Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

Structural and magnetic properties of Fex–C1−x nanocomposite thin films

Abstract: Iron–carbon nanocomposite thin films with iron concentrations ranging from 12 to 74 at. % were grown by ion-beam sputtering codeposition at different substrate temperatures. The microstructure of the films was characterized by transmission electron microscopy, atomic force microscopy, extended x-ray absorption fine structure, and grazing incidence small-angle x-ray scattering. A granular morphology consisting of body-centered-cubic iron-rich nanoparticles (2–5 nm in diameter and 3–8 nm in height with a relatively sharp size distribution) regularly distributed within a more or less graphitic matrix was obtained. Structural properties as well as magnetic ones were found to depend strongly on composition, substrate temperature, and postdeposition treatments (annealing or Ar+ irradiation). The temperature dependence of the susceptibility exhibited a superparamagnetic response with blocking temperature in the range of 13–180 K. The maximum low-temperature (5 K) coercivity value obtained in this study was 850 O...

CO Coverage/Oxidation Correlated with PtRu Electrocatalyst Particle Morphology in 0.3 M Methanol by In Situ XAS

Abstract: In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Pt L{sub 3} and Ru K edges, were carried out on three different carbon-supported PtRu electrocatalysts in an electrochemical cell in 1 M HClO{sub 4} with 0.3 M methanol. The CO and OH adsorbate coverage on Pt and Ru were determined as a function of the applied potential via the novel delta XANES technique, and the particle morphology was determined from the EXAFS and a modeling technique. Both the bifunctional and direct CO oxidation mechanisms, the latter enhanced by electronic ligand effects, were evident for all three electrocatalysts; however, the dominant mechanism depended critically on the particle size and morphology. Both the Ru island size and overall cluster size had a very large effect on the CO oxidation mechanism and activation of water, with the bifunctional mechanism dominating for more monodispersed Ru islands, and the direct surface ligand effect dominating in the presence of larger Ru islands.

Surface extended-x-ray-absorption-fine-structure study of oxygen interaction with Al(111) surfaces

Abstract: We have investigated the oxidation of Al(111) surfaces by means of polarization-dependent surface extended-x-ray-absorption-fine-structure (EXAFS) measurements. The experiments were performed above the oxygen $K$ absorption edge (\ensuremath{\sim}535 eV) using the total electron-yield detection technique. The oxidation range from monolayer chemisorption corresponding to an ordered oxygen (1 \ifmmode\times\else\texttimes\fi{} 1) overlayer to the formation of a thick surface layer of amorphous oxide has been studied. For the chemisorbed monolayer (100-150-L ${\mathrm{O}}_{2}$) we find a strong polarization dependence of the EXAFS signal which allows the determination of the O-O intraoverlayer separation 2.90 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}. The O-Al chemisorption bond length has been determined to be 1.79 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}. Within experimental error we find the same O-Al distance for the initial oxide-like phase which accompanies the chemisorbed phase. At higher exposures (1000-L ${\mathrm{O}}_{2}$) the vanishing low-energy-electron-diffraction pattern is accompanied by an increase in the O-Al bond length (1.88 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{}). This value is also observed for heavily oxidized surfaces. The O $K$ edge shows the formation of a strong "white-line"-like threshold resonance with increasing oxygen exposure. This resonance is attributed to transitions to localized final states in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-like molecular units. It is pointed out that detailed analysis of the absorption threshold may by itself provide information on the local structure of surface complexes. Models for the progressive oxidation process of Al(111) surfaces implied by our data are discussed.

A Si K-edge EXAFS/XANES study of sodium silicate glasses

Abstract: Si K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) have been obtained on a series of sodium silicate glasses containing 15–40 mol% Na2O. The XANES reveals that, with the addition of Na2O, the SiSi bond distance distribution decreases and there are increased contributions from multiple scattering beyond the second coordination sphere. These responses imply that some degree of network ordering occurs with alkali addition. The EXAFS data indicate that the SiO bond distance increases from 1.61 ± 0.02 A for amorphous SiO2 to 1.66 ± 0.02 A for 30 mol% added Na2O. For Na2O > 30 mol%, the SiO bond distance decreases. The SiO bond distance changes indicate that, for ≤ 30 mol% Na2O, network depolymerisation effects on the SiO bond dominate any effect from increased non-bridging oxygen (NBO) formation. For compositions > 30 mol% Na2O, increased NBO formation has the dominant effect on the SiO bond distance. This may indicate that the microsegregation of network modifiers from network formers, as predicted by molecular dynamics studies, is significant at ≥ 30 mol% added Na2O.