Pd-Pt nanoalloy transformation pathways at the atomic scale

Nanoalloys have attracted considerable attention for their wide applications in materials, optics, catalysis, and biomedicine, which largely rely on their composition-, size-, and shape-dependent properties. Because these properties change dynamically with working conditions, the knowledge of the complex transformation pathway of nanoalloys is highly demanded. Herein, we combined the in-situ aberration-corrected scanning transmission electron microscopy and multiscale modeling to fully resolve the whole transformation trajectory of a bimetallic nanoalloy (Pd-Pt) at the atomic level. The transformation from core-shell to solid-solution structure is a multistep and temperature-dependent pathway, which includes monometallic-surface refacetting, bimetallic-surface refacetting, and alloy mixing, owing to the different atomic activation barriers of surface diffusion and bulk migration. The critical role of shell thickness in determining the transformation pathway was also revealed and explained. In particular, a unique partial core-shell structure with an anisotropic surface pattern was observed in the annealing process of ultrathin core-shell nanoparticles. This study offers a fundamental insight into the structure evolution of nanoalloys, which is beneficial for the development of functionalized nanoparticles with kinetic stability.