A transmission electron microscopy study of CoFe₂O₄ ferrite nanoparticles in silica aerogel matrix using HREM and STEM imaging and EDX spectroscopy and EELS

Magnetic nanocomposite materials consisting of 5 and 10 wt% CoFe ₂O₄ nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe₂O₄-10wt% sample, bright-field scanning transmission electron microscopy (BF STEM) and high-resolution transmission electron microscopy (HREM) images showed distinct, rounded CoFe₂O₄ nanoparticles, with typical diameters of roughly 8 nm. For the CoFe₂O₄-5wt% sample, BF STEM images and energy dispersive X-ray (EDX) measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 ± 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe₂O₄, and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe₂O₄-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than ± 0.5 nm in analysis position. BF STEM images obtained for the CoFe₂O ₄-10wt% sample showed many "needle-like" nanostructures that typically have a length of ∼ 10 nm and a width of ∼ 1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 ± 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials.