Reaction of water with MgO(100) surfaces. Part II: Synchrotron photoemission studies of defective surfaces

Synchrotron-based photoemission spectroscopy (PES), low energy electron diffraction, X-ray reflectivity, and optical and atomic force microscopy were used to study the reaction of water vapor [at pressures p(H2O) less than or equal to 2.0 x 10(-5) Torr for 3 min] with UHV cleaved MgO(100) surfaces with different levels of surface defects. The surfaces studied included flat and stepped single-crystal surfaces, wavy polycrystalline surfaces, and Ar+-sputtered flat single-crystal surfaces. AFM and optical microscopy studies revealed that these surfaces have very different surface step densities. The threshold p(H2O) value for hydroxylation of the flat MgO(100) surface is approximate to 10(-4) Torr (independent of exposure time), and reactions below this threshold occur dominantly at surface defect sites, which provides a means of estimating surface defect densities. Our results show that water dissociatively chemisorbs at defect sites on MgO(100) surfaces at the residual gas pressure of the UHV chamber (<1 x 10(-10) Torr). At p(H2O) less than or equal to 2 x 10(-5) Torr the coverage of hydroxyls is estimated to be approximately 5%, 20%, 20%, and 35% of a monolayer for the flat, stepped, wavy, and Ar+-sputtered MgO(100) surfaces, respectively (here we define one monolayer as one hydroxyl per surface site, or one water molecule, or two hydroxyls, per surface unit cell); these hydroxyl coverage values are used as a measure of surface defect densities. The defects are dominantly steps on cleaved surfaces, except for the Arc-sputtered surface which should contain a significant number of point defects as well. Thermodynamic calculations were carried out to estimate the lower limit p(H2O) for hydroxylation of surface defect sites, and the calculated p(H2O) for dissociative chemisorption of water on defect sites is <10(-11) Torr, which is consistent with experimental observations. (C) 1998 Published by Elsevier Science B.V. All rights reserved.