Insights from Molecular Dynamics Simulations

Letter pubs.acs.org/JPCL

Thermal Stability of Co−Pt and Co−Au Core−Shell Structured Nanoparticles: Insights from Molecular Dynamics Simulations Yu-Hua Wen,*,† Rao Huang,† Gui-Fang Shao,‡ and Shi-Gang Sun§ †

Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, China ‡ 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: Co−Pt and Co−Au core−shell nanoparticles were heated by molecular dynamics simulations to investigate their thermal stability. Two core structures, that is, hcp Co and fcc Co, have been addressed. The results demonstrate that the hcp−fcc phase transition happens in the hcp-Co-core/fcc-Ptshell nanoparticle, while it is absent in the hcp-Co-core/fcc-Au-shell one. The stacking faults appear in both Pt and Au shells despite different structures of the Co core. The Co core and Pt shell concurrently melt and present an identical melting point in both Co−Pt core−shell nanoparticles. However, typical two-stage melting occurs in both Co−Au core−shell nanoparticles. Furthermore, the Au shell in the hcp-Co-core/fcc-Au-shell nanoparticle exhibits a lower melting point than that in the fcc-Co-core/fcc-Au-shell one, while the melting points are closely equal for both hcp and fcc Co cores. All of these observations suggest that their thermal stability strongly depends on the structure of the core and the element of the shell.

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drastic reduction of its magnetic properties.12−14 Besides, a Co core covered by other noble metals to form core−shell structures such as Co@Au,15−20 Co@Ag,21 and Co@Pd22,23 nanoparticles was also synthesized and extensively investigated, in which desired catalytic, magnetic, and optical properties were achieved. As is known, higher temperature is inevitably involved not only in the synthesis and postprocessing processes of these nanoparticles but also in applications such as high-temperature catalytic reactions. Naturally, their structures and properties may be changed significantly with increasing temperature. Investigations of the thermal stability of these nanoparticles are hence extremely necessary, which are helpful both for suppressing their sintering and coarsening during preparation and for stabilizing their structures in applications. However, relevant studies of the thermal behavior of Co cores covered by noble metal shells have been rarely reported to date. In general, the noble metal shell is an fcc structure. The structure of the Co core, however, may exhibit the hcp or fcc phase, depending on the synthesis procedures.9−14 In practice, random intergrowth of the two types of crystalline structures of Co is commonly due to the rather small energy difference between them.24 Different core structures will have strong effects on the stability and physical and chemical properties of core−shell structured nanoparticles. However, it still remains unclear how the

oble metals, such as Pt, Pd, Au, and Ag, are of fundamental importance to a wide range of fields including catalysis, optics, electrical engineering, biomedical, and more.1,2 However, the low reserve and extremely high price of raw materials severely limit their widespread applications. Among all of the solution strategies to improve their performance and utilization efficiency, forming core−shell structures in which non-noble metals are covered by noble metal shells has become an efficient approach and has received tremendous attentions recently.3,4 Introducing the non-noble metals into the noble metallic system not only can greatly decrease the total cost but also is an effective method to achieve bifunctional or multifunctional properties by the synergistic structural and electronic effects of two or more metals (via the so-called strain and ligand effects).5−23 In all of the promising candidates for core material, 3d transition metal cobalt (Co) is a peculiar choice generally because of its good thermal stability (high melting point), ferromagnetic property, and optional crystallographic structures [hexagonal close packed (hcp) and face-centered cubic (fcc) structures], which may bring versatile and tunable properties.7−23 It has been verified that Co−Pt core−shell (Co@Pt) nanoparticles exhibit excellent oxygen reduction activity, large uniaxial anisotropy, and high chemical stability, which make them display extensive applications in the fields of electronics, catalysis, and high-density storage devices.7−14 Meanwhile, the architecture of Co@Pt structured nanoparticles is also beneficial for protection of the Co core from the etching of the acid environment when they are used in fuel cells as catalysts and from avoiding oxidation, which causes © 2017 American Chemical Society

Received: July 21, 2017 Accepted: August 24, 2017 Published: August 24, 2017 4273

DOI: 10.1021/acs.jpclett.7b01880 J. Phys. Chem. Lett. 2017, 8, 4273−4278

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The Journal of Physical Chemistry Letters

ature-dependent potential energy for Co@Pt and Co@Au nanoparticles. The heat capacity, which was deduced from the potential energy of the system,27 is also presented in this figure. As one can find, the potential energy increases almost linearly with temperature at low temperatures (typically below 1400 K for Co@Pt and 750 K for Co@Au nanoparticles), showing an invariable heat capacity. Furthermore, for both Co@Pt and Co@Au systems, the potential energy of nanoparticle with a hcp Co core is slightly higher than that of the nanoparticle with an fcc Co core (about 0.008 eV/atom for Co@Pt systems and about 0.002 eV/atom for Co@Au systems), indicating that the latter has better structural stability. This result is expectable because in the latter the crystal structures of both the core and the shell are fcc lattices and easy to match each other in the core/shell interface compared to the hcp core. With further increased temperature, significant differences occur in Co@Pt and Co@Au systems (see Figure 1a,b). For the F−Co@Pt nanoparticle, an abrupt rise in the potential energy appears at a temperature around 1700 K after a continuously linear increase, corresponding to a sharp peak in the heat capacity curve. In general, the melting temperature (Tm) is defined as the temperature at which the heat capacity reaches its maximum.27 Therefore, the melting point of the F− Co@Pt nanoparticle is ascertained to be 1720 K. Beyond the overall melting, the potential energy presents an abrupt reduction, which should be attributed to fast mutual diffusion between Pt and Co atoms after melting. For the H−Co@Pt nanoparticle, an appreciable reduction in energy at 1520 K, previous to melting, can be found from Figure 1a. Considering that pure Co can transform from hcp into fcc structure during the heating process,28 the energy decrease may be associated with this structural change (see the in-depth discussion in the following). After this reduction, its energy curve is similar to that of the F−Co@Pt nanoparticle. Note that the melting point of the H−Co@Pt nanoparticle is also 1720 K. From the simulation results of pure Co and Pt nanoparticles (not shown here), we can confirm that their melting points are respectively 1600 and 1930 K. Evidently, the melting point of the Co@Pt nanoparticle just lies between these two values. Similar results have been frequently verified in other core−shell structured nanoparticles.29,30 On the other hand, for the Co@Au system, one can find that the potential energy continuously rises with increasing temperature and no energy reduction occurs (see Figure 1b). Different from the Co@Pt case, two distinct peaks can be observed in the heat capacity curve. Considering the remarkable difference in the melting points of the pure Au and the pure Co nanoparticles (1040 and 1600 K, respectively), the Au shell will melt prior to the Co core. Therefore, the first peak should be attributed to melting of the Au shell, and the second peak should be associated with melting of the Co core, displaying a typical “two-stage” melting.31 According to the peak position of the heat capacity, the melting temperature is deduced to be 950 K for the Au shell and 1470 K for the Co core in the F−Co@Au nanoparticle, while 910 K for the Au shell and 1480 K for the Co core in the H−Co@Au one. Besides, there is no energy reduction observed in the Co@Au nanoparticles after overall melting, implying that the mixing of Au and Co atoms does not decrease the energy of the system. This difference between Co@Pt and Co@Au nanoparticles should be understandable because the enthalpy of mixing is positive for the Co−Au system while negative for the Co−Pt system.32 To sum up, the thermal stability and melting points

different core structures affect their core/shell interfaces and, further, their thermal stability. In this Letter, we present a detailed molecular dynamics (MD) investigation of the thermal stability of Co@Pt and Co@ Au nanoparticles in which both core structures (hcp and fcc) are considered. Besides the widespread applications, the reason why Pt and Au were chosen as shell materials is that they are contiguous elements in the periodic table and have similar atomic radii but prominently different melting points.25 From MD simulations, we found that the hcp−fcc phase transition occurs in the hcp Co core of the Co@Pt nanoparticle, while it is absent in the Co@Au one. Moreover, the core and shell concurrently melt in the Co@Pt nanoparticle, whereas typical two-stage melting happens in the Co@Au one. These results display the possibility of tuning thermodynamic behaviors by controlling core structures as well as by choosing shell elements. The demonstrated structure−property relationship provides practical guidance to both the design and utilization of new types of noble-metallic-based nanomaterials. Considering that Pt and Au are fcc metals, two types of Co@ Pt and Co@Au nanoparticles with different core structures, that is, hcp-Co-core/fcc-Pt-shell (denoted as H−Co@Pt), fcc-Cocore/fcc-Pt-shell (denoted as F−Co@Pt), hcp-Co-core/fcc-Aushell (denoted as H−Co@Au), and fcc-Co-core/fcc-Au-shell (denoted as F−Co@Au), were constructed in this work. Their diameters are about 10 nm, and Co/Pt and Co/Au atomic ratios were equally set to be 1. Besides, pure hcp Co, fcc Pt, and fcc Au nanoparticles with the same size were constructed for facilitating comparison with the bimetallic ones. In order to examine the thermal stability of these nanoparticles, MD simulations were employed to heat them until overall melting. The procedures of MD simulations are described in detail in the Experimental Section. Generally, some important information concerning the thermodynamic properties, the structural change, the initiation, and the progress of phase transition can be extracted from the recorded relevant data during MD simulations. Experimentally, an effective way to detect the solid−solid or solid−liquid phase transition is to examine the caloric curve (i.e., energy versus temperature) of the system.26 Figure 1 illustrates the temper-

Figure 1. Potential energy and specific heat capacity as a function of temperature for (a) Co@Pt and (b) Co@Au nanoparticles. Note that the dashed line corresponds to the heat capacity. 4274

DOI: 10.1021/acs.jpclett.7b01880 J. Phys. Chem. Lett. 2017, 8, 4273−4278

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atoms on the surface of solid particles may facilitate the dissolution of adsorbates, enabling the appearance of different chemical processes.26 Finally, it should be noted that the melting points of the core and shell, deduced from the Lindemann index curves in Figure 2, are consistent with those from the caloric curves in Figure 1. According to the aforementioned analyses, the temperature dependence of energy and the Lindemann index of the nanoparticles composed of an fcc Co core and an fcc Pt (or Au) shell are analogous to that of other fcc bimetallic core−shell nanoparticles in our previous studies.29−31,36 However, as for the particles with hcp cores, we can find that both the energy and Lindemann index decrease before melting in the H−Co@ Pt nanoparticle, and the Au shell in the H−Co@Au nanoparticle presents an appreciably lower melting point than that in the F−Co@Au one. Because the only difference between the two Co@Au nanoparticles lies in the crystalline structure of the Co core, it is reasonable to attribute the diverse results to the unharmonious lattice between the hcp core and the fcc shell. In order to detect the structural changes at the atomic level and shed light on the associated behaviors, we investigated the structural evolutions of Co@Pt and Co@Au nanoparticles during MD simulations of continuous heating. Here, we introduce the common neighbor analysis (CNA) proposed by Honeycutt and Andersen37 to monitor the local structures in the nanoparticles. This analysis assigns four indices, ijkl, to each pair of atoms that have common neighbors and provides a description of the local environment of the pair. All bonded pairs in the fcc crystal are of type 1421, whereas the hcp crystal has equal numbers of types 1421 and 1422. Because the pairs beside types 1421 and 1422 do not reveal some useful information for Pt, Co, and Au metals, we have classified atoms into three categories by using the CNA method. Atoms in a local fcc order are considered to be fcc atoms. Atoms in a local hcp order are classified as hcp atoms whose regular occurrence in an fcc crystal is regarded as the structure of stacking faults. Atoms in all other local orders are considered to be “other” atoms. Figure 3 presents the results of structural analyses by the CNA method. As seen in Figure 3a, the majority of Pt atoms (about 60%) belong to fcc atoms at 0 K, and the remainder belong to “other” atoms. These so-called other atoms are usually distributed at the surface and core−shell interface. No hcp atom is found in the Pt shell at ground state. However, at 50 K, a large number of hcp atoms (about 10% or more of Pt atoms) that occur in the shell and fcc atoms correspondingly decrease, implying that some of the fcc atoms have transformed into hcp ones. These hcp atoms gather together in a series of two adjacent {111} planes to form the structure of stacking faults in the Pt shell (see Figure 3e). According to the theory of crystal dislocations, intrinsic stacking faults, which appear as two adjacent {111} planes of hcp atoms, could be left after a Shockley partial dislocation nucleates and propagates through an fcc crystal.38 Therefore, it can be deduced that the Shockley partial dislocations should be activated at 50 K. These stacking faults are found in both F−Co@Pt and H−Co@Pt nanoparticles (see Figure 3e,f) because of the large mismatch between the Pt lattice (lattice constant a = 3.924 Å) and both fcc Co (a = 3.548 Å) and hcp Co (a = 2.507 and c = 4.070 Å). With the increased temperature, both fcc and hcp Pt atoms gradually reduce and completely disappear until overall melting. The same phenomena have also been observed in Au shells (a

of core−shell structured nanoparticles strongly depend on both the composition and the crystalline structure. Next, we explore the detailed melting mechanism and associated behaviors of these nanoparticles. The Lindemann melting criterion has been usually used to examine the initiation and development of melting.33 This criterion states that a solid melts when the dimensionless Lindemann index exceeds a critical value, demonstrating good agreement with the Born criterion.34 The Lindemann criterion suggests that melting of the nanoparticle happens when the index reaches 0.03−0.05.35 By investigating the Lindemann index curves of pure Co, Pt, and Au nanoparticles, we find that the appropriate critical value of the Lindemann index is 0.058, 0.048, and 0.041 for Co, Pt, and Au, respectively. The temperature-dependent Lindemann indices were calculated for Co@Pt and Co@Au nanoparticles, as respectively shown in Figure 2a,b. It should be noted that the atoms in the

Figure 2. Dependence of Lindemann indices on temperature for (a) Co@Pt and (b) Co@Au nanoparticles. Note that the horizontal dashed line indicates the critical value of the Lindemann index.

core and the shell are separately computed. One can find from Figure 2a that the Lindemann index presents a sharp rise when the Co@Pt nanoparticle melts, similar to what happens in the energy curve of Figure 1a. The temperature at which the sharp rise occurs is the same for Co and Pt, indicating that the core and the shell melt simultaneously. Moreover, the Lindemann index of Co decreases at about 1520 K, similar to the energy change in Figure 1a. For the Co@Au nanoparticles, the Lindemann indices of Au and Co exhibit different temperature dependences. It can be found from Figure 2b that the Lindemann index of Au exceeds its critical value at 910 K for the H−Co@Au nanoparticle and 940 K for the F−Co@Au nanoparticle, signifying melting of the Au shell. This verifies again that the melting point of the Au shell in the H−Co@Au nanoparticle is lower than that of the Au shell in the F−Co@Au one. By comparison, the Lindemann index of Co exceeds its critical value until 1450 K. Between 950 and 1450 K, both Co@ Au nanoparticles are regarded as a solid Co particle encapsidated by liquid-like Au, presenting a coexistence of solid and liquid states in such a broad temperature region. This morphology is chemically interesting because the liquid metal 4275

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Figure 3. Temperature-dependent percentage of atoms in Co@Pt [(a) Pt shell and (b) Co core] and Co@Au [(c) Au shell and (d) Co core] nanoparticles. The coloring denotes the type of core−shell nanoparticle: black, H−Co@Pt; red, F−Co@Pt; blue, H−Co@Au; and green, F−Co@ Au. The shape denotes the type of atoms: square, fcc atom; circle, hcp atom; and triangle, other atom. (e−h) Snapshots of the cross section of core− shell nanoparticles taken at three representative temperatures.

the hcp−fcc phase transition in the H−Co@Au nanoparticle should be attributed to the fact that the existence of the Au shell remarkably lowers the thermal stability of the hcp Co core and makes the Co atoms become disordered before the critical temperature for them to transform into the fcc structure. Finally, we discuss why the Au shell in the H−Co@Au nanoparticle has a lower melting point than that in the F−Co@ Au one. It can be found from Figure 3c that the proportions of both fcc and hcp atoms in the former are about 3% lower than that in the latter at low temperatures. On the contrary, the proportion of other atoms in the former is approximately 6% higher than that in the latter. It means that the number of fully coordinated atoms in the former is remarkably less than that in the latter. According to our previous studies, surface premelting generally starts at the locations where atoms have low coordinations.39,40 Consequently, the high percentage of lowcoordinated atoms in the H−Co@Au nanoparticle tends to facilitate surface premelting and thus results in a lower melting temperature. This indicates that an fcc core covered by an fcc shell may form a more thermally stable morphology compared with a hcp core covered by an fcc shell with the same composition, which is also reflected from Figure 1. In summary, we investigated core structure-dependent thermal properties of Co@Pt and Co@Au nanoparticles by using MD simulations. Two types of Co core, that is, the hcp core and the fcc core, were comparatively considered. The hcp−fcc phase transition in the H−Co@Pt nanoparticle was observed at 1530 K, but it was absent in the H−Co@Au one. The stacking faults occurred in both Pt and Au shells despite different structures of the Co core. The Co core and Pt shell were found to concurrently melt and presented the same melting point in both Co@Pt nanoparticles. However, typical

= 4.078 Å) in both types of Co@Au nanoparticles (see Figure 3c). Figure 3b illustrates the temperature-dependent CNA results of the Co core in Co@Pt nanoparticles, displaying significant differences from those of the Pt shell. For the fcc core, the majority of Co atoms (over 80%) belong to fcc atoms, and a minor number of hcp atoms (about 2%) are found. These hcp atoms are randomly distributed in the core and do not form ordered structures (see Figure 3f). Their existence thus does not reveal any effective information. For the hcp core, most Co atoms belong to hcp atoms at low temperatures, and their number gradually lowers with enhanced temperature (typically before 1500 K; see Figure 3b). During this period, there are some fcc atoms (less than 0.5% of Co atoms) dispersed around the core/shell interface at the temperature (see Figure 3b,e). However, at 1530 K, an abrupt rise in the percentage of fcc atoms (up to 12.9%) is observed. At the same time, the proportion of hcp atoms decreases from 13.7% at 1490 K to 3.7% at 1530 K. Moreover, the number of other atoms also remarkably lowers at this temperature. These newly formed fcc atoms aggregate into small clusters in the core (see Figure 3g). This implies that the transition from hcp to fcc phase happens in the Co core. Meanwhile, it can be noted that the temperature of this solid−solid phase transition of the Co core is prominently higher than that of bulk Co. This may result from the fact that the Co core is under the confinement of the Pt shell with a higher melting point. In contrast, the hcp−fcc transition is not observed in the H−Co@Au nanoparticle, as indicated by Figure 3d. Considering that the phase transition occurs at around 1530 K in the H−Co@Pt nanoparticle while the Co core melts at 1480 K and the Au shell melts at 910 K in the H−Co@Au nanoparticle (see Figure 3h), the absence of 4276

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The Journal of Physical Chemistry Letters two-stage melting happened in both Co@Au nanoparticles. Analyses of the energy and Lindemann index showed that the Au shell in the H−Co@Au nanoparticle has a lower melting point than that in the F−Co@Au one, while the melting points of the hcp and fcc Co core are closely equal. These observations suggest that for core−shell structured nanoparticles their thermal stability relies heavily on not only the crystal structure of the core but also the element of the shell. Our study thus provides valuable insights into the design and application of core−shell structured nanoparticles composed of noble metals and non-noble transition metals.

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ACKNOWLEDGMENTS

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REFERENCES

This work is supported by the National Natural Science Foundation of China (Grants 51271156 and 11474234) and the Fundamental Research Funds for the Central Universities of China (Grant 20720150023).

(1) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (2) Guo, S. J.; Wang, E. K. Noble Metal Nanomaterials: Controllable Synthesis and Application in Fuel Cells and Analytical Sensors. Nano Today 2011, 6, 240−264. (3) Ghosh Chaudhuri, R.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373−2433. (4) Long, N. V.; Yang, Y.; Minh Thi, C.; Minh, N. V.; Cao, Y. Q.; Nogami, M. The Development of Mixture, Alloy, and Core−Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion in Low-Temperature Fuel Cells. Nano Energy 2013, 2, 636−676. (5) Zhang, X. B.; Yan, J. M.; Han, S.; Shioyama, H.; Xu, Q. Magnetically Recyclable Fe@Pt Core−Shell Nanoparticles and Their Use as Electrocatalysts for Ammonia Borane Oxidation: The Role of Crystallinity of the Core. J. Am. Chem. Soc. 2009, 131, 2778−2779. (6) Panizon, E.; Ferrando, R. Strain−Induced Restructuring of the Surface in Core@Shell Nanoalloys. Nanoscale 2016, 8, 15911−15919. (7) Beard, K. D.; Borrelli, D.; Cramer, A. M.; Blom, D.; Van Zee, J. W.; Monnier, J. R. Preparation and Structural Analysis of Carbon− Supported Co Core/Pt Shell Electrocatalysts Using Electroless Deposition Methods. ACS Nano 2009, 3, 2841−2853. (8) Cantane, D. A.; Oliveira, F. E. R.; Santos, S. F.; Lima, F. H. B. Synthesis of Pt−Based Hollow Nanoparticles Using CarbonSupported Co@Pt and Ni@Pt Core−Shell Structures as Templates: Electrocatalytic Activity for the Oxygen Reduction Reaction. Appl. Catal., B 2013, 136-137, 351−360. (9) Yang, X. J.; Cheng, F. Y.; Tao, Z. L.; Chen, J. Hydrolytic Dehydrogenation of Ammonia Borane Catalyzed by Carbon Supported Co Core−Pt Shell Nanoparticles. J. Power Sources 2011, 196, 2785−2789. (10) Varade, D.; Haraguchi, K. Clay-Supported Novel Bimetallic Core−Shell Co−Pt and Ni−Pt Nanocrystals with High Catalytic Activities. Phys. Chem. Chem. Phys. 2014, 16, 25770−25774. (11) Mei, H.; Wu, W. Q.; Yu, B. B.; Li, Y. B.; Wu, H. M.; Wang, S. F.; Xia, Q. H. Non−Enzymatic Sensing of Glucose at Neutral pH Values Using a Glassy Carbon Electrode Modified with Carbon Supported Co@Pt Core−Shell Nanoparticles. Microchim. Acta 2015, 182, 1869− 1875. (12) Kristian, N.; Yu, Y. L.; Lee, J. M.; Liu, X. W.; Wang, X. Synthesis and Characterization of CoCore−PtShell Electrocatalyst Prepared by Spontaneous Replacement Reaction for Oxygen Reduction Reaction. Electrochim. Acta 2010, 56, 1000−1007. (13) Ali, S.; Ahmed, R.; Sohail, M.; Khan, S. A.; Ansari, M. S. Co@Pt Core−Shell Nanoparticles Supported on Carbon Nanotubes as Promising Catalyst for Methanol Electro−Oxidation. J. Ind. Eng. Chem. 2015, 28, 344−350. (14) Kuttiyiel, K. A.; Choi, Y. M.; Hwang, S. M.; Park, G. G.; Yang, T. H.; Su, D.; Sasaki, K.; Liu, P.; Adzic, R. R. Enhancement of the Oxygen Reduction on Nitride Stabilized Pt−M (M = Fe, Co, and Ni) Core−Shell Nanoparticle Electrocatalysts. Nano Energy 2015, 13, 442−449. (15) Bao, Y. P.; Calderon, H.; Krishnan, K. M. Synthesis and Characterization of Magnetic−Optical Co−Au Core−Shell Nanoparticles. J. Phys. Chem. C 2007, 111, 1941−1944. (16) Mandal, S.; Krishnan, K. M. CocoreAushell Nanoparticles: Evolution of Magnetic Properties in the Displacement Reaction. J. Mater. Chem. 2007, 17, 372−376.

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EXPERIMENTAL SECTION Co@Pt, Co@Au, Co, Pt, and Au nanoparticles were constructed from large single crystal by using a certain cutoff radius. For the convenience of comparison, the diameters of all nanoparticles were set to be about 10 nm. The Gupta potentials, based on the second-moment approximation of the tight-binding scheme (TB-SMA), were employed to describe the interatomic interactions of metals. Their parameters were optimized according to bulk properties (cohesion energy, lattice parameter, and elastic constants) of pure elements and thermodynamic properties of their alloys (mixing and segregation enthalpies).32 To examine the thermal stability of these nanoparticles, we employed 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 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 MD (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 2400 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. 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 Nose−Hoover thermostat41 and Berendsen approach42 to maintain the nanoparticle system at the desired temperature and ambient pressure. The equations of atomic motion were integrated by the Verlet-velocity algorithm with a 1 fs time step.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu-Hua Wen: 0000-0002-0578-7506 Rao Huang: 0000-0001-9913-2123 Shi-Gang Sun: 0000-0003-2327-4090 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 4277

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The Journal of Physical Chemistry Letters (17) Song, Y. J.; Ding, J.; Wang, Y. H. Shell-Dependent Evolution of Optical and Magnetic Properties of Co@Au Core−Shell Nanoparticles. J. Phys. Chem. C 2012, 116, 11343−11350. (18) Mayoral, A.; Llamosa, D.; Huttel, Y. A Novel Co@Au Structure Formed in Bimetallic Core@Shell Nanoparticles. Chem. Commun. 2015, 51, 8442−8445. (19) Llamosa Perez, D.; Espinosa, A.; Martínez, L.; Roman, E.; Ballesteros, C.; Mayoral, A.; Garcia-Hernandez, M.; Huttel, Y. Thermal Diffusion at Nanoscale: From CoAu Alloy Nanoparticles to Co@Au Core/Shell Structures. J. Phys. Chem. C 2013, 117, 3101−3108. (20) Munoz-Sandoval, E.; Perea-Lopez, N.; Lima-Juarez, R.; LabradaDelgado, G. J.; Rivera-Escoto, B. A.; Zamudio, A.; Silva-Pereyra, H. G.; Robles-Avila, E.; Terrones, M. Synthesis, Characterization and Magnetic Properties of Co@Au Core−Shell Nanoparticles Encapsulated by Nitrogen-Doped Multiwall Carbon Nanotubes. Carbon 2014, 77, 722−737. (21) Wang, L.; Clavero, C.; Huba, Z.; Carroll, K. J.; Carpenter, E. E.; Gu, D. F.; Lukaszew, R. A. Plasmonics and Enhanced Magneto− Optics in Core-Shell Co−Ag Nanoparticles. Nano Lett. 2011, 11, 1237−1240. (22) Shao, M. H.; Sasaki, K.; Marinkovic, N. S.; Zhang, L. H.; Adzic, R. R. Synthesis and Characterization of Platinum Monolayer Oxygen− Reduction Electrocatalysts with Co−Pd Core−Shell Nanoparticle Supports. Electrochem. Commun. 2007, 9, 2848−2853. (23) Shen, K.; Chen, L.; Long, J. L.; Zhong, W.; Li, Y. W. MOFs− Templated Co@Pd Core−Shell NPs Embedded in N−Doped Carbon Matrix with Superior Hydrogenation Activities. ACS Catal. 2015, 5, 5264−5271. (24) Lee, B. W.; Alsenz, R.; Ignatiev, A.; Van Hove, M. A. Surface Structures of the Two Allotropic Phases of Cobalt. Phys. Rev. B: Condens. Matter Mater. Phys. 1978, 17, 1510−1520. (25) Kittel, C. Introduction to Solid State Physics, 7th ed; John Wiley & Sons: New York, 1996. (26) Cao, B. P.; Starace, A. K.; Judd, O. H.; Jarrold, M. F. Melting Dramatically Enhances the Reactivity of Aluminum Nanoclusters. J. Am. Chem. Soc. 2009, 131, 2446−2447. (27) 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. (28) Frey, F.; Schneider, J.; Prandl, W.; Zeyen, C.; Ziebeck, K. The HCP−FCC Transition in Pure Co Investigated by Neutron Scattering. J. Phys. F: Met. Phys. 1979, 9, 603−616. (29) 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. (30) 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. (31) 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. (32) Cortes-Huerto, R.; Sondon, T.; Saul, A. Molecular Dynamics Simulations of the Formation of 1D Spin−Valves from Stretched Au− Co and Pt−Co Nanowires. J. Phys.: Condens. Matter 2014, 26, 474206. (33) Shibuta, Y.; Suzuki, T. Melting and Nucleation of Iron Nanoparticles: A Molecular Dynamics Study. Chem. Phys. Lett. 2007, 445, 265−270. (34) Jin, Z. H.; Gumbsch, P.; Lu, K.; Ma, E. Melting Mechanisms at the Limit of Superheating. Phys. Rev. Lett. 2001, 87, 055703. (35) 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. (36) Huang, R.; Wen, Y. H.; Shao, G. F.; Sun, S. G. Insight into the Melting Behavior of Au−Pt Core−Shell Nanoparticles from Atomistic Simulations. J. Phys. Chem. C 2013, 117, 4278−4286.

(37) Honeycutt, J. D.; Andersen, H. C. Molecular Dynamics Study of Melting and Freezing of Small Lennard−Jones Clusters. J. Phys. Chem. 1987, 91, 4950−4963. (38) Schiotz, J.; Vegge, T.; Di Tolla, F. D.; Jacobsen, K. W. Atomic− Scale Simulations of the Mechanical Deformation of Nanocrystalline Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 11971− 11983. (39) Wen, Y. H.; Huang, R.; Zeng, X. M.; Shao, G. F.; Sun, S. G. Tetrahexahedral Pt−Pd Alloy Nanocatalysts with High−Index Facets: An Atomistic Perspective on Thermodynamic and Shape Stabilities. J. Mater. Chem. A 2014, 2, 1375−1382. (40) Huang, R.; Wen, Y. H.; Zhu, Z. Z.; Sun, S. G. Atomic−Scale Insights into Structural and Thermodynamic Stability of Pd−Ni Bimetallic Nanoparticles. Phys. Chem. Chem. Phys. 2016, 18, 9847− 9854. (41) Evans, D. L.; Holian, B. L. The Nose−Hoover Thermostat. J. Chem. Phys. 1985, 83, 4069−4074. (42) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684−3690.

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DOI: 10.1021/acs.jpclett.7b01880 J. Phys. Chem. Lett. 2017, 8, 4273−4278