Strain-induced restructuring of the surface in core@shell nanoalloys

A general trend towards surface reconstructions which eliminate open facets in favour of close-packed facets is found for a wide class of A@B nanoalloys with atomically thin shells (B-skin structures), by means of a combination of atomistic and ab initio density-functional calculations. This class comprises Co@Au, Co@Pt, Ni@Pt, Co@Ag, Ni@Ag, Cu@Ag, Cu@Au, Rh@Au, Ni@Rh and Ni@Pd nanoalloys, in which core atoms are smaller than shell atoms. The reconstruction can induce a global restructuring of the nanoparticles, with different parts transforming in a concerted way and causing the emergence of pyritohedral structures from fcc nanoalloys, and of chiral decahedra. The instability of open facets is rationalized in terms of the equilibration of atomic pressures in strained structures. The surface reconstruction can have important effects on nanoalloy properties, as shown for hydrogen adsorption on Ni@Pd. Stress-driven surface reconstructions in core@shell nanoalloys cause the disappearance of open facets to the advantage of closed-packed facets.