Singular behaviour of atomic ordering in Pt-Co nanocubes starting from core-shell configurations

The ordered structure of platinum-cobalt (Pt-Co) alloy nanoparticles has been studied actively because the structure influences their magnetic and catalytic properties. On the Pt-Co alloy's surface, Pt atoms preferentially segregate during annealing to reduce the surface energy. Such surface segregation has been shown to promote the formation of an ordered structure near the surface of Pt-Co thin films. Although this phenomenon seems also useful to control the nanoparticle structure, this has not been observed. Here, we have studied the ordered structure in annealed Pt@Co core-shell nanoparticles using a scanning transmission electron microscope. The nanoparticles were chemically synthesized, and their structural changes after annealing at 600 degrees C, 700 degrees C, and 800 degrees C for 3 h were observed. After being annealed at 600 degrees C and 800 degrees C, the particles contained the L1(2)-Pt3Co ordered structure. The structure seems reasonable considering an initial Pt : Co ratio of similar to 4 : 1. However, we found that the L1(0)-PtCo structure was formed near the nanoparticle surface after annealing at 700 degrees C. The L1(0)-PtCo structure was thought to be formed from the surface segregation of Pt atoms and insufficient diffusion of Pt and Co atoms to mix them in the particle overall.