Unusual Effect of an Electron Beam on the Formation of Core/Shell (Co/CoO) Nanoparticles Differing by Their Crystalline Structures

In this study, an unusual effect of the electron beam in transmission electron microscopy (TEM) on the formation of Co/CoO core/shell structures is developed through careful in situ TEM/scanning TEM (STEM) analysis. By feature of the nanoscale precision of this approach, the electron beam-irradiated Co nanoparticles reveals remarkable resistance to oxidation compared to those without irradiation treatment. Moreover, the irradiated hcp single domain Co nanocrystals result in Co/CoO core/shell nanoparticles after oxidation, instead of the CoO hollow nanoparticles without irradiation treatment. This study highlights the electron beam can also play a role in nanoscale Kirkendall effect, in addition to the nanocrystallinity and 2D ordering effect that we have recently demonstrated. By careful in situ STEM-EELS (electron energy-loss spectroscopy) studies of the Co nanoparticles, it was found that the deliberately irradiated nanoparticles undergo an outward diffusion process of Co ions, forming an oxide layer with O species produced by the carboxylic group covalently bound to the Co atoms of the surface