X-ray Absorption Fine Structure Data Research Articles

Systematic investigation of relativistic effects in EXAFS data analysis

Modern XAFS (x-ray absorption fine structure) data analysis is based on accurate multiple-scattering (MS) calculations of the x-ray absorption cross section, usually carried out solving the nonrelativistic Schr\"odinger equation for complex effective optical muffin-tin potentials describing the scattering of the atoms. The introduction of relativistic effects in extended XAFS (EXAFS) multiple-scattering calculations has been described in several papers and shown to be important for heavy atoms. However, few examples of applications and detailed studies of relativistic effects were given so far. In this work, we have performed a systematic investigation of relativistic corrections in systems of increasing atomic number, using a reliable simulation scheme recently developed and based on the incorporation of a pseudo-Schr\"odinger equation effectively replacing the Dirac relativistic form. Calculations have been put to a test in 12 different pure-element condensed-state systems, with the atomic number ranging from $Z=10$ for crystalline Ne to $Z=90$ for crystalline Th. The importance of accounting for relativistic effects has been highlighted for elements with $Z\ensuremath{\gtrsim}60$, as ones for which relativistic corrections for amplitudes of calculated XAFS MS signals exceed $10%$. The size of relativistic effects for calculated higher-order XAFS signal (with respect to the dominant single-scattering first-neighbor signal) has been shown, taking as example the ${L}_{3}$-edge spectra of crystalline Au and Pb. The size of relativistic effects for the $K$ and ${L}_{3}$ edges has been also evaluated, showing a slight increase of relativistic corrections for the ${L}_{3}$ edge. The improvement in the accuracy of XAFS simulations has been demonstrated comparing the results obtained for structural refinements of the ${L}_{3}$ edge of crystalline Au at 300 K.

Strain-Assisted Single Pt Sites on High-Curvature MoS 2 Surface for Ultrasensitive H 2 S Sensing

Strain-Assisted Single Pt Sites on High-Curvature MoS ₂ Surface for Ultrasensitive H ₂ S Sensing

Synthesis and structural re-investigation of Fe2TeO6 using synchrotron x-rays

Fe2TeO6 (FTO), an antiferromagnetic magnetoelectric material which crystallizes in tetragonal (P42/mnm, 136) crystal structure, shows close to room temperature interesting magnetic interactions. To understand the complexity in its magnetism, a huge variation in its magnetic properties has been reported by various groups, which have been attributed to the presence of Fe2O3 (an impure phase) in the final FTO compound. Herein, synthesis using alternate ingredients and Synchrotron structural investigation of single phasic FTO are presented. Out of the several initial ingredients (Fe2O3, Fe3O4 with TeO2, Te(OH)6) in solid state reaction route, only the reaction between Fe3O4 and TeO2 results in the single phasic FTO (not reported so far), while the others (Fe2O3 with TeO2 or Te(OH)6) result in certain amount of Fe2O3 impurity. Rietveld refinement on Synchrotron X-ray Diffraction data and Fe K edge Extended X-ray Absorption Fine Structure data provide the overall similarity between the average crystalline structure and local structure of FTO, indicating the intrinsic nature of it’s complex magnetic behavior. Copyright © 2017 VBRI Press.

Efficient Electronic Modulation of g-C 3 N 4 Photocatalyst by Implanting Atomically Dispersed Ag 1 -N 3 for Extremely High Hydrogen Evolution Rates

Efficient Electronic Modulation of g-C ₃ N ₄ Photocatalyst by Implanting Atomically Dispersed Ag ₁ -N ₃ for Extremely High Hydrogen Evolution Rates

High-temperature treatments of niobium under high vacuum, dilute air- and nitrogen-atmospheres as investigated by in situ X-ray absorption spectroscopy.

Niobium metal foils were heat-treated at 900°C under different conditions and in situ investigated with time-resolved X-ray absorption fine-structure (EXAFS and XANES) measurements. The present study aims to mimic the conditions usually applied for heat treatments of Nb materials used for superconducting radiofrequency cavities, in order to better understand the evolving processes during vacuum annealing as well as for heat treatments in controlled dilute gases. Annealing in vacuum in a commercially available cell showed a substantial amount of oxidation, so that a designated new cell was designed and realized, allowing treatments under clean high-vacuum conditions as well as under well controllable gas atmospheres. The experiments performed under vacuum demonstrated that the original structure of the Nb foils is preserved, while a detailed evaluation of the X-ray absorption fine-structure data acquired during treatments in dilute air atmospheres (10-5 mbar to 10-3 mbar) revealed a linear oxidation with the time of the treatment, and an oxidation rate proportional to the oxygen (air) pressure. The structure of the oxide appears to be very similar to that of polycrystalline NbO. The cell also permits controlled exposures to other reactive gases at elevated temperatures; here the Nb foils were exposed to dilute nitrogen atmospheres after a pre-conditioning of the studied Nb material for one hour under high-vacuum conditions, in order to imitate typical conditions used for nitrogen doping of cavity materials. Clear structural changes induced by the N2 exposure were found; however, no evidence for the formation of niobium nitride could be derived from the EXAFS and XANES experiments. The presented results establish the feasibility to study the structural changes of the Nb materials in situ during heat treatments in reactive gases with temporal resolution, which are important to better understand the underlaying mechanisms and the dynamics of phase formation during those heat treatments in more detail.

Fluorescence-detected quick-scanning X-ray absorption spectroscopy.

Time-resolved X-ray absorption spectroscopy (XAS) offers the possibility to monitor the state of materials during chemical reactions. While this technique has been established for transmission measurements for a number of years, XAS measurements in fluorescence mode are challenging because of limitations in signal collection as well as detectors. Nevertheless, measurements in fluorescence mode are often the only option to study complex materials containing heavy matrices or in samples where the element of interest is in low concentration. Here, it has been demonstrated that high-quality quick-scanning full extended X-ray absorption fine-structure data can be readily obtained with sub-second time resolution in fluorescence mode, even for highly diluted samples. It has also been demonstrated that in challenging samples, where transmission measurements are not feasible, quick fluorescence can yield significant insight in reaction kinetics. By studying the fast high-temperature oxidation of a reduced LaFe0.8Ni0.8O3 perovskite type, an example where the perovskite matrix elements prevent measurements in fluorescence, it is shown that it is now possible to follow the state of Ni in situ at a 3 s time resolution.

Irradiation effect on alloy GH3535 revealed by X-ray absorption fine structure

The effects of ion irradiation on the atomic local structure of GH3535, a nickel based alloy and candidate of pressure vessel material for molten salt reactor, were investigated by X-ray absorption fine structure (XAFS). We investigated the local structure of the three major solid solution atoms: Mo, Cr, and Fe under the irradiation of two species of ions: He+ and Ni13+. The changes to the coordination number of these atoms after irradiation were determined from XAFS data. The results reveal that ion irradiation causes selective damage to the elements in alloy. No matter for He+ and Ni13+, Cr is the most vulnerable atom in the alloy to the irradiation with the most decrease in its local coordination number. Mo, on the contrary, changes little under either type of ion irradiation. The irradiation damage to Fe lies in between. The degree of ion damage, however, is closely related to the ion species and energy. XRD results show slight increase of lattice constant in the alloy after irradiation with Ni13+, while the lattice remains unchanged after irradiation with He+. The MD simulation in pure nickel system suggests the lattice expanse effect by the competition between interstitials and vacancies.

MiXAFS: a program for X-ray absorption fine-structure data analysis.

A new program called miXAFS for the analysis of X-ray absorption fine-structure (XAFS) data is presented. miXAFS can analyze the XAFS functions simultaneously for all measured X-ray absorption edges of the constituent elements in a sample under the constraints for the structural parameters over the edges. The program provides a surface plot of the R-factor as a function of two structural parameters, which is useful to validate the optimized structural parameters. The structural parameters can be obtained from the XAFS data in a few steps using the setting file and batch process. The program, which is coded in MATLAB and freely available, runs on Macintosh and Windows operating systems. It has a graphical user interface and loads experimental data and XAFS functions in a variety of ASCII data formats.

Effect of Protonated Amine Molecules on the Oxygen Reduction Reaction on Metal-Nitrogen-Carbon-Based Catalysts

Non-precious metal electrocatalysts based on pyrolyzed metal, nitrogen, and carbon (MNC) are viewed as an inexpensive replacement for platinum-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. One of the enduring issues in the field of MNC catalysis is identifying the exact active structure responsible for the ORR. Many ORR active sites have been proposed recently, such as transition metal coordinated to (i) four pyrrolic nitrogens, (ii) four pyridinic nitrogens, and (iii) four pyridinic nitrogens interacting with one protonated nitrogen; among these, the latter is viewed as the most promising active site for the ORR. In this study, we have synthesized a manganese-based MNC catalyst (MnNx/C). EPR and X-ray absorption fine structure (XAFS) analysis indicated the presence of four nitrogens around the Mn(II) ion. The ORR performance of an MnNx/C catalyst was recorded in the presence of the disodium salt of ethylenediaminetetraacetic acid (EDTA-Na2), ethylene diamine (ED), and combination of ED and acetic acid (AA). The presence of EDTA-Na2 and ED + AA in the electrolyte solution maximizes the availability of protonated amine-N around the catalyst. As a consequence, we noticed significant improvement in the ORR kinetics in H2SO4 (10−4, 10−6 N) and NaOH (10−6 to 10−2 N) electrolyte solutions. The improvement in the onset potential for the ORR ranged between 80 and 160 mV as the pH was changed from 4 to 12. Based on XAFS data and ORR polarization in the presence of EDTA-Na2 and ED + AA, we believe that the MnN4 moiety interacting with the protonated amine is the most probable active site contributing to ORR activity in the H2SO4 (10−4, 10−6 N) and NaOH (10−6 to 10−2 N) electrolyte solutions.

Local structure of Ge quantum dots determined by combined numerical analysis of EXAFS and XANES data

The sensitivity of X-ray absorption near-edge structure (XANES) to the local symmetry has been investigated in small (∼4 nm) matrix-free Ge quantum dots. The FDMNES package was used to calculate the theoretical XANES spectra that were compared with the experimental data of as-prepared and annealed nanoparticles. It was found that XANES data for an as-prepared sample can only be adequately described if the second coordination shell of the diamond-type structural model is included in the FDMNES calculations. This is in contrast to the extended X-ray absorption fine-structure data that show only the first-shell signal. These results suggest that, despite the high degree of disorder and a large surface-to-volume ratio, as-prepared small Ge quantum dots retain the diamond-type symmetry beyond the first shell. Furthermore, we utilized this sensitivity of XANES to the local symmetry to study annealed Ge quantum dots and found evidence for significant structural distortion which we attribute to the existence of surface disorder in the annealed oxygen-free Ge quantum dots.

The chemical transformation of hydrocarbons to carbon using SO3 sources

To develop low-cost carbon fiber, The Dow Chemical Company and others have explored the sulfonation of polyethylene fibers as an alternative to the incumbent poly(acrylonitrile) route. Although the process of polyethylene sulfonation and subsequent thermal carbonization in an inert atmosphere has been known to provide carbonaceous material for over 35years, we have found the chemical understanding of this transformation to be insufficient. Herein, we report a series of studies that have led to our current understanding for both the sulfonation and subsequent thermal treatment steps. Sulfonation of hydrocarbon model compounds yields completely conjugated and functionalized products. Spectroscopic data suggest that sulfonated polyethylene is similar; containing extended conjugated systems with sulfonic acids and various other oxygen-containing functional groups. Near-Edge X-ray Absorption Fine Structure data have identified that between 150 and 200°C the polymer undergoes a cross-linking step, while evolved gas analysis has identified concomitant release of SO2 and H2O. Above 600°C, H2 is produced and a graphenic carbon microstructure is obtained.

Investigation of Cationic Distribution in Zn1−x Ni x Fe2O4 (x = 0.0 to 1.0) by Employing X-ray Absorption Fine Structure Spectroscopy

In the present study, x-ray absorption fine structure (XAFS) spectroscopy has been employed to investigate the cationic distribution in Zn1−xNixFe2O4 (x = 0.0 to 1.0) spinel ferrites. The phase purity and lattice parameters of freshly synthesized Zn1−xNixFe2O4 were verified by laboratory x-ray diffraction analysis. It is observed that lattice parameters decrease with increase in the Ni concentration, while the system retains cubic symmetry in Fd3m space group. To investigate the local structural environment and cationic distribution, XAFS data were collected at room temperature for the Fe, Zn, and Ni K-edges in Zn1−xNixFe2O4, using the x-ray absorption facilities of the Elettra synchrotron source, Trieste, Italy. In the case of Fe, pre-edge features provide valuable information regarding the octahedral or tetrahedral site occupancy, while Zn and Ni edges do not show any appreciable change in the near-edge region. From extended x-ray absorption fine structure (EXAFS) spectroscopy, it is observed that all Zn and Ni atoms occupy tetrahedral and octahedral sites, respectively, whereas Fe atoms occupy octahedral sites in ZnFe2O4 but for the other compositions are present at both octahedral and tetrahedral sites. The occupancy of Fe at octahedral sites decreases with increasing Ni concentration in Zn1−xNixFe2O4. These results demonstrate that the local environment of Zn does not show significant variations as the changes at second nearest neighbors are due to Fe and Ni, which have similar scattering factors. However, variations in the Ni surroundings are due to the change of Fe and Zn ratio at A sites, having a relatively larger difference in scattering factor. However, the changes observed in the EXAFS spectra of Fe are a consequence of variation in the occupancy of Fe at tetrahedral and octahedral sites.

<i>In Situ</i> XAFS Methods for Characterizing Catalyst Structure in Proton Exchange Membrane Fuel Cell

We established and developed an in situ X-ray absorption fine structure (XAFS) experimental testing device for characterizing hydrogen-oxygen proton exchange membrane fuel cells (PEMFC) on XAFS beamline BL14W1 at the Shanghai Synchrotron Radiation Facility (SSRF). XAFS data were collected under the operating state of the fuel cell with Pt/C and Pd/C as the cathode and anode catalysts, respectively, while the cell current-voltage (J-V) Curve and power density curves were monitored. Changes in the oxidation states of the Pt/C catalyst were observed during the reaction process at different potentials. Strong Pt-O bonds on the surfaces of the Pt were found to be induced at high potential; this may hinder the performance of Pt and reduce its oxygen reduction reaction (ORR) activity. The study also verified the reliability and feasibility of the herein established experimental apparatus and technique.

Surface Structural Reconstruction for Optical Response in Iodine-Modified TiO2 Photocatalyst System

We report an alternative mechanism for the optical response of an iodine-modified anatase TiO2 photocatalyst material. Unlike the general realization that the iodine atom provides impurity levels within the optical band gap, we suggest that a distorted surface structure plays a dominant role. Anatase pure TiO2 and iodine-modified TiO2 (1-TiO2) nanocrystals were synthesized by a simple solvothermal method, and I-TiO2 exhibited an extended absorption edge up to 550 nm. Employing iodine K-edge X-ray absorption fine structure (XAFS), we demonstrate that iodine is not incorporated into the nanoparticle interior but exists in the form of IO3- groups at the surface. Furthermore, this IO3- cluster adsorption largely induces a disordered structure, as revealed by Ti 2p(3/2) X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Ti K-edge XAFS data fitting. Density functional theory calculation indicates that this distorted structure can form a midgap state, which should be responsible for its excellent optical property. This finding represents a promising route for improving the optical response of the nanophotocatalyst system via surface treatment.

Unraveling the Impurity Location and Binding in Heavily Doped Semiconductor Nanocrystals: The Case of Cu in InAs Nanocrystals

The doping of colloidal semiconductor nanocrystals (NCs) presents an additional knob beyond size and shape for controlling the electronic properties. An important problem for doping with aliovalent elements is associated with resolving the location of the dopant and its structural surrounding within small NCs, an issue directly connected with self-purification. Here we used a postsynthesis diffusion-based doping method for introducing Cu impurities into InAs quantum dots. X-ray absorption fine structure (XAFS) spectroscopy experiments along with first-principle density functional theory (DFT) calculations were used to probe the impurity sites. The concentration dependence was investigated for a wide range of doping levels, helping to derive a self-consistent picture where the Cu impurity occupies an interstitial site within the InAs lattice. Moreover, at extremely high doping levels, Cu–Cu interactions are identified in the XAFS data. This structural model is supported by X-ray diffraction data, along wit...

Effect of Pore Size and Nickel Content of Ni-MCM-41 on Catalytic Activity for Ethene Dimerization and Local Structures of Nickel Ions

The catalytic activity of nickel ion-loaded mesoporous silica MCM-41 (Ni-M41) for ethene dimerization was investigated as a function of the pore size and the amount of nickel. In addition, the silica wall and the loading of the nickel species were characterized. The Ni-M41 samples with smaller pore size and higher Si/Ni ratio exhibited greater reaction rate constants. The Fourier transform infrared (FT-IR) spectra indicated the formation of 2:1 nickel phyllosilicate-like species along the pore wall. Furthermore, the IR band at approximately 570 cm–1 and the X-ray absorption fine structure (XAFS) spectra indicated the existence of five-membered rings consisting of Si–O on the M41 pore wall in addition to the typical six-membered ones. On the basis of the UV–vis–NIR diffuse reflectance (UV–vis–NIR DR), FT-IR, and XAFS data, we propose that the three- and four-coordinated Ni2+ ions lie on the five- and six-membered Si–O rings of silica, respectively. Nitrogen monoxide was employed as a probe molecule in the ...

Distinctive Arsenic(V) Trapping Modes by Magnetite Nanoparticles Induced by Different Sorption Processes

Arsenic sorption onto iron oxide spinels such as magnetite may contribute to arsenic immobilization at redox fronts in soils, sediments, and aquifers, as well as in putative remediation and water treatment technologies. We have investigated As(V) speciation resulting from different sorption processes on magnetite nanoparticles, including both adsorption and precipitation, using X-ray absorption fine structure (XAFS) spectroscopy and transmission electron microscopy (TEM). XAFS results suggest that AsO(4) tetrahedra form predominantly inner-sphere bidentate corner-sharing ((2)C) complexes and outer-sphere complexes on magnetite in the adsorption experiments. In the precipitation experiments, an increasing fraction of AsO(4) tetrahedra appears to be incorporated in clusters having a magnetite-like local structure with increasing As loading, the remaining fraction of As being adsorbed at the surface of magnetite particles. In the sample with the highest As loading (15.7 μmol/m(2)) XAFS data indicate that As(V) is fully incorporated in such clusters. Such processes help to explain the significantly higher arsenic uptake in precipitation samples compared to those generated in adsorption experiments. In addition, for the precipitation samples, TEM observations indicate the formation of amorphous coatings and small (~3 nm) nanoparticles associated with larger (~20-40 nm) magnetite nanoparticles, which are absent in the adsorption samples. These results suggest that As(V) could form complexes at the surfaces of the small nanoparticles and could be progressively incorporated in their structure with increasing As loading. These results provide some of the fundamental knowledge about As(V)-magnetite interactions that is essential for developing effective water treatment technologies for arsenic.

Pyrovanadolysis, a Pyrophosphorolysis-like Reaction Mediated by Pyrovanadate, Mn2+, and DNA Polymerase of Bacteriophage T7

DNA polymerases catalyze the 3'-5'-pyrophosphorolysis of a DNA primer annealed to a DNA template in the presence of pyrophosphate (PP(i)). In this reversal of the polymerization reaction, deoxynucleotides in DNA are converted to deoxynucleoside 5'-triphosphates. Based on the charge, size, and geometry of the oxygen connecting the two phosphorus atoms of PP(i), a variety of compounds was examined for their ability to carry out a reaction similar to pyrophosphorolysis. We describe a manganese-mediated pyrophosphorolysis-like activity using pyrovanadate (VV) catalyzed by the DNA polymerase of bacteriophage T7. We designate this reaction pyrovanadolysis. X-ray absorption spectroscopy reveals a shorter Mn-V distance of the polymerase-VV complex than the Mn-P distance of the polymerase-PP(i) complex. This structural arrangement at the active site accounts for the enzymatic activation by Mn-VV. We propose that the Mn(2+), larger than Mg(2+), fits the polymerase active site to mediate binding of VV into the active site of the polymerase. Our results may be the first documentation that vanadium can substitute for phosphorus in biological processes.

X-ray absorption spectroscopic evidence for the formation of Pb(II) inner-sphere adsorption complexes and precipitates at the alkaline soil-water interface

Adsorption mechanisms of Pb on soil with high CaCO3 content were investigated by combined batch sorption and X-ray absorption fine structure (XAFS). Date from the batch equilibrium studies showed that Pb sorption was nonlinear and was well fitted to Langmiur isotherm. The XAFS data indicated that Pb could be adsorbed via the inner-sphere complex, the precipitation of calcium carbonate containing Pb (PbCaCO3), and outer-sphere Pb sorption complex. The formations of inner-sphere complexes and PbCaCO3 implied strong metal interactions with the surfaces the mechanistic reason for the affinity of Pb for CaCO3 as observed in macroscopic studies. At low metal concentration, 500 mg x L(-1) of initial Pb, radial distance of the first-shell Pb-O (R1) was 0.169 2 nm, however, at 1 000 mg x L(-1) of initial Pb, the R1 was 0.166 8 nm. These revealed that the percentage of inner-sphere complexes increased when the initial Pb was increased from 500 to 1 000 mg x L(-1).

X-ray absorption fine structure (XAFS) study of the effects of pH on Pb(II) sorption by soil

Combined batch sorption and in situ X-ray absorption fine structure (XAFS) provide direct assessment of the mechanisms for Pb(II) sorption at the soil-water interface under different pH conditions. The XAFS data indicated that the innersphere Pb sorption complex with ionic character (Pb4 (OH)4(4+)) dominated the Pb surface speciation, and the outer-sphere Pb sorption complex and the precipitation of calcium carbonate containing Pb(PbCaCO3) were also involved in the adsorption samples. Coordination number and radial distance of the first-shell Pb-O decreased from 0.172 7 to 0.166 6 nm and the percentage of inner-sphere complexes increased when the initial pH changed from 6.0 to 8.5, indicating that the mechanism of Pb(II) sorption by the soil was pH-dependent.