Extended X-ray Absorption Fine Structure Results Research Articles

Effects of Fe Ions, Ultraviolet Irradiation, and Heating on Microscopic Structures of Black Lacquer Films.

The chemical reaction between Fe and lacquer has been used to create the black color in lacquer coatings since ancient times. Here, the effects of Fe ion addition, UV irradiation, and heating on the microscopic structures of black lacquer films were investigated by using X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), Fourier transform-infrared spectroscopy (FT-IR), small-angle X-ray scattering (SAXS), and small angle neutron scattering (SANS). The EXAFS result indicated that heating and UV irradiation made the coordination structure of Fe3+ in the lacquer nonuniform, and that heating caused the greatest nonuniformity. The FT-IR, SAXS, and SANS results demonstrated that the microscopic structural changes in the black lacquer films were induced by both heating and UV irradiation, but the changes were different. Heating caused a substantial structural change on the nanoscale, and UV irradiation mainly caused changes in the molecular binding mode. The results provide important knowledge for analyzing archeological lacquer samples and for developing lacquer-based materials. This work also demonstrates the utility of the complementary use of XANES, EXAFS, FT-IR, SAXS, and SANS for nondestructive analysis of black lacquer in precious cultural relics.

Combined experimental and theoretical studies of some uranium(VI) complexes derived from imidazole-based carbenes

A series of bidentate, viz., 1,1'-(1,2-ethylene)-3,3'-dimethyldiimidazoline-2,2'-diylidene (L1), 1-methyl-3-(2-pyridylmethyl)-imidazoline-2-ylidene (L2) and tridentate, viz., 1,3-bis(2-pyridyl)-imidazoline-2-ylidene (L3) ligands have been obtained from their corresponding imidazolium salts through deprotonation reactions. Treatment of UO2Cl2(THF)3 with one equivalent L3 produces air-stable U(VI)-carbene complex 3, characterized by elemental analysis, FTIR, NMR spectroscopy as well as extended X-ray absorption fine structure (EXAFS) analysis. EXAFS result indicates that the ligand is bonded through one C and two N-atoms to the uranium atom. Attempts to synthesize uranyl complexes derived from L1 and L2 were also successful but these complexes are decomposed quickly within one hour at room temperature. 3 produces pure UO2 powder when heated under an argon atmosphere from room temperature to 600 °C with constant heating rate of 5 °C/min. The solid-state UV–Vis spectrum of the compound shows absorption peaks at 332 and 450 nm. The excitation spectrum of 3 (at λ em = 520 nm) exhibits two almost symmetrical peaks at 273 and 368 nm. Density functional theory-based quantum mechanical calculations indicate that a partial covalent interaction exists between the carbene C and U while a weak non-covalent interaction exists between carbene N and U atoms.

The Electrochemical Growth of 2D Metals on Insulators Using a Graphene Interface

2D confined metals (2D-M) under heteroepitaxial relations with graphene (Gr) allow for the potential control of chemical properties of the 2D-M by an external electric field. If this external electric field is supplied by an insulating substrate (such as a dielectric or a ferroelectric), then it can enable a non-Faradaic manipulation of electrocatalytic reactions via the M-Gr electrocatalyst system. In this work, we will explore the architecture of a 2D metal-graphene (2D M-Gr), where the substrate is supplying an electric field as well as fulfilling the role of the insulating substrate (dielectric) in the system. The insulating substrate will carry an embedded electron charge at the interface between two dielectric (DE) layers, close to the surface of the metal, allowing for electronic influence on the metal. In this system, graphene acts as an imperfect screener of electric fields allowing the dielectric/ferroelectric-generated field to influence the metal. We find that providing an approximate potential of 0.1 V yields a significant increase in the hydrogen evolution current within a 2D platinum/graphene (2D Pt/Gr) system. Extended X-ray absorption fine structure (EXAFS) studies are used to examine 2D M-Gr epitaxy. EXAFS results indicate a strain condition imposed by Gr on the metal grown, where Gr is constraining nearest-neighbor distances through epitaxial directions. Experimental research on the strain induced in graphene-templated Pt monolayers by Alamgir et al. found that a single graphene layer induces a 3.5% compressive strain on the Pt adlayer deposited onto it, increasing the catalytic activity of the system In the 2D M-Gr system, our strained, epitaxially grown metal on Gr will be demonstrated on an Au/Gr/DE and Pd/Gr/DE system.

Effects of Sn-ion irradiation on local structure and flux pinning properties of MgB2 thin films

We studied the effects of tin (Sn) ion irradiation on the local structure and flux pinning properties of 412-nm-thick MgB2 thin films. Ions with 2 MeV incident energy were irradiated onto MgB2 thin films at room temperature and various dose levels from 2 × 1012 to 7 × 1013 atoms/cm2. X-ray diffraction and extended X-ray absorption fine-structure results confirmed extended average bond lengths and increased mean-square relative displacement that correlated with crystalline disorder serving as artificial pinning centers induced by ion irradiation. While the superconducting transition temperature (Tc) exhibited a decreasing trend, the magnetization Jc (H) of the irradiated films measured at both 5 K and 20 K were remarkably enhanced compared to that of pristine films at high applied magnetic fields. The decreases in exponent β from 1.1 and 1.13 to 0.69 and 0.62 after irradiation at 5 K and 20 K, respectively, as the dose increased to 5 × 1013 atoms/cm2 indicates a shift in the pinning mechanism from weak collective pinning to strong plastic pinning according to collective pinning theory. In addition, the scaling behavior of the flux pinning force density (Fp) in the irradiated MgB2 films showed a crossover from surface pinning to normal point pinning. These results suggest that intermediate-energy irradiation by heavy ions such as Sn generates effective pinning sources for improving Jc(H) in MgB2 superconductors.

Strain-Assisted Single Pt Sites on High-Curvature MoS 2 Surface for Ultrasensitive H 2 S Sensing

Strain-Assisted Single Pt Sites on High-Curvature MoS ₂ Surface for Ultrasensitive H ₂ S Sensing

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

Local structural distortions in SnTe investigated by EXAFS

Extended x-ray absorption fine structure (EXAFS) has been measured at the K edge of Sn in SnTe in the temperature range from 5 to 480 K. EXAFS results are consistent with the presence of a local rhombohedral distortion in the full temperature range from 5 to 300 K, even well above the ferroelectric transition temperature, suggesting a partial order-disorder character of the transition. At and above 300 K, the anomalous behaviour of the third and fourth EXAFS cumulants reveals a modification of the anharmonicity of the effective pair potential, possibly connected with the softening of high frequency modes or to the presence of multiple phases.

Homogeneous-like Alkyne Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation, which is currently accomplished by modified palladium catalysts. Herein, we rep...

The Enhancement of Selectivity and Activity for Two-Electron Oxygen Reduction Reaction by Tuned Oxygen Defects on Amorphous Hydroxide Catalysts

Amorphous catalysts, thanks to their unique coordinately unsaturated properties and abundant of defect sites, incline to possess higher activity and selectivity than their crystalline counterparts....

Study on crystallographic and electronic structure of micrometre-scale ZnO and ZnO:B rods via X-ray absorption fine-structure spectroscopy.

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra were recorded to investigate the electronic structure and local crystal structure of ZnO and ZnO:B powders produced via hydrothermal synthesis. ZnO and ZnO:B grow as micrometre-scale rods with hexagonal shape, as confirmed by scanning electron microscopy micrographs. The number of broken ZnO:B rods increases with increasing B concentration, as observed in the images, due to B atoms locating in between the Zn and O atoms which weakens and/or breaks the Zn-O bonds. However, no disorder within thecrystallographic structure of ZnO upon B doping is observed from X-ray diffraction results, which were supported by EXAFS results. To determine the atomic locations of boron atoms in the crystal structure and their influence on the zinc atoms, EXAFS data were fitted with calculated spectra using the crystal structure parameters obtained from the crystallographic analysis of the samples. EXAFS data fitting and complementary k-weight analysis revealed the positions of the B atoms - their positions were determined to be in between the Zn and Oatoms.

Investigation of the structure-luminescence relationship in ZnAlEu layered double hydroxides intercalated with nitrate and benzenecarboxylates

Rare Earth-containing materials exhibit interesting luminescence properties, such as a spectral profile with narrow emission lines and a well understood crystal field dependence. These features make them appealing materials for molecular probing. Here, the structural properties governing the luminescence of Eu3+ ions hosted in layered double hydroxides (LDH) are investigated. Eu3+-substituted LDH have been shown to present photoluminescence properties dependent on the anionic species intercalated between the hydroxide layers. This feature is shown by the characterization and comparison between LDH intercalated with NO3− and a set of benzenecarboxylates: trimesate (benzene-1,3,5-tricarboxylate, BTC), trimellitate (benzene-1,2,4-tricarboxylate, TLA) and terephthalate (benzene-1,4-dicarboxylate, TERE). Successful LDH formation and intercalation with the respective anion was confirmed by powder X-ray diffraction (PXRD). The coordination structure of Eu3+ has been investigated by Extended X-ray Absorption Fine Structure (EXAFS) experiments, showing the incorporation of this activator in the hydroxide layers of the LDH in an 7/8-fold Oxygen-rich environment. The systems here investigated present very similar coordination, except for the presence of a water related Oxygen at around 3.0 Å in the nitrate-intercalated LDH, absent in the benzenecarboxylate-intercalated materials. The photoluminescence properties of all samples were analysed on the basis of the Judd-Ofelt theory, showing that the polycarboxylate- and NO3−-intercalated LDH can be grouped into two distinct classes with different sets of Judd-Ofelt intensity parameters (Ω2 and Ω4). This has been rationalized based on the EXAFS results. Symmetry distortions in the first coordination shell of Eu are also expected from the observed changes in Ω2 and Ω4. This distinctive feature allows one to probe interlayer modifications in LDH through changes in the luminescence properties of lanthanide activators hosted in their hydroxide layers, opening up new opportunities for in-situ investigation of anion exchange reactions in these materials.

X-ray absorption spectroscopy study of Ga-doping in reactively sputtered ZnO films

Ga-doped ZnO (GZO) films were grown by reactive co-sputtering of a Zn-GaAs target (3% area coverage of Zn by GaAs) at 375 °C with different partial pressures of oxygen. The resistivity of GZO films increases drastically from ≲10−3 Ω-cm to ~ 0.2 Ω-cm as the O2 in the sputtering atmosphere is changed from 5 % to 6 %. The films grown at 5% or lower O2 have the Ga/Zn ratio of ~ 0.1 and display low resistivity with carrier concentration ~ 1021 cm−3, while the films grown at 6% or higher O2 have lower Ga/Zn ratio of ~ 0.01 and display high resistivity with carrier concentration ~ 1019 cm−3. Extended X-ray absorption fine structure (EXAFS) measurements at Zn and Ga K-edges reveal substitutional incorporation of Ga in low resistivity films. X ray photoelectron spectroscopy (XPS) studies reveal a substantial decrease in oxygen vacancies together with increase in oxygen interstitials, in high resistivity GZO films. EXAFS measurements of the high resistivity films show a substantial increase in Ga-O and Ga-Zn bond distances and Ga-O coordination, indicating incorporation of oxygen interstitials in the vicinity of Ga. The comparison of measured and simulated X–ray absorption near edge spectra at O K-edge, together with XPS and EXAFS results, provides adequate experimental evidence of the presence of acceptor type (GaZn+Oi) defect complex, which cause carrier compensation in high resistivity films.

Structural studies on transition metal ion complexes of polyethylene oxide-natural rubber block copolymers

Two shot solution polymerised NR/PEO block copolymer (BC) was used as an absorbent in this study. This polymer has got polyethylene oxide (PEO) immobilised on hydrophobic natural rubber and it was used for complexation studies with the selected 3d transition metal ions. The prepared complexes were subjected to various analytical techniques such as energy dispersive X-ray spectroscopy (EDX), Fourier Transform infrared spectroscopy(FTIR), Raman spectroscopy, X-ray diffraction (XRD) studies, extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near edge spectroscopy (XANES). EDX analysis confirms presence of the respective metal ion in each complex. FTIR spectroscopy reveals the 72 helical conformation of polyethylene oxide segments in BC which is retained with some deformation upon complexation. The BC-metal ion interaction is confirmed by broadening of the C-O-C triplet peak. XRD analysis revealed that PEO lattice undergoes expansion during complexation inorder to accommodate the respective metal ion. From the EXAFS results it was observed that each metal ion shows only one peak that corresponds to the oxygen shell indicating the presence of only one type of metal ion bonding. The EXAFS gives hexa coordinated pattern for Co(II), Ni(II) and Zn(II) complexes while a tetra coordination for the Cu(II) complex. Metal-oxygen distance in a given complex is constant and unique which varies with the metal ion. XANES shows a distorted octahedral symmetry (Oh) with sp3d2 hybridisation for the hexa coordinated complexes and a square planar symmetry with dsp2 hybridisation for the tetra coordinated complex. Feasibility of 1 s → 3d transition confirms +2 oxidation state of the metal ions. The combined result of EXAFS and FTIR shows the best fit structure of the complexes in which metal ions are encapsulated within the PEO helical tunnel.

Differences in local structures around zirconium atoms in Eu2Zr2O7 and La2Zr2O7

Extended X-ray absorption fine structure (EXAFS) analyses of the Eu2Zr2O7 and La2Zr2O7 pyrochlore composition oxides were performed to examine differences in the local structures around the Zr atoms. In the EXAFS analyses of Eu2Zr2O7, the amplitude reduction factor of Zr16c-O8a was considerably large, indicating that a part of the oxygen vacancy site (8a) was occupied by oxide ions. Furthermore, the occupancy of the 8a site determined by Rietveld refinement of the X-ray diffraction data for Eu2Zr2O7 was 0.32(2), which was consistent with the EXAFS results. From the EXAFS analyses of La2Zr2O7, oscillation in the high wavenumber k-range sharpened with decreasing temperature, and the mean square relative displacement (MSRD) values of the ZrO peak in the R-space were small compared to those of Eu2Zr2O7. Therefore, the disorder in the local structures around the Zr atoms of La2Zr2O7 was determined to be smaller than that of Eu2Zr2O7.

Mercury Trithiolate Binding (HgS3) to a de Novo Designed Cyclic Decapeptide with Three Preoriented Cysteine Side Chains.

Mercury(II) is an unphysiological soft ion with high binding affinity for thiolate ligands. Its toxicity lies in the interactions with low molecular weight thiols including glutathione and cysteine-containing proteins that disrupt the thiol balance and alter vital functions. However, mercury can also be detoxified via interactions with Hg(II)-responsive regulatory proteins such as MerR, which coordinates Hg(II) with three cysteine residues in a trigonal planar fashion (HgS3 coordination). The model cyclodecapeptide P3C, c(GCTCSGCSRP) was designed to promote Hg(II) chelation in a HgS3 coordination environment through the parallel orientation of three cysteine side chains. The binding motif is derived from the dicysteine P2C cyclodecapeptide validated previously as a model for d10 metal transporters containing the binding sequence CxxC. The formation of the mononuclear HgP3C complex with a HgS3 coordination is demonstrated using electrospray ionization mass spectrometry, UV absorption, and 199Hg NMR. Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the Hg(II) coordination environment is T-shaped with two short Hg-S distances at 2.45 Å and one longer distance at 2.60 Å. The solution structure of the HgP3C complex was refined based on 1H-1H NMR constraints and EXAFS results. The cyclic peptide scaffold has a rectangular shape with the three binding cysteine side chains pointing toward Hg(II). The HgP3CH complex has a p Ka of 4.3, indicating that the HgS3 coordination mode is stable over a large range of pH. This low p Ka value suggests that the preorientation of the three cysteine groups is particularly well-achieved for Hg(II) trithiolate coordination in P3C.

X-ray absorption spectroscopy investigation of relationship between Mg vacancy and magnetic properties of MgO powder

This paper reports on the ferromagnetism of MgO powder and degraded powder at room temperature. The starting material, Mg(OH)2, was analyzed by X-ray fluorescence (XRF) spectrometry and then calcined at 450 °C for 5 h to obtain fresh MgO powder. The calcined powder was separated into two batches (i) kept in vacuum sealed (MgO-F) and (ii) kept at ambient environment (30–32 °C at 45–55% relative humidity) for 2 days (MgO-D). The samples were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and X-ray absorption spectroscopy (XAS) including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XRD and EXAFS confirmed that MgO phase in MgO-F. Both Mg(OH)2 and MgO compositions were found in MgO-D. The origin of ferromagnetism at room temperature of MgO-F and MgO-D is attributed to the Mg vacancies in the MgO structure that revealed and verified by XANES and EXAFS results.

English

Chromium(III), copper(II) and lead(II) are among the heavy metals produced and released in large amounts by anthropogenic sources worldwide, including Burkina Faso. Previous studies have demonstrated the successful application of domestic natural mixed clays for the removal of these metal ions as a cheap and environmentally friendly method. Qualitative mineralogical characterization of the clays revealed that they consist of kaolinite, illite, orthrose and quartz, and minor quantities of albite and montmorillonite. pHPZC for the clays, as determined by potentiometric titrations, are in the range 6.8 to 7.3. In this study, the interactions of chromium(III), copper(II) and lead(II) ions with these clay minerals were examined by the use of extended X-ray absorption fine structure (EXAFS) spectroscopy. Cr3+ forms tetrameric hydrolysis complexes on the mineral surface with a Cr–O bond distance of 1.98 A, and two Cr···Cr distances at 3.02 and 3.62 A. This is indicative of a tetrameric [Cr4(OH)6(H2O)12]6+ entity bound to the clay mineral surface. A distance of 3.17 A, regarded as a Cr···Fe distance, indicates that one Cr3+ ion in the hydrolyzed tetramer binds to two oxygens in the mineral surface which are bound to either one or two iron(III) ions in the surface. Pb2+ binds two oxygen atoms at an average bond distance of 2.31 A, with a significant contribution of linear multiple scattering from the PbO2 entity. The EXAFS results of Cr3+ sorption are consistent with the presence of a hydrolysis product of polymeric Cu2+ species with a surface complex or precipitate. Key words: Sorption, extended X-ray absorption fine structure (EXAFS) spectroscopy, clay, precipitation, Burkina Faso.

The interaction between molten gallium and the hydrocarbon medium induced by ultrasonic energy—can gallium carbide be formed?

AbstractUltrasonic irradiation of molten gallium in organic liquids (decane, dodecane, etc.) results in dispersion of the gallium into nanometric spheres. These were examined by several analytical methods XRD, DSC, Raman and IR spectroscopy) as well as electron microscopy (SEM, TEM) and found to be composed of Ga and C. The DSC analysis indicates that the Ga has possibly reacted with carbon, while the Raman spectrum of the product demonstrates a strong additional peak that could not be identified. This work explores the possibility that the product is gallium carbide or another gallium‐carbon complex. To investigate the nature of the product, we performed detailed extended X‐ray absorption fine structure (EXAFS) and X‐ray absorption near‐edge structure (XANES) analyses. On the basis of DSC, IR, and Raman it appear to be formation of GaC, whereas the analysis by EXAFS and XANES demonstrated that the gallium is found to be in a higher reduced state (almost metallic), supported by carbon. The question that remains open in addition to the one related to the formation of galium carbide is whether a complex structure, including oxygen contamination is involved in the layers surrounding the Ga as indicated by the EXAFS results.

Arsenate and Selenate Scavenging by Basaluminite: Insights into the Reactivity of Aluminum Phases in Acid Mine Drainage.

Basaluminite precipitation may play an important role in the behavior of trace elements in water and sediments affected by acid mine drainage and acid sulfate soils. In this study, the affinity of basaluminite and schwertmannite for arsenate and selenate is compared, and the coordination geometries of these oxyanions in both structures are reported. Batch isotherm experiments were conducted to examine the sorption capacity of synthetic schwertmannite and basaluminite and the potential competitive effect of sulfate. In addition, synchrotron-based techniques such as differential pair distribution function (d-PDF) analysis and extended X-ray absorption fine structure (EXAFS) were used to determine the local structure of As(V) and Se(VI) complexes. The results show that oxyanion exchange with structural sulfate was the main mechanism for removal of selenate, whereas arsenate was removed by a combination of surface complexes and oxyanion exchange. The arsenate adsorption capacity of basaluminite was 2 times higher than that of schwertmannite and 3 times higher than that of selenate in both phases. The sulfate:arsenate and sulfate:selenate exchange ratios were 1:2 and 1:1, respectively. High sulfate concentrations in the solutions did not show a competitive effect on arsenate sorption capacity but had a strong impact on selenate uptake, suggesting some kind of specific interaction for arsenate. Both d-PDF and EXAFS results indicated that the bidentate binuclear inner sphere was the most probable type of ligand for arsenate on both phases and for selenate on schwertmannite, whereas selenate forms outer-sphere complexes in the aluminum octahedral interlayer of basaluminite. Overall, these results show a strong affinity of poorly crystalline aluminum phases such as basaluminite for As(V) and Se(VI) oxyanions, with adsorption capacities on the same order of magnitude as those of iron oxides. The results obtained in this study are relevant to the understanding of trace element behavior in environments affected by acid water, potentially opening new research lines focused on remediation by natural attenuation processes or engineered water treatment systems.

Structural analysis of Tm3 + doped As–S–Ga glasses by Raman and EXAFS spectroscopy

Structural properties of Tm3+ doped As–S–Ga glasses were investigated using Raman spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at As K-edge, Ga K-edge and Tm L3-edge. The Raman spectra are interpreted on the basis of comparison with the well-known Raman spectra of As2S3 glasses the structure of which mainly consists of pyramidal [AsS3/2] units interconnected by S–S chains. Incorporation of Ga2S3 in As2S3 glass matrix induced the dissociation of S–S chains in behalf of the formation of tetrahedra [GaS4]. Moreover, introduction of Tm into As–S–Ga glasses forms Tm–S bonds through the further dissociation of [AsS3/2] pyramidal units. The deficiency of S in the glasses is compensated with the formation of As4S4 structural units. The EXAFS results indicate that Ga–S bond distance in the tetrahedra [GaS4] decreases from 2.29Å to 2.26Å with the increase of Tm concentration in the glass. Meanwhile, Tm–S bond distance increases from 2.66Å to 2.71Å approaching to that of crystalline Tm2S3 and simultaneously, the coordination number of Tm gradually decreases from 7 to 4.