Characterization of Alumina Surface Contaminated Sulfur Compounds Studied by Tunneling Spectroscopy

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Analyses of alumina surfaces were investigated by inelastic electron tunneling spectroscopy. The tunneling spectra of adsorbed species for organic thioacids, thioureas, thioamides and esters onto the alumina, and of coated silicon or metal film were measured by using Al/alumina(sample or film)/Pb tunneling junction. The following results were obtained from the analysis of vibration spectra.(1)On the clean alumina surfaces, there were many OH-groups, both free and hydrogen bonded, and a few aluminum cations, i. e., Lewis' acid points.(2)Methane and benzen sulfonic acids were adsorbed as sulfonic ions onto alumina surfaces by reaction with surface OH groups. Thioureas and thioamides were found to be adsorbed on the surface by the interaction between the nitrogen atoms of these molecules and Al^+.(3)The analysis of vibration spectra for alkyl benzensulfonates and alkyl phosphites indicated that these esters were decomposed by their reaction with OH groups producing sulfonate and phosphonate ions on aluminum ion, perpendicularly. On the other hand, alkyl carboxylates were found to have two decomposition processes. One is the severing of the bond between the alkyl group and oxygen in the ester molecule, producing alkoxide ions which were adsorbed on the alumina surface. The other is the severing of the bond between the alkyl group and oxygen in the esters ; the carboxylate ions produced were adsorbed on the surface.(4)The evaporated silicon thin film on alumina was found to form many SiH_X on the surface. Inelastic electron tunneling spectra for copper or iron thin films on alumina showed that no signals were observed due to the copper or iron metals. This is interesting phenomena in the analysis of adsorbed compounds onto metal surfaces.