Assembly and structure of Ni/NiO core-shell nanoparticles

► We report an investigation of structure in Ni/NiO core-shell nanoparticles (NP). ► NP have been realized with a gas aggregation source and by controlled oxidation. ► A HR-TEM analysis was performed, also with exit wave reconstruction method. ► Ni NP core has a regular multitwinned Mckay icosahedral structure. ► NiO forms island with direct or opposite (twinned) stacking on Ni (1 1 1) surfaces.