Comparison of magnetic properties and high-temperature phase stability of phosphate- and oleic acid-capped iron oxide nanoparticles.

We study the influence of dynamic capping of Fe₃O₄ nanoparticles with phosphate and oleic acid, on their structure, magnetic properties and thermal stability of magnetic nanoparticles. It is observed that the phosphate coating on iron oxide lowers the dipole-dipole interaction significantly, as compared to oleic acid capping. The Mössbauer results show that the spin canting order of oxidized shell and the mean hyperfine field values follow the order Fe0 (uncoated) > FeOA (oleic acid capped) > FP1 (phosphate capped). The uncoated Fe₃O₄ nanoparticle is non-stoichiometric in nature due to oxidation, whereas FP1 and FeOA are of the correct stoichiometry. Mössbauer and photoacoustic spectroscopic studies on air-annealed phosphate-coated magnetite nanoparticles confirm that the magnetic iron oxide phase is preserved up to 833 K and a complete conversion of Fe₃O₄ into the non-magnetic hematite phase occurs at 1173 K. The iron oxide air annealed at 833 K is found to have a shell of orthorhombic α-Fe₂O₃ over the magnetite core. However, in oleic acid-coated nanoparticles, the magnetic to non-magnetic phase transformation commences at 623 K and the conversion was complete at 823 K. The photoacoustic spectra of the air-annealed phosphate-coated Fe₃O₄ particles showed a flipping of the absorption intensity between 500−700 nm and 800−1000 nm, due to the conversion of Fe₃O₄ to γ-Fe₂O₃ at 923 and γ-Fe₂O₃ to α-Fe₂O₃ at 1173 K, respectively. The γ-Fe₂O₃ showed an intense absorption peak above 750 nm, whereas the α-Fe₂O₃ showed a peak broadening in the wavelength range of 600-700 nm, in addition to the strong peaks at a wavelength above 750 nm. This study suggests that the photoacoustic spectroscopy can distinguish clearly the three polymorphs of iron oxide i.e., Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃. Our results confirm the ability of phosphate-capped iron oxide particles to retard the oxidation of Fe²⁺ contents during the crystal growth process. [ABSTRACT FROM AUTHOR]