Reaction of water vapor with α-Al2O3(0001) and α-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 (α-Al2O3) and single-crystal, thin-film hematite (α-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 min, with total exposure ranging from 1.8 L to 1.8×1010 L) at constant p(H2O). A two-stage reaction was observed for each oxide surface, with dissociative chemisorption of water occurring mainly at defect sites below threshold pressures of ≈1 Torr and ≈10−4 Torr for the corundum and hematite (0001) surfaces, respectively. Extensive hydroxylation of these surfaces was found to occur above the respective threshold pressures. Longer exposures to water vapor below these threshold pressures did not result in increased hydroxylation; however, longer exposures above these threshold pressures resulted in increased hydroxylation, as expected. The threshold pressure of α-Al2O3 was accurately predicted using a simple equilibrium thermodynamic model for the conversion of corundum to gibbsite (Al(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 completely understood, but may be due to the presence of a passivating layer of Fe-hydroxide that reduces the surface energy of the hydroxylated hematite (0001).