K. Veltruská

Interaction of Au with CeO2(111): A photoemission study

We have studied the adsorption of low dimensional gold on ceria, produced by evaporation onto the surface. The interaction of gold with CeO(2)(111) layers was investigated with x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and resonant photoelectron spectroscopy (RPES). Gold was deposited in steps onto a 1.5 nm thick CeO(2)(111) layer epitaxially grown on a Cu(111) substrate. The RPES showed a partial Ce(4+)-->Ce(3+) reduction, observed as a resonant enhancement of the 4f level of the Ce(3+) species. This can be explained by possible creation of a new Au(+) ionic state. The observed effects are stronger for Au deposition at room temperature than at 250 degrees C. The obtained results are in agreement with already published density functional theory calculations reporting weakening of bond between the oxygen and the Ce atoms in ceria caused by the presence of gold.

Epitaxial Cubic Ce2O3 Films via Ce-CeO2 Interfacial Reaction.

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO2 films. The method yields bulk-truncated layers of cubic c-Ce2O3. Compared to CeO2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce2O3(111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO2(111). In addition, c-Ce2O3 layers represent an experimental realization of a normally unstable polymorph of Ce2O3. During interfacial reaction, c-Ce2O3 nucleates on the interface between CeO2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal-oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.

XPS, ISS and TPD study of Pd–Sn interactions on Pd–SnOx systems

Abstract The sensitivity and selectivity of SnO2 based gas sensors could be improved by doping of small amount of transition metals. In this work we used X-ray photoelectron spectroscopy, ion scattering spectroscopy, and thermal desorption techniques to investigate Pd evaporated on SnOx thin layer substrate, prepared by spray pyrolysis. The evolution of Pd/SnOx layer morphology with increasing amount of Pd deposits was studied using the XPS inelastic background shape analysis. The observations are compared to the results obtained from natural SnO2 crystal and metallic Sn substrates. A strong Pd–Sn bimetallic interaction was observed, resulting in the formation of PdSn alloy of noble metal-like electronic structure. This feature also corresponds to the presence of two CO desorption states with low energy peaks at approximately 390 K. The relation of our results with the operation mechanism of gas detection are discussed.

CO adsorption on palladium model catalysts: XPS Pd–Al2O3 interaction study

The metal–substrate interaction (MSI) represents one of the most important effects determining the properties of supported catalysts. It is expected to be one of the driving forces of the size effect in catalysis. In this work we investigated the MSI by X-ray photoelectron spectroscopy (XPS) in the case of small Pd particles deposited on γ- and α-alumina substrates. We compared the binding energy and initial state variations as a function of Pd coverage. The initial state has been found to be shifted to higher positive values for a more strongly interacting substrate (γ-Al2O3), whilst for the inert-like sapphire substrate the MSI was less important. The initial state shift value was associated with the electron transfer in the substrate clusters direction. It was shown that the substrate–metal charge transfer could be responsible for partial CO dissociation on γ-Al2O3 supported Pd particles.

XPS and ESD study of carbon and oxygen chemistry on TiZrV NEG

Abstract In this work, properties of TiZrV getter films prepared on stainless-steel substrates by magnetron sputtering were investigated. Changes of sample surface during thermal activation and interaction of the activated getter with CO were studied by means of XPS and ESD of neutrals. The XPS measurements reflect the disappearance of the superficial oxide layer coveing air-exposed TiZrV surfaces and the transformation of graphitic carbon to carbides during thermal activation. The dissociative CO adsorption on activated TiZrV getter was observed. Neutral products of electron-stimulated desorption were registered using quadrupole mass spectrometer. The ESD yield of CO and H 2 was measured during thermal activation as well as during CO and H 2 exposure of activated getter. The difference between the ESD yields for CO and H 2 after the exposure indicates possibly different pumping mechanisms for these gases. Hydrogen seems to be incorporated into the bulk while carbon monoxide stays chemisorbed on the getter surface. Although detectable activation proceeds already at temperatures above 160°C, the activation process is reasonably fast (a few hours) at higher temperatures (above 200°C).

Study of Pd–In interaction during Pd deposition on pyrolytically prepared In2O3

Abstract In2O3 belongs to the group of metal oxides used successfully for gas detection, with sensitivity for CO and H2 comparable with that of SnO2 and excellent for O3. The sensing properties of these sensors could be greatly improved by adding small amount of transition metals such as Pd, Pt, Au, etc. In this work, we used X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) to study the growth of Pd particles on In2O3 substrates prepared by spray pyrolysis method. Surface stoichiometry and its changes during Ar+ ion bombardment used for surface cleaning were checked with XPS and ISS. The Pd growth was studied stepwise in situ using the micro-electron beam evaporation source (MEBES) with controlled evaporation rate. XPS and ISS measurements were carried out with special attention to metal–substrate interaction (MSI), which was observed for Pd–In and resulted in the formation of Pd–In alloy with noble metal-like electronic structure. Further, we studied reduction of these samples after oxidation in air. A lower reduction temperature than with bulk PdO was observed. The results are compared with the experiment of Pd deposition performed on a pure In foil.

Optical characterization of diamond like carbon films using multi-sample modification of variable angle spectroscopic ellipsometry

Abstract In the first part, thicknesses and spectral dependences of the optical constants of the diamond like carbon (DLC) films were determined without any parameterization in the range 240–830 nm by multi-sample modification of variable angle spectroscopic ellipsometry (VASE). DLC films were prepared by plasma enhanced chemical vapor deposition (PECVD) onto silicon single crystal substrates. It was shown that the influence of some defects of the DLC films on the optical constants is suppressed by applying the method of multi-sample modification of VASE. In the second part, the spectral dependences of the determined optical constants were interpreted using a recently developed model of dispersion based on the modified Lorentz oscillator. Two modified Lorentz oscillators corresponding to both π→π* and σ→σ* interband transitions were taken into account. Within this model of dispersion of the optical constants, the concept of the band gap and the existence of localized energy states within the band gap were taken into account as well.

XPS and Ellipsometric Study of DLC/Silicon Interface

Diamond-like carbon (DLC) films were prepared by plasma-enhanced chemical vapour deposition from the mixture of methane and argon on silicon substrates. Films were characterised by multi-sample modification of variable angle spectroscopic ellipsometry. Ellipsometry showed that there is a transition interlayer between the DLC film and the silicon substrate that cannot be attributed to a thin silicon dioxide layer but rather to amorphous silicon and/or a modified oxide layer. TRIM calculations revealed that argon or carbon ions could not produce a significant layer of amorphous silicon because the depth of target atom displacements is below the thickness of a native oxide layer. The chemical composition of a DLC film profile including a DLC/silicon interface was studied by X-ray photoelectron spectroscopy (XPS) coupled with argon sputtering of the 34 nm thick DLC film. The DLC/silicon interface composed of less than 6% of oxygen and a gradually decreasing and increasing carbon and silicon percentage, respectively.

Pd/Al2O3 interaction: the influence of ionicity character of different alumina surfaces

Abstract The nature of the bonding between Pd atoms and alumina surfaces was investigated by analysing the oxygen photoelectron induced Auger KLL transitions. The activity of oxide surfaces was analysed in terms of the surface ability for electronic exchange with the metal adlayer. The comparative study of the metal–substrate interaction on different types of Pd supports (α-Al 2 O 3 , γ-Al 2 O 3 and thin alumina layers on polycrystalline Al foil) is presented in this paper. The surface atomic composition of all substrates after cleaned bombardment procedure was determined by ion scattering spectroscopy. The ionicity of the oxygen sites was observed to be a function of the oxide thickness and stoichiometry. It was found that the reduction of the oxide surfaces caused the increase of the I (KL 23 L 23 )/ I (KL 1 L 23 ) intensity ratio, i.e. the increase of the ionicity. The opposite tendency for this ratio was observed during Pd deposition on thin alumina oxide. The possible explanations of such behaviour (charge transfer to Pd particle and/or creation of the Al–Pd alloy) are discussed.

CO adsorption on Al2O3-supported Pd clusters: XPS study

Abstract The adsorption of carbon monoxide on small alumina-supported Pd particles have been studied by X-ray photoelectron spectroscopy (XPS). The results showed clearly different surface properties of bulk metal and supported clusters. The effect of partial CO dissociation was observed on Pd deposited on γ-alumina and on clean aluminium, but not on small Pd particles prepared on thin amorphous oxide film on Al substrate. CO–Pd interaction was determined from C 1s photoelectron spectra that exhibited two CO-related components. The dissociation activity was monitored as a rise of C 1s signal at 285 eV, while the molecularly adsorbed CO exhibited the intensity at 287 eV. The study showed that beside the particle size, the metal–substrate interaction (MSI) plays an important role in CO–Pd adsorption process.