temperatures above 600 K are reduced to a constant value, which confirms occupation of the void space by Ag atoms and stability of created CS structures. However, by increasing the temperature, the number of Ag−Ag bonds decreases, while the number of Au−Ag bonds remains approximately constant. This indicates the start of melting in the shell region before melting of the whole nanocluster. In order to further monitor the structural evolution of Au@ void@Ag YSNs, we applied the common neighbor analysis (CNA)97 to characterize the local crystal structure. Each bond that connects a central atom and its nearby neighbors is characterized by a set of four characteristic numbers ijkl. These numbers depend on whether the atoms are nearest (1) or nextnearest (2) neighbors (i), the number of nearest neighbors that they have in common (j), the number of bonds among these common neighbors (k), and the number of bonds in the longest continuous chain of bonds connecting the common neighbors (l).98The different types of pairs are associated with different types of local order. All bonded pairs in a fcc crystal are of type 1421, whereas a hcp crystal has equal numbers of types 1421 and 1422. In this work, each atom in a nanocluster can be classified according to its local crystal structure. Atoms in a local fcc order were considered to be fcc atoms; atoms in a local hcp order were classified as hcp atoms; and atoms in all other local orders were collectively considered to be “other” atoms because they did not reveal any useful structural information for the nanocluster. In this study, the CNA is given as percentages. We have presented CNA percents as a function of temperature for Au@void@Ag YSNs with variable shells in Figure 3. As can be seen in this figure, for the simulated nanoclusters, percents of hcp and fcc atoms at 1 K are less than 1% due to the small size of the core and small coordinaion number of Ag atoms at the shell region and also the existence of void space between these regions. At temperature ranges between 200 to 350 K, a sudden increase of fcc and hcp atoms accompanied by an abrupt decrease of other atoms indicates that an unstable Au@void@Ag structure tends to be converted to a more stable structure with higher local order by collapse of the void space when Ag atoms enter into it; see the snapshots of Figure 3. Also, it can be observed that by increasing the shell size, which is accompanied by an increase of the void space, rearrangement of Ag atoms in that space is taken place in such a way that interatomic interactions become more strengthened and the percents of fcc and hcp atoms are increased. Therefore, the Au13@void@Ag252 nano-

nanoclusters configurational energies and heat capacities are decreased from 200 to 350 K, which indicates structural transformations and instability of yolk−shell structures at higher temperatures. Also, this instability of the yolk−shell regime can be observed from Figure S1 of the Supporting Information, which indicates the volume of void space as a function of temperature, and its results are in good agreement with the mentioned discussion. We have also presented the melting temperatures in Table 1. It can be observed that for Table 1. Melting Points of Au@Void@Ag YSNs with Variable Shells and Cores composition (variable shell)

melting point (K)

composition (variable core)

melting point (K)

Au13@void@Ag92 Au13@void@Ag162 Au13@void@Ag252

620 680 700

Au13@void@Ag252 Au55@void@Ag252 Au147@void@Ag252

700 770 820

Au@void@Ag YSNs containing the cores with the same size, an increase of shell size, which is accompanied by an increase of void size and the number of Ag atoms in the shell, leads to an increase of the melting point. Because the influence of the surface of the nanocluster on its stability and melting point has been confirmed by several previous studies,94,95 in order to better understand and interpret the energy and heat capacity curves, changes in percent of surface atoms during the heating process for Au@void@Ag YSNs with different shell sizes are given in Figure S2 of the Supporting Information. As shown in this figure, the percent of Ag surface atoms is approximately constant for larger shell sizes, which resembles creation of CS structures at temperatures below 350 K and instability of the yolk−shell structure under these circumstances. Therefore, by increasing the temperature, Ag atoms tend to diffuse toward the void and completely occupy the void space. Because Ag atoms in the shell have lower coordination numbers, the shell structure has less stability than the core. Therefore, Ag atoms tend to diffuse toward the core and fill the void in order to increase their coordination number and reach higher stability, which leads to a new morphology of the nanocluster with higher thermodynamic stability. It can be seen that Au13@void@Ag252 nanoclusters have the lowest configurational energy and highest thermodynamic stability. However, Au13@void@Ag92 with the lowest void space has the highest configurational energy and lowest thermodynamic stability. It seems that an increase of shell size and void space provides better opportunity for the nanocluster in order to reconstruct and reach a more stable structure. Moreover, Figure S2 indicates that at the melting point of the Au13@void@Ag92 nanocluster with the lowest void space an increase of Au atoms in the surface is accompanied by a decrease of surface Ag atoms, which demonstrates easier diffusion of Au atoms to occupy surface sites due to the smallest size of this cluster, whereas for larger ones, this variation of Ag surface and Au core atoms is not significant due to the stability of the created CS structures. The increase of thermodynamic stability by shell size also can be understood from excess energy, the Δ parameter, which can be defined as96 Δ=

Au−Ag coh coh − NAuεAu − NAgεAg Etotal

(NAu + NAg)2/3

(1) 2992

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other atoms (64.9%) has the highest stability and melting point in comparison with the other ones (for Au13@void@Ag92, Pfcc ≈ 1.9%, Phcp = 2.9%, and Pother = 95.2%, and for Au13@void@ Ag162, Pfcc ≈ 0, Phcp = 19.4%, and Pother = 80.0%). By increasing the temperature, a sudden increase of other atoms and a decrease of fcc and hcp atoms to 0% indicates melting of the nanocluster. In order to examine trends in chemical order as a function of temperature, it is convenient to define an order parameter. The extent of mixing (chemical order parameter), σ, can be defined as99 σ=

NA−A + NB−B − NA−B NA−A + NB−B + NA−B

(2)

where NA−A and NB−B refer to the number of A−A and B−B nearest-neighbor bonds within the binary cluster and NA−B is the number of nearest-neighbor A−B bonds in it. σ = 1 indicates complete separation of core and shell regions, σ ≈ 0 stands for disordered mixing, and σ < 0 indicates more ordered mixing, such as layering and onion-like configurations. The variations of σ versus temperature for Au@void@Ag YSNs with variable shells are represented in Figure S5 of the Supporting Information. In this figure, σ values below 600 K are approximately constant, which indicates stability of created CS structures. The strong drop of σ values at lower temperatures resembles occupation of void space and creation of CS structures. By increasing the temperature, fluctuations of the σ parameter are seen, which can be related to variations of Ag−Ag, Ag−Au, and Au−Au bonds during the heating process and the start of melting. Near the melting and at the melting point, a small decrease of the σ parameter confirms diffusion of Au atoms toward the shell region and an increase of Ag−Au interactions accompanied by a decrease of Ag−Ag and Au−Au bonds, which indicates creation of a structure with disordered mixing. In order to better determine the melting behavior of the simulated nanoclusters, snapshots from their cross sections at different temperatures are illustrated in Figure 4. As is obvious, the melting process starts from the shell region and then extends to the other parts. This temperature-induced phase

Figure 3. CNA percentages as a function of temperature for Au@ void@Ag YSNs with variable shells. The corresponding snapshots of cross sections of the nanoclusters are presented as the insets (the first is the structure at 1 K, the second is the solid−solid transformation of the structure, and the last is the distribution of other, hcp, and fcc atoms at the solid−solid transformation). Coloring denotes the type of atom: violet, Ag atom; yellow, Au atom; blue, other atom; pink, hcp atom; and light green, fcc atom.

cluser with the largest number of fcc and hcp atoms (respectively 12.5 and 22.6%) and the lowest number of

Figure 4. Snapshots of cross sections of Au@void@Ag YSNs with variable shells including (a) Au13@void@Ag92, (b) Au13@void@Ag162, and (c) Au13@void@Ag252, taken at six representative temperatures during the heating process. Coloring denotes the type of atom: violet, solid Ag atom; yellow, solid Au atom; red, liquid Ag atom; and green, liquid Au atom. 2993

DOI: 10.1021/acs.jpclett.7b00978 J. Phys. Chem. Lett. 2017, 8, 2990−2998

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The Journal of Physical Chemistry Letters transition provides an opportunity for Au atoms to diffuse toward the surface of the nanocluster. This diffusion at higher temperatures leads to more deformation of the nanocluster. Also, for nanoclusters with larger shell sizes, there are more atoms in the solid state, which implies their thermodynamic stability, which is in good agreement with the mentioned results. Moreover, creation of structures with disordered mixing at the melting point can be observed in the snapshots of Figure 4. The radial distribution function (RDF or g(r)) is one of the most important structural quantities for characterization of a system. The RDF is defined as100

( (

1 N r± g (r ) = ρ V r±

Δr 2 Δr 2

) )

(3)

Figure 5. Configurational energies and isochoric heat capacities per atom versus temperature for Au@void@Ag YSNs with variable cores.

( Δ2r ) is the number of atoms in the interval Δr Δr r ± 2 , V (r ± 2 ) is the volume of this interval, and ρ is the where N r ±

local density of atoms. The Ag−Ag, Au−Au, and Au−Ag RDFs for Au@void@Ag YSNs with variable shells at the initial state at 1 K, at the melting point, and at 100 K after melting point are presented in Figures S6−S8 of the Supporting Information. We have also showed the corresponding snapshots of these nanoclusters as the insets in Figure S6. In these figures, it can be observed that RDF peaks of metal−metal pairs for all of the simulated nanoclusters become broader at the melting point due to the loss of their initial ordered structure. For RDF plots of Ag−Ag and Ag−Au pairs in Figures S6 and S7, the pattern and position of peaks at 1 K are similar, which indicates the similarity of shell structures with different sizes. Also, the heights of the first RDF peak for Ag−Ag pairs are smaller for the nanoclusters with larger shell sizes, which indicates their higher shell density because g(r) has a reverse relation with density.101 At the melting point and 100 K after that, the number of RDF peaks decreased, which indicates loss of structural order of the solid state to a disordered structure of the liquid phase. Also, Figure S8 exhibits that at the melting point and 100 K after, by increase of shell size, heights of RDF peaks of Ag−Au pairs are decreased because for nanoclusters with larger shell sizes the number of collapsed Ag atoms toward the void space is higher, which leads to an increase of density and reduces the g(r) value. It is noticeable that beside RDFs, radial chemical distribution functions (RCDFs) can be helpful, too.101,102 However, due to the asymmetric location of the void space inside of the nanocluster, RCDFs may not give helpful results for YSNs simulated in this study. (II) Ef fect of Core Size. In order to study the effect of core size on the thermal stability and melting behavior of Au@ void@Ag nanoclusters, similar to Figure 2, configurational energies and Cv values per atom of Au13@void@Ag252, Au55@ void@Ag252, and Au147@void@Ag252 nanoclusters versus temperature are presented in Figure 5. Also, melting points of these nanoclusters are given in Table 1. Considering these results, it can be concluded that the melting points of simulated nanoclusters are increased by their core size. In order to interpret this result, structural changes during the heating process are investigated using CNA plots. The percents of fcc, hcp, and other atoms throughout the heating process for Au@ void@Ag nanoclusters with different core sizes are illustrated in Figure 6. At is obvious from this figure, nanoclusters with

Figure 6. CNA percentages as a function of temperature for Au@ void@Ag YSNs with variable cores. The corresponding snapshots of cross sections of the nanoclusters are presented as the insets (the first is the structure at 1 K, the second is the solid−solid transformation of the structure, and the last is the distribution of other, hcp, and fcc atoms at the solid−solid transformation). Coloring denotes the type of atom: violet, Ag atom; yellow, Au atom; blue, other atom; pink, hcp atom; and light green, fcc atom.

different core sizes exhibit different behavior in CNA plots. For Au13@void@Ag252 at 1 K, percents of fcc and hcp local crystals are negligible due to the small size of the core and small coordinaion number of Ag atoms at the shell region and also due to the existence of void space between these regions. By increasing the temperature up to 350 K, a sudden increase of fcc and hcp atoms indicates collapse of Ag atoms toward the void space and filling it with a more stable morphology. Also, by decreasing the core size, the sudden increase of fcc and hcp 2994

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Figure 7. Snapshots of cross sections of Au@void@Ag YSNs with variable cores including (a) Au13@void@Ag252, (b) Au55@void@Ag252, and (c) Au147@void@Ag252, taken at six representative temperatures during the heating process. Coloring denotes the type of atom: violet, solid Ag atom; yellow, solid Au atom; red, liquid Ag atom; and green, liquid Au atom.

S11 of the Supporting Information. The σ < 1 value at temperatures below 600 K resembles the stability of created CS strctures and occupation of the void space at lower temperatures. The sudden fall of the σ parameter after 600 K also indicates more mixing of Au and Ag atoms in order to diffuse these atoms in opposite directions. Also, it can be observed that by increasing the core size, the decrease of the σ parameter at the melting point is more obvious, which indicates the tendency of the nanoclusters with larger core sizes to create structures with more disordered mixing. The Ag−Ag, Au−Au, and Au−Ag RDFs for Au@void@Ag YSNs with variable cores at the initial state at 1 K, at the melting point, and at 100 K after the melting point are presented in Figures S12−S14 of the Supporting Information. We have also showed the corresponding snapshots of these nanoclusters as the insets in Figure S12. It can be observed in Figure S12 that by increasing the core size, the peaks of Ag−Ag interactions are decreased, which can be related to the decrease of void space and increase of density for larger core sizes. Moreover, Figure S13 indicates that at the melting point and after that, a decrease of Au atoms in the core and decrease of the core density leads to a decrease of the Au−Au first RDF peak. It is noticeable that, despite the existence of available investigations on synthesis, characterization, and chemical activities of YSNs, their thermal behavior has been unknown. Therefore, molecular simulation methods can give important information for these types of nanomaterials. A similar problem exists for other types of new nanomaterials such as hollow bimetallic nanoparticles.103 In this study, effects of sizes of the core and shell regions on thermodynamic stability and structural transformations of Au@ void@Ag yolk−shell nanoclusters were investigated by MD simulation. The results indicated that all of the simulated nanoclusters are unstable at temperatures below 350 K. As the void is lost at approximately 350 K, the melting point is concerned with only core@shell nanoparticles, where the shell thickness is altered. In these nanoparticles, Ag shell atoms with smaller coordination numbers are collapsed toward the void space and fill it in order to increase their coordination numbers

atoms at 350 K disappeared, and before 600 K, these values are approximately constant for Au55@void@Ag252 and Au147@ void@Ag252 nanoclusters. This can be related to more rapid collpase of void space for larger core sizes, which leads to creation of stable CS structures at low temperatures because by increasing the core size, smaller void space is available (see Figure S1 of the Supporting Information). Because Au−Au interactions are stronger than Ag−Ag ones, it is expected that by increasing the Au concentration, the melting point of the nanocluster can be increased. This is in good agreement with data of Figure 5 in which a Au13@void@Ag252 < Au55@void@ Ag252 < Au147@void@Ag252 trend is obtained for thermodynamic stability. In Figure 7, snapshots of cross sections of Au@ void@Ag nanoclusters with different core sizes at six temperatures including 1 K, before melting, and at the melting point are presented. It can be concluded that melting starts from the shell region, generally. After the melting point, disordered mixing is observed, which is in good agreement with data of Figure 6. The changes in the percent of surface atoms of Au@void@ Ag YSNs with different core sizes during the heating process are given in Figure S9 of the Supporting Information. Before 600 K, the percent of surface atoms remains constant. However, after 600 K, a considerable decrease of surface Ag atoms accompanied by a valuable increase of Au surface atoms for nanoclusters with larger core sizes indicates diffusion of Au atoms toward the surface and migration of Ag atoms to the core in order to create a mixed structure near the melting point in such a way that the Au147@void@Ag252 nanocluster with the largest number of Au atoms exhibits the largest percent of surface Au atoms at the melting point. The thermal changes of the numbers of various metal−metal bonds for Au@void@Ag YSNs with different core sizes throughout the heating process are illustrated in Figure S10 of the Supporting Information. The sudden increase of Ag−Au bonds and the decrease of Au−Au ones near the melting point for nanoclusters with larger core sizes confirm the tendency of Au atoms to occupy the shell region at higher temperatures. The changes of the σ parameter versus temperature for Au@ void@Ag YSNs with different core sizes are shown in Figure 2995

DOI: 10.1021/acs.jpclett.7b00978 J. Phys. Chem. Lett. 2017, 8, 2990−2998

Letter

The Journal of Physical Chemistry Letters

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and create a more stable CS structure. Alternatively, Au atoms at higher temperatures tend to diffuse toward the surface of the nanocluster. Also, the Au147@void@Ag252 nanocluster with the largest shell and core size and minimum void space creates the most stable structure and largest melting point. Therefore, shell and core sizes of yolk−shell nanoclusters are two important cooperative factors that effect their thermodynamic stability and melting behavior. These results are expected to be useful for construction and production of YSNs with special stabilities and also for better understanding of thermal behavior of yolk− shell bimetallic nanocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00978. Void space, percents of surface atoms, numbers of metal−metal bonds, Δ parameter, chemical order parameter versus temperature for simulated nanoclusters with different shell and core sizes, RDF plot for metal− metal pairs, and details of quantum Sutton−Chen potentials for the related metal−metal interactions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98 915 3008670. Fax: +98 571 400332. ORCID

Hamed Akbarzadeh: 0000-0001-6880-9365 Amir Nasser Shamkhali: 0000-0003-3376-3672 Notes

The authors declare no competing financial interest.

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REFERENCES

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