Co@CoO@Au core-multi-shell nanocrystals.

We report on the chemical synthesis, structural and compositional characterization, and hierarchical organization of Co-core, concentric CoO–Au multi-shell nanocrystals (Co@CoO@Au). Based on electron microscopy, magnetometry and spectroscopy experiments, we present compelling evidence for the formation of the Au outer shell on CoO, challenging the common assumption that the reduction reaction to form Au shells can only occur if the cobalt surface has not oxidized. Our findings suggest that the presence of a metal shell surrounding a transition-metal core nanocrystal following such a reduction reaction cannot be taken as evidence that the transition metal oxide is absent from the surface of the nanocrystal core. We find that Au shell growth can produce Co@CoO@Au nanocrystals possessing five-fold twinning symmetry and we suggest that their growth is facilitated through self nucleation and coalescence of Au particles. [ABSTRACT FROM AUTHOR]