Cu interactions with α-Al2O3(0001): effects of surface hydroxyl groups versus dehydroxylation by Ar-ion sputtering

X-ray photoelectron spectroscopy studies and first principles calculations compare Cu adsorption on heavily hydroxylated α-Al2O3(0001) with dehydroxylated surfaces produced by Ar+ sputtering followed by annealing in O2. Annealing a cleaned sapphire sample with an O2 partial pressure of ∼5×10−6 Torr removes most contaminants, but leaves a surface with ∼0.4 ML (ML: monolayer) carbon and ∼0.4 ML OH. Subsequent light (6 min) Ar-ion sputtering at 1 keV reduces the carbon to undetectable levels but does not dehydroxylate the surface. Further sputtering at higher Ar-ion excitation energies (>2 keV) partially dehydroxylates the surface, whereas 5 keV Ar-ion sputtering creates oxygen vacancies in the surface region. Further annealing in O2 repairs the oxygen vacancies in the top layers, but those beneath the surface remain. Deposition of Cu on the hydroxylated surface at 300 K results in a maximum copper(I) coverage of ∼0.35 ML, in agreement with theoretical predictions. Maximum copper(I) coverage at 300 K decreases with decreasing surface hydroxylation. Exposure of a partially dehydroxylated sapphire(0001) surface to either 2 Torr H2O vapor or air results in recovery of surface hydroxylation, which in turn increases the maximum copper(I) coverage. These results demonstrate that the hydroxyl surface coverage critically affects the ability of Cu to ‘wet’ sapphire(0001) at 300 K. In addition, first principles density functional calculations show that while an ad-OH species stabilizes copper(I) at room temperature, any nearby in-surface OH does the opposite.