Edge X-ray Absorption Spectroscopy Research Articles

PH impact on reductive dechlorination of cis-dichloroethylene by Fe precipitates: An X-ray absorption spectroscopy study

The pH impact on reductive dechlorination of cis-dichloroethylene (cis-DCE) was investigated using in situ Fe precipitates formed under iron-rich sulfate-reducing conditions. The dechlorination rate of cis-DCE increased with pH, which was attributed to changes in the solid-phase Fe concentration, the composition of Fe minerals, and the surface speciation of Fe minerals. With increasing pH, larger quantities of Fe minerals, having much greater reactivity than dissolved Fe(II), were produced. Fe–K edge X-ray absorption spectroscopy (XAS) analysis of Fe precipitates revealed the presence of multiple Fe phases with their composition varying with pH. Correlation analyses were performed to examine how the solid-phase Fe concentration, the composition of Fe minerals, and their surface speciation were linked with the cis-DCE dechlorination rate. Such analyses revealed that neither mackinawite (FeS) nor magnetite (Fe3O4) was reactive with cis-DCE dechlorination, but that Fe (oxyhydr)oxides including green rusts and Fe(OH)2 were reactive. Based on a proposed model of the surface acidity of Fe minerals, the increasing deprotonated surface Fe(II) groups with pH correlated well with the enhanced cis-DCE dechlorination.

A disordered nanoparticle model for 6-line ferrihydrite

Much of the bioavailable and geochemically reactive iron in aerobic, circumneutral settings is frequently found in the form of nanoscale particles of a hydrated iron(III) oxyhydroxide phase known as ferrihydrite. Developing useful structural descriptions of defective nanophases such as ferrihydrite has long posed significant challenges. Recently, Michel et al. (2007, 2010) proposed a structural model for ferrihydrite in place of the long-accepted model of Drits et al. (1993). Both models reproduce to high accuracy certain forms of X-ray scattering data from powdered ferrihydrite. However, discrepancies remain that we hypothesized are due to forms of structural disorder not easily represented by existing models. To test this hypothesis, we performed a novel structural analysis of total X-ray scattering data acquired from 6-line ferrihydrite. We generated three candidate whole-nanoparticle models of ferrihydrite composed of a two-phase Drits model, the Michel model, and a hybrid phase based on a single-phase Drits model that incorporated tetrahedral Fe sites, creating a lattice in which the Michel model was one of many possible topologies. We implemented a reverse Monte Carlo (RMC) approach to explore alternative configurations of iron occupancies plus structural disorder, and to refine the nanoparticle structure using both the reciprocal and real-space forms of the X-ray scattering data. We additionally used oxygen K -edge X-ray absorption spectroscopy to semi-quantitatively assess the ratio of protonated:non-protonated oxygen sites in an iron(III) oxides. This analysis provides independent evidence for a significantly lower OH:O stoichiometric ratio for ferrihydrite than for goethite, further constraining the RMC models. The hybrid structure model gave better agreement to the experimental total scattering data than nanoparticles based upon either the Michel or Drits models. Models that incorporated tetrahedrally coordinated iron sites consistently achieved better matches to the data than models containing face-sharing octahedra. Long-range vacancy disorder was essential for optimum fits to the scattering data, highlighting the utility of whole-nanoparticle models in place of unit-cell models with random distributions of iron vacancies. The RMC-derived structures do not satisfy all experimental constraints on composition and structure. Nevertheless this work illustrates that a suitably constrained RMC method applied to whole-nanoparticle models can be an effective approach for exploring disorder in nanocrystalline materials.

Alpha particle damage in biotite characterized by microfocus X-ray diffraction and Fe K-edge X-ray absorption spectroscopy

AbstractCombined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total α-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe–Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the 24He2+, the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.

Electronic structure of the hole-doped delafossite oxides CuCr1−xMgxO2

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr${}_{1\ensuremath{-}x}$Mg${}_{x}$O${}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.03$) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation $+U$ ($\mathrm{LDA}+U$) scheme. Cr/Cu 3$p$-$3d$ resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr $3d$ character with a minor contribution from the Cu $3d$ through Cu $3d\ensuremath{-}$O $2p\ensuremath{-}$Cr $3d$ hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr $3d$ character. Cr $2p$ PES and $L$-edge XAS spectra exhibit typical Cr${}^{3+}$ features for all $x$, while the Cu $L$-edge XAS spectra exhibited a systematic change with $x$. This indicates now that the Cu valence is monovalent at $x=0$ and the doped hole should have Cu $3d$ character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu${}^{+}$ ($3{d}^{10}$) and Cu${}^{2+}$ ($3{d}^{9}$) like initial states in Cu $2p$-$3d$ resonant PES spectrum of at $x=0$, while Cu $2p$ PES spectra with no doubt shows the Cu${}^{+}$ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu $4s$ state, but also finite Cu $3d,4s\ensuremath{-}$Cr $3d$ charge transfer via O $2p$ states in the ground-state electronic configuration.

Comment on “Femtosecond laser-induced modification of potassium-magnesium silicate glasses: An analysis of structural changes by near edge x-ray absorption spectroscopy” [Appl. Phys. Lett. 100, 224101 (2012)

potassium-magnesium silicate glasses: An analysis of structural changes by near edge x-ray absorption spectroscopy” [Appl. Phys. Lett. 100, 224101 (2012)] Delphine Cabaret, St! ephanie Rossano, and Nicolas Trcera Institut de Min! eralogie et de Physique des Milieux Condens! es, Universit! e Pierre et Marie Curie (UPMC), UMR CNRS 7590, c. c. 115, 4 Place Jussieu, 75252 Paris Cedex 05, France Laboratoire G! eomat! eriaux et Environnement, Universit! e Paris EST, EA 4508, 5 Bd Descartes, Champs sur Marne, 77454 Noisy-Champs Cedex 2, France Synchrotron SOLEIL, BP 48, 91192 Gif sur Yvette, France

Response to “Comment on ‘Femtosecond laser-induced modification of potassium-magnesium silicate glasses: An analysis of structural changes by near edge x-ray absorption spectroscopy’” [Appl. Phys. Lett. 102, 196101 (2013)

First Page

On-the-fly Scan: Improving the Performance of Absorption Spectrum Measurement

The efficiency of conventional point-to-point scan is limited by overheads from mechanical system and data acquisition system. With network-friendly development interface provided by commercial software and hardware, the DAQ system, the position encoding system, the undulator, and the monochromator system can be easily controlled by synchronizing various instruments for on-the-fly scanning operation. In this work, an on-the-fly scanning method is developed for absorption spectrum measurement under the Labview's programing environment. We have tested its performance on Dragon type beamlines at NSRRC. For a given energy resolution and a S/N level our preliminary results show that the time required to obtain a spectrum of Near Edge X-ray Absorption Fine Structure (NEXAFS) and X-ray Absorption Spectroscopy (XAS) using the on-the-fly scanning system is about 10 times and 20 times shorter than those using the point-to-point scanning with bending magnet and undulator sources, respectively. The higher efficiency gained from the on-the-fly scanning system results from minimizing system communication time and mechanical system overheads. The new improved technique should significantly contribute to the study and measurement of photon dose sensitive samples.

X-ray absorption and magnetic circular dichroism characterization of Fe-doped [formula omitted] thin films

We report x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements of both paramagnetic and ferromagnetic Ti1−xFexO2−δ thin films. Fe L3,2 edge XAS spectra showed that the Fe ions in our samples were in mixed valence states and that the Fe2+/Fe3+ ratio decreased with increasing oxygen partial pressure. Finite XMCD signals were observed only for the Fe2+ state in the ferromagnetic sample. These findings indicate that the observed ferromagnetism in Ti1−xFexO2−δ originates from ferromagnetic interactions between Fe2+ local spins.

Characterization of thermally treated Co2+-exchanged zeolite X

This study investigated thermal stabilization of Co2+-exchanged zeolite X (CoX) using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and leaching tests. From SEM-EDX analysis, cobalt was dispersed randomly at ≤600°C, suggesting its presence as an extraframework cation in exchange sites. At ≥800°C, cobalt was locally concentrated with Al on the vitreous surface. Consistent with such observations, XRD data indicated that CoX maintained the zeolite framework at ≤600°C, and that it became vitrified and transformed to nepheline (NaAlSiO4(s)) and cobalt aluminate (CoAl2O4(s)) at ≥800°C. Cobalt-K edge XAS was subjected to both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analyses. In XANES spectra, the pre-edge peaks and edge-shoulders, characteristic of 4-fold coordinated cobalt (e.g., CoAl2O4(s)), were not evident at ≤600°C, but such features were strong at ≥800°C. The EXAFS spectra of CoX at ≤600°C lacked in the coordination shells beyond the first CoO shell. In contrast, CoX at ≥800°C showed the EXAFS spectra similar to CoAl2O4(s). Taken together, cobalt was likely present as 6-fold coordinated Co2+ in exchange sites at ≤600°C and mainly incorporated into a non-exchangeable CoAl2O4-like phase in both vitreous and crystalline forms at ≥800°C. In agreement with this proposition, leaching tests with concentrated CaCl2 solutions supported the greater stability of cobalt at ≥800°C.

High temperature extended x-ray absorption fine structure study of multiferroic BiFeO3

Local atomic structure modifications around Fe atoms in polycrystalline multiferroic BiFeO3 are studied by Fe K edge x-ray absorption spectroscopy as a function of temperature across the Néel temperature (TN = 643 K) in order to reveal local structure modifications related to the magnetic transition. This work demonstrates that on crossing TN the local structure around Fe shows peculiar changes: the Fe–O bond lengths get shorter, the ligand symmetry increases and the Fe–O bond length disorder (σ2) deviates from Debye behaviour. These results suggest that the structural transition at the ferroelectric Curie temperature (TC = 1103 K) is anticipated by early local rearrangement of the structure starting already at TN.

Femtosecond laser-induced modification of potassium-magnesium silicate glasses: An analysis of structural changes by near edge x-ray absorption spectroscopy

The effects of femtosecond laser pulse irradiation on the glass structure of alkaline silicate glasses were investigated by x-ray absorption near edge structure spectroscopy using the beamline of the Physikalisch-Technische Bundesanstalt at the electron synchrotron BESSY II in Berlin (Germany) by analyzing the magnesium K-edge absorption peak for different laser fluences. The application of fluences above the material modification threshold (2.1 J/cm2) leads to a characteristic shift of ∼1.0 eV in the K-edge revealing a reduced (∼3%) mean magnesium bond length to the ligated oxygen ions (Mg-O) along with a reduced average coordination number of the Mg ions.

Formation of Layered Fe(II)–Al(III)-Hydroxides during Reaction of Fe(II) with Aluminum Oxide

The reactivity of aqueous Fe(II) with aluminum oxide in anoxic solutions was investigated with batch kinetic experiments combined with Fe K edge X-ray absorption spectroscopy measurements to characterize Fe(II) sorption products. Formation of Fe(II)-Al(III)-layered double hydroxides with an octahedral sheet structure similar to nikischerite (NaFe(II)(6) Al(3)(SO(4))(2)(OH)(18) (H(2)O)(12)) was observed within a few hours during sorption at pH 7.5 and aqueous Fe(II) concentrations of 1-3 mM. These Fe(II) phases are composed of brucite-like Fe(II)(OH)(2) sheets with partial substitution of Al(III) for Fe(II), charge balanced by anions coordinated along the basal planes. Their fast rate of formation suggests that these previously unrecognized Fe(II) phases, which are structurally and compositionally similar to green rust, may be an important sink of Fe(II) in suboxic and anoxic geochemical environments, and impact the fate of structurally compatible trace metals, such as Co(II), Ni(II), and Zn(II), as well as redox-reactive species including Cr(VI) and U(VI). Further studies are required to assess the thermodynamics, formation kinetics, and stability of these Fe(II) minerals under field conditions.

Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite

Ferrihydrite is a common iron hydroxide nanomineral commonly found in soils, sediments, and surface waters. Reactivity with this important environmental surface often controls the fate and mobility of both essential nutrients and inorganic contaminants. Despite the critical role of ferrihydrite in environmental geochemistry, its structure is still debated. In this work, we apply bulk sensitive Fe L edge X-ray absorption spectroscopy to study the crystal field environment of the Fe in ferrihydrite and other Fe oxides of known structure. This direct probe of the local electronic structure provides verification of the presence of tetrahedrally coordinated Fe(III) in the structure of ferrihydrite and puts to rest the controversy on this issue.

Element-specific and bulk magnetism, electronic, and crystal structures of La0.70Ca0.30Mn1−xCrxO3

The magnetic interactions in La${}_{0.70}$Ca${}_{0.30}$Mn${}_{1\ensuremath{-}x}$Cr${}_{x}$O${}_{3}$ ($x=0.15$, 0.50, and 0.70) are investigated by x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), high-resolution x-ray powder diffraction, and bulk magnetization measurements. XAS in the Mn and Cr ${L}_{2,3}$ edges support stable single valent Cr${}^{3+}$ ions and a varying Mn valence state with $x$, while the O $K$ edge XAS spectrum reveals local maxima in the O $2p$ density of states close to the Fermi level due to mixing with Mn and Cr $3d$ states. A robust antiferromagnetic state is found for $x=0.70$ below ${T}_{N}=258$ K. For $x=0.15$, combined XMCD and bulk magnetization measurements indicate a fully polarized ferrimagnetic state for the Mn and Cr spins below ${T}_{c}=224$ K. For $x=0.50$, a reduced ferrimagnetic component dominated by Mn spins is present below ${T}_{c}=154$ K. No evidence of lattice anomalies due to cooperative charge and orbital orderings is found by x-ray diffraction for all samples. The magnetic properties of this system are rationalized in terms of a competition of ferromagnetic Mn-Mn double exchange and antiferromagnetic Cr-Cr and Cr-Mn superexchange interactions.

BaVS3 probed by V L edge x-ray absorption spectroscopy

Polarization dependent vanadium L edge x-ray absorption spectra of BaVS3 single crystals are measured in the four phases of the compound. The difference between signals with the polarizations E⊥c and E ∥ c (linear dichroism) changes with temperature. Besides increasing the intensity of one of the maxima, a new structure appears in the pre-edge region below the metal–insulator transition. More careful examination brings to light that the changes start already with pretransitional charge density wave fluctuations. Simple symmetry analysis suggests that the effect is related to rearrangements in the Eg and A1g states, and is compatible with the formation of four inequivalent V-sites along the V–S chain.

Using a Dual Plasma Process to Produce Cobalt-Polypyrrole Catalysts for the Oxygen Reduction Reaction in Fuel Cells: II. Analysing the Chemical Structure of the Films

The chemical structure of cobalt–polypyrrole – produced by a dual plasma process – is analyzed by means of X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption spectroscopy (NEXAFS), X-ray diffraction (XRD), energy-dispersive X-Ray spectroscopy (EDX) and extended X-ray absorption spectroscopy (EXAFS). It is shown that only nanoparticles of a size of 3 nm with the low temperature crystal structure of cobalt are present within the compound. Besides that, cobalt–nitrogen and carbon–oxygen structures are observed. Furthermore, more and more cobalt–nitrogen structures are produced when increasing the magnetron power. Linking the information on the chemical structure to the results about the catalytic activity of the films – which are presented in part I of this contribution – it is concluded that the cobalt–nitrogen structures are the probable catalytically active sites. The cobalt–nitrogen bond length is calculated as 2.09 Å and the carbon–nitrogen bond length as 1.38 Å.

Binding of HgIIto High-Affinity Sites on Bacteria Inhibits Reduction to Hg0by Mixed FeII/IIIPhases

Magnetite and green rust have been shown to reduce aqueous Hg(II) to Hg(0). In this study, we tested the ability of magnetite and green rust to reduce Hg(II) sorbed to 2 g · L(-1) of biomass (Bacillus subtilis), at high (50 μM) and low (5 μM) Hg loadings and at pH 6.5 and 5.0. At high Hg:biomass loading, where Hg(II) binding to biomass is predominantly through carboxyl functional groups, Hg L(III)-edge X-ray absorption spectroscopy showed reduction of Hg(II) to Hg(0) by magnetite. Reduction occurred within 2 h and 2 d at pH 6.5 and 5.0, respectively. At low Hg:biomass loading, where Hg(II) binds to biomass via sulfhydryl functional groups, Hg(II) was not reduced by magnetite at pH 6.5 or 5.0 after 2 months of reaction. Green rust, which is generally a stronger reductant than magnetite, reduced about 20% of the total Hg(II) bound to biomass via sulfhydryl groups to Hg(0) in 2 d. These results suggest that Hg(II) binding to carboxyl groups does not significantly inhibit the reduction of Hg(II) by magnetite. However, the binding of Hg(II) to biomass via sulfhydryl groups severely inhibits the ability of mixed Fe(II/III) phases like magnetite and green rust to reduce Hg(II) to Hg(0). The mobility of heavy metal contaminants in aquatic and terrestrial environments is greatly influenced by their speciation, especially their oxidation state. In the case of Hg, reduction of Hg(II) to Hg(0) can increase Hg mobility because of the volatility of Hg(0). Since Hg is typically present in aquatic and terrestrial systems at low concentrations, binding of Hg(II) to high-affinity sites on bacteria could have important implications for the potential reduction of Hg(II) to Hg(0) and the overall mobility of Hg in biostimulated subsurface environments.

Electronic Structure Investigation of Highly Compressed Aluminum withKEdge Absorption Spectroscopy

The electronic structure evolution of highly compressed aluminum has been investigated using time resolved K edge x-ray absorption spectroscopy. A long laser pulse (500 ps, I(L)≈8×10(13) W/cm(2)) was used to create a uniform shock. A second ps pulse (I(L)≈10(17) W/cm(2)) generated an ultrashort broadband x-ray source near the Al K edge. The main target was designed to probe aluminum at reshocked conditions up to now unexplored (3 times the solid density and temperatures around 8 eV). The hydrodynamical conditions were obtained using rear side visible diagnostics. Data were compared to ab initio and dense plasma calculations, indicating potential improvements in either description. This comparison shows that x-ray-absorption near-edge structure measurements provide a unique capability to probe matter at these extreme conditions and severally constrains theoretical approaches currently used.

X-Ray Absorption SpectroscopicStudies of Platinum Speciation in Fresh and Road Aged Light-Duty Diesel Vehicle Emission Control Catalysts

The species present in a variety of fresh and road aged light-duty diesel catalysts were determined by platinum L3 edge X-ray absorption spectroscopy (XAS). It was found that it is not sufficient to use the analysis of X-ray absorption near edge structure (XANES) alone to determine the nature of species present in fresh and road aged catalysts. Detailed analysis of the extended X-ray absorption fine structure (EXAFS) revealed the presence of a mixture of oxidic and metallic species in the fresh catalysts. Metallic components were predominantly found in the road aged catalysts. The present study did not find any chloroplatinate species in the systems investigated.

Understanding the Electronic Structure of 4d Metal Complexes: From Molecular Spinors to L-Edge Spectra of a di-Ru Catalyst

L(2,3)-edge X-ray absorption spectroscopy (XAS) has demonstrated unique capabilities for the analysis of the electronic structure of di-Ru complexes such as the blue dimer cis,cis-[Ru(III)(2)O(H(2)O)(2)(bpy)(4)](4+) water oxidation catalyst. Spectra of the blue dimer and the monomeric [Ru(NH(3))(6)](3+) model complex show considerably different splitting of the Ru L(2,3) absorption edge, which reflects changes in the relative energies of the Ru 4d orbitals caused by hybridization with a bridging ligand and spin-orbit coupling effects. To aid the interpretation of spectroscopic data, we developed a new approach, which computes L(2,3)-edges XAS spectra as dipole transitions between molecular spinors of 4d transition metal complexes. This allows for careful inclusion of the spin-orbit coupling effects and the hybridization of the Ru 4d and ligand orbitals. The obtained theoretical Ru L(2,3)-edge spectra are in close agreement with experiment. Critically, existing single-electron methods (FEFF, FDMNES) broadly used to simulate XAS could not reproduce the experimental Ru L-edge spectra for the [Ru(NH(3))(6)](3+) model complex nor for the blue dimer, while charge transfer multiplet (CTM) calculations were not applicable due to the complexity and low symmetry of the blue dimer water oxidation catalyst. We demonstrated that L-edge spectroscopy is informative for analysis of bridging metal complexes. The developed computational approach enhances L-edge spectroscopy as a tool for analysis of the electronic structures of complexes, materials, catalysts, and reactive intermediates with 4d transition metals.