X-ray Absorption Near-edge Structure Features Research Articles

Nonlocal XANES pre-edge feature of FeTiO3 ilmenite-type at Ti and Fe K-edge

The pre-K-edge feature of Ti and Fe XANES of ilmenite FeTiO3 were investigated. The electronic structure of FeTiO3 was examined based on the partial density of electronic state distribution on both local and nonlocal atoms. A strong contribution of nonlocal metal-metal intervalence electronic states of Ti and Fe pre-K-edge was observed in X-ray absorption near-edge structure (XANES) measurement. This observation agrees well with the calculated local state density. The nonlocal excitation transitions in the pre-edge feature contribute to local pre-K-edge at both Ti and Fe. Ti pre-K-edge consists of highly hybridized Ti-3d(t2g) with a small part of dipole-allowed 4p and a small 4s density transition state feature which is predominated by nonlocal of Fe-3d mixed with 4p state delocalized via O-2p as the main component peak. The peak feature at Ti-3d(eg) energy state is assigned to hybridization of Ti-3d(eg) and Ti-4p state contributed with nonlocal of the hybridized Fe-sp and the Ti-pd state. Fe pre-K-edge shows two main prominent transition peaks of hybridized Fe-pd state and the delocalized local Fe-sp with nonlocal Ti-3d(t2g) and 4p mixing states centered at 7112.38 and 7116.48 eV, respectively. The local and nonlocal excited transition energy states and density of both Ti pre-K-edge and Fe pre-K-edge features are corresponded with each other through the O-2p state transition pathway.

Supervised Machine-Learning-Based Determination of Three-Dimensional Structure of Metallic Nanoparticles.

Tracking the structure of heterogeneous catalysts under operando conditions remains a challenge due to the paucity of experimental techniques that can provide atomic-level information for catalytic metal species. Here we report on the use of X-ray absorption near-edge structure (XANES) spectroscopy and supervised machine learning (SML) for refining the 3D geometry of metal catalysts. SML is used to unravel the hidden relationship between the XANES features and catalyst geometry. To train our SML method, we rely on ab initio XANES simulations. Our approach allows one to solve the structure of a metal catalyst from its experimental XANES, as demonstrated here by reconstructing the average size, shape, and morphology of well-defined platinum nanoparticles. This method is applicable to the determination of the nanoparticle structure in operando studies and can be generalized to other nanoscale systems. It also allows on-the-fly XANES analysis and is a promising approach for high-throughput and time-dependent studies.

A structural study of bone changes in knee osteoarthritis by synchrotron-based X-ray fluorescence and X-ray absorption spectroscopy techniques

Osteoarthritis (OA) is characterized by degeneration of articular cartilage and thickening of subchondral bone. The present study investigated the changing of biochemical components of cartilage and bone compared between normal and OA people. Using Synchrotron-based X-ray fluorescence (SR-XRF) and X-ray absorption spectroscopy (XAS) techniquesincluding X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were employed for the bone changes in kneeosteoarthritisstudies. The bone samples were collected from various osteoarthritis patients with both male and female in the ages range between 20 and 74 years old. SR-XRF results excited at 4240 eV for Ca elements show a majority three main groups, based on their XRF intensities, 20–36 years, 40–60 years and over 70 years, respectively. By employing XAS techniques, XANES features can be used to clearly explain in term of electronic transitions occurring in bone samples which are affected from osteoarthritis symptoms. Moreover, a structural change around Ca ions in bone samples is obviously obtained by EXAFS results indicating an increase of Ca-amorphous phase when the ages increase.

XAFS study of aqua (diethylenetriamine)(isonicotinato)-copper(II) complex - inference of square-pyramidal geometry

X-ray absorption fine structure (XAFS) of aqua (diethylenetriamine) (isonicotinato) copper(II) complex has been investigated. The crystal structure of the complex is unavailable. The X-ray absorption near edge structure (XANES) spectrum of the complex has been compared with those of the standard compounds to estimate the coordination geometry and oxidation state of copper. This study indicates that the copper center has +2 oxidation state and that it may be penta or hexa-coordinated. The spectrum has been further compared with those of the earlier studied complexes having square-pyramidal geometry. Similarity has been observed between them indicating that the complex should have penta-coordinated square-pyramidal geometry. The same inference has been obtained from the study of the characteristic pre-edge and XANES features. This inference has been further examined by analysis of the extended X-ray absorption fine structure (EXAFS) spectrum. The experimental EXAFS data has been fitted with three different theoretical models for tetra-coordination, penta-coordination, and hexa-coordination. The tetra model is unable to provide a satisfactory fit so it has been ignored. Although the penta and hexa models give comparable results, the parameters obtained indicate that the penta-coordinated model fits better to the experimental data than the hexa-coordinated model. This penta-coordinated model has been further fitted to the experimental data by allowing the coordination number to vary. Such fitting also shows that the penta-coordinated model is a satisfactory model for the studied complex. The obtained values of mean-square disorder indicate that the force constants are different for the equatorial and axial bonds. Copyright © 2014 John Wiley & Sons, Ltd.

Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy

The valence states of Mn in mixed-valent layer and tunnel structure manganese dioxides (MnO2), usually referred to as phyllomanganates and tectomanganates, can be measured by X-ray absorption near-edge structure (XANES) spectroscopy with a precision and accuracy that are difficult to estimate owing to the paucity of well-characterized standards. A compilation of the Mn K-edge XANES spectra of most naturally occurring manganates, synthetic analogs of known structure and chemical composition, and pure-valence phase species is presented and made available as an open source. We intend this compilation to serve as a basis for the spectroscopic determination of the fractions of the Mn 2+, 3+, and 4+ valences in mixed-valent manganates and phase mixtures. The XANES derivatives of tectomanganates and phyllomanganates with no or little Mn3+ in the MnO2 layer exhibit intensities, shapes, and relative energy positions of the main features characteristics of a particular valence composition. For these compounds, valence fractions can be derived using linear combination fitting analysis. Best quantitative results are obtained when the unknown spectrum is fit to a weighted sum of all reference spectra in the database with the fractions of species constrained to be non-negative (Combo method). The accuracy of the average valence is estimated to 0.04 v.u. in the range of 3+ to 4+, and decreases when the proportion of divalent Mn is higher than 15%. The accuracy of the method is also lower in (layer Mn3+, Mn4+) manganates, because the XANES features are affected non-additively by the amount and distribution of the Jahn-Teller Mn3+ cations. The merit of the Combo method for the determination of manganese valence sums relative to the methods based on calibration curves is discussed.

ChemInform Abstract: XANES Study of Underpotential Deposited Copper on Carbon-Supported Platinum.

Abstract Underpotential deposited (UPD) copper on carbon-supported platinum was investigated in-situ in 0.5 M H2SO4 at 0.05 V (SCE) by XANES (X-ray Absorption Near Edge Structure). By taking XANES spectra at the Cu K-edge and the Pt LIII-edge it was possible to determine the valence state of the copper and observe modifications in the electronic structure of the platinum on adsorption of copper. The XANES for UPD copper shows that the adsorbed copper had an oxidation state close to Cu+. XANES features indicate strongly a tetrahedral coordination for the adsorbed Cu species. A reduction in intensity of the white line in the platinum XANES is consistent with a partial filling of empty Pt d-band vacancies on adsorption of copper. Thus, UPD species can modify the electronic structure of a platinum catalyst. The adsorbed Cu+ species are apparently associated with HSO−4 ions.

X-ray linear dichroism of defects in GaN:Mg using hard x-ray nanoprobe

In this study, we report the application of synchrotron radiation nanoprobe techniques to the structural analysis of pyramidal defects in Mg doped GaN. A combination of fluorescence mapping with spectroscopic techniques enabled us to examine not only the elemental nature but also their crystallographic orientation on the submicron scale. Our observations provide strong evidence for the diffusion and subsequent segregation of background impurities of Fe and Cr on the sample surface, with the absence of pronounced sequential stacking compared with Ga atoms. The strong polarization-dependent x-ray absorption near-edge structure features showed the preservation of the hexagonal crystalline structure in both defect-free and hexagonal pyramids. From the spatially resolved x-ray linear dichroism, no preferential disorder was observed in the direction parallel or perpendicular to the crystal growth.

Correlation between local structure and molar ratio of Au (III) complexes in aqueous solution: An XAS investigation

The local and geometrical structure around gold (III) e.g., Au 3+ ions in aqueous solution with different OH −/Cl − molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCl n (OH) 4− n ] − solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl − ligands by OH − ligands. A square–planar geometry for [AuCl n (OH) 4− n ] − with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH −/Cl − molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.

X-ray Absorption Spectroscopy of Hemes and Hemeproteins in Solution: Multiple Scattering Analysis

A full quantitative analysis of Fe K-edge X-ray absorption spectra has been performed for hemes in two porphynato complexes, that is, iron(III) tetraphenylporphyrin chloride (Fe(III)TPPCl) and iron(III) tetraphenylporphyrin bis(imidazole) (Fe(III)TPP(Imid)2), in two protein complexes whose X-ray structure is known at atomic resolution (1.0 A), that is, ferrous deoxy-myoglobin (Fe(II)Mb) and ferric aquo-myoglobin (Fe(III)MbH2O), and in ferric cyano-myoglobin (Fe(III)MbCN), whose X-ray structure is known at lower resolution (1.4 A). The analysis has been performed via the multiple scattering approach, starting from a muffin tin approximation of the molecular potential. The Fe-heme structure has been obtained by analyzing independently the Extended X-ray Absorption Fine Structure (EXAFS) region and the X-ray Absorption Near Edge Structure (XANES) region. The EXAFS structural results are in full agreement with the crystallographic values of the models, with an accuracy of +/- 0.02 A for Fe-ligand distances, and +/-6 degrees for angular parameters. All the XANES features above the theoretical zero energy (in the lower rising edge) are well accounted for by single-channel calculations, for both Fe(II) and Fe(III) hemes, and the Fe-N p distance is determined with the same accuracy as EXAFS. XANES evaluations of Fe-5th and Fe-6th ligand distances are determined with 0.04-0.07 A accuracy; a small discrepancy with EXAFS (0.01 to 0.05 A beyond the statistical error), is found for protein compounds. Concerns from statistical correlation among parameters and multiple minima in the parameter space are discussed. As expected, the XANES accuracy is slightly lower than what was found for polarized XANES on Fe(III)MbCN single crystal (0.03-0.04 A), and states the actual state-of-the-art of XANES analysis when used to extract heme-normal parameters in a solution spectrum dominated by heme-plane scattering.

Mn-rich clusters in GaN: Hexagonal or cubic symmetry?

In this study, we report the application of synchrotron radiation microprobe to the analysis of Mn-rich clusters in GaN. From the Mn and Ga fluorescence line intensity ratio, an average Mn concentration of 11% was deduced. A combination of fluorescence mapping with spectroscopic techniques enabled us to examine not only the cluster elemental nature but also their crystallographic orientation on the submicron scale. The strong polarization-dependent x-ray absorption near-edge structure features showed the preservation of the hexagonal symmetry in both cluster-free and Mn-rich regions. However, from the x-ray absorption data taken inside the clusters, a preferential disorder was observed in the direction parallel to the crystal growth rather than perpendicular to it.

Nickel site distribution and clustering in synthetic double-chain silicates by experimental and theoretical XANES spectroscopy

The crystal-chemical role of Ni in synthetic potassium-richterites within the join ${\mathrm{K}(\mathrm{CaNa})\mathrm{Mg}}_{4}{\mathrm{Ni}[\mathrm{Si}}_{8}{\mathrm{O}}_{22}]{(\mathrm{OH})}_{2}{\ensuremath{-}\mathrm{K}(\mathrm{CaNa})\mathrm{Ni}}_{5}[{\mathrm{Si}}_{8}{\mathrm{O}}_{22}]{(\mathrm{OH})}_{2}$ has been investigated by XAS (x-ray-absorption spectroscopy). From a structural point of view these materials are classified as double-chain silicates and belong to the amphiboles group. X-ray absorption near-edge structure (XANES) spectra recorded at the $\mathrm{Ni}K$ edge show variations that can be related to the Ni-Mg substitution at the octahedral $M$ sites of these amphi-boles. Theoretical XANES spectra, calculated on the basis of the multiple-scattering theory, reproduce very well the experimental spectra, and allow us to understand the relationships between XANES features and the chemical and structural environments around the photoabsorbing ion. The combination of XAS experiments and theoretical calculations provides information on the Ni partitioning between the three octahedral sites and on the local chemistry around the absorber.

Hydrogen Chemisorption on Silica-Supported Pt Clusters: In Situ X-ray Absorption Spectroscopy

Hydrogen chemisorption on small silica-supported Pt clusters was investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy. The clusters were found to exhibit a bulklike Pt first nearest neighbor (NN) distance (2.76 A) and low disorder while covered by chemisorbed hydrogen. In contrast, bare Pt clusters produced by heating in vacuo at 300 °C are characterized by a contracted Pt NN distance (2.66 A) and greater disorder. These effects are reversed by re-exposure of the bare Pt clusters to H2 at 25 °C. The metal−support interface is characterized by a short Pt−O distance, irrespective of the presence of chemisorbed hydrogen. An apparent L3 edge shift of 0.8 eV relative to bulk Pt is observed for the hydrogen-covered clusters. This shift is attributed to a decrease in the Pt L3 edge resonance (white line) intensity, as no corresponding shift is observed at the L2 edge. A hydrogen-related L2,3 XANES feature at 9 eV ap...

X-ray absorption at the Ca K edge in natural-garnet solid solutions: A full-multiple-scattering investigation.

We present an x-ray absorption spectroscopy study at the Ca K edge in several natural-garnet materials with calcium content ranging between 0.2 and 2.9 atoms per formula unit. Comparison of experimental spectra to theoretical calculations based on multiple-scattering theory provides a reliable tool to monitor local environment modifications around the Ca site along the series. Throughout the analysis, special attention has been devoted to determine both the minimum size of the simulation system able to reproduce all the experimental x-ray absorption near-edge structure features, and the best choice for the exchange-correlation contribution to the final-state potential of the photoabsorption process to be used in the ab initio calculations.

X-ray absorption studies of Sr, La and Nd in T- and T′- phase compounds

A detailed theoretical analysis of X-ray-absorption near edge structure (XANES) of La 2CuO 4, La 2− x Sr x CuO 4 and Nd 2CuO 4 compounds in T- and T′-phases at the Sr, La and Nd K-, L 1- and L 3-edges is presented. The calculations, obtained in the framework of multiple scattering theory, are in good agreement with the experimental data. Our analysis demonstrates that XANES features are due both to the geometrical coordination around the photoabsorber and type of final state probed by the photoelectron.

Platinum-Mordenite Catalysts for n-Hexane Isomerization: Characterization by X-Ray Absorption Spectroscopy and Chemical Probes

Abstract Platinum-mordenite (Pt-MOR) catalysts were prepared from NH 4 -MOR by ion exchange with [Pt II (NH 3 ) 4 ][OH] 2 , calcination in O 2 at 350°C, and reduction in H 2 at 350°C. The resultant Pt-H-MOR was active for n -hexane isomerization and hydrocracking via bifunctional catalysis at 240-300°C and 1 atm. The observed activation energies for C 6 branched-isomer formation are unusually low, suggesting that the isomerization rates were controlled by pore diffusion. A Pt-KH-MOR catalyst was prepared by ion exchange with aqueous KNO 3 and re-reduction at 350°C; elemental analysis evidenced 90% exchange of protons for K + ions. The product distribution and observed activation energies for C 6 branched-isomer formation over Pt-KH-MOR are consistent with n -hexane isomerization via bifunctional catalysis. Hydrocracking was strongly suppressed, and light hydrocarbons were formed primarily by Pt-catalyzed hydrogenolysis. From in situ extended X-ray absorption fine structure spectroscopy and H 2 temperature-programmed desorption, we conclude that the Pt-MOR catalysts consist of small Pt clusters hosted within the mordenite crystals. The Pt L III X-ray absorption near-edge structure (XANES) spectra of Pt-H-MOR and Pt-KH-MOR are closely similar, suggesting that the electronic structure of the Pt clusters is unaffected by mordenite acid-base chemistry. For the freshly reduced catalysts, a XANES feature at 10 eV relative energy is assigned to Pt-H antibonding states. The infrared spectrum of CO adsorbed on Pt-H-MOR contains an intense band at 2084 cm −1 , which is assigned to linear CO moieties on Pt clusters. A small peak at 2124 cm −1 is assigned to isolated Pt I -CO species, which we infer are formed by oxidative fragmentation of Pt clusters. The infrared spectrum of CO adsorbed on Pt-KH-MOR evidences a red shift of the linear CO band, which we suggest is due to electrostatic interactions between carbonyl O atoms and nearby K + ions.

Fluorescence Yield Xanes and Exafs Experiments: Application to Highly Dilute and Surface Samples

The development of intense synchrotron radiation sources during the last twenty years has enabled several types of x-ray spectroscopy and scattering techniques to come into practical use. One of the most significant methods for the characterization of extremely dilute samples is high resolution x-ray absorption-edge spectroscopy. The technique is usually divided into two separate methods according to whether the x-ray absorption near edge structure (XANES) or the extended x-ray absorption fine structure (EXAFS) is analyzed. XANES features are due mainly to bound-state electronic transitions just below, and on the low energy side of the edge, and to multiple scattering resonances on the top of the edge and at somewhat higher energies. EXAFS features are oscillations due to ejected photoelectron back scattering interference processes in the close vicinity of the absorber atom. XANES analysis is used to determine atom valence, atom site distortion from regular geometries, and other details of the atom site. EXAFS features can be analyzed to recover interatomic distances between the absorber atom and its first few shells of neighbors, as well as the number and types of these neighbors. Together these techniques can provide an atomspecific probe of die short-range structure within almost any type of condensed matter.

An XANES study of carbides of molybdenum and tungsten

The X-ray absorption near edge structure (XANES) of molybdenum and tungsten compounds has been studied by using synchrotron radiation. Substantial difference in XANES features between metals and their carbides suggested significant difference in electronic structure between them. The shift in edge position indicated charge transfer from metal to carbon when a carbide is formed. In light of a modern band theory, the larger white line areas for carbides could be interpreted as due to some of the unoccupied d-states which are not measured by x-ray white line area.

XANES study of underpotential deposited copper on carbon-supported platinum

Underpotential deposited (UPD) copper on carbon-supported platinum was investigated in-situ in 0.5 M H 2SO 4 at 0.05 V (SCE) by XANES (X-ray Absorption Near Edge Structure). By taking XANES spectra at the Cu K-edge and the Pt L III-edge it was possible to determine the valence state of the copper and observe modifications in the electronic structure of the platinum on adsorption of copper. The XANES for UPD copper shows that the adsorbed copper had an oxidation state close to Cu +. XANES features indicate strongly a tetrahedral coordination for the adsorbed Cu species. A reduction in intensity of the white line in the platinum XANES is consistent with a partial filling of empty Pt d-band vacancies on adsorption of copper. Thus, UPD species can modify the electronic structure of a platinum catalyst. The adsorbed Cu + species are apparently associated with HSO − 4 ions.