K-edge X-ray Absorption Fine Structure Research Articles

Structure Determination, Mechanical Properties, Thermal Stability of Co2MoB4 and Fe2MoB4.

The precise determination of atomic position of materials is critical for understanding the relationship between structure and properties, especially for compounds with light elements of boron and single or multiple transition metals. In this work, the single crystal X-ray diffraction is employed to analyze the atomic positions of Co2MoB4 and Fe2MoB4 with a Ta3B4-type structure, and it is found that the lengths of B-B bonds connecting the two zig-zag boron chains are 1.86 Å and 1.87 Å, but previously unreported 1.4 Å. Co and Fe atoms occupy the same crystallographic position in lattice for the doped samples and the valence is close to the metal itself, and Co/Fe K-edge X-ray Absorption Fine Structure(XAFS) spectra of borides with different ratios of Co to Fe are collected to detect the local environment and chemical valence of Co and Fe. Vickers hardness and nano indentation measurements are performed, together with the Density Functional Theory (DFT) calculations. Finally, Co2MoB4 possess better thermal stability than Fe2MoB4 evaluated by Thermogravimetric Differential Thermal Analysis (TG-DTA) results.

Antimony speciation, phytochelatin stimulation and toxicity in plants

Antimony (Sb) is a toxic metalloid that has been listed as a priority pollutant. The environmental impacts of Sb have recently attracted attention, but its phytotoxicity and biological transformation remain poorly understood. In this study, Sb speciation and transformation in plant roots was quantified by Sb K-edge X-ray absorption spectroscopy. In addition, the phytotoxicity of antimonate (SbV) on six plant species was assessed by measuring plant photosynthesis, growth, and phytochelatin production induced by SbV. Linear combination fitting of the Sb K-edge X-ray absorption near-edge structure (XANES) spectra indicated reduction of SbV was limited to ∼5–33% of Sb. The data confirmed that Sb-polygalacturonic acid was the predominant chemical form in all plant species (up to 95%), indicating Sb was primarily bound to the cell walls of plant roots. Shell fitting of Sb K-edge X-ray absorption fine-structure (EXAFS) spectra confirmed Sb–O and Sb–C were the dominant scattering paths. The fitting indicated that SbV was bound to hydroxyl functional groups of cell walls, via development of a local coordination environment analogous to Sb-polygalacturonic acid. This is the first study to demonstrate the key role of plant cell walls in Sb metabolism.

A polycrystalline diamond micro-detector for X-ray absorption fine-structure measurements.

The microminiaturization of detectors used to record the intensity of X-ray beams is very favorable for combined X-ray experimental techniques. In this paper, chemical-vapor-deposited (CVD) polycrystalline diamond film was used to fabricate a micro-detector owing to its well controlled size, good thermostability, and appropriate conductivity. The preparation process and the main components of the CVD diamond micro-detector are described. The external dimensions of the packaged CVD diamond micro-detector are 15 mm × 7.8 mm × 5.8 mm. To demonstrate the performance of the detector, K-edge X-ray absorption fine-structure (XAFS) spectra of Cr, Fe, Cu, and Se foils were collected using the CVD diamond micro-detector and routine ion chamber. These XAFS measurements were performed at beamline 1W2B of Beijing Synchrotron Radiation Facility, covering an energy range from 5.5 to 13.5 keV. By comparison, it can be seen that the CVD diamond micro-detector shows a more excellent performance than the routine ion-chamber in recording these XAFS spectra. The successful application of the CVD diamond micro-detector in XAFS measurements shows its feasibility in recording X-ray intensity.

The new X-ray absorption fine-structure beamline with sub-second time resolution at the Taiwan Photon Source.

The new TPS 44A beamline at the Taiwan Photon Source, located at the National Synchrotron Radiation Research Center, is presented. This beamline is equipped with a new quick-scanning monochromator (Q-Mono), which can provide both conventional step-by-step scans (s-scans) and on-the-fly scans (q-scans) for X-ray absorption fine-structure (XAFS) spectroscopy experiments, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectral measurements. Ti and Te K-edge XAFS spectra were used to demonstrate the capability of collecting spectra at the limits of the working energy range. The Ni and Cu K-edge XAFS spectra for a Cu-doped Pt/Ni nanocomposite were acquired to test the performance of the newly commissioned beamline. Pt L3- and Ru K-edge quick-scanning XAFS (QXAFS) spectra for standard Pt and Ru foils, respectively, revealed the stability of the q-scan technique. The results also demonstrated the beamline's ability to collect XAFS spectra on a sub-second timescale. Furthermore, a Zn(s)|Zn2+(aq)|Cu(s) system was tested to indicate that the states of the Zn electrode could be observed in real time for charging and discharging conditions using an in situ/operando setup combined with QXAFS measurements.

Hydroxyl-Group Identification Using O K-Edge XAFS in Porous Glass Fabricated by Hydrothermal Reaction and Low-Temperature Foaming.

Porous glass was prepared by the hydrothermal reaction of sodium borosilicate glass, and oxygen-ion characterization was used to identify the hydroxyl groups in its surface area. A substantial amount of “water” was introduced into the ionic structure as either OH− groups or H2O molecules through the hydrothermal reaction. When the hydrothermally treated glass was reheated at normal pressures, a porous structure was formed due to the low-temperature foaming resulting from the evaporation of H2O molecules and softening of the glass. Although it was expected that the OH− groups would remain in the porous glass, their distribution required clarification. Oxygen K-edge X-ray absorption fine structure (XAFS) spectroscopy enables the bonding states of oxygen ions in the surface area and interior to be characterized using the electron yield (EY) and fluorescence yield (FY) mode, respectively. The presence of OH− groups was detected in the O K-edge XAFS spectrum of the porous glass prepared by hydrothermal reaction with a corresponding pre-edge peak energy of 533.1 eV. In addition, comparison of the XAFS spectra obtained in the EY and FY modes revealed that the OH− groups were mainly distributed in the surface area (depths of several tens of nanometers).

Operando Observation of Cobalt Catalysts for Water Splitting By X-Ray and Infrared Absorption Spectroscopy

Recently, electrochemical hydrogen production by water splitting has been receiving attention with regard to achieving a sustainable society. For commercial applications, improving overall water splitting efficiency has been required for the development of highly active oxygen evolution electrocatalysts. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. In this situation, we have developed many operando observation techniques for XAFS and infrared absorption spectroscopy using attenuated total reflection mode (ATR-IR) to observe the oxygen evolution electrocatalysts. For example, hard X-ray XAFS can observe the electronic states and local structures of metal species in the catalysts. On the other hand, tender and soft X-ray XAFS and ATR-IR can reveal the chemical states of light elements in the catalysts. Therefore, we investigated the reaction mechanism of Co-KPi and similar catalysts for water splitting using various kinds of operando XAS and ATR-IR spectroscopy (Figure 1). At first, we prepared for Co-KPi and some similar cobalt catalysts in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution by electrodeposition method. It was confirmed that these cobalt catalysts are composed of the edge-sharing CoO6 octahedral structure (CoOOH) by Co K-edge XAFS using hard X-ray. Next, operando O K-edge XAFS spectra for cobalt catalysts were taken using soft X-ray under electrochemical control. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). The CoO2 peak was obviously observed for Co-KPi of highly active OER electrocatalyst, compared with Co-KOAc and Co-KCl. Finally, operando P K-edge XAFS and ATR-IR revealed the phosphate ion is bounded on the cobalt clusters to stabilize the CoO2 structure. Therefore, we revealed that Co-KPi can function as highly active OER electrocatalyst due to the presence of CoO2 strucutre stabilized by phosphate ion. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). Figure 1

Titanium local coordination environments in Cretaceous–Paleogene and Devonian–Carboniferous boundary sediments as a possible marker for large meteorite impact

The local coordination environments of Ti in Cretaceous–Paleogene (K–Pg) from Stevns Klint and Devonian–Carboniferous (D–C) boundary from western part of Cat Co Beach on Cat Ba Island sediments are studied by K-edge X-ray absorption fine structure (XAFS) in order to provide local atomic information by X-ray absorption near edge structure (XANES) and coordination environments by extended X-ray absorption fine structure (EXAFS). Ti K-edge XAFS spectra in bulk part of K–Pg and D–C boundary sediments are compared with those of reference materials such as TiO2 (rutile, anatase and brookite polymorph), CaTiO3, MgTiO3, SrTiO3, PbTiO3, moldavite-brownish, moldavite-green, suevite from Ries crater, impactite, obsidian and Kilauea volcanic glass. The shapes of XANES and EXAFS spectra in K–Pg sediments are similar to those in suevite. Suevite was formed under meteorite impact and its glass component was formed under high temperature and high pressure. Similarities of XANES and EXAFS between K–Pg sediments and suevite indicate that formation process of K–Pg sediments is related to a meteorite impact event. On the other hand, the shape of XANES spectrum in D–C sediments is similar to those in anatase and obsidian. However, the shape of EXAFS spectra in D–C sediments is similar to those in obsidian, rather than anatase. Coordination environments of Ti in D–C sediments suggest that the original glass-like local environments were changed to anatase-like local environments by devitrification. This leads to the conclusion that the analysis of the atomic coordination environments of Ti in boundary sediments can in principle be used as a marker of large meteorite impact, though this Ti local environmental information is actually lost due to the devitrification phenomenon to anatase by quenching process or long-time diagenesis. This may be compensated by the XAFS analysis of Zr because local coordination environments of Zr in same analytical point of K–Pg sediments were not affected by diagenesis (Tobase et al., J Miner Petrol Sci 110:88–91, 2015a).

Adsorption mechanisms of chromate and phosphate on hydrotalcite: A combination of macroscopic and spectroscopic studies

Hydrotalcite (HT) is a layered double hydroxide (LDH), which is considered as a potential adsorbent to remove anion contaminants. In this study, adsorption of chromate (CrO4) and phosphate (PO4) on HT was conducted at various pH and temperatures. Related adsorption mechanisms were determined via the isotherm, kinetic, and competitive adsorption studies as well as the Cr K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The maximum adsorption capacities for CrO4 and PO4 on HT were 0.16 and 0.23 mmol g−1. Regarding adsorption kinetics, CrO4 and PO4 adsorption on HT could be well described by the second order model, and the rate coefficient of CrO4 and PO4 on HT decreased significantly with the increasing pH from 5 to 9. The adsorption kinetics for CrO4 and PO4 were divided into fast and slow stages with the boundary at 15 min. This biphasic adsorption behavior might be partially attributed to multiple reactive pathways including anion exchange and surface complexation. Fitting results of Cr K-edge EXAFS analysis showed a direct bonding between CrO4 and Al on HT surfaces. Such a surface complexation appeared to be the rate-limiting step for CrO4 adsorption on HT. By contrast, the diffusion through the hydrated interlayer space of HT was the major rate-limiting step for PO4. This study determined the adsorption behaviors of CrO4 and PO4 on HT, including the initial transfer process and the subsequent adsorption mechanisms. Such information could improve the strategy to use HT as the potential adsorbent for the remediation of anionic pollutants.

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.

Cobalt Catalysts for Oxygen Evolution Studied By Operando Soft X-Ray Absorption Spectroscopy

Recently, hydrogen production system by electrochemical water splitting has received attractive attention to realize a sustainable society that does not depend on fossil fuels. For commercial applications, the improvement of overall water splitting efficiency has been required by the development of highly active oxygen evolution electrocatalyst. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. Thus, we have investigated the reaction mechanism by measuring operando O K-edge XAFS spectra for Co-KPiand similar catalysts under electrochemical potential control. Electrochemical O K-edge XAFS measurements using soft X-rays were performed with transmission mode at BL3U in the UVSOR Synchrotron, according to the previous works [3]. A home-made electrochemical cell was used with Au/Cr/SiC thin film substrates as working electrodes, a Pt counter electrode, and a Ag/AgCl reference electrode. Co-KPi electrocatalyst was electrodeposited on the Au/Cr/SiC working electrode in potassium phosphate buffered electrolyte containing Co(NO3)2. The O K-edge XAFS spectra for Co-KPi catalyst were taken under electrode potential control in potassium phosphate buffered electrolyte (Figure 1). At 0.0 V, an absorption peak attributed to oxygen species of CoOOH was detected around 531 eV. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). Next, to discuss the relationship between spectroscopic result and catalytic activity, the O K-edge XAFS spectra were taken for the low active cobalt oxide catalysts with CoOOH local structures electrodeposited in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution. Then, the intensities of CoO2 species for Co-KOAc and Co-KCl were much lower than that for Co-KPi. Therefore, we found that Co-KPi can function as highly active OER electrocatalysts due to the presence of CoO2species. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). [3] (a) M. Nagasaka et al, J. Phys. Chem. C 117 16343 (2013). (b) M. Yoshida et al, J. Phys. Chem. C 119, 19279 (2015). Figure 1

Analyses of Charge/Discharge Mechanism of Fe-Containing Li2S-Based Positive Electrode Material Using X-Ray Absorption and Scattering Measurements

1. Introduction Lithium sulfide (Li2S) is one of the promising cathode active materials for high-energy rechargeable lithium batteries because of its high theoretical capacity (ca. 1170 mAh・g-1) [1]. However, this material shows both electronically and ionically resistive property, which gives rise to relatively low electrochemical performance in the cells [2]. Several attempts have been performed to enhance the conductivity of Li2S, such as forming composites with several metals (Li2S-Fe, Li2S-Cu) [1,2] or carbon [3,4]. Recently, we have tried to prepare Fe-containing Li2S-based positive electrode materials (Li8FeS5) and found that the cells showed the discharge capacity of ca. 330 mAh・g-1 and it was enlarged to ca. 730 mAh・g-1after the pre-cycling treatment [5]. However, its charge/discharge mechanism and the effect of the pre-cycling treatment were still unclear. In the present work, we analyzed the structure of Li8FeS5for Li extraction/insertion reactions with/without pre-cycling treatment using X-ray absorption and scattering measurements. 2. Experimental Li2S and FeS with a molar ratio of 4 : 1 were blended thoroughly, and the mixture was then treated by the SPS process at 600oC [5]. The resulting pellet was ground and mixed with carbon (acetylene black) powder with a weight ratio of 9 : 1, and then it was mechanically milled for 8 h to yield the Li8FeS5sample. The obtained Li8FeS5 sample was characterized by X-ray diffraction, as well as inductively coupled plasma (ICP) and atomic absorption spectroscopy. Electrochemical lithium extraction / insertion reactions (charge/discharge) were carried out using lithium coin-type cells with 1M LiPF6 / (EC + DMC) electrolyte at a current density of 46.7 mA・g-1(corresponding to 0.04C) initially with charging with and without stepwise pre-cycling treatment [5]. Fe K-edge X-ray absorption fine structure (XAFS) of the composite cathode materials before and after charge/discharge cycling, sealed in argon filled aluminum laminate bags, were measured using a laboratory-type spectrometer (Technos EXAC-820) and S K-edge XAFS of the materials were measured in vacuum chamber by the beamline BL-10 in SR center of Ritsumeikan University, respectively. We also carried out the X-ray scattering measurements at the beamline BL28XU in SPring-8, and the obtained data were conducted by pair distribution function (PDF) analysis [6]. 3. Results and Discussion The obtained Li8FeS5 sample was black in color, and the XRD pattern showed that it consisted of low-crystalline Li2S (antifluorite structure) [5]. The atomic ratio, estimated by ICP and atomic absorption spectroscopy, was Li : Fe : S = 7.8 : 1.0 : 4.8 [5]. The radial structure functions around S atoms for the Li8FeS5 sample, calculated from EXAFS spectra, showed irreversible changes without pre-cycling treatment, while they showed reversible changes after the pre-cycling treatment. And the radial structure functions around Fe atoms showed that the first neighbor peak shifted to shorter distance and the amplitude decreased significantly after the initial charge without pre-cycling treatment. Although the peak position and the amplitude reversibly changed to the initial ones after the subsequent discharge, actually little change was observed in the second cycling. These are consistent with the charge/discharge performances of the cells, where they showed relatively low initial capacity and very rapid cycle degradation. On the contrary, the reversible shift of the distances and change of amplitudes of the first neighbor peaks were observed for the charge/discharge cycling after the pre-cycling treatment. Such stabilized local structure around Fe atoms for Li extraction/insertion reactions might be assigned to the Fe atoms that occupy partially Li site of low-crystalline Li2S lattice with antifluorite structure, which would be formed during the pre-cycling treatment. Probably it would result in the relatively higher capacity and improved cycle performance of the cells. We also carried out the PDF analyses of the Li8FeS5sample with/without pre-cycling treatment, and the calculated radial distribution functions were compared with those obtained by the EXAFS analyses, which will be presented in the Conference.

High-resolution Measurement of Contact Ion-pair Structures in Aqueous RbCl Solutions from the Simultaneous Corefinement of their Rb and Cl K-edge XAFS and XRD Spectra

In concentrated solutions of aqueous RbCl, all of the Rb+ and Cl– ions exist as contact ion pairs. This full structural assessment is derived from the refinement of three independent experimental measurements: the Rb and Cl K-edge X-ray absorption fine structure (XAFS) and the X-ray diffraction spectra (XRD). This simultaneous refinement of the XAFS and XRD data provides high accuracy since each method probes the structure of different local regions about the ions with high sensitivity. At high RbCl concentration (6 mol·kg–1) the solution is dominated by Rb+–Cl– contact ion pairs yielding an average of 1.5 pairs at an Rb–Cl distance of 3.24 A. Upon formation of these ion pairs, approximately 1.1 waters molecules are displaced from the Rb+ and 1.4 water molecules from Cl–. The hydration shells about both the cation and anion are also determined. These results greatly improve the understanding of monovalent ions and provide a basis for testing the Rb+–Cl– interaction potentials used in molecular dynamics (MD) simulation.

Structural Relationship between CoO6Cluster and Phosphate Species in a Cobalt–Phosphate Water Oxidation Catalyst Investigated by Co and P K-edge XAFS

A cobalt–phosphate (Co–Pi) oxygen evolution cocatalyst on a SrTiO3 photoelectrode was investigated by Co and P K-edge X-ray absorption fine structure (XAFS) spectroscopy. Formation of an edge-shari...

Fe K-Edge X-ray Absorption Fine Structure Determination of γ-Al2 O3 -Supported Iron-Oxide Species.

The structure of FeOx species supported on γ-Al2 O3 was investigated by using Fe K-edge X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) measurements. The samples were prepared through the impregnation of iron nitrate on Al2 O3 and co-gelation of aluminum and iron sulfates. The dependence of the XRD patterns on Fe loading revealed the formation of α-Fe2 O3 particles at an Fe loading of above 10 wt %, whereas the formation of iron-oxide crystals was not observed at Fe loadings of less than 9.0 wt %. The Fe K-edge XAFS was characterized by a clear pre-edge peak, which indicated that the FeO coordination structure deviates from central symmetry and that the degree of FeOFe bond formation is significantly lower than that in bulk samples at low Fe loading (<9.0 wt %). Fe K-edge extended XAFS oscillations of the samples with low Fe loadings were explained by assuming an isolated iron-oxide monomer on the γ-Al2 O3 surface.

A new spectroelectrochemical cell for in situ measurement of Pt and Au K-edge X-ray absorption fine structure.

A new spectroelectrochemical cell to investigate the structure of Pt/Au nanoclusters using Pt and Au K-edge X-ray absorption fine structure (XAFS) measurements under the electrochemical conditions is developed. K-edge XAFS measurements for Pt and Au require a sample as thick as 1-2 cm, which prevents homogeneous potential distribution. We can measure in situ Pt and Au K-edge XAFS spectra and determine reasonable electrochemical surface areas using our developed spectroelectrochemical cell. This work provides a new approach to analyze Pt/Au core-shell nanoclusters. The new cell is designed to be applied to both spectra with high absorption-edge energies such as the K-edge of Pt and Au and those with low absorption-edge energy such as Pt L-edge.

Surface Structural Reconstruction for Optical Response in Iodine-Modified TiO2 Photocatalyst System

We report an alternative mechanism for the optical response of an iodine-modified anatase TiO2 photocatalyst material. Unlike the general realization that the iodine atom provides impurity levels within the optical band gap, we suggest that a distorted surface structure plays a dominant role. Anatase pure TiO2 and iodine-modified TiO2 (1-TiO2) nanocrystals were synthesized by a simple solvothermal method, and I-TiO2 exhibited an extended absorption edge up to 550 nm. Employing iodine K-edge X-ray absorption fine structure (XAFS), we demonstrate that iodine is not incorporated into the nanoparticle interior but exists in the form of IO3- groups at the surface. Furthermore, this IO3- cluster adsorption largely induces a disordered structure, as revealed by Ti 2p(3/2) X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Ti K-edge XAFS data fitting. Density functional theory calculation indicates that this distorted structure can form a midgap state, which should be responsible for its excellent optical property. This finding represents a promising route for improving the optical response of the nanophotocatalyst system via surface treatment.

K-Edge X-ray Absorption Fine Structure Analysis of Pt/Au Core–Shell Electrocatalyst: Evidence for Short Pt–Pt Distance

A carbon-supported Pt-shell Au-core electrocatalyst (Pt/Au/C) was prepared by sequential deposition of Pt ions on the surface of Au nanoparticles supported on carbon. The area-specific activity of the oxygen reduction reaction (ORR) for the prepared Pt/Au/C in 0.1 M HClO4 aqueous solution was approximately 2 times higher than that for a commercial carbon-supported Pt electrocatalyst (Pt/C). The core–shell structure was confirmed using electrochemical methods and Pt and Au K-edge X-ray absorption fine structure (XAFS) analysis. XAFS analyses indicated that the Pt–Pt bond distance for the Pt/Au/C catalyst was shorter than that for Pt foil and the Pt/C catalyst. In addition, the Au–Au distance was much shorter than that for Au foil. The reason for the high ORR activity of Pt/Au/C is considered to be shorter Pt–Pt bond distance as compared to that of Pt/C.

Current Status of AIST X-ray-Absorption-Spectroscopy (XAFS) Instrument with 100-Pixel Superconducting-Tunnel-Junction Array Detector

We constructed a fluorescence yield X-ray absorption fine structure (XAFS) apparatus for the measurement of local structure around trace light element impurities in light element matrices. Our XAFS instrument with a 100-pixel STJ detector is now in the stage of routine operation and open to users at a synchrotron radiation facility (KEK PF). The average energy resolution of the 100 pixels is 11.8 $$\pm $$ 0.6 at 400 eV in a sensitive area of 1 mm $$^{2}$$ . As an example of functional materials, we successfully measured nitrogen K-edge XAFS spectra of a compound semiconductor, 4H-SiC, with a nitrogen dopant concentration of 300 ppm. The faint N-K line was clearly separated from the strong C-K line.

In Situ Electrochemical XAFS Studies on an Iron Fluoride High-Capacity Cathode Material for Rechargeable Lithium Batteries

The reactions and structural evolution of FeF3 during cell cycling are investigated in an in situ cell by using Fe K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The results of X-ray absorption near-edge structure spectroscopic analysis demonstrate that there are three stages in the reaction of FeF3 with Li: (1) a two-phase intercalation reaction in the range of x = 0 to 0.46 Li, (2) a single-phase intercalation reaction in the range of x = 0.46 to 0.92 Li, and (3) a conversion reaction in the range of x = 0.92 to 2.78 Li. The coordination numbers (CNs) and bond lengths of the Fe–F bonds or Fe–Fe bonds for the lithiated FeF3 are obtained by performing XAFS fitting. The splitting trends of the Fe–F bond lengths and the Fe–F CNs in the range of x = 0 to 0.92 Li support the proposal that R-3c-structured FeF3 is transformed into R3c-structured Li0.92FeF3 after the intercalation of 0.92 equiv. of Li, and that the intermediate Li0.46FeF3 may be R3-structured. The small Fe–Fe CN of Li2.78FeF3 indica...

Electrochemical lithium intercalation into Bi2Sr2CaCu2O8+δ

We have prepared Li-intercalated LixBi2Sr2CaCu2O8+δ (x =0-2.0) samples by using electrochemical method, and performed synchrotron x-ray diffraction, Cu K-edge x-ray absorption fine structure (XAFS), and magnetic susceptibility measurements. With increasing x, a- and c-lattice parameters monotonically increase, which shows lithium intercalation into Bi2Sr2CaCu2O8+δ. Accompanied by the lithium insertion, the valence of Cu ion changes from Cu2+/Cu3+ to Cu1+/Cu2+ to realize charge neutrality. This change of the valence was detected by Cu K-edge XAFS measurement. A clear increase of spectral weight that corresponds to 1s→ 4pπ(3d10L) was observed at around 8982 eV with x. The superconducting (SC) transition temperature TC significantly changes from 74 K for x = 0 to 90 K for x = 0.8, which is attributed to modified density of states by the decrease of hole concentration. A volume fraction of the superconducting phase was 1-2 % for x ≥ 0.6 implying phase separation where Li-rich non SC phase and Li-poor SC phase coexist. Such a phase separation is universally seen in electrode active materials.