X-ray Absorption Fine Structure Investigation Research Articles

In situ/operando plug-flow fixed-bed cell for synchrotron PXRD and XAFS investigations at high temperature, pressure, controlled gas atmosphere and ultra-fast heating.

A plug-flow fixed-bed cell for synchrotron powder X-ray diffraction (PXRD) and X-ray absorption fine structure (XAFS) idoneous for the study of heterogeneous catalysts at high temperature, pressure and under gas flow is designed, constructed and demonstrated. The operating conditions up to 1000°C and 50 bar are ensured by a set of mass flow controllers, pressure regulators and two infra-red lamps that constitute a robust and ultra-fast heating and cooling method. The performance of the system and cell for carbon dioxide hydrogenation reactions under specified temperatures, gas flows and pressures is demonstrated both for PXRD and XAFS at the P02.1 (PXRD) and the P64 (XAFS) beamlines of the Deutsches Elektronen-Synchrotron (DESY).

Local structure investigation of Co-Fe-Si-B ribbons by extended X-ray absorption fine-structure spectroscopy.

In the present work, extended X-ray absorption fine-structure (EXAFS) investigations of Co69FexSi21-xB10 (x = 3, 5, 7) glassy ribbons were performed at the Co K-edge. The magnitude of the first peak of the Fourier transforms of theEXAFS signals is found to increase monotonically with increasing Si concentrations indicating the formation of the localized ordered structure at the atomic scale. The Co-Si coordination number (CN) increases at the expense of the CN of Co/Fe. Smaller interatomic distances are observed in the glassy phase compared with that in the crystalline phase which promotes the stability of the glassy phase. Calculations of the thermodynamic parameter (PHSS), cohesive energy (EC) and the atomic radius difference (δ) parameter show that the alloy composition Co69Fe3Si18B10 has a good glass-forming ability (GFA) with the highest CN of Si compared with other compositions. A linear correlation of CN with that of the GFA parameter (PHSS) exists and the CN also plays a crucial role in the GFA of the glassy alloys. This parameter should be considered in developing different GFA criteria.

Ti K-edge XAFS investigation of lithium migration in lithium titanium oxide anode material under charge and discharge cycle

Changes in Ti K-edge X-ray absorption fine structure (XAFS) spectra of lithium titanium oxide (LTO) during the charge discharge cycle of lithium ion battery were investigated by using a specially designed cell for in situ X-ray measurement. Thought LTO is famous for zero-strain lattice during the charge/discharge cycle, the significant spectral changes might be attributed to Li migration in the lattice. The experimental results were compared with the theoretical simulation, and the spectral changes were caused both by the slight changes in oxygen position parameter that affected titanium neighbors and Li migration.

Enhanced sequestration of selenite in water by nanoscale zero valent iron immobilization on carbon nanotubes by a combined batch, XPS and XAFS investigation

In this paper, nanoscale zero-valent iron (NZVI) immobilized on carbon nanotube (CNT) composite (NZVI/CNT) was prepared, characterized and used for the sequestration of Se(IV) in water. The structural analysis revealed that NZVI were uniformly immobilized on CNT surfaces, and thus the oxidation and aggregation of NZVI was obviously minimized. Compared to bare NZVI, the NZVI/CNT exhibited much higher efficiency on Se(IV) sequestration due to the good synergistic effect between CNT adsorption and NZVI reduction. The results from X-ray absorption fine structure (XAFS) revealed that Se(IV) could be almost completely reduced into Se(0)/Se(-II) by NZVI/CNT, while partial reduction occurred on NZVI with a trace of Se(IV) adsorbed on the corrosion products. Besides, the role of pH in Se(IV) sequestration on NZVI/CNT was lower than that on NZVI due to the buffering effect of CNT. Moreover, CNT immobilization could also weaken the inhibition effect of humic acid (HA) on the removal of Se(IV) by NZVI, since CNT showed strong adsorption for HA that blocks potential reactivity. The excellent performance of NZVI/CNT offer a promising material for detoxification of Se(IV) from wastewater.

X-Ray Absorption Fine Structure Investigation of Copper(II) Mixed Ligand Complexes with Pyridinedicarboxylic Acid as Primary Ligand

The X-ray absorption fine structure (XAFS) spectra at the K-edge of the copper complexes Cu(PDC)(Mim)3 H2O (1) and Cu(PDC)2(EA)2H2O (2) (where PDC – Pyridine-2,3-dicarboxylic acid, Mim – 2-methylimidazole, and EA – ethyl acetate) have been investigated. The experimental extended X-ray absorption fine structure data of complex 1 have been analyzed by fitting the theoretical model generated from its own crystallographic data. The crystallographic data for complex 2 are not available. It has been found by comparing the intensity of the pre-edge peaks and X-ray absorption near edge structure features of complexes 1 and 2 that both complexes possess square pyramidal geometry around the copper centers and thus complex 2 is analogous to complex 1. Hence, the theoretical model generated for complex 1 has been fitted to the experimental EXAFS data of complex 2 to determine the structural parameters of complex 2. The coordination geometry of both complexes has been depicted. Further, the chemical shifts have been used to determine the oxidation state as well as to estimate the effective nuclear charge on the copper atom.

Lead Stabilization in Soil Amended with Lime Waste: An Extended X-ray Absorption Fine Structure (EXAFS) Investigation

To determine Pb species in soils following the immobilization process, sequential extraction has been used despite the possibility of overestimating Pb species from unintended reactions during chemical extraction. Meanwhile, the application of extended X-ray absorption fine structure (EXAFS) has been shown to provide a more precise result than chemical extraction. In this study, the immobilization of Pb in contaminated soils treated with liming materials such as oyster shell (OS) or eggshell (ES) was evaluated with thermodynamic modelling and EXAFS analysis. Thermodynamic modelling by visual MINTEQ predicted the precipitation of Pb(OH)₂ in OS and ES treated soils. In particular, the values of saturation index (SI) for Pb(OH)₂ in OS (SI=0.286) and ES (SI=0.453) treated soils were greater than in the control soil (SI=0.281). Linear combination fitting (LCF) analysis confirmed the presence of C 12 H 10 O 14 Pb 3 (lead citrate, 44.7%) by citric acid from plant root, Pb-gibbsite (Pb adsorbed gibbsite, 26.4%), and Pb-kaolinite (Pb adsorbed kaolinite, 20.3%) in the control soil. On the other hand, Pb(OH)₂ (16.8%), Pb-gibbsite (39.3%), and Pb-kaolinite (25.6%) were observed in the OS treated soil and Pb(OH)₂ (55.2%) and Pb-gibbsite (33.8%) were also confirmed in the ES treated soil. Our results indicate that the treatment with OS and ES immobilizes Pb by adsorption of Pb onto the soil minerals as a result of the increase in soil negative charge and the formation of stable Pb(OH)₂ under high pH condition of soils.

Local structure and nanoscale homogeneity of CeO2–ZrO2: differences and similarities to parent oxides revealed by luminescence with temporal and spectral resolution

Although homogeneity at the atomic level of CeO2-ZrO2 with a Ce/Zr atomic ratio close to unity is considered to be one of the main causes for the increased total oxygen storage capacity (OSC), the characterization approaches of homogeneity remain a major challenge. We propose a simple, yet effective method, to assess both structural and compositional homogeneity of CeO2-ZrO2 by using Eu(3+) luminescence measured with time and dual spectral resolution (emission and excitation). For Eu(3+)-CeO2-ZrO2 calcined at 750 °C, the X-ray diffraction, Raman and High-Resolution Transmission Electron Microscopy data converge to a single pseudo-cubic phase. However, the evolution of Eu(3+)-delayed luminescence from cubic ceria-like to tetragonal zirconia-like emission reveals the formation of CeO2- and ZrO2-rich nanodomains and provides evidence for early phase separation. For Eu(3+)-CeO2-ZrO2 calcined at 1000 °C, the emission of Eu(3+) reveals both structural and compositional inhomogeneity. Our study identifies the differences between the local structure properties of CeO2 and ZrO2 parent oxides and CeO2-ZrO2 mixed oxide, also confirming the special chemical environment of the oxygen atoms in the mixed oxide as reported earlier by Extended X-ray Absorption Fine Structure investigations.

Sol–gel preparation of efficient red phosphor Mg2TiO4:Mn4+ and XAFS investigation on the substitution of Mn4+ for Ti4+

An efficient red phosphor based on the substitution of Mn4+ for Ti4+ in the lattice of Mg2TiO4 was prepared through a sol–gel route. The phosphor was characterized by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and UV-vis spectroscopy and the luminescent properties of the samples with varying Mn4+ concentrations were investigated. The photoluminescence of the sol–gel prepared Mg2TiO4:Mn4+ phosphor was compared with the corresponding sample prepared via solid-state reaction and the commercial CaAlSiN3:Eu2+. X-ray absorption fine structure (XAFS) investigation reveals the oxidation state and coordination environment of the Mn ion in the crystal structure of Mg2TiO4 and indicates the substitution of Mn4+ for Ti-sites. Moreover, a decrease in lattice parameter was observed with the increase of Mn4+ content in Mg2TiO4, further confirming the replacement of Ti4+ ions by smaller Mn4+ ions in the host lattice.

XAFS and Dielectric Study for Pb(Fe1/2Nb1/2)O3-PbTiO3 Crystals

Multiferroic Pb(Fe1/2Nb1/2)O3-PbTiO3 (PFNT) system exhibits the co-operative phenomena with the coexistence of electric and magnetic orders. Through the temperature-dependent dielectric measurements, powder XRD and Raman scattering studies, combining the x-ray absorption fine-structure (XAFS) investigation, the dielectric properties and structures of Pb(Fe1/2Nb1/2)1-xTixO3 (x = 0.07 and 0.48) crystals were investigated. The frequency-dependent dielectric anomaly was revealed, which is significantly correlated to the local structure differences and the Fe2+/Fe3+ hoping. The presented results will be useful for understanding and exploring the structure/property relationship beyond this material.

Synthesis of selenium doted pyrite single crystals prepared by chemical vapor transport

Selenium is of environmental concern due to a possible exposure of 79Se radionuclide from a high-level nuclear waste repository to the biosphere or enlarged concentrations in soils and lakes or wetlands, threatening to health of vertebrates. Pyrite on the contrary is the most common iron sulfide on earth's surface and noted for the incorporation of trace elements into its structure. In this study, we report the development of a high temperature synthesis of pure selenium doted pyrite, using solid AlBr3, Fe0, S0 and Se0 by means of a chemical vapor transport reaction in a 2-zone furnace with a gradient of 700°C to 600°C in a two reaction step. The amount of 1.1wt% of homogeneously distributed selenium became incorporated into pyrite single-crystals with a size up to 0.5cm in diameter. X-ray absorption fine-structure investigations were performed for the determination of the valence state and structural bonding of incorporated Se into pyrite. These reveal the predominant incorporation of Se0 into pyrite structure without further bonding. This result indicates that the formation of a FeSxSey condensed matter, accompanied with a substitution of sulfur by selenium, is not likely.

X-ray Absorption Fine Structure Investigation of the Low Temperature Grown ZnCoO Films

Temperature Grown ZnCoO Films A. Wolskaa,∗, M.T. Klepka, B.S. Witkowski, M.I. Šukasiewicz, E. Guziewicz, M. Godlewski a Institute of Physics PAS, al. Lotnikow 32/46, 02-668, Warsaw, Poland b Dept. Math. and Natural Sciences College of Science, Cardinal S. Wyszy«ski Univ., Warsaw, Poland ZnO based diluted magnetic semiconductors are intensively investigated for possible spintronic applications. In the present work we investigate the ZnCoO layers grown at low temperature by atomic layer deposition. The local atomic structure of a series of layers with di erent Co concentration is investigated by the X-ray absorption ne structure measurements. Two groups of ZnCoO layers are investigated the ones with an uniform Co distribution and highly nonuniform lms. For uniform samples we observe that a majority of Co atoms is built into the ZnO matrix substituting the Zn atoms. In contrast, for the nonuniform samples, metallic Co inclusions are also observed. These results are in strong correlation with the magnetic properties of the lms studied separately. Samples with the uniform Co distribution (Co substitutes Zn in ZnO) are paramagnetic, whereas the nonuniform ones show a ferromagnetic response.

Extended X‐Ray Absorption Fine Structure Investigation of Carbon Stabilized Expanded Austenite and Carbides in Stainless Steel AISI 316

AbstractLow temperature carburized AISI 316 stainless steel ‐ carbon expanded austenite ‐ was investigated with EXAFS and synchrotron diffraction together with synthesized carbides of the type M3C2, M7C3 and M23C6. It was found that the chemical environment of carbon expanded austenite is not associated with any of the investigated carbides, that carbon has a strong affinity for chromium, i.e. short range order, and that carbon is in solid solution.

Excitons at the B K edge of boron nitride nanotubes probed by x-ray absorptionspectroscopy

We have performed a near-edge x-ray absorption fine-structure (NEXAFS) investigation ofmulti-walled boron nitride nanotubes (BNNTs). We show that the one-dimensionality ofBNNTs is clearly evident in the B K edge spectrum, while the N K edge spectrum issimilar to that of layered hexagonal BN (h-BN). We observe a sharp feature at theσ* onset of the B K edge, which we ascribe to a core exciton state. We also report acomparison with spectra taken after an ammonia plasma treatment, showing that the B Kedge becomes indistinguishable from that of h-BN, due to the breaking of the tubular orderand the formation of small h-BN clusters.

Stepwise formation of P-cluster in nitrogenase MoFe protein

The P-cluster of nitrogenase is one of the most complex biological metallocenters known to date. Despite the recent advances in the chemical synthesis of P-cluster topologs, the biosynthetic mechanism of P-cluster has not been well defined. Here, we present a combined biochemical, electron paramagnetic resonance, and X-ray absorption spectroscopy/extended X-ray absorption fine-structure investigation of the maturation process of P-clusters in DeltanifH molybdenum-iron (MoFe) protein. Our data indicate that the previously identified, [Fe(4)S(4)]-like cluster pairs in DeltanifH MoFe protein are indeed the precursors to P-clusters, which can be reductively coupled into the mature [Fe(8)S(7)] structures in the presence of Fe protein, MgATP, and dithionite. Moreover, our observation of a biphasic maturation pattern of P-clusters in DeltanifH MoFe protein provides dynamic proof for the previously hypothesized, stepwise assembly mechanism of the two P-clusters in the alpha(2)beta(2)-tetrameric MoFe protein, i.e., one P-cluster is formed in one alphabeta dimer before the other in the second alphabeta dimer.

Impacts of Chemical Amendment and Plant Growth on Lead Speciation and Enzyme Activities in a Shooting Range Soil: An X‐ray Absorption Fine Structure Investigation

In situ chemical immobilization is a practical remediation technology for metal-contaminated soils because of its capability to reduce cost and environmental impacts. We assessed the immobilization effects of poultry waste amendment and plant growth (Panicum maximum Jacq.) on Pb speciation and enzyme activities in shooting range soils. Soil contaminated with Pb was obtained from the top 20 cm of a shooting range. To evaluate Pb mobility in the soil profile treated with plants and immobilizing amendment, we used large columns filled with Pb-contaminated soil (0-20 cm, surface soils) and non-contaminated soil (20-75 cm, subsurface soils). The column study demonstrated that the amendment reduced the toxicity characteristic leaching procedure-extractable Pb in the surface soil by 90% of the Control soil. Lead mobility from the surface to subsurface profiles was significantly attenuated by plant growth but was promoted by the amendment without plant application. The extended X-ray absorption fine structure analysis revealed that the amendment reduced the proportion of PbCO(3) and Pb-organic complexes and transformed them into a more geochemically stable species of Pb(5)(PO(4))(3)Cl with 30 to 35% of the total Pb species. Applications of plant and amendment increased activities of dehydrogenase and phosphatase in the surface soil with 2.7- and 1.1-fold greater than those in Control, respectively. The use of amendments in combination with plant growth may have potential as an integrated remediation strategy that enables Pb immobilization and soil biological restoration in shooting range soils.

Near-Edge X-Ray Absorption Fine-Structure Investigation of Graphene

We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single-layer graphene we observe a splitting of the pi resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.

Speciation of deeply buriedTiOxnanolayers with grazing-incidence x-ray fluorescence combined with a near-edge x-ray absorption fine-structure investigation

Nondestructive methods based on electron emission may encounter serious difficulties when probing the chemical state of deeply buried nanolayers due to restricted information depth. The purpose of the present work is to evaluate to which extent photon emission can overcome these restrictions. Grazing-incidence x-ray fluorescence combined with a near-edge x-ray absorption fine-structure investigation (GIXRF-NEXAFS) offers access to depth-resolving analysis of buried nanolayers with respect to both the chemical speciation and the layer composition. By varying the angle of incidence, the penetration depth can be tuned from a few to several hundreds of nanometers. The information depth of the emitted fluorescence radiation is in the same general range as the soft x-ray regime. Initial measurements were performed on nominally 30 nm thick titanium nanolayers oxidized to different extents and buried below 5 nm carbon layers. These layered structures were produced by means of ion beam sputtering deposition. The plane grating monochromator beamline for undulator radiation in the laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II provides tunable radiation of both well-known flux and high spectral purity for GIXRF-NEXAFS studies. The current results confirm that GIXRF-NEXAFS has the potential to substantially contribute to the speciation of deeply buried nanolayers. The analysis of measurements at a constant incident angle demonstrated that it is not possible to find an angle of incidence for the NEXAFS region to ensure a stable penetration depth. However, appropriate angular corrections can ensure a constant mean penetration depth, in particular, in the vicinity of absorption edges.

Low-Temperature Scanning Tunneling Microscopy and Near-Edge X-ray Absorption Fine Structure Investigations of Molecular Orientation of Copper(II) Phthalocyanine Thin Films at Organic Heterojunction Interfaces

Controlling the molecular orientation in p-n organic heterostructures is one of the key challenges in organic electronics that needs to be solved to improve device performance. In situ low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure measurements are used to investigate the molecular orientation of copper(II) phthalocyanine (CuPc) thin films at the interface of a p-n organic heterojunction comprising CuPc on top of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). It is found that CuPc molecules adsorb flat on lying-down PTCDA thin films with their π-conjugated molecular plane parallel to the substrate surfaces. The preferential orientation of CuPc thin films is determined by the directional π-π interaction at the CuPc/PTCDA interface, which also gives rise to the lying-down configuration of CuPc thin films on highly ordered pyrolytic graphite.

Extended X-ray Absorption Fine Structure Investigation of Adsorption and Separation Phenomena of Metal Ions in Organic Resin

Analytical technique using organic resins has already been well-developed, and its applications are employed in various fields; nevertheless, the chemical phenomena occurring inside the resin remain unclear for the most part. In the present study, we apply EXAFS spectroscopy to elucidate the adsorption and separation phenomena of metal ions by organic resin. That is, the chemical species of trivalent lanthanides (Ln(III)) adsorbed in a tertiary pyridine resin from hydrochloric acid and nitric acid solutions have been determined by EXAFS. The results in HCl solutions suggest that Ln(III) ions are partly dehydrated in the resin phase, enabling the pyridine groups of the resin and chloride ions to coordinate to the Ln(III) ions in their primary coordination sphere. On the other hand, Ln(III) ions are tightly coordinated by several nitrate ions in HNO3 solutions and they keep forming the nitrate complex even in the resin phase. The lighter Ln of Nd tends to form an anionic nitrate complex, [Nd(NO3)4.nH2O]-, in the resin phase, while the middle Ln of Sm exists as a cationic nitrate complex, [Sm(NO3)2.nH2O]+, for the most part. On the basis of these EXAFS results, the adsorption and separation mechanisms of the pyridine resin in HCl solutions are interpreted as the direct coordination of pyridine groups to metal ions, while the mechanisms in HNO3 solutions are mainly dominated by the anion-exchange reaction between the protonated pyridine groups and the anionic nitrate complexes of Ln(III). The obtained results demonstrate that the hydration of metal ions weakens, and instead, other complexations are enhanced in the resin phase.

EXAFS study of U(VI) uptake by calcium silicate hydrates

Among the different cement minerals, calcium silicate hydrates (C-S-H) are the prime candidates for heavy metal binding because of their abundance and appropriate structure. Immobilization processes of heavy metals by cementitious materials, and in particular C-S-H phases, thus play an important role in multibarrier concepts developed worldwide for the safe disposal of hazardous and radioactive wastes. In this study, the uptake of U(VI) by C-S-H has been investigated using X-ray absorption fine structure (XAFS) spectroscopy. C-S-H phases were synthesized using two different procedures: One is based on the mixing of CaO and SiO2 solids (“direct reaction” method); for the other one starting solutions of Ca and Si are used (“solution reaction” method). XAFS investigations were carried out on samples doped with U(VI). U(VI) was either sorbed onto previously precipitated C-S-H phases (sorption samples) or added during C-S-H synthesis (coprecipitation samples). The coordination environment of U(VI) in the sorption samples was found to be independent of the procedure used for C-S-H synthesis. A split equatorial oxygen shell (Oeq1: R=2.23–2.27 Å; Oeq2: R=2.36–2.45 Å), neighboring silicon atoms at short (R=3.07–3.11 Å) and long (R=3.71–3.77 Å) distances, and neighboring Ca atoms (R=3.77–3.81 and 4.15–4.29 Å) were observed for all the samples. The structural parameters resemble those reported for uranophane. The coordination environment of U(VI) in the coprecipitation samples depends on the method used for C-S-H synthesis, and further, the spectra differ from those determined for the sorption samples. UU backscattering contributions were observed in the samples prepared using the direct reaction method, whereas no split equatorial shell appeared in the samples prepared using the solution reaction method.