Synthesis of oxidation-resistant metal nanoparticles via atomic layer deposition

Oxidation-resistant iron nanoparticles were produced via the decomposition of iron oxalate particles followed by in situ passivation with ultrathin alumina films deposited by atomic layer deposition (ALD). Decomposition and passivation were carried out in a fluidized bed reactor at low pressure and under mechanical agitation. Films were deposited using self-limiting, sequential surface reactions of trimethylaluminium and water. Thermo-gravimetric studies determined that two different mechanisms were involved in the decomposition of iron oxalate. The iron nanoparticles were highly pure as verified by x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Reduction of particle size was achieved via a cryogenic milling process and enhanced by the natural breakage of porous oxalate particles during processing. Synthesized iron particles had a primary size in the range of 50-80 nm. As demonstrated by high-resolution transmission electron microscopy (HRTEM), highly conformal alumina films were deposited on individual nanoparticles with an average growth rate of 0.16 nm/cycle. Superior oxidation resistance at 427 deg C was achieved after particles were coated with an 8 nm alumina film. After the coating process, the particles were ferromagnetic.