Inelastic electron tunnelling spectroscopy of selected alcohols and amines on plasma-grown aluminium oxide

Abstract

The surface chemical properties of plasma-grown aluminium oxide are explored by inelastic electron tunnelling spectroscopy. Spectra of benzyl alcohol, benzylamine, 2-phenylethylamine, diphenylamine and ethanol, all adsorbed from the vapour onto the oxide at room temperature, are reported. The ethanol gives mainly an ethoxide species upon adsorption and the liberated proton remains on the surface and enhances the surface hydroxyl concentration. Benzyl alcohol behaves differently. Two possible adsorption mechanisms are proposed for it and for benzylamine. Either (a) a Lewis acid–Lewis base coordination of the adsorbed species at a surface aluminium cation, with attendant substantial charge redistribution, leads to strong hydrogen-bond formation involving adjacent surface oxide centres or (b) Brønsted acid protonation of the adsorbate by the surface occurs. Both may lead to broad, downshifted ν(OH) and ν(NH) modes like those seen in the spectra. The other two amines, though less readily adsorbed, give confirmation of this behaviour.