Hello Dr. Prabhin

You could try a double displacement type of preparation technique. Using aqueous solutions of cobalt nitrate (hexahydrate) and potassium hydroxide, you can precipitate cobalt oxide. To tune the size of the precipitate, changing the pH, use of complexing agents like EDTA and even changing solution temperature may help. Nickel yields nano-sized oxide by the same technique, I should expect cobalt to do the same.

This article can be helpful: Article Manganese oxides and their application to metal ion and cont...

I did reverse titration to make ZrO2 nanoparticles once. As originally the metal ions are fully dissolved in solution, the titration rate and agitation speed can control the fine oxide growth. Maybe you can add some kind of dispersant later to stabilize the nanoparticles in your suspension at a targeted concentration.

Thank you Siva, Aminul and Zhongfen for the informations. I will try double displacement preparation technique .