Structure of the hydrated α-Al2O3 (0001) surface

The physical and chemical properties of the hydrated alpha-Al2O3 (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 alpha-Al2O3 and gamma-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 alpha-Al2O3 (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.