Thermal Stability of Platinum-Cobalt Bimetallic Nanoparticles

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Thermal Stability of Platinum−Cobalt Bimetallic Nanoparticles: Chemically Disordered Alloys, Ordered Intermetallics, and Core− Shell Structures Rao Huang,† Gui-Fang Shao,‡ Yang Zhang,§ and Yu-Hua Wen*,† †

Institute of Theoretical Physics and Astrophysics, Department of Physics, and ‡Research Center for Cloud Computing and Big Data, Department of Automation, Xiamen University, Xiamen 361005, China § Department of Applied Physics, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: Pt−Co bimetallic nanoparticles are promising candidates for Pt-based nanocatalysts and magnetic-storage materials. By using molecular dynamics simulations, we here present a detailed examination on the thermal stabilities of Pt−Co bimetallic nanoparticles with three configurations including chemically disordered alloy, ordered intermetallics, and core−shell structures. It has been revealed that ordered intermetallic nanoparticles possess better structural and thermal stability than disordered alloyed ones for both Pt3Co and PtCo systems, and Pt3Co−Pt core−shell nanoparticles exhibit the highest melting points and the best thermal stability among Pt−Co bimetallic nanoparticles, although their meltings all initiate at the surface and evolve inward with increasing temperatures. In contrast, Co−Pt core− shell nanoparticles display the worst thermal stability compared with the aforementioned nanoparticles. Furthermore, their melting initiates in the core and extends outward surface, showing a typical two-stage melting mode. The solid−solid phase transition is discovered in Co core before its melting. This work demonstrates the importance of composition distribution to tuning the properties of binary nanoparticles. KEYWORDS: metallic nanoparticle, thermal stability, alloy, core−shell structure, molecular dynamics

1. INTRODUCTION With rapidly increasing energy demands and gradual depletion of fossil fuel reserves, the exploration of energy conversion devices with high efficiency and low emissions has been greatly urged during the past decade. Fuel cells, such as polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are considered to be promising candidates to meet these requirements.1,2 Platinum (Pt) has been frequently used as the main catalyst in fuel cells as well as petrochemical and automobile industries in the past decades.3 However, the limited resources of Pt and its high cost become a significant barrier to its widespread applications. Moreover, Pt is prone to being poisoned by carbon monoxide when used as anode material, and it cannot exhibit completely satisfactory activity for the oxygen reduction reaction (ORR) when used as cathode material.4 To achieve highly active catalysts with reduced Pt loading while enhanced catalytic efficiency, considerable exploration has been made on Pt-based bi- and multimetallic catalysts by alloying Pt with transition metals.5−7 In general, Pt-based catalysts exhibit different properties from their monometallic counterparts owing to the synergistic effects of geometry and electronic effects between metals.5,8 Therefore, their catalytic performances can be virtually tunable because an additional degree of freedom for modifying the geometric and electronic structures is applicable by changing their composition and size.8 Among Pt-based catalysts, Pt−Co systems have © XXXX American Chemical Society

attracted tremendous interest because of their relatively high ORR activity and stability.8−29 The available studies have verified that alloying Co with Pt at the ratio of 1:3 gives higher catalytic activity than many transition metals with Pt of different compositions.9,10 The observed improvement in activity after alloying has generally been attributed to the modification of surface structure and electronic structure including a lower oxidation state of Pt, which can suppress the formation of Pt oxide, a shorter Pt−Pt bond distance that favors adsorption of oxygen molecules, increased 5d orbital vacancies, and the formation of a thin Pt skin on the surface of alloy composite.11−16 Additionally, Pt−Co nanoparticles have been considered as the promising candidates for applications in ultrahigh-density data storage devices because of their magnetism, large uniaxial anisotropy, and high chemical stability.17−22 These favorable factors make Pt−Co nanoparticles display prospective applications in fields of catalysis, electronics, magnetic recording media, and biotechnology. To date, Pt−Co bimetallic nanoparticles with various structures and morphologies have been experimentally synthesized by chemical and physical approaches.8−29 According to the atomic distributions of elements, these nanoparticles Received: January 26, 2017 Accepted: March 28, 2017 Published: March 28, 2017 A

DOI: 10.1021/acsami.7b01337 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of Pt−Co bimetallic nanoparticles: (a) disordered Pt3Co alloy (Pt3Co-DA), (b) disordered PtCo alloy (PtCo-DA), (c) ordered Pt3Co L12 intermetallics (Pt3Co-OI), (d) ordered PtCo L10 intermetallics (PtCo-OI), (e) Co−Pt core−shell structure (Co@Pt), and (f) Pt3Co−Pt core−shell structure (Pt3Co@Pt). Coloring denotes the type of atom: blue, Pt atom; bright green, Co atom.

ordered intermetallics, and core−shell structures. Therefore, they could exhibit catalytic, electrical, magnetic, and optical properties remarkably different from those of their monometallic counterparts. As is known, the thermal stability of bimetallic and multimetallic nanoparticles is an important issue that needs to be clarified mainly owing to the following four aspects. (i) Metallic nanoparticles tend to aggregate into larger ones when the ambient temperature exceeds the Tammann temperature.33 Hence, to reasonably control the synthesis and work temperature is of technological significance for suppressing the sintering and coarsening of the nanoparticles. (ii) The composition distribution may vary with temperatures due to the differences of alloying elements in cohesive energy, surface energy, atomic radius, and diffusivity. (iii) Bimetallic and multimetallic nanoparticles, which are formed at low temperature by applying chemical reduction methods, generally have incomplete mixing. Therefore, to understand their thermodynamic behaviors at different temperatures is crucial for achieving good alloying.8 (iv) The order−disorder phase transition has been discovered in bimetallic nanoparticles. It includes the solid−solid transformation such as the transition from ordered L10 structure to disordered alloy in Pt−Co nanoparticles27 and solid−liquid transformation in metallic nanoparticles.34−36 These transitions all belong to the thermally driven behavior and generally take place at elevated temperatures. To sum up, an exploration on the thermal stability of Pt−Co bimetallic nanoparticles is considerably helpful not only for suppressing their sintering and coarsening but also for understanding the structure−temperature relationship and thus stabilizing their structures at high temperatures. Despite the continuously growing reports on synthesis, characterization, and performance of Pt−Co bimetallic nanoparticles, there has still lacked an elaborate study on their

may be classified as chemically disordered alloy (solid solution), ordered intermetallics, and core−shell structures, depending on the preparation strategy and route. Numerous researchers have reported that Pt−Co (typical Pt3Co, PtCo, and PtCo3) alloy nanoparticles were chemically disordered, and most of them were coated by Pt-skin surface layer due to the thermally driven preferential surface segregation of Pt or preferential dissolution of the non-noble component during catalyst processing and operation of the fuel cell.8,9,11−18,20−26 These nanoparticles all possess face-centered cubic (fcc) crystal phases with a concomitant lattice contraction for Pt due to the incorporation of Co atoms. Meanwhile, structurally ordered intermetallic Pt− Co bimetallic nanoparticles have also been reported.10,19,27 For example, ordered Pt3Co intermetallic cores (L12 structure), covered by a 2−3 atomic-layer thick Pt shell, exhibited over 200% increase in mass activity and over 300% increase in specific activity compared with disordered Pt3Co alloy nanoparticles and the Pt/C catalysts.10 Besides, the ordered Pt−Co intermetallic nanoparticles (L10 structure) have also been prepared experimentally.19,27 The long-range order structures in these nanoparticles exhibit a large coercivity owing to the rotation magnetization, which makes them become one of the candidates for future high-density magnetic storage media. Apart from the aforementioned two types of bimetallic nanoparticles, another type which has been successfully synthesized is the Co-core/Pt-shell (Co@Pt) structure.7,17,28,29 The electrochemical tests showed that the core−shell nanoparticles exhibit superior activity and durability for methanol oxidation than Pt-alloy catalysts.28,29 In addition, the stable existence of the ordered intermetallics and core−shell structures has also been theoretically confirmed.30−32 As stated above, Pt−Co bimetallic nanoparticles present diverse structures and morphologies including disordered alloy, B


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Their values were A = 0.242 eV, ξ = 2.506 eV, p = 11.14, q = 3.68 for Pt and A = 0.189 eV, ξ = 1.907 eV, p = 8.80, q = 2.96 for Co.38 Besides, A = 0.245 eV, ξ = 2.386 eV, p = 9.97, and q = 3.32 were used to describe the interatomic action between Pt and Co.39 The cutoff radius for both metals was fixed at the fifth-neighbor distance of Pt atom. Computational Details. As is known, the real environments where metallic nanoparticle catalysts work are complicated and diverse. Because of the lack of the effective potentials to describe the interatomic interactions between metals and their surrounding materials such as water, oxygen, and hydrogen, or hydroxyl ions, in this study all the nanoparticles were placed in vacuum to explore their intrinsic thermal properties. To examine the thermal stability of Pt− Co bimetallic nanoparticles, we have employed molecular dynamics (MD) approaches to investigate their structural evolutions under different temperatures. Upon starting MD simulations, the conjugategradient techniques were first used to minimize the total energy of the nanoparticles at zero temperature. After initial energy minimization, the nanoparticles were subjected to a continuous heating process. To make the simulations more realistic, constant temperature and pressure molecular dynamics (NPT-MD) were employed to allow energy and volume fluctuations. These nanoparticles experienced the heating process consisting of a series of NPT-MD simulations from 0 to 2100 K with a temperature increment of 50 K. A smaller step of 10 K, however, was adopted to examine the melting behavior more accurately when the temperature was close to the melting point. Based on our previous experiences,34−36 the MD simulations were carried out for 200 ps at each temperature, during which atomic coordinates, velocities, and potential energies were extracted for calculation of the statistical quantities in the last 25 ps. We respectively employed the Nosé−Hoover thermostat40 and Berendsen approach41 to maintain the nanoparticle system at desired temperature and ambient pressure. The equations of atomic motion were integrated by the Verlet-velocity algorithm42 with 1 fs time step.

thermal stability and associated thermodynamic behaviors. In this article, we address on the thermal stability and structural evolution of Pt−Co bimetallic nanoparticles under heating process by molecular dynamics simulations. Different structures and morphologies including chemically disordered alloy (solid solution), ordered intermetallics such as L10 and L12 structures, and core−shell structures are considered for Pt−Co bimetallic nanoparticles. A comparison of results with pure Pt and Co nanoparticles is also presented. This article is structured as follows. Section 2 concisely describes the simulation methods. Section 3 presents the calculated results, discussion, and comparison with available studies. The main conclusions are summarized in section 4.

2. EXPERIMENTAL SECTION Model Construction. In general, Pt−Co bimetallic nanoparticles can be constructed from large single crystal by using a certain cutoff radius. For the convenience of comparison, the total number of atoms in these nanoparticles was set to be about 20 500 (varying a little with lattice structures). To investigate the influence of structural order degree, we have considered disordered Pt−Co alloy (including Pt3Co and PtCo, respectively denoted as Pt3Co-DA and PtCo-DA) and ordered intermetallics (including Pt3Co L12 and PtCo L10 structures, respectively denoted as Pt3Co-OI and PtCo-OI), as illustrated in Figures 1a−d. Note that Pt and Co atoms in the Pt−Co alloyed nanoparticles are distributed randomly in the fcc lattice (see Figures 1a,b), consistent w ith the experimental characterizations.5,9,11,13−16,23,26 Considering that core−shell structures have been synthesized experimentally in Pt−Co nanoparticles,7,17,28,29 here we have also modeled Co−Pt core−shell (denoted as Co@Pt) nanoparticles, as illustrated in Figure 1e. Note that in Co@Pt nanoparticle Co atoms are arranged in hexagonal close packing (hcp) lattice and Pt ones are arranged in fcc lattice, in agreement with experimental results.17,28 Furthermore, the atomic ratio of Pt and Co in core−shell structure was set to be 1:1 for comparison with PtCo disordered alloy and ordered intermetallic nanoparticles. Pt@Co nanoparticle was not considered in this work because this morphology is chemically uninteresting due to its poor catalytic properties and has not been experimentally reported up to date.37 Lastly, considering that a novel type of Pt−Co nanocatalysts, which is composed of ordered Pt3Co intermetallic cores with two or three atomic layer thick Pt shell, has been successfully prepared and exhibited superior activity for the ORRs,10 it has also been modeled in this work, as illustrated in Figure 1f. Besides, both Pt and Co monometallic nanoparticles were constructed for comparison with Pt−Co bimetallic ones (not shown in Figure 1). Potential Description. To describe the interatomic interactions of metals, here we have employed the Gupta potentials, which were extensively adopted in the past decades to examine a variety of problems in materials science by atomistic simulations. The Gupta potentials for the metal bond are based on the second-moment approximation of the tight-binding scheme (TB-SMA), showing a good description of thermal and transport properties of transition metals and their alloys.38 They contain Born−Mayer-type repulsive pair interaction and many-body attractive effective band energy. The total energy for a system of N atoms can be written as

3. RESULTS AND DISCUSSION To examine the thermodynamic stability of Pt−Co bimetallic nanoparticles, we recorded relevant data during MD simulations of heating processes. By analyzing these data, we may acquire some important information concerning the thermodynamic properties, the structural evolution, the initiation, and progress of phase transition. Experimentally, one effective way to trace the phase transition (such as solid− solid or solid−liquid transition) is to examine the caloric curve (i.e., energy versus temperature) of the system.43,44 Nowadays, the caloric curves are frequently used to investigate the melting behavior of bulk and nanostructured materials both theoretically and experimentally.34−36,43−45 Figure 2 illustrates the temperature dependence of potential energy for various Pt−Co bimetallic nanoparticles. As a comparison, the results of pure Pt and Co nanoparticles are also presented in this figure. It can be found that the potential energy increases almost linearly with temperature at low temperatures, followed by a sharp change in a certain temperature region. Evidently, the potential energy of disordered alloy is higher than that of ordered intermetallics with the same composition for both Pt3Co and PtCo nanoparticles at the same temperature, indicating that the ordered intermetallics have better stability. Moreover, Co@Pt core−shell structure possesses higher energy than PtCo disordered alloy with the same composition. This signifies that the Co@Pt structure should be least favored among the three types of configurations in Figure 1. Compared with the monotonically rising energy with increased temperature for Pt− Co disordered alloy and ordered intermetallic nanoparticles, the situation is also different for the Co@Pt nanoparticle: Following the continuously increased energy, an abrupt

⎧ ⎫1/2 ⎡ ⎛ rij ⎡ ⎞⎤ ⎪ ⎛ rij ⎞⎤⎪ 2 ⎢ ⎢ ⎥ ⎥ ⎨ U = ∑ A exp − p⎜ − 1⎟ − ∑ ξ exp − 2q⎜ − 1⎟ ⎬ ⎢⎣ ⎝ r0 ⎢⎣ ⎠⎥⎦ ⎪ ⎝ r0 ⎠⎥⎦⎪ ⎩ j≠1 ⎭ i=1 (1) N

in which rij denotes the distance between the ith and jth atoms and r0 is the first-neighbor distance in the metal. ξ is an effective hopping integral, and q describes its dependence on the relative interatomic distance. For pure metal species (Pt or Co), the parameters A, ξ, p, and q are determined by fitting to the experimental values of the cohesive energy, lattice parameter, and independent elastic constants. C


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was deduced from the potential energy of the system according to the equation45 Cp(T ) =

dU 3 + R gc dT 2

(2)

where U is the potential energy of the system and Rgc is 8.314 J/(mol K). Figure 2 also illustrates the specific heat capacity of Pt−Co bimetallic nanoparticles at various temperatures. According to eq 2, the melting temperature of nanoparticle can be well ascertained. First, the melting points of pure Pt and Co nanoparticles are identified to be 1900 and 1120 K, respectively. Evidently, their melting points are all remarkably lower than those of bulk counterparts (2045 K for Pt and 1770 K for Co).46 The lowering melting point should be attributed to high surface-volume ratio in nanosized particles and the much lower surface premelting temperature, which have been widely verified both experimentally and theoretically.47 Second, it can be identified that the melting point of disordered alloy nanoparticle is lower than that of ordered intermetallic one. For example, the melting points are 1600 and 1310 K for the Pt3Co-DA and the PtCo-DA nanoparticles, while they are 1630 and 1430 K for the Pt3Co-OI and the PtCo-OI ones. Therefore, it can be found from both the potential energy curve and melting point that the ordered intermetallic nanoparticles display better structural and thermal stabilities than the chemically disordered ones for the same composition. Furthermore, higher Pt ratio is beneficial to improving the thermal stability of Pt−Co bimetallic nanoparticles. For core− shell structures, the melting point is well identified to be 1260 and 1710 K for the Co@Pt and the Pt3Co@Pt nanoparticles. This indicates that the addition of Pt into Co core to form L12 ordered structure can remarkably enhance the thermal stability of Co−Pt core−shell structures. Available experiments have also confirmed that Pt3Co@Pt nanoparticles can maintain their structures very well at high temperatures.10 As is mentioned above, the thermal stability and melting points of Pt−Co bimetallic nanoparticles strongly depend on

Figure 2. Temperature dependence of potential energy and specific heat capacity for different Pt−Co bimetallic nanoparticles. Note that the dashed lines correspond to the heat capacity.

reduction in the potential energy occurs at temperatures above the solid−liquid transition. This reduction of energy should be attributed to the fast mutual diffusion between Pt and Co atoms after melting and attractive Pt−Co interactions. However, for the PtCo-OI, PtCo-DA, and Co@Pt nanoparticles, their energy curves completely overlap with each other beyond overall melting, implying that they should have the same structure and composition distribution at liquid state. Meanwhile, the Pt3Co@Pt nanoparticle shows the similar energy curve as the alloyed and monometallic nanoparticles, different from the Co@Pt nanoparticle. This diversity may be associated with their different melting mechanisms, which will be discussed later. The solid−liquid phase transition is usually accompanied by the sharp rise of the energy and the abrupt peak of heat capacity due to the absorption of latent heat. Therefore, the melting temperature (Tm) is generally defined as a temperature at which the heat capacity reaches its maximum.43−45 The heat capacity

Figure 3. Temperature-dependent Lindemann indices of nanoparticles for (a) pure Pt and Co, (b) Pt3Co disordered alloy and ordered intermetallics, (c) PtCo disordered alloy and ordered intermetallics, and (d) Co@Pt and Pt3Co@Pt structures. Note that the dashed line indicates the critical value of Lindemann index of 0.05 for both Pt and Co. D


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that the Lindemann index curve of Pt3Co@Pt nanoparticle is similar to those of Pt−Co disordered alloyed and ordered intermetallic nanoparticles, which signifies the identical melting mode. However, the Co@Pt nanoparticle presents remarkable differences in Lindemann index curves compared with those of other Pt−Co bimetallic ones. For the Co@Pt nanoparticles, according to the Lindemann curves in Figure 3d, the melting point is 1270 K for the Co core and 1380 K for the Pt shell. Furthermore, the enhanced mutual diffusivity of Pt and Co atoms results in the sharp rise of Lindemann index for Pt at the temperature where Co core melts and the sharp decrease of Lindemann index for Co at the temperature where Pt shell melts (see Figure 3d). The most interesting thing here is that the Co core has melted prior to the Pt shell. In general, the melting initiates at the surface (or shell) of core−shell structured nanoparticles even the core is composed of a metallic element with a lower melting point. For examples, our previous studies on Pd@Pt and Au@Pt nanoparticles showed that the melting processes start from the outermost layers of Pt shell, and the core and shell synchronously melt at the overall melting although both Pd and Au have noticeably lower melting points than Pt.34,35,52 However, this abnormal phenomenon, i.e., the core melts prior to the shell in the Co@Pt nanoparticles, has not been observed in other core− shell structured nanoparticles. This abnormal melting behavior, on one hand, is inseparable from the remarkable difference of melting point between Pt and Co. On the other hand, considered that the hcp−fcc phase transition of Co may happen approximately at 700 K,53 it could also affect the melting behavior of Co and Co-based materials. Therefore, we explored the structural evolution of both pure Co and Co@Pt nanoparticles during continuous heating. Here, the common neighbor analysis (CNA)54 was employed to characterize the local crystal structure by examining the bonds between an atom and its nearest neighbors. Utilizing this analysis, all the atoms in a nanoparticle were classified into three categories: atoms in a local fcc order were considered to be fcc atoms; atoms in a local hcp order were classified as hcp atoms; atoms in all other local orders were considered to be “other” atoms. The results of structural analysis are presented in Figure 4. As seen in Figure 4a, no fcc atom appeared in pure hcp Co nanoparticle during the heating, indicating that no phase transition happened. However, for the Co@Pt nanoparticle, a peak of the fcc atom curve can be clearly identified in the temperature range 700−800 K (see Figure 4b). This signifies that the original hcp Co in the core did undergo a

the composition and atomic distribution of Pt and Co. Therefore, it is necessary to explore the melting mechanism and associated behaviors of these nanoparticles. In order to investigate the onset and evolution of melting, the Lindemann melting criterion has been widely employed in theoretical studies.47 This criterion states that a solid melts when the dimensionless Lindemann index exceeds a critical value and has been verified to be in good agreement with the Born criterion.48 For a system of N atoms, the local Lindemann index of the ith atom is defined as the root-mean-squared bond length fluctuation as49 1 δi = N−1

∑ j≠i

⟨R ij 2⟩ − ⟨R ij⟩2 ⟨R ij⟩

(3)

and the system-averaged Lindemann index can be obtained by δ̅ =

1 N

∑ δi i

(4)

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 happens as the index reaches its critical value in the range of 0.1−0.15, depending on materials. However, a smaller critical index of 0.03−0.05 was frequently used in nanoparticles owing to the relaxed constraint of surface atoms.50 The dependences of Lindemann indices on temperature were respectively calculated for pure Pt, pure Co, and Pt−Co bimetallic nanoparticles, as illustrated in Figure 3. It can be seen from Figure 3a that there occurs a sharp rise in the Lindemann index curve when the nanoparticle melts, similar to what happens in energy curve in Figure 2. By investigating the curves of pure Pt and Co nanoparticles, we find that 0.05 should be an appropriate critical value of Lindemann index for both Pt and Co. According to the Lindemann criterion, the nanoparticles will melt into liquid when the system-averaged Lindemann index exceeds this critical value. Therefore, the melting temperature can also be defined as the temperature at which the averaged Lindemann index exceeds its critical value. One can find that the melting points of pure Pt and Co nanoparticles, deduced from the Lindemann index curves, are in good agreement with those from the energy and heat capacity curves. For Pt−Co bimetallic nanoparticles, the Lindemann indices were respectively statistically averaged for Pt and Co atoms, as illustrated in Figures 3b−d. As seen from Figures 3b,c, for both disordered alloy and ordered intermetallics, the Lindemann index curves of Pt and Co present a steep rise at the same temperature, indicating that they have synchronously melted. Moreover, the melting temperatures deduced from the Lindemann index curves in Figures 3b,c are well consistent with those from the caloric curves in Figure 2. The monotonous and synchronous changes of Lindemann indices for Pt and Co indicate that the melting progresses in Pt−Co disordered alloyed and ordered intermetallic nanoparticles are similar to those in pure Pt, Co, and other monometallic nanoparticles; that is, the melting initiates at the surface of nanoparticle and progressively spreads into the core, finally leading to the overall melting at the melting point.47,51 Figure 3d shows the temperature-dependent Lindemann indices of core−shell structured nanoparticles. It can be seen

Figure 4. Temperature-dependent percentage of atoms in (a) pure Co and (b) Co@Pt nanoparticles. Note that in the Co@Pt case only the Co atoms were counted. E


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Figure 5. Snapshots of cross sections of Pt−Co bimetallic nanoparticles taken at five representative temperatures: (a) PtCo disordered alloy and (b) Co@Pt structures. Coloring denotes the type of atom: red, Pt Lindemann atom (Pt-LA); pink, Co Lindemann atom (Co-LA); blue, Pt nonLindemann atom (Pt-nLA); and bright green, Co non-Lindemann atom (Co-nLA).

this time, the majority of Co atoms in the core have transformed into Lindemann atoms, indicating that the core substantially becomes liquid. It is noted that this temperature is remarkably higher than the melting point of pure Co nanoparticle, showing that Co nanoparticle, coated by Pt layers to form Co@Pt structure, possesses higher melting point and exhibits superheating. This liquid-core/solid-shell structure can be maintained up to 1350 K. With further increased temperature, the core/shell interface begins to be destroyed, and some of molten Co atoms diffuse into the shell even surface through the broken interface. At the same time, a large number of Pt atoms have also come into the core region. As a result, the melting of the local surface has been found at 1360 K. This melting rapidly expands along the surface, finally leading the overall melting of the whole nanoparticle, as shown in the snapshots at 1370 and 1390 K in Figure 5b.

solid−solid structural transition into fcc structure prior to its melting. This phase transition is absent in pure Co nanoparticle because the pure Co nanoparticle is placed in a vacuum, and its surface is free and unconstrained. As the temperature rises, the surface becomes disordered at about 700 K, which can be identified from the dropping percentage of hcp atoms in Figure 4a. Therefore, it is reasonable that the hcp−fcc phase transition does not occur in the pure Co nanoparticle. In order to visualize the melting process at atomic level and shed light on the melting behavior, we have investigated structural evolutions of Pt−Co bimetallic nanoparticles during MD simulation of continuous heating. In this work, we employed the concept of Lindemann atom, which has been previously adopted in the study of melting of surface-free Lennard-Jones crystal.47,48 The atom is defined as Lindemann atom if its Lindemann index exceeds the critical value (0.05 for Pt and Co here); otherwise, it is classified as a non-Lindemann atom. Considering that the Lindemann index curves are roughly analogous for Pt−Co disordered alloy, ordered intermetallic, and Pt3Co@Pt nanoparticles, their melting behaviors should be similar. Therefore, only the results of Pt3Co disordered alloy nanoparticle have been presented as a representative. Figure 5 illustrates the atomistic snapshots of Pt3Co disordered alloy and Co@Pt nanoparticles at five representative temperatures. It can be seen from Figure 5a that the melting initiates at particle surfaces at 1570 K. The initiation of melting nucleation is inhomogeneous on the surface. This inhomogeneous nucleation has also been found in monometallic nanoparticles.51 Its presence is attributed to the local structural instability during heating.48 With the temperature increased up to 1590 K, a liquid layer forms on the surface. This liquid layer gradually thickens and extends into the core region with further elevated temperature, eventually leading to the occurrence of a whole liquid particle, as successively demonstrated in the snapshots at 1600, 1610, and 1620 K of Figure 5a. Figure 5b illustrates the melting process of Co@Pt nanoparticle. One can find that the melting originates from the core region close to the core/shell interface at 1260 K. It can be seen that the atomic arrangement in the region where the premelting occurs is different from that in other core region. The ordered crystalline structure has been destroyed in the premelting region. At 1280 K, the melting has already expanded to the whole core except the core/shell interface. At

4. CONCLUSIONS In summary, we have performed a detailed examination on thermal stability of Pt−Co bimetallic nanoparticles using MD simulations. Three different structures, i.e., disordered alloy (Pt3Co and PtCo solid solution), ordered intermetallics (Pt3Co L12 and PtCo L10 structures), and core−shell configurations (Co@Pt, and Pt3Co@Pt), have been addressed. The results reveal that ordered intermetallic nanoparticles present better structural and thermal stability than disordered alloyed ones for both Pt3Co and PtCo. However, their melting processes were analogous; that is, the melting initiates on the surface and develops inward with increasing temperature, finally resulting in overall melting at the melting point. Despite the similar melting mode with aforementioned nanoparticles, Pt3Co@Pt core− shell nanoparticle exhibits the highest melting point and the best thermal stability among the three types of Pt−Co bimetallic nanoparticles. By contrast, Co@Pt nanoparticle displays the lowest melting point and the worst thermal stability. Furthermore, the melting initiates in the core and extends outward the shell, displaying a typical two-stage melting characteristic. The abnormal melting behavior should be attributed to the hcp−fcc phase transition of Co core as well as the noticeable difference in melting points between Pt and Co. This study provides insights into the structure and stability of Pt−Co bimetallic nanoparticles and motivates further design and synthesis of bimetallic even multimetallic nanoparticle F


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catalysts with both excellent performance and outstanding stability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.H.W.). ORCID

Yu-Hua Wen: 0000-0002-0578-7506 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51271156 and 11474234), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130121110012), and the Fundamental Research Funds for the Central Universities of China (Grant No. 20720150023).

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Thermal Stability of Platinum-Cobalt Bimetallic Nanoparticles

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