Effect of Surfactants on the Structure, Phase Composition, and Magnetic Properties of FexSy Nanoparticles Synthesized by Thermal Decomposition

The effect of surfactants on the structure, morphology, and magnetic properties of Fe x S y iron-sulfide nanoparticles synthesized by thermal decomposition is studied. Oleylamine, hexadecylamine, and octadecylamine are used as surfactants. It is established by X-ray and electron-diffraction analysis combined with Mössbauer spectroscopy that, in samples 1 and 2 prepared using oleylamine and hexadecylamine, respectively, the Fe 3 S 4 greigite phase dominates, with an inverse spinel structure isostructural to the iron oxide Fe 3 O 4 magnetite with minor Fe 9 S 11 impurities. Deviations in the distribution of iron cations over the tetrahedral and octahedral sites relative to the bulk greigite crystals are observed. The nanoparticles synthesized using octadecylamine (sample 3) are found to be multiphase with a greigite fraction of ~20%. In all three cases, as showed the results of transmission electron microscopy and Fourier transform infrared spectroscopy together with thermogravimetry analysis, the magnetic nanoparticles have an organic shell chemically bonded to their magnetic core, which prevents the agglomeration of the particles. This shell is much more massive in samples 2 and 3. The magnetization values for samples 1 and 2 are similar to those of greigite nanoparticles reported in publications, while the magnetization of sample 3 is several times lower, in accordance with the greigite fraction in it. The combination of fairly high magnetization with a massive organic shell allows one to consider hexadecylamine to be a promising surfactant for the synthesis of iron-sulfide nanoparticles protected from external impact and agglomeration.