| Abstract
| - We have developed crystalline nanoarchitectures of iron oxide that exhibit superparamagneticbehavior while still retaining the desirable bicontinuous pore−solid networks and monolithic nature of anaerogel. Iron oxide aerogels are initially produced in an X-ray-amorphous, high-surface-area form, byadapting 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 eitherFe3O4 (magnetite) or γ-Fe2O3 (maghemite) phases, as verified by electron microscopy, X-ray and electrondiffraction, 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 variousaerogel forms, and facilitates the differentiation of Fe3O4 and γ-Fe2O3 components, which are difficult toassign using only diffraction methods. These nanocrystalline, magnetic forms retain the inherentcharacteristics of aerogels, including high surface area (>140 m2 g-1), through-connected porosityconcentrated in the mesopore size range (2−50 nm), and nanoscale particle sizes (7−18 nm). On thebasis of this synthetic and processing protocol, we produce multifunctional nanostructured materials witheffective control of the pore−solid architecture, the nanocrystalline phase, and subsequent magneticproperties.
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