Magnetic studies of mesoporous nanostructured iron oxide materials synthesized by one-step soft-templating

Abstract

A combined magnetization and ⁵⁷Fe spin-echo nuclear magnetic resonance (NMR) study has been carried out on mesoporous nanostructured materials consisting of the magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) phases. Two series of samples were synthesized using a recently developed one-step soft-templating approach with systematic variations in calcination temperature and reaction atmosphere. Nuclear magnetic resonance has been shown to be a valuable tool for distinguishing between the two magnetic iron oxide spinel phases, Fe₃O₄ and γ-Fe₂O₃, on the nanoscale as well as monitoring phase transformation resulting from oxidation. For the Fe₃O₄ and γ-Fe₂O₃ phases, peaks in the NMR spectra are attributed to Fe in the tetrahedral (A) sites and octahedral (B) sites. The magnetic field dependence of the peaks was observed and confirmed the site assignments. Fe₃O₄ on a nanoscale readily oxidizes to form γ-Fe₂O₃ and this was clearly evident in the NMR spectra. As evidenced by transmission electron microscope (TEM) images, the porous mesostructure for the iron oxide materials is formed by a random close-packed aggregation of nanoparticles; correspondingly, superparamagnetic behavior was observed in the magnetic measurements. Although X-ray diffraction (XRD) shows the spinel structure for the Fe₃O₄ and γ-Fe₂O₃ phases, unlike NMR, it is difficult to distinguish between the two phases with XRD. Nitrogen sorption isotherms characterize the mesoporous structures of the materials, and yield BET surface area values and limited BJH pore size distribution curves.