X-ray absorption fine structure spectroscopic study of uranium nitrides

Uranium mononitride (UN), sesquinitride (U₂N₃) and dinitride (UN₂) were characterized by extended X-Ray absorption fine structure spectroscopy. Analysis on UN indicate the presence of three uranium shells at distances of 3.46(3), 4.89(5) and 6.01(6) Å and a nitrogen shell at a distance of 2.46(2) Å. For U₂N₃, two absorbing uranium atoms at different crystallographic positions are present in the structure. One of the uranium atoms is surrounded by nitrogen atoms at 2.28(2) Å and by uranium atoms at 3.66(4) and 3.95(4) Å. The second type of uranium atom is surrounded by nitrogen atoms at 2.33(2) and 2.64(3) Å and by uranium atoms at 3.66(4), 3.95(4) and 5.31(5) Å. Results on UN₂ indicate two uranium shells at 3.71(4) and 5.32(5) Å and two nitrogen shells at 2.28(2) and 4.34(4) Å. The lattice parameters of UN, U₂N₃ and UN₂ unit cells were respectively determined to be 4.89(5), 10.62(10) and 5.32(5) Å. Those results are well in agreement with those obtained by X-Ray diffraction analysis.     

X-ray absorption fine structure of artificial antigens for cadmium

Immunoassay technology as a quick and large-scale screening method to detect metal ions in foods and environmental samples has rapidly been developed due to several advantages over conventional instrument-intensive methods. Unlike biomacromolecule, metal ions are haptens without immunogenicity, so successful preparation of artificial antigens is the first critical step for establishing immunoassay methods for them. In the current paper, cadmium ions were conjugated to BSA and OVA, respectively, using bifunctional chelator, p-SCN-Bn-DTPA. The ultraviolet analysis indicated that the maximum absorption peak of Cd–p-SCN-DTPA–BSA and Cd–p-SCN-DTPA–OVA had a small peak shift and an apparent absorbance increase compared to that of BSA and OVA, and the extents of substitution of ?-amino in both conjugates were 51.2% and 58.6%, respectively. In addition, the EXAFS of conjugates implied that Cd²⁺ coordinated with N and O atoms of DTPA in artificial antigens, the coordination type and number of Cd–DTPA, Cd–p-SCN-Bn-DTPA–BSA, Cd–p-SCN-Bn-DTPA–OVA were the same. XANES region and geometries of the three compounds were also same. These results implied that the three antigens had the similar local structure and atomic geometry.This was the first time that the XAFS was attempted for the identification of artificial heavy metal ion antigens.     

Near-edge X-ray absorption fine structure investigations of order in carbon nanotube-based systems

Probing order in nanotube systems is of fundamental importance in devising applications of these tubes in field emission applications as well as for components of composite materials. We use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to qualitatively and quantitatively study the degree of order and alignment in a wide range of carbon nanotube-based systems, including single-walled carbon nanotube (SWNT) powder, SWNT films, and aligned multiwalled carbon nanotubes. The results are compared to analogous data obtained from a highly ordered pyrolytic graphite (HOPG) sample.     

Near-edge X-ray absorption fine structure spectroscopy-assisted purification of single-walled carbon nanotubes

Single-walled carbon nanotubes were produced by the conventional arc discharge method, and purified with a two-step treatment. First, the raw soot containing single-walled carbon nanotubes was burned up at ca. 350 °C in air to remove amorphous carbon, and then it was treated by strong acidic solvents to remove metal catalysts. Near-edge X-ray absorption fine structure spectroscopy was applied to analyze the defects on single-walled carbon nanotubes in whole purification process, so the experimental conditions can be optimized, and finally high-purity single-walled carbon nanotubes were obtained as revealed by various spectroscopic characterizations such as scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Raman spectroscopy.     

Conversion electron yield X-ray absorption fine structure measurements under atmospheric conditions

The detection of a sample current under atmospheric conditions was applied to XAFS measurements of a Ni foil and NiO powders, and the feasibility of the conversion electron yield (CEY) detection was examined. The obtained XAFS spectra were compared with those obtained using the conventional transmission method; it is concluded that fairly good statistics can be obtained using the CEY method and that the obtained spectra show an identical structure to those obtained using the transmission method.     

Oxygen K-edge X-ray absorption fine structure studies of vacuum-deposited ice films

Multilayer ice films deposited on polycrystalline Au(111) substrates at 90 and 153 K under ultra-high-vacuum conditions are studied using O K-edge X-ray absorption fine structure spectroscopy in the surface-sensitive partial electron yield mode. Both near-edge and extended oscillatory fine structures are analyzed in combination with the theoretical real-space full-multiple-scattering simulations based on the FEFF8 code. The experimental data consistently indicate that the local structure of the near-surface regions in ice films at both substrate temperatures resembles that of high-density crystalline modifications of ice (e.g., ice II, ice III, or high-density amorphous ice). In addition, the ice films deposited at 153 K most probably contain a minor fraction of low-density cubic ice (I(c) phase).     

Molecular characterization of brominated persistent pollutants using extended X-ray absorption fine structure (EXAFS) spectroscopy

X-ray absorption fine structure (EXAFS) spectroscopy spectra were collected for three brominated persistent pollutants: 6-bromo-2,4,5-trichlorophenol (BrTriClP), pentabromophenol (PentaBrP) and 3,3′,5,5′-tetrabromobisphenol A (TBBA). The substances were selected to be symmetrical (BrTriClP and TBBA) or asymmetrical (PentaBrP) with respect to the atomic Br positions and to differ in the number of bromine and other halide atoms, as well as their relative positions. The asymmetrical PentaBrP was modelled with special detail as not all bromine atoms have identical coordination environments. The studied substances displayed unique EXAFS spectra, which could be used to determine the molecular structure in fair detail. We conclude that EXAFS spectroscopy is a suitable technique for molecular characterization of the comparatively complex molecules within the class of compounds of brominated organic persistent pollutants. A detailed understanding of the EXAFS spectra of the pure compounds opens up possibilities to study the interactions with soil and sediment matrices by means of EXAFS spectroscopy. Figure Brominated organic persistent pollutants are characterized by EXAFS spectroscopy     

X-ray absorption near edge structure and extended X-ray absorption fine structure analysis of Fe(II) aqueous and acetone solutions

X-ray absorption spectroscopy measurements were used to determine the structure of the first coordination shell of Fe(II) ions in aqueous and acetone based solutions. Extended X-ray absorption fine structure analysis coupled with ab initio X-ray absorption near edge structure calculations confirms the octahedral coordination of the iron ion in water based solution. Data collected for acetone rich solutions can be reproduced assuming coexistence of the octahedral Fe(H(2)O)(6)(2+) and tetrahedral [FeCl(4)](2-) complexes. Distortion of the tetrahedral coordination of ion was detected in some of the acetone based solutions.     

Hydration of counterions in cation exchange resins studied by X-ray absorption fine structure

X-Ray absorption fine structure has revealed that K(+) is partially desolvated when transferred into the interior of cation-exchange resin, while Rb(+) and Sr(2+) keep their first hydration structures.     

Geometric structure determination of N694C lipoxygenase: a comparative near-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure study

The mononuclear nonheme iron active site of N694C soybean lipoxygenase (sLO1) has been investigated in the resting ferrous form using a combination of Fe-K-pre-edge, near-edge (using the minuit X-ray absorption near-edge full multiple-scattering approach), and extended X-ray absorption fine structure (EXAFS) methods. The results indicate that the active site is six-coordinate (6C) with a large perturbation in the first-shell bond distances in comparison to the more ordered octahedral site in wild-type sLO1. Upon mutation of the asparagine to cysteine, the short Fe-O interaction with asparagine is replaced by a weak Fe-(H(2)O), which leads to a distorted 6C site with an effective 5C ligand field. In addition, it is shown that near-edge multiple scattering analysis can give important three-dimensional structural information, which usually cannot be accessed using EXAFS analysis. It is further shown that, relative to EXAFS, near-edge analysis is more sensitive to partial coordination numbers and can be potentially used as a tool for structure determination in a mixture of chemical species.     

Solid-state near-edge X-ray absorption fine structure spectra of glycine in various charge states

The experimental solid-state near-edge X-ray absorption fine structure spectra for a series of glycine-related samples including alpha-glycine, beta-glycine, glycinium chloride, glycinium trifluoroacetate, and sodium glycinate at the C, N, and O K-edges measured under identical conditions are reported and compared. An assignment of spectral features for alpha-glycine is proposed on the basis of extended theoretical simulations of polarization-dependent spectra performed within the real-space multiple-scattering formalism explicitly taking into account the intermolecular environment of a glycine molecule in a crystal.     

Determination of chemical form of antimony in contaminated soil around a smelter using X-ray absorption fine structure.

Only limited information is available about the behavior of antimony (Sb) in contaminated soils. However, understanding the behavior of Sb in contaminated soils is important, because the toxicity or solubility of this element depends on its chemical state. In this study, we investigated the levels of Sb and the chemical forms of Sb in the soil around a smelter using X-ray absorption fine structure (XAFS) spectra. The highest Sb concentration in the contaminated soil was 2900 mg/kg dry soil. According to Sb-K edge X-ray absorption near edge (XANE) spectra, the Sb in the soil was in the form of Sb(V) compounds. The similarity of extended XAFS (EXAFS) spectra suggests that Sb speciation was independent of the sampling site, which indicates that Sb or Sb2O3 emitted from the smelter was converted into Sb(V) compounds in the soil.     

Near-edge X-ray absorption fine structure spectroscopy of diamondoid thiol monolayers on gold

Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 +/- 0.05 and 0.16 +/- 0.04 eV, respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different degrees of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond nanoparticles.     

Near edge X-ray absorption fine structure spectroscopy of bacterial hydroxamate siderophores in aqueous solutions

X-ray absorption spectroscopy (XAS) is widely used to explore the coordination environments and structures of metal complexes in aqueous solutions and disordered phases. Although soft-XAS studies on gaseous phases, solid phases and their interfaces have shown that XAS is a versatile tool in studying the functional group composition of organic molecules, the application of XAS to studying aqueous solutions is seriously limited because of experimental difficulties. In this report, using a modified synchrotron endstation geometry, we show how soft-XAS was used to study the changes in electronic states of reactive functional groups in a bacterial macromolecule, desferrioxamine B (desB, a hydroxamate siderophore) and its structural analogue (acetohydroxamic acid (aHa)). We collected C, N, and O near edge X-ray absorption fine structure (NEXAFS) spectra of these molecules in aqueous solutions and complemented their spectral interpretation with calculated X-ray spectra of "hydrated" aHa. The experimental spectra of desB are similar to those for aHa at the C, N, and O K-edges. In addition, the electronic transitions of amide and hydroxamate functional groups in the macromolecule can be distinguished from the N spectra. Small energy differences in the pi*(C=O)NO and the transitions at the C- and N-edges of aHa and desB indicate that the substituent attached to N in desB ((CH2)n) determines the electron density in the (C=O)NO core. As the solution pH increased, the pi*(C=O)NO transition of the hydroxamate group of these two molecules exhibit energy shifts at the C-, N-, and O-edges, which are consistent with increased electron delocalization in the (C=O)NO core of aHa (and desB), predicted from the calculations. The spectra of the aHa(H2O)3- anion also provide evidence for partial N-deprotonation at pH values usually attributed to an O-acid. These results indicate that soft-XAS is well suited for studying the electronic states of different functional groups in aqueous organic macromolecules.     

Variable incidence angle X-ray absorption fine structure spectroscopy: a zirconia film study

Variable incidence angle X-ray absorption fine structure (VIAXAFS) spectroscopy offers a non-destructive ability to investigate film nano-structures. This technique was applied, spanning sample-beam angles from a grazing to normal incidence on a film obtained by zirconia sputtering on flat sample of stainless steel. X-ray absorption fine structure analysis on the Zr K edge identified chemical, defects and fractal structures through the film depth. VIAXAFS revealed occurrence of zirconium monoxide fractions at the surface a reduced state of zirconium oxide vs. the zirconium dioxide bulk. The discussion underlines that the technique may quantify the profile of various sub-layers, nano-pores, dislocations, vacancies or defect features.     

New sights into the electrochemical interface provided by in situ X-ray absorption fine structure and surface X-ray scattering

Various important processes, such as electron transfer reactions, adsorption/desorption, solvation/desolvation, and formation/cleavage of chemical bonds, take place at electrolyte/electrode interfaces during electrocatalytic reactions. Those processes have been understood on the basis of changes in the surface composition, atomic arrangement, and molecular and electronic structures of the interfaces by using various in situ analysis techniques. To date, in situ analysis and observation of those interfacial processes at an ideal single-crystal surface are indispensable not only for fundamental understanding of the reaction mechanism but also for rational design of the highly efficient and durable electrocatalytic materials. Here, historical and recent progress of in situ studies on electrocatalytic reactions is briefly reviewed with a focus on two major techniques, X-ray absorption fine structure and surface X-ray scattering.     

Bound site of Mo atoms and its local structure in a Mo/HY catalyst characterized by extended X-ray absorption fine structure and density functional calculation

The bound site of Mo atoms and its local structure in a Mo/HY catalyst have been determined by detailed analysis of extended X-ray absorption fine structure (EXAFS). Molybdenum was introduced in the supercage of HY zeolite by cycles of saturated adsorption of Mo(CO)6 at room temperature and subsequent thermal decomposition at 573 K. Two Mo atoms per supercage were immobilized in each CVD-thermal treatment cycle. The Mo loading increased linearly with the cycles up to three cycles at saturation, where six Mo atoms were supported. Temperature-programmed decomposition of the adsorbed Mo(CO)6 was also characterized by GC, QMS, and FT-IR, respectively. The EXAFS analysis including multiple scattering based on theoretical calculations revealed that Mo bound with two oxygen atoms connects to Al, where one of the two oxygen atoms had been associated with a proton. The bound site is called the S(III)' site. The zeolite framework was significantly distorted by the introduction of low-valent Mo, resulting in isolation of the [MoO2Al] unit from the surrounding zeolite framework due to a quasi-disruption of Si-O bonds adjacent to the unit. In the mild oxidation of the low-valent Mo/HY sample two Mo=O bonds were newly formed and the position of Mo was displaced by 0.06 nm so that the distortion of zeolite framework around the Al atom was relieved. The structures were also supported by DFT calculations. This study is the first example that the position of metal cation in zeolite was determined unambiguously by the EXAFS analysis.     

Surface structure and chemical switching of thioctic acid adsorbed on Au(111) as observed using near-edge X-ray absorption fine structure

Thioctic acid (alpha-lipoic acid) is a molecule with a large disulfide-containing base, a short alkyl chain with four CH2 units, and a carboxyl termination. Self-assembled monolayer (SAM) films ofthioctic acid adsorbed on Au(111) have been investigated with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) to determine film quality, bonding, and morphology. Using standard preparation protocols for SAMs, that is, dissolving thioctic acid in ethanol and exposing gold to the solution, results in poor films. These films are highly disordered, contain a mixture of carboxyl and carboxylate terminations, have more than monolayer coverage, and exhibit unbound disulfide. Conversely, forming films by dissolving 1 mmol thioctic acid into 5% acetic acid in ethanol (as previously reported with carboxyl-terminated alkanethiols) forms ordered monolayers with small amounts of unbound sulfur. NEXAFS indicates tilted over endgroups with the carboxyl group normal on average 38 degrees from the surface normal. Slight angle-dependent intensity modulations in other features indicate alkyl chains statistically more upright than prostrate on the surface. Reflection-absorption Fourier transform infrared (RA-FTIR) spectra indicate hydrogen bonding between neighboring molecules. In such well-formed monolayers, a stark reorientation occurs upon deprotonation of the endgroup by rinsing in a KOH solution. The carboxylate plane normal is now about 66 degrees from sample normal, a much more upright orientation. Data indicate this reorientation may also cause a more upright orientation to the alkyl portion of the molecules.