Atomistic Study of Creation of Bimetallic Clusters by Coalescence
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1087-V08-01
Atomistic Study of Creation of Bimetallic Clusters by Coalescence
S. Mizuno, and K. Shintani Department of Mechanical Engineering and Intelligent Systems, University of ElectroCommunications, 1-5-1 Chofugaoka, Chofu, Tokyo, 182-8585, Japan
ABSTRACT Coalescence of the two clusters of different metallic elements is studied using moleculardynamics simulation. All the pair combinations of the elements Au, Ag, Pt, and Pd are considered. It is concluded a united cluster becomes one among a core-shell cluster, an alloyed cluster, and an epitaxially-joined cluster, and the size of original two clusters and the lattice mismatch between them are two primary factors that determine the morphology of the united cluster. INTRODUCTION Metallic clusters or nanoparticles show excellent performance as catalysts because of their high surface-to-volume ratio. An inert-gas aggregation (IGA) source is an experimental method by which clusters are produced. In such a method, cluster coalescence is one of growth modes of clusters [1], whereas coalescence of supported Pt clusters is one of processes causing the loss of electrochemically active surface area of the catalyst in fuel cell electrodes [2]. Furthermore, bimetallic clusters attract much attention of researchers in recent years because of their novel physical and chemical properties. At coalescence of two metallic clusters of different species, alloying or core-shell structuring tends to occur spontaneously. Resulting alloyed clusters or core-shell clusters will behave as unique catalysts. Ascencio et al. [3,4] synthesized colloidal dispersions of Au-Cu alloy nanoparticles by an improved simultaneous reduction method, and characterized their structures by high-resolution transmission electron microscopy. Using inverse micelle method, Li et al. [5] synthesized Au-Ag nanoparticles, and revealed their internal structures by scanning transmission electron microscopy. These experiments invoked the following analytical investigations. Hendy et al. [6] studied the coalescence of facetted lead clusters using molecular dynamics (MD) with the empirical glue potential under the condition of an IGA source. Ding et al. [7] investigated the size dependence of coalescence and melting of iron clusters using a many-body interaction potential based on the second moment approximation of the tight-binding model. Combining the Gupta potential and bond order analysis, Chen and Johnston [8] performed MD simulations for the martensitic transformation of Au(core)-Ag(shell) clusters. However, under what conditions alloyed or core-shell clusters are produced through the coalescence of two clusters of different elements is an open question. In order to address this problem, in this paper, we study morphological evolution of two metallic clusters of different elements at coalescence using molecular-dynamics simulation. All the pair combinations of the elements Au, Ag, Pt, and Pd are considered. The relevant physical constants of the four elements
are shown in table I, according to which we notic
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