Preparation of Magnetic Nanoparticles via a Chemically Induced Transition: Role of Treating Solution’s Temperature

Using FeOOH/Mg(OH)₂ as precursor and FeCl₂ as the treating solution, we prepared γ-Fe₂O₃ based nanoparticles. The FeCl₂ treating solution catalyzes the chemical reactions, dismutation and oxygenation, leading to the formation of products FeCl₃ and Fe₂O₃, respectively. The treating solution (FeCl₂) accelerates dehydration of the FeOOH compound in the precursor and transforms it into the initial seed crystallite γ-Fe₂O₃. Fe₂O₃ grows epitaxially on the initial seed crystallite γ-Fe₂O₃. The epitaxial layer has a magnetically silent surface, which does not have any magnetization contribution toward the breaking of crystal symmetry. FeCl₃ would be absorbed to form the FeCl₃·6H₂O surface layer outside the particles to form γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles. When the treating solution's temperature is below 70 °C, the dehydration reaction of FeOOH is incomplete and the as-prepared samples are a mixture of both FeOOH and γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles. As the treating solution's temperature increases from 70 to 90 °C, the contents of both FeCl₃·6H₂O and the epitaxial Fe₂O₃ increased in totality.