Size control of magnetite nanoparticles in hydrothermal synthesis by coexistence of lactate and sulfate ions

The size of magnetite nanoparticles was controlled by means of coexistence effects of anions in the starting solution of organic solvent-free hydrothermal synthesis via coprecipitation. As the coexisting anions, lactate ion, which forms relatively stable complex with ferric ion, and sulfate ion were used. The hydrothermal synthesis was carried out at various concentrations of lactate and sulfate ions. The obtained magnetite nanoparticles were characterized by a scanning electron microscope, transmission electron microscope, powder X-ray diffractometer, dynamic light scattering particle size analyzer and superconducting quantum interference device. The formation mechanism of magnetite nanoparticles in this system was investigated based on the experimental results. The coexisting anions remarkably influenced both formation of crystalline nuclei and dispersion stabilization of formed precipitates. The particle size depended strongly on the concentration of lactate and sulfate ions. It has been clarified that lactate ion has the effect to decrease the particle size and sulfate ion promotes the particle growth. By adjusting the concentration of lactate and sulfate ions properly as the operating factor, we could control successfully the particle size from 9.5 to 38.6 nm in median size.