Extended X-ray Absorption Fine Structure Method Research Articles

Taking a different road: following Ag25 and Au25 cluster activation via in situ differential pair distribution function analysis.

Synchrotron X-ray total scattering measurements and accompanying pair distribution function (PDF) analyses are an excellent characterization technique to complement both transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy methods for detailed structural studies of atom-precise metal clusters. Herein, we study the thermal activation of Au25(SR)18- and Ag25(SR)18- clusters on alumina supports via in situ differential PDF (dPDF) analyses to compare structural changes in the metal clusters upon thermal activation in air. The metal-metal interatomic distances in Au25(SR)18- and Ag25(SR)18- clusters as measured by the dPDF method are comparable with those measured via single-crystal crystallographic and EXAFS methods. Compared to EXAFS analysis, in situ dPDF data can also provide high-temperature, non-element specific, longer range structural information with excellent temporal resolution. TEM and dPDF results show that Ag25(SR)18 systems behave significantly differently than analogous Au25(SR)18 systems upon thermal activation. Atom-precise Au clusters on alumina supports show continuous growth in particle size with increasing activation temperature due to particle coalescence upon thermal deprotection, and grow to an average size of 11.2 ± 2.1 nm for samples thermally activated at 650 °C. Conversely, analogous Ag clusters on alumina supports show particle size growth to mid-sized particles (3.2 ± 0.4 nm) at activation temperatures of 450 °C, beyond which the Ag particles then undergo thermal degradation to give smaller Ag clusters with an average size of 1.4 ± 0.2 nm for samples thermally activated at 650 °C. The significant difference in the behaviours of atom-precise, thiolate-protected Au and Ag clusters upon thermal activation emphasizes the development of distinct activation protocols for different metal cluster systems.

EXAFS of titanium LIII edge using a compact laboratory system based on a laser-plasma soft X-ray source

We present results of applying a compact laboratory system based on a laser-plasma soft X-ray (SXR) source to the extended X-ray absorption fine structure (EXAFS) spectroscopy of titanium in the vicinity of the LIII absorption edge. The source, operating with a plasma produced as a result of irradiation a krypton/helium double-stream gas-puff target, was optimized for an intense emission in the spectral range between 200 and 700 eV. A broad SXR spectrum and high photon flux are indispensable in the EXAFS studies. The experimental setup assured the simultaneous acquisition of both the reference and the absorption spectra. A grazing-incidence flat-field spectrometer was used to record the spectra. The absorption spectra of a thin (200-nm thick) titanium sample revealed the features in the EXAFS region allowing for quite accurate determination of the radial distances between the atoms. Results are in good agreement with the output of numerical modeling based on photoelectron wave function scattering and with the data reported for using a synchrotron source. This confirms the suitability of the source for using it in the standard EXAFS method.

Exploring the Distribution of Zn2+ in Inner and Outer Helmholtz Planes of the Electrical Double Layer of Soil based on Wien Effect

Core Ideas Wien effect measurements assess adsorbed cations distribution over the Helmholtz planes of the electrical double layer. Assessing cation distributions in the electrical double layer improves bioavailability predictions for plant nutrients and pollutant metals. Most of the zinc adsorption takes place in the inner Helmholtz plane layer and occurs via chemical interactions. The classical electrical double layer (EDL) theory explains the interaction between ions and charged soil colloidal particles, playing an importance role in the bioavailability and toxicity of ions in soils, which are of agricultural and environmental concern. A new approach based on extrapolating measurements of suspension Wien effect is suggested to determine the adsorbed ions distribution in the different compartments of the EDL. The new approach was applied to assess Zn2+ distribution in the EDL of Zn‐saturated soil colloids. The results showed that more than 84% of Zn2+ was adsorbed via chemical interactions and located in the inner Helmholtz plane layer. The remaining Zn2+ fraction was electrically adsorbed and distributed in the outer Helmholtz plane layer (0.3 to 2.1%) and in the Gouy–Chapman diffuse layer (7.4 to 15.0%). Independently, the results of linear combination fitting of extended x‐ray absorption fine structure (EXAFS) spectra indicated that the contents of outer‐sphere Zn in one paddy soil and the boggy soil were consistent with the proportions of Zn2+ electrostatically interacting with the soil colloids based on the extrapolation of the suspension Wien effect method, while the contents of outer‐sphere Zn in a second paddy soil, the yellow cinnamon soil, and the yellow brown soil was overestimated by the EXAFS method. The approach proposed in this paper to assess specific and electrostatic adsorption deepens our understanding of metal sorption processes in soils and reinforces the adequacy of the Grahame–Stern–Gouy–Chapman model of the EDL to describe the ion distribution in the solid‐water interface.

Solvent-Driven Gate Opening in MOF-76-Ce: Effect on CO2 Adsorption.

A cerium-based metal-organic framework with MOF-76 topology has been synthesized by a very simple and fast solvothermal method that has been tested for a one gram yield. Variable-temperature powder XRD and X-ray absorption data, analyzed by Rietveld and multiple-scattering extended X-ray absorption fine-structure methods, revealed high thermal stability and the presence of three different stable structures. X-ray absorption near-edge structure and FTIR spectroscopy probed the presence of cerium(III), which was characterized by coordinatively unsaturated sites that, however, played no major role in carbon dioxide adsorption. The material revealed excellent carbon dioxide adsorption properties: the highest gravimetric capacity of 15 wt% was observed at 1.1 bar in the case of the sample activated at 250 °C in vacuum, whereas the strongest interaction energy of 35 kJ mol(-1) was observed for the sample activated at 150 °C. Negligible nitrogen uptake of the sample activated at 150 °C indicates that this material is a promising candidate for nitrogen/carbon dioxide separation purposes.

Local atomic structure of Zr–Cu and Zr–Cu–Al amorphous alloys investigated by EXAFS method

We report on extended X-ray absorption fine structure (EXAFS) study of rapidly quenched Zr–Cu and Zr–Cu–Al glassy alloys. The local atomic order around Zr and Cu atoms was investigated. From the EXAFS data fitting the values of coordination number, interatomic distances and mean square atomic displacement were obtained for wide range of compositions. It was found that icosahedral symmetry rather than that of corresponding crystalline analogs dominates in the local atomic structure of Zr–Cu and Zr–Cu–Al amorphous alloys. Judging from bonding preferences we conclude that addition of Al as an alloying element results in considerable deviation from random mixing behavior observed in binary Zr–Cu alloys.

The structure study of amorphous nanocrystalline nanocomposite films of germanium by AFM and EXAFS methods

AbstractThe amorphous nanocrystalline semiconductor nanocomposites obtained by evaporation in ultrahigh vacuum at different substrate temperatures show anomalous electro‐physical properties. It may be connected with the structure of clusters which may consist of both crystalline and amorphous phases. The volume portion of the amorphous phase to the nanosized particles can be obtained by the extended x‐ray absorption fine structure (EXAFS) method. In this work we investigate the structure changes of thin nanocomposite films of Ge obtained at condensation temperatures in the range of 25–250 °C by Atomic force microscopy (AFM) and EXAFS methods. Using the method described by Valeev et al. it was shown that the sizes of nanocrystallites in an amorphous matrix vary essentially according to obtaining conditions (according to AFM data); their content vary from 10 to 90%. Copyright © 2008 John Wiley & Sons, Ltd.

Structural study of highly dispersed cerium dioxides

Cerium dioxide samples of different dispersivity, obtained by different methods, have been investigated by X-ray diffraction (XRD) analysis, full-profile analysis of diffraction patterns, calculation of radial distribution function (RDF) of electron density, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. According to the XRD data, the samples studied can be identified in the fluorite structure with conventional unit-cell parameters. However, the local structure of highly dispersed samples, determined by the RDF and EXAFS methods, changes: the coordination spheres for the Ce-O distances are represented by two distances.

Application of EXAFS and XANES Methods in Coordination Chemistry of Carbohydrates and Their Derivatives

A summary is presented of the results obtained on metal complexes of carbohydrates (aldoses, ketoses and mono-, di- and polysaccharides) and their derivatives (aldonic and alduronic acids, polyalcohols, amino sugars, and amino acid sugar adducts). The structural parameters (coordination number, bond distance, and Debye–Waller factor) measured by extended X-ray absorption fine structure (EXAFS) spectroscopy obtained earlier on transition metals [Cu(II), Fe(III), Ni(II), Zn(II), Co(II), Ag(I), Mn in different oxidation states and organotin(IV)] complexes are discussed. In addition we report here the preparation of Bu2Sn(IV)2+ complexes of rutin {3-[6-O-(6-deoxy-α-L-mannopyranosyl)-(β-D-glucopyranosyl)oxy]-2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4H-1-benzopyran-4-one} and quercetin (containing {O,O} donor atoms) in ligand to metal ratios of 1:1 and 1:2. The analytical data revealed the formation of well-defined complexes. The local structures of the complexes were determined by an EXAFS method. It was found that the Sn–O bond distances in the first coordination sphere are closed the values found in Et2Sn(IV) complexes with nonprotected carbohydrates.

Dependence of EXAFS (Extended X-Ray Absorption Fine Structure) Parameters of Iodide Anions in Various Solvents upon a Solvent Parameter

Abstract X-ray absorption spectra of I K-edge have been obtained for iodide ions dissolved in various solvents. Oscillation amplitudes of the extended X-ray absorption fine structure (EXAFS) evaluated from the peak intensities in the Fourier-transform spectra were found to correlate with the Mayer-Gutmann acceptor number (AN) of solvents studied. The correlation is quite similar to that for bromide ions, which has already been reported by the same authors (H. Tanida, H. Sakane, and I. Watanabe, J. Chem. Soc., Dalton Trans., 1994, 2321). Such results confirm that the EXAFS method is not only a tool to determine geometrical structures in liquids, but can also be used for evaluating solute-solvent interaction strength, similarly to AN. The I…O distances in protic solvents and I…C in aprotic solvents were determined to be 3.4—3.5 and 3.7—3.8 Å, respectively. In protic solvents, the larger the AN values, the shorter the I…O distances and the larger the co-ordination numbers of I-, which were determined to be 1—4. In aprotic solvents, the co-ordination numbers of I- were found to be 4—8. It seems that aprotic solvents having two methyl groups in a molecule are able to solvate I- through two methyl groups, which is similar to the conclusion reported for Br-.

Accurate Bond Length Determination by EXAFS Method

The Extended X-ray Absorption Fine Structure (EXAFS) spectra of pure iron, pure platinum and intermetallic compound Fe 2 Zr, whose strtuctures were well krown, have been measured at lower temperatures (20 K–300 K), and the accuracy of the interatomic-distance determination has been estimated. In order to increase the accuracy in determining the values of the structural parameters, the FABM technique proposed by Teo et al. has been applied. In our results, however the accuracy of the interatomic distance under various temperature could be improved almost an order of magnitude by assuming that the inflection point on the absorption edge represents the edge position.