Production of nanostructured iron oxide particles via aerosol decomposition

Nanostructured iron oxide particles with average size below 100 nm were produced by aerosol decomposition method starting from an aqueous iron nitrate solution. Air, nitrogen, or mixture of hydrogen (7 %) and nitrogen were used as the carrier gas. Gas-phase particle number size distributions were determined with a differential mobility analyzer. Particle morphology and crystallinity were studied with scanning (SEM) and transmission (TEM) electron microscopes. Crystalline phase composition of the particles was studied with X-ray diffraction (XRD). The average gas-phase diameter of particles produced in air or N₂ reduced from 80 to 47 nm when temperature was increased from 500 to 1100 °C. In H₂ rich environment, the reduction of average size was larger, from 80 nm at 500 °C to 45 nm already at 900 °C. SEM results showed that very small crystallites (5–10 nm) were formed on the surface of the particles produced in N₂ at 500 °C. When the processing temperature was increased to 700, 900 and 1100 °C, the crystallites on the particle surfaces were grown to 15–30, 30–60 and 60–180 nm, respectively. TEM results show that very small particles (< 50 nm) were single crystals and larger particles were polycrystalline with crystallite size of about 50 nm at 700 °C in H₂/N₂. Magnetite particles were produced from aqueous iron nitrate solution at 500 °C in H₂/N₂ and at 900 °C in N₂ according to XRD results.