Surface-Sandwich Segregation Phenomena in Bimetallic Ag-Ni and Pd-Ni Nanoparticles: A Molecular Dynamics Study

摘要

By molecular dynamics simulation it is shown that interdiffusion in the initial f.c.c. Ag-core (≈ 28 at. %) - Ni-shell (≈72 at. %) and Ni-core (≈34 at. %) - Pd-shell (≈66 at. %) nanoparticles can lead to surface—sandwich segregation. It is observed that there is a separation of the initial Ag-Ni core-shell structure into Ag-core - Ni-intermediate shell - Ag-disperse surface monolayer. The initial crystal Ni-Pd core-shell structure transforms to the core of a non-crystalline Pd-rich solid solution with quite strongly developed icosahedral short-range order, which is covered by a surface—sandwich shell, where Ni atoms are located in the centres of interpenetrating icosahedra of a subsurface Kagome net layer while the Pd atoms occupy the vertices of the icosahedra and cover this Ni layer from inside and outside. We demonstrate that under certain conditions a surface-sandwich segregation phenomenon at the nanoscale can be observed in systems with completely different phase diagrams in the bulk states: in systems displaying the extremely low mutual solubility as in the Ag-Ni system, or in systems exhibiting a continuous mutual solid solubility like the Pd-Ni system.