Atomic-scale observation of ordered structure induced by surface segregation in annealed Pt@Co core-shell nanoparticles

The ordered structure of binary alloy nanoparticles determines their magnetic and catalytic characteristics. In the alloys after annealing, one of the components preferentially segregates on the surface to reduce surface energy. This surface segregation has been known as a factor in the construction of an ordered phase near the surface. However, the segregation-induced ordering has not been observed for nanoparticles. Here, platinum@cobalt (Pt@Co) core-shell nanoparticles were synthesized, and their structural changes after annealing at 600{\deg}C, 700{\deg}C, and 800{\deg}C for 3 hours were observed by a scanning transmission electron microscope. We discovered an L10-PtCo structure near the surface at 700{\deg}C, which was unexpected given the initial Pt:Co ratio of about 4:1. The L10-PtCo structure was considered to form due to surface segregation of Pt atoms and diffusion insufficient to mix Pt and Co atoms in the particle overall because the structure did not form at 600{\deg}C and 800{\deg}C.