A High-Resolution Photoemission Study of Nanoscale Aluminum Oxide Films on NiAl(110)

• Published in: Langmuir Vol. 21; no. 18; pp. 8312 - 8318
• Main Authors: Mulligan, Andrew, Dhanak, Vin, Kadodwala, Malcolm
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 30-08-2005
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Surface physical chemistry; Aluminium oxide; High resolution; Oxygen; Annealing; Oxide layer; Film; Oxides; Decomposition; Crystal face; X ray; Amorphous phase; Aluminium Alloys; Substrate; Crystallites; Adsorption; Oxidation; Diffusion; Nickel Alloys; Alumina; Room temperature
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0509753

Using high-resolution soft X-ray photoemission, Al 2p, we have been able to quantify the relative populations of tetrahedrally (Altet) and octahedrally (Aloct) coordinated Al3+ in three distinct phases of nanoscale aluminum oxide films on NiAl(110). We have hence determined the bulk alumina phases that the nanoscale films most resemble. Adsorption of oxygen at room temperature produces a layer which predominately (90%) contains Altet and is analogous to the amorphous bulk phase of alumina. Annealing this layer results in an Al enrichment of the oxide layer, through the diffusion of metal from the substrate, and an increase in the relative amount of Aloct, producing a γ-alumina-like layer with a relative Aloct/Altet occupancy of 28 ± 3%/72 ± 3%. Oxygen adsorption at 823 K also produces a γ-like phase, with a relative Aloct/Altet occupancy of 27 ± 3%/73 ± 3%, although this layer is thicker than that formed at room temperature. Both oxidation methods produce γ-alumina layers that display poor translational order. However, these poorly ordered layers have a relative Aloct/Altet occupation similar to that of well-ordered oxide films produced using different oxidation conditions in previous studies. Both γ layers undergo partial decomposition upon annealing to 1273 K, producing an α-alumina-like oxide, which contains only Aloct, and is highly deficient in Al. There are significant oxide-free areas within the α-alumina oxide layer, which is characteristic of crystallite formation. Repeated cycles of oxidation and annealing to 1273 K do not produce a homogeneous film, but they do make the α-like oxide more Al rich.