Nanocrystallinity and the Ordering of Nanoparticles in 2D Superlattices: Controlled Formation of either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals.

Here it is demonstrated that the di!usion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen di!usion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward di!usion of Co ions is favored over the inward di!usion of oxygen, producing hollow CoO single-domain nanocrystals.

Source : Nanocrystallinity and the Ordering of Nanoparticles in 2D Superlattices: Controlled Formation of either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals. 
Z.Yang, I.Lisiecki, M. Walls, M.P. Pileni ACS Nano. , 2013,  7, 1342–1350.