Stability of Au-Pd Core-Shell Nanoparticles

Article pubs.acs.org/JPCC

Stability of Au−Pd Core−Shell Nanoparticles Carlos Fernández-Navarro*,† and Sergio Mejía-Rosales‡ †

Universidad Autónoma de Nuevo León, Preparatoria No. 25, Escobedo, Nuevo León, Mexico 66054 Facultad de Ciencias Físico-Matemáticas, Centro de Investigación en Ciencias Físico-Matemáticas (CICFIM), Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, Mexico 66455

‡

S Supporting Information *

ABSTRACT: The stability of Au−Pd alloys with sizes close to 3 nm and icosahedral, decahedral, and truncated octahedral geometries with AucorePdshell and PdcoreAushell elemental distributions have been studied using canonical molecular dynamics simulations. The analysis of excess energy show that the PdcoreAushell ordering is more stable than the AucorePdshell for particles of this size, while the analysis of the order parameter Q6 revealed that some of the particles with AucorePdshell ordering exhibited geometric and structural changes previous to melting of the particles. Analysis of the local density of the species revealed that these changes are due to diffusion of Pd atoms into the inner core of the particles. The geometry and structure of all of the particles with PdcoreAushell were preserved until just before the solid−liquid transition, as well as showing a lower melting temperature than the AucorePdshell particles.

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INTRODUCTION Metal nanoparticles have been continuously studied in the last decades, mostly because of the interest of using these systems in catalysis,1 sensors,2 magnetic devices, and optoelectronics.3,4 In particular, bimetallic nanoparticles have attracted the attention of both experimentalists and theoreticians due to the improvement of many of their properties compared against their monometallic counterparts,5,6 in a synergy based on the coexistence of the two metals present in the particles.7 Another reason for the increasing interest in these systems is the fact that their physical and chemical properties depend in great measure on the size, shape, composition, and local distribution of the chemical species, which opens the possibility of tuning these properties by design. Using the most recently developed synthesis techniques, it is posible to produce a great variety of nanoparticle shapes, such as truncated octahedral,8 decahedra,9 icosahedra,10 nanorods,11 tetrahedra,12 and others, either formed by homogeneous random alloys13 or by core−shell structures.14,15 The use of electron microscopy, in particular, of Z-contrast imaging techniques, has allowed determination that many of the bimetallic particles produced by several methods have in many of the cases a core−shell structure.6,16 The use of highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) has allowed investigation of the distribution of specific sites in the surface of a particle; in nanoparticles produced by the polyol method, it was found that Pd atoms were preferentially distributed at the surface as isolated atoms surrounded by a hexagonal array of Au atoms. These experimental findings agreed well with theoretical results from molecular dynamics (MD) simulations.17 These isolated © 2017 American Chemical Society

Pd sites may be of relevance in the catalytic activity of the nanoparticles. Several nanoalloys have been studied either within a theoretical approach18,19 or experimentally.20,21 From the theoretical perspective, for example, the structural rearrangement of Au−Pd small clusters on a titania substrate has been investigated using MD under spin-polarized Kohn−Sham density functional theory (DFT).22 The authors found that at finite temperature the Pd atoms migrated to the outer surface of the cluster, producing potential active sites. In a global optimization of Pd−Au bimetallic clusters using the Gupta many-body empirical potential, Pittaway et al.23 determined the lowest-energy structures in clusters of up to 50 atoms, finding that in clusters with Au−50%Pd50% some segregation of Au to the surface lowers the energy of the cluster, without the cluster becoming a core−shell structure. In another work where global optimization searches were made on 98-atom Au−Pd clusters using a Gupta potential, three different parametrizations were compared, finding that the parametrizations based on DFT calculations and on experimental bulk properties produce clusters with a high degree of Au−Pd mixing, while the parametrization based on the average of the pure Pd−Pd and Au−Au parameters produces core−shell configurations.24 In this same study, basin hopping Monte Carlo optimization followed by DFT relaxation produced FCC-HCP and Marks decahedra as the lowest-energy structural motifs, consistent with experimental observations made by the same group. All of these studies remark the intrinsic complexity of the energy Received: May 11, 2017 Revised: August 29, 2017 Published: September 6, 2017 21658

DOI: 10.1021/acs.jpcc.7b04564 J. Phys. Chem. C 2017, 121, 21658−21664

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The Journal of Physical Chemistry C

considered particles with a number of atoms that correspond to a size of approximately 3 nm.

landscape on Au−Pd clusters and nanoparticles and the difficulty in establishing which of the possible homotops of a given size and composition are the most energetically stable. Other studies contemplate the evolution of Au−Pd clusters supported in nanotubes and graphite,25 the study of growth mechanisms of Pd atoms on Au seeds by grand canonical simulations,26 and the melting of crown-jewel arrangements in Au−Pd clusters by MD simulations.27 From the experimental perspective, for example, Shoujie et al.28 synthesized Cu@Au particles with a range of sizes of 7.1 ± 0.5 nm, using a sequential reduction method and implementing a set of first-principles calculations in order to determine the stability of these particles. Deogratias et al.29 synthesized Ag−Pt particles of around 5 nm used for the oxygen reduction reaction (ORR) and used HAADF-STEM to determine that the particles had a core−shell structure. Pérez-Tijerina et al.,10 using an inert gas condensation system, produced Au−Pd nanoparticles with sizes of 1, 3, and 5 nm, finding that the particles were mostly icosahedral, with less frequent production of decahedra. Among the group of bimetallic nanoalloys, the Au−Pd system has been one of the most frequently studied due to its potential use in catalysis. This nanoalloy has been used in the direct synthesis of hydrogen peroxide (H2O2), benzyl alcohol hydrogenation,30 alcohol to aldehyde oxidation,6,31,32 CO oxidation, hydrocarbon hydrogenation, vinyl acetate synthesis,33−35 and hydrogen absorption.36 The electrocatalytic performance by Aucore−Pdshell relatively large nanoparticles (around 19 nm) has been optimized with particles synthesized by ascorbic acid reduction using Au particles as seeds, in ethanol oxidation reactions; it was found that the activity of the particles is strongly correlated to the density of defects at the surface of the particles, in what is an example of the relevance of both composition and structure at the surface.37 In a novel experimental setting, Aucore−Pdshell’s were prepared by the synthesis of Aucore−Agshell particles and the posterior replacement of Ag by Pd via a galvanic replacement reaction;38 the authors report an improvement in activity and durability in catalyzing ORR, likely due to the lattice stresses produced by Ag removal. Recently, it has been demonstrated that it is possible to improve the catalytic activity of the Au−Pd alloy by increasing the concentration of Pd up to a certain value and that for higher Pd concentrations its catalytic activity gets reduced.30,31 Thus, it is expected that the maximum catalytic activity in a particle with a fixed relative concentration of Au and Pd is related to the size of the core in a core−shell system because this size will determine the fraction of atoms of a particular species that will remain available to be located at the surface of the particle. Furthermore, because catalysts are usually used at relatively large temperatures, it is important to understand the structural stability of the particles as they are subjected to heating processes.39 Taking these facts into consideration, the aim of this work is to implement a series of MD simulations at several temperatures in order to determine the behavior of the structural stability of Au−Pd core−shell nanoparticles when the relative concentrations of the species are varied. The study considers both Aucore−Pdshell and Pdcore−Aushell particles and several geometries because these particles are important in catalytic and optical applications.40−42 Because it is known that properties are strongly dependent on size in particles with diameters ranging between 1 and 3 nm,43 in this study we

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METHODS Three geometries were considered in this study: icosahedra (Ih), Marks decahedra (Dh), and truncated octahedra (TO). All of the particles were formed by concentric layers, and the number of atoms per layer at each geometry is described in Table 1. Each particle is identified using the number of layers Table 1. Size and Composition of the Icosahedral, Decahedral, and Truncated Octahedral Particles Considered in This Study particle

Ih and Dh

TO

Au1Pd4 Au2Pd3 Au3Pd2 Au4Pd1 Pd1Au4 Pd2Au3 Pd3Au2 Pd4Au1

Au13Pd548 (Au3%Pd97%) Au55Pd506 (Au10%Pd90%) Au147Pd414 (Au26%Pd74%) Au309Pd252 (Au53%Pd47%) Pd13Au548 (Pd3%Au97%) Pd55Au506 (Pd10%Au90%) Pd147Au414 (Pd26%Au74%) Pd309Au252 (Pd53%Au47%)

Au6Pd580 (Au1%Pd99%) Au44Pd542 (Au8%Pd92%) Au140Pd446 (Au24%Pd76%) Au314Pd272 (Au54%Pd46%) Pd6Au580 (Pd1%Au99%) Pd44Au542 (Pd8%Au92%) Pd140Au446 (Pd24%Au76%) Pd314Au272 (Pd54%Au46%)

formed exclusively by Au or Pd atoms in the following way: Aucore−Pdshell and Pdcore−Aushell, in such a way that, for example, Au1Pd4 is the label that identifies a particle with a core of one layer made of Au and an external shell formed by four layers of Pd. Figure 1 shows three examples of the geometries covered in this study.

Figure 1. Geometries considered in this study. (a) Icosahedron; (b) truncated octahedron; c) Marks decahedron. The representations in the lower row show the internal composition of the core−shell particles.

The particles considered in this study were Au1Pd4, Au2Pd3, Au3Pd2, Au4Pd1, Pd1Au4, Pd2Au3, Pd3Au2, and Pd4Au1. Table 2 shows the total number of atoms of Au and Pd both at the core and at the external shell. At an initial stage, the particles had an ideal geometry, with the interatomic distances assigned using the nominal bulk Table 2. Number of Layers Composing the Icosahedral, Decahedral, and Truncated Octahedral Particles of This Study

21659

geometry

number of atoms per layer

number of layers

Ih and Dh TO

1, 12, 42, 92, 162, 252 6, 38, 96, 174, 272

5 4


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contribution of surface and volume to the total energy of a nanoparticle and acts as a measure of which the core−shell structure will be energetically more favorable among others for a specific size. In order to identify the structural changes while a particle is heated, we calculated the bond order parameter Q6, defined as48

interatomic distances of Au and implementing a MD thermalization stage at 300 K to allow the structures to relax their bond lengths before starting the analysis of the dynamics of the particles. In these thermalization runs, there was no atomic diffusion, and the geometry of the particles was preserved. Afterward, the particles were subjected to a heating process at 300 K, increasing the temperature at 100 K intervals, until the particles reached their fusion temperatures. The MD simulations were performed using the DL_POLY 2.20 code,44 with no periodic conditions, in the canonical ensemble and implementing a Nosé−Hoover thermostat in order to keep the temperature constant along each simulation. The motion equations were integrated using the velocity Verlet algorithm with a time step of 1.5 fs. Each canonical simulation was 3.5 × 106 steps long (equivalent to 5.25 ns of dynamics), using the first 1 × 106 steps for thermal equilibration of the system. Each particle was subjected to this heating process once. The interaction energy was calculated using the Sutton− Chen (SC) metal interaction model ⎡ 1 U (r ) = ε ⎢ ⎢⎣ 2

⎤ ⎛ a ⎞n ∑ ⎜⎜ ⎟⎟ − c ρi ⎥⎥ r ⎦ j ≠ i ⎝ ij ⎠

6

Q6 =

N

Q 6m =

Table 3. Parameters of the SC Potential for the Au−Au, Pd− Pd, and Au−Pd Interactions m

ε (meV)

c

a (Å)

8 6 7

7.8052 3.2864 5.0647

53.581 148.205

4.0651 3.8813 3.9721

The stability of the particles was monitored through the excess energy,47 defined as ⎛ E(AuN ) E(PdN ) ⎞ 1 −n Eexc = ⎜E(Au mPd n) − m ⎟ ⎝ N N ⎠N

(5)

RESULTS The evolution of the Q6 parameter is shown in Figure 2a−f. In the panels, we also show characteristic geometries at different temperatures, corresponding to the Au4Pd1 and Pd4Au1 particles. The initial structure of the particles is relatively stable before reaching the fusion temperature, with the exception of the icosahedral Au4Pd1 particle (panel (a)) that suffers an evident structural transformation when the temperature increases from 700 to 900 K. This transformation is driven by diffusion of Au to the surface that, as can be noted in the particle represented in the figure, has started at 700 K. At 800 K, the geometry of the particle has migrated from icosahedral to a fcc-like structure, with both (111) and (100) surfaces, similar to a Marks decahedron. This is analogous to the solid− solid structural transition from icosahedral to fcc reported in Pt clusters supported on MgO,49 although in the latter case the transition is preceded by a transition of the interface with the MgO layer toward a (100) arrangement, unlike the case of the Au4Pd1 particle, which is not suspended in any substrate; on the other hand, the transition in the Au4Pd1 particle is driven by diffusion of Au toward the surface, unlike the monometallic Pt cluster. Another transformation happens to the Pd4Au1 of Dh geometry that experiences a slight change in the value of Q6 at 400 K but rapidly takes back the value that characterizes decahedral structures. This slight change in Q6 is likely due to local rearrangements on the (100) faces of the particles, as can be noted in the image of the particle at 400 K. In all of the particles, the solid-to-liquid transition is marked by an abrupt change in the slope of the curves; the melting temperatures coincide with those marked by abrupt changes in the corresponding caloric curves and, as expected, these critical temperatures are smaller than the melting temperatures of Au and Pd at bulk. Both the caloric curves (see Figures 4 and S1 in Supporting Information) and the graphs of Q6 (plotted in Figure 2) show

The first term of 1 corresponds to a pairwise repulsive interaction, while the second term considers a cohesive manybody contribution to the total energy. The parameter a is a characteristic interatomic distance, ε has energy units, c is a dimensionless parameter, and n and m are integers (in monometallic systems) such that n > m. The power law form was chosen because in this way the potential is scalable, in the sense that two different metals described by the same set of n and m can be converted one into the other by rescaling of the energy and length units. Details on the SC potential and its use in alloyed systems can be found in ref 45. The parametrization of the SC potential used in this work is the one due to Cağin et al.;46 these parameters were calculated taking quantum effects under consideration, in such a way that zero-point frequencies and elastic constants are appropriately described. The values of these parameters are shown in Table 3.

n

N

∑i = 1 Nnb(i)

■

(2)

11 12 11.5

N (i)

∑i = 1 ∑ j =nb1 Y6m(rij)

Here, Nnb(i) is the number of first neighbors of the ith atom, rij is the magnitude of the vector that binds the i and j atoms, and Y6m(rij) are the spherical harmonics associated with the bond formed between the i and j atoms. It was considered that a pair of atoms are nearest neighbors one of the other if they are separated by a distance of 3.5 Å or smaller, corresponding to the distance of the first minimum in the pair correlation function for Au. For a fcc perfect structure Qfcc 6 = 0.575, while in a perfect decahedron QDh 6 = 0.43 and in a perfect icosahedron QIh 6 = 0.167. In the limit of the liquid state, Q6 drops to values close to 0.

where

Au−Au Pd−Pd Au−Pd

(4)

where

(1)

⎛ a ⎞m ρi = ∑ ⎜⎜ ⎟⎟ r j ≠ i ⎝ ij ⎠

4π [ ∑ | Q 6m|2 ]1/2 13 m =−6

(3)

where E(AumPdn) is the total energy of the particle with m Au atoms and n Pd atoms and E(AuN) and E(PdN) are the energies of a Au particle and a Pd particle, respectively, such that m + n = N. Defined in this way, the excess energy considers the 21660


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Figure 2. (a−f) Q6 order parameter as a function of temperature for the particles considered in this study. (g) Excess energy for the Aucore−Pdshell particles analyzed in (a−c). (h) Excess energy for the Pdcore−Aushell particles analyzed in (d−f). (i) Excess energy for homogeneous nanoalloys, presented for the sake of comparison with (g) and (h). All of the values of Eexc were measured at 300 K.

Nevertheless, it is known that experimentally it is possible to synthesize Aucore−Pdshell or random alloys depending on the synthesis conditions,15 and this can be explained by the positive but relatively small values of Eexc in these structures; see Figure 2g,i. Figure 3 shows the behavior of the energy excess as a function of temperature, measured while the particles are heated until they reach a value of T close to the melting transition. As can be noted, for most of the cases, the value of Eexc is kept almost constant when the temperature is increased, and this behavior corresponds to a linear increase in the total configurational energy of the particles (see the Supporting Information), evidencing low diffusion of the elements and almost no change in the geometry of the particles. Nevertheless, there are several interesting exceptions. In the case of Pdcore− Aushell icosahedral particles, Figure 3d, Eexc decreases gradually starting at around 600 K, in agreement with a slight increase in the rate of change of the configurational energy, produced by the diffusion of some Au atoms on the surface while the particle keeps its overall icosahedral geometry (see, for example, the representations of the Pd4Au1 particle in Figure 2d). Other exceptions are the Au3Pd2 and the Au4Pd1 particles with icosahedral and decahedral geometries that show abrupt changes in the slope of their curves. These changes coincide with the sudden fall in the slope of the configurational energy

that the melting temperature increases with the number of Pd atoms and that the Aucore−Pdshell’s have overall higher melting temperatures than the Pdcore−Aushell particles. Even though the Aucore−Pdshell particles have a positive excess energy while the excess energy in Pdcore−Aushell particles is negative, mostly due to the difference in surface energy, the Aucore−Pdshell particles must overcome a potential barrier to break the Au−Pd bonds at the interface and start the diffusion of Au atoms to the surface, which implies a need for a larger thermal energy for this diffusion to start. The Pdcore−Aushell particles have already a low surface energy, which implies that there is no need for Au atoms to diffuse toward the surface, and thus, the thermal energy needed for melting is not as high as that in the Aucore− Pdshell cases, hence the lower melting temperature. The increase of the melting temperature with the number of Pd atoms has been observed in diverse studies on Au−Pd alloys.17,50 The excess energy results are shown in Figure 2g−i for the simulations performed at 300 K. Here, the negative values of Eexc indicate that the Pdcore−Aushell is more stable than the Aucore−Pdshell configuration, in agreement with previous studies.50−52 Figure 2I shows the results for Eexc in particles with random distributions of the two metals. As can be noted, the Pdcore−Aushell core−shell structures are energetically more stable that the random alloys,52 but the range of values for random alloys is the same as the range for Aucore−Pdshell. 21661


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Figure 3. Excess energy Eexc as a function of the temperature of the particle. (a) Aucore−Pdshell icosahedron; (b) Aucore−Pdshell truncated octahedron; (c) Aucore−Pdshell decahedron; (d) Pdcore−Aushell icosahedron; (e) Pdcore−Aushell truncated octahedron; (f) Pdcore−Aushell decahedron.

While the plot of Q6 (Figure 2) for the icosahedral Au3Pd2 shows that the particle does not experience relevant structural changes, the diffusion analysis shows that the palladium atoms start to diffuse at a temperature of 500 K (Figure 4a), while the local density analysis shows that at a temperature of 800 K the number of Pd atoms positioned at a distance between 4 and 9 Å from the center of mass of the particle increases, while the number of Au atoms at distances larger than 10 Å increases as well (see Figure 5b). Figure 5c,d shows the behavior of the diffusion and local density at 800 K for the Au4Pd1 icosahedral particle. The diffusion analysis shows that at temperatures higher than 600 K both elements have a large mobility, and this coincides with the change in the behavior of Q6 that marks a transformation from an icosahedral structure to a decahedral structure. This transformation has been previously observed and reported elsewhere.53,54 From local density analysis, it was found that the number of Pd atoms located at distances between 5 and 10 Å increases at this temperature and that the number of Au atoms close to the surface increases as well. The diffusion of Pd to the inner volume of the particle attenuates compression of the inner bonds, releasing internal stresses in the particle55 and provoking the structural transformation that explains the change in the behavior of Q6.

curves (see Figure 4), which indicates a negative specific heat due to a structural change prior to the melting transition. In

Figure 4. Caloric curves corresponding to the icosahedral (left graph) and decahedral (right graph) Aucore−Pdshell particles considered in this study.

order to explain this behavior, we performed an analysis of the diffusion and the local density of the chemical species. For particles other than Au3Pd2 and Au4Pd1, the local density analysis indicates that the core−shell ordering is kept without significant changes until the particles reach their melting temperature, behavior that is also consistent with the behavior of Q6 and Eexc. The diffusion analysis shows that these particles have a solid core but a liquid-like outer shell at temperatures close to the melting transition. 21662


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the species but also on the geometry of the particles and the distribution of the metals. We found that, in general, the melting temperatures are higher in Aucore−Pdshell particles than those in Pdcore−Aushell particles. By analysis of the Q6 order parameter, it was determined that the particles conserved their original geometry, with the exception of the Au4Pd1 icosahedral particle that transforms its geometry into a decahedral motif, driven by the migration of surface Pd atoms into the interior of the particle in a substitutional diffusion. Our study shows that the structural stability of the core−shell particles, as measured by the excess energy, is a complex property that depends on geometry, the local distribution of species, and relative concentrations, as has been stated in a previous study with smaller particles.57 Our results suggest that the diffusion of Pd atoms at high temperatures plays an important role in the structural stability of the particles, a role that must be taken into consideration in the design of Au−Pd nanoalloys for specific applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04564. Caloric curves obtained for the particles considered in this study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 5. Diffusion (left graphs) and local density (right graphs) of Au (black dots) and Pd (red dots) atoms throughout the heating process. In the local density plots, the dotted line denotes the original borderline between the core and shell of the particle. (a,b) Au3Pd2 icosahedron; (c,d) Au4Pd1 icosahedron; (e,f) Au4Pd1 decahedron.

ORCID

Carlos Fernández-Navarro: 0000-0002-2107-2236 Sergio Mejía-Rosales: 0000-0003-0053-2632 Notes

The authors declare no competing financial interest.

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Local density analysis for the Au4Pd1 decahedral particle at 800 K (Figure 5f) shows that the number of Pd atoms increases at this temperature. The diffusion of Pd (Figure 4e) starts at 400 K, in agreement with a slight change in the value of Q6. The segregation of Pd atoms toward the surface is due both to the difference in size between the two species and to the difference in surface energy (Au: 1.63 J/m2; Pd: 2.05 J/m2).8 This segregation phenomenon has been observed in previous experimental and theoretical studies.20,56 From the Q6, diffusion, and local density analysis, it can be stated that the change in Eexc observed in the Au3Pd2 and Au4Pd1 particles with icosahedral geometry is due to the diffusion of Pd atoms that provokes a drop in the configurational energy, as can be noted in the caloric curves presented in Figure 4, while the change in the value of Eexc in the Au4Pd1 decahedral particle is due to geometric changes that have no significant effect on the values of Q6.

ACKNOWLEDGMENTS The authors acknowledge support from the National Council of Science and Technology (CONACYT) through the Basic Science Grant 168813 and support from UANL through the PAICYT Grant CE335-15.

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CONCLUSIONS In this work, we have investigated the structural stability of Au−Pd nanoalloys approximately 3 nm in size with icosahedral, decahedral, and truncated octahedral geometries and with Aucore−Pdshell and Pdcore−Aushell elemental distributions using canonical MD simulations. The melting temperatures were determined through the behavior of the caloric curves and order parameters Q6. The results show that the melting temperature depends not only on the relative concentration of 21663


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