Juan P. Holgado

Study of CeO2 XPS spectra by factor analysis: reduction of CeO2

Abstract The evaluation of the reduction state of CeO2 by XPS is still controversial because of the presence of more than 10 peaks in the Ce(3d) photoemission spectra due to Ce3+ and Ce4+, some of them at similar binding energies. This makes difficult conventional fitting using nonlinear least-square (NLLSF) methods with Gaussian–Lorentzian curves that require the knowledge of the number of bands. In the present work, we have tried to overcome this problem using factor analysis (FA), a method that has been applied with success to other spectroscopic problems. In order to obtain confident results for the individual shapes of the components, (the basis for a correct quantification) we have combined target testing (TT) and iterative target transformation factor analysis (ITTFA). We have shown that the combined use of these two methods results in a more accurate evaluation (ca. ±2%) of the reduction state of partially reduced CeO2. The election of the “real factors” corresponding to Ce4+ and Ce3+ species is discussed in detail.

XPS study of oxidation processes of CeOx defective layers

Abstract A CeO 2 thin film has been subjected to Ar + bombardment at 298 K to induce the reduction of its outmost layers by preferential removal of oxygen. An XPS study of the altered layer at normal and grazing angle has been carried out. The Factor Analysis (FA) of the XPS spectra of this Ar + reduced film shows that it has a stoichiometry close to Ce 2 O 3 , being Ce 3+ the dominant species at both collection angles. Simultaneously, the O1s spectra depict a lateral peak whose relative intensity is higher for those spectra recorded at grazing angle. Exposure to successive doses of O 2 at 298 K of the reduced layers produces the increase of the O/Ce ratio and a progressive reoxidation of Ce 3+ into Ce 4+ as determined by FA of the Ce3d spectra. Simultaneously, the lateral component at the O1s peak also decreases, thus discarding that it is due to surface contamination by –OH or similar species, as previously suggested in the literature. After exposure to a high pressure of oxygen (ca. 1 Torr), the XPS spectrum obtained at a normal collection angle shows an almost complete oxidation of the film to CeO 2 . However, in the spectrum at grazing angle, Ce 3+ species and the lateral component of oxygen are still detected. The lateral O1s component is tentatively attributed to oxygen ions with unusual coordinations in a defective CeO x ( x 3+ ions might be due to fully coordinated species. Enrichment of the surface of the defective cerium oxide with these oxygen species seems to be a result of the same structural rearrangements that favour the observed stabilization of Ce 3+ species at the surface.

Preparation of transparent and conductive Al-doped ZnO thin films by ECR plasma enhanced CVD

The objective of this work is to optimise the deposition conditions of aluminium-doped ZnO by ECR plasma enhanced CVD to produce transparent and conductive thin films. Films were deposited by the reaction of Zn(C 2 H 5 ) 2 and Al(CH 3 ) 3 with pure oxygen plasma or mixtures of oxygen and hydrogen plasmas, obtained by an ECR downstream plasma source operated at low pressure (0.1-1 Pa). Control of the resistivity and UV-Vis transparency of the films was achieved by changing the aluminium dose in the film, the preparation temperature and the composition of the plasma (O 2 :H 2 ratio). Optical constants were determined by transmittance data in the UV-Vis region, all samples presenting very good optical characteristics (96-92% transmittance in visible range). On the other hand, resistivities, as obtained at room temperature by a four-point probe, ranged from > 10 +7 to 6×10 -3 Ω-cm, depending on the preparation conditions. Thus, the lowest resistivities were obtained for samples prepared with a plasma of H 2 +O 2 (2.0:1.5). Composition, microstructure, cristallinity and thickness of the films were characterised by XRF, XPS, RBS and XRD.

An XPS study of the mixing effects induced by ion bombardment in composite oxides

Abstract XPS has been used to study MxOy-TiO2 (MxOy: CeO2 and Al2O3) composite systems in powder form submitted to bombardment with ions (Ar+ and O+2) of 3.5 keV kinetic energy. In CeO2-TiO2 submitted to Ar+ bombardment it is found that cerium is reduced to Ce3+. This species remains stable against re-oxidation by exposure to O2, which contrasts with the behaviour of Ce3+ in partially reduced CeO2 where under similar conditions cerium is oxidized to Ce4+. Reduction to Ce3+ is also observed after O+2 bombardment which points to the fact that the cerium ions are mixed within the titanium oxide lattice and this is the reason for the stabilization of the Ce3+ state. In TiO2-Al2O3 the existence of mixing of the two components upon ion bombardment is shown by an accurate assessment of the photoelectron and Auger peaks of the spectra and the evaluation of the Auger parameters of the different elements. The use of XPS as a tool to show the existence of mixing in composite oxides after ion bombardment is discussed.

Determination of texture by infrared spectroscopy in titanium oxide-anatase thin films

A theoretical model to determine the effective dielectric tensor of heterogeneous materials composed by anisotropic microcrystallites has been introduced to explain the infrared spectral features of textured thin films of uniaxial materials as the function of a textural parameter. This theoretical treatment is able to satisfactorily reproduce the experimental absorbance spectra of TiO2–anatase thin films chosen as a model system. Comparison of texture data obtained from infrared spectroscopy and x-ray diffraction are in good agreement which support the validity of the proposed model.

Ar stabilisation of the cubic/tetragonal phases of ZrO2 in thin films prepared by ion beam induced chemical vapour deposition

Abstract ZrO2 thin films have been prepared at room temperature by ion beam induced chemical vapour deposition (IBICVD). Two different sets of samples have been prepared depending on whether O2+ or mixtures O2++Ar+ ions were used for the decomposition of the precursor. The structure, microstructure, surface roughness and optical properties (i.e. refraction index) have been determined for the two sets of samples. The ‘as prepared’ samples were very compact and dense and had a very low surface roughness. The (O2+)–ZrO2 samples were transparent and had a high index of refraction (n=2.20 at λ=660 nm). On the contrary, the (O2+–Ar+)–ZrO2 films showed a greyish aspect after preparation, probably because of the existence of lattice defects in their structure. These defects were removed by annealing in air at T≥573 K. Then, the films became transparent and had a slightly lower refraction index than that of the O2+–ZrO2 films subjected to the same thermal treatments. For the (O2+–Ar+)–ZrO2 thin films, it was shown by X-ray photoelectron spectroscopy (XPS) and Rutherford back scattering (RBS) that Ar (4 at.%) remained incorporated within the oxide lattice even after annealing at T=773 K. The incorporation of Ar atoms within the ZrO2 network induced significant modifications in the crystallographic structure of the films. Thus, while X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) showed that the (O2+)–ZrO2 films consist of a mixture of monoclinic and tetragonal phases of zirconia, the (O2+–Ar+)–ZrO2 films depicted, at 298 K, a high degree of amorphisation. However, these films yielded a cubic/tetragonal phase after annealing at T>573 K. The role of the embedded Ar in inducing these structural changes is discussed.

An XPS study of the Ar+-induced reduction of Ni2+ in NiO and Ni-Si oxide systems

The reduction effect on the Ni2+ species induced by 3.5 keV Ar+ bombardment of NiO particles dispersed on SiO2 and on NiSiO3 as well as on two NiO powder samples with different grain sizes has been quantitatively studied by XPS. In all the cases reduction of the Ni2+ species to metallic Ni° has been observed. However, it is also shown that Ar+-bombardment-induced reduction of Ni2+ is strongly enhanced by the presence of Si4+ cations, which on the other hand remain stable.

Optical and crystallisation behaviour of TiO2 and V/TiO2 thin films prepared by plasma and ion beam assisted methods

This paper reports the synthesis of TiO2 and V/TiO2 thin films, prepared by plasma enhanced and ion beam induced chemical vapour deposition procedures. Thin films with different contents of vanadium from a few to some tenth percents of this element have been prepared. Vanadium oxide thin films were also prepared for comparison. Morphological, structural and chemical characterisation of the samples was carried out by means of several techniques including scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Optical properties of these samples were studied by ellipsometry and ultraviolet–visible absorption spectroscopy. It has been shown that the absorption threshold of V/TiO2 thin films shifts continuously towards the visible by increasing the V content. On the basis of this behaviour it is proposed the use of these thin films as optical filters in the UV–vis region of the spectrum. In the ‘as prepared’ samples vanadium is homogeneously distributed within an amorphous Ti–O structure. After annealing in air at T>673 K, both segregation of vanadium as vanadium oxide and crystallisation of the TiO2 into the rutile structure take place. It is realised that infrared spectroscopy can be a complementary technique of XRD to determine the crystallisation behaviour of the samples. The observed structural modifications are accompanied by a change in the light absorption properties of the films that can now be interpreted as due to the superposition of the absorption spectra of TiO2 and vanadium oxide. In the ion beam assisted films, only slight changes in their refraction index (n) are found as a function of the vanadium content and/or the annealing treatments. The changes were greater for the less compact films obtained by plasma deposition, indicating that the main parameter controlling the value of n is the density of the films.

Degradation of LaMnO3−y surface layer in LaMnO3−y/metal interface

We report electrical measurements showing the degradation processes of LaMnO3−y (LaMnO) in LaMnO/normal metal interface in both point contact and planar-type junctions. The time evolution of the junction resistance is characterized by the appearance of a second maximum in the resistance versus temperature dependence (R–T) that occurs at a temperature lower than that for the metal–insulator transition in the bulk. These effects are explained in terms of the formation of a depleted interface layer in LaMnO caused by an out-diffusion of oxygen from the manganite surface to the normal metal. This was confirmed by XPS measurements. Similar results on LaSrMnO3−y/metal interfaces were also obtained.

Sonogashira cross-coupling and homocoupling on a silver surface:chlorobenzene and phenylacetylene on Ag(100)

Scanning tunneling microscopy, temperature-programmed reaction, near-edge X-ray absorption fine structure spectroscopy, and density functional theory calculations were used to study the adsorption and reactions of phenylacetylene and chlorobenzene on Ag(100). In the absence of solvent molecules and additives, these molecules underwent homocoupling and Sonogashira cross-coupling in an unambiguously heterogeneous mode. Of particular interest is the use of silver, previously unexplored, and chlorobenzene-normally regarded as relatively inert in such reactions. Both molecules adopt an essentially flat-lying conformation for which the observed and calculated adsorption energies are in reasonable agreement. Their magnitudes indicate that in both cases adsorption is predominantly due to dispersion forces for which interaction nevertheless leads to chemical activation and reaction. Both adsorbates exhibited pronounced island formation, thought to limit chemical activity under the conditions used and posited to occur at island boundaries, as was indeed observed in the case of phenylacetylene. The implications of these findings for the development of practical catalytic systems are considered.