X-ray Absorption Spectroscopy Techniques Research Articles

X-ray absorption spectroscopy investigations on radioactive matter using MARS beamline at SOLEIL synchrotron

Abstract The MARS beamline at the SOLEIL synchrotron is dedicated to the characterization of radioactive material samples. One great advantage of the beamline is the possibility to characterize about 380 radionuclides by different X-ray techniques in the same place. This facility is unique in Europe. A wide energy range from around 3.5 keV to 36 keV K-edges from K to Cs, and L3 edges from Cd to Am and beyond can be used. The MARS beamline is optimized for X-ray absorption spectroscopy techniques (XANES/EXAFS), powder diffraction (XRD) but x-ray fluorescence (XRF) analysis, High Energy Resolution Fluorescence Detected -XAS (HERFD-XAS), X-ray Emission (XES) and μ-XAS/XRD are also possible. A description of the beamline as well as its performances are given in a first part. Then some scientific examples of XAS studies from users are presented which cover a wide variety of topics in radiochemistry and nuclear materials.

Formation of Li2MnO3 investigated by in situ synchrotron probes

Both in situ high-energy X-ray diffraction and in situ X-ray absorption spectroscopy were used to investigate the structural evolution of materials during the solid-state synthesis of Li2MnO3. Combing X-ray absorption spectroscopy and factor analysis techniques, we were able to capture the spectrum and evolution of an intermediate phase (MnO2) that could not be detected by the diffraction technique. Meanwhile, the X-ray diffraction data clearly showed the anisotropic crystallization of Li2MnO3 during sintering above 600 °C.

Transition Elements and Nucleation in Glasses Using X‐ray Absorption Spectroscopy

Selected results on the environment around Zr, Ti, and Ni in glasses and glass‐ceramics using X‐ray absorption spectroscopy (XAS) are presented to illustrate how this technique can provide information on the nucleation/crystallization processes. These elements actively participate to nucleation and, additionally, some of them bring new optical properties to the material. We show that extended X‐ray absorption fine structure (EXAFS) and X‐ray absorption near edge structure (XANES) information, at K‐edge or L2,3‐edges, can give short and medium range structural and bonding information. Nucleation activity of transition elements cannot be concluded from their local environment within the investigated parent glasses (at least for Zr and Ti, the main nucleating agents). On the contrary, medium range organization is important to understand nucleation processes with the evidence of heterogeneities or structural fluctuations that can be directly related to the first crystallizing phases. New developments in the spatial resolution or in situ high temperature measurements show the utility of the XAS technique with environmental changes around transition elements that can be detected prior to macroscopic crystallization. This can be useful to determine whether a specific ion (e.g., Ni2+) enters the crystalline phase and in which site, allowing a better control of the temperature or crystallization kinetics to optimize the material properties.

High energy resolution off-resonant spectroscopy for x-ray absorption spectra free of self-absorption effects.

X-ray emission spectra recorded in the off-resonant regime carry information on the density of unoccupied states. It is known that by employing the Kramers-Heisenberg formalism, the high energy resolution off-resonant spectroscopy (HEROS) is equivalent to the x-ray absorption spectroscopy (XAS) technique and provides the same electronic state information. Moreover, in the present Letter we demonstrate that the shape of HEROS spectra is not modified by self-absorption effects. Therefore, in contrast to the fluorescence-based XAS techniques, the recorded shape of the spectra is independent of the sample concentration or thickness. The HEROS may thus be used as an experimental technique when precise information about specific absorption features and their strengths is crucial for chemical speciation or theoretical evaluation.

10 cm x 10 cm Single Gas Electron Multiplier (GEM) X-ray Fluorescence Detector for Dilute Elements

We have built and tested a 10 cm × 10 cm single Gas Electron Multiplier (GEM) X-ray detector to probe dilute amounts of Fe in a prepared sample. The detector uses Argon/Carbon Dioxide (75/25) gas mixture flowing at a slow rate through a leak proof Plexi-glass enclosure held together by O-rings and screws. The Fluorescence X-ray emitted by the element under test is directed through a Mylar window into the drift region of the detector where abundant gas is flowing. The ionized electrons are separated, drifted into the high electric field of the GEM, and multiplied by impact ionization. The amplified negatively charged electrons are collected and further amplified by a Keithley amplifier to probe the absorption edge of the element under test using X-ray absorption spectroscopy technique. The results show that the GEM detector provided good results with less noise as compared with a Silicon drift detector (SDD).

Unique effect of Cu(II) in the metal-induced amyloid formation of β-2-microglobulin.

β-2-Microglobulin (β2m) forms amyloid fibrils in the joints of patients undergoing hemodialysis treatment as a result of kidney failure. In the presence of stoichiometric amounts of Cu(II), β2m self-associates into discrete oligomeric species, including dimers, tetramers, and hexamers, before ultimately forming amyloid fibrils that contain no copper. To improve our understanding of whether Cu(II) is unique in its ability to induce β2m amyloid formation and to delineate the coordinative interactions that allow Cu(II) to exert its effect, we have examined the binding of Ni(II) and Zn(II) to β2m and the resulting influence that these metals have on β2m aggregation. We find that, in contrast to Cu(II), Ni(II) does not induce the oligomerization or aggregation of β2m, while Zn(II) promotes oligomerization but not amyloid fibril formation. Using X-ray absorption spectroscopy and new mass spectrometry-related techniques, we find that different binding modes are responsible for the different effects of Ni(II) and Zn(II). By comparing the binding modes of Cu(II) with Ni(II), we find that Cu(II) binding to Asp59 and the backbone amide between the first two residues of β2m are important for allowing the formation of amyloid-competent oligomers, as Ni(II) appears not to bind these sites on the protein. The oligomers formed in the presence of Zn(II) are permitted by this metal’s ability to bridge two β2m units via His51. These oligomers, however, are not able to progress to form amyloid fibrils because Zn(II) does not induce the required structural changes near the N-terminus and His31.

Local structure determination of substitutional elements in Ca3Co4−xMxO9(M = Fe, Cr, Ga) using X-ray absorption spectroscopy

Ca3Co4O9 is one of the most promising thermoelectric oxide materials at high temperature. Its structure consists of two misfit layers: the CaOCoOCaO rocksalt-type (RS) layer and the CdI2-type CoO2 layer. In this paper, we reported the synthesis of single phase Ca3Co4−xMxO9 (where M = Fe, Cr, and Ga). Thermoelectric measurements showed the variation of thermoelectric properties depending on the charge state of the substitutional elements. We showed the direct evidence for the location of the substitutional elements in the system using an X-ray absorption spectroscopy technique. The extended X-ray absorption fine structure spectra were fitted with the models. Also, the X-ray absorption near edge structure spectra were compared with the simulated spectra from the first principle calculation. The data analysis show that for Fe and Cr substitution, the Fe and Cr atoms are more likely to be located in the RS layer rather than in the CoO2 as presumed previously. For the Ga case, Ga atoms accommodate both sites; still the majority stays in the RS layer. This is supported by the calculation of the total energy of the system, which showed that the total energy is lower when the substituted elements were located in the RS layer.

In-situ reflection-XANES study of ZDDP and MoDTC lubricant films formed on steel and diamond like carbon (DLC) surfaces

Chemical characterisation of boundary lubricated interfaces is essential for developing mechanistic models that describe lubricant additive interactions with the surface and their effect on tribological performance. In this study the potential for using the synchrotron-based reflection mode X-ray absorption spectroscopy (XAS) technique for in-situ chemical characterisation of lubricant films has been studied. Thermal films formed from zinc dialkyl dithio phosphate (ZDDP) and molybdenum dialkyl dithio carbamate (MoDTC) lubricant additives have been formed and analysed in-situ using the X-ray absorption near edge structure (XANES) spectroscopy technique. The surface sensitivity of this approach was improved by doing the analysis in reflection mode, enabling analysis of only top layer (up to around 10nm) of the solid surface. In addition, in-lubro analysis of pre-formed tribofilms from the same additives was done using non-vacuum conditions. The results are discussed in conjunction with XANES and X-ray photoelectron spectroscopy (XPS) analysis of similar additives published in the literature. The results obtained are consistent with the existing ZDDP and MoDTC literature and provide some new insight into intermediate species not reported before. The advantages and disadvantages of the developed XANES methodology for in-situ surface chemical analysis of lubricated conditions are discussed.

Development of in situ soft X-ray absorption spectroscopic technique under high temperature and controlled atmosphere

Abstract In this study, in situ soft X-ray absorption spectroscopy (soft XAS) technique, which enables us to analyze electronic structures of oxides at elevated temperatures while controlling atmospheric conditions, was developed. The technique was applied to investigate the electronic structures of La0.6Sr0.4CoO3 − δ. X-ray absorption spectra at the Co L-edges and the O K-edge were measured in the temperature range from room temperature to 873 K and the p(O2) range from 10− 4 to 10− 2 bar. The developed in situ soft XAS technique is found to be effective for evaluating electronic structures of oxides under controlled temperature and p(O2).

Huge Photoresistance in Transparent and Conductive Indium Titanium Oxide Films Prepared by Electron Beam–Physical Vapor Deposition

Transparent and conductive indium titanium oxide (ITiO) films have been obtained by electron beam physical vapour deposition with Ti content from 5 at % up to 28 at %. X-ray absorption spectroscopy techniques have been used to identify the local environment of Ti ions. Even at the lowest concentrations Ti is not incorporated into the In2O3 structure but forms clusters of a Ti-In mixed oxide that present a distorted rutile TiO2 short-range order. The optical transmittance of the annealed samples reaches 95 % and no significant variation of the gap energy (around 3.7 eV) is observed with Ti content. The electronic conductivity under light irradiation is studied evidencing a huge photo-resistance in the samples with Ti content above 22 at % reaching more than two orders of magnitude for the 26 at % Ti under illumination with few μW/cm(2) at 365 nm. Hall and conductivity results are analyzed using a model that takes into account both electron and hole carriers as well as the conductivity enhancement by carrier photogeneration. The electron carrier density decreases with Ti content while its mobility increases up to values of 1000 cm(2)/(V s). Oxygen annealed ITiO films obtained by this technique with Ti content below 10 at % have properties adequate as transparent semiconductors and those with Ti content higher than 22 at % have exceptional photoresistive properties relevant for numerous applications.

Investigation of Non-Precious Metal CoN4-Based Oxygen Reduction Catalyst by Electrochemical and X-ray Absorption Spectroscopy Techniques

Electrochemical and X-ray absorption techniques are applied to determine the influence of pyrolysis and nitrogen-containing precursor loading on the catalytic activity and structure of carbon-supported cobalt triethylenetetramine (CoTETA/C) catalyst. These CoTETA/C catalysts with different nitrogen-containing precursor loadings are synthesized from pyrolyzing cobalt triethylenetetramine supported on carbon at 800°C in Ar atmosphere. For comparison, the as-prepared (without pyrolysis) is also studied. Cyclic voltammograms and rotating disk electrode measurements indicate that the as-prepared presents very low activity, while the CoTETA/C catalysts with different nitrogen-containing precursor loadings show good catalytic activity and the maximum is found for CoTETA/C catalyst with Co:TETA = 1:4. X-ray absorption measurements at Co and N K edges show that after pyrolysis the CoN4 centers are no longer detected along with little nitrogen atoms inserted into carbon matrix and nanometallic α-Co clusters are formed. X-ray diffraction also confirms the presence of nanometallic α-Co particles on the catalysts.

Synchrotron X-ray absorption spectroscopy analysis of arsenic chemical speciation in human nail clippings

Environmental context Chronic ingestion of arsenic leads to its accumulation in keratinous tissues, which can represent a risk factor for developing cancer. We use synchrotron X-ray absorption spectroscopy to investigate chemical bonding of arsenic in the keratins from nail clippings of volunteers from areas in Atlantic Canada with low-to-moderate arsenic contamination of drinking water. The study helps our understanding of arsenic metabolism and its role in cancer development. Abstract Drinking water aquifers in many areas of the world have naturally elevated levels of inorganic arsenic exceeding the World Health Organization limit. Arsenic concentrations in human nail clippings are commonly used as a biomarker of exposure to this toxic element. However, the chemical form of arsenic accumulated in nail tissues is not well determined. We employed synchrotron microprobe and bulk X-ray absorption spectroscopy techniques to analyse the concentration and chemical speciation of arsenic in the finger- and toenail clippings of volunteers living in the vicinity of Sackville, New Brunswick, Canada. This area is known to have low-to-moderately elevated levels of arsenic in ground water. Arsenic species in clippings were represented by three main groups, distinguished by the As-K near-edge X-ray absorption fine structure spectra: (1) AsIII type, which can be fitted as a mixture of As bound to thiols, and also to oxygen or methyl groups, with a small contribution from AsV species, (2) AsV type, best represented by fitting arsenate in aqueous solution and (3) The AsIII+AsV mixture type. The high proportion (%) of sulfur-bound arsenic species most likely corresponds to binding between arsenic (in its trivalent and, to a lesser extent, pentavalent forms) and cysteine residues in the sulfur-rich fraction of keratin and keratin-associated proteins. Further work is needed to explore whether these chemical species could be used as toxicity biomarkers of human exposure to elevated levels of As in drinking water.

Understanding the Rate Capability of High‐Energy‐Density Li‐Rich Layered Li1.2Ni0.15Co0.1Mn0.55O2 Cathode Materials

The high‐energy‐density, Li‐rich layered materials, i.e., xLiMO2(1‐x)Li2MnO3, are promising candidate cathode materials for electric energy storage in plug‐in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). The relatively low rate capability is one of the major problems that need to be resolved for these materials. To gain insight into the key factors that limit the rate capability, in situ X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) studies of the cathode material, Li1.2Ni0.15Co0.1Mn0.55O2 [0.5Li(Ni0.375Co0.25 Mn0.375)O2·0.5Li2MnO3], are carried out. The partial capacity contributed by different structural components and transition metal elements is elucidated and correlated with local structure changes. The characteristic reaction kinetics for each element are identified using a novel time‐resolved XAS technique. Direct experimental evidence is obtained showing that Mn sites have much poorer reaction kinetics both before and after the initial activation of Li2MnO3, compared to Ni and Co. These results indicate that Li2MnO3 may be the key component that limits the rate capability of Li‐rich layered materials and provide guidance for designing Li‐rich layered materials with the desired balance of energy density and rate capability for different applications.

In situ XAS study of an improved natural phosphate catalyst for hydrogen production by reforming of methane

Some nickel catalysts supported on natural phosphate (NP) have been tested for the dry methane reforming reaction. Although the original impregnated 15%Ni/NP catalyst has no activity at all, the modification of the support by mechano-chemical and/or acid treatment strongly improved the catalytic performance, yielding a series of very active and stable catalysts. The chemical and physical characterization by X-ray diffraction (XRD), temperature programmed reduction (TPR), in situ X-ray absorption spectroscopy (XAS) and other techniques have shown that these treatments mainly modify the interaction between the nickel phase and the support surface. The nickel ions occupy calcium position in the surface of the phosphate phase, which stabilizes and improves the dispersion of nickel species. The final reduced catalysts present a much better dispersed metallic phase interacting with the NP surface, which has been identified as responsible for the observed outstanding catalytic performances.

Local structure of core/shell nanocrystalline particles produced by surfactant-assisted ball milling of Fe powder in paraffin

Surfactant-assisted ball milling of the Fe powder in paraffin has been used for fabrication of core–shell nanocrystalline particles. The local atomic structure of the bulk and surface layers of the mechanically milled particles has been studied using X-ray absorption spectroscopic techniques with synchrotron radiation from the DORIS storage ring at DESY, Hamburg. Regardless of milling environment composition, the as-prepared powders were shown to be characterized by a significant drop in the EXAFS signal intensity and coordination numbers of the Fe–Fe pairs due to the formation of nanocrystalline state in the particles. It has been shown that an addition of perfluorononanoic acid as a surfactant has a more prominent effect on the structure of the shell layers. The effect is revealed as an appearance of light element atoms (O, F, C) in the local atomic environment of the Fe atoms due to formation of oxide, carbide and adsorbed structures of different types in the particle shell.

Evidence for core–shell nanoclusters in oxygen dispersion strengthened steels measured using X-ray absorption spectroscopy

Nanostructured ferritic alloys (NFA) dispersion strengthened by an ultra high density of Y–Ti–O enriched nano-features (NF) exhibit superior creep strength and the potential for high resistance to radiation damage. However, the detailed character of the NF, that precipitate from solid solution during hot consolidation of metallic powders mechanically alloyed with Y2O3, are not well understood. In order to clarify the nature of the NF, X-ray absorption spectroscopy (XAS) technique, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were used to characterize the local structure of the Ti and Y atoms in both NFA powders and consolidated alloys. The powders were characterized in the as-received, as-milled and after annealing milled powders at 850, 1000 and 1150°C. The consolidated alloys included powders hot isostatic pressed (HIPed) at 1150°C and commercial vendor alloys, MA957 and J12YWT. The NFA XAS data were compared various Ti and Y-oxide standards. The XANES and EXAFS spectra for the annealed and HIPed powders are similar and show high temperature heat treatments shift the Y and Ti to more oxidized states that are consistent with combinations of Y2Ti2O7 and, especially, TiO. However, the MA957 and J12YWT and annealed–consolidated powder data differ. The commercial vendor alloys results more closely resemble the as-milled powder data and all show that a significant fraction of substitutional Ti remains dissolved in the (BCC) ferrite matrix.

Structure of PtnNi Nanoparticles Electrocatalysts Investigated by X-ray Absorption Spectroscopy

Carbon supported Pt-Ni nanoparticles (NPs) electrocatalysts with nominal Pt/Ni atomic ratios of 3:1, 2:1, and 1:1, denoted as Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C, respectively, were obtained by a modified polyol process. The structure of PtnNi/C (n = 3, 2, I) electrocatalysts was studied by using an X-ray absorption spectroscopy technique combined with X-ray diffraction and transmission electron microscopy. Significantly, the NPs of Pt3Ni/C in air are demonstrated to have a quasi core-shell structure consisting of a core of metallic Pt surrounded by many small NiOx clusters. In contrast, both Pt2Ni/C and Pt1Ni/C are demonstrated to have an alloy structure with partial oxidation on the surface. Under the atmosphere of H-2 at 393 K, the PtnNi/C became the expected bimetallic alloy. At last, we discuss the effect of Ni amount on the structure of PtnNi/C and estimate their possible catalytic activity for methanol oxidation reaction. Our results further confirmed that the structure of Pt-Ni NPs can be influenced by Ni amount and this effect may be enlarged by environment effects.

In Situ Ru K-Edge X-ray Absorption Spectroscopy Study of Methanol Oxidation Mechanisms on Model Submonolayer Ru on Pt Nanoparticle Electrocatalyst

In situ X-ray absorption spectroscopy (XAS) with electrochemical reaction control has enabled a detailed investigation of the mechanism of the methanol electrooxidation by a bimetallic PtRu catalyst. By the use of an original electrodeposition technique, ca. 0.3 monolayer of Ru was deposited on the surface of unsupported Pt nanoparticles (Ru@Pt). The presence of Ru atoms only at the surface of nanoparticles turns a bulk sensitive XAS technique into a surface methodology which permits correlation of the X-ray absorption fine structure at the Ru K-edge to the role of Ru atoms in the methanol oxidation process. In situ XAS spectra of the Ru@Pt nanoparticles were collected at various electrode potentials in background electrolyte, and then in 1 M solution of methanol (CH3OH) in the same electrolyte. Significant differences in the catalyst state have been revealed between these two environments. In the background electrolyte, Ru gradually oxidizes from mostly metallic to a Ru(III)/Ru(IV) mixture at the highest potentials. In the presence of CH3OH, the Ru oxidation state remains a mixture of metallic and Ru(III) even at the highest potentials. CO-type species were found adsorbed on Ru atoms at all potentials and coadsorbed with OH species at potentials 0.175 V vs Ag/AgCl and higher with steady number of near neighbors. The same potential correlates to the beginning of methanol oxidation reaction observed electrochemically. Therefore coadsorption of CO and OH groups on Ru atoms appears to be critical in the methanol oxidation process. The need for OH groups for CO removal from Pt sites was previously suggested by the bifunctional CH3OH oxidation mechanism; however, occupancy of Ru atoms with CO species and direct structural observations of the coadsorption of CO and OH on the same Ru atom is a novel finding resulting from this study. Other changes in the catalyst environment were observed, including decreased Ru–Ru and Ru–Pt bond distances, increased numbers of Ru–Pt near neighbors, and decreased number of Ru–Ru near neighbors.

Probing local structure of pyrochlore lead zinc niobate with synchrotron x-ray absorption spectroscopy technique

Local structure of lead zinc niobate (PZN) ceramic, synthesized via B-site oxide precursor route in atmospheric pressure, was investigated using synchrotron x-ray absorption spectroscopy (XAS) technique. The x-ray absorption near-edge structure (XANES) simulation was first carried out. The XANES simulation results indicate that the PZN ceramic is in pyrochlore phase having Zn2+ substituted on Nb5+ site. Afterwards, the extended x-ray absorption fine structure (EXAFS) analysis was performed to extract the bond length information between Zn2+ and its neighboring atoms. From the EXAFS fitting, the bond length between Zn2+ and Pb2+ in the pyrochlore phase was found to be longer than the previously reported bond length in the perovskite phase. Further, with the radial distribution information of Zn2+'s neighboring atoms, the formation energies along the precursor-to-pyrochlore and precursor-to-perovskite reaction paths were calculated using the density functional theory (DFT). The calculated results show that the formation energy of the perovskite phase is noticeably higher than that of the pyrochlore phase, which is influenced by the presence of energetic Pb2+ lone pair, as the perovskite phase has shorter Zn2+ to Pb2+ bonding. This therefore suggests the steric hindrance of Pb2+ lone pair and the mutual interactions between Pb2+ lone pair and Zn2+ are main causes of the instability of lead zinc niobate in the perovskite structure and confirm the efficacy of XAS and DFT analysis in revealing local structural details of complex pyrochlore materials.

High resolution scanning tunnelling microscopy and extended x-ray-absorption fine structure study of the (5 × 3) silicide structure on Cu(001)

Using low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), and x-ray absorption spectroscopy (XAS) techniques, we have studied the first steps of silicon adsorption onto Cu (001) single crystal substrate. For low coverage (∼0.5 ML) and after annealing at 100 °C, STM images and LEED patterns reveal the formation of an ordered quasi commensurate (5×3) superstructure. From a quantitative analysis of XAS data, we extract the Si-Cu distance and detail the local atomic arrangement of the (5×3) structure.