G. Apai

Electronic and vibrational properties of hydroxylated and dehydroxylated thin Al2O3 films

Abstract The electronic and vibrational properties of planar thin-films (20–30 A) of aluminum oxide on a Ru(001) substrate were studied using X-ray photoelectron (XPS), Auger, and high resolution electron energy loss (HREEL) spectroscopies. Hydroxylated Al 2 O 3 films, prepared in water/oxygen mixtures or produced by exposure of dehydroxylated Al 2 O 3 films to water vapor, exhibit OH stretching vibrations at ∼3730 cm −1 , characteristic of isolated hydroxyl groups. It is particularly significant that the hydroxyl group stability on planar Al 2 O 3 films, oxidized at pressures between several Torr and 1 atm, is shown to be comparable to that for alumina powders. This is taken as a strong indication that the planar oxide structure is similar to that of powder alumina surfaces. XPS and Auger spectra for the thin-film aluminas indicate Auger parameters and valence band densities-of-state comparable to those of powdered aluminas. The aluminum Auger parameter shifts from γ-like to α-like at least 200 K below transition temperatures for micron thick films, and 350 K below the bulk transition temperatures. Our 30 A films are substantially thinner, and so the Auger parameter may be indicative of a low temperature phase transformation, but is certainly characteristic of a change in the local cation environment from a mixture of tetrahedral and octahedral cation occupation to only octahedral coordination. The surface phonon spectra of aluminas containing no excess Al 0 produce the characteristic three-peak vibrational spectrum. Calculations performed using the dielectric theory of electron energy loss spectroscopy confirm this characteristic phonon loss spectrum to be composed of both longitudinal (LO) and transverse (TO) surface optic modes.

Defect structure of clean and chlorinated aluminum oxide films probed by methanol chemisorption

Abstract We have shown that clean γ- and α-Al2O3 films, prepared under relatively high pressure oxidation conditions, display methanol chemisorption properties and reactivity that are typical of powder alumina catalysts. Detailed vibrational and electronic information can be obtained for adsorbates on these thin-film oxides and is consistent with the surface chemistry expected of traditional powder oxides. The low activity toward dissociation of methanol and the low methoxy stability characteristic of our alumina films are in contrast to the highly active nature of UHV-oxidized aluminum surfaces, which have been used extensively to model alumina surfaces. Because high activation energies for dissociative chemisorption on oxides hinder traditional UHV approaches, we have studied methanol reactions effectively under higher pressure conditions. Reaction in several Torr of methanol at 425 K produced the stable formate species, which shows the similarity of our alumina films to powdered alumina surfaces. Chlorinated Al2O3 films, however, stabilized the methoxy species under similar conditions. A Fourier de-Poissonization technique, which removed oxide multiple and combination losses, significantly improved identification of adsorbate features in the HREELS data. There is a substantial opportunity for further examination of the stability and decomposition pathways of methoxy and related intermediates formed under such reaction conditions.

Nucleation and growth of vacuum-deposited metal aggregates studied by electron microscopy

Abstract Data are presented on the kinetics of growth of metal aggregates formed in the initial stages of vacuum deposition on amorphous substrates. These data suggest a refinement of the model used to describe earlier results. The model involves active sites at which the metal aggregates nucleate and diffusion into the substrate of atoms incident at locations removed from these active sites. The essential feature of the model is the concept of a capture zone of constant width surrounding each active site or growing metal aggregate. Metal atoms incident on the capture zone can migrate to the associated cluster, and become incorporated therein. This modification produces a model which is analyzed to give growth curves with only one adjustable parameter: the width of the capture zone. Data for Ag, Au, Pd, and Pt on vacuum-deposited carbon are all fit with a capture-zone width of 6.5 A.

Electronic, vibrational and chemical properties of alumina surfaces☆

Abstract We have correlated the electronic, vibrational and chemical structure of a series of alumina films with the objective of obtaining a more unified understanding of microscopic parameters observed during the transition from an amorphous alumina through to the thermodynamically favored α-Al 2 O 2 phase. Local electronic structure is characterized with a combination of Auger parameter measurements with X-ray induced valence and core-level photoemission. Complementary information concerning long-range order is derived from surface optical phonon HREEL spectra interpreted through dielectric theory calculations. Hydroxyl stability and chemisorption of methanol provide chemical insight into the surface defect and acidic properties of the films. Such studies illustrate the potential for obtaining fundamental spectral and kinetic information from these model systems which may facilitate the interpretation of surface chemistry and defect behavior of traditional alumina powders.

The interaction of NH3 with ordered Pt surfaces

Abstract The interaction of ammonia with ordered Pt surface was studies by means of surface core-level photoemission and tight-binding-type calculations. Clean Pt surfaces have distinguishable surface and bulk components of the 4f 7/2 core level. The 4f 7/2 surface component is shifted to lower binding energy (-0.32 eV) than the bulk on the clean (111) surface, but in the presence of ammonia the surface peak is shifted to positive binding energy (0.7 eV). This result is unexpected, since it indicates a depletion of d-electron density on Pt atoms attached to NH 3 , in contrast to common assumptions of NH 3 as a net donor. Thin-film calculations show this depletion in the form of rehybridization of sp with d electrons on the Pt atom. The mixing of p z orbitals with the d band leads to a dipole moment perpendicular to the surface, which in addition to the static dipole of ammonia is also a major factor in the decrease in work function upon chemisorption.