Influence of synthesis conditions on the structure and size of magnetite nanoparticles in co-precipitation method without stabilizer in one system

Unique properties of magnetite (Fe3O4) nanoparticles (MNPs) have promoted wide applications in various fields, but specifying and defining preparation conditions of MNPs are still challenges for the desired monodispersed size. We discussed systematically the influence of synthesis conditions on the structure and size of MNPs in co-precipitation method without stabilizer in one system. NH4OH/Fe3+ ratio, Fe3+/Fe2+ ratio, initial concentration of HCl, total concentration of iron ions, reaction temperature, aging time and washing times were investigated. The surface of magnetite crystal was easily oxidized at high NH4OH/Fe3+ ratio, and the high pH value of the reaction system can induce the large crystalline size. The hydrodynamic diameter of MNPs was efficiently controlled by the equilibrium of DLVO forces and non-DLVO forces. An appropriate initial concentration of HCl and higher reaction temperature can reduce the crystalline size due to the decelerated nucleation rate. The aging time and the total concentration of iron affected the crystalline size positively. Noticeably, pH value of MNPs dispersion in distilled water would remarkably reduce after each washing by centrifugation due to surface absorption of hydroxy group. This research provided the fundamental support for the reproducible preparation of MNPs in co-precipitation method.