Interdiffusion and surface-sandwich ordering in initial Ni-core–Pd-shell nanoparticle

Using molecular dynamics simulation (∼1 μs) in combination with the embedded atom method we have investigated interdiffusion and structural transformations at 1000 K in an initial core–shell nanoparticle (diameter ∼4.5 nm). This starting particle has the f.c.c. structure in which a core of Ni atoms (≈34%) is surrounded by a shell of Pd atoms (≈66%). It is found that in such nanoparticles reactive diffusion accompanying nucleation and growth of a Pd2Ni ordering surface-sandwich structure takes place. In this structure, the Ni atoms mostly accumulate in a layer just below the surface and, at the same time, are located in the centres of interpenetrating icosahedra to generate a subsurface shell as a Kagomé net. Meanwhile, the Pd atoms occupy the vertices of the icosahedra and cover this Ni layer from the inside and outside as well as being located in the core of the nanoparticle forming (according to the alloy composition) a Pd-rich solid solution with the remaining Ni atoms. The total atomic fraction involved in building up the surface–sandwich shell of the nanoparticle in the form of the Ni Kagomé net layer covered on both side by Pd atoms is estimated at ∼70%. These findings open up a range of opportunities for the experimental synthesis and study of new kinds of Pd–Ni nanostructures exhibiting Pd2Ni surface-sandwich ordering along with properties that may differ significantly from the corresponding bulk Pd–Ni alloys. Some of these opportunities are discussed.