Effects of pH Value, Reaction Time, and Filling Pressure on the Hydrothermal Synthesis of ZnFe2O4 Nanoparticles

We investigated the effect of the pH value, reaction time, and filling pressure on the preparation of ZnFe₂O₄ (ZFO) nanoparticles using a hydrothermal synthesis. X-ray diffraction showed that all three parameters should be controlled in order to prepare ZFO nanoparticles without impurities. Furthermore, the nanoparticle size reduced from 60 to 5 nm when the filling pressure was increased from 50% to 70%. The X-ray photoelectron spectroscopy showed that the ZFO nanoparticles actually possess a mixed spinel structure due to the coexistence of Fe²⁺ and Fe³⁺, and there were significant amounts of oxygen vacancies near the surface. The cation inversion and surface effect worked together to determine the magnetic properties of the ZFO nanoparticles. The ZFO nanoparticles of 60 nm sizes showed a much enhanced room temperature ferromagnetism together with a larger coercivity compared with that of the ZFO nanoparticles of 5 nm sizes. Our results provide valuable information regarding the optimization of the hydrothermal synthesis of ferrite nanoparticles for desired applications.