REACTION OF WATER-VAPOR WITH ALPHA-AL2O3(0001) AND ALPHA-FE2O3(0001) SURFACES - SYNCHROTRON X-RAY PHOTOEMISSION-STUDIES AND THERMODYNAMIC CALCULATIONS

X-ray photoemission experiments were carried out to study the reaction of water vapor with clean (0001) surfaces of single-crystal corundum (alpha-Al2O3) and single-crystal, thin-film hematite (alpha-Fe2O3) at constant reaction time (3 min) as a function of water vapor pressure [ p(H2O)] and, in separate experiments, at longer reaction times (=30 mi n, with total exposure ranging from 1.8 L to 1.8 x 10(10) L) at consta nt p(H2O). A two-stage reaction was observed for each oxide surface, w ith dissociative chemisorption of water occurring mainly at defect sit es below threshold pressures of approximate to 1 Torr and approximate to 10(-4) Torr for the corundum and hematite (0001) surfaces, respecti vely. Extensive hydroxylation of these surfaces was found to occur abo ve the respective threshold pressures. Longer exposures to water vapor below these threshold pressures did not result in increased hydroxyla tion; however, longer exposures above these threshold pressures result ed in increased hydroxylation, as expected. The threshold pressure of alpha-Al2O3 was accurately predicted using a simple equilibrium thermo dynamic model for the conversion of corundum to gibbsite (AI(OH)(3)). In contrast, the measured threshold pressure for hematite is more than five orders of magnitude lower than the predicted threshold pressure for the conversion of hematite to FeOOH or Fe(OH)(3). This difference between observation and prediction for hematite (0001) is not complete ly understood, but may be due to the presence of a passivating layer o f Fe-hydroxide that reduces the surface energy of the hydroxylated hem atite (0001). (C) 1998 Published by Elsevier Science B.V. All rights r eserved.