The best and easiest way to prepare ZVNI is via borohydride reduction technique. It yields evenly dispersed nanometer sized nanoparticles. To a solution of any iron salt, a pinch of NaBH4 is added and centrifuged. To prevent oxidation, the ZVNI is capped with CMC by centrifuging at 15,000 rpm.
I think Dey has given you nice suggestion. Also if you want to use sol gel method by maintaining the staichiometric ratios of the precursor and reducing agents. i am sure you will get result and will be chief also.
Further, by using sol gel method you can tailor the particle size