Electron-beam physical vapor deposition of iron nanoparticles and their thermal stability in the Fe–O  system

Abstract

Chemically pure (ligand-free) 5–70 nm iron nanoparticles were synthesized by electron-beam physical vapor deposition (EB-PVD) in a porous NaCl matrix. The influence of chemical composition, substrate temperature, and isothermal treatment on the dimensional, structural, and phase composition of the nanoparticles in the Fe–O system was studied. For this purpose, independent molecular fluxes of Fe and NaCl were applied to a stationary substrate by electron-beam physical vapor deposition (EB-PVD) at substrate temperatures of 45–400 °C to produce 3–30 wt.% Fe–NaCl condensates. With increasing iron content of the condensate, substrate temperature, and heat treatment temperature, the average size of particles in the Fe-O system becomes greater. To study the oxidation kinetics of iron nanoparticles, the condensates produced at a substrate temperature of 45 °C were isothermally treated in air at 200–650 °C. The condensates were examined by scanning electron microscopy, transmission electron microscopy, and X-ray phase diffraction, and aqueous solutions of condensates by dynamic light scattering. Being of small size and thus having high adsorption capacity for air and moisture oxygen, iron exothermally oxidized in the condensate when the vacuum chamber depressurized and the condensate separated from the substrate. At different substrate temperatures and after heat treatment, the nanoparticles may contain pure iron and iron oxides Fe₃O₄ and Fe₂O₃. In the studied condensates, the a-Fe phase is present only when the iron content is more than 20 at.%. This is explained by the fact that not all nanoparticles (crystallites) have time to oxidize to the Fe₃O₄ phase with increase in the sizes. In addition, Fe₃O₄ nanoparticles additionally adsorb oxygen. The ratio of the atomic percentage of oxygen to the atomic percentage of iron depends on the amount of iron, decreases with increasing iron content of the condensate, and even exceeds this value for the stoichiometric composition of Fe₂O₃, being equal to 1.5. The study shows that the EB-PVD method is universal in the use of inorganic materials for the synthesis and retention of pure nanoparticles of metals and their oxides.