Nanocrystalline Iron Oxide Aerogels as Mesoporous Magnetic Architectures

We have developed crystalline nanoarchitectures of iron oxide that exhibit superparamagnetic behavior while still retaining the desirable bicontinuous pore−solid networks and monolithic nature of an aerogel. Iron oxide aerogels are initially produced in an X-ray-amorphous, high-surface-area form, by adapting recently established sol−gel methods using Fe(III) salts and epoxide-based proton scavengers. Controlled temperature/atmosphere treatments convert the as-prepared iron oxide aerogels into nanocrystalline forms with the inverse spinel structure. As a function of the bathing gas, treatment temperature, and treatment history, these nanocrystalline forms can be reversibly tuned to predominantly exhibit either Fe3O4 (magnetite) or γ-Fe2O3 (maghemite) phases, as verified by electron microscopy, X-ray and electron diffraction, microprobe Raman spectroscopy, and magnetic analysis. Peak deconvolution of the Raman-active Fe−O bands yields valuable information on the local structure and vacancy content of the various aerogel forms, and facilitates the differentiation of Fe3O4 and γ-Fe2O3 components, which are difficult to assign using only diffraction methods. These nanocrystalline, magnetic forms retain the inherent characteristics of aerogels, including high surface area (>140 m2 g-1), through-connected porosity concentrated in the mesopore size range (2−50 nm), and nanoscale particle sizes (7−18 nm). On the basis of this synthetic and processing protocol, we produce multifunctional nanostructured materials with effective control of the pore−solid architecture, the nanocrystalline phase, and subsequent magnetic properties.