Mechanochemical formation of superparamagnetic magnetite nanoparticles from ferrihydrite over a wide range of pH environments

We have developed a new aqueous-phase synthesis of highly crystalline superparamagnetic magnetite nanoparticles using mechanochemical effects under environmentally friendly conditions (i.e., at room temperature and without organic solvent). In this method, aqueous suspensions of ferric hydroxide (ferrihydrite) at pH values between 2.9 and 13.3 were used as the starting material (precursor) and milled in a stainless steel horizontal tumbling ball mill. Although the milling was performed without heating and in the absence of any reducing agent, magnetite nanoparticles with a diameter of 10–20 nm were formed. This result implies that ferric ions were reduced to ferrous ions during the milling, resulting in the formation of ferrous hydroxide from ferric hydroxide. The initial pH of the precursor suspensions had a minimal effect on properties of the magnetite nanoparticles thus produced, showing that this process is unaffected by variations in the initial pH. The formation of magnetite is described using oxidation–reduction reactions, in which the corrosion of steel plays an important role, and the initial pH greatly affects the reaction mechanism of magnetite formation.