Stability Control of AgPd@Pt Trimetallic Nanoparticles via Ag-Pd Core

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Materials and Interfaces

Stability Control of AgPd@Pt Trimetallic Nanoparticles via AgPd Core Structure and Composition: A Molecular Dynamics Study Hamed Akbarzadeh, Mohsen Abbaspour, Esmat Mehrjouei, and Maliheh Kamrani Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00447 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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Industrial & Engineering Chemistry Research

Stability Control of AgPd@Pt Trimetallic Nanoparticles via Ag-Pd Core Structure and Composition: A Molecular Dynamics Study Hamed Akbarzadeh1*, Mohsen Abbaspour1, Esmat Mehrjouei1, Maliheh Kamrani1

Department of Chemistry, Faculty of Basic Sciences, Hakim Sabzevari University, 96179- 76487 Sabzevar, Iran

*Correspondence addressed: Email: [email protected] Tel.: +98 5144013339

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this research, we have simulated AgPd@Pt nanoparticles with different core structures (including the fcc, bcc, amorphous, phase-separated, and ordered structures) and different core compositions in the heating process. Our investigations showed that the phase-separated core nanocluster has higher melting temperature and therefore, it has more thermal stability than other clusters while the bcc core nanocluster has lower melting temperature. The melting point also increases by increasing the Pd mole fraction in the cluster core. The results also showed a decrease in the configurational energy, CV, and surface energy before the melting point which is due to the diffusion of the Ag and Pd atoms to the clusters surface before the melting points. By increasing the temperature more, the different atoms of the clusters mix and form alloy structures. These results have been also approved by the radial chemical distribution function (RCDF). In order to present a complete structural investigation, the atomic strain distribution has been calculated for AgPd@Pt nanoclusters with the various core structures and compositions. The results showed that the palladium and silver atoms in the nanocluster core have more strain than the platinum atoms in the nanocluster shell. The tensile strain also exists in the cluster core and the compressive strain exists in the nanocluster shell.


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1. Introduction The alloying of monometallic nanoparticle (NP) with a second metal is a method to enhance its catalytic activity1. It has been reported that bimetallic NPs, composed of two different metals, exhibit considerable catalytic properties in comparison to monometallic NPs. Also, studies have exhibited that trimetallic NPs have superior catalytic activities compared with mono- and bimetallic counterparts2-5. There are many investigations about bimetallic nanoparticles6-12 whereas there are a few reports for structure and stability of trimetallic NPs including Pd, Ag, and Pt in the literature3,13-16. Also, there are rare theoretical studies about the trimetallic nanoparticles17-24. Tao et al. studied the stable structure of tetra hexahedral Pt-X-Cu (X=Au, Ag, Pd, and Rh) trimetallic NPs via adopting quantum Sutton-Chen (Q-SC) many body potentials, and an improved genetic algorithm.17 They also investigated the structural stability and segregation behavior of the Pt-alloy NPs with different sizes and compositions. Their results showed that Pt atoms have a certain tendency to segregate on the surface, while Ag, Pd, Au, and Cu atoms have a strong tendency to segregate on the surface. Wu et al. investigated the structural characterization of metal dopants in trimetallic M-Pd-Pt (M= Au and Ag) nanoclusters which are optimized by adaptive immune optimization algorithm18. They reported that the structural stability of Au-Pd-Pt and Ag-Pd-Pt nanoclusters is different. They also showed that the Pd and Pt atomic distribution in Ag-Pd-Pt and Au-Pd-Pt is similar, but Au atoms grow more closely than Ag atoms. Wu et al. investigated geometrical and energetic properties of

trimetallic Au-Pd-Pt nanoclusters by using the Gupta potential and an adaptive immune optimization algorithm (AIOA)19. They exhibited that Au and Pt atoms have tendency to be positioned at the surface and center of Au-Pd-Pt, respectively, in such a way that Pd atoms have more tendency to distribute into Au atoms. Akbarzadeh et al. studied the thermal, structural and dynamical properties of trimetallic Au-Cu-Ni nanoclusters under heating and cooling processes by molecular dynamics (MD) simulation20. They found that Au-Cu-Ni nanoclusters form homogeneous configurations. In another work, Akbarzadeh et al. used MD simulation in order to study the heating and cooling processes of the Ag@CoFe trimetallic nanoclusters21. They indicated that the Ag and Co atoms locate at the surface and the core of the nanocluster, respectively. They also investigated the effect of addition of Co atoms to Ag@Fe bimetallic nanoclusters. Their results showed that the increasing of Co concentration enhances the thermal stability of the Ag3 ACS Paragon Plus Environment


Co-Fe trimetallic nanoclusters. Liu at al. studied the effects of the size and composition on the atomic structure of trimetallic Pt-Pd-Rh nanoparticles via the improved genetic algorithm (GA) and by using the quantum Sutton-Chen many body potentials22. Their results indicated that Pd and Pt atoms preferred to be located on the surface while Rh atoms preferred to be located in the core. Recently, Akbarzadeh et al. investigated the thermodynamic stability of Pd@Au@Pd three-shell nanoparticles and the effects of nanocluster size and composition on their stability via MD simulation 23. Their results demonstrated that the melting point of Pd@Au@Pd NPs enhances with increasing of NPs size and decreasing of Au concentration. Akbarzadeh et al. also studied the thermodynamic stability of AgPd@Pt nanoclusters with different morphologies of cuboctahedron (CO), icosahedron (Ih), decahedron (Dh), octahedron (Oh), and Marks-decahedron (m-Dh) by MD simulation24. They found that the thermal stability of AgPd@Pt NPs is dependent on their morphologies and obey the following trend: m-Dh > Oh > Dh > CO> Ih. It is also shown that the surface effect is the most important effect on the thermal stability of these nanoclusters. In the recent years, the study of metallic NPs, in particular Pt- and Pd-based NPs, have attracted a wide attention due to their higher activity in various catalytic reactions such as fuel cells25-30. It has been demonstrated introducing a cheap metal such as Ag at the core of these NPs decrease the cost of metallic catalysts and demonstrate superior catalytic activity3,31. It has been also exhibited that the properties of bimetallic and trimetallic NPs can be tuned by composition, size, and atomic ordering32-34. Despite these studies, there is not any investigation about the effect of structure and composition of the core on the thermal stability of trimetallic NPs. It is obvious that these effects can exhibit important properties changes of the trimetallic NPs such as the stability which can be useful for catalytic applications. Therefore, in this work we have investigated the effect of structure and composition of the nanocluster core on the thermal and structural stability of AgPd@Pt coreshell trimetallic NPs3 via MD simulation.

2. Simulation method In this study, our main aim is to investigate the influence of structure and composition of the cluster core on the thermal stability of AgPd@Pt core-shell trimetallic nanoparticles. In order to reach this aim, cuboctahedral AgPd@Pt nanoclusters with 923 atoms were selected for MD simulations. This study can be divided into two steps. In each step, five types NPs were considered. totally, ten 4 ACS Paragon Plus Environment

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trimetallic NPs were examined. Step 1: in order to investigate the effect of AgPd core structure, Ag0.5Pd0.5@Pt with five difference structures (face centered cubic (fcc), body centered cubic (bcc), amorphous, phase-separated, and ordered core) were considered. Note that in all these NPs, Pt atoms in the cluster shell have been kept in fcc lattice. In fcc, bcc, and amorphous cores, Ag and Pd atoms are distributed randomly. The number of Pt atoms in the shell was 362 which it is equal for all the AgPd@Pt NPs. Step 2: in order to investigate the effect of core composition, five Ag/Pd ratios, i.e., Ag@Pt, Ag0.7Pd0.3@Pt, Ag0.5Pd0.5@Pt, Ag0.3Pd0.7@Pt, and Pd@Pt were considered. Note that Ag and Pd atoms in these nanoclusters are distributed randomly in the core and the number of Pt atoms in the shell was 362 which it is equal for all the AgPd@Pt nanoclusters. The schematic illustrations of AgPd@Pt NPs with various core structures and compositions have been presented in Fig. 1. To reach the stable structures, all the NPs have been heated from 1 K to 300 K by the increment of 50 K. After full relaxation, the different types of NPs have been subjected to a continuous heating from 300 K to 2000 K by the increment of 50 K. Around the melting temperature, a smaller increment of 10 K has been also adopted to examine the melting temperature more accurately. Every simulation of heating process has been performed for 1 ns. The time step was also 1 fs. The DL_POLY classic software35 was employed to simulate the melting behavior of the nanoclusters. All MD simulations were carried out in canonical ensemble (NVT) by using NoséHoover thermostat36,37. The equations of atomic motion were investigated by Verlet-leapfrog algorithm38 with a time step of 1 fs. The QSC many body potential39,40 was adopted to describe the interatomic interactions. Table 1 shows the QSC parameters of Pd-Pd, Ag-Ag, and Pt-Pt potentials. For Ag-Pt, Ag-Pd, and Pd-Pt potentials, the geometric mean has been used for the energy parameter 𝜀 and the arithmetic mean has been used for the parameters a, n, and m.

3. Results and discussion 3.1 Effect of the core structure The configurational energy of the AgPd@Pt nanoclusters with the different core structures including the fcc, bcc, amorphous, phase-separated, and ordered structures have been presented in 5 ACS Paragon Plus Environment


Fig. 2. As this figure shows, the configurational energy almost changes linearly by increasing the temperature and then, it shows a sudden change at the melting point which indicates a solid-liquid phase transition. To determine the melting temperatures with more precision, we have also computed the heat capacity at constant volume (CV) and presented in Fig. 2. As Fig. 2 shows, the melting temperatures can be better distinguished by the sharp peaks in the CV plots. The obtained melting points of the AgPd@Pt clusters have been presented in Fig. 3. This figure shows that the phase-separated core nanocluster has higher melting temperature and therefore, it has more thermal stability than other clusters while the bcc core nanocluster has lower melting temperature and less thermal stability than other nanoclusters. It is also shown that all trimetallic AgPd@Pt nanoclusters have lower melting temperatures than the corresponding bulk metals which is due to the higher surface/volume ratio in the nanoclusters than the bulk metals.41,42 As can be seen in Fig. 2, all of the nanoclusters show a decrease of configurational energy and a negative peak of CV before the melting point. In order to more investigate this phenomenon, we have calculated the surfaces energies of the different AgPd@Pt nanoclusters during the heating process and presented in Fig. 4. According to this figure, the surface energies of the different clusters decrease before the melting points. These results are in agreement with the configurational energy and CV results in Fig. 2. The schematic illustration of the AgPd@Pt trimetallic nanoclusters with various core structures have been also presented during the heating process in Fig. 5. As can be seen in Fig. 5, the Ag and Pd atoms diffuse to the clusters surface before the melting points. This migration of Ag and Pd atoms to the surface decreases the surface energy of the cluster because silver and palladium have smaller surface energies than platinum. By increasing the temperature more, the different atoms of the clusters mix and form alloy structures. It is also shown that the Ag atoms have faster diffusion than the Pd atoms. This result can be better observed in the phase-separated core structure in Fig. 5. It should be also noted that the structure of AgPd@Pt nanoalloy is not the most stable configuration from the energetic point of view. Therefore, it is expected the mobility of internal atoms is activated at high temperatures but below melting and Ag and Pt atoms move to the surface and energy decreases. Therefore, a negative peak of CV before the melting point is expected to be observed. In order to investigate the chemical ordering of the different atoms within the AgPd@Pt nanoclusters, we have calculated the radial chemical distribution function (RCDF)43 of the phase6 ACS Paragon Plus Environment

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separated core structure and presented in Fig. 6. The initial core-shell structure can be distinguished in the cluster at 300 K. The Ag and Pd atoms diffuse from the cluster core to the cluster surface by increasing temperature whereas the Pt atoms diffuse from the surface to the core. By increasing the temperature more, the different atoms mix and form the alloy disordered structure before the melting point. This behavior can be drastically observed after the melting point. The similar trend of RCDF has been observed for the different nanoclusters. To characterize the local nanoclusters structures, we have calculated the common neighbor analysis (CNA)44 for the clusters and presented (as the percent) in Fig. 7. The CNA has been frequently used to examine the structural change under mechanical deformation and heating process45,46. The snapshots of cross sections of distributions of fcc, hcp, bcc, and other atoms in the different AgPd@Pt trimetallic nanoclusters have been presented during the heating process in Fig. 8. According to Figs. 7 and 8, the amount of the ordered atoms (the atoms with fcc, hcp, and bcc structures) decreases while the amount of the disordered atoms (other atoms) increases during the heating process which indicate the gradual melting of the nanoclusters. At the melting points, the amount of the ordered atoms is zero which are in good agreement with the configurational energies and CV plots in Fig. 2. It is also shown in Fig. 8 that the amount of the other atoms increases from the clusters surface toward the clusters core which indicates that the melting of the clusters initiates from their surfaces. It is also shown in Figs. 7 and 8 that a sudden growth of the fcc atoms can be observed for the amorphous core structure at the range of 300-400 K. This structure transformation from the amorphous to the fcc at low temperature shows the instability of the amorphous structure. It is also shown that the bcc atoms in the bcc core structure transform to fcc structure at the range of 300-400 K. Our results are also in good agreement with the experimental results47-51 of trimetallic nanoclusters. These experimental works reported the synthesis of trimetallic nanoalloys in the fcc and ordered structures. For instance, Khanal et al.47 synthesized and characterized AuCu/Pt trimetallic nanoparticles by scanning transmission electron microscopy (TEM) technique. They concluded that the Cu, Au, and Pt metals share the fcc structure. Zhang et al.48 synthesized Pt-Pd-Au ternary alloy nanoparticles on graphene sheets. The nanoclusters were further characterized by TEM, energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which indicated that the Pt-Pd-Au nanoparticles were ternary alloys with fcc 7 ACS Paragon Plus Environment


structure. Li and Takahashi49 prepared Fe-Co-Ni ternary nanoalloys and then investigated their structures by XRD and TEM. They found that the nanoparticles progressively have a fcc, bcc and a mixture of fcc and bcc structure. Wang et al.50 synthesized structurally ordered platinum–cobalt core@shell nanoclusters with enhanced stability and activity as oxygen reduction electrocatalysts. They showed that the core@shell Pt3Co@Pt/C nanoparticles are more active and durable than the disordered Pt3Co=C alloy. Kuttiyiel et al.51 also reported the synthesis of a structurally ordered Au-Pd-Co nanoparticles which exhibit catalytic activities in both acid and alkaline media. The strain effect is very important for the nanoalloys structure.52,53 Therefore, we have calculated the atomic strain distribution for AgPd@Pt clusters with the different core structures at 300 K and presented in Fig. 9. According to this figure, Pd and Ag atoms in the nanocluster core have more strain than the platinum atoms in the nanocluster shell. This result has been better found in Fig. 10. According to Figs. 9 and 10, the tensile strain exists in the nanocluster core and the compressive strain exists in the nanocluster shell. It should be noted that in the normal core-shell structures, the nanocluster core is under compressive strain and the nanocluster surface is under tensile strain. The reason of this different behavior is due to the misfit between palladium and silver atoms in the nanocluster core and the platinum atoms in the nanocluster shell because the silver atoms in the nanocluster core has the highest lattice constant. To accommodate this misfit, the platinum atoms in the cluster shell tend to shrink whereas the silver and palladium atoms in the cluster core expand. Thus, the silver and palladium atoms in the nanocluster core suffer the tensile strain and diffuse to the nanocluster shell. This behavior represents itself as the transformation in the structure before the melting temperature. It is also possible to calculate the local stress54 of the different atoms in the AgPd@Pt nanocluster to show that the Pd and Ag atoms inside the cluster core undergo more stress than the platinum atoms in the cluster shell. Thus, the silver and palladium atoms in the nanocluster core diffuse to the nanocluster shell to release the stress.

3.2 Effect of the core composition The configurational energy of the AgPd@Pt clusters with the different compositions of silver and palladium in the cluster core have been presented in Fig. 11. The sharp change in the configurational energy and CV indicates a solid-liquid phase transition. The obtained melting points of the AgPd@Pt clusters were presented in Fig. 12. As this figure shows, the melting point 8 ACS Paragon Plus Environment

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and thermal stability of the clusters increase by increasing the Pd mole fraction (and decreasing the Ag mole fraction) in the cluster core. This is due to the fact that the cohesive energy of Pd is greater than Ag (The cohesive energy of Ag and Pd is 2.95 and 3.89 eV, respectively). It is also shown in Fig. 11 that the trimetallic nanoclusters show a decrease of configurational energy and CV before the melting point whereas the bimetallic clusters do not show such clear decrease. In order to more investigate this phenomenon, we have also calculated the surfaces energies of the different nanoclusters during the heating process and presented in Fig. 13. According to this figure, the surface energies of the trimetallic clusters decrease before the melting temperatures (such as the configurational energy and CV results) but, we cannot observe such decrease in the surface energy of the bimetallic clusters. The snapshots of the different nanoclusters have been also presented during the heating process in Fig. 14. This figure shows that the silver and palladium atoms diffuse to the clusters surface before the melting points which decrease the surface energy of the cluster. Due to the diffusion of both Ag and Pd atoms to the surface of the trimetallic clusters, the amount of decrease of the surface energy is greater than the bimetallic clusters in which one of these metals (Ag or Pd) migrates to the cluster surface. We have also calculated the RCDF plot for the Ag0.5Pd0.5@Pt cluster and presented in Fig. 15. According to this figure, the silver and palladium atoms diffuse from the cluster core to the cluster surface whereas the Pt atoms diffuse from the surface to the core by increasing the temperature and finally, the different atoms mix and form the alloy disordered structure. Figure 16 shows the temperature dependence of fcc, bcc, hcp, and other atoms in the AgPd@Pt clusters during the heating process has been presented in Fig. 16. We have also presented the snapshots of cross sections of distributions of hcp, fcc, bcc, and other atoms in the different clusters in the heating process in Fig. 17. According to these figures, the amount of the ordered atoms decreases from the cluster surface while the amount of the disordered atoms increases during the heating process. Finally, we have calculated the atomic strain distribution for AgPd@Pt clusters with the different core compositions at 300 K and presented in Fig. 18. This figure shows that the Pd and Ag atoms in the nanocluster core have more strain than the Pt atoms in the nanocluster shell. This can be better found in Fig. 19. Therefore, the tensile strain exists in the nanocluster core and the compressive strain exists in the nanocluster shell. This result is due to the misfit between Pd and 9 ACS Paragon Plus Environment


Ag atoms in the nanocluster core and the platinum atoms in the nanocluster shell because the silver atoms in the nanocluster core has the highest lattice constant. To accommodate this misfit, the platinum atoms in the cluster shell tend to shrink whereas the silver and palladium atoms in the cluster core expand. Thus, the silver and palladium atoms in the nanocluster core suffer the tensile strain and diffuse to the nanocluster shell. This behavior represents itself as the transformation in the structure before the melting temperature55-60.

4. Conclusion We have simulated the trimetallic nanoclusters of AgPd@Pt with the different core structures (including the fcc, bcc, amorphous, phase-separated, and ordered structures) in the heating process. Our investigations of the core structure showed that the phase-separated core nanocluster has higher melting temperature and therefore, it has more thermal stability than other clusters while the bcc core nanocluster has lower melting temperature and less thermal stability than other nanoclusters. The results also showed a decrease in the configurational energy, CV, and surface energy before the melting point which is due to the diffusion of the Ag and Pd atoms to the clusters surface before the melting points. By increasing the temperature more, the different atoms of the clusters mix and form alloy structures. These results have been also approved by the RCDF plots. We have also shown that the amount of the ordered atoms (the atoms with fcc, hcp, and bcc structures) decreases while the amount of the disordered atoms increases during the heating process. Also, the amount of the other atoms increases from the clusters surface toward the clusters core which indicates the surface melting. In order to present a complete structural investigation, we have also calculated the atomic strain distribution for AgPd@Pt clusters with the different core structures. The results showed that the silver and palladium atoms in the nanocluster core have more strain than the platinum atoms in the nanocluster shell. Thus, the tensile strain exists in the nanocluster core and the compressive strain exists in the nanocluster shell. This result is due to the misfit between Pd and Ag atoms in the nanocluster core and the platinum atoms in the nanocluster shell. To accommodate this misfit, the platinum atoms in the cluster shell tend to shrink whereas the silver and palladium atoms in the cluster core expand. Thus, the silver and palladium atoms in


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the nanocluster core diffuse to the nanocluster shell. This behavior represents itself as the transformation in the structure before the melting temperature. Our investigations of the core composition showed that the melting point and thermal stability of the clusters increase by increasing the Pd mole fraction in the cluster core. The trimetallic nanoclusters also show decreases of configurational energy, CV, and surface energy before the melting point whereas the bimetallic clusters do not show such decrease which can be due to the diffusion of both Ag and Pd atoms to the surface of the trimetallic clusters. These results have been also approved by the RCDF plots. We have also shown that the amount of the ordered atoms decreases from the surface of the cluster whereas the amount of the disordered atoms increases during the heating process. The atomic strain distribution for AgPd@Pt clusters with the different core compositions have been also studied. The results showed that the Pd and Ag atoms in the nanocluster core have more strain than the Pt atoms in the nanocluster shell and therefore, the Ag and Pd atoms in the nanocluster core diffuse to the nanocluster shell. The structure transformation before the melting temperature approves this phenomenon.

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Table 1: Sutton-Chen potential parameters for the Pd-Pd, Ag-Ag and Pt-Pt interactions. QSC

𝜀 (eV)

a (Å)

n

m

c

Pd-Pd

0.0032864

3.8813

12

6

148.205

Ag-Ag

0.0039450

4.0691

11

6

96.524

Pt-Pt

0.0097894

3.9163

11

7

71.336

Figure captions Figure 1. Schematic illustration of (top) AgPd@Pt NPs with various core structures consisting of (a) fcc core, (b) bcc core, (c) amorphous core, (d) ordered core, and (e) phase-separated core and (bottom) AgPd@Pt NPs with various core compositions consisting of (f) Ag@Pt, (g) Ag0.7Pd0.3@Pt, (h) Ag0.5Pd0.5@Pt, (i) Ag0.3Pd0.7@Pt, and (j) Pd@Pt. Coloring denotes type of atom: blue, Pt atom; violet, Pd atom; and grey, Ag atom.



Figure 2. Potential energy (solid lines) and specific heat capacity (dashed lines) of AgPd@Pt trimetallic nanoclusters with various core structures as a function of temperature. Figure 3. Melting points of AgPd@Pt trimetallic nanoclusters with various core structures. Figure 4. The surface energy of AgPd@Pt trimetallic nanoclusters with various core structures. Figure 5. Schematic illustration of AgPd@Pt trimetallic nanoclusters with various core structures consisting of (a) fcc core, (b) bcc core, (c) amorphous core, (d) ordered core, and (e) phaseseparated core during the heating process. Coloring denotes type of atom: blue, Pt atom; violet, Pd atom; and grey, Ag atom. Figure 6. RCDF during the melting of AgPd@Pt trimetallic nanocluster with phase-separated core structure (Different atomic types are distinguishable by different colors: blue, Pt; violet, Pd, and grey, Ag). Figure 7. Temperature-dependent percentage of fcc, hcp, bcc, and other atoms for AgPd@Pt trimetallic nanoclusters with various core structures. Figure 8. Snapshots of cross sections of distribution of fcc, hcp, bcc, and other atoms in AgPd@Pt trimetallic nanoclusters with various core structures consisting of (a) fcc core, (b) bcc core, (c) amorphous core, (d) ordered core, and (e) phase-separated core during the heating process. Coloring denotes type of atom: green, fcc atom; red, hcp atom; blue, bcc atom; and light grey, other atom. Figure 9. Distribution of atomic strain for AgPd@Pt trimetallic nanoclusters with various core structures at 300 K. Note that the horizontal axis denots the distance between the atom and the particle center of mass. The horizontal dashed line indicate the zero value.

Figure 10. Strain maps of AgPd@Pt trimetallic nanoclusters with various core structures consisting of (a) fcc core, (b) bcc core, (c) amorphous core, (d) ordered core, and (e) phaseseparated core at 300 K. Each cluster is shown in two views: (left) strain map of the cluster surface, and (right) strain map of the cluster cross-section. Atoms are colored according to their local strain. The start value and end value in the color maps indicates minimum and maximum values of the strain. 18 ACS Paragon Plus Environment

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Figure 11. Potential energy (solid lines) and specific heat capacity (dashed lines) of AgPd@Pt trimetallic nanoclusters with various core compositions as a function of temperature. Figure 12. Melting points of AgPd@Pt trimetallic nanoclusters with various core compositions. Figure 13. The surface energy of AgPd@Pt trimetallic nanoclusters with various core compositions. Figure 14. Schematic illustration of AgPd@Pt trimetallic nanoclusters with various core compositions consisting of (a) Ag@Pt, (b) Ag0.7Pd0.3@Pt, (c) Ag0.5Pd0.5@Pt, (d) Ag0.3Pd0.7@Pt, and (e) Pd@Pt during the heating process. Coloring denotes type of atom: blue, Pt atom; violet, Pd atom; and grey, Ag atom. Figure 15. RCDF during the melting of Ag0.5Pd0.5@Pt trimetallic nanocluster (Different atomic types are distinguishable by different colors: blue, Pt; violet, Pd, and grey, Ag) Figure 16. Temperature-dependent percentage of fcc, hcp, bcc, and other atoms for AgPd@Pt trimetallic nanoclusters with various core compositions. Figure 17. Snapshots of cross sections of distribution of fcc, hcp, bcc, and other atoms in AgPd@Pt trimetallic nanoclusters with various core compositions consisting of (a) Ag@Pt, (b) Ag0.7Pd0.3@Pt, (c) Ag0.5Pd0.5@Pt, (d) Ag0.3Pd0.7@Pt, and (e) Pd@Pt during the heating process. Coloring denotes type of atom: green, fcc atom; red, hcp atom; blue, bcc atom; and light grey, other atom. Figure 18. Distribution of atomic strain for AgPd@Pt trimetallic nanoclusters with various core cmpositions at 300 K. Note that the horizontal axis denots the distance between the atom and the particle center of mass. The horizontal dashed line indicate the zero value.

Figure 19. Strain maps of AgPd@Pt trimetallic nanoclusters with various core compositions consisting of (a) Ag@Pt, (b) Ag0.7Pd0.3@Pt, (c) Ag0.5Pd0.5@Pt, (d) Ag0.3Pd0.7@Pt, and (e) Pd@Pt at 300 K. Each cluster is shown in two views: (left) strain map of the cluster surface, and (right) strain map of the cluster cross-section. Atoms are colored according to their local strain. The start value and end value in the color maps indicates minimum and maximum values of the strain.



Figure 1


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Figure 2

-3.4

-3.6

Fcc core Bcc core Amorphous core Phase-separated core Ordered core

0.006

0.004

-3.8 0.002 -4.0 0.000 -4.2 -0.002 -4.4 300

600

900

1200

1500

Temperature (K)


1800

2100

Heat capacity per atom (eV/K)

Configurational energy per atom (eV/atom)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Figure 3

1600

1400

1200

1210

1200

1150

1140

1100

1000

800

Core structure


ore Bc cc

ho us Am orp

de Or

co re

red co re

cc ore Fc

se -se pa rat ed

co

re

600

Ph a

Melting point (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 39

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Figure 4

0.00018

Fcc core Bcc core Amorphous core Ordered core Phase-separated core

0.00016

Surface energy (eV/Å2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.00014 0.00012 0.00010 0.00008 0.00006 0.00004 0.00002 300

600

900

1200

1500

Temperature (K)


1800

2100


Figure 5


Page 24 of 39

Page 25 of 39

Figure 6

16

Ag Pd Pt

Phase-separated core 14 12

Radius(Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 8 6 4 2 0 200

400

600

800

1000

1200

Temperature (K)


1400


Figure 7

0 96

300

600

900

1200 1500 1800 2100

Fcc core

64 32 0

Bcc core

90

Percent of atom types

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 30 0

Amorphous core

117 78 39 0 96

Ordered core

64 32 0 99

Phase-separated core Other Fcc Hcp Bcc

66 33 0 0

300

600

900

1200 1500 1800 2100

Temperature (K)


Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8



Figure 9

0 1.14

4

8

12

16

0.00 -0.57 0.34 0.17

20

Ag Pd Pt

Fcc core

0.57

Bcc core

0.00 -0.17

Strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

0.190

Amorphous core

0.095 0.000 -0.095 0.048 0.024

Phase-separated core

0.000 -0.024 0.064 0.032

Ordered core

0.000 -0.032 0

4

8

12

r(Å)


16

20

Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10



Figure 11

0.012

-3.7

Ag@Pt Ag0.7Pd0.3@Pt Ag0.5Pd0.5@Pt Ag0.3Pd0.7@Pt Pd@Pt

-3.8 -3.9 -4.0

0.010 0.008 0.006

-4.1

0.004

-4.2

0.002

-4.3

0.000

-4.4

-0.002 300

600

900

1200

1500

Temperature (K)


1800

2100

Heat capacity per atom (eV/K)

Configurational energy per atom (eV/atom)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 39

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Figure 12

1600

1250

Pd @P t

1200

1200

1240

.7@ Pt

1400

1100 1020 1000

800

Pd 0 0.3 Ag

Ag

Pd 0 0.7 Ag

0.5 Pd 0.5 @P t

.3@ Pt

@P t

600

Ag

Melting point (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Core composition



Figure 13

0.00020

Surface energy (eV/Å2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

0.00015

0.00010

0.00005

Ag@Pt Ag0.7Pd0.3@Pt Ag0.5Pd0.5@Pt

0.00000

Ag0.3Pd0.7@Pt Pd@Pt

-0.00005 300

600

900

1200

1500

Temperature (K)


1800

2100

Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 14



Figure 15

16

Ag Pd Pt

Ag0.5Pd0.5@Pt 14 12

Radius(Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

10 8 6 4 2 0 200

400

600

800

1000

1200

Temperature (K)


1400

Page 35 of 39

Figure 16

0

300

600

900

1200 1500 1800 2100

Ag@Pt

96 64 32 0

Ag0.7Pd0.3@Pt

96

Percent of atom types

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


64 32 0

Ag0.5Pd0.5@Pt

96 64 32 0

Ag0.3Pd0.7@Pt

96 64 32 0 96

Pd@Pt Other Fcc Hcp Bcc

64 32 0 0

300

600

900

1200 1500 1800 2100

Temperature (K)



Figure 17


Page 36 of 39

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Figure 18

0

4

8

12

0.026

16

20

Ag@Pt

0.013 0.000 -0.013 0.028

Ag0.7Pd0.3@Pt

0.014 0.000 -0.014

Strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ag0.5Pd0.5@Pt

1.54 0.77 0.00 0.036

Ag0.3Pd0.7@Pt

0.018 0.000 -0.018 0.0000

Pd@Pt

-0.0047 -0.0094 -0.0141 0

4

8

12

r(Å)


16

20


Figure 19


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TOC


Stability Control of AgPd@Pt Trimetallic Nanoparticles via Ag-Pd Core

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