Shell-mediated control of surface chemistry of highly stoichiometric magnetite nanoparticles

Abstract

Magnetite (Fe₃O₄) nanoparticles are one of the most studied nanomaterials for different nanotechnological and biomedical applications. However, Fe₃O₄ nanomaterials gradually oxidize to maghemite (γ-Fe₂O₃) under conventional environmental conditions leading to changes in their functional properties that determine their performance in many applications. Here we propose a novel strategy to control the surface chemistry of monodisperse 12 nm magnetite nanoparticles by means of a 3 nm-thick Zn-ferrite epitaxial coating in core/shell nanostructures. We have carried out a combined Mössbauer spectroscopy, dc magnetometry, X-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy study on iron oxide and Fe₃O₄/Zn_0.6Fe_2.4O₄ core/shell nanoparticles aged under ambient conditions for 6 months. Our results reveal that while the aged iron oxide nanoparticles consist of a mixture of γ-Fe₂O₃ and Fe₃O₄, the Zn-ferrite-coating preserves a highly stoichiometric Fe₃O₄ core. Therefore, the aged core/shell nanoparticles present a sharp Verwey transition, an increased saturation magnetization and the possibility of tuning the effective anisotropy through exchange-coupling at the core/shell interface. The inhibition of the oxidation of the Fe₃O₄ cores can be accounted for in terms of the chemical nature of the shell layer and an epitaxial crystal symmetry matching between the core and the shell.