Crystalline Magnetic Gels and Aerogels Combining Large Surface Areas and Magnetic Moments

The production of materials that simultaneously combine large surface areas and high crystallinities is a major challenge. Conventional sol-gel chemistry strategies to produce high-surface-area gels and aerogels generally result in amorphous or poorly crystalline materials. To attain proper crystallinities, materials are exposed to relatively high annealing temperatures that result in significant surface losses. This is a particularly limiting issue in the production of high-surface-area magnetic aerogels owing to the strong relationship between crystallinity and magnetic moment. To overcome this limitation, we demonstrate here the gelation of preformed magnetic crystalline nanodomains to produce magnetic aerogels with high surface area, crystallinity, and magnetic moment. To exemplify this strategy, we use colloidal maghemite nanocrystals as gel building blocks and an epoxide group as the gelation agent. After drying from supercritical CO₂, aerogels show surface areas close to 200 m² g^-1 and a well-defined maghemite crystal structure that provides saturation magnetizations close to 60 emu g^-1. For comparison, the gelation of hydrated iron chloride with propylene oxide provides amorphous iron oxide gels with slightly larger surface areas, 225 m² g^-1, but very low magnetization, below 2 emu g^-1. Thermal treatment at 400 °C is necessary to crystallize the material, which results in a surface area loss down to 87 m² g^-1, well below the values obtained from the nanocrystal building blocks.