New molecular insights into the stability of Ni–Pd hollow nanoparticles

Hamed Akbarzadeh*a, Esmat Mehrjoueia, Amir Nasser Shamkhalib, Mohsen Abbaspoura, Sirous Salemia and Samira Ramezanzadeha
aDepartment of Chemistry, Faculty of Basic Sciences, Hakim Sabzevari University, 96179-76487 Sabzevar, Iran. E-mail: akbarzadehhamed@yahoo.com; Fax: +98571 400332; Tel: +98915 3008670
bDepartment of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, 56199-11367 Ardabil, Iran

Received 28th June 2017 , Accepted 14th August 2017

First published on 17th August 2017


In this study, the thermal behaviors of pure Ni and Pd as well as Ni@Pd, and Pd@Ni hollow nanoclusters were investigated by MD simulations. The Ni@Pd hollow nanoclusters exhibited more thermodynamic stability and a higher melting point than the Pd@Ni ones. This result is opposite to the trend demonstrated by the corresponding bulk materials, which could be related to the effect of the hollow core. Due to the small difference between the melting points of bulk Pd and Ni, a two-step melting behavior was not observed for the hollow Pd–Ni nanoclusters. The differences between the thermodynamic stabilities of the simulated nanoclusters were related to the concentration of Pd atoms in the shell and Ni atoms in the core regions due to the lower surface energy of Pd atoms and the higher cohesive and binding energy of Ni atoms. Also, a larger nanocluster size led to a faster diffusion of Pd atoms toward the shell of the nanocluster. Moreover, the diffusion of Pd atoms to the surface and Ni atoms to the core region for Pd@Ni nanoclusters near the melting point and the increase in the ordered atoms under these circumstances led to a higher melting point of this nanocluster in comparison with the Ni@Pd nanoclusters. These results indicate the potential for the future construction of nanocatalysts based on bimetallic nanoclusters with core–shell hollow structures.


Introduction

In recent years, nanoparticles (NPs) have attracted a lot of attention because of their unique physical and chemical properties resulting from their nano-dimensions that cannot be observed in bulk materials. Also, among NPs, bimetallic nanoclusters (NCs) containing two different elements, especially ones with a core–shell structure, have received considerable interest because it has been shown that the properties of the core can be changed by growing an extra shell. In other words, core–shell bimetallic nanoclusters have demonstrated improved thermal stability and novel catalytic properties in comparison with monometallic nanoclusters.1–17 In addition, there is an important economic advantage in the synthesis of core–shell nanoclusters, whereby reduced production costs are possible as the expensive and active metal atoms are located on the surface of an inexpensive core.18–20

The structure of core–shell NPs is suitable for producing different types of architectures, including a continuous porous shell, multiple shells, yolk–shell, and porous core and hollow core structures.21,22 Nanostructures with hollow cavities, which can be obtained by core removal, have attracted more and more interest from researchers worldwide over the past few decades due to their superior properties, including uniform size, well-defined shape with a large empty cavity, high specific surface area, and lower density, and advantages such as reduction of metal usage and thus a reduction of costs, and also their wide range of potential applications, such as drug delivery, gene delivery, bio-imaging, electrode materials, lithium-ion batteries, gas sensors, and catalysts.23–37 Therefore, several attempts have been made by experimental researchers to synthesize hollow NPs (HNPs).38–44

In the field of catalysis, special attention has been paid to metal NPs because they can provide a high number of active surface sites, and therefore could make good catalysts.45–53 However in comparison among metal NPs, hollow structures with the same size show more catalytic activity due to the fact that their exterior and interior surfaces can interact with the reactants. For example, Pt–Pd nanoclusters with a porous dendritic shell and hollow core are able to enhance the catalytic activity for the methanol oxidation reaction (MOR) in comparison with Pt nanocatalysts.42 In addition, nest-like Ni1−xPtx (x = 0.00, 0.12) hollow spheres with sub-micrometer sizes have been utilized to improve the yield from the NH3BH3 reaction, which releases H2, due to the high specific surface area of the hollow spheres.54 Furthermore, bimetallic hollow Pd–Co nanospheres have shown superior catalytic activities for the Sonogashira reaction in aqueous media.55 Another example is the synthesis of hollow Pd spheres with enhanced catalytic activities in Suzuki coupling reactions, and which can be reused many times, even after seven times in one study, without the loss of catalytic activity.56 Also, Lu et al. synthesized hollow Au–Pt nanospheres with porous shells as superior electrocatalysts for ethylene glycol oxidation in an alkaline medium.57 Shang et al. successfully prepared carbon-supported trimetallic Ni–Pd–Au hollow NPs through a galvanic displacement reaction. These researchers showed that Ni–Pd–Au/C exhibits superior catalytic activity and stability for methanol electro-oxidation in comparison with Pd/C and Pt/C.58 Rao and Radhakrishna demonstrated the in situ synthesis of hollow Ag–Pd NPs with high catalytic activity for the Suzuki–Miyaura reaction.59 Liang and co-workers reported that Pt hollow nanospheres have a higher electrocatalytic activity in comparison with solid Pt NPs of the same size, which could be related to their higher surface area.60 In another work, Peng et al. reported an electrochemical synthesis of Pt hollow cubic nanoboxes, which then exhibited enhanced catalytic activity for the methanol oxidation reaction (MOR).61 Chen et al. recently developed hollow Ru NPs with enhanced catalytic activity in the dehydrogenation of ammonia borane in comparison to monometallic solid Ru NPs due to the small Ru crystal size and its high specific surface area.62

Recently, magnetic core–shell NPs have attracted significant attention due to their specific properties, which suggest possible applications in catalysis, as magnetic carriers for drug targeting, hyperthermia, bio-sensors, magnetic fluids, magnetic resonance imaging, data storage, magnetic refrigeration, and magnetic bio-separation.63–67 Ni-based nanoclusters, such as Ni–Au,68 Ni–Pd,69 Ni–Fe,70 Ni–Cu,71 and Ni–Co,71 have been studied for their important catalytic and magnetic properties. Among the Ni-based nanoclusters, Ni–Pd ferromagnetic bimetallic nanoclusters have drawn considerable interest due to their wide ranging applications in hydrogen storage materials, hydrogen sensors, medicine, microelectronics, and as an efficient catalyst in reactions such as hydrogenation.72–78 The synthesis of Ni–Pd NPs with Pd-core/Ni shell,79 Ni-shell/Pd-core,80 and Ni–Pd alloy81 structures has been reported, with the structures determined by various spectroscopic methods, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray adsorption fine structure (EXAFS). Also, recently a hollow structure was prepared for Ni–Pd NPs based on the galvanic replacement method. These NPs exhibited triple-enzyme mimetic activities, i.e., oxidase-like activity, peroxidas-like activity, and catalase-like activity, which were used successfully for the colorimetric biosensing of glucose.82 However, the thermodynamic properties and melting behaviors of hollow structure NPs, such as hollow core–shell Ni–Pd NPs, are still not well known or understood, which is particularly important for their applications as new nanocatalysts. Due to the small scales and complex structures of hollow NPs, understanding their structural properties from experimental data is difficult. Therefore, it is necessary to use molecular dynamics (MD) simulations to investigate the hollow bimetallic nanoclusters. Huang et al. investigated the thermal behavior of hollow Au@Pt and Pt@Au core–shell NPs by molecular dynamics simulations. Their results indicated a two stage melting process for both NPs, but with two different melting modes. After the complete melting, both the Pt@Au and Au@Pt NPs transform into a mixing alloy in which the shell is enriched with Au atoms.83 Despite this study, no MD study has been completed for hollow-structured NPs that has included the effect of cluster size on their thermal behavior.

In this study, for the first time, we investigated the melting behavior of hollow core–shell Ni@Pd and Pd@Ni NPs with different sizes by molecular dynamics simulations. Since icosahedral NPs present increased catalytic and magnetic properties, in this work we selected the icosahedral Ni–Pd nanoclusters for the simulations.84–86

Simulation method

Molecular dynamics (MD) simulations were carried out for Ni0.5@Pd0.5 and Pd0.5@Ni0.5 core–shell nanoclusters with hollow structures. In this study, nanoclusters with an icosahedral morphology and total numbers of atoms N = 561, and 923 were selected. These structures were obtained by core removal from nanoclusters with 506 and 776 atoms, and are presented in Fig. 1. It is noticeable that in the hollow core–shell nanoclusters, the hollow cavity is surrounded by the core and the core is surrounded by the shell. In this study, hollow pure Ni and pure Pd nanoclusters were also considered in order for a better comparison of the results.
image file: c7qi00370f-f1.tif
Fig. 1 Atomic arrangement of (a) hollow Pd@Ni nanoclusters and (b) hollow Ni@Pd nanoclusters with the sizes of N = 506 and 776 atoms. Schematic illustrations of the mentioned core–shell nanoclusters with hollow structures are shown on the right (dark green, Ni atom; dark blue, Pd atom).

Dl_POLY 4.03 software87 was used for the MD simulations. All the MD simulations were performed in a canonical ensemble (NVT) with constant temperature and volume. The temperature of the systems was controlled by a Nosé-Hoover thermostat.88,89 The Verlet-leapfrog algorithm90 was used to integrate the equations of motion. A cutoff radius for all of the simulations was selected as 15 Å. Also, the time step was set as 1 fs. In order to investigate the thermal behavior, the nanoclusters were heated from 1 to 1700 K with a temperature interval of 10 K.

In this study, the Gupta potential91–93 was employed to describe the metal–metal interactions. The Gupta potential is based on the second moment approximation to tight-binding theory and is constructed from an attractive many-body (Vm) term and a repulsive pair (Vr) term:

 
image file: c7qi00370f-t1.tif(1)
where
 
image file: c7qi00370f-t2.tif(2)
and
 
image file: c7qi00370f-t3.tif(3)

In these equations, rij represents the distance between atoms i and j, while r0 is the nearest-neighbor distance in the bulk (in Å). The Gupta potential parameters (A, ζ, p, and q) were fitted to the bulk properties of each metal (such as the cohesive energy, lattice parameters, and independent elastic constants at 0 K). The parameters for the Gupta potential are shown in Table 1. For Ni–Pd interactions, the geometric mean was used to calculate the parameters A and ζ, while the arithmetic mean was used for the remaining parameters.

Table 1 Gupta potential parameters for the Ni–Ni, Pd–Pd, and Ni–Pd interactions. The parameters are taken from ref. 93 for Ni and ref. 94 for Pd
Parameters r0 (Å) A (eV) ζ (eV) p q
Ni–Ni 2.4911 0.0376 1.070 16.990 1.189
Pd–Pd 2.7485 0.1746 1.718 10.867 3.742
Ni–Pd 2.6198 0.0810 1.356 13.928 2.465


The detailed thermodynamic properties and structural changes during heating were investigated through the caloric curves, heat capacities, common neighbor analysis, number of surface atoms, and order parameters.

Results and discussion

The potential energies per atom were calculated at different temperatures for the Pd@Ni, Ni@Pd, pure Pd, and pure Ni nanoclusters containing N = 506 and 776 atoms, and are presented in Fig. 2 and 3, respectively. According to these figures, one can identify a 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 specific heat capacity at constant volume (Cv) per atom was calculated for the nanoclusters during the heating process, and the results are shown in Fig. 2 and 3. The melting temperature is defined as the maximum of the peak heat capacity.94–97 As can be seen from Fig. 2 and 3, the maximum values of the heat capacities correspond to the temperatures for which a sharp increase in the total potential energy curves can be observed. The obtained melting temperatures are given in Table 2.
image file: c7qi00370f-f2.tif
Fig. 2 Temperature dependence of the potential energy (solid lines) and the corresponding heat capacities (dashed lines) of hollow monometallic and bimetallic NPs with N = 506.

image file: c7qi00370f-f3.tif
Fig. 3 Temperature dependence of the potential energy (solid lines) and the corresponding heat capacities (dashed lines) of hollow monometallic and bimetallic NPs with N = 776.
Table 2 Melting points of hollow monometallic and bimetallic NPs with different sizes (N = 506 and 776)
Composition Melting point (K) Composition Melting point (K)
Pure Pd/N = 506 950 Pure Pd/N = 776 1000
Ni@Pd/N = 506 1100 Ni@Pd/N = 776 1250
Pd@Ni/N = 506 1250 Pd@Ni/N = 776 1300
Pure Ni/N = 506 1350 Pure Ni/N = 776 1450


On the basis of the results in Fig. 2 and 3 and Table 2, it was found that the Ni nanocluster with the least potential energy has the highest thermodynamic stability and melting point. The opposite result is seen for the Pd nanocluster with the highest potential energy, which had the lowest thermodynamic stability and melting point. The melting points of the bulks of Pd and Ni were 1828, and 1728 K, respectively. It seems that the mentioned unexpected melting temperatures for the simulated Ni and Pd nanoclusters can be related to their hollow structures, whereby the higher cohesive and binding energies of Ni over Pd can increase the thermodynamic stability of the hollow Ni nanoclusters, surprisingly. The cohesive and binding energies of Ni and Pd are given in Table 3. Also, it was observed that in Ni@Pd and Pd@Ni nanoclusters, the absolute value of the internal energy and melting point increased in comparison with pure Pd nanoclusters. This phenomenon indicates that the alloying of Pd nanoclusters with Ni atoms increases their thermal stability. Moreover, comparing Fig. 2 and 3, one can see that larger nanoclusters have lower potential energies and hence higher stabilities and melting points. It is noticeable that for Ni–Pd hollow nanoclusters, due to the similar melting points of the bulk Ni and Pd metals, two-step melting is not observed (this two-step melting was reported for Pt–Au hollow nanoclusters in previous studies83).

Table 3 Some elemental properties of Pd and Ni
Element Ecoh (eV) Esurf (J m−2) Atomic radius (Å) Electronegativity Binding energy (eV)
Pd 3.89 2.05 1.37 2.20 0.68
Ni 4.44 2.45 1.24 1.80 1.29


As observed earlier, the hollow Pd@Ni nanoclusters have a higher melting point than the hollow Ni@Pd ones (see Table 2). In order to explain the reason for this phenomenon, we explored the structural evolution of these NPs during continuous heating and applied the common neighbor analysis (CNA)98 to characterize the local crystal structures of the simulated NPs. Each bond that connects a central atom and its nearest neighbors was characterized by a set of four characteristic numbers ijkl. These numbers depend on whether the atoms are nearest (1) or next-nearest (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). The different types of pairs are associated with different types of local order. All the bonded pairs in an fcc crystal are of the type 1421, whereas an hcp crystal has equal numbers of type 1421 and 1422. Both types 1441 and 1661 are the main bonded pairs in a bcc crystal. CNA has been frequently used to examine the structural changes under mechanical deformation and heating processes.99,100

In this study, each atom in a nanocluster was classified according to its local crystal structure. All the atoms were classified into four categories by the CNA approach. Atoms in a local fcc order were considered to be fcc atoms; atoms in a local bcc order were considered to be bcc atoms; atoms in a local hcp order were classified as hcp atoms whose occurrence in an fcc crystal is generally regarded as the structure of stacking faults; atoms in all other local orders were considered to be other atoms. The percentages of fcc, hcp, and other atoms throughout the heating process for the hollow monometallic and bimetallic NPs with N = 576 and 776 are illustrated in Fig. 4 and 5, respectively. As shown in Fig. 4, pure Pd nanoclusters have the least percentage of fcc and hcp atoms and the highest percentages of other atoms, in comparison with the other nanoclusters. By increasing the temperature, the resulting higher thermal vibrations and motions of Pd atoms lead to deviations from the equilibrium positions. Then, the percentages of fcc and hcp atoms are decreased continuously, until at melting point, all of the local fcc and hcp structures are destroyed, which leads to a disordered structure with 100% of the other atoms after the melting point. The fall in the percentages of the fcc and hcp structures to zero is also evidence for the solid–liquid phase transition, which is in good agreement with the data in Fig. 2 and 3. Unlike pure Pd nanoclusters, the pure Ni nanoclusters with N = 576 indicated a sudden increase in hcp atoms at 500 K. In order to better interpret this phenomenon, snapshots of the simulated nanoclusters with configurations of Pd and Ni atoms, accompanied with distributions of fcc, hcp, bcc, and other atoms at different temperatures are illustrated in Fig. 6 and 7, respectively, for N = 506 and 776. As is obvious from Fig. 6(a) that for pure Pd nanoclusters, other atoms are distributed on their surface and inside the hollow core at 300 K. Since Pd atoms have a lower surface energy, they tend to occupy regions with smaller coordination numbers. Therefore, the existence of Pd atoms leads to stabilization of the hollow region in such a way that the stability remains even at higher temperatures up to near the melting point. With stabilization of the hollow structure, other atoms are present in the highest percentage. Hence, the thermodynamic stability of the Pd nanoclusters is decreased and at the melting point, a sudden collapse of the hollow structure occurs. Unlike pure Pd nanoclusters, for Ni nanoclusters with N = 506, due to the higher binding and cohesive energies of Ni atoms, regions with higher coordination number are more favored to be occupied. Therefore, at 500 K, the hollow region is quickly filled and the percentage of hcp atoms are increased (see Fig. 6(b)). Therefore, Ni nanoclusters with the highest percentage of fcc and hcp atoms (Pfcc = 12.5%, Phcp = 27.5%) and the smallest number of other atoms have the highest melting point and thermodynamic stability. For Ni@Pd nanoclusters with N = 506, at 500 K, the Ni atoms inside the nanoclusters fill the hollow cavity and create local hcp and fcc structures, which is obvious from the sudden increase in fcc and hcp percentage in Fig. 4. By increasing the temperature to the melting point, the percentages of fcc and hcp atoms smoothly decreased, which was expected (see Fig. 6(c)). However, a different behavior was observed for Pd@Ni nanocluster with the same size. For Pd@Ni nanoclusters, the existence of Pd atoms with a small surface energy inside, stabilizes the hollow core up to 800 K, and thus most of the atoms are the other type. At 900 K, the Ni atoms with a higher cohesive energy migrate to the core region and fill the cavity which explains the abrupt increase in the fcc percentage in Fig. 4. Therefore, the existence of fcc ordering in the core region leads to an increase in the thermal stability of Pd@Ni nanoclusters and their melting point in comparison with Ni@Pd. By considering Fig. 5 and 7, it can be concluded that similar phenomena occur for the nanoclusters with N = 776. It is noticeable that the melting points reported in Table 2 were obtained by considering both the caloric curves and CNA analysis, because some other peaks in the heat capacity curves occur at temperatures which cannot be introduced as the melting points in the CNA approach.


image file: c7qi00370f-f4.tif
Fig. 4 Temperature-dependent percentage of fcc, hcp, and other atoms in the hollow monometallic and bimetallic NPs with N = 506.

image file: c7qi00370f-f5.tif
Fig. 5 Temperature-dependent percentage of fcc, hcp, and other atoms in the hollow monometallic and bimetallic NPs with N = 776.

image file: c7qi00370f-f6.tif
Fig. 6 Snapshots of cross-sections of the atomic arrangement of hollow NPs with N = 506 including (a) Pd, (b) Ni, (c) Ni@Pd, and (d) Pd@Ni, and also the distribution of fcc, hcp, bcc, and other atoms at different representative temperatures during the heating process. Coloring denotes type of atom: dark blue, Pd atom; dark green, Ni atom; red, hcp atom; light green, fcc atom; light blue, bcc atom; and light gray, other atom.

image file: c7qi00370f-f7.tif
Fig. 7 Snapshots of cross-sections of the atomic arrangement of hollow NPs with N = 776 including (a) Pd, (b) Ni, (c) Ni@Pd, and (d) Pd@Ni, and also the distribution of fcc, hcp, bcc, and other atoms at different representative temperatures during the heating process. Coloring denotes type of atom: dark blue, Pd atom; dark green, Ni atom; red, hcp atom; light green, fcc atom; light blue, bcc atom; and light gray, other atom.

Since, most of the catalytic activities are related to the surface of the nanoclusters and also as many of the catalytic reactions take place at high temperatures, the redistribution of atoms throughout the heating process affect the surface and catalytic activity of these materials. Therefore, it is important to study the atomic distribution of the surface atoms of NPs. The number of surface atoms at different temperatures for hollow nanoclusters with N = 506 and 776 are shown in Fig. 8 and 9, respectively. For all of the simulated nanoclusters, especially the pure Ni and Pd ones, the number of surface atoms is increased with temperature, which indicates an extension of their structure by temperature in such a way that at higher temperatures the nanoclusters are converted to a uniform ball (see Fig. 6 and 7). As is obvious from these figures, the Ni@Pd nanoclusters are stable at higher temperatures and the positions of the Ni and Pd atoms are retained, approximately. However, for Pd@Ni NPs, due to the larger atomic radii of Pd atoms and the existence of structural strain, the thermal stability is decreased, and during the heating process, Pd atoms tend to move from the core toward the surface, whereas Ni atoms migrate in the reverse direction. These movements of Ni and Pd atoms decrease the mentioned structural strain. The mentioned movements of atoms for Pd@Ni nanoclusters with N = 506 are slower than those with N = 776. This phenomenon is considered a result of the void space. For N = 776, the void space collapses at lower temperature (400 K); however, for N = 506, the filling of the void space takes place at higher temperature (900 K). Therefore, for N = 506, diffusion of Pd atoms toward the surface and filling of the void space occur simultaneously, which leads to a slower migration of Pd atoms to the shell region. Therefore, for N = 506, at 900 K a sudden jump is observed for the number of surface atoms, whereas w smoother jump is seen for Pd@Ni nanoclusters with N = 776 at 1200 K (see Fig. 8 and 9). By considering these facts, the differences between the thermodynamic stabilities of the simulated Ni–Pd NPs can be summarized using the data in Table 3. The lower surface energy of Pd atoms can create a shell with lower surface energy and increase the thermodynamic stability of the nanocluster. Also, the more cohesive and binding energy of Ni atoms are more favored for the core region with the maximum number of Ni–Ni bonds and higher coordination number. Also, the smaller size of Ni atoms in comparison with Pd ones (10% size mismatch) reduces the compressive strain in the core region. Moreover, the electronegativity of Pd is higher than that of Ni. Since nanoclusters prefer charge concentration in regions with a smaller coordination number, therefore Pd atoms with higher electronegativity are favored to be distributed on the shell of the nanocluster.


image file: c7qi00370f-f8.tif
Fig. 8 Number of surface atoms plotted against temperature for hollow monometallic and bimetallic NPs with N = 506.

image file: c7qi00370f-f9.tif
Fig. 9 Number of surface atoms plotted against temperature for hollow monometallic and bimetallic NPs with N = 776.

The order parameter σ was also calculated for the simulated nanoclusters and was defined by:101

 
image file: c7qi00370f-t4.tif(4)
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. Here, σ ≈ 1 indicates complete segregation, while σ ≈ 0 for mixing, and σ ≤ 0 indicates a layer-like order. The σ parameter versus temperature for the hollow nanoclusters, including pure Ni and Pd, Ni@Pd and Pd@Ni with N = 506 and 776 are presented in Fig. 10 and 11, respectively. For pure nanoclusters, the σ values are always equal to one. For N = 506, the Ni@Pd NP σ parameter is approximately constant, which resembles its thermal stability. However, for Pd@Ni NP, diffusion of the Pd atoms from the core to the shell region starts from 800 K, which leads to a decrease in the σ parameter, which is due to an increase in the Ni–Pd bonds during the diffusion process. At higher temperatures, the concentration of Pd atoms on the shell of the nanocluster results from the increase in the σ parameter. Finally, at 1200 K, the σ parameter reaches its initial value, σ ≈ 0.5. At this temperature, all the Pd atoms are concentrated in the shell region and a new Ni@Pd core–shell structure is obtained. This new structure has a stable form at higher temperatures with a constant value of σ. For Ni@Pd nanocluster with N = 776, the initial decrease of σ within the temperature range 400 K to 500 K indicates surface melting in the shell region and a slight mixing of Ni and Pd atoms at this condition (see Fig. 7(c)). For the Pd@Ni nanoclusters with N = 776, similar to N = 506, diffusion of the Pd atoms toward the shell region starts in the early stages of the heating process (from 400 K), which lowers the σ parameter. Again at 1200 K, all of the Pd atoms are concentrated on the surface and a new Ni@Pd core–shell structure is created with the sudden increase of σ to σ = 0.5.


image file: c7qi00370f-f10.tif
Fig. 10 Order parameter (σ) as a function of temperature for hollow monometallic and bimetallic NPs with N = 506.

image file: c7qi00370f-f11.tif
Fig. 11 Order parameter (σ) as a function of temperature for hollow monometallic and bimetallic NPs with N = 776.

Conclusion

The aim of this work was to investigate the thermal and structural properties of pure Ni and Pd and Ni@Pd and Pd@Ni nanoclusters with core–shell hollow structures by molecular dynamics simulations. For hollow bimetallic nanoclusters, two sizes with 506 and 776 atoms were considered with equal mole fractions of Ni and Pd. Also, various methods were used to better analyze the simulation results, including curves, heat capacities, common neighbor analysis, number of surface atoms, and ordering parameters. Using these approaches, the effects of the nanocluster size on the structural and thermal properties were investigated. Moreover, the structural transformations during the heating process in the simulated nanoclusters were also determined.

Conflicts of interest

There are no conflicts to declare.

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