Scanning Transmission Electron Microscopy Study of the Evolution of Needle-Like Nanostructures in CoFe2O4 and NiFe2O4 Silica Nanocomposite Aerogels

Magnetic nanocomposite materials consisting of 10 wt % CoFe2O4 or NiFe2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. A 100-kV aberration-corrected scanning transmission electron microscope (STEM) has been used to study these materials, and bright field and high angle annular dark field images show that after heat treatment at both 450 and 900 degrees C, they contain needle-like nanostructures similar to 1 nm in width and 10 nm in length. High resolution STEM images show that the needle-like nanostructures have a layered internal structure with typical interlayer spacings of 0.33 +/- 0.02 nm. Electron energy loss spectroscopy using a 0.13-nm diameter probe gives information on the composition of these nanostructures. The results presented here for samples heat treated at 450 degrees C are consistent with needle-like nanostructures arising from Co and Ni silicate hydroxides which are separate from the also present Fe-containing phase of ferrihydrite nanoparticles. Samples heat treated at 900 degrees C have previously been shown to contain round ferrite nanoparticles similar to 8 nm in diameter. The results presented here are consistent with the needle-like nanostructures being transformed into ferrite-like phases after heat treatment at 900 degrees C, and the needle-like nanostructures are often found attached to round ferrite nanoparticles.