Preparation of iron oxide and iron oxide/silicon oxide nanoparticles via water-in-oil microemulsion

A new water-in-oil (W/O) reverse microemulsion system was developed to prepare silica-coated iron oxide nanoparticles, where Igepal CO-520 was used as the surfactant and cyclohexane as the oil phase. As a comparison, coprecipitation method was also employed to synthesize the nanoparicles with hydrothermal posttreatment to enhance the crystallinity of the iron oxide nanoparticles. The iron oxide nanoparticles prepared via the microemulsion and co-precipitation at ambient temperature are magnetite (Fe₃O ₄) and have poor crystallinity with the size of 5-10nm and 10-20nm, respectively. The iron oxide particles via microemulsion grew in size to ∼60nm after hydrothermal treatment at 200°C and transformed to hematite (α-Fe₂O₃) at 140°C. Large size distribution (14-180nm) was observed for nanoparticles prepared by coprecipitation after post-hydrothermal treatment at 200°C and maghemite (γ-Fe ₂O₃) phase was formed at 140°C - 160°C and transformed into the hematite at 200°C. Silicon oxide coated iron oxide nanocomposites were also synthesized in-situ by ammonia-catalyzed tetraethyl orthosilicate (TEOS) hydrolysis and condensation after forming the iron oxide nanoparticles in the microemulsion system. TEM with EDS showed that the iron oxides were coated by a thin layer of silica.