TEM study of the structural dependence of the epitaxial passive oxide films on crystal facets in polyhedral nanoparticles of chromium

Nanocubes and partially truncated rhombic dodecahedral nanoparticles of Cr have been studied by electron diffraction, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is found that the nanoparticles of Cr are enclosed by the epitaxial passive oxide films. The oxide films on 100% truncated nanocubes of Cr with only one kind of facets, {1 0 0} facets, are face-centered cubic (fcc) structured Cr2O3 with a lattice constant of 0.407 nm. There are two kinds of oxide films in partially truncated nanoparticles of Cr with two kinds of crystallographic facets, {1 0 0} and {1 1 0}. The same fcc Cr2O3 is found on the {1 0 0} facets while the rhombohedral α-Cr2O3 is found on the {1 1 0} facets. This is similar to the two kinds of oxides, fcc and rhombohedral Fe2O3, which have also been observed in polyhedral nanoparticles of Fe. These passive Cr2O3, found in nanoparticles of Cr which have remained unchanged in water for four years, may have important implications for protective oxide films involving Cr.