DFT characterization of coverage dependent molecular water adsorption modes on alpha-Al2O3(0001)

Ab initio density functional theory was used to investigate the stable and metastable states of adsorbed molecular water on the alpha-Al2O3(0001) surface as a function of coverage. The atoms of the dry surface undergo pronounced inward relaxations with respect to their bulk positions. At low coverages (Theta <= 0.5) water adsorbs nearly parallel to the surface plane, with an 0 atom atop a surface Al. The adsorption is mainly due to the donation from water lone pairs into vacant p orbital of surface Al, drawing the surface Al outward. With increasing coverage, water adsorption atop Al competes with an alternative configuration with water bound through H to surface oxygen. These two competing modes generate a variety of distinct but nearly isoenergetic adsorption modes that terminate in a hexagonal, ice-like layer at a coverage of two water molecules per surface Al. The binding energy per water molecule is maximized in the two limits of coverage, but deviates from this extreme only slightly at intermediate coverages. At no coverage is the water binding great enough to overcome the energetic preference for water to dissociatively adsorb. (c) 2007 Elsevier B.V. All rights reserved.