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International audience; Angle-resolved X-ray photoelectron spectroscopy (ARXPS) experiments were performed to study in situ the reaction at the film–substrate interface during metal organic chemical vapor deposition (MOCVD) growth of TiO2 thin films deposited on the silicon substrate. The in-depth distribution of chemical species was determined using several ARXPS thickness calculation models considering either single or bilayer systems. By the comparison of two single-layermodels, the presence of a second layer composed of silicon oxidewas evidenced. High-resolution transmission electron microscopy (HRTEM) observations confirmed the stratification of the film in two layers, as well as the perfect wetting of the surface by TiO2, irrespective of the model used. The contribution of the ARXPS bilayer model was demonstrated by the agreement of the values of film thickness obtained by both techniques. Nevertheless, the correlation between each layer's thickness was only possible by taking into account chemical species mixing. Indeed, the presence of oxidised silicon in the upper TiO2 layer, in addition to that of the SiOx interlayer, has to be considered to interpret ARXPS results in a correct way.

International audience; Homogeneous collections of Pd−Ni core−shell nanoparticles have been prepared by decomposition of metal−organic compounds and studied by several electron microscopy techniques: transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), energy-filtered microscopy (EFTEM), and by X-ray photoelectron spectroscopy (XPS). The physical and chemical properties of the Pd shell are supposed to depend on its electronic properties, which are influenced by the presence of the Ni core and by the deformation in the Pd lattice. Here, the interfacial structure of Pd/Ni and the lattice deformations in the core and the shell are studied in detail. The catalytic properties of the pure metal and the bimetallic particles, toward CO oxidation, have been investigated.

The electronic structure of titanium dioxide surfaces having undergone different preparations leading to different stoichiometries and crystallinities has been studied using resonant photoemission spectroscopy. Valence band photoemission spectra through the Ti 3p–3d/4s absorption edge between 45 and 55 eV were measured and allowed a characterization of defects present at the surface as well as of the quality of the surface organization. Indeed, from the comparison of the resonance results obtained for each kind of surface with the LEED patterns on the one hand and the corresponding Ti 2p core level lines on the other hand, it was evidenced that the high binding energy part of the valence band seems rather related to the ordering of the surface than to the amount of defects.

International audience; The ability to determine the nature and the occurrence of defects is a central need of ceramic surface chemistry. In titanium oxides, the Ti-LMV Auger decays line shape is very sensitive to the titanium degree of oxidation, and has long been empirically used as a qualitative probe of the stoichiometry. In the present work, resonant Auger and resonant valence-band measurements at the Ti-L2,3 edges in TiO2, TiO2–x and metallic titanium provide a clear evidence that the evolutions of the Ti-LMV Auger line shape are due to drastic changes in the valence-band profile and in the probability of L2L3V Coster-Kronig decay processes when a fraction of titanium ions is reduced. Both are shown to be consequences of the filling of Ti-3d states near the Fermi level on defects sites. These conclusions are confirmed by Auger-photoelectron coincidence experiments.

TiO2/TiNxOy δ-doping structures were grown on the top of (1 1 0)TiO2 rutile substrates by low pressure metal-organic vapor phase epitaxy (LP-MOVPE) technique at 750 °C. The samples were analyzed by high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and X-ray diffraction techniques (rocking curves and φ-scans). The presence of satellites in the (1 1 0)TiO2 rocking curve revealed the epitaxial growth of 10 period δ-doping structures. The thickness of the TiO2 layers, 84 nm, was deduced from the satellites period. HRTEM observations showed around 1.5 nm thick δ-doping layers, where the presence of nitrogen was detected by EELS. The analysis of the Bragg surface diffraction peaks observed in the φ-scans points to an almost negligible strain in this sample which was confirmed by substrate curvature radius measurements.

Molybdenum was deposited on blue (i.e. non-stoichiometric) TiO2(1 1 0) surface using a very low deposition rate (less than 0.05 eqML min−1). The resulting deposit was investigated by means of X-ray photoelectron diffraction (XPD), LEED and XPS. Just after deposition, the film is mainly constituted of metallic molybdenum, contains oxygen homogeneously dispersed through the whole deposit and the broad features detected in XPD scans are interpreted as a coarse epitaxy between TiO2(1 1 0) surface and the (0 0 1) face of bcc molybdenum. The orientation relationship is: Mo(1 0 0)[0 0 1]//TiO2(1 1 0)[0 0 1]. After annealing the deposit at 673 K, XPD scans become sharper and epitaxy is achieved even if no LEED pattern is observed. Moreover, the annealing induces a noticeable decrease of the oxygen content inside the film.

Quadrupolar transitions can play an important role in X‐ray absorption spectroscopy, especially when it is used for magnetic measurements, like in X‐ray Magnetic Circular Dichroism or Resonant Magnetic Scattering. We show here that resonantly excited Ti KL2,3L2,3 Auger spectra of TiO2 (110) carry a clear signature of quadrupolar transitions from the 1s to localized eg and t2g d‐like states. The quadrupolar nature of the observed additional spectator lines are clearly demonstrated by their angular dependence, and their intensity is used to locate and quantify the quadrupolar transitions in the absorption spectrum.

International audience; We report the percolation behaviour of the conductivity of titanium oxynitride films grown by low-pressure metal-organic chemical vapour deposition, composed of TiNxOy mixed with TiO2. The usual DC parameters (t, s and Φc), obtained from the effective media theory equations, are compared to the universal values (s = sun while t < tun because of the film anisotropy). This is the first example of an electrical continuum percolation applied to columnar films with chemically similar conducting and insulating units (non-heterogeneous percolation) whose mixing is based upon the growth temperature during the film growth.

The two components of the Ti ${L}_{23}{M}_{23}V$ Auger transition recorded on a stoichiometric rutile crystal are identified as ${L}_{2}{M}_{23}V$ and ${L}_{3}{M}_{23}V$ contributions. This assignment is evidenced by concordant data relative to resonances of the LMV decay at the Ti ${L}_{23}$ thresholds and to Auger emission recorded in coincidence with the ${2}_{1/2}$ and ${2}_{3/2}$ photoemission at a photon energy far above the Ti ${L}_{23}$ edges. The ${L}_{3}{M}_{23}V$ transition is shown to follow either the direct photoexcitation of a ${2}_{3/2}$ electron or the fast Coster-Kronig decay of a ${2}_{1/2}$ photohole. Although specific LMV contributions related to valence orbitals are identified, the long-suggested dual description of the ${L}_{23}{M}_{23}V$ Auger line as intra-atomic and interatomic transitions are discarded.

Physical review letters 88(24), 243001 (2002). doi:10.1103/PhysRevLett.88.243001, Published by APS, College Park, Md.

Surface laser treatment of commercially pure titanium plates was performed in air using two different Nd : YAG sources delivering pulses of 5 and 35 ns. The laser fluence conditions were set to obtain with each source either yellow or blue surface layers. Nuclear reaction analysis (NRA) was used to quantify the amount of light elements in the formed layers. Titanium oxinitrides, containing different amounts of oxygen and nitrogen, were mainly found, except in the case of long pulses and high laser fluence, which led to the growth of titanium dioxide. The structure of the layers was studied by x-ray diffraction and Raman spectroscopy. In addition, reflectance spectra showed the transition from a metal-like behaviour to an insulating TiO2-like behaviour as a function of the treatment conditions.Modelling of the laser–target interaction on the basis of the Semak model was performed to understand the different compositions and properties of the layers. Numerical calculations showed that vaporization dominates in the case of short pulses, whereas a liquid-ablation regime is achieved in the case of 35 ns long pulses.

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The adsorption site of sodium atoms deposited onto a clean TiO2(110) surface has been determined by EXAFS. The best result is obtained for an 'in-between' site where the sodium is bound to two bridging oxygen atoms at 2.25 A and one in-plane one at 2.40 A in full agreement with DFT calculations. At higher coverage the site becomes a hollow site where Na is equidistant to the three oxygen atoms at 2.30 A.

A Glow Discharge Optical Emission Spectrometry device has been designed for glove box adaptation in CEA Valduc to analyze carbon, hydrogen, nitrogen and oxygen at low concentration in nuclear materials. Vacuum ultraviolet region of the spectrum (between 120 and 160 nm) was chosen for light elements analysis with the aim to avoid optical interferences with matrix emission lines (20000 lines in the visible region for plutonium element). For this purpose, two different systems of collection have been developed: a first based on two focusing lenses and a second having an optical interface based on mirrors. In the present paper, the whole set-up integrated in a glove box is described. Optical limits, such as absorption and chromatic and geometric aberrations, of the first system of collection and transmission of light (two MgF2 focusing lenses), are presented, as well as the new optical interface based on mirrors developed in order to reduce the defects of the lens system. Calibration graphs performed for carbon, nitrogen and oxygen in non-nuclear materials were plotted with the lenses' system and the corresponding sensitivities were calculated. First results obtained with the new optical box are also presented. This system allows an increase in the number of photons transmitted from the source to the analyser. Numerical calculations and first experimental results particularly prove its efficiency for the transmission of the hydrogen line (121 nm).

A great many characterizations have been carried out on NiMo catalysts by means of various techniques. However, little is known about their real surface structure. Molybdenum deposition was performed under uhv on cleaved NiO(100) surfaces. The growth mode was determined by Auger electron spectroscopy, reflection high energy electron diffraction and secondary ion mass spectrometry. The overgrowth of a poorly crystallized MoO 2 film is evidenced for deposition at room temperature. Upon thermal treatments the nucleation and growth of nickel islands occur within an amorphous MoO x layer with x > 2. Despite the relative simplicity of this non-supported catalyst, one can consider that the superficial phases obtained in this study may be found at the surface of real NiMo catalysts.

The interaction of oxygen with (001) and (111) Ni surfaces is studied mainly by static SIMS. The presence of OH groups on the surface is revealed. The reactivities of these NiO surfaces and that of NiO(001) with respect of hydrogen and hydrogenated species are studied.

TiO2 powder in either the rutile or the anatase form do not generate ammonia when they are subjected to ultraviolet irradiation under a wet nitrogen flux. In order to undergo this photochemical reaction, the samples first have to be annealed under air at atmospheric, high temperature. This treatment generates defect states in the band gap of the semiconductor and in the hydroxyl groups on the surface. The effects of thermal pretreatment on the various characteristics of the TiO2 powders such as specific area, crystal structure, morphology and stoichiometry have been studied. The nature of the hydroxyl groups on TiO2 surface is determined by IR spectrometry and by temperature-programmed desorption.

The first stages in the growth of a nickel deposit on to the (100) face of TiO2 was investigated using mainly Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). Auger intensities of both adsorbate (Ni) and substrate (Ti, O) were followed versus deposition time. A layer-by-layer growth of three nickel layers is obtained followed by a nucleation stage. The attenuation coefficients for adsorbate and substrate Auger peaks through a monolayer of adsorbate are calculated. In dynamic SIMS experiments secondary ionic intensities of the deposit and of the substrate were measured during sputtering. By using the sequential layer sputtering (SLS) model for the sputtering process, the morphology of the deposit obtained by AES is confirmed.

The influence of traces of fluorine on secondary ion emission of hydroxylated TiO2 (100) surfaces is studied. The kinetics of the hydroxylation reaction is greatly enhanced by fluorine. The saturation population of one hydroxyl is not modified by the presence of fluorine. At last fluorine modifies drastically the secondary ion emission especially that of negative ions.

The structure and stability of different crystallographic phases of tungstentrioxide and the effect of potassium doping have been studied using ab initiocalcuations, both in the bulk and at the surface. The study was motivatedby experimental works on the growth of WO3 nanorods on a mica substrate.The nanorods adopt a hexagonal phase, which is metastable in bulk WO3.The first part of the manuscript reports the study of bulk WO3. For thehexagonal and monoclinique phase, the atomic structure is deterimined andcompared with different models that have been proposed in the experimentalliterature. The relative stability of the two phases is calculated as a functionof potassium doping. From this an approximate phase diagram of KxWO3is obtained which shows that the the hexaongal phase is stable for x between3 and 35%. It is also shown that the WO3 lattice distortions disappearfore sufficient K concentration. These results explain the stabilization of thehexagonal phase in the nanorods through insertion of K atoms from the micasurface.In the second part of the manuscript, a sytematic study of the surfaces ofWO3 in the monoclinic and hexagaonal phase is presented. For each phase,the three surface orientations of lowest Miller indices are considered. All nonreconstructedterminations and several types of reconstructions are analysedin terms of polarity, stability and atomic structure. In the monoclinic phase,the (010) surface with a c(2x2) reconstruction is most stable, in agreementwith experiment. For the hexagonal phase, the surface with lowest energy(0.02 eV/Å2) is c(1x1)O2(1120). It is found that the stability of the differentorientations is very sensitive to potassium doping. For hexagonal KxWO3with x = 13 , the (0001) surface is most stable with a KO3 termination and asurface energy of 0.05 eV/Å2., Nous avons étudié par calcul ab initio la structure et la stabilité de différentes phases cristallographiques du trioxyde de tungstène en volume et ensurface, ainsi que l'effet du dopage par le potassium. L'étude a été motivéepar des travaux expérimentaux sur la croissance de nano-bâtonnets WO3 surun substrat de mica ; ces nanostructures adoptent la phase hexagonale, mé-tastable dans le WO3 massif. La première partie du manuscrit est consacrée àl'étude du volume. Les structures atomiques des phases hexagonales et monocliniquessont déterminées et comparées aux différents modèles proposés dansla littérature expérimentale. La stabilité relative de ces deux phases est calculée en fonction du taux de potassium permettant d'obtenir un diagrammede phase approximatif de KxWO3 montrant que la phase hexagonale est stablepour x compris entre 3 et 35%. Par ailleurs, les distorsions disparaissentpour des concentrations de K suffisamment élevées. Ces résultats expliquentla stabilisation de la phase hexagonale dans les nano-bâtonnets par l'insertiond'atomes de K de la surface du mica. Dans la seconde partie, une étudesystématique des surfaces, en phase monoclinique et hexagonale, est présentée. Pour chaque phase, les trois orientations de surface de plus faibles indicesde Miller sont considérées. Toutes les terminaisons non-reconstruitesainsi que plusieurs types de reconstructions ont été analysées en terme depolarité, stabilité et structure atomique. Pour la phase monoclinique, l'orientation(010) avec reconstruction c(2x2) est la surface la plus stable, enaccord avec des données expérimentales. Quant à la phase hexagonale, c'estla surface c(1x1)O2(1120) qui a la plus petite énergie de surface (0.02 eV/Å2).Nous trouvons que la stabilité des différentes orientations est très sensible audopage par le potassium. En effet, pour KxWO3 avec x = 13 , c'est l'orientation (0001) qui est la plus stable, avec une terminaison KO3 et une énergiede surface de 0.05 eV/Å2.