X-ray Absorption Near-edge Spectroscopy Spectra Research Articles

Effects of Mg Local Structure on Mg K-edge XANES Spectra of MgxZn1-xO Alloy: A First-principles Study

Previously, the measured synchrotron x-ray absorption near-edge spectroscopy (XANES) spectra of MgxZn1-xO alloy (MZO) samples were fitted by using two crystal structure models, i.e., four-fold wurtzite and six-fold rocksalt, with limited success [Appl. Phys. Lett. 99, 261901 (2011)]. For Zn K-edge, the two bases were sufficient for fitting the XANES spectra of intermediate alloy compositions; indicating that majority of the Zn atoms are either four-fold or six-fold coordinated. However, for Mg K-edge, clear deficiency of the fitting can be detected for the intermediate alloy compositions. In this work, first-principles calculations based on density functional theory were carried out to examine other plausible forms of Mg in the alloy. Based on our results, we suggested that the deficiency of the fitting results on Mg K-edge could be attributed to the formation of an unbuckled wurtzite (HX) structure, where Mg atoms are five-fold coordinated. For pure MgO, the HX structure was predicted to be stable under certain tensile strain conditions [Phys. Rev. B 77, 024104 (2008)].

Microbeam X-ray Absorption Near-Edge Spectroscopy study of the oxidation of Fe and Nb in zirconium alloy oxide layers

Alloy optimization of zirconium based alloys used for nuclear fuel cladding is key to increasing corrosion resistance and reducing hydrogen pickup. The mechanism by which alloying elements influence these processes is investigated by focusing on the chemical state evolution of two alloying elements, Fe and Nb, when incorporated into the growing oxide layers of various production zirconium alloys – Zircaloy-4, ZIRLO® and Zr–2.5Nb – as well as a model alloy – Zr–0.4Fe–0.2Cr. X-ray Absorption Near-Edge Spectroscopy (XANES) measurements to determine the evolution of their oxidation states is performed using micro-beam synchrotron radiation on cross sectional oxide samples. A thin (∼12μm) cross-sectional sample of Zircaloy-4 oxide was also designed and fabricated to differentiate the signal coming from the Fe in solid solution from the signal coming from the Fe in precipitates. The XANES spectra were fitted using a combination of standards, to determine the evolution of the oxidized fractions of Fe and Nb in the oxide as function of distance from the oxide/metal interface. The results show that the oxidation of Fe and Nb in the oxide layer is delayed relative to that of Zr. Both the second phase precipitates and solid solution Fe atoms were initially incorporated in metallic form into the oxide layer, although it appears that Fe in solid solution oxidizes first. It is shown that after a given distance from the metal/oxide interface (which is alloy dependent), the alloying elements start to oxidize. Qualitative TEM examinations of precipitates embedded in zirconium oxide layers correlate well with the quantitative XANES results. These results allow a discussion of a qualitative oxidation model of Fe and Nb in Zr alloys.

Unraveling the Dynamic Nature of a CuO/CeO2 Catalyst for CO Oxidation in Operando: A Combined Study of XANES (Fluorescence) and DRIFTS

The redox chemistry and CO oxidation (2CO + O2 → 2CO2) activity of catalysts generated by the dispersion of CuO on CeO2 nanorods were investigated using a multitechnique approach. Combined measurements of time-resolved X-ray absorption near-edge spectroscopy (XANES) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in one setup were made possible with the development of a novel reaction cell in which fluorescence mode detection was applied to collect the XANES spectra. This is the first reported example using XANES in a similar technique combination. With the assistance of parallel time-resolved X-ray diffraction (XRD) measurements under operando conditions, we successfully probed the redox behavior of CuO/CeO2 under CO reduction, constant-flow (steady-state) CO oxidation and during CO/O2 cycling reactions. A strong copper ↔ ceria synergistic effect was observed in the CuO/CeO2 catalyst. Surface Cu(I) species were found to exhibit a strong correlation with the catalyst activity for the CO oxidation reaction. By analysis of phase transformations as well as changes in oxidation state during the nonsteady states in the CO/O2 cycling reaction, we collected information on the relative transformation rates of key species. Elementary steps in the mechanism for the CO oxidation reaction are proposed based on the understandings gained from the XANES/DRIFTS operando studies.

The Influence of Chemical Compounds on the Sulfur K-Edge X-Ray Absorption near Edge Spectrum of the Vulcanized Rubber

The vulcanized styrene-butadiene rubber (SBR) was studied using X-ray absorption near edge spectroscopy (XANES). The effect of the sulfur containing molecules left over in the sample on the precision of the sulfur K-edge XANES spectrum was focused. The six rubber compounds used here were vulcanized with different curing systems including convention, semi-efficiency and efficiency. For each compound, the measurements were preformed on two different samples which were the untreated and treated chloroform samples. The results showed when compounds were cured with conventional or semi-efficient vulcanization system, the XANES spectra between the untreated and treated sample was changed with very high sulfur to accelerator ratio. On the other hand, the tremendous dissimilarity was observed when compounds were cured with the efficient vulcanization system. Thus the sample treatment was necessary when using XANES to identify sulfur crosslink.

Nature of Pd and Ti Metals in the Structure of CMAS Glass and Ceramics

The structure of electrically conductive CMAS-TiO2-Pd glass and ceramics was investigated by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). The XANES spectra of Ti do not show any significant difference between the glasses ceramized in air or in a reducing “forming” gas, as well as between Pd-containing versus Pd-free samples, nor between surface versus bulk of the glass-ceramic samples. However, EXAFS and XANES data recorded at the Pd K-edge show significant dependences on whether the glass-ceramic was ceramized in air or in “forming” gas. The XPS spectra of Ti 2p core-level electrons for glasses ceramized in air or “forming” gas also show a strong difference depending on whether the samples did or did not contain Pd. STEM mapping confirms the existence of grains in the form of main crystalline phases identified with XRD, and also reveals the existence of Pd nanoparticles in glasses ceramized in both air and in “forming” gas.

A XANES study of LiVPO4F: a factor analysis approach

Evolving factor analysis (EFA) of X-ray absorption near-edge spectroscopy (XANES) data is shown to be a useful tool to understand the phase relationships and compositional ranges of stability in the LiVPO4F-VPO4F system. EFA was used to calculate the concentration of phases versus state-of-charge in a lithium-ion battery and true XANES spectra. The results of EFA showed that, indeed, three phases were present during cycling of a LiVPO4F∥Li cell: LiVPO4F, LixVPO4F, and VPO4F. In contrast to what was reported by others, the second phase was not a fixed composition with x = 0.67, but, instead, existed over a range of lithium stoichiometry, x = 0.25 to 0.80. EFA results also showed that the reactions leading to these phases are reversible.

Characterization of (La,Sr)(Ti,Co)O3 Oxides for Symmetrical Solid Oxide Fuel Cell Electrodes

In this work, a combined synchrotron X-ray diffraction (XPD), X-ray absorption near edge spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) study of La0.4Sr0.6Ti1-yCoyO3±δ (0 ≤ y ≤ 0.5), intended as symmetric solid oxide fuel cells electrode, is presented. Charge compensation mechanisms were characterized by room temperature measurements; LSTC has a perovskite pseudocubic structure with an A-site vacancy concentration related with Co doping level. This result seems to be triggered by the fact that Ti remains in a higher formal valence (around 4+) independent of the Co concentrations. On the contrary, Co K-edge XANES spectra show a dependency with the doping level but this seems to be related to structural changes. In-situ measurements simulating a symmetric SOFC working environment show that LSTC undergoes a rhombohedral to cubic perovskite phase transition with critical temperature dependent of the structural distortion level.

The Effect of Synthetic Method and Annealing Temperature on Metal Site Preference in Al1–xGaxFeO3

Magnetoelectric materials couple both magnetic and electronic properties, making them attractive for use in multifunctional devices. The magnetoelectric AFeO3 compounds (Pna2(1); A = Al, Ga) have received attention as the properties of the system depend on composition as well as the synthetic method used. Al(1-x)Ga(x)FeO3. (0 ≤ x ≤ 1) was synthesized by the sol-gel and coprecipitation methods and studied by X-ray absorption near-edge spectroscopy (XANES). Al L(2,3-), Ga K-, and Fe K-edge XANES spectra were collected to examine how the average metal coordination number (CN) changes with the synthetic method. Al and Fe were found to prefer octahedral sites, while Ga prefers the tetrahedral site. It was found that composition played a larger role in determining site occupancies than synthetic method. Samples made by the sol-gel or ceramic methods (reported previously; Walker, J. D. S.; Grosvenor, A. P. J. Solid State Chem. 2013, 197, 147-153) showed smaller spectral changes than samples made via the coprecipitation method. This is attributed to greater ion mobility in samples synthesized via coprecipitation as the reactants do not have a long-range polymeric or oxide network during synthesis like samples synthesized via the sol-gel or ceramic method. Increasing annealing temperature increases the average coordination number of Al, and to a lesser extent Ga, while the average coordination number of Fe decreases. This study indicates that greater disorder is observed when the Al(1-x)Ga(x)FeO3. compounds have high Al content, and when annealed at higher temperatures.

Influence of Chloride and Fe(II) Content on the Reduction of Hg(II) by Magnetite

Abiotic reduction of inorganic mercury by natural organic matter and native soils is well-known, and recently there is evidence that reduced iron (Fe) species, such as magnetite, green rust, and Fe sulfides, can also reduce Hg(II). Here, we evaluated the reduction of Hg(II) by magnetites with varying Fe(II) content in both the absence and presence of chloride. Specifically, we evaluated whether magnetite stoichiometry (x = Fe(II)/Fe(III)) influences the rate of Hg(II) reduction and formation of products. In the absence of chloride, reduction of Hg(II) to Hg(0) is observed over a range of magnetite stoichiometries (0.29 < x < 0.50) in purged headspace reactors and unpurged low headspace reactors, as evidenced by Hg recovery in a volatile product trap solution and Hg L(III)-edge X-ray absorption near edge spectroscopy (XANES). In the presence of chloride, however, XANES spectra indicate the formation of a metastable Hg(I) calomel species (Hg2Cl2) from the reduction of Hg(II). Interestingly, Hg(I) species are only observed for the more oxidized magnetite particles that contain lower Fe(II) content (x < 0.42). For the more reduced magnetite particles (x ≥ 0.42), Hg(II) is reduced to Hg(0) even in the presence of high chloride concentrations. As previously observed for nitroaromatic compounds and uranium, magnetite stoichiometry appears to influence the rate of Hg(II) reduction (both in the presence and absence of chloride) confirming that it is important to consider magnetite stoichiometry when assessing the fate of contaminants in Fe-rich subsurface environments.

The new ID21 XANES full-field end-station at ESRF

A new X-ray absorption near-edge spectroscopy (XANES) full-field imaging station has been developed, installed and tested on beamline ID21 at the European Synchrotron Radiation Facility (ESRF). The set-up operates in the 2-9 keV energy range and allows for the simultaneous acquisition of up to 4.106 XANES spectra over large sample areas with preserved sub-micron spatial resolution. The versatile set-up is compatible with various types of cameras and magnifying objectives. It accommodates spatial resolutions ranging from 0.3 μm to 1.4 μm and fields of view from 600 μm up to 2 mm. The range of potential applications is broad: from geology, cultural heritage, environmental sciences to medicine.

A case for oxygen deficiency in Gd2Ti2−xZrxO7 pyrochlore-type oxides

Pyrochlore-type oxides having the formula Gd2Ti2−xZrxO7 have been studied in the past as potential candidates for the sequestration of nuclear waste elements owing to their chemical robustness and increased resistance to radiation induced damage with greater Zr incorporation. To be used for nuclear waste sequestration applications, all aspects of these materials need to be understood. This is especially the case for the stoichiometry of the materials and the effect of crystal defects. X-ray absorption near-edge spectroscopy (XANES) has been used here to study the local chemical environment of the metal atoms depending on composition. Analysis of Ti K-edge XANES spectra has shown that Gd2Ti2−xZrxO7 (0⩽x<2) can be slightly O-deficient, and that the level of O-deficiency increases with greater Zr incorporation.

Evolution of the local structure at the phase transition in CeO2-Gd2O3solid solutions

We investigated the local environment of the Ce and Gd ions in one of the most important oxygen ion conductors, the CeO${}_{2}$-Gd${}_{2}$O${}_{3}$ solid solution, using ${L}_{\mathrm{III}}$ edge extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray absorption near edge spectroscopy (XANES). The average Gd-O distance, as deduced by EXAFS, decreases gradually with increasing Gd content, whereas a sharp decrease in the average Ce-O distance occurs at 25 mol$%$ Gd, accompanying the appearance of structural features characteristic of double fluorite symmetry. An abrupt change in the local environment of Ce, rather than of Gd, is also supported by the XANES spectra. Efforts at stabilizing the oxygen conducting $Fm$-3$m$ phase in CeO${}_{2}$-Gd${}_{2}$O${}_{3}$ solid solutions should therefore concentrate on tailoring the local environment of the Ce ion. We further suggest that our data may provide an explanation for the recently discovered giant electrostriction effect in Ce${}_{0}$.${}_{8}$Gd${}_{0}$.${}_{2}$O${}_{1.9}$. Since this composition is at the limit of stability of the fluorite phase, local distortions in Ce${}_{0}$.${}_{8}$Gd${}_{0}$.${}_{2}$O${}_{1.9}$ may be modulated by an external electric field more readily than those of the other solid solutions.

An investigation of pyrochlore-type oxides (Yb2Ti2−xFexO7-δ) by XANES

Yb2Ti2O7, which adopts the pyrochlore-type structure, has a large number of applications due to its remarkable chemical and physical properties. Some of these properties (e.g., ionic conduction) can be improved by introducing oxygen vacancies. The effect of composition on the structural and electronic properties needs to be understood to further develop these materials. The Yb2Ti2−xFexO7-δ (0≤x≤0.2) system was synthesized by conventional solid-state reaction methods and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the examination of the effects of substitution on metal charge and coordination number (CN). Analysis of the transition metal K-edge and L2,3-edge spectra from the Yb2Ti2−xFexO7-δ system allowed for the elucidation of the oxidation state and CN of Fe and Ti metal with increasing Fe content. Fe3+, rather than Fe4+, substitutes for Ti4+ in Yb2Ti2−xFexO7-δ. As x increases, a greater oxygen deficiency lowers the metal CN, as observed by the increase in intensity of the pre-edge region of the Fe K- and Ti K-edge XANES spectra, and by analysis of the Fe L3- and Ti L3-edge XANES spectra. The presence of oxygen defects may make this material appropriate for fuel cell applications, as has been suggested previously.

L2,3-edge x-ray absorption near-edge spectroscopy analysis of photoisomerism in solid ruthenium–sulfur dioxide complexes

Ru L2,3 edge x-ray absorption near-edge spectroscopy spectra of four compounds [Ru(SO2)(NH3)4X]Y (X = water, Y = (±)-camphorsulfonate2; X =water, Y = tosylate2; X =isonicotinamide, Y = tosylate2; X =pyridine, Y = triflate2) are reported before and after optical excitation, which causes photoinduced linkage isomerism of the SO2 ligand. Principal component analysis reveals consistent changes in the spectra upon photoisomerization, suggesting a partial oxidation of the Ru ion. These results demonstrate that x-ray absorption near-edge spectroscopy is an effective means of probing photoinduced linkage isomerism.

An X-ray absorption spectroscopic study of the effect of bond covalency on the electronic structure of Gd2Ti2−xSnxO7

The titanate and stannate pyrochlore-type oxides have been investigated because of their potential applications in different fields. Pyrochlore-type oxides exhibit a wide variety of properties such as fast ionic conduction, resistance to radiation induced structural damage, and ferro- and antiferro-magnetism. These properties mainly depend on the metal-oxygen bonding interactions and electronic structure of the materials, both of which can change with composition. The Gd2Ti(2-x)Sn(x)O7 (0 ≤ x ≤ 2) system was synthesized by the ceramic method and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the examination of the effect of substitution on bonding and the electronic structure of materials. Examination of metal K- and L3-edge XANES spectra from the Gd2Ti(2-x)Sn(x)O7 system allowed for the elucidation of how the metal-oxygen bond covalency effects the electronic structure of these materials with increasing Sn content. The ionic character of the Ti-O and Gd-O bonds increases while the Sn-O bond becomes more covalent as x increases in the formula, and resulted in changes in energy and/or line shape of the spectra. This study shows that the bonding interactions between metal and oxygen vary strongly with composition, which may affect the fast ionic conduction and resistance to radiation induced structural damage that has been previously reported for these materials. It was also observed in this study that the intensity of the intersite-hybrid peaks found in the pre-edge region of the Ti K-edge XANES spectra resulting from excitations of 1s electrons into next-nearest neighbour Ti 3d states decreased significantly with increasing Sn incorporation.

Depth profile investigation of the incorporated iron atoms during Kr+ ion beam sputtering on Si (001)

We investigate the incorporation of iron atoms during nano-patterning of Si surfaces induced by 2keV Kr+ ion beam erosion under an off-normal incidence angle of 15°. Considering the low penetration depth of the ions, we have used X-ray reflectivity (XRR) and X-ray absorption near edge spectroscopy (XANES) under grazing-incidence angles in order to determine the depth profile and phase composition of the incorporated iron atoms in the near surface region, complemented by secondary ion mass spectrometry and atomic force microscopy. XRR analysis shows the accumulation of metallic atoms within a near surface layer of a few nanometer thickness. We verify that surface pattern formation takes place only when the co-sputtered Fe concentration exceeds a certain limit. For high Fe concentration, the ripple formation is accompanied by the enhancement of Fe close to the surface, whereas no Fe enhancement is found for low Fe concentration at samples with smooth surfaces. Modeling of the measured XANES spectra reveals the appearance of different silicide phases with decreasing Fe content from the top towards the volume.

Characterization of thermally treated Co2+-exchanged zeolite X

This study investigated thermal stabilization of Co2+-exchanged zeolite X (CoX) using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and leaching tests. From SEM-EDX analysis, cobalt was dispersed randomly at ≤600°C, suggesting its presence as an extraframework cation in exchange sites. At ≥800°C, cobalt was locally concentrated with Al on the vitreous surface. Consistent with such observations, XRD data indicated that CoX maintained the zeolite framework at ≤600°C, and that it became vitrified and transformed to nepheline (NaAlSiO4(s)) and cobalt aluminate (CoAl2O4(s)) at ≥800°C. Cobalt-K edge XAS was subjected to both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analyses. In XANES spectra, the pre-edge peaks and edge-shoulders, characteristic of 4-fold coordinated cobalt (e.g., CoAl2O4(s)), were not evident at ≤600°C, but such features were strong at ≥800°C. The EXAFS spectra of CoX at ≤600°C lacked in the coordination shells beyond the first CoO shell. In contrast, CoX at ≥800°C showed the EXAFS spectra similar to CoAl2O4(s). Taken together, cobalt was likely present as 6-fold coordinated Co2+ in exchange sites at ≤600°C and mainly incorporated into a non-exchangeable CoAl2O4-like phase in both vitreous and crystalline forms at ≥800°C. In agreement with this proposition, leaching tests with concentrated CaCl2 solutions supported the greater stability of cobalt at ≥800°C.

Structure of epitaxial (Fe,N) codoped rutile TiO2thin films by x-ray absorption

Homoepitaxial thin films of Fe:TiO${}_{2}$ and (Fe,N):TiO${}_{2}$ were deposited on rutile(110) by molecular beam epitaxy. X-ray absorption near edge spectroscopy (XANES) spectra were collected at the Ti L-edge, Fe L-edge, Ti K-edge, O K-edge, and N K-edge. No evidence of structural disorder associated with a high concentration of oxygen vacancies is observed. Substitution of Fe for Ti could be inferred, and secondary phases such as Fe${}_{2}$O${}_{3}$, Fe${}_{3}$O${}_{4}$, and FeTiO${}_{3}$ can be ruled out. The similarity of the N K-edge spectra to O, and the presence of a strong x-ray linear dichroism signal for the N K-edge, indicates that N is substitutional for O in the rutile lattice and is not present as a secondary phase such as TiN. Simulations of the XANES spectra confirm substitution, although N appears to be present in more than one local environment. Neither Fe:TiO${}_{2}$ nor (Fe,N):TiO${}_{2}$ exhibit intrinsic room temperature ferromagnetism, despite the presence of mixed valent Fe(II)/Fe(III) in the reduced (Fe,N):TiO${}_{2}$ film.

Hard X-ray Fluorescence Measurements of Heteroepitaxial Solid Oxide Fuel Cell Cathode Materials

Strontium doped lanthanum manganite (La0.8Sr0.2)0.95MnO3 (LSM) and lanthanum strontium cobalt ferrite La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) thin films were grown by pulsed laser deposition (PLD) on single crystal substrates. Total X-ray Reflection Fluorescence (TXRF) reveals the evolution of surface composition due to annealing at 800 C. X-ray Absorption Near Edge Spectroscopy (XANES) provides insight into the local electronic structure of the cations. LSM manganese concentration is higher at the surface than in the bulk in as-deposited films. Annealing further enhances surface manganese segregation and irreversibly changes the manganese XANES spectra. Surface strontium is greatly enhanced in LSCF samples quenched from 800 C, however the strontium is reabsorbed into the film when allowed to cool slowly.

Interionic Interactions in Imidazolic Ionic Liquids Probed by Soft X-ray Absorption Spectroscopy

This work investigates pure ionic liquids (ILs) derived from an imidazolium ring with different carbonic chains and halides or bis(trifluoromethanesulfonilimide) (TFSI(-)) as anions, using X-ray absorption near edge spectroscopy (XANES) at different energies (N, S, O, F, and Cl edges) to probe the interionic interactions. XANES data show that the interaction with the anion is weaker when the cation is an imidazolium than when the salt is formed by smaller cations, as lithium, independently of the length of the carbonic chain attached to the imidazolium cation. The results also show that, for all studied ILs, it is not observed any influence of the anion on the XANES spectra of the cation, nor the opposite. 1-Methylimidazolium with Cl(-), a small and strongly coordinating anion, presents in the N K XANES spectrum a splitting of the band corresponding to nitrogen in the imidazolic ring, indicating two different chemical environments. For this cation with TFSI(-), on the contrary, this splitting was not observed, showing that the anion has a weaker interaction with the imidazolic ring, even without a lateral carbonic chain.