Investigation of Possible Formation of Au@M (M = Cu, Ir, Pt, and Rh

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Investigation of possible formation of Au@M (M= Cu, Ir, Pt, and Rh) core-shell nanoclusters in a condensationcoalescence process using molecular dynamics simulations Mohsen Abbaspour, Hamed Akbarzadeh, Sirous Salemi, and Samira Lotfi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03724 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Industrial & Engineering Chemistry Research

Investigation of possible formation of Au@M (M= Cu, Ir, Pt, and Rh) core-shell nanoclusters in a condensation-coalescence process using molecular dynamics simulations

Mohsen Abbaspour,* Hamed Akbarzadeh, Sirous Salemi, Samira Lotfi Department of Chemistry, Hakim Sabzevari University, 96179- 76487Sabzevar, Iran

Abstract In this work, the possible formation of Au@Pt, Au@Cu, Au@Rh, and Au@Ir clusters during the vapor condensation-coalescence process has been investigated by inserting icosahedral gold nanocluster into copper, platinum, rhodium, and iridium vapor environments. We have studied some properties of the created nanoclusters at different simulation times. Our results showed that the stability of the formed core-shell structures obeys the following order: Pure Au >Au@Cu > Au@Rh> Au@Ir which means that the positioning of rhodium, iridium, and copper on the gold cluster is unfavorable. The Au@Pt cluster was also the most stable cluster. The results also showed that the Au@Pt and Au@Rh are not pure core-shell structures but they are (Au/Pt mixed)@Pt and (Au/Rh mixed)@Rh structures. The structural investigations also indicated that the initial icosahedral morphology of the gold cluster was disappeared whereas the formed coreshell nanoclusters had fcc-like and hcp-like morphologies. The thermal investigations showed that the nanoclusters become more spherical by increasing the temperature. The gold atoms also migrate to the cluster surface but the metal surface atoms diffuse to the inner cluster layers by increasing the temperature.

*Correspondence addressed: Email: [email protected] 1 ACS Paragon Plus Environment

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1. Introduction Core-shell bimetallic clusters, with the different metals in the core and the shell, have recently received a significant amount of interest due to their unique chemical and physical properties which are different from those of the monometallic components.1-4 According to the benefits of the core-shell nanoclusters, core-shell Au-based nanoclusters can achieve high stability and activity by controlling their electronic, composition, shape, and geometric properties.5 Gold is a good candidate to stabilize the catalysts in the fuel cell systems.6 It has been shown that the gold nanocluster can stabilize platinum to decrease dissolution in the acidic environment.7,8 Moreover, the positioning of gold atoms in clusters cores can increase Raman scattering because of its surface plasmon resonance in the surface enhanced Raman scattering (SERS) which is used for electrochemical reaction observations and bio-applications.9-12 The Au-based coreshell nanoclusters (especially, by using Pt as the catalytic) are used for oxygen reduction reactions.5 Au-Rh core-shell nanoclusters are used as catalyst for hydrogenation which is more efficient and affordable than pure Rh. In addition to this, bimetallic nanoclusters of Au-Cu have also many applications but are mainly used in medical sensors and biomedicine.13 One of the commonly used techniques of nanocluster synthesis is to evaporate metal atoms and condensate them into a vapor atmosphere. Theoretically, we can synthesize nanoparticles with special chemical composition, size, and structure. The factors of the experimental vapor condensation process can be also controlled.14-16 However, the nanoclusters formed in the gas condensation are multi-structured and non-spherical and their shapes are also discontinuous. Therefore, production of clusters with specific structure and shape is an important stage toward the more versatile application of the nanoclusters. But, the generation of the clusters with specific morphology is a formidable technical task.16 Molecular dynamics (MD) simulation is a real solution to study the structure, dynamics, shape, and growing nanoparticle and thus, it can specify the important atomistic processes occur in this process.17 Several simulations have been applied to the process of vapor condensation procedure consisting of production of monometallic clusters and nanoalloys.18-22 However, a complete study of nucleation process from the vapor phase is not currently available.23 Many of these simulation studies have also reported that they could not generate core-shell structures.20-22 There is no or very rarely simulation or experimental gas condensation work which claims that the 2 ACS Paragon Plus Environment

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core-shell nanoparticles have been obtained during the condensation process. Just recently, we reported the formation of Rh@Au, Ir@Au, Pt@Au, and Cu@Au core-shell nanoclusters into the gas-phase condensation process using MD simulations.23 In this paper, the possible formation of Au@Pt, Au@Cu, Au@Rh, and Au@Ir clusters during the vapor condensation process has been investigated by inserting icosahedral gold nanocluster into the metal vapor environments. In fact, the positioning of gold atoms as the cluster core (which has smaller surface energy than the other metals) would be very interesting. We have studied structure and some thermodynamic properties of the nanoclusters created at different simulation times.

2. Simulation Details In these simulations, the cubic cells with dimensions of 100×100×100 Å3 were constructed with a homogeneous gas including 2000 atoms of vapor Cu, Rh, Ir, or Pt. The initial density of metal vapor was 0.00198 (Å-3). The gold icosahedral cluster with 309 atoms set up in the center of the cells at 300 K. We have selected the icosahedra structure as the initial geometries of the gold nanoclusters because the icosahedral morphologies are constructed based on to the magic numbers which are more stable than other morphologies for different metal nanoclusters24,25 . Some studies have shown that the fcc and decahedral structures of gold clusters are more stable than other structures.26,27 We have shown in the next sections (in accordance with our previous study23 ) that the initial morphology of the gold cluster positioned into the different metal vapors is vanished after some initial simulation times and therefore, it has negligible effect on the obtained simulation results. The snapshots of the gold nanocluster into the Cu, Rh, Ir, and Pt vapor atoms have been presented at 2.25, 2.5, 3, 4, and 20 ns in Figure1 and in Figures S1 to S3 in the supporting information. Unlike the conventional gas-phase condensation process, we have not used an inert gas in the condensation process. Our previous investigations showed that the final obtained results of the core@shell clusters with and without an inert-gas are almost similar.23

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After 2.25 ns

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2.5 ns

3 ns

4 ns

6 ns

20 ns

Figure 1. Snapshots of the Au nanocluster into the Ir vapor at various simulation times.


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As Figures1, S1, S2, and S3 show, some of the vapor atoms of Rh, Ir, Cu, and Pt encounter to the gold nanocluster by increasing the time which results in the enlargement of the gold nanocluster. Some of the vapor atoms also strike and join together which results in the condensation procedure and the production of the pure metal clusters. Thus, the pressure of the metal vapor reduces by the simulation time. By increasing the time, the Au-metal core-shell nanocluster grows. The number of formed pure gold clusters also reduces and their size increases by the time. These observations are in good agreement with the experimental and theoretical works on the condensation of the metal vapors.28 In this study, we will concentrate only on the Au-metal core-shell clusters. All of the MD simulations have been carried out in the NVT ensemble by DL_POLY code29 . Also, several previous works have used the NVT ensemble in the gas condensation procedure using MD simulations.30-33 We have recently used the NVT ensemble in the simulation of gasphase condensation procedure consisting of Pt, Rh, Ir, and Cu clusters into Au vapor.23 We have also used the periodic boundary conditions at three dimensions. To solve the equations of motions, the Verlet-Leapfrog algorithm were also used. The cutoff distance was 12 Å with 1fs time-step. The simulation time for every condensation process was 20 ns. Many previous works have also used the nanosecond scale 28,34 and the similar density ranges35 for their simulations of the condensation process. The obtained results are not based on single trajectories and several simulations were approved our results for the systems. For the Au-Au, Au-M, and M-M interactions, the quantum Sutton-Chen (QSC) potentials have been used36,37 . The QSC model has been successfully and widely applied in molecular dynamics simulations of nanoclusters in good agreement with the experiments.38,39 The QSC model is:

U tot

1 a       2 i  j j  rij 

n   1   c  i 2    i  

(1)

The first term is the pair repulsion interaction model and the second term shows the metallic bonding energy corresponding to the local density, ρi : a  i    j  i  rij

   

m

(2)



In Eq. (1), c is a dimensionless factor, rij is the distance between atoms i and j, and ε and a are parameters related to energy and length, respectively. To compute the interaction between Au and M, we have used the geometric mean for the energy parameter ε and the arithmetic mean for the remaining parameters.

3. Results and discussion 3.1 Analysis of Thermodynamic parameters Total energy (per atom) for Au@Ir, Au@Pt, Au@Cu, Au@Rh, and pure gold clusters formed in the vapor condensation procedure have been given at 3, 4, 6, and 20 ns in Figure 2. For comparison, the gold clusters produced in the pure gold vapor (without the inserted external nanocluster) have been also given. The solid symbols in this figure indicate the biggest core@shell nanoclusters in each system. According to Figure 2, the order of the absolute values of total energy of the produced nanoclusters is as: EAu@Ir> EAu@Pt> EAu@Rh> EAu >EAu@Cu . This trend is the same as the order of the cohesive energy values of the bulk metals40 : EIrcoh> EPtcoh > ERhcoh > EAucoh > ECucoh. It is also shown in Figure 2 that the core-shell nanocluster in all systems (with the exception of Au@Cu system) has smaller value of the total energy than the other pure clusters. This is because of the fact that the cohesive energy of gold is smaller than bulk Ir, Pt, and Rh metals but it is greater than Cu.


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Figure 2. Total energy for the different core-shell clusters created in the vapor condensation at various times.

To study the relative stability of the formed nanoclusters in the vapor condensation, the Δ value was computed from the equation41,42 : 

Coh Etot  N Au Au  N M  MCoh N surf

(3)



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where Etot is the total energy of the clusters obtained by the MD simulations. NAu and  Au are Coh

the number and the bulk cohesive energy of Au atoms and NM and  M are the number and the Coh

cohesive energy of the M atoms (M= Cu, Pt, Ir, and Rh). Nsurf is the number of the cluster surface atoms. As the nanocluster is more stable its Δ value becomes smaller42. The Δ value was calculated for the created clusters at the various times in Figure 3. According to Figure 3, the Δ for the pure metal nanoclusters obeys the order: ΔAu< ΔCu< ΔPt< ΔRh< ΔIr. The reason of this order of stability of the pure metal clusters can be interpreted using the surface energy. If we compare the experimental surface energy of the corresponding bulk metals43 : ESIr > ESRh > ESPt > ESCu > ESCu, we can found that the smaller the surface energy the more stable the cluster. If we observe the stability of the core-shell nanoclusters at 3 ns, we can found that almost the same order of the stability of pure nanoclusters has been also existed for the core-shell clusters (with the exception of the Au@Pt cluster): ΔAu< ΔAu@Cu< ΔAu@Rh< ΔAu@Ir. This means that the existence of the Rh, Cu, and Ir metals on the Au cluster is unfavorable. It is also shown in each system that the ∆ parameter of the formed core-shell cluster is smaller (and thus is more stable) than the pure metal clusters. This result can be also interpreted using the surface energy. As we have presented the number of the produced clusters, the different number of atoms in cluster, and their percent of different surface atoms in Table 1 and Tables S1 to S3, the produced core-shell clusters are not really pure core-shell clusters and some of the gold atoms have been appeared on the clusters surfaces. Due to the smaller surface energy of Au than other metals, the resulted core-shell clusters have smaller surface energy than the pure metal clusters and therefore, they are more stable. There is also another important point about the stability of Au@Pt cluster which is the most stable cluster than other clusters.


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Figure 3. The Δ value for the various created clusters at the different simulation times.


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Table 1. The number of total atoms, number of each kind of atom, their molar fractions, number of the surface atoms, and percent of the surface atoms for the Au@Cu nanoclusters (Copper atoms are in red and gold atoms are in yellow). time (ns)

Snapshot

Ntot

NCu

NAu

xCu

xAu

Nsur

Nsur

%Nsur

%Nsur

Cu

Au

Cu

Au

3ns

491

185

306

0.38

0.62

96

119

44.6512

55.3488

4ns

1969

1661

308

0.84

0.16

462

117

79.7927

20.2073

6ns

2309

2000

309

0.87

0.13

566

102

84.7305

15.2695

2309

2000

309

0.87

0.13

542

98

84.6875

15.3125

20ns

We have also computed the number of the clusters and their atoms in the different systems at different times and given in Figure 4. As Figure 4a illustrates, the number of the produced nanoclusters in Au@Ir and Au@Pt systems is more than Au@Cu and Au@Rh systems. Almost, the reverse trend can be observed in Figure 4b for the number of the atoms in the nanoclusters in the different systems because when the more number of clusters are formed then, their number of atoms become smaller (the total number of atoms in the different systems is equal). The number of atoms in the formed core@shell clusters of Au@Ir and Au@Pt is also less than the Au@Cu 10 ACS Paragon Plus Environment

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and Au@Rh clusters (see for example compare the total number of atoms at 4 ns for the different clusters in Tables 1, S1, S2, and S3). In Au@Ir system, the vapor Ir atoms have more tendency to align together (rather than join to the Au cluster) to form the pure Ir nanoclusters. This is because of the greater cohesive energy of Ir atoms than Au (and also other atoms) and therefore, the number of the Ir nanoclusters is more than other systems in Figure 4a. It is also shown that by increasing the time, the number of the produced nanoclusters decreases due to the coalescence between the clusters and also their number of atoms increases. The rate of decreasing of the number of Pt and Ir clusters become slower than the Cu and Rh nanoclusters by elevating the simulation time. This result is because of the slower coalescence process in Au@Ir and Au@Pt systems than Au@Cu and Au@Rh systems which is due to the greater interactions and heavier masses of Ir and Pt than Cu and Rh atoms.



10 Au@Cu Au@Ir Au@Pt Au@Rh

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Figure 4. The number of the formed nanoclusters and their atoms in the different systems at different simulation times.


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3.2 Structural analysis To investigate the structural characteristic of the formed clusters, the radial distribution function (RDF or g(r)) for the nanoclusters has been calculated using the following formula44 : g (r ) 

1

2

  r  r  r  i

j i

i

(4)

j

In Eq. (4), the angle brackets indicate a configurational average. The Au-Au, M-M, and Au-M RDFs for the last nanoclusters (N= 2309 atoms) produced have been given after the time of 20 ns at 300 K in Figure 5. According to this figure, the Ir-Ir RDFs (in the g(r)MM plot) are higher than other RDF peaks which is because of the stronger Ir-Ir interactions than other interactions. Because the various atoms used in our simulations have the different lattice constants (sizes) their positions of the RDF peaks are different43 : LCu (3.61 Å) < LRh (3.80 Å)

Investigation of Possible Formation of Au@M (M = Cu, Ir, Pt, and Rh

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