Synthesis of core-shell nanostructures of Co₃O ₄@SiO₂ with controlled shell thickness (5-20 nm) and hollow shells of silica

Synthesis of uniform silica shell over Co₃O₄ nanoparticles was carried out using the colloidal solutions of Tergitol and cyclohexane. The shell could be controlled to a thickness of up to 20 nm by varying different parameters such as the amount of tetraethylorthosilicate, concentration of Co₃O₄ nanoparticles, reaction time and the presence of water and 1-octanol. Control of the amount of water (required for hydrolysis) appears to be the key factor for controlling the shell thickness. The methodology used is suitable to form shell over nanoparticles (present in powder form; synthesized at high temperature) which have high degree of agglomeration. Hollow shells of silica were obtained by the dissolution of the oxide core of Co₃O₄@SiO₂ core-shell nanostructures. The composition of these core-shell nanostructures was confirmed by high-resolution transmission electron microscopy and elemental mapping by energy dispersive X-ray analysis. The hollow shells were characterized by using TEM, EDX and IR. Electron paramagnetic resonance studies of the core-shell nanostructures indicate the presence of free radicals on silica shell due to the presence of dangling bonds in the silica. Increase in the magnetic susceptibility was observed for these core-shell nanostructures.