1 Kinetic Map For Destabilization Of Pt-Skin Au ... - ACS Publications

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Kinetic Map for Destabilization of Pt-Skin Au Nanoparticles via Atomic Scale Rearrangements Mantha Sai Pavan Jagannath, Srikanth Divi, and Abhijit Chatterjee* Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai 400076 India

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S Supporting Information *

ABSTRACT: A commonly used strategy to enhance the mass activity of Ptbased catalysts involves the synthesis of Au nanoparticles (NPs) with a monolayer-thick Pt-skin layer. The synergistic effect of Au and Pt results in a higher catalytic activity and better Pt utilization. However, the stability of the Pt-skin layer is questionable as our recent equilibrium Monte Carlo simulations predict that eventually the surface Pt is replaced by Au. The role of Au during destabilization of Pt-skin in vacuum and solution is investigated with the help of molecular dynamics. Different starting Au−Pt arrangements are studied mimicking various NP synthesis approaches. Beyond a critical number of atoms in a Pt cluster, the ideal Pt monolayer rapidly transforms to a three-dimensional (3D) Pt cluster. This is supported by our model predicting transition from the Pt monolayer to Volmer−Weber growth in the Au−Pt system. At room temperature, Pt atoms move into the subsurface layer at second timescales mainly via the exchange mechanism involving Au atoms or Au climbing on top of Pt. For all practical purposes, the experimental “Pt-skin” Au NPs may actually correspond to a single layer of surface Au over subsurface Pt layers. Presence of large 3D Pt clusters may slowdown the climbing of Au atoms on Pt, thereby delaying the formation of Au-skin.

1. INTRODUCTION Pt nanoparticles (NPs) play an important role in the area of catalysis1 and low-temperature fuel cells,2 for example, direct methanol fuel cells. In recent years, the catalytically active monolayer (ML) Pt-skin bimetallic NPs3,4 have generated a significant interest within the community because of their potential to alleviate the relatively high cost and low stability of pure Pt NPs. Considerable focus has been given to the synthesis of ML Pt-skin NPs with Au present in the core using a variety of experimental methods.3,5−12 These synthesis approaches attempt to stabilize the Pt-skin while exploiting the synergistic effects arising from the interactions between Au and Pt.13−17 Density functional theory (DFT) calculations have attributed the high catalytic activity to the presence of a unique Pt-skin layer.18−21 However, segregation studies based on DFT22,23 and classical interatomic potentials24,25 paint a different picture of the elemental distribution in Au−Pt NPs. Monte Carlo (MC) simulations suggest that at equilibrium, Au forms a skin layer in Au1−x−Ptx (0 < x < 0.4) NPs, and Pt is present in the subsurface layers.24,25 This is expected because Au has a lower surface energy than Pt. Au atoms are also present in the core (see the equilibrium structure in the right panel of Figure 1). However, the main limitation of these computational studies is that they focus on thermodynamically equilibrated Au−Pt NPs,24,26 which are in principle attained postsynthesis after a long period of time. In this study, we investigate the mechanisms for destabilization of the Pt-skin layer during high-temperature annealing and in solution synthesis, and their associated timescales. © XXXX American Chemical Society

Figure 1. Examples of Pt-skin layer NPs2 formed via (a) deposition of Pt atoms on the Au NP, (b) galvanic displacement of ML Cu by Pt, and (c) deposition of Pt clusters. The equilibrium Au−Pt NP structure is shown on the right.

Experimentally probing whether the synthesized Au@Pt core−shell NPs indeed possess a stable Pt-skin layer can be challenging. Most experimental characterization techniques,4,27 for example, EDX and X-ray photoelectron spectroscopy have a resolution of 5 nm, and the aberration-corrected scanning Received: June 26, 2018 Revised: October 5, 2018 Published: October 25, 2018 A

DOI: 10.1021/acs.jpcc.8b06102 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

performed at 400−800 K. Thereafter, activation barriers are calculated using the nudged elastic band (NEB)47 once the initial and final states are known. High-temperature MD simulations are also performed in solution using temperature as a parameter to accelerate the dynamics. In the case of solution, one can determine the timescales of evolution at lower temperatures by projecting the dynamics. Similar ideas of using high-temperature MD calculations have been employed in accelerated MD techniques.48,49 The outline of the paper is as follows. The computational methodology used is presented in Section 2. Results are presented in Section 3, followed by conclusions in Section 4.

transmission electron microscopy (STEM) can resolve the top two layers. Aberration-corrected STEM characterization reveals that Pt-rich surface layers of as-prepared Pt0.5−Au0.5 can become rich in Au after heat treatment in air for 30 min.27 Au−Pt NPs have been reported to form alloyed, segregated, and partially alloyed Au@Pt and Pt@Au core−shell structures as well as Pickering structures.28−32 The evolution of the elemental distribution within a Au−Pt NP, which determines its long-term catalytic activity and stability, still remains poorly understood.28,33−35 Pt is believed to form an ideal ML on the Au extended surface. Recently, it was shown that Pt MLs on Au NPs behave differently from Pt on single crystal extended surfaces of Au.36 Two regimes were encountered, namely, a monoatomic-thick layer Pt on Au NPs is formed at a low Pt coverage, and three-dimensional (3D) Pt nanoclusters are formed beyond 0.5ML coverages. Given the lower surface energy of Au, these experiments do not explain the role of Au. Equilibrium MC simulations do neither reveal the timescales at which the Au-skin structures are formed nor do they provide the mechanism for evolution toward these equilibrium structures via compositional rearrangements. It is important to ascertain whether Pt-skin structures can survive for long, especially in situations where thermal treatment is employed for removing stabilizing agents used in colloidal synthesis and codeposition.2,27,37 Understanding of the nanoscale effects on destabilization of the Ptskin structure is also essential for catalyst design and optimization. It confirms the timescales for which the standard picture of a ML Pt-skin layer on top of the Au core, which is still being used by modeling and experimental groups, is accurate. Several groups have succeeded in synthesizing Pt-skin Au NPs using high temperatures, surfactants, electrical energy, layer-by-layer Pt grown on Au via replacement of overpotentially deposited Ni MLs38 and/or sacrificial templates,6,13 Cu under potential deposition followed by Pt galvanic displacement,39,40 spontaneous (electroless) deposition of Pt on Au NPs,41−44 selective dealloying of bimetallic NPs,45 and Pt deposition on Au cores using capping agents.46 In particular, we focus on deposition of Pt atoms/clusters on Au NPs while ignoring the presence of a third element and considering different starting Au−Pt geometries of the Au@Pt(ML) core− shell and Pt island clusters on Au NPs as shown in left panels of Figure 1. Because Pt deposition proceeds atom-by-atom, a ML of Pt is slowly formed (Figure 1a). Similarly, other starting structures of interest include incomplete Pt-skin (e.g., ones formed when incomplete underpotentially deposited Cu shell is formed followed by galvanic displacement of Cu with Pt), complete Pt-skin layer (Figure 1b) and Pt cluster deposition on the Au NP (Figure 1c). We begin by probing the Volmer−Weber growth mode in Pt-skin Au NPs. The basis is a simplified model using effective Au−Au, Au−Pt, and Pt−Pt interactions obtained from a more accurate embedded atom method (EAM) potential. The model provides insights into transition from the ML Pt-film to 3D Pt cluster formation. Next, assuming that the starting Ptskin layer in Figure 1 can be reached, we investigate its stability using molecular dynamics (MD). Mechanisms leading to the formation of equilibrium structure (right panel of Figure 1) from these starting structures are sought. To mimic the heat treatment of NPs typically between 523 and 673 K in an inert atmosphere, we perform MD calculations at 600 K temperature in vacuum. Because activated events happen frequently at MD timescales at higher temperatures, calculations are

2. COMPUTATIONAL METHODOLOGY 2.1. System Setup. We consider truncated octahedron NPs of size 2.7, 4.3, 9.1, and 10.7 nm (containing 711, 2735, 24 695, and 39 871 atoms, respectively). The truncated octahedron shape has energy lower in comparison to other NP shapes. A truncated octahedron NP possesses 8 {111} facets, 6 {100} facets, 12 {111}−{111} edges, 24 {111}− {100} edges, and 24 vertices. The behavior of the {100} and {111} facets is studied separately. Periodic boundary conditions are employed. The NP is placed inside a periodic box of sufficient size such that its interaction with its periodic image is negligible. Because the MD calculations are performed in vacuum, this corresponds to the situation where the NP is taken out of the solvent and placed in an inert gas, for example, N2 or Ar, for heat treatment. Pt atoms are introduced at the NP surface to mimic experimental synthesis conditions for Ptskin Au NPs. As mentioned earlier, the following cases are considered (i) few Pt adsorbed atoms (adatoms) on Au, (ii) Pt clusters and incomplete layer (less than 1ML thickness) on Au, and (iii) complete Pt ML on Au. In case of solvent simulations, the NP was solvated in a periodic box such that the solvent region is 30 Å thick. The number of solvent molecules in the box was consistent with the density of water at room temperature. A short MD calculation at 300 K was performed to equilibrate the system. Calculations involving extended surfaces described later were performed in a similar manner. 2.2. Molecular Dynamics. Classical MD simulations are performed in the canonical (NVT) ensemble. A Langevin thermostat is used to maintain a constant temperature. Atomic scale events, which are thermally activated, are observed more frequently at higher temperatures. At the same time, these temperatures are not high enough to cause the NPs to melt. Furthermore, the solvent also does not undergo a phase transition because of the use of a constant periodic box size. See ref 49 for discussion on the use of high-temperature MD calculations for accelerating rare events in metal−solvent systems. The MD step size is 4 fs. Timescales of 10−40 ns are typically accessed in the simulation. The same set of calculations was performed with different random seeds to better understand the dominant kinetic pathways in the system. Materials evolution can be described in terms of state-tostate transitions where each state corresponds to a minimum in the potential energy landscape.50 The rate constant kp is calculated from the Arrhenius rate expression (kp = νp exp(−Ep/kBT)) assuming that the pre-exponential factor νp = 1013 s−1 and the activation barrier Ep are calculated using the climbing image NEB method47 in vacuum. This approach was useful for estimating rate constants at 300 or 600 K (typical B


The Journal of Physical Chemistry C annealing temperature) from the 800 K calculations. Optimization with the NEB was performed using the global L-BFGS method and a spring constant of 0.1 eV/Å2. Even at high temperatures, short MD calculations reveal only some of the many possible evolution mechanisms. Therefore, a small subset of pathways discovered using high-temperature MD provides an upper bound of the timescales of evolution at low temperatures. Diffusion mechanisms such as Au climbing on top of Pt atoms/surface (described later) are difficult to study with the NEB because of the complex nature of the concerted atomic moves. To overcome this issue, a sequence of frames was extracted from the MD trajectory at regular intervals (for every 0.1−3 ps MD time interval). NEB calculations were performed with consecutive extracted frames as long as the frames correspond to different energy minima. Thereafter, the reaction coordinates were stitched together to provide the composite minimum energy path. This approach treats Au climbing as a series of events occurring between geometrically distant initial and final states. 2.3. Interatomic Potentials. The embedded atom method (EAM) was employed with our MD simulations. The total potential energy of the system from the EAM is given by51 Et =

1 ∑ ϕ(rij)+∑ Fi(ρi ) 2 i,j i

LJ 12 6 VM −S(rij) = 4εM−S((σM−S/ rij) − (σM−S/ rij) )

(5)

with σM−S = 2.95 Å and ϵM−S/kB = 61.6 K (0.5 kJ/mol). Section S2 shows that the interfacial energy calculated using the interatomic potentials described above is in good agreement with experiments. 2.4. Model for Volmer−Weber Growth Mode. 2.4.1. Driving Force for 3D Pt Formation on Extended (111) Surface. We calculate the energy change associated with a one-layer Pt structure converting to a multilayer Pt structure using a simplified model. In this model, the energy change, which provides the driving force for such a transformation, is obtained as a function of NPt the total number of Pt atoms and number of Pt layers present. Denoting NLPt as the number of Pt atoms in layer L of the multilayered structure, we require that

(1)

∑ χ(rij) j≠i

(4)

Here, we choose σS−S = 2.95 Å and ϵS−S/kB = 61.6 K (0.5 kJ/ mol). LJ potentials were developed for M−S systems, wherein both metal−metal (M−M) and M−S interactions are described by the LJ form.55 In these approaches, the M−S interaction parameter ϵM−S is obtained using a mixing rule such as ϵM−S = (ϵM−MϵS−S)1/2. Typical values of ϵM−M and ϵS−S used are 1 and 0.5 kJ/mol, respectively.55 The parameter σ for Au and Pt lies between 2.8 and 2.96 Å.55 Unfortunately, activation barriers obtained with metal surface diffusion using the LJ potential for the M−M interactions have not been tested. Therefore, we use the LJ potential for describing the M−S and S−S interactions but the EAM is used to describe the M−M interactions. The M−S interaction used is

where ϕ(rij) is the two-body potential between pairs of atoms i and j, F(ρi) is the energy required to embed an atom i, and the electron density term at atom i is denoted by ρi. The density term contains contributions from other atoms j, that is ρi =

12 6 iji ij σ yz yzz jjjj σS−S yzz z S−S z j j j z j z = 4εS−Sjjjj z − jj z zz jjj rij zz j rij zz zzz { k {{ kk

VSLJ−S(rij)

Article

NPt =

(2)

∑ NPtL

(6)

L

where χ(rij) is the electron density contribution of the j-th atom on the i-th atom. The pure metal EAM potentials of ref 52 are in good agreement with literature values for lattice constants, activation barrier for various atomic moves, and cohesive and surface energies. For the alloy system, the heat of mixing of the Au−Pt random alloy is correctly obtained using the parameterization for the Au−Pt system from our earlier work in ref 53, which applies the mixing rule Ä ÉÑ Ñ 1 ÅÅ 1 ϕ Au−Pt(r ) = ÅÅÅÅθϕ Au−Au + ϕPt−Pt ÑÑÑÑ ÑÖ (3) 2 ÅÇ θ

A regular hexagon of edge dL atoms will contain (see Figure S2 of the Supporting Information) NPtL = 3dL(dL − 1) + 1

(7)

NLPt

atoms. In general, may not result in a regular hexagon, and there are many ways in which the Pt atoms can arrange themselves. The edge length of the equivalent (regular) hexagon is determined from NLPt by solving eq 7, that is, dL = (3 + 12NPtL − 3 )/6 (see Figure S2 in the Supporting Information). The admissible values of NLPt are determined by the geometrical constraint, such that N1Pt > N2Pt > N3Pt. In case of a two-layer structure, the number of atoms in layer 2 can be increased by reducing the size of layer 1. Figure S3 in the Supporting Information shows an example of a complete second layer in a two-layer Pt cluster. To capture the energy of the multilayered structure, we introduce a term called effective number of layers in a cluster, which is denoted as Leq. A single Pt layer will have Leq = 1. The effective number of layers is calculated as

for calculating the pair potential term ϕAu−Pt in terms of the pure metal pair potentials. θ is termed as the mixing parameter. The optimal value of θ, namely 0.707, was found by fitting θ to experimental heat of mixing data of the Au−Pt alloy system.53 See Section S1 of the Supporting Information for more discussion on accuracy of the EAM potential. Tight-binding, DFT, and electronic structure-based solvation models54 have been employed with water. However, employing these computationally expensive electronic structure calculations makes the problem intractable. As a result, a standard approximation used in the modeling of metal−solvent (M−S) systems involves representing a solvent molecule by a single coarse-grained particle.55,56 Solvent−solvent (S−S) particles separated by distance rij interact via the LennardJones (LJ) potential

Leq = 1 + +

C

NPt2 1 NPt +3−

1 12NPt −3

NPt3 NPt2 + 3 −

12NPt2 − 3

(8) DOI: 10.1021/acs.jpcc.8b06102 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Figure S3 in the Supporting Information shows that for a NPt2

1 NPt

one can proceed to compute the energy change. In addition, the Au surface on which the multilayered Pt has been placed also needs to be accounted for in the energy calculation. The energy ei for a Au atom when Pt atoms are present above it is given by eAu(Cb) and eAu(Ce) otherwise, analogous to the notation used for the Pt atoms. Table 1 can be used to calculate the energy of the multilayered structure with respect to a single-layered hexagon both containing NPt atoms as follows

1 12NPt

= +3− − 3 . Leq complete second Pt layer = 2, when a complete second Pt layer is formed. The last term in eq 8 is ignored when the third layer is absent. For the purpose of calculating the energy, it is useful to classify atoms in the hexagon as the vertex, edge, and center (nonvertex/edge) sites denoted as V, E, and C sites, respectively. For a perfect hexagon of size NLPt, the number of Pt atoms lying at the hexagon vertex NLPt(V) = 6, edge

NPtL (E) =

12NPtL − 3 − 9,

and

ij j 1 1 ΔE(Leq ) = jjjNPt eAu(Cb) + (NPt − NPt )eAu(Ce) jj k yz z + ∑ EL(NPtL )zzz − (NPteAu(Cb) + E1(NPt)) zz L = 1,2,3 {

center

NPtL (C) = NPtL + 3 − 12NPtL − 3 . When NL+1 Pt > 0 such that a hexagon is formed, the sites in layer L that are covered by the hexagon are denoted as Cb (here b stands for buried). The number of such sites is given by NLPt(Cb) = NL+1 Pt . The remaining sites NLPt − NL+1 Pt in layer L are treated as exposed surface sites. The number of such sites is given by

(11)

In eq 11, the Au surface layer in contact with Pt has an energy contribution N1PteAu(Cb) + (NPt − N1Pt)eAu(Ce) where N1PteAu(Cb) is the number of Au atoms covered by Pt in the first layer and NPt − N1Pt denotes the number of exposed Au surface atoms. The energy of the single-layered hexagonal Pt structure is NPteAu(Cb) + E1(NPt). 2.4.2. Driving Force for 3D Pt Formation on Extended (100) Surface. The procedure used for the (100) surface is similar to the (111) case. Instead of a hexagonal cluster, a perfect square cluster is considered with NPt denoting the total number of Pt atoms. A square of edge dL atoms will contain NLPt atoms. The edge length of an equivalent square is given by

NPtL (Ce) = NPtL + 3 − 12NPtL − 3 − NPtL + 1. The total energy of the system within the context of the EAM potential can be written in terms of atomic energy contributions Et =

∑ ei

(9)

i 1 ∑ 2 j

ϕ(rij) + Fi(ρi ) contains Here, the atomic energy ei = many-body terms. Replacing ei with an effective environment-dependent energy eliminates the need for explicitly calculating the many-body terms of the EAM potential. To obtain the effective ei , representative structures were constructed, for example, 1−3 Pt layers on extended Au(111) such that each Pt layer is in the form of a hexagon (see schematic in Figure S3 of the Supporting Information). After minimizing the energy Et, the values of ei for Pt and Au atoms were directly obtained in terms of their layer position and coordination (edge, vertex, or center atoms) from our EAM code. Note that such an analysis is only performed with vacuum. Au was present only as a slab, and no Au was present within the Pt layers or on top of Pt. Table 1 lists the values of

dL =

NPtL (E) = 4 NPtL − 8 a n d NPtL (C) = NPtL + 4 − 4 NPtL . When the number of Pt atoms in layer L + 1 is NL+1 Pt , we write N P Lt ( C b ) = N P Lt + 1 and

NPtL (Ce) = NL + 4 − 4 NPtL − NPtL + 1. The effective number of layers for Pt on Au(100) is calculated as Leq = 1 +

Table 1. Effective Atomic Energy in eV for Different Environments in the Hexagon-Shaped Multilayered Cluster on a (111) Surface: V = Hexagon Vertex, E = Hexagon Edge, Ce = Hexagon Center (Exposed to Vacuum), Cb = Hexagon Center (Buried under Pt) L=1 L=2 L=3

eLPt(V)

eLPt(E)

eLPt(Ce)

eLPt(Cb)

−3.65 −3.96 −4.02 eAu(Ce)

−4.00 −4.40 −4.45

−4.65 −4.95 −4.97 eAu(Cb)

−5.55 −5.66

−3.544

ξ= V,E,C ,C

e

NPt3 NPt2 + 4 − 4 NPt2

Table 2. Atomic Energy in eV for Different Environments in the Square-Shaped Multilayered Cluster on a (100) Surface: V = Square Vertex, E = Square Edge, Ce = Square Center (Exposed to Vacuum), Cb = Square Center (Buried under Pt)

L=1 L=2 L=3

NPtL (ξ)ePtL (ξ) b

1 1 NPt + 4 − 4 NPt

+

Considering a maximum of 3 Pt layers, the energy change upon formation of the layered structure is calculated with the help of eqs 10 and 11. Values of eAu and ePt are provided in Table 2. 2.4.3. Multilayer Pt FormationA Comparison of Extended (111) and (100) Surfaces. The average atomic energy change ΔE/NPt versus Leq for the {111} and {100}

−4.025

∑

NPt2

(12)

effective ei while specifying the layer position and coordination. e3Pt(Cb) is not reported because a maximum of three layers is considered in this analysis. A similar approach was employed with the Au(100) surface in vacuum. Now, writing the energy associated with a particular layer as EL(NPtL ) =

NPtL . O n t h e b a s i s o f t h i s N PLt ( V ) = 4 ,

eLPt(V)

eLPt(E)

eLPt(Ce)

eLPt(Cb)

−3.59 −3.88 −4.03 eAu(Ce)

−3.99 −4.34 −4.46

−4.42 −4.79 −4.89 eAu(Cb)

−5.46 −5.51

−3.454

(10) D

−3.841 DOI: 10.1021/acs.jpcc.8b06102 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION Dynamical behavior observed with different starting structures is presented in order of increasing complexity. Mechanisms observed in vacuum and solution are identical. Unless explicitly mentioned, the values provided are for vacuum conditions. Additional snapshots of the NP obtained in vacuum and solution are presented in Sections S4 and S5 of the Supporting Information, respectively. 3.1. Low Pt Coverage. Consider the deposition of Pt atoms on the Au NP (Figure 1a). In typical experiments, Pt deposition happens over several minutes to hours. We study the initial rearrangement associated with clusters of 1 or more Pt adatoms on the Au NP at 600 K to show that the Pt adatoms either move into surface or subsurface positions by displacing Au atoms. Only two-dimensional clusters are studied in an attempt to replicate the behavior with the Ptskin layer. Figure 3a,b demonstrates the behavior of a single Pt adatom on a (100) and (111) facet of a Au NP, respectively. In both

surface is plotted in Figure 2. MATLAB codes are provided in Section S3 of the Supporting Information. Different starting

Figure 2. Change in energy per Pt atom as a function of number of Pt layers formed on (a) extended {111} and (b) extended {100} surfaces.

NPt values are considered. Pt atom energy is lower when it is present in the second layer (see Table 1), that is, the Pt−Au contact is less favored than the Pt−Pt contact. As a result, ΔE < 0 for Leq ≥ 2. Figure 2a reveals a competition between high surface energy of the multilayered structures and favorable atomic energies due to the reduced Pt−Au contact as Leq increases. For NPt < 25, a single-layered Pt structure is stable. At NPt = 27, although a complete two-layered structure formation is energetically favorable, a barrier needs to be overcome to attain this structure. Smaller second-layered Pt clusters obtained with Leq < 1.8 are energetically unfavorable because of the higher surface energy associated with these structures. It is for this reason that we do not observe two-layer structures for small Pt clusters. A complete two-layer Pt structure is more stable than 1 layer Pt when 27 < NPt < 217. The barrier for the single-layered to complete two-layer structure shifts toward the left, which implies that a smaller second layer cluster needs to be formed to overcome the barrier. For Pt clusters greater than 27 atoms, our MD simulations indeed show that the formation of the two-layer structure is not spontaneous. A complete two-layer structure can be rapidly formed when NPt > 150. For clusters containing 1000 or more Pt atoms, we expect three-layered structures to be preferred. Simulations of larger NPs, for example, 30 nm diameter, that can support 1000 or more Pt atoms were not performed in work. We conclude that multilayered Pt structures on Au(111) become energetically favorable once NPt is greater than 25 adatoms. A corollary is that single-layer Pt on small Au NPs (e.g., 25 atom large Pt cluster) and forms multiple-layered Pt structure. Climbing a multilayer Pt structure entails a larger activation barrier. To highlight these aspects, we have considered larger-sized Au NPs of 4.3, 5.9, and 9.1 nm size (Figure 7). In Figure 7a,b, a 19 Pt adatom cluster is present on top of a {111} facet. Because the Pt cluster is close to the NP edge in Figure 7a, the undercoordinated Au atoms at the NP edge detach/peel-off and are able to climb the Pt cluster. This does not happen in Figure 7b where the Pt cluster is present in the middle of the facet. The Pt cluster remains intact at the nanosecond timescales. In contrast, a larger Pt cluster containing 120 atoms in a single layer is transformed to a two-layer Pt (see Figure 7c), analogous to a Volmer−Weber growth mode.4 These results are consistent with the critical Pt cluster size discussed in Section 2.4. Au adatoms are formed at longer timescales via detachment of Au atoms from the NP edge (path A or B in Figure 7d, also activation barrier of 1.6 eV in Figures S13 and S14 of the Supporting Information). After hopping on the Au surface (activation barrier of 0.05−0.8 eV depending on the surface as seen in Figures S15 and S16 of the Supporting Information),

(also see Figure S4 of the Supporting Information). The Au atoms participating in the pop-out mechanism cover the newly formed Pt cluster. The movement of Au atoms on top of the Pt cluster results in a slightly distorted NP. The activation barrier encountered is 0.8 eV (Figure 5b). At 800 K, we observe the same mechanism as in 600 K (Figure 5c). However, because the kinetics is much faster at the high temperature, it allows the NP regain its original shape from the distorted one at nanosecond timescales (Figure S5 in the Supporting Information). A similar pop-out mechanism is observed in the {100} facet. Four Pt atoms were placed as shown in Figure 5c. The Pt atoms form a single cluster once the Au atoms lying in between them have popped out (Figure S3 in the Supporting Information). Pt clusters are not formed when the Pt atoms are separated by a larger distance (Figures 5d and S7 in the Supporting Information). In such cases, the evolution proceeds by Pt atoms migrating into the subsurface region involving the long-exchange mechanism as discussed earlier in Figure 4 or the climbing mechanism. When small Pt clusters are present in the surface, the Pt cluster can move into the subsurface region via the Au climbing mechanism (Figure 6). In the Au climbing

Figure 6. (a−d) Examples of Au climbing on a Pt cluster and their energy diagrams on the right handside.

mechanism, Au atoms one-by-one climb on top of the Pt clusters. Unlike the pop-out mechanism, which is a single-step many-atom move, the climbing mechanism is a multistep process. The initial barrier involves detachment of a Au atom from the NP edge. Once a Au atom detaches, several undercoordinated atoms are formed at the NP edge which can also detach. The resulting Au adatoms diffuse on the NP surface to reach the Pt cluster. Thereafter, they climb on top of the Pt cluster. Factors that influence the rates of the climbing mechanism include (i) size of the Pt cluster, (ii) coordination G


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Figure 8. Pt climbing on the {111} facet with a 4.3 nm NP happens at picosecond timescales, which results in the formation of two Pt layers.

Figure 9. Starting with a complete Pt ML on a Au NP, the Au climbing mechanism causes Au to appear at the surface at nanosecond timescales. Figure 7. (a−c) Behavior of Pt clusters of different sizes on large Au(111) surfaces. Pt can form either 1 or 2 layers. (d) Detachment of Au from the NP edge and climbing on two layers of Pt.

top of other Pt atoms on the {111} and {100} facets forming a two Pt layer structure. This exposes Au atoms at the NP edge. Thereafter, Au atoms detach from the edge and climb on the Pt facets. Au climbing on the two-layer Pt {111} facet is slow because of the large Schwoebel barrier. On the other hand, Au and Pt climb rapidly on the Pt cluster in the {100} facet. Figure 9 shows that {100} facets has a higher Au surface coverage than the {111} surface. After 100 ns, the surface coverage of Au atoms is 80% at 800 K compared to 40% coverage at 600 K indicating that the process is thermally activated. The key steps in the destabilization of the Pt-skin Au NP include: (a) rapid formation of the two-layer Pt cluster, (b) appearance of Au at NP edges, followed by detachment of Au atoms from the NP edges, (c) climbing of Au and Pt atoms on the Pt cluster alongside exchange and pop-out moves. Movies S1 and S2 in the Supporting Information show a 10.7 nm NP in vacuum and solvent, respectively, at 600 K. The large size of the initial Pt ML causes the multilayer Pt formation in steps. First, small multilayer Pt rafts form whose edges are decorated with Au. Next, these Pt rafts skim the surface adding more Pt atoms to their layers and exposing Au atoms below. Small clusters of Au can be seen moving on top of the multilayer Pt surface. These Au clusters do not climb down the Pt layers. 3.5. Au Climbing Rate in Vacuum and Solution at Room Temperature. Several hundred independent MD calculations were performed with single-layer Pt on extended Au(111) and (100) surfaces at 600, 700, and 800 K in solution as shown in Figure 10. The initial structure is shown in the top left of panel (a,d) for (111) and (100) surfaces, respectively. The use of small Au slabs (12 × 8 × 5 and 8 × 8 × 8 unit cells for (111) and (100) surfaces, respectively, with bottom Au layer frozen) enables estimation of Au climbing rates at these temperatures from the MD simulations.

the Au adatoms arrive at the Pt cluster edge and stick to it. Occasionally, the Au adatoms climb on the Pt cluster by overcoming a Schwoebel-type barrier (path C and D in Figure 7d). For a single-layer Pt cluster, the climbing barrier is nearly 0.6 eV (rate constant of 100 μs−1 at 600 K) on the (100) surface and 0.9 eV (rate constant of 0.28 μs−1) on the (111) surface as shown in Figures S17 and S18 of the Supporting Information. For a two-layer Pt cluster, the climbing barrier is nearly 0.6 eV (rate constant of 100 μs−1 at 600 K) on the (100) surface and 1.4 eV (rate constant of 17.5 s−1) on the (111) surface as shown in Figures S19 and S20 of the Supporting Information. These rates were calculated assuming a pre-exponential factor of 1013 s−1 in the Arrhenius rate expression. In other words, the rate limiting step for the Au climbing on a Pt adatom cluster on the (100) or (111) surface is Au detachment from the NP edge, which has a rate constant as low as 0.2 s−1 associated with it. The two-layer Pt formation is observed more clearly in Figure 8, where a 90-atom Pt layer on the (111) facet of a 4.3 nm NP is shown. The Pt atoms in the middle of the cluster are first destabilized. Gradually, few Pt atoms climb on top of the existing Pt layer to form a two-layered Pt structure. The second Pt layer grows with time. In conjunction, Au atoms also climb on top of the two-layer Pt structure. The simulation was performed at 600 K for 80 ps. Ultimately, a complete two-layer Pt cluster is formed. 3.4. Pt-Skin Layer on a Au NP. Finally, we consider a Au NP with a complete Pt-skin reminiscent of the experimentally prepared core−shell Au@Pt structures (see Figure 9). MD simulations were performed with small (2.7 nm) NPs at 600− 800 K. The behavior observed can be understood in terms of the previous cases. At short timescales, the Pt atoms climb on H


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Figure 10. Independent MD trajectories showing Au climbing on the Pt cluster on the (a−c) extended (111) surface and (d−f) extended (100) surface in the solvent. The solvent is not shown for clarity. (g−h) Corresponding Arrhenius plots for the climbing rates. Closed circles denoted measured rates and open circles denote predicted rates.

Figure 11. Kinetic map describing the evolution of starting Pt-skin structures toward near-to-equilibrium structures. Timescales at 600 K are shown in arrows. The increasing surface Pt coverage is shown from top to bottom. In general, a combination of mechanisms would be seen depending on the Pt cluster size.

One Au atom was placed next to the Pt layer. The Au atom moves rapidly along the edge of the Pt cluster. After the multilayer Pt structure forms, the Au atom can climb the Pt layer (panel a,b). In some cases, the Au surface atoms pop out

leaving a surface vacancy behind (panel c). The (100) surface behaves similar, however, more Au surface atom pop-out events are witnessed. The waiting time required for the Au atom to climb is noted. The Au climbing rate k is estimated I


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The Journal of Physical Chemistry C using the maximum likelihood approach, that is, k = N/t. Here, N denotes the number of climbing events observed, and t is the sum of waiting times. The Arrhenius plots shown in (panels g,h) confirms that Au climbing is a thermally activated event. Similar MD calculations were performed with the slabs in vacuum. From this analysis, the pre-exponential factor and activation barrier for Au climbing on the (111) surface are 5.3 ps−1 and 0.53 eV (in comparison to 130 ps−1 and 0.7 eV in vacuum), respectively. The pre-exponential factor and activation barrier for Au climbing on the (100) surface are 54 ps−1 and 0.72 eV (in comparison to 1.24 ps−1 and 0.42 eV in vacuum), respectively. Using the Arrhenius rate expression, Au climbing rates in solution on (111) and (100) surfaces are found to be 6 and 107 ms−1 at 300 K, respectively. On the basis of these observations, one can develop kinetic maps that describe the path taken by the (sub-ML-to-ML) Ptskin Au NP as it destabilizes and approaches the equilibrium structure. Figure 11 shows the kinetic map for different starting Pt-skin structures. The main difference between small (4 nm) NPs is that Au atoms at the edge of smaller NPs can detach more easily from the edge to form Au adatoms, which proceed to climb on top of the Pt clusters. Figure 10 summarizes the behavior for four different Pt cluster sizes. A notable feature of the kinetic map is that the slowest evolution happens when two-layer Pt is formed and Au climbing on the Pt cluster proceeds at second timescales. Because thermal annealing is typically performed for up to 30 min or more in experiments,27 it is likely that a significant fraction of the Pt atoms deposited on the NP surface are accommodated within the surface.

motivate similar studies on synthesis of other nanostructured materials.

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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.8b06102.

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Evolution of Pt-skin Au NPs in solvent and vacuum, Volmer−Weber growth model, and validation of the interatomic potential (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Abhijit Chatterjee: 0000-0002-3747-8433 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author acknowledges support from the Science and Engineering Research Board, Department of Science and Technology grant nos. EMR/2017/001520 and SB/S3/CE/ 022/2014 and the Indian National Science Academy grant no. SP/YSP/120/2015/307.

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REFERENCES

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4. CONCLUSIONS In recent years, Pt-skin Au core NPs have generated a significant interest in the catalysis community. The picture of the NP structure often presented in the literature involves a NP core containing Au and a surface layer of Pt atoms. This structure is used to explain the catalytic activity of the NP. However, the stability of Pt at the surface of the Au NP has not been adequately studied so far. Our investigations of the stability of Au@Pt core−shell NPs using MD simulations provide a different dynamical picture of the Pt-skin NPs. Different starting NP configurations believed to exist in synthesis conditions such as during atom-by-atom deposition, cluster deposition, presence of a single Pt layer formed via Galvanic displacement of another metal, and so forth, are studied with MD. We find that irrespective of how Pt is placed on the NP, the Pt-skin is inherently unstable on the surface of the Au NP and tends to diffuse to the subsurface in the timescales of seconds or shorter. The diffusion mechanisms can vary depending on the NP size, facet, and amount of Pt deposited. On the basis of this understanding, a kinetic map is presented which describes the main mechanisms by which Pt will finally arrive at the subsurface layer. This work provides a new fundamental understanding of how fast the experimental synthesis conditions can drive the as-prepared NPs toward the equilibrium structure. This work highlights the need to incorporate MD calculations during the study of the catalytic activity using techniques such as DFT. Finally, this work constitutes one of the first attempts to understand dynamic evolution of elemental distribution within these BNPs during/postsynthesis thus making these simulations relevant to experimentalists. We hope this work will J


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