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

The unique properties of magnetite (Fe₃O₄) nanoparticles (MNPs) have promoted a wide range of applications in various fields, but specifying and defining the preparation conditions of MNPs with the desired monodispersed size are still challenges. This paper discusses systematically the influence of synthesis conditions on the structure and size of MNPs in the coprecipitation method without a stabilizer in one system. The ammonium hydroxide (NH₄OH)/iron (III) (Fe³⁺) ratio, iron (III)/iron (II) (Fe²⁺) ratio, initial concentration of hydrochloric acid (HCl), total concentration of iron ions, reaction temperature, aging time and washing times were investigated. The surface of magnetite crystals was easily oxidized at high ammonium hydroxide/iron (III) ratios, and the high pH value of the reaction system can induce a large crystalline size. The hydrodynamic diameter of MNPs was efficiently controlled by the equilibrium between Derjaguin–Landau–Verwey–Overbeek (DLVO) forces and non-DLVO forces. An appropriate initial concentration of hydrochloric acid and a 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, the pH value of the MNP dispersion in distilled water would remarkably reduce after each washing by centrifugation due to surface absorption of the hydroxy group. This research provided fundamental support for the reproducible preparation of MNPs in the coprecipitation method.