Low-temperature synthesis of redispersible iron oxide nanoparticles under atmospheric pressure and ultradense reagent concentration

Highly dispersed α-Fe2O3 nanoparticles ca. 3 to 8 nm in diameter were prepared at atmospheric pressure, low temperature, and at an ultradense reagent concentration by titrating an aqueous ammonia solution into a dense iron oleate/toluene mixture. A transparent suspension was obtained by redispersing the prepared particles in nonpolar solvents since they were redispersible to primary particles without aggregate formations. The prepared particles were characterized by TEM, XRD, and FT-IR, and their dispersion stability in organic solvents was determined by dynamic light scattering (DLS) and viscosity measurements. In order to analyze the formation process of the highly dispersed α-Fe2O3 nanoparticles, time-course measurements of DLS and viscosity during the nanoparticle synthesis in toluene were carried out. A significant increase in the suspension viscosity and the formation of an aggregated structure were observed as soon as the titration of the aqueous ammonia solution. The suspension viscosity and aggregated particle size gradually reduced with continuous vigorous stirring; finally, α-Fe2O3 nanoparticles that were completely redispersible in nonpolar solvents were obtained after ca. 24 h. The particle size could be controlled by the synthesis temperature, and such redispersible α-Fe2O3 nanoparticles were obtained even when the reagent concentration was increased to 2.8 mol/L.