PNNL

Abstract

We show that monolayer quantities of Co metal, deposited in ultrahigh vacuum, wet the alpha-Al2O3(0001) surface as prepared by oxygen plasma cleaning at room temperature. While this cleaning procedure removes adventitious carbon, it does not remove the monolayer of hydroxyls resulting from dissociative chemisorption of water vapor accompanying air exposure. O 1s core-level spectra taken at grazing emission establish the presence of the OH monolayer, which is manifested in the presence of a peak ~1.7eV to higher binding energy relative to the lattice oxygen peak. After 0.3 ML Co deposition at 300K, the OH O 1s peak intensity is greatly reduced. After interaction with the surface, ~2/3 of the Co in this dose is metallic, while the remainder is Co(II). A 0.8 ML Co deposition results in a larger metallic to ionic Co ratio. First-principles density functional theory has been used to investigate several possible surface reactions that give rise to wetting. All that emit water are endothermic. However, those that emit H2 are exothermic. Initial Co adatoms promote the removal of two H atoms per Co, and each donates two electrons to the remaining O- ions as cation sites are occupied. Exothermicity is great for the isolated process (i.e. a single Co reacting with the surface). However, the presence of residual OH and metallic Co following 0.3 ML deposition suggests metallic Co must also be present to catalyze the reaction. Metallic Co adataoms exhibit strong d-d binding to oxidized Co(II), thus producing wetting.