Surface EXAFS via differential electron yield

Surface‐sensitive analysis via extended X‐ray absorption fine‐structure (EXAFS) spectroscopy is demonstrated using a thickness‐defined SiO₂ (12.4 nm)/Si sample. The proposed method exploits the differential electron yield (DEY) method wherein Auger electrons escaping from a sample surface are detected by an electron analyzer. The DEY method removes local intensity changes in the EXAFS spectra caused by photoelectrons crossing the Auger peak during X‐ray energy sweeps, enabling EXAFS analysis through Fourier transformation of wide‐energy‐range spectral oscillations. The Si K‐edge DEY X‐ray absorption near‐edge structure (XANES) spectrum appears to comprise high amounts of SiO₂ and low Si content, suggesting an analysis depth, as expressed using the inelastic mean free path of electrons in general electron spectroscopy, of approximately 4.2 nm. The first nearest neighbor (Si—O) distance derived from the Fourier transform of the Si K‐edge DEY‐EXAFS oscillation is 1.63 Å. This value is within the reported values of bulk SiO₂, showing that DEY can be used to detect a surface layer of 12.4 nm thickness with an analysis depth of approximately 4.2 nm and enable `surface EXAFS' analysis using Fourier transformation.     

In situ X‐ray absorption spectroscopic studies of copper in a copper‐rich sludge during electrokinetic treatments

Speciation of copper in a copper‐rich chemical‐mechanical polishing sludge during electrokinetic treatment has been studied by in situ extended X‐ray absorption fine structure (EXAFS) and X‐ray absorption near‐edge structure (XANES) spectroscopy. The least‐squares‐fitted XANES spectra indicate that the main copper species in the sludge are Cu(OH)₂ (74%), nanosize CuO (20–60 nm) (13%) and CuO (>100 nm) (13%). The average bond distance and coordination number (CN) of Cu—O are 1.96 Å and 3.5, respectively. Under electrokinetic treatment (5 V cm^?1) for 120 min, about 85% of the copper is dissolved in the electrolyte, 13% of which is migrated and enriched on the cathode. Notably the copper nanoparticles in the sludge can also migrate to the cathode under the electric field. By in situ EXAFS, it is found that during the electrokinetic treatment the bond distance and CN of Cu—O are increased by 0.1 Å and 0.9, respectively.     

Extraction of local coordination structure in a low‐concentration uranyl system by XANES

Obtaining structural information of uranyl species at an atomic/molecular scale is a critical step to control and predict their physical and chemical properties. To obtain such information, experimental and theoretical L₃‐edge X‐ray absorption near‐edge structure (XANES) spectra of uranium were studied systematically for uranyl complexes. It was demonstrated that the bond lengths (R) in the uranyl species and relative energy positions (ΔE) of the XANES were determined as follows: ΔE₁ = 168.3/R(U—O_ax)² ? 38.5 (for the axial plane) and ΔE₂ = 428.4/R(U—O_eq)² ? 37.1 (for the equatorial plane). These formulae could be used to directly extract the distances between the uranium absorber and oxygen ligand atoms in the axial and equatorial planes of uranyl ions based on the U L₃‐edge XANES experimental data. In addition, the relative weights were estimated for each configuration derived from the water molecule and nitrate ligand based on the obtained average equatorial coordination bond lengths in a series of uranyl nitrate complexes with progressively varied nitrate concentrations. Results obtained from XANES analysis were identical to that from extended X‐ray absorption fine‐structure (EXAFS) analysis. XANES analysis is applicable to ubiquitous uranyl–ligand complexes, such as the uranyl–carbonate complex. Most importantly, the XANES research method could be extended to low‐concentration uranyl systems, as indicated by the results of the uranyl–amidoximate complex (～40 p.p.m. uranium). Quantitative XANES analysis, a reliable and straightforward method, provides a simplified approach applied to the structural chemistry of actinides.     

Insight into growth of Au–Pt bimetallic nanoparticles: an in situ XAS study

Au–Pt bimetallic nanoparticles have been synthesized through a one‐pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X‐ray absorption spectroscopy (XAS) and UV–Vis spectroscopy at the energy‐dispersive EXAFS beamline (BL‐08) at Indus‐2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X‐ray near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L₃‐edges. While the XANES spectra of the precursors provide real‐time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core–shell‐type configuration of a bimetallic alloy in a one‐pot synthesis method. The core–shell‐type structure of the nanoparticles has also been confirmed by ex situ energy‐dispersive spectroscopy line‐scan and X‐ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.     

XANES analysis of spectral properties and structures of arsenate adsorption on TiO2 surfaces

X‐ray absorption near‐edge structure (XANES) of arsenate adsorption on TiO₂ surfaces was calculated using self‐consistent multiple‐scattering methods, allowing a structural analysis of experimental spectra. A quantitative analysis of the effect of disorder revealed that the broadening and weakening of the characteristic absorption in experimental XANES was due to the structural disorder of the arsenate–TiO₂ adsorption system. The success with calculating the scattering amplitude of a specific set of paths using the path expansion approach enables the scattering contributions of different coordination shells to the XANES to be sorted out. The results showed that the scattering resonances from high‐level shells inherently overlapped onto the first‐shell scattering amplitudes, and formed the fine structures in the XANES region. A variation in one oscillatory feature could be due to several structural changes affecting specific single/multiple‐scattering amplitudes. Therefore, direct assignments of spectral features with structural elements should be based on adequate theoretical analysis.     

Alternative difference analysis scheme combining R‐space EXAFS fit with global optimization XANES fit for X‐ray transient absorption spectroscopy

Time‐resolved X‐ray absorption spectroscopy (TR‐XAS), based on the laser‐pump/X‐ray‐probe method, is powerful in capturing the change of the geometrical and electronic structure of the absorbing atom upon excitation. TR‐XAS data analysis is generally performed on the laser‐on minus laser‐off difference spectrum. Here, a new analysis scheme is presented for the TR‐XAS difference fitting in both the extended X‐ray absorption fine‐structure (EXAFS) and the X‐ray absorption near‐edge structure (XANES) regions. R‐space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non‐derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR‐XAS difference analysis of Fe(phen)₃ spin crossover complex and yielded reliable distance change and excitation population.     

Pressure study of monoclinic ReO2 up to 1.2 GPa using X‐ray absorption spectroscopy and X‐ray diffraction

The crystal and local atomic structure of monoclinic ReO₂ (α‐ReO₂) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X‐ray absorption spectroscopy and high‐resolution synchrotron X‐ray powder diffraction and a home‐built B₄C anvil pressure cell developed for this purpose. Extended X‐ray absorption fine‐structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re—Re distances and a distorted ReO₆ octahedron in the α‐ReO₂ structure. X‐ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the α‐phase, demonstrating a modulation of the Re—Re distances. The relatively small portion of the diffraction pattern accessed in the pressure‐dependent measurements does not allow for a detailed study of the crystal structure of α‐ReO₂ under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re—Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that α‐ReO₂ is a possible inner pressure gauge for future experiments up to 1.2 GPa.     

Early commissioning results for spectroscopic X‐ray Nano‐Imaging Beamline BL 7C sXNI at PLS‐II

For spectral imaging of chemical distributions using X‐ray absorption near‐edge structure (XANES) spectra, a modified double‐crystal monochromator, a focusing plane mirrors system and a newly developed fluorescence‐type X‐ray beam‐position monitoring and feedback system have been implemented. This major hardware upgrade provides a sufficiently stable X‐ray source during energy scanning of more than hundreds of eV for acquisition of reliable XANES spectra in two‐dimensional and three‐dimensional images. In recent pilot studies discussed in this paper, heavy‐metal uptake by plant roots in vivo and iron's phase distribution in the lithium–iron–phosphate cathode of a lithium‐ion battery have been imaged. Also, the spatial resolution of computed tomography has been improved from 70 nm to 55 nm by means of run‐out correction and application of a reconstruction algorithm.     

Effect of gamma irradiation on X‐ray absorption and photoelectron spectroscopy of Nd‐doped phosphate glass

X‐ray absorption near‐edge structure (XANES) and X‐ray photoelectron spectroscopy (XPS) of Nd‐doped phosphate glasses have been studied before and after gamma irradiation. The intensity and the location of the white line peak of the L₃‐edge XANES of Nd are found to be dependent on the ratio O/Nd in the glass matrix. Gamma irradiation changes the elemental concentration of atoms in the glass matrix, which affects the peak intensity of the white line due to changes in the covalence of the chemical bonds with Nd atoms in the glass (structural changes). Sharpening of the Nd 3d_5/2 peak profile in XPS spectra indicates a deficiency of oxygen in the glasses after gamma irradiation, which is supported by energy‐dispersive X‐ray spectroscopy measurements. The ratio of non‐bridging oxygen to total oxygen in the glass after gamma radiation has been found to be correlated to the concentration of defects in the glass samples, which are responsible for its radiation resistance as well as for its coloration.     

Three‐dimensional imaging of chemical phase transformations at the nanoscale with full‐field transmission X‐ray microscopy

The ability to probe morphology and phase distribution in complex systems at multiple length scales unravels the interplay of nano‐ and micrometer‐scale factors at the origin of macroscopic behavior. While different electron‐ and X‐ray‐based imaging techniques can be combined with spectroscopy at high resolutions, owing to experimental time limitations the resulting fields of view are too small to be representative of a composite sample. Here a new X‐ray imaging set‐up is proposed, combining full‐field transmission X‐ray microscopy (TXM) with X‐ray absorption near‐edge structure (XANES) spectroscopy to follow two‐dimensional and three‐dimensional morphological and chemical changes in large volumes at high resolution (tens of nanometers). TXM XANES imaging offers chemical speciation at the nanoscale in thick samples (>20 µm) with minimal preparation requirements. Further, its high throughput allows the analysis of large areas (up to millimeters) in minutes to a few hours. Proof of concept is provided using battery electrodes, although its versatility will lead to impact in a number of diverse research fields.     

Coordination changes in crystalline and vitreous GeO2

Abstract

High pressure x ray absorption spectroscopy (XAS) has been performed up to 29 GPa on crystalline and amorphous GeO₂. The modification of the x ray absorption near edge structure (XANES) as well as the variation of the Ge-O distance, indicate that the coordination of Ge changes from 4 to 6 above 6.5 GPa. The transition is confirmed by Raman spectroscopy.     

Local structure of uncapped and capped InGaN/GaN quantum dots

The local structure around the indium atoms in uncapped and capped In_xGa_1?xN quantum dots has been studied by In K‐edge extended X‐ray absorption fine structure (EXAFS) spectroscopy. The samples were grown by metal organic vapour phase epitaxy. The EXAFS was successfully applied to study the structural properties of buried quantum dots which are not optically active. The analysis revealed that capping the quantum dots with GaN does not affect the bond distances of the In—N and In—Ga, but makes the In—In distance shorter by 0.04 Å.     

High‐accuracy X‐ray absorption spectra from mM solutions of nickel (II) complexes with multiple solutions using transmission XAS

A new approach is introduced for determining X‐ray absorption spectroscopy (XAS) spectra on absolute and relative scales using multiple solutions with different concentrations by the characterization and correction of experimental systematics. This hybrid technique is a development of standard X‐ray absorption fine structure (XAFS) along the lines of the high‐accuracy X‐ray extended range technique (XERT) but with applicability to solutions, dilute systems and cold cell environments. This methodology has been applied to determining absolute XAS of bis(N‐n‐propyl‐salicylaldiminato) nickel(II) and bis(N‐i‐propyl‐salicylaldiminato) nickel(II) complexes with square planar and tetrahedral structures in 15 mM and 1.5 mM dilute solutions. It is demonstrated that transmission XAS from dilute systems can provide excellent X‐ray absorption near‐edge structure (XANES) and XAFS spectra, and that transmission measurements can provide accurate measurement of subtle differences including coordination geometries. For the first time, (transmission) XAS of the isomers have been determined from low‐concentration solutions on an absolute scale with a 1–5% accuracy, and with relative precision of 0.1% to 0.2% in the active XANES and XAFS regions after inclusion of systematic corrections.     

X‐ray absorption spectroscopic study on gold particles formed on titania and alumina

X‐ray absorption near‐edge structure (XANES) measurements near the Au L₃ edge were made on Au(III) complex ions adsorbed on titania and alumina without a specific reducing agent. Compared with the XANES spectrum of a pure gold foil, the gold adsorbed on titania and alumina was found to be reduced to Au(0). The XANES method could obtain spectra of gold particles less than 1 nm in diameter, although a UV–visible absorption spectrum was difficult to observe with such samples. Copyright © 2003 John Wiley & Sons, Ltd.     

Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

Temperature‐dependent EXAFS study of the local structure and lattice dynamics in cubic Y2O3

The local structure and lattice dynamics in cubic Y₂O₃ were studied at the Y K‐edge by X‐ray absorption spectroscopy in the temperature range from 300 to 1273 K. The temperature dependence of the extended X‐ray absorption fine structure was successfully interpreted using classical molecular dynamics and a novel reverse Monte Carlo method, coupled with the evolutionary algorithm. The obtained results allowed the temperature dependence of the yttria atomic structure to be followed up to ～6 Å and to validate two force‐field models.     

Solid energy calibration standards for P K‐edge XANES: electronic structure analysis of PPh4Br

P K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K‐edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K‐edge X‐ray absorption spectroscopy, which has well established energy calibration standards for Cl (Cs₂CuCl₄) and S (Na₂S₂O₃·5H₂O), but not neighboring P. This paper presents a review of common P K‐edge XANES energy calibration standards and analysis of PPh₄Br as a potential alternative. The P K‐edge XANES region of commercially available PPh₄Br revealed a single, highly resolved pre‐edge feature with a maximum at 2146.96 eV. PPh₄Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh₃ rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh₃ and PPh₄⁺ revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time‐dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre‐edge feature in the P K‐edge XANES spectrum of PPh₄Br was assigned to P 1s → P‐C π* transitions, whereas those at higher energy were P 1s → P‐C σ*. Overall, the analysis suggests that PPh₄Br is an excellent alternative to other solid energy calibration standards commonly used in P K‐edge XANES experiments.     

Fast XANES fluorescence imaging using a Maia detector

A new fast X‐ray absorption spectroscopy scanning method was recently implemented at the Hard X‐ray Microprobe endstation P06, PETRA III, DESY, utilizing a Maia detector. Spectromicroscopy maps were acquired with spectra for X‐ray absorption near‐edge structure (XANES) acquisition in the sub‐second regime. The method combines XANES measurements with raster‐scanning of the sample through the focused beam. The order of the scanning sequence of the axes, one beam energy axis and two (or more) spatial axes, is a variable experimental parameter and, depending on it, the dwell at each location can be either single and continuous (if the energy axis is the inner loop) or in shorter discontinuous intervals (if a spatial axis is innermost). The combination of improved spatial and temporal resolution may be necessary for rapidly changing samples, e.g. for following in operando chemical reactions or samples highly susceptible to beam damage where the rapid collection of single XANES spectra avoids issues with the emergence of chemical changes developing from latent damage. This paper compares data sets collected on a specially designed test pattern and a geological thin‐section scanning the energy as inner, middle and outer axis in the sequence. The XANES data of all three scanning schemes is found to show excellent agreement down to the single‐pixel level.     

Phosphorus L2,3‐edge XANES: overview of reference compounds

Synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy is becoming an increasingly used tool for the element speciation in complex samples. For phosphorus (P) almost all XANES measurements have been carried out at the K‐edge. The small number of distinctive features at the P K‐edge makes in some cases the identification of different P forms difficult or impossible. As indicated by a few previous studies, the P L_2,3‐edge spectra were richer in spectral features than those of the P K‐edge. However, experimentally consistent spectra of a wide range of reference compounds have not been published so far. In this study a library of spectral features is presented for a number of mineral P, organic P and P‐bearing minerals for fingerprinting identification. Furthermore, the effect of radiation damage is shown for three compounds and measures are proposed to reduce it. The spectra library provided lays a basis for the identification of individual P forms in samples of unknown composition for a variety of scientific areas.     

Electronic structure effects on B K‐edge XANES of minerals

In order to assess the usability of X‐ray absorption near‐edge structure (XANES) for studying the structure of BO_n‐containing materials, the dependence of theoretical XANES at the B K‐edge on the way the scattering potential is constructed is investigated. Real‐space multiple‐scattering calculations are performed for self‐consistent and non‐self‐consistent potentials and for different ways of dealing with the core hole. It is found that in order to reproduce the principal XANES features it is sufficient to use a non‐self‐consistent potential with a relaxed and screened core hole. Employing theoretical modelling of XANES for studying the structure of boron‐containing glasses is thus possible. The core hole affects the spectrum significantly, especially in the pre‐edge region. In contrast to minerals, B K‐edge XANES of BPO₄ can be reproduced only if a self‐consistent potential is employed.