From Oxygen to Oxide: First-Principles Study of Some Key Aspects

Doktorsavhandling, 2003

Having important technological applications and interesting fundamental science aspects, oxidation of aluminum has received impressive attention. Technologically, the main reason for the great interest is the fact that the oxide film which forms on aluminum protects the surface against further oxidation and corrosion. From a fundamental point of view, the simple electronic structure of aluminum makes it a prototype system for oxidation. A close interplay between theoretical studies and experiments has provided a proper atomic-scale explanation to many key aspects of aluminum oxidation. However, there are still some unsettled issues, which remain to be solved in the future. This thesis is a theoretical study of some key aspects of aluminum oxidation. On one hand, adsorption of diatomic molecules on the (111) surface of aluminum, forming the initial stage of aluminum oxidation, is elucidated. On the other hand, a new one-dimensional electron gas (1DEG) is predicted and characterized on the surface of the metastable κ-alumina. First-principles density-functional-theory (DFT) calculations, using the plane-wave pseudopotential method, make the basis for our study and are used as inputs for phenomenological models. The main objective of the adsorption study is to provide a proper explanation of the sticking behavior and a deeper understanding of molecular chemisorption. The sticking behavior is accounted for in a diabatic charge-transfer model, based on electronically excited states, calculated from first-principles. The latter is achieved by developing a new DFT-extended ΔSCF method. Molecular intermediates are predicted for several diatomic molecules, e.g., O2, NO and CO, revitalizing concepts as chemical hardness and molecular asymmetry. Such molecular states are apt for molecular trapping, molecular flipping, and abstraction. The latter has been confirmed experimentally for O2 on Al(111) and explains the huge separation (≥ 80Å) between thermally adsorbed O-atoms on the same surface. These concerted experimental observations and theoretical studies demonstrate the virtue of electron-structure-trend analysis. The materials-theory prediction of an 1DEG on the (00-1) surface of κ-alumina is made on the basis of DFT calculations. Furthermore, the Peierls metal-insulator instability of this 1DEG is characterized by means of a tight-binding model, with DFT calculations as input parameters. It is concluded that this 1DEG is metallic over a temperature range of [1,800] K and exhibits a high electrical conductivity, which should make it a very interesting manifestation of a 1DEG.