PNNL

Abstract

The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some uncertainties. Exposure of most clean metals to air or oxygen results in the nearly instant formation of an oxide layer. Because this initial layer normally forms in a relatively uncontrollable manner, the atomic level understanding of the initial oxidation is limited in comparison to the abundant experimental observation and theoretical derivation on thickening of oxidation layer on metal surface at high temperature. We report in this letter mMicrostructurale characterization of iron nanoparticles oxide passivated with iron oxide shell nanoparticleswere studied using high resolution transmission electron microscopy (HRTEM) and high angle annular dark-filed (HAADF) imaging in aberration corrected scanning transmission electron microcopy (STEM). Voids were readily observed on both small single crystal ?-Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticle has been engineered for Co at higher temperatures1, they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some controversy. For example, void formation has been noticed in the single crystal ?-Fe nanoparticles as a consequence of iron outward diffusion during the initial oxidation at room temperature. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particles larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation provides new insight for tailoring of metal/metal-oxide core-shell structured nanoparticles for applications related to optics, magnetism, and nanoelectronics.