Spontaneous formation of hollow cobalt oxide nanoparticles by the Kirkendall effect at room temperature at the water–air interface

Abstract

We observe the spontaneous formation of hollow cobalt oxide nanoparticles at room temperature, indicating an enhancement of the solid-state diffusion at the nanoscale. Single crystal cobalt nanoparticles covered by a hydrophobic organic layer were transformed spontaneously into CoO hollow nanoparticles when deposited on the water–air interface in a matter of a few hours. The presence of water modifies the reactivity on the nanoparticle surface favoring the formation of the hollow structure; otherwise Co–CoO core–shell nanoparticles are obtained. The CoO hollow nanoparticles are formed only in an intermediate state, and after a period of time these structures finally undergo disintegration to form minor CoO entities.