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(H ₂O)≤2.0×10 ^-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(H ₂O) value for hydroxylation of the flat MgO(100) surface is ≈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 (<1×10 ^-10 Torr). At p(H ₂O)≤2×10 ^-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 Ar ⁺-sputtered surface which should contain a significant number of point defects as well. Thermodynamic calculations were carried out to estimate the lower limit p(H ₂O) for hydroxylation of surface defect sites, and the calculated p(H ₂O) for dissociative chemisorption of water on defect sites is <10 ^-11 Torr, which is consistent with experimental observations.