J.E. Crowell

Surface sensitive spectroscopic study of the interaction of oxygen with Al(111) - low temperature chemisorption and oxidation

Abstract The interaction of an atomically clean Al(111) surface with O 2 has been studied using a combination of electron energy loss spectroscopy (EELS) and Auger spectroscopy (AES). Oxygen dissociatively adsorbs and occupies both surface and subsurface binding sites under all exposure conditions in the temperature range 122–700 K. Surface sites are preferentially occupied at low exposures, while higher exposures increasingly favor population of subsurface sites. Studies of O 2 adsorption at temperatures as low as 131 K have shown that formation of Al 2 O 3 occurs at high oxygen exposures. The Al 2 O 3 produced exhibits a 54 eV Auger transition and a characteristic vibrational spectrum with loss features at 430, 645, and 880 cm −1 . Argon ion bombardment of thin monolayer level Al 2 O 3 layers leads to preferential loss of Al 2 O 3 and a reduction in the subsurface-to-surface oxygen ratio. Electron bombardment of similar, thin Al 2 O 3 layers is ineffective in inducing desorption of surface species, whereas thick Al 2 O 3 layers are strongly influenced by electron bombardment, as judged from AES behavior. Qualitative models for O 2 adsorption, oxidative annealing, and damage by ion and electron bombardment are given.

The adsorption and thermal decomposition of water on clean and oxygen‐predosed Al(111)

The adsorption of water on both clean and oxygen‐predosed Al(111) has been studied by vibrational spectroscopy using electron energy loss spectroscopy (EELS). At 130 K, adsorption on either surface is competitively associative and dissociative. The dominant dissociation product is a hydroxyl species. On the clean surface, adsorption is predominantly molecular, while in the presence of oxygen, adsorption is predominantly dissociative. In contrast to the low temperature behavior, adsorption of water on clean Al(111) at 300 K is completely dissociative, resulting in oxygen adsorption and surface oxidation. Adsorbed hydroxyl species can be produced at 300 K by prolonged water exposure. Upon heating a low‐temperature water layer adsorbed on either surface, molecular water desorption and further decomposition both occur. The production of adsorbed hydroxyl species from water reaches a maximum at 250 K on the clean surface and at 350 K on the oxygen‐predosed surface. The hydroxyl species decompose above these te...

The metal-metal oxide interface: A study of thermally-activated diffusion at the Ni/Al2O3 interface using electron spectroscopies

The behavior of Ni atoms deposited on well-characterized Al2O3 films, prepared by oxidizing an atomically clean Al(111) surface, has been studied using high-resolution electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). At 200 K, the deposited Ni forms three-dimensional Ni clusters on the Al2O3/Al(111) substrate. Interaction of these Ni metal clusters with surface Al-O species is directly detected by observation of the preferential modification of surface phonon mode frequencies and intensities in contrast to the behavior of a bulk phonon mode of the Al2O3 substrate. Heating the Ni/Al2O3 deposit above 200 K gives rise to two thermally induced processes: Process, I, smoothing (below ∼400 K) of the three-dimensional Ni clusters and Process II, diffusion (400–700 K) of the Ni overlayer through channels in the Al2O3 film. The metallic aluminum beneath the Al2O3 film is found to provide a major driving force for the inward diffusion of the Ni overlayer by Ni-Al alloy formation. This work represents the first application of EELS to the study of transport processes in a thin film system.

A vibrational study of the adsorption and decomposition of formic acid and surface formate on Al(111)

The adsorption and decomposition of formate and formic acid have been studied on the Al(111) surface using high resolution electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). Formic acid reacts with clean Al(111) at 120 K to form a surface formate species. Molecular adsorption of formic acid occurs at 120 K only after saturation of the surface formate layer has been reached. Off‐specular vibrational measurements have determined that the formate species is symmetrically bridge bonded through both oxygen atoms with C2v symmetry. A lower symmetry formate species of C1 symmetry is produced upon heating or electron bombardment of a condensed formic acid layer. Thermal or electron induced decomposition of a formate or a condensed formic acid layer is controlled by oxygen incorporation into the aluminum lattice, and results in complete decomposition and production of a carbon and oxygen covered surface. Only hydrogen is evolved from a formate‐covered Al(111) surface.

Assignment of a surface vibrational mode by chemical means: Modification of the lattice modes of Al2O3 by a surface reaction with H2O

The adsorption of water on a well‐characterized Al2O3 film produced by oxidizing an Al(111) surface has been studied using electron energy loss spectroscopy (EELS). Water reacts with this oxide layer to form surface hydroxyl species characterized by a sharp O–H (O–D) stretching vibration at 3720 (2740) cm−1. The assignment of the three‐peak vibrational spectrum of Al2O3 to surface and bulk Al–O modes is confirmed by chemical means based on the observation that the 625 cm−1 loss feature assigned to a surface Al–O stretching vibration is preferentially affected upon formation of surface hydroxyl groups. The remaining two bulk modes are uninfluenced by surface hydroxylation. The surface hydroxyl species can be quantitatively removed by electron stimulated desorption (ESD), reversing the spectroscopic changes observed upon exposure of Al2O3 to water, further substantiating the vibrational assignments.

An EELS and TPD study of the adsorption and decomposition of acetic acid on the Al(111) surface

Abstract The adsorption and decomposition of acetic acid on Al(111) have been studied using electron energy loss spectroscopy (EELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). Acetic acid reacts with clean Al(111) at 120 K to form a surface acetate species. The adsorbed acetate bonds to the surface in a symmetric configuration with C s symmetry at 120 K. The adsorption of molecular acetic acid occurs at this temperature only after saturation of the surface acetate layer; this physisorbed multilayer desorbs molecularly at 167 K. Thermal decomposition of the adsorbed acetate leads to a carbon- and oxygen-covered surface; the only detectable thermal decomposition product is H 2 . Electron irradiation induces a similar decomposition process of the surface acetate.

Ni cluster chemistry on Al2O3—A vibrational eels study using chemisorbed CO on a model catalyst: Ni/Al2O3/Al(111)

The interaction of CO molecules with a model catalyst, Ni/Al 2 O 3 /Al(111), has been investigated using electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD) in the temperature range of 200–600K. The model supported Ni catalyst is produced by Ni evaporation onto a thin Al 2 O 3 film supported on an Al(111) surface. The effect of the Al 2 O 3 /Al(111) substrate on the adsorption of CO on Ni clusters and the effect of the Ni particle size on the CO binding sites are discussed. The thermally induced desorption and decomposition of the adsorbed CO layer on Ni/Al 2 O 3 /Al(111) have also been investigated. The EELS technique is shown to be very powerful in this type of study, since its wide vibrational frequency range allows one to simultaneously study the interaction of the Ni particles with the Al 2 O 3 /Al(111) substrate and the adsorption of CO molecules on the supported Ni particles.

Infrared spectroscopic observations of hydrogen bonding and Fermi resonance of adsorbed methyl chloride on alumina surfaces

The physical adsorption of methyl chloride onto Al2O3 surfaces containing surface OH groups has been studied using transmission infrared spectroscopy. Methyl chloride reversibly bonds via hydrogen bonding to the surface hydroxyl groups with a spectroscopically measured heat of adsorption of −3.37±0.38 kcal mol−1. Physisorption of methyl chloride results in a significant reduction in the intensity of the rotational wings of the methyl chloride absorption bands relative to the gas phase, a small downward frequency shift in their band centers, and substantial effects on the ν(OH) region of the alumina spectrum due to hydrogen bonding of surface OH groups with methyl chloride. Fermi resonances are observed for adsorbed methyl chloride and result in the observation of the overtones of both the symmetric and asymmetric methyl deformation modes. It is postulated that the hydrogen‐bonded CH3Cl species possess rotational (or other) degrees of freedom which lead to a high entropy in the adsorbed layer, compared to ...

The adsorption and decomposition of carboxylic acids on Al (111)

Electron energy loss Spectroscopy has been used to investigate the interaction of formic acid and acetic acid with Al(111). Both carboxylic acids react with clean Al(111) at 120 K to form the corresponding surface carboxylate species, formate and acetate. Both carboxylates bond in a symmetric bridging geometry, formate with C2v symmetry, acetate with Cs symmetry. Molecular adsorption of the carboxylic acid occurs at 120 K only after saturation of the surface carboxylate layer has been reached. Thermal or electron induced decomposition is controlled by oxygen incorporation into the aluminum lattice. This decomposition leads ultimately to a carbon and oxygen covered surface that exhibits a vibrational spectrum characteristic of aluminum oxide.

Infrared spectroscopic study of the rotation of chemisorbed methoxy species on an alumina surface

We present experimental and calculated vibration–rotation spectra as a function of temperature for the methoxy species (–OCH3 and –OCD3) chemisorbed on an alumina surface. The axis of rotation is the C–O bond axis. The model for our calculations is that of free rotation, and we describe the methods employed here in full detail. The qualitative agreement between the calculated and experimental spectra suggests that the adsorbed methoxy species is undergoing free rotational motion about the C–O bond axis.