Adsorption of H₂O on a Single-Crystal α-Al₂O₃(0001) Surface

Abstract

The adsorption of H2O on a single-crystal α-Al2O3(0001) surface was examined using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) techniques. α-Al2O3(0001) models the surface of Al2O3 exhaust particles generated by solid rocket motors that may affect the stratospheric ozone layer. After cleaning and annealing to 1100 K, the α-Al2O3(0001) surface displayed a well-defined hexagonal (1 × 1) low-energy electron diffraction (LEED) pattern. Absolute hydroxyl coverages on this α-Al2O3(0001) single-crystal surface were determined using H2O LITD signals. Hydroxylation by the dissociative adsorption of H2O was differentiated from molecular H2O adsorption using TPD studies with isotopically labeled H218O. For H2O dissociative adsorption at 300 K, the initial sticking coefficient was S ≈ 0.1. The H2O sticking coefficient decreased nearly exponentially with hydroxyl coverage, and the hydroxyl coverage saturated at ϑOH = 0.5 × 1015 OH groups/cm2 after a H2O exposure of >1010 langmuir. For constant H2O exposures performed at different H2O pressures, the resulting hydroxyl coverage also increased with H2O pressure suggesting collisionally activated H2O adsorption. On the basis of these H2O adsorption results, α-Al2O3 rocket exhaust particles in the stratosphere should be hydroxylated at coverages of ϑOH ≈ 0.3 × 1015 OH groups/cm2. H2O adsorption on α-Al2O3(0001) was also investigated using a H2O plasma. Plasma hydroxylation yielded much larger hydroxyl coverages of ϑOH = 3.6 × 1015 OH groups/cm2 and destroyed the hexagonal LEED pattern after one plasma exposure. Larger hydroxyl coverages were measured after consecutive H2O plasma exposures indicating that plasma hydroxylation progressively roughens the α-Al2O3(0001) surface.