Surface-sandwich segregation phenomena in bimetallic Ag-Ni and Pd-Ni nanoparticles: a molecular dynamics study

By molecular dynamics simulation it is shown that interdiffusion in the initial f.c.c. Agcore (≈ 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 Kagomé 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.