Structural and magnetic characterisation of Co and Ni silicate hydroxides in bulk and in nanostructures within silica aerogels

X-ray diffraction, transmission electron microscopy, x-ray absorption spectroscopy and magnetic measurements were used to investigate highly porous silica aerogels containing iron and cobalt or iron and nickel, calcined at 450°C, which are intermediates in the formation of CoFe2O4-SiO2 and NiFe2O4-SiO2 aerogel nanocomposites. Aerogels without iron and pure Co and Ni silicate hydroxides were also studied. The aerogel samples containing iron and cobalt or iron and nickel, present two separate phases one containing iron in the form of ferrihydrite, the other one being either Co or Ni silicate hydroxides. The silicate hydroxides are present in the aerogels in form of nanoparticles with a 1-dimensional appearance in electron microscopy images, which are therefore anisotropic nanostructures, such as nanoneedles or nanosheets. The study of the magnetic properties indicates that the coercivity is greatly modified by size and shape of the particles going from pure Co or Ni silicate hydroxides to the aerogel samples containing only nickel or only cobalt. Finally, in the samples containing also iron, the total magnetic behavior is influenced both by the presence of ferrihydrite and by the possible diffusion of iron in the nanostructures of cobalt or nickel silicate hydroxide.