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.
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