Showing papers on "Extended X-ray absorption fine structure published in 2007"

A synthetic precedent for the [FeIV2(μ-O)2] diamond core proposed for methane monooxygenase intermediate Q

Abstract: Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe(IV)(2)(mu-O)(2)] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc(+/0)) has been performed in MeCN at -40 degrees C on a valence-delocalized [Fe(III)Fe(IV)(mu-O)(2)(L(b))(2)](3+) complex (1b) (E(1/2) = 760 mV versus Fc(+/0)). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with lambda(max) at 485 nm (9,800 M(-1).cm(-1)) and 875 nm (2,200 M(-1).cm(-1)). The 4.2 K Mossbauer spectrum of 2b exhibits a quadrupole doublet with delta = -0.04(1) mm.s(-1) and DeltaE(Q) = 2.09(2) mm.s(-1), parameters typical of an iron(IV) center. The Mossbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe(2)(mu-O)(2)] core at 674 cm(-1), downshifting 30 cm(-1) upon (18)O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 A and an Fe scatterer at 2.73 A. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [Fe(IV)(2)(mu-O)(2)(L(b))(2)](4+), the first synthetic complex with an [Fe(IV)(2)(mu-O)(2)] core. A comparison of 2b and its mononuclear analog [Fe(IV)(O)(L(b))(NCMe)](2+) (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak C H bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.

Nearest-neighbor oxygen distances in liquid water and ice observed by x-ray Raman based extended x-ray absorption fine structure.

Abstract: We report the nearest-neighbor oxygen-oxygen radial distribution function (NN O-O RDF) of room temperature liquid water and polycrystalline ice Ih (-16.8 degrees C) obtained by x-ray Raman based extended x-ray absorption fine structure (EXAFS) spectroscopy. The spectra of the two systems were taken under identical experimental conditions using the same procedures to obtain the NN O-O RDFs. This protocol ensured a measurement of the relative distance distribution with very small systematic errors. The NN O-O RDF of water is found to be more asymmetric (tail extending to longer distances) with longer average distance (2.81 A for water and 2.76 A for ice) but a slightly shorter peak position (2.70 A for water and 2.71 A for ice). The refinement also showed a small but significant contribution from the linear O-H-O multiple scattering signal. The high sensitivity to short range distances of the EXAFS probe will set further restrictions to the range of possible models of liquid water.

Coordination of Sr and Mg in calcite and aragonite

Abstract: Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeo-environmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2%) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5% dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg–O bond distances of ~2.12 A, which are much smaller than the Ca–O bond distance of 2.35 A but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain nanodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.08 A) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite.

13 – Superlubricity of Molybdenum Disulfide

Abstract: Publisher Summary This chapter investigates the chemical composition of molybdenum disulfide (MoS2) coatings. High resolution TEM (HR-TEM) and extended X-ray absorption fine structure (EXAFS) are used to investigate the short-range order in the MoS2 crystallites of the coating. HR-TEM studies show that most of MoS2 crystallites exhibit many imperfections including faults, kinks, and curvature. Most of the vacuum-deposited MoS2 coatings contain significant amounts of oxygen in their structure, mainly because small amounts of water are present during the sputtering process. The oxygen incorporation affects the crystalline structure, the orientation, and the film morphology. A typical shift of the (100) peak is observed in X-ray diffraction and is strongly correlated with the oxygen content in the film. This shift has been attributed to the (100) lattice contraction. It is also correlated with a lattice expansion of the (002) basal planes in the c direction. For higher values of the oxygen content, molybdenum oxides (MoOy) are formed.

Surface complex structures modelled with quantum chemical calculations: carbonate, phosphate, sulphate, arsenate and arsenite

Abstract: Summary Hybrid molecular orbital/density functional theory (MO/DFT) calculations on molecular clusters were used to model infrared (IR) vibrational frequencies and interatomic distances obtained via extended X-ray absorption fine structure (EXAFS) spectroscopy. Molecular clusters were found to provide good agreement with experimental observations for the oxyanions carbonate, phosphate, sulphate, arsenate and arsenite. The results show a consistent tendency to form bidentate bridging surface complexes at low pH, but the protonation and hydration states play a significant role in the results obtained from calculation as various protonation states of monodentate surface complexes are also predicted to be stable as pH increases (i.e. the number of H þ ions in the model are decreased). A method for estimating the Gibbs free energy of adsorption (DGads) is discussed to complement the comparisons of experimental and theoretical spectroscopic parameters. Calculated DGads values were consistent with the interpretations based on modelling observed spectra.

Speciation of Fe in silicate glasses and melts by in-situ XANES spectroscopy

Abstract: In-situ X-ray absorption spectroscopy at the Fe K -edge was used to characterize the local structural environment of Fe 2+ and Fe 3+ in silicate melts at high temperature (up to 1050 °C) in comparison to their quenched glassy analog at room temperature. Measurements were performed on binary alkali-silicate compositions and on haplogranitic compositions, which were doped with about 5 wt% Fe 2 O 3 . Changes in the structural environment of Fe were evaluated by analyzing both the pre-edge feature and the first maximum of the EXAFS of the spectra. In most cases, the spectra collected at high temperature differed from those of the quenched samples. At reducing conditions, the melts showed slightly higher amounts of low-coordinated Fe 2+ than their glassy counterparts. This finding is consistent with results of earlier studies (e.g., Jackson et al. 1993), but the observed change in speciation is smaller than reported by these authors. At oxidizing conditions, glasses and melts displayed a more heterogeneous behavior. The spectra of alkali-silicate compositions indicate higher amounts of low-coordinated Fe 3+ in the melt, whereas no significant difference between melt and glass was observed for Fe 3+ in haplogranitic compositions, even if the latter are peralkaline. The amount of non-bridging O atoms in the glass/melt system appears to play an important role particularly for Fe 3+ . However, more complex relationships between Fe and other structural components, especially Al, are possible.

Optical absorption studies of copper phthalocyanine thin films

Abstract: The optical absorption of thermally evaporated copper phthalocyanine (CuPc) was studied in the UV-VIS-NIR region. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthalocyanine molecules, namely, the Soret ( B ) and the Q -band . The Q -band shows its characteristic splitting (Davydov splitting) of the main absorption peak in the metal phthalocyanine correlates with the relative tendencies of the metal to out-of-plane bonding. Some of the important spectral characteristics such as the molar extinction coefficient (e molar ), the oscillator strength ( f) , the electric dipole strength ( q 2 ) and absorption half-bandwidth (Δ λ ) of the principle optical transitions were evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.95±0.03 and 1.55±0.02 eV. The spectra of the infrared absorption allow characterization of vibration modes for the powder, as-deposited material and thin films of CuPc annealed at 423 K for two hours. Discussion of the obtained results and their comparison with the previous published data are also given.

Structural characterization of the Co2FeZ (Z=Al, Si, Ga, and Ge) Heusler compounds by x-ray diffraction and extended x-ray absorption fine structure spectroscopy

Abstract: This work reports on the structure of Fe containing, Co2-based Heusler compounds that are suitable for magnetoelectronic applications. The compounds Co2FeZ (where Z=Al, Si, Ga, and Ge) were investigated using the x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For compounds containing Ga or Ge, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the EXAFS data indicated that both compounds crystallize in the L21 structure.

Formation and Oxidation of Nanosized Metal Particles by Electrochemical Reaction of Li and Na with NiCo2O4: X-ray Absorption Spectroscopic Study

Abstract: X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to investigate the electrochemical reactions of NiCo2O4 in lithium and sodium test cells. Nanosized particles of NiCo2O4 were prepared by the thermal decomposition of a mixed oxalate precursor and used as active electrode material. Ni K and Co K XANES results give evidence of the successive steps in the reduction mechanism of the oxide during the first cell discharge. In a first step NiCo2O4 reacts with lithium and sodium and the reduction of both Ni3+ and Co3+ to the 2+ oxidation state is shown by a peak shift to lower energy values. As a result, sodium oxide or lithium oxide and the transition metal monoxides are formed. Second, metallic products are formed, in which the average coordination numbers of the nearest neighbors for Ni and Co derived from the EXAFS data are 4 and 5, respectively, associated with ca. 1 nm particle size. These novel ultrafine particles are stabilized in the Li2O matrix of the discharged electrodes. Reoxidation of the metallic products up to a divalent state of Ni and Co in the form of monoxides is achieved during the charge process of the electrochemical cells.

Doped V2O5-Based Cathode Materials: Where Does the Doping Metal Go? An X-ray Absorption Spectroscopy Study

Abstract: X-ray absorption spectroscopy (XAS) has been used to probe the local structure of doped vanadium pentoxide materials prepared through sol–gel processes. The doped samples have been analyzed at the vanadium K-edge as well as at the doping metal K-edge (copper and zinc). The presence of two metal sites gives two independent absorption signals, even if we are investigating the same compound; for this reason the reliability of the analysis is enhanced. The strategy in determining the local structure will be discussed. The local structural modifications resulting from low to high doping concentration in the best performing copper-doped V2O5 aerogel like material is reported. Similar measurements were done using zinc as doping metal, in both aerogel-like and xerogel form. The EXAFS analysis shows that the vanadium atomic environment is not altered by the doping metal insertion and that the doping metals (Cu and Zn) are found to be in the same site. Copper and zinc are 4-fold coordinated by almost coplanar oxyge...

Fundamentals of Nanoscale Film Analysis

Abstract: An Overview: Concepts, Units, and the Bohr Atom.- Atomic Collisions and Backscattering Spectrometry.- Energy Loss of Light Ions and Backscattering Depth Profiles.- Sputter Depth Profiles and Secondary Ion Mass Spectroscopy.- Ion Channeling.- Electron-Electron Interactions and the Depth Sensitivity of Electron Spectroscopies.- X-ray Diffraction.- Electron Diffraction.- Photon Absorption in Solids and EXAFS.- X-ray Photoelectron Spectroscopy.- Radiative Transitions and the Electron Microprobe.- Nonradiative Transitions and Auger Electron Spectroscopy.- Nuclear Techniques: Activation Analysis and Prompt Radiation Analysis.- Scanning Probe Microscopy.

Electrical and magnetic properties of chemically derived nanocrystalline cobalt ferrite

Abstract: Nanocrystalline cobalt ferrite particles of 8nm grain size were synthesized by coprecipitation technique and subsequently suitably heat treated to obtain higher grain sizes The experimentally observed changes in the dc electrical conductivity and Curie temperature with heat treatment have been attributed to the changes in the cation distributions as obtained from the Mossbauer and extended x-ray absorption fine structure (EXAFS) measurements and to the grain size The activation energies for conduction as determined from the Arrhenius plots suggest that the conductivity is due to hopping of both electrons and holes The observed decrease in conductivity when the grain size is increased from 8to92nm is clearly due to the predominant effect of migration of some of the Fe3+ ions from octahedral to tetrahedral sites, as is evident from in-field Mossbauer and EXAFS measurements But the higher conductivity of the 102 and 123nm particles compared to that of the 92nm particles is attributed to the higher grain

Active Ti Species in TiCl3-Doped NaAlH4. Mechanism for Catalyst Deactivation

Abstract: The nature of the active Ti species in TiCl3-doped NaAlH4, a promising hydrogen storage material, was studied as a function of the desorption temperature with Ti K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, Ti K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and X-ray diffraction (XRD). In the freshly prepared sample, Ti was amorphous and surrounded by 4.8 Al atoms divided between two shells at 2.71 and 2.89 A. In the next shell, 1.9 Ti atoms were detected at 3.52 A. It was concluded that 30% of Ti was incorporated into the surface of Al crystallites and 70% of Ti occupied interstitials in the NaAlH4 lattice, possibly forming trimeric, triangular Ti entities. After hydrogen desorption at 125 °C, NaAlH4 decomposed and the Ti−Al coordination number increased from 4.8 to 8.5. We propose that all Ti is incorporated into the surface layer of the formed Al. After the material was heated to 225 °C, the local structure of Ti, as inferred from EXAFS and XANES spectroscopy,...

Synthesis and characterization of the Keggin-type ruthenium-nitrido derivative [PW11O39{RuN}]4- and evidence of its electrophilic reactivity.

Abstract: The ruthenium-nitrido POM derivative [PW11O39{RuVIN}]4- has been synthesized by reaction between [PW11O39]7- and [RuVINCl5]2- or [RuVINCl4]-. Its molecular structure has been confirmed from multinuclear 31P and 183W NMR spectroscopy together with an EXAFS study, while the oxidation state of the ruthenium bearing the nitrido ligand has been inferred both from 183W NMR and XANES analysis at the Ru-K edge. The potential of [PW11O39{RuVIN}]4- in N-atom transfer reactions has been demonstrated through reaction with triphenylphosphine, which ultimately leads to the release of the bis(triphenylphosphane)iminium cation [PPh3NPPh3]+ through several intermediates, among which the phosphoraniminato derivative [PW11O39{RuVNPh3}]3- has been structurally characterized. Its unusual oxidation state is in accordance with its EPR spectrum.

Anatase-TiO2 nanomaterials: analysis of key parameters controlling crystallization.

Abstract: Nanoparticulated TiO2 materials with anatase structure were synthesized by using a microemulsion method. The structural characteristics of the amorphous solid precursors and their evolution during thermal treatments were studied by using X-ray absorption structure (X-ray absorption near edge structure XANES and extended X-ray absorption fine structure EXAFS), XRD-PDF (X-ray diffraction-pair distribution function), and infrared spectroscopy. Concerning the precursor materials, XANES and EXAFS showed a local order closely related to that of the anatase structure but containing defective, undercoordinated Ti5c4+ species in addition to normal Ti6c4+ species. The PDF technique detects differences among samples in the local order (below 1 nm) and showed that primary particle size varies throughout the amorphous precursor series. The physical interpretation of results concerning the amorphous materials and their evolution under thermal treatment gives conclusive evidence that local, intraparticle ordering variations determine the temperature for the onset of the nucleation process and drive the solid behavior through the whole crystallization process. The significance of this result in the context of current crystallization theories of oxide-based nanocrystalline solids is discussed.

Uranyl Coordination in Ionic Liquids: The Competition between Ionic Liquid Anions, Uranyl Counterions, and Cl- Anions Investigated by Extended X-ray Absorption Fine Structure and UV−Visible Spectroscopies and Molecular Dynamics Simulations

Abstract: The first coordination sphere of the uranyl cation in room-temperature ionic liquids (ILs) results from the competition between its initially bound counterions, the IL anions, and other anions (e.g., present as impurities or added to the solution). We present a joined spectroscopic (UV−visible and extended X-ray absorption fine structure)-simulation study of the coordination of uranyl initially introduced either as UO2X2 salts (X- = nitrate NO3-, triflate TfO-, perchlorate ClO4-) or as UO2(SO4) in a series of imidazolium-based ILs (C4mimA, A- = PF6-, Tf2N-, BF4- and C4mim = 1-methyl-3-butyl-imidazolium) as well as in the Me3NBuTf2N IL. The solubility and dissociation of the uranyl salts are found to depend on the nature of X- and A-. The addition of Cl- anions promotes the solubilization of the nitrate and triflate salts in the C4mimPF6 and the C4mimBF4 ILs via the formation of chloro complexes, also formed with other salts. The first coordination sphere of uranyl is further investigated by molecular dyna...

Preparation, Characterization, and Electrochemical Properties of Pure and Composite LaNi0.6Fe0.4O3-Based Cathodes for IT-SOFC

Abstract: Recently LaNi1–xFexO3 materials have been indicated as good candidates for cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs), and LaNi0.6Fe0.4O3 (LNF) showed the highest specific conductivity in the LaNi1–xFexO3 series. Here we report the results of our investigation on the performances of LNF, synthesized by the coprecipitation method. A detailed local structure analysis, performed using X-ray absorption near edge spectroscopy (XANES ) and extended X-ray absorption fine structure (EXAFS) techniques, indicated that both Fe and Ni are in the (III) oxidation state with coordination numbers of ∼6, suggesting the absence of appreciable amount of oxygen vacancies. The fresh LNF material was used as a precursor for LNF–YSZ (yttria-stabilized zirconia) and LNF–SDC (samaria-doped ceria) electrodes. In the case of the LNF–YSZ nanocomposite, even a short (5 h) treatment at 1000 °C induced a strong reaction between the cathode components, leading to the formation of an undesirable La2Zr2O7 insu...

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.

Measuring water vapor in hydrocarbons

Abstract: Low concentrations of water vapor within a background of one or more hydrocarbon gases may be detected and quantified using a differential absorption spectrometer. A dehydrated sample of the gas is used as a background sample whose absorption spectrum allows elimination of absorption features not due to water vapor in the gas. Absorption spectra may recorded using tunable diode lasers as the light source, these lasers may have a wavelength bandwidth that is narrower than the water vapor absorption feature used for the differential absorption spectral analysis.

Methanol oxidation catalysis and substructure of PtRu bimetallic nanoparticles

Abstract: Catalytic material of PtRu nanoparticles supported on carbon (PtRu/C) for direct methanol fuel cells was synthesized by a polyol reduction method. Addition of phosphorus was effective for downsizing PtRu particles and improving their catalytic activity. The activity obtained was six times of that of a commercial catalysis. The samples were analyzed by techniques of X-ray absorption fine structure (XAFS) at Pt L III -edge and Ru K-edge, transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF). These results indicated a core–shell structure consisting of a Pt-rich core and Ru-rich shell. By examining coordination numbers determined by XAFS analysis, we found a clear correlation between the catalytic activity and the Pt–Ru atomic pair frequency occurring on the particle surface, which supports the “bi-functional mechanism”.

Structures and magnetic properties of wurtzite Zn1−xCoxO dilute magnetic semiconductor nanocomposites

Abstract: Cobalt-doped ZnO dilute magnetic semiconductor nanocomposites Zn1−xCoxO with Co concentrations from 0.02 to 0.25 were prepared by the sol-gel method. The magnetic measurement shows paramagnetic behavior for all the samples. The structures of these composites were investigated by x-ray diffraction and fluorescence x-ray absorption fine structure spectroscopy. It is indicated that at low Co concentration (x⩽0.05), the Co atoms are incorporated into the ZnO lattice and located at the substitutional sites of the Zn atoms. At higher Co doping concentration (x⩾0.10), the secondary phase Co3O4 is precipitated. Correlating the magnetic behavior with the structural properties of the Zn1−xCoxO nanocomposites, the authors interpret the paramagnetism to be intrinsic in nature as a result of the low effective doping of Co in ZnO and the lack of oxygen vacancies.