ACIDO-BASIC PROPERTIES OF SIMPLE OXIDE SURFACES .3. SYSTEMATICS OF H+ AND OH- ADSORPTION

Thanks to a self-consistent tight binding approach, applied to clusters embedded in a Madelung field created by an infinite array of charges at the lattice sites, we have modeled the acido-basic properties of various simple oxide surfaces: BaO(001), SrO(001), CaO(001), MgO(001), rutile-TiO2(110), alpha-quartz SiO2(0001). We have studied H+, OH- and dissociated H2O adsorption characteristics: adsorption energy, charge transfer and density of states. We have proved that the decreasing basic character of the oxygen sites along the series comes from the larger covalency of the compounds, which opposes the proton adsorption, while the increasing acid character of the surface cations comes from their lower and lower electropositivity and from their smaller and smaller ionic radii. This detailed study completes the model that we had elaborated previously [Surf. Sci. 262 (1992) 245 and 259], and confirms the dominant role of covalent effects, as opposed to electrostatic effects, in the acido-basic properties of these insulating oxides.