Characterization of the iron oxide phases formed during the synthesis of core-shell FexOy@C nanoparticles modified with Ag

Core-shell Fe_xO_y@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). Fe_xO_y@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO₃)₃ · 9H₂O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO₃ in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe₃O₄ and γ-Fe₂O₃. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid Fe_xO_y@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe₃O₄@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the Fe_xO_y@C-Ag NPs is discussed.