Pt-bimetallic (PtM where M=Fe, Co, Ni and so on) nanoparticles (NPs) have attracted great interest because of their outstanding activity for the oxygen reduction reaction (ORR) in fuel cell applications 1, 2. This work may pave the way towards designing catalysts through post-synthesis annealing for optimized catalytic performance. Furthermore, a comprehensive interplay among phase evolution, surface faceting and elemental inter-diffusion is revealed, and supported by atomistic simulations. Here, by tracking the same individual Pt 3Co nanoparticles during in situ annealing in a scanning transmission electron microscope, we directly discern five distinct stages of surface elemental rearrangements in Pt 3Co nanoparticles at the atomic scale: initial random (alloy) elemental distribution surface platinum-skin-layer formation nucleation of structurally ordered domains ordered framework development and, finally, initiation of amorphization.

Understanding the complete dynamic response of surface structure and chemistry to thermal treatments at the atomic scale is imperative for the rational design of catalyst nanoparticles. The catalytic performance of nanoparticles is primarily determined by the precise nature of the surface and near-surface atomic configurations, which can be tailored by post-synthesis annealing effectively and straightforwardly.

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