Reaction of water with MgO(100) surfaces. Part I:: Synchrotron X-ray photoemission studies of low-defect surfaces

Abstract

Synchrotron-based photoemission spectroscopy and low-energy electron diffraction were used to study the reaction of water vapor at 300 K and different water vapor pressures, p(H 2O), ranging from 5×10 −9 Torr to 10 −3 Torr (3 min exposure at each pressure), with vacuum-cleaved MgO(100) surfaces that had low (5–10%) defect densities. The O 1s, Mg 2p, and O 2s/VB spectra were acquired at photon energies chosen to optimize surface sensitivity. O 1s and O 2s/VB spectra are sensitive to water adsorption onto MgO(100) even at very low water vapor exposures [<10 −8 Torr for 3 min (<1.8 L)], whereas Mg 2p spectra show significant changes only for high exposures [≥10 −4 Torr for 3 min (≥1.8×10 4 L)]. Comparison of these spectra with similar spectra of MgO(100) surfaces immersed in bulk water and of polycrystalline Mg(OH) 2 indicates that water chemisorbs dissociatively in two distinct stages on “low defect” MgO(100) surfaces, forming surface hydroxyl groups. The first stage occurs at water vapor exposures ≤3×10 −5 Torr for 3 min (≤5.4×10 3 L) or for 30 min (≤5.4×10 4L) and involves a relatively fast reaction with surface defects (corner and edge-step sites and point defects) comprising 5–10% of the surface sites, in agreement with recent first-principles electronic structure calculations. The second stage occurs at higher water vapor pressures (≥10 −4 Torr for 3 min) and involves dissociative chemisorption of water on terrace sites, which is not predicted by recent first-principles calculations. The apparent sticking coefficient for the first reaction stage (≥0.16) is about four orders of magnitude larger than that for the second reaction stage (≥3×10 −5), suggesting that the second reaction stage requires significantly more activation energy than the first stage. Our results also suggest that the hydroxylation reaction is not sensitive to exposure time below a threshold pressure of ≈10 −4 Torr. Although both kinetic and thermodynamic interpretations are possible, a thermodynamic analysis of the hydroxylation reaction (using bulk solid free energies) predicts approximately the same threshold pressure as observed. After the surface is fully hydroxylated, additional water can be physisorbed on the hydroxyl layer. Analysis of O 1s spectra taken from the same surface but at different photon energies indicates that hydroxyls are formed predominantly on the surface and not in the bulk under these exposure conditions. Our experimental data also show that the 4–6 eV electrons used to mitigate surface change during the photoemission experiments have no effect on the dissociation of water on the MgO(100) surface.