Structure of the Hydrated α-Al 2 O 3 (0001) Surface

Abstract

The physical and chemical properties of the hydrated α-Al ₂ O ₃ (0001) surface are important for understanding the reactivity of natural and synthetic aluminum-containing oxides. The structure of this surface was determined in the presence of water vapor at 300 kelvin by crystal truncation rod diffraction at a third-generation synchrotron x-ray source. The fully hydrated surface is oxygen terminated, with a 53% contracted double Al layer directly below. The structure is an intermediate between α-Al ₂ O ₃ and γ-Al(OH) ₃ , a fully hydroxylated form of alumina. A semiordered oxygen layer about 2.3 angstroms above the terminal oxygen layer is interpreted as adsorbed water. The clean α-Al ₂ O ₃ (0001) surface, in contrast, is Al terminated and significantly relaxed relative to the bulk structure. These differences explain the different reactivities of the clean and hydroxylated surfaces.