Transmission electron microscopic investigation of an ordered Al₂O₃ film on NiAl(110)

We have prepared under ultrahigh vacuum conditions a NiAl(110) wedge which was oxidized such that, after oxidation, small support-free Al2O3 crystallites were formed at the edge of the wedge, and the remaining part of the wedge was covered by a thin well-ordered Al2O3 film. This film, which has been characterized in great detail in earlier studies [J. Libuda et al., Surf. Sci. 318 (1994) 61], was transferred in air to a transmission electron microscope (TEM) and studied with respect to its geometric structure. The particular sample preparation allows for a direct comparison between the structure of the film and the bulk aluminum oxide. We find that the supported oxide exhibits a γ-Al2O3-like structure similar to that observed previously under UHV conditions, indicating that exposure to air has only a limited influence on the film. The lattice constant of the oxide film agrees within ∼2% with that reported for bulk Al2O3. The domain structure of the alumina film as found in the ultrahigh vacuum experiments was also identified using TEM of the supported layer. The support-free oxide formed at the edge of the wedge exhibits the structure of the supported film formed under UHV conditions with deviations of the lattice constants of ∼7% in one direction and ∼1% in the other