Alumina surfaces and interfaces under non-ultrahigh vacuum conditions

Abstract

This paper is a review of studies of the structures and reactivities of ordered alumina surfaces under ultrahigh vacuum (UHV; < 10 −8 Torr), ambient (>1 Torr), and intermediate (10 −7–10 −1 Torr) pressure conditions. Most ordered alumina films—including α - Al 2O 3(0001) and transitional phase thin films grown on single-crystal substrates, are Al-terminated, but do not dissociate H 2O or many other small molecules (e.g., CH 3OH, NH 3) at room temperature under UHV conditions. Under ambient conditions, the α - Al 2O 3(0001) surface becomes OH-terminated, with an overlayer of physisorbed molecular H 2O stabilized by hydrogen bonding interactions with the OH substrate layer. The reactivity under ambient conditions is consistent with theoretical predictions of cooperative dissociation pathways for H 2O on this surface with low activation barriers, and is also consistent with desorption studies indicating that high fractional surface coverages of H 2O ( θ H 2 O ∼ 1 ) should only be observed at pressures of ∼1 Torr or higher. Surprisingly, cooperative H 2O interactions have also been observed at the surfaces of ordered films grown on Ni 3Al(111) and (110), and on NiAl(100) substrates at intermediate pressures orders of magnitude below ambient; P H 2O >10 −7 Torr, 300 K. Under these conditions, a cooperative reaction is apparently initiated at defect sites, resulting in strong surface rearrangement. NO appears to exhibit analogous behavior to H 2O, albeit at UHV pressures and at 100 K, where NO dimers form at surface defect sites. These data indicate that cooperative surface reactions occur at transitional phase alumina surfaces at pressures that are orders of magnitude below what one would expect based on straightforward thermodynamics and kinetics calculations, and point to the importance of surface defect sites for initiating reactions that eventually affect the entire surface. There is suggestive evidence that H 2O exposure also leads to the incorporation of interstitial atomic hydrogen in transitional phase alumina films. Results at ambient and intermediate pressures indicate that these pressure regimes form distinct reaction environments, with reaction pathways on alumina that differ qualitatively from each other as well as from results observed in UHV.