Structure of the hydrated alpha-Al2O3 (0001) surface

The physical and chemical properties of the hydrated alpha-Al2O3 (0001) sur face are important for understanding the reactivity of natural and syntheti c aluminum-containing oxides. The structure of this surface was determined in the presence of water vapor at 300 kelvin by crystal truncation rod diff raction at a third-generation synchrotron x-ray source. The fully hydrated surface is oxygen terminated, with a 53% contracted double Al Layer directl y below. The structure is an intermediate between alpha-Al2O3 and gamma-Al( OH)(3), a fully hydroxylated form of alumina. A semiordered oxygen Layer ab out 2.3 angstroms above the terminal oxygen Layer is interpreted as adsorbe d water. The clean alpha-Al2O3 (0001) surface, in contrast, is AL terminate d and significantly relaxed relative to the bulk structure. These differenc es explain the different reactivities of the clean and hydroxylated surface s.