EXAFS Signal Research Articles

On EXAFS Debye-Waller factor and recent advances

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter-atomic equilibrium distance. By comparing the first-shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi-harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.

Local Ordering Around Tetrahedral Co2+ in Silicate Glasses

Alkali lime silicate glasses containing 5 wt% of CoO were investigated by Co K‐edge XANES and EXAFS and optical absorption spectroscopy. Our results reveal the presence of tetrahedral Co2+ connected with the glass network, with a IVCo–O–Si angle of 134°. Changing the alkali from K+ to Na+ induces an increase of the local disorder around Co2+, as shown by a decrease of the contribution from the second neighbors in the EXAFS signal. We propose two models for interpreting the structure of the second shell of neighbors. Our results provide a structural basis for rationalizing the optical properties of Co2+ species in glasses.

The Role of Different Types of CuO in CuO–CeO2/Al2O3 for Total Oxidation

Different types of copper oxide in CuO–CeO2/Al2O3 were investigated structurally and catalytically. Standard preparation lead to the presence of CuO monocrystals and Ce1−xCuxO2−x as evidenced by XRD, HRTEM and EDX. Washing with nitric acid removed the monocrystals, while leaving the Ce1−xCuxO2−x solid solution. The Fourier transformed Cu K patterns for the HNO3-washed catalyst showed a diminished second Cu shell, characteristic for Ce1−xCuxO2−x as well as for X-ray amorphous nano-sized CuAl2O4 spinel phase. XANES analysis and first shell Cu fitting confirmed this CuAl2O4 like structure outside Ce1−xCuxO2−x. By modeling the Ce K EXAFS signal in the washed sample, the amount of Cu in Ce1−xCuxO2−x was determined to be 18 ± 6 %. Total oxidation of CO and C3H8 as model reactions showed little difference in activity between non-washed and washed CuO–CeO2/Al2O3, indicating that CuO monocrystals were hardly active at temperatures below 450 °C. XRD performed in situ during TPR and TPO showed that CeO2 in Ce1−xCuxO2−x added to the redox capacity of both CuO–CeO2/Al2O3 samples as it was partially reduced at 300 °C. At higher temperatures, the CuO monocrystals also participated in reaction, leading to a higher activity for the oxidation of CH4.

X-ray absorption investigation of titanium oxynitride nanoparticles obtained from laser pyrolysis

This work presents a structural study by X-ray Absorption Spectroscopy (XAS) and Electron Energy-Loss Spectroscopy (EELS) of complex titanium oxynitride nanoparticles (Ti(O,N)), synthesized by laser pyrolysis from titanium tetraisopropoxide and ammonia as precursors. Previous structural characterizations obtained by XRD and XPS have shown that the nanoparticles present a TiO type face-centered cubic (FCC) structure but with three different oxidation degree for titanium. The synthesis of this kind of titanium oxide or oxynitride nanoparticles is very unusual. Moreover, their properties are highly dependent of their structure. EELS spectrum-imaging data were therefore used for mapping the different chemical species. These measurements reveal that the nanoparticles are composed of a FCC solid solution of nitrogen and oxygen in titanium. The local structure around Ti was then studied. XANES measurements show an absorption threshold corresponding to a global valence state between Ti3+ and Ti4+, with a pre-edge structure characteristic of a mix between a face-centered cubic (FCC) structure and a disordered TiO2 structure whereas the EXAFS signal is dominated by the contribution of the FCC structure. Oxidative heat-treatments have been performed from 250 to 450°C in order to follow the transition towards the dioxide phase. EELS measurements show that the oxidation occurs from the surface of the nanoparticles. XAS show that this transition does not involve any other crystallographic phase than TiO2, mainly in its anatase form, and seems to occur via a dioxide disordered phase. The FCC contribution completely disappears at 450°C where the sample reaches a complete TiO2 structure. Moreover, a correlation is established between the structural evolution and optical properties.

Short range order around Sc atoms in Fe90Sc10nanoglasses using fluorescence X-ray absorption spectroscopy

X-ray absorption spectroscopy has been applied to probe the local structure of FeSc nanostructured amorphous solids around Sc atoms. At the Sc K-edge, the Fe 90 Sc 10 nanoglass is characterized by a very short range order (EXAFS oscillations are only present up to k = 8 A -1 ). A drastic difference in the phase and the amplitude of the nanoglass EXAFS signal compared to the amorphous ribbon indicating a different local structure around Sc atoms in both structures is observed. In addition, the changes in the local structure around Sc atoms are more significant than around Fe atoms.

Interpretation of the Ni K-edge EXAFS in nanocrystalline nickel oxide using molecular dynamics simulations

Analysis of atomic structure at the nanoscale is a challenging task, complicated by relaxation phenomena and thermal disorder. In this work, the x-ray absorption spectroscopy at the Ni K-edge was used to address this problem in nanocrystalline NiO (nano-NiO) at 300 K. The analysis of the first two coordination shells using conventional two-shell single-scattering approximation allowed us to determine the expansion of the average lattice but contraction of the Ni―O bonds in the first coordination shell in nano-NiO in comparison with the bulk nickel oxide. The EXAFS signal generated within the first six coordination shells (up to ~ 6.5 Å) was successfully interpreted using classical molecular dynamics and ab initio multiple-scattering EXAFS theory. We found that simple rigid-ion force-field model is able to describe the structure relaxation and dynamics in both bulk and nano-NiO. Such approach requires less parameters than conventional EXAFS analysis and allows accounting explicitly for thermal effects and many-atom distribution functions. Finally, we showed that the EXAFS signal is rather sensitive to small variations of the force-field model and, thus, the agreement between the experimental and calculated EXAFS signals can be used for the force-field model optimization.

Apparently Regular Octahedral Coordination of Ag(II) in IF6[Ag(SbF6)3

AbstractThe crystal structure of IF6[Ag(SbF6)3] has been determined by X‐ray single crystal and X‐ray powder diffraction analysis. The structure consists from [Ag(SbF6)3]– chains and [IF6]+ cations placed between them. Ag2+ is found in regular octahedral coordination of six fluorine atoms [Ag–F = 6 × 2.170(12) Å]. EXAFS analysis of Jahn–Teller distortions in IF6[Ag(SbF6)3] shows that Ag2+ surrounding is elongated octahedral at room temperature. EXAFS signals are fitted with four short and two long Ag–F distances [4 × 2.11(1) Å; 2 × 2.40(2) Å]. This result is in apparent contradiction with X‐ray diffraction studies where the [AgF6] moiety appears to be a regular octahedra. The Raman spectrum of IF6[Ag(SbF6)3] is in agreement with the presence of regular IF6 and deformed SbF6 groups.

Extended X-ray absorption fine structure of the [Fe]-hydrogenase Hmd active site

Hydrogenases are enzymes that catalyze the reversible oxidation of molecular hydrogen. Although their structure and catalytic mechanism are of considerable applied interest as models for the development of efficient catalysts for hydrogen fueled processes, the understanding of how hydrogenases react with H2 is only in its infancy. Two of the three known types of hydrogenases are iron-sulfur proteins that contain a dinuclear metal center, either [NiFe] or [FeFe]. In contrast, [Fe]-hydrogenase is the only mononuclear hydrogenase and thus a perfect system for studying the structural and electronic determinants of these enzymes. Here we summarize recent improvements in modeling based on the EXAFS signal and the geometric structure of this metalloenzyme in its as isolated or reconstituted form. The individual contributions to the EXAFS resulting in two different structural models are presented and discussed. Inspired by the new crystal structure, we show an advanced EXAFS model for the enzyme from Methanothermobacter marburgensis.

Structure of [Ru(bpy)n(AP)(6-2n)]2+ homogeneous complexes: DFT calculation vs. EXAFS

We used EXAFS and DFT calculations to investigate the structure of [Ru(bpy)(AP)4]2+ and [Ru(bpy)2(AP)2]2+ (bpy=2-2'-bipyridyne, AP=4-aminopyridyne) in aqueous solution (10 mM). These derivatives are of potential interest since, upon direct irradiation, they can form reactive aqua-species able to bind to macromolecules. An attempt has been made to determine with EXAFS the structure of the photodissociation product of the [Ru(bpy)2(AP)2]2+ complex, where a water molecule fill the coordination vacancy left by an AP ligand resulting in [Ru(bpy)2(AP)(H2O)]2+. Unfortunately, co-presence in the experimental sample of both original and photodissociated complexes, causes the failure of the analysis. This failure was due to the structural complexity of both systems and to the similarity in their EXAFS signals. This work underlines the potentialities and the limits of EXAFS spectroscopy when dealing with highly diluted samples where the local environment of the adsorbing atom is characterized by structured ligands: the local environment of Ru is correctly reproduced when dealing with homogeneous samples, while the co-presence of two or more different species makes the data analysis highly critical.

Quantum mechanics–molecular dynamics approach to the interpretation of x-rayabsorption spectra

The quantum mechanics–molecular dynamics approach to the simulation ofconfiguration-averaged EXAFS spectra is proposed, and its application isdiscussed for the example of the Ti K-edge EXAFS spectrum in cubic perovskiteSrTiO3. Proper use of ab initio quantum mechanics allows a number of empirical parameters, usedin the molecular dynamics simulation, to be reduced, whereas the molecular dynamicsallows us to account for temperature effects. All together, the approach provides a way ofaccounting for static and dynamic disorder in EXAFS signals from the coordination shellsabove the first one, where many-atom (multiple-scattering) effects are often important.

Correlating structure and ion recognition properties of [Ni(salen)]-based polymer films

Abstract The structural origins of ion recognition by electrochemically addressable poly[Ni( salen )] thin films are explored using in situ X-ray spectroscopy. XANES and EXAFS provided the local environment (nearest and next-nearest neighbours) around the Ni atom and solution-derived Ba 2+ bound to the film. The Ni is covalently bound to two N and two O donors in square planar geometry, irrespective of film redox state and the presence (or absence) of bound Ba 2+ . The role of the Ni is purely structural; dramatic changes in i – E response accompanying Ba 2+ uptake are assigned to the delocalised poly( salen ) polymer spine. Ba 2+ is trapped in a pseudo-crown formed by two methoxy O donors and two O donors shared with the Ni atom. The Ba 2+ EXAFS signal from thick films (10 μm) is significantly below that anticipated from electrochemical observations on thin films ( 2+ access to the outer region of the film; surface sensitive XPS data confirm this. Combination of spectroscopic and electrochemical data suggest that, for exposure times of ca. 10 3 s, only sites in the outer ca. 1 μm of the film are occupied; the implied diffusion coefficient of 10 −11 cm 2 s −1 is consistent with a relatively compact solvated film.

Evidence for double-electron excitations in theL3-edge x-ray absorption spectra of actinides

A double-electron excitation in ${L}_{3}$-edge x-ray absorption spectra of actinides has been observed for the first time. The investigation of this effect is presented. Actinide species in valence states IV and III were investigated by using experimental data of ${\mathrm{Th}}^{4+}$, ${\mathrm{U}}^{4+}$, ${\mathrm{Np}}^{4+}$, ${\mathrm{Pu}}^{3+}$, and ${\mathrm{Am}}^{3+}$ hydrates. The double-electron excitation was identified as a ${L}_{3}{N}_{6,7}$ shake-up effect. Energy positions of the double-electron features were found in good agreement with the $Z+1$ approximation. The $LN$ resonance intensity corresponds with the resonance of the $L$ edge. This has been observed by comparing the ${L}_{1}$, ${L}_{2}$, and ${L}_{3}$ XANES spectra of ${\mathrm{Th}}^{4+}$, as well as the ${L}_{3}$ XANES spectra of Np in oxidation states IV, V, VI, and VII. Influences of the double-electron excitation on the EXAFS signal are discussed.

Continuous Cauchy wavelet transform of XAFS spectra

The continuous Cauchy wavelet transform (CCWT) is applied to the analysis of XAFS spectra. Thanks to that method, XANES and EXAFS signals can be visualized in three-dimensions: the wavevector (k), the interatomic distance uncorrected for phase-shifts (R′) and the CCWT modulus (corresponding to the continuous decomposition of the amplitude terms). Applied to EXAFS spectra, the CCWT analysis provides straightforward qualitative information related to the k-range of each “R′-EXAFS” contribution. Such information is particularly useful to perform next nearest-neighbors identification, despite the presence of spectral artifacts such as multiple-scattering features, multi-electronic excitations or noise. When applied to XANES spectra, the CCWT analysis helps highly to measure the “spectral limit” between XANES and EXAFS regions, as well as the energy range required to model properly next-nearest neighbors. To further illustrate the potential of CCWT analyses applied to XAFS spectra, we present examples related to: (1) a XANES spectrum collected at the Ti K-edge for titanite (CaTiSiO5); (2) an experimental Au LIII-edge EXAFS spectrum for gold sorbed on goethite (FeO(OH)).

Local Structure of Liquid Te Studied by Neutron Diffraction and EXAFS

Abstract Neutron diffraction measurements for liquid (l-) Te have been performed. The first minimum after the first peak of g(r) is well above zero and filled up with temperature, while the second peak diminishes substantially and flattens out at high temperature. The temperature variations of g(r) are analyzed combining the structural information available from our EXAFS results. The curve fitting of g(r) for l-Te predicts that two different inter-chain components coexist. The first corresponds to the short-distance inter-chain component around 3.2Å and the second corresponds to the long-distance inter-chain component around 3.8Å. The filling of the first minimum around 3.2Å at high temperature is clearly related to the metallization. EXAFS analysis on the basis of the model g(r) indicates that the short-distance inter-chain component (P2) responsible for the metallic conduction in l-Te as well as the nearest intra-chain component (P1) are required for fitting the oscillatory spectrum in the region of short-range sensitivity of the EXAFS signal. It is concluded that the S-M transition observed for l-Te arises from the transformation to the densely packed configuration composed of shortened chain molecules.

Interaction Between Pt(acac) 2 and Alumina Surfaces Studied by XAS

The anchoring and decomposition mechanisms of platinum(II) bis-acetylacetonate on alumina surfaces are studied by X-ray absorption spectroscopy at the platinum LIII edge. A distinction is made between highly reactive surfaces which are partially dehydroxylated and exhibit coordinatively unsaturated surface sites, and deactivated surfaces which are covered by a monolayer of OH groups. The samples are studied after three stages of a wet impregnation synthesis: after drying at room temperature, drying at 120 °C and calcination at 350 °C. The XANES signal and the filtered EXAFS signals of the first and second coordination shells around platinum were analyzed. Two different mechanisms are discussed for the two types of supports.

X-Ray Absorption and Diffraction Studies of Pr3+, Tb3+ and Er3+-Activated Silica Gels

Rare-earth (Pr3+, Tb3+, Er3+) doped silica xerogels were studied by x-ray absorption spectroscopy and x-ray diffraction. A change of the local environment around rare-earth ions upon xerogel densification at 900–950°C and co-doping with aluminum ions was determined from the rare-earths L3-edge EXAFS signals. The densification process induces a decrease of the coordination number and a compression and deformation of the first coordination shell, composed of oxygen atoms. The second coordination shell, composed of silicon and/or aluminum ions, also experiences some modification, which is attributed mainly to a shortening of the shell radius. No evidence of clustering of rare-earth ions upon densification was observed. X-ray diffraction data on Tb-doped gels confirm the EXAFS results.

New EXAFS Measurements by XEOL and TEY on Porous Silicon

Results of an EXAFS investigation on porous Silicon carried out by X-ray Excited Optical Luminescence (XEOL) and Total Electron Yield (TEY) techniques, at the Si K absorption edge, are reported. For the first time XEOL spectra of porous silicon have been recorded in a wide energy range (1800–2500 eV) and EXAFS signals have been singled out from them. Simultaneous TEY and XEOL measurements yield to different results: in particular TEY-EXAFS is sensitive up to the third coordination shell of Si, while XEOL-EXAFS reveals only the contributions of the first two coordination shells; moreover they show a different dependence on changes of the etching parameters. This evidences the sensitivity of XEOL technique to the local structure of the quantum confined luminescent sites. The dependence of the light emission properties on the main preparation parameters and their influence on the short-range structure of red and yellow porous silicon samples are also investigated.

EXAFS as a tool for catalyst characterization: a review of the data analysis methods

A review of the EXAFS data analysis methods is presented. A detailed description of the EXAFS signal extraction and the Fourier transform of the data are discussed. The procedure for determining interatomic distances, coordination numbers and disorder effects from EXAFS data is described. This paper also discusses the data analysis statistics. Finally, one example of catalyst characterization by the EXAFS technique is reported.

EXAFS and XANES characterization of silicate-oxide nanocomposites

X ray absorption edges and preedges together with Fourier transforms of the EXAFS signal of the LaCo(fsa)2en,NO3 complex ((fsa)2en ˭N,N′-bis(3-carboxysalicydene)ethylenediamine) alone or intercalated in montmorillonite and taeniolite have been studied as a function of temperature. A preferential orientation of the complex has been found when it is intercalated in the silicates. Upon applying thermal treatment, it undergoes similar decomposition whatever it is free or in taeniolite. Due to the thermal stability of taeniolite layers, a taeniolite-LaCoO3 layered nanocomposite could be formed. On the other hand, we have shown that being in montmorillonite, the cobalt oxide phase is reduced progressively above 270°C.

XAFS Analysis of Jahn-Teller Distortions in Tris(1,2-Diaminoethane)Copper(II) Sulfate Cu(en)3SO4

XAFS analysis Jahn-Teller distortions in tris(1,2-diaminoethane)copper(II) sulfate, Cu(en) 3 SO 4 (en 1,2-diaminoethane) shows that Cu(II) is elongated octahedral at room and at low temperature: the XANES spectra are similar and the EXAFS signals are fitted with four short and two long Cu-N distances. This result is in apparent contradiction with X-ray diffraction studies: [Cu(en) 3 ] 2+ appears regular octahedral at 300 K and compressed at 120 K. The observed EXAFS is clearly incompatible with the EXAFS simulations using FEFF 6 code, with the diffraction coordinates. On the contrary, a good agreement is obtained for the simulations Ni(en) 3 SO 4 , where Ni is not subject to Jahn-Teller effect. Similar results are obtained with the hexanitrocuprate(II) K 2 PbCu(NO 2 ) 6 and the hexanitronickelate(II) K 2 PbNi(NO 2 ) 6 . The results are in agreement with a dynamic Jahn-Teller effect in elongated octahedra [Cu(en) 3 ] 2+ and [Cu(NO 2 ) 6 ] 4- at high temperature and with a static Jahn-Teller effect combined with an antiferrodistortive order at low temperature.