Mass transport and structural transformations in core@shell Cu@Ni nanoparticles

Core@shell Cu@Ni nanoalloys are of interest for applications to catalysis, magnetism and flexible electronics. The Cu@Ni chemical arrangement is however strongly out of equilibrium, because the two metals show some preferential tendency to mix instead of forming phase-separated structures, and Cu atoms tend to segregate to the surface because of the lower cohesion and surface energy of Cu with respect to Ni. Here we study the evolution towards equilibrium of Cu@Ni nanoparticles by molecular dynamics simulations. We consider different sizes from about 150 to about 1000 atoms. The simulations allow to single out the mechanisms by which Cu atoms leave the core and then pass through the Ni shell to reach the nanoparticle surface. Some of these mechanisms involve a single Cu atom moving to the surface, while other mechanisms bring many Cu atoms to the surface at the same time. The latter mechanisms are associated to deep structural transformations of the entire nanoparticle. The simulations show also that the detachment of a Cu atom from the core involves the formation of a vacancy, which may migrate within the nanoparticle. We calculate the energy barriers for vacancy diffusion in pure Ni and Cu nanoparticles and in Cu@Ni nanoalloys, obtaining a good agreement with the available experimental data. • In core@shell Cu@Ni nanoparticles the evolution towards equilibrium is different depending on nanoparticle size. • In small nanoparticles, most Cu atoms reach the surface at the same time by nanoparticle restructuring. • In larger nanoparticles, Cu atoms reach the surface mostly one by one. • The diffusion of Cu atoms in the Ni shell occurs by collective mechanisms in which vacancies are formed. • The diffusion barrier for vacancies is calculated finding good agreement with experimental data.