Morphology and oxide shell structure of iron nanoparticles grown by sputter-gas-aggregation

The crystal faceting planes and oxide coating structures of core–shell structured iron/iron-oxide nanoparticles synthesized by a sputter-gas-aggregation process were studied using transmission electron microscopy (TEM), electron diffraction and Wulff shape construction. The particles grown by this process and deposited on a support in a room temperature process have been compared with particles grown and deposited at high temperature as reported in the literature. It has been found that the Fe nanoparticles formed at RT are invariantly faceted on the {100} lattice planes and truncated by the {110} planes at different degrees. A substantial fraction of particles are confined only by the 6{100} planes (not truncated by the {110} planes); this contrasts with the Fe particles formed at high temperature (HT) for which a predominance of {110} planes has been reported. Furthermore, at RT no particle was identified to be only confined by the 12{110} planes, which is relatively common for the particles formed at HT. The Fe cubes defined by the 6{100} planes show a characteristic inward relaxation along the and directions and the reason for this behaviour is not fully understood. The oxide shell on the Fe{100} plane maintains an orientation relationship: and , which is the same as the oxide formed on a bulk Fe(001) through thermal oxidation. Orientation of the oxide that forms on the Fe{110} facets differs from that on Fe{001}: therefore, properties of core–shell structured Fe nanoparticle faceted primarily with one type of lattice plane may be fully different from that faceted with another type of lattice plane.