O.L.J. Gijzeman

XPS analysis of palladium oxide layers and particles

The thermal reduction of both oxidised palladium foil and SiO2Si(100) supported palladium oxide particles, ranging in size fro 3.5 to 13 nm, was investigated with XPS. Equations were derived for the XPS intensities, measured at normal emission angles, of the particles which consisted of a metallic core and an oxidic skin. By applying these equations on the spectra measured after each reduction step, the particle size and the size of the metallic core were calculated. Measurements on palladium foil showed that the oxide layer thickness decreases linearly with the reduction time up to the last monolayer oxide. The reduction rate of the surface oxide is about eight times lower than the reduction rate of the bulk oxide. The growth of the metallic core in palladium oxide particles appeared to be linearly proportional to the surface area. The reduction rate of the smallest particles was comparable to the reduction rate of the surface oxide of the palladium foil. The larger particles behave identical to the palladium foil.

Adsorption of oxygen and surface oxide formation on Pd(111) and Pd foil studied with ellipsometry, LEED, AES and XPS

Abstract The interaction of oxygen with Pd(111) and polycrystalline palladium foil has been studied with ellipsometry, LEED, AES and XPS in the temperature range of 300 to 770 K and pressures up to 1 Pa. Ellipsometry was used to monitor the adsorption of oxygen and gave indication for the formation of a surface oxide at higher temperatures ( T ≥ 470 K) and pressures ( p ≥ 10 −4 Pa). The presence of a surface oxide is supported by a complex LEED pattern, ascribed to a square lattice with a = 7.5 ± 0.5A and domains in six orientations. It was not possible to match this structure with a simple overlayer structure on the (111) plane or with an unreconstructed crystal plane of PdO. XPS measurements on palladium foil, after the same treatment, showed the presence of ≈0.5 ML PdO on the surface. Bulk oxide was not formed. The amount of oxygen on the surface could not be determined with AES because during AES the electron beam easily removed adsorbed oxygen, especially on Pd(111). On palladium foil the oxygen is removed less effectively by the electron beam, which indicates that oxygen is bound more tightly to defects. Bulk palladium oxide, produced by heating the palladium foil in air, was not affected by the electron beam, even at high current densities. The ellipsometric parameters δΔ and δΨ never exceeded 0.40° and 0.08° respectively, both on Pd(111) and palladium foil. This indicates that the diffusion of oxygen is limited to surface layer(s) under the conditions studied. Diffusion to the bulk did not occur.

Interaction of methane with Ni(111) and Ni(100); diffusion of carbon into nickel through the (100) surface; An aes-leed study

The interaction of methane with Ni(100) results in the deposition of carbon in a carbidic form onto the surface. The sticking coefficient is initially 5 × 10−9 and does not depend on temperature in the range 474–563 K. This surface carbide consists of two different types with respect to their stability. A first kind may be made to diffuse into the bulk between 626–670 K, similar to experiments on Ni(110). A second kind is more tightly bound and leads to a segregation type of behaviour. Methane does not interact with Ni(111).

Surface roughness effects in quantitative XPS: magic angle for determining overlayer thickness

Abstract The use of X-ray photoelectron spectroscopy (XPS) as a technique for non-destructive depth profiling of technical samples is often hindered by their roughness. In this paper, we elaborate on earlier work where we reported on the apparent existence of a ‘magic’ angle for the determination of the thickness of uniform overlayers on rough substrates. Simple calculations for fully three-dimensional model rough surfaces strongly suggest that the thickness of an overlayer on a rough substrate can be determined accurately without taking the roughness into account in the analysis of the XPS intensities, the neglect of roughness effects leading to an average error less than 10% in the determined thickness.

Ellipsometry-LEED study of oxygen adsorption and the carbon monoxide- oxygen interaction on Ag(110)

Abstract The adsorption of oxygen on Ag(110) has been studied by ellipsometry and LEED. Oxygen pressures varied between 10 −6 and 5 × 10 −5 Torr and the crystal temperature between room temperature and 200° C. The change in the ellipsometric parameter Δ was found to be proportional to the oxygen coverage derived from ( n × 1) superstructures in LEED. Initial sticking coefficients are about 5 × 10 −4 and the maximum coverage is 0.5 O atom Ag atom. The reaction of CO with adsorbed oxygen was studied between room temperature and 200°C at CO pressures between 10 −7 and 4 × 10 −6 Torr. The decrease in oxygen coverage is slow at high oxygen coverages and accelerates at lower coverages. Two distinct rate constants may be derived from a simple model, each proportional to the CO pressure. LEED evidence points to considerable freedom of motion for O-atoms along the troughs of the Ag(110) face. Above 200°C oxygen desorption takes place. Ellipsometric measurements indicate that at these temperatures either a surface rearrangement takes place, or that the bonding character of oxygen to the substrate changes. Isosteric heats of adsorption were estimated for crystal temperatures between 240 and 340°C.

Surface and subsurface oxygen on Cu(111), Cu(111)-Fe and Cu(110) and their influence on the reduction with CO and H2

Abstract We present a model for the reaction of adsorbed oxygen on Cu(110), Cu(111) and Cu(111)-Fe with hydrogen and carbon monoxide. The basic assumption is that two types of oxygen exist on these three surfaces, which has been demonstrated so far for Cu(110) only. Surface oxygen is removed by CO while the other type converts to surface oxygen. Reduction experiments with hydrogen on surface oxygen show that the reaction is first-order in the hydrogen pressure for Cu(111) but half-order for Cu(111)-Fe. This occurs already in the presence of very small amounts of iron, which facilitate the dissociation of hydrogen.

CO interaction with Fe(100): Effects of carbon and oxygen adlayers on co adsorption isotherms

Abstract The interaction of CO with clean Fe(100) at 300 K is shown to occur in two distinct stages. namely: (i) Dissociation of CO, leading to a carbon and oxygen coverage of approximately 0.25 monolayer respectively as indicated by AES and LEED, and (ii) reversible molecular CO adsorption which has been studied with ellipsometry. The isosteric heat of adsorption is dependent on the CO coverage and equals 100 ± 5 kJ mol at zero coverage. CO adsorption isotherms were recorded on a carbon covered Fe(100) surface and yielded a coverage independent isosteric heat of adsorption of 77 ± 4 kJ mol . No CO dissociation was observed on this surface. From CO adsorption isotherms studies on different adlayers, containing carbon and/or oxygen on Fe(100) at 300 K, it can be concluded that less CO is adsorbed when the surface contains more oxygen. A quantitative description of the CO isotherms can only be given by assuming the adsorbing surface to be heterogeneous.

The influence of different catalyzers in hot-wire CVD for the deposition of polycrystalline silicon thin films

Polycrystalline silicon thin films grown by hot-wire chemical vapour deposition, using tungsten and tantalum as filament material, show different material properties. The different filament materials can cause this difference, due to different surface reactions and catalytic properties. X-Ray photoelectron spectroscopy on used-filaments shows a much lower silicon content in the near-surface region of the tantalum filaments, as compared to the tungsten ones. Furthermore, it is shown that the silicon content on the tungsten filament increases linearly with time, while the silicon content on the tantalum filament saturates quickly. A comparison of the silicon contents on the different filaments shows the possible presence of a liquid phase on the tungsten filaments surface during deposition. This liquid phase can cause the short lifetime of tungsten filaments compared to tantalum ones.

The adsorption and decomposition of carbon monoxide on Ni(100) and the oxidation of the surface carbide by oxygen

The interaction of carbon monoxide with Ni(100) has been studied by ellipsometry and Auger electron spectroscopy. Bombardment by electrons of a relatively high energy (2500 eV) leads to the disproportionation of the adsorbed CO (2 COad → Cad + CO2g ). The rate of oxidation of this surface carbide is , where hc is the carbon 272 eV Auger peak height, n=0.5 and the apparent activation energy Eact =13.3 kcal/mole. This relation is valid at 200–400°C and at oxygen pressures of 5 × 10−9−8 × 10−7 Torr.

Dissolution of carbon into nickel through the (110) surface

Abstract The dissolution of carbon through the Ni(110) surface has been studied by means of AES and LEED. Reproducible kinetic data were obtained in the temperature range 615–660 K after the crystal had been annealed at 870 K. These data are interpreted with a model in which bulk diffusion from a constant plane source just beneath the surface is rate limiting. The calculated carbon concentrations are compared with literature data on the solubility of graphite in nickel.