Reaction of nanometer-sized Cu particles with a SiO2 substrate

The thermal stability of nanometer-sized Cu particles on a 400–500 nm thick SiO2 layer on top of a Si(100) substrate was studied after annealing in ultrahigh vacuum up to 620¿°C. Atomic force microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and Auger electron spectroscopy measurements clearly show that Cu-silicide islands are formed. A direct reaction of Cu with the SiO2 support is assumed, which is facilitated by a fairly strong metal-support interaction and by the wetting behavior of the silicide islands. Exposure to air at room temperature results in regeneration of the annealed Cu/SiO2 system

Mechanism of neutralization in low-energy He+ ion scattering from carbidic and graphitic carbon species on rhenium

In general, the absence of matrix effects in low-energy ion scattering makes quantification possible. Ion fractions, important in quantification, are obtained by measuring scattered ion yields as a function of primary energy. Differences in ion fraction and final energy (strong matrix effect) are observed in 1-3.5 keV He+ scattering from graphitic and carbidic carbon on a polycrystalline rhenium ribbon. These results are explained by a special quasi-resonance neutralization for graphite because of the large width of the sp band of graphite, extending towards the He 1s energy level

Strong matrix effect in low-energy He+ ion scattering from carbon

In low-energy ion scattering the contribution of neutralization processes to the scattered ion yield is very important in quantification. Neutralization of low-energy (1-3.5 keV) He+ ions by carbon is found to be much stronger for graphitic than for carbidic carbon. The ion fraction for graphitic carbon for 2.5 keV 3He+ scattering over 136° is about 60 times lower than that for carbidic carbon. For the 4He+ isotope the effect is even larger. Such a strong matrix effect for one element has not been measured before in low-energy (1-3.5 keV) inert-gas ion scattering. The neutralization behaviour is discussed in terms of a special quasi-resonant neutralization process for graphite

Preparation of a rhodium catalyst from rhodium trichloride on a flat, conducting alumina support studied with static secondary ion mass spectrometry and monochromatic x-ray photoelectron spectroscopy

A Rh catalyst was prepd. by electrostatic adsorption of RhCl3-derived species in aq. soln. on a model support, consisting of a 4-5 mm thick layer of Al oxide on an Al foil. The conversion of the Rh precursor species into metallic Rh was studied by monochromatic XPS and static SIMS. Freshly prepd. catalysts contain adsorbed Rh-complexes with only one chloro ligand; this is explained by a mechanism in which chloro ligands of the initially adsorbed complex, [RhCln(OH)4-n(H2O)2]-, are displaced by surface OH groups. The Rh-Cl species decomp. at redn. temps. 200 Deg are needed to remove Cl from the alumina support. [on SciFinder (R)

Oxidation of carbidic carbon on a rhodium surface

Different mechanisms of at. carbon and oxygen recombination on a rhodium surface are studied with Auger electron spectroscopy (AES) and XPS. The kinetics of adsorbed carbidic carbon oxidn. (carbon coverage qC ~ 0.1-0.3 ML(monolayer)) by gas-phase oxygen that proceeds by a Langmuir-Hinshelwood reaction mechanism, provides the value of the activation energy for recombination (Erecact ~ 170 +- 20 kJ/mol). Erecact depends slightly on the carbon coverage. An Eley-Rideal type of reaction was obsd. for adsorbed oxygen and at. gas-phase carbon recombination which occurs in a dynamic regime. The low value found for the activation energy (near zero) is consistent with the mechanism that this exothermic reaction is too fast for energy dissipation into the substrate; the energy is mainly transferred into translational, vibrational energy of CO. [on SciFinder (R)

Application of low-energy noble-gas ion scattering to the quantitative surface compositional analysis of binary alloys and metal oxides

Low-energy noble-gas ion scattering (LEIS) probes the outermost atomic layer of a material, but a quantitative compositional analysis of this layer is not straightforward. It is demonstrated that quantification by calibration can be done, assuming that ion fractions and shielding effects are the same for the reference sample and sample of interest. These assumptions are critically evaluated and LEIS experiments on binary alloys and metal oxides are presented that can partly verify these assumptions. The LEIS measurements of a Cu–Au alloy and CuO powder as a function of initial energy indicate the absence of matrix effects in the ion fractions after scattering from the metal atoms in these systems. In metal oxides, shielding of surface metal atoms by shadowing/blocking and ion neutralization by neighbouring atoms can significantly influence the quantification of the metal atom concentration and is determined by the local atomic arrangement as illustrated by LEIS experiments of CuO and ZnO samples

A surface science study of model catalysts. 1. Quantitative surface analysis of wet-chemically prepared Cu/SiO2 model catalysts

Cu/SiO2 model catalysts containing nanometer-sized Cu particles on a flat silica model support were wetchemically prepared and characterized in detail by a variety of surface science techniques. The particle size and shape, particle number density, metal surface coverage, total metal loading, and oxidation state of the particles were determined by ultrahigh vacuum atomic force microscopy, electron microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. Deposition of a Cu precursor on a flat Si wafer with a SiO2 top layer by spin-coating was followed by calcination in air at 450 °C. This preparation method produces both homogeneously distributed hemispherical CuO particles with an average height of 8 nm and highly dispersed oxidic Cu species. Subsequent reduction in hydrogen at 250 °C results in metallic and more rounded Cu particles with an average height of 8 nm

Quantification in low-energy ion scattering: elemental sensitivity factors and charge exchange processes

For quantitative analysis in Low Energy Ion Scattering (LEIS), elemental sensitivity factors are needed. Here, these factors are presented for 1 keV He+ scattering from a number of pure metals. The influence of different properties of the target on neutralization of the incident ions has been studied and elemental specific neutralization constants (characteristic velocities) have been measured. An enhanced neutralization was found for elements for which ionization plays an important role. A qualitative explanation is given for this observation