The unusual surface chemistry of α-Al2O3 (0001)

We report on the influence of heat treatment on the surface chemistry of an alpha-alumina crystal. We compare its electrical double layer behaviour with that of 150 nm diameter alpha-Al2O3 particles. Surface spectroscopy and zeta potential studies are used to understand the changes in surface chemistry. The pH(pzc) of an alpha-Al2O3 (0001) single crystal (similar to 4) is more acidic than that of alpha-Al2O3 particles (8.5), a difference explained by the dominance of Al2OH surface groups on the single crystals and their charging behaviour. Heat treatment of the alumina surface causes a substantial decrease in the number of surface OH groups. Heating at 500 degrees C decreases the surface density of hydroxyl groups. Heating at 1050 degrees C also affects surface morphology and surface chemistry. The increased magnitude of the zeta potential and the pH(pzc) shift to lower pH suggest a surface reconstruction and the appearance of more acidic aluminium sites.