Improvement of Thermal Stability of Maghemite Nanoparticles Coated with Oleic Acid and Oleylamine Molecules: Investigations under Laser Irradiation

We investigated the influence of the coating of maghemite nanoparticles (NPs) with oleic acid and oleylamine molecules on the thermal stability of maghemite and on the γ → α-Fe₂O₃ phase transformation. The uncoated maghemite NPs were synthesized by coprecipitation and the coated NPs by thermal decomposition of organometallic precursors. The morphology and size of the coated NPs were characterized by transmission electron microscopy and magnetic and structural properties by ⁵⁷Fe Mössbauer and Raman spectroscopies. The phase stability of coated maghemite NPs was examined under in situ laser irradiation by Raman spectroscopy. The results indicate that coated γ-Fe₂O₃ NPs are thermally more stable than the uncoated NPs: the phase transformation of maghemite into hematite was observed at 15 mW for uncoated NPs of 4 nm, whereas it occurs at 120 mW for the coated NPs of similar size. The analysis of the Raman baseline profile reveals clearly that the surface coating of maghemite NPs results both in reducing the number of surface defects of nanoparticles and in delaying this phase transition