Absorption Fine Structure Spectroscopy Research Articles

Speciation of sulphur in soils and soil particles by X‐ray spectromicroscopy

SummaryCurrent wet chemical methods for the speciation of sulphur (S) in soils are inaccurate and do not allow one to assess the S speciation of individual soil particles and colloids. X‐ray microscopy and Near Edge X‐ray Absorption Fine structure Spectroscopy (NEXAFS) can be used to study individual species of S at the K‐adsorption edge. We have used these techniques to identify and quantify S species in bulk soil, soil particles and colloids from Oh and Bh horizons of two forested Podzols. The partitioning of soil sulphur as determined on bulk samples of the Oh horizons by X‐ray spectromicroscopy agreed fairly well with the results of a conventional S speciation for the soil at Schluchsee, and reasonably well for that at Rotherdbach. The NEXAFS analyses on individual soil particles revealed that they are richer in reduced organic sulphur than the bulk soil for the Schluchsee Oh and richer in sulphate for Rotherdbach Oh. The techniques can be used reliably to separate and quantify sulphur species with different oxidation states in the soil. The combination of X‐ray transmission and sulphur fluorescence images with unfocused and focused NEXAFS spectra at the K‐adsorption edge of sulphur at specific microsites allowed us to compare the distribution of S species in bulk soil with that of distinct soil particles and soil colloids. Moreover, we can use it to assess the spatial distribution of different S species on soil particles on a scale of a few hundred nanometres.

The nature of arsenic in uranium mill tailings by X-ray absorption spectroscopy

In order to understand the evolving world of environmental issues, the ability to characterize and predict the stability and bioavailability of heavy metal contaminants in mine waste is becoming increasingly more important. X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were used to characterize a series of synthetic and natural samples associated with mine tailings processing. XANES was shown to be excellent as a tool to rapidly differentiate oxidation states of arsenic within the samples. The EXAFS spectra provided information on the mineralogy of the precipitated raffinate and tailings and showed that these samples are composed of a mixture of amorphous ferric arsenates, adsorbed arsenates and a mixture of other poorly ordered arsenates.

Investigation of the Ligand Exchange Behavior of Square‐Planar Nickel(II) Complexes by X‐ray Absorption Spectroscopy and X‐ray Diffraction

AbstractThe local structure of nickel and bromine in three square‐planar nickel(II) complexes [(2,2′‐bipyridyl)(bromo)(mesityl)nickel(II)] (1), [(bromo)(N,N‐diisopropyl‐1,2‐ethanediimine)(mesityl)nickel(II)] (2), [(bromo)(mesityl)bis(triphenylphosphane)nickel(II) (3)] and their ligand exchange behavior towards several solvents has been determined by means of X‐ray Absorption Near Edge Spectroscopy (XANES), Extended X‐ray Absorption Fine Structure Spectroscopy (EXAFS), X‐ray Diffraction (XRD), multinuclear NMR and UV/Vis spectroscopy. Several square‐planar and octahedral nickel(II) complexes were prepared and spectroscopically characterized as reference compounds. The exchange behavior strongly depends on the nature of the incoming donor ligand (solvent molecule). In some cases the substitution of one or more ligands was accompanied by conservation of the square‐planar geometry. In other solvents the geometry changed from square‐planar to octahedral. The structural information obtained is important to the further development of such nickel(II) complexes as catalysts for various applications. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Anomalous Composition Dependence of Optical Energies of MBE-grown InGaN

ABSTRACTA direct comparison of the optical energies of MBE- and MOVPE-grown InxGa1-xN epilayers of similar InN content is performed for the first time. The InN fraction in the 7 MBE samples examined ranged from x ∼ 0.11 to x ∼ 0.35 while the range in available MOVPE epilayers is [0, 0.4]. Wavelength Dispersive X-ray (WDX) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were used to measure composition and local structure (alloy character) of the samples. Cathodoluminescence (CL) spectroscopy in situ, ex situ photoluminescence (PL) mapping and large-area optical absorption spectroscopy were used to measure various optical energies. The composition dependence of the optical energies is determined by the growth method. The absorption bandgap and luminescence peak energies vary linearly with x for both growth methods, suggesting a near-zero value of the bowing parameter. But the energy intercept at zero InN content in MOVPE samples is close to the wurtzite-GaN bandgap of 3.4 eV at room temperature, as expected, while the equivalent for MBE samples falls near 3.2 eV.

X-ray absorption spectroscopy, simulation and modeling of Si-DLC films

Amorphous silicon-containing diamond-like carbon (Si-DLC) films were deposited on silicon wafers by Ar+ Ion Beam Assisted Deposition (IBAD) at various energy conditions. The films were examined with X-ray Absorption Near Edge Structure (XANES) spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The Si K-edge X-ray Absorption Spectroscopy (XAS) results indicate that Si-DLC films have an amorphous structure, where each Si atom is coordinated to four carbon atoms or CHn groups. This short-range order, where a Si atom is surrounded by four C atoms, was found in all Si-DLC films. The XANES spectra do not indicate Si coordination to oxygen atoms or phenyl rings, which are present in the precursor material. A structural model of Si-DLC is proposed based on XAS findings. Simulated X-ray absorption spectra of the model produced by FEFF8 show a good resemblance to the experimental data.

An exafs study of nanocrystalline thin films

This work will show the results of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on thin films of ZrO2:16%Y and CeO2, (with a particle size ranging from 4 nm to 330 nm), prepared on a sapphire substrate. The EXAFS measurements will reflect the true nature of the ZrO2 and CeO2 nanocrystallites and will enable meaningful conclusions to be drawn about the levels of disorder at the atomic level.

Complex behavior in quaternary zirconias for inert matrix fuel: what do these materials look like at the nanometer scale?

Conventional crystallographic analysis of cubic stabilised zirconias (CSZ) over a wide range of stabilisers and stoichiometries has shown O atom displacements along lattice vectors and cation disorder with irreproducible characteristics. However, because diffraction is only sensitive to the average long-range order and biased towards periodic and symmetric distortions, it is essential to supplement it with probes of short-range order that may reveal additional, element-specific details about local ordering. We have performed X-ray Absorption Fine Structure (XAFS) spectroscopy measurements on a series of Er-Y-(U-Ce)-Zr oxides to determine the local environment around each of the cations. Results from these experiments demonstrate that not only are there significant element- and composition-dependent distortions away from the crystallographic structure but also interactions between specific pairs of the metal ions indicative of cooperative behaviour. This suggests that some of the metal ions are not randomly, isomorphically substituted into the lattice but instead have tendencies to form nanometer-scale clusters and networks. In this respect, substantial differences are seen in the influence of Ce and U on the Y-centered distortions, consistent with different roles for these two elements in inert matrix fuel (IMF) matrices. This type of nanometer-scale heterogeneity is typically coupled to the phase stability and other properties of other materials and may therefore be important to these zirconias in their IMF application.

DYNAMICS AND LOCAL STRUCTURE OF COLOSSAL MAGNETORESISTANCE MANGANITES

We report Extended X-ray Absorption Fine Structure and anelastic spectroscopy measurements on hole doped manganese oxides La 1-x Ca x MnO 3 which present the colossal magnetoresistance effect. EXAFS measurements were realized both in the absence and presence of an applied magnetic field of 1.1 Tesla, in a wide temperature range (between 330 and 77 K) and at various dopings (x = 0.25 and x = 0.33). The magnetic field orders the magnetic moments so favouring the electron mobility and the reduction of Mn-O octahedra distortions. We observe the presence of four short and two long Mn-O distances (1.93 and 2.05Å respectively) above and also below the metal-insulator phase transition. The overall distortion decreases but does not completely disappear in the metallic phase suggesting the possible coexistence of metallic and insulating regions at low temperatures. The magnetic field reduces the lattice distortions showing evidence of a microscopic counterpart of the macroscopic colossal magnetoresistance. We also present preliminary anelastic relaxation spectra in a wide temperature range from 900 K to 1 K on a sample with x = 0.40, in order to study the structural phase transitions and the lattice dynamics. A double peak has been observed at the metal-insulator transition in the imaginary part of Young's modulus. This double peak indicates that the metal-insulator transition could be a more complex phenomenon than a simple second order phase transition. In particular the peak at lower temperatures can be connected with the possible presence of inhomogeneous phase structures. Another intense dissipation peak has been observed corresponding to the structural orthorhombic-trigonal transition around 750 K.

NEXAFS and ARUP Spectroscopy of an Organic Single Crystal: α-Perylene

We have studied the applicability of Near Edge X-ray Absorption Fine Structure (NEXAFS) and Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) on insulating organic molecular crystals, using single crystals of α-perylene as an example. The influences of sample charging were investigated. By employing suitable compensation techniques in combination with partial yield detection, the charging problem could be solved satisfactorily for NEXAFS and partially for ARUPS. The techniques to be described allowed us to obtain NEXAFS spectra with a fairly high energy resolution of δE = 0.4 eV. The spectra so obtained for α-perylene single crystals display several sharp peaks whose intensities strongly depend on the polarization angle of the incident synchrotron radiation. From the angular dependence we derive an average angle of inclination of the molecular planes relative to the (001) cleavage plane of 85°±5° which is in excellent agreement with the angle determined from a full X-ray crystal structure analysis. The various peaks are tentatively assigned to different transitions from C Is levels to π* and [sgrave]* final states; some spectral details even reflect influences by the crystal structure. It is noteworthy that useful sample thickness can be anywhere between monolayer and macroscopic dimensions.

Characterization of plasma nitrided pure titanium by X-ray absorption spectroscopy

To date, the exact nature of the plasma nitriding mechanism and the role of energetic particle bombardment are not well understood. The purpose of this work has been to obtain a more detailed knowledge about the evolution of the plasma nitrided surface layer as a function of the energy of the bombarding particles. Nitrided layers were produced at the surface of pure titanium specimens at various flux energies by Intensified Plasma-Assisted Processing (IPAP), a triode plasma technique developed in our laboratory. X-ray Absorption Near Edge Structure (XANES) spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy were used to characterize the local structure of the titanium nitride layers. Cross sections of the processed specimens were studied by Auger electron spectroscopy and electron microscopy. The results showed that increasing flux energy promotes the formation of a well-ordered TiN layer at the surface. Low flux energies produce significantly lower fractions of the TiN phase at the surface, as well as thinner nitrided layers. A structural model was suggested and quantitatively tested based on the XANES and EXAFS measurements.

Real-Time Characterization of Biogeochemical Reduction of Cr(VI) on Basalt Surfaces by SR-FTIR Imaging

Synchrotron radiation-based (SR) Fourier-transform infrared (FTIR) spectromicroscopy in the mid-infrared region is a surface analytical technique that can provide direct insights into the localization and real-time mechanisms for the reduction of the (CrO4)2- chromate [Cr(VI)] species on surfaces of geologic materials. Time-resolved SR-FTIR spectra indicate that, in the presence of endoliths (mineral-inhabiting microorganisms), microbial reduction of Cr(VI) to Cr(III) compounds on basaltic mineral surfaces is the key mechanism of Cr(VI) transformation. It proceeds in at least a two-step reaction with Cr(V) compounds as possible intermediate products, with the reduction of Cr(VI) increasing during the concomitant biodegradation of a dilute organic vapor (toluene). Analyses of spatially resolved SR-FTIR spectra show that the maximum reduction of Cr(VI) to Cr(III) compounds occurs on surfaces densely populated by microorganisms. The oxidation state of Cr(III) compounds was confirmed by micro-x-ray absorption fine-structure spectroscopy. Both the time- and space-resolved SR-FTIR spectra show that in the absence of endoliths, Cr(VI) reduction is insignificant. With this effort, the potential use of SR-FTIR spectromicroscopy in providing mechanistic information of reduction of Cr(VI) has been demonstrated. This method can now be expanded to examine other IR-amenable microbial/chemical contaminant systems.

Tribo-chemistry at the head/disk interface

Tribo-chemical studies at the head/disk interface (HDI) were conducted on hydrogenated (CH/sub x/), nitrogenated (CN/sub x/), and cathodic-arc amorphous hard carbon disk samples coated with perfluoropolyether ZDOL and X1P/ZDOL lubricant. The studies involved drag tests with uncoated and carbon-coated Al/sub 2/O/sub 3/-TiC sliders and thermal desorption experiments in an ultra-high vacuum (UHV) tribochamber followed with a surface chemistry analysis by X-ray Photo Emission Electron Microscopy (X-PEEM) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The friction and catalytic decomposition mechanisms of ZDOL are described, as well as the tribo-chemical performance of cathodic-arc carbon overcoats coated with ZDOL, and data demonstrating the chemical alteration of the lubricant and carbon overcoat are also presented.

Surface Interactions of Actinide Ions with Geologic Materials Studied by XAFS

ABSTRACTWe have investigated the interaction of the actinyl ion, , with silica, alumina, and montmorillonite surfaces under ambient atmosphere and aqueous conditions using x-ray Absorption Fine Structure (XAFS) Spectroscopy. In acid solution (pH ∼ 3.5), the uranyl ion shows a strong interaction with the silica and alumina surfaces, and a relatively weak association with the montmorillonite surface. The extent of direct surface interaction is determined by comparing structural distortions in the equatorial bonding environment of the uranyl ion relative to the structure of a “free” uranyl aquo complex. Based on this formalism, surface complexation on silica and alumina occurs through an inner-sphere mechanism with surface oxygen atoms binding directly to the equatorial region of the uranyl ion. In contrast, sorption on montmorillonite occurs by an outer sphere mechanism in which the uranyl ion retains the simple aquo complex structure and binds to the surface via ion-exchange. In near-neutral solutions (pH ∼ 6), sorption on all of the materials is dominated by an inner-sphere mechanism. The formation of surface oligomeric species is also observed on silica and alumina.

NEXAFS measurements of the molecular ordering in the boundary layers of liquid crystalline free standing films

By near edge X-ray absorption fine-structure (NEXAFS) spectroscopy a finite molecular tilt angle in the surface layer of a free standing film in the liquid crystalline smectic A phase of C7 was directly detected. Analysis of the angular dependent intensities of the oxygen K edge NEXAFS spectra leads to an average tilt angle of the molecules in the surface layer of about 34°, which is characteristic for the bulk smectic C* phase of C7.

Structural Characterisation of Condensed Molecular Materials using Polarised NEXAFS Spectroscopy

The basic principals underpinning the technique of polarised Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy using synchrotron radiation are described with particular relevance to the technique's application in the structural characterisation of organic surfaces and interfaces. Experimental data, recorded in the ultra-soft spectral region close to the C and N K-edges (ca. 200eV to 500eV) on beamline U1 A at the Brookhaven National Synchrotron Light Source (NSLS) is presented including applications to thin films of long chain hydrocarbons, LB films, anti-corrosion coatings and organic single crystals.

Exafs study of the Nd ion lattice position in Nd:MgO: LiNbO3 and Nd:ZnO:LiNbO3

The local order around Nd3+ ions in Nd:MgO:LiNbO3 and Nd:ZnO:LiNbO3 codoped samples has been investigated by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Independently from the codoping atom, the main position of the Nd ions has been determined to be near the Li lattice site.

ChemInform Abstract: Investigation of Aquo and Chloro Complexes of UO2+2, NpO+2, Np4+, and Pu3+ by X‐Ray Absorption Fine Structure Spectroscopy.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

On Extended X-Rays Absorption Fine Structure Characterisation of Iron Platinum Clusters

ABSTRACTPresent paper reports on Extended X-rays Absorption Fine Structure (EXAFS) Spectroscopy characterisation of Fe/Pt clusters. Samples of different nominal iron contents (FePt4 and FePt2) have been prepared and characterised by direct measurements of absorption coefficient at Pt L3 and Fe K edges. Catalysts were prepared by precipitation from suitable mixing (in the appropriate stoichiometry) of mesitylene suspentions containing small clusters of either platinum or iron. Single species suspention, in turn, were prepared by vapor metal deposition.Catalitic activity of prepared samples was tested on cinnammic aldehyde. As a general result, platinum clusters exhibit activity in the hydrogenation of both carbon double bond and aldehyde group with a preference for the latter. Iron addition to the pure platinum catalyst reverts the selectivity while leaving the catalitic activity almost unchanged.The present paper represents a temptative to achieve structural information on Fe and Pt short range environments, thus providing insights on the dynamics of the catalytic process.

Chemical Speciation by NEXAFS Spectromicroscopy: Insights from Molecular Modelling of Polymers

AbstractNear Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy of polymers performed in a scanning transmission X-ray microscope (STXM) can provide chemical speciation with <0.1 gm spatial resolution in imaging mode. The core excitation spectra of molecular compounds that are structural analogues of polymers help interpret the NEXAFS spectra of polymers. The effect of nt-delocalization on polymer NEXAFS is discussed and illustrated by comparison to molecular spectra. Extended Htickel calculations are particularly useful for providing insight into the relationship between chemical structure and the molecular and polymer spectra. We report the interpretation of experimental NEXAFS spectra of polyethylene terephthalate (PET). Molecular models indicate that NEXAFS will be sensitive to structural isomerization in polyester polymers. We demonstrate the capability of NEXAFS to distinguish hard-segment and soft-segment phase segregation in polyurethanes.

New Experimental Approaches for the Study of Polymer/Metal Interphases

Abstract In this short review we present recent experiments which can be used to infer the structural parameters of ultrathin polyimide and polyamide acid films as a function of distance from the substrate surface. The polyimide films are prepared by the Langmuir-Blodgett technique in a layer-by-layer fashion, and the orientation of the pyromellitic imide unit in the polyimide macromolecules is determined as a function of film thickness by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Subsequent delamination experiments on these Langmuir-Blodgett (LG) deposited polyimide films reveal that the locus of failure does not occur in a “weak boundary-layer” adjacent to the silicon substrate as expected from delamination experiments with macroscopically thick films. As a non-destructive method to study the orientation of polymer molecules during film growth, second harmonic generation (SHG) experiments on the deposition of polyamide acid (PAA) on gold and silver surfaces will be briefly describ...