An experimental investigation of aqueous-phase synthesis of magnetite nanoparticles via mechanochemical reduction of goethite

A newly developed mechanochemical process for the simple aqueous phase synthesis of crystalline magnetite nanoparticles has been experimentally investigated. In this process, a suspension of ferric hydroxide precursor is milled at room temperature using a horizontal tumbling ball mill consisting of a stainless steel pot and balls. Ferric hydroxide is transformed to magnetite without the use of a reducing agent. As a model starting material for the investigation, a pH-adjusted suspension of crystalline goethite was used. As the milling time increased, goethite disappeared along with the simultaneous formation of magnetite. A single phase of magnetite was obtained after 16 h of milling. A reaction mechanism for the formation of magnetite has been proposed based on oxidation–reduction reactions, in which the corrosion of iron in the pot and balls plays an important role. Free electrons are generated by the release of ferrous ions from the stainless steel in an anodic reaction, which then reduce goethite to ferrous hydroxide in a cathodic reaction. The solid phase reaction between ferrous hydroxide and goethite produces magnetite. Not only could the mechanochemical effect induced by the collision of balls accelerate the corrosion even under alkaline conditions, it can also promote the formation and crystallization of magnetite.