Growth mechanisms and oxidation resistance of gold-coated iron nanoparticles

We report the chemical synthesis of Fe-core/Au-shell nanoparticles by a reverse micelle method and the investigation of their growth mechanisms and oxidation-resistant characteristics. The core-shell structure and the presence of the Fe and Au phases have been confirmed by transmission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Mössbauer spectroscopy, and inductively coupled plasma techniques. Additionally, atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) have been used to study details of the growth processes. The Au shell grows by nucleating on the Fe-core surface before coalescing. The magnetic moments of such nanoparticles, in the loose powder form, decrease over time due to oxidation. The less than ideal oxidation resistance of the Au shell may have been caused by the rough Au surfaces. However, in pressed pellet form, electrical transport measurements show that the particles are fairly stable, as the resistance of the pellet does not change appreciably over time.