WATER CHEMISTRY ON SURFACE DEFECT SITES - CHEMIDISSOCIATION VERSUS PHYSISORPTION ON MGO(001)

The following paper presents the results of a theoretical study that probed the chemistry of water at structural defects on the MgO (001) surface. The computational technique used was periodic Hartree-Fock (PHF) theory with density functional based correlation corrections. The adsorption energies for water adsorbed on isolated corner, edge, and surface sites on the MgO surface were compared to the hydroxylation energies for the same sites. As stated in a previous paper, the binding of water to the perfect surface is exothermic by 4.1-5.6 kcal/mol whereas hydroxylating the perfect surface was endothermic by 24.5 kcal/mol. At step-edge sites, the process of water adsorption is exothermic and comparable in magnitude to the hydroxylation of these sites. The binding energies associated with water bound to the step-edge are 6.5-10.5 kcal/mol, and hydroxylation of this site is exothermic by 7.3 kcal/mol. At corner sites we find a strong preference for hydroxylation. The binding of water to a corner is exothermic by 20.7 kcal/mol, and hydroxylation is exothermic by 67.3 kcal/mol. Mulliken populations indicate that the formation of a hydroxylated surface is governed by the stability of the hydroxyl bond where a hydrogen is bonded to a surface oxygen ion. As the coordination number of this oxygen binding site decreases, its ionic character also decreases, and it forms a more stable bond with the incoming hydrogen. This trend is confirmed by the densities of states for these sites. Finally, hydroxylation of the perfect (001) surface was examined as a function of lattice dilation. It was determined that, as the lattice constant increases, hydroxylation becomes more energetically favorable. This may be important-in interpreting experimental thin-film results where the lattice constant of the substrate upon which the MgO film is deposited is slightly larger than that of bulk MgO.