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

Abstract

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 °C, 700 °C, and 800 °C for 3 h were observed. After being annealed at 600 °C and 800 °C, the particles contained the L1₂–Pt₃Co ordered structure. The structure seems reasonable considering an initial Pt : Co ratio of ∼4 : 1. However, we found that the L1₀–PtCo structure was formed near the nanoparticle surface after annealing at 700 °C. The L1₀–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.