Showing papers on "Extended X-ray absorption fine structure published in 2012"

Interaction between Eu(III) and Graphene Oxide Nanosheets Investigated by Batch and Extended X-ray Absorption Fine Structure Spectroscopy and by Modeling Techniques

Abstract: The interaction mechanism between Eu(II) and graphene oxide nanosheets (GONS) was investigated by batch and extended X-ray absorption fine structure (EXAFS) spectroscopy and by modeling techniques. The effects of pH, ionic strength, and temperature on Eu(III) adsorption on GONS were evaluated. The results indicated that ionic strength had no effect on Eu(III) adsorption on GONS. The maximum adsorption capacity of Eu(III) on GONS at pH 6.0 and T = 298 K was calculated to be 175.44 mg.g(-1), much higher than any currently reported. The thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that Eu(III) adsorption on GONS was an endothermic and spontaneous process. Results of EXAFS spectral analysis indicated that Eu(III) was bound to similar to 6-7 O atoms at a bond distance of similar to 2.44 angstrom in the first coordination shell. The value of Eu C bond distance confirmed the formation of inner-sphere surface complexes on GONS. Surface complexation modeling gave an excellent fit with the predominant mononuclear monodentate >SOEu2+ and binuclear bidentate (>SO)(2)Eu-2(OH)(2)(2+) complexes. This paper highlights the application of GONS as a suitable material for the preconcentration and removal of trivalent lanthanides and actinides from aqueous solutions in environmental pollution management.

Spectroscopic Characterization of Mixed Fe–Ni Oxide Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Electrolytes

Abstract: Mixed Fe−Ni oxide electrocatalysts for the oxygen evolution reaction in alkaline electrolytes were synthesized using three different approaches: evaporation induced self-assembly, hard templating, and dip-coating. For each synthesis method, a peak in oxygen evolution activity was observed near 10 mol % Fe content, where the mixed metal oxide was substantially more active than the parent metal oxide electrocatalysts. X-ray diffraction (XRD) analysis showed the formation of a mixed NiO/NiFe2O4 phase at low Fe concentrations, and formation of Fe2O3 at compositions above 25 mol % Fe. Raman vibrational spectroscopy confirmed the formation of NiFe2O4, and did not detect Fe2O3 in the electrocatalysts containing up to 20 mol % Fe. X-ray absorption near edge structure (XANES) showed the Fe in the mixed oxides to be predominantly in the +3 oxidation state. Extended X-ray absorption fine structure (EXAFS) showed changes in the Fe coordination shells under electrochemical oxygen evolution conditions. Temperature programmed reaction spectroscopy showed the mixed oxide surfaces also have superior oxidation activity for methanol oxidation, and that the reactivity of the mixed oxide surface is substantially different than that of the parent metal oxide surfaces. Overall, the NiFe2O4 phase is implicated in having a significant role in improving the oxygen evolution activity of the mixed metal oxide systems.

Applications of extended X-ray absorption fine-structure spectroscopy to studies of bimetallic nanoparticle catalysts

Abstract: Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to study short range order in heterometallic alloys for almost four decades. In this critical review, experimental, theoretical and data analytical approaches are revisited to examine their power, and limitations, in studies of bimetallic nanocatalysts. This article covers the basics of EXAFS experiments, data analysis, and modelling of nanoscale clusters. It demonstrates that, in the best case scenario, quantitative information about the nanocatalyst's size, shape, details of core–shell architecture, as well as static and dynamic disorder in metal–metal bond lengths can be obtained. The article also emphasizes the main challenge accompanying such insights: the need to account for the statistical nature of the EXAFS technique, and discusses corrective strategies.

Spectroelectrochemistry: Theory And Practice

Abstract: 1. Introduction.- 1. Motivations for Spectroelectrochemistry.- 2. Methodologies Available.- 3. Computer-Based Data Processing.- 4. The Future.- References.- 2. X-Ray Techniques.- 1. Historical Background.- 1.1. Ultrahigh Vacuum Techniques.- 1.2. X-Ray Techniques for Surface Study.- 1.2.1. Scattering Methods.- 1.2.2. Absorption Techniques.- 1.3. Neutron Scattering.- 2. Theory-The Interaction of X-Rays with Matter.- 2.1. X-Ray Scattering.- 2.2. X-Ray Absorption.- 3. Experimental Details.- 3.1. In Situ X-Ray Diffraction.- 3.1.1. X-Ray Detection Methods.- 3.1.2. X-Ray Sources.- 3.1.3. Cell Design.- 3.1.4. The Experiment.- 3.2. In Situ X-Ray Absorption Studies.- 4. Applications.- 4.1. In Situ X-Ray Diffraction.- 4.2. EXAFS Studies.- List of Symbols.- References.- 3. Photoemission Phenomena at Metallic and Semiconducting Electrodes.- 1. Introduction.- 1.1. Some General Features of Photoelectronic Emission.- 1.2. Reaction Step Models for Photoemission.- 2. Theoretical: Metals.- 2.1. Fowler's Theory for Metal/Vacuum Interfaces.- 2.2. Tunneling through the Potential Barrier.- 2.3. Quantum Mechanical Photoemission Theories for the Metal/Vacuum and Metal/Electrolyte Interfaces.- 2.4. Optical Polarization and Crystal Epitaxy Effects.- 2.5. Role of the Electrical Double Layer.- 3. Theoretical: Semiconductors.- 3.1. Kane's Theory for Semiconductor/Vacuum Interfaces.- 3.2. Gurevich's Quantum Mechanical $$ \frac{3} {2} $$ Law for In Situ Photoemission.- 3.3. Bockris and Uosaki Treatment.- 3.4. Hot Carrier Effects: The Nozik-Williams Model.- 4. Experimental Techniques.- 4.1. Choice of Scavanger and Electrolyte.- 4.2. Cell Design and Electrode Preparation.- 4.3. Optics, Apparatus, and Methods.- 5. Conclusions.- 5.1. Physical Mechanistic Studies.- 5.2. Solvated Electron Chemistry.- References.- 4. UV-Visible Reflectance Spectroscopy.- 1. Introduction.- 2. Physical Optics.- 2.1. Optical Constants.- 2.2. The Reflectivity of an Interface.- 2.3. Three-Phase System and Linear Approximation.- 2.4. Nonlocal Optics.- 3. Experimental.- 3.1. Arrangements for Determining ?R/R.- 3.2. Electrochemical Cells and Electrodes.- 4. The Metal/Electrolyte Interface.- 4.1. Electroreflectance Studies of the Metal Surface.- 4.2. Surface States at the Metal/Electrolyte Interface.- 4.3. Surface Plasmon Studies.- 4.4. Double-Layer Contributions to Electroreflectance.- 5. Chemisorption and Film Formation.- 5.1. Oxides.- 5.2. Ions and Molecules.- 5.3. Metal Adsorbates.- 5.4. Metal Film Formation.- 6. Summary and Outlook.- Appendix I.- Appendix II.- List of Symbols.- References.- 5. Infrared Reflectance Spectroscopy.- 1. Introduction and Historical Survey.- 2. Theory of Reflection-Absorption Spectroscopy.- 2.1. Propagation of an Electromagnetic Plane Wave.- 2.2. Fundamentals of Absorption Spectroscopy. Selection Rules.- 2.3. Specular Reflection. Application to Reflection-Absorption Spectroscopy. Surface Selection Rules.- 3. Experimental Techniques.- 3.1. Dispersive Spectrometers.- 3.1.1. Optical Components Used in Infrared Spectrometers Specially Designed for External Reflectance Spectroscopy.- 3.1.2. Signal Detection and Processing.- 3.1.3. Techniques for External Reflectance Spectroscopy.- 3.1.4. Internal Reflection Spectroscopy.- 3.2. Fourier Transform Infrared Spectroscopy (FTIRS).- 3.2.1. Principle of FTIR Spectrometers.- 3.2.2. Use for External Reflection Measurements.- 3.2.3. Use for Internal Reflection.- 3.3. Design of the Spectroelectrochemical Cell.- 3.3.1. Electrochemical Cells for External Reflection.- 3.3.2. Electrochemical Cells for Internal Reflection.- 3.4. Discussion of the Techniques.- 4. Applications to Selected Examples.- 4.1. General Survey.- 4.2. Adsorption of Hydrogen on Platinum in Acid Media.- 4.2.1. Why This Example?.- 4.2.2. Experimental Conditions and Data Acquisition.- 4.2.3. Interpretation of the Results.- 4.3. Adsorption of Carbon Monoxide on Noble Metals in Aqueous Media.- 4.3.1. Choice of This Example.- 4.3.2. Adsorption of CO on Platinum Electrodes.- 4.3.3. Adsorption of CO on Palladium.- 4.3.4. Infrared Bands of Adsorbed CO.- 4.4. Adsorbed Intermediates in Electrocatalysis.- 4.4.1. Chemisorption of Methanol at a Platinum Electrode.- 4.4.2. Chemisorption of Formic Acid at Platinum, Rhodium, and Gold Electrodes.- 4.4.3. Chemisorption of Ethanol at a Platinum Electrode.- 4.5. Investigations in Nonaqueous Solvents and Detection of the Intermediates Formed in the Vicinity of the Electrode Surface.- 4.5.1. Choice of Examples.- 4.5.2. Spectra of Adsorbed Species in Nonaqueous Media.- 4.5.3. Observation of Anion and Cation Radicals.- 5. Conclusions.- References.- 6. Surface-Enhanced Raman Scattering.- 1. Overview.- 1.1. Introduction.- 1.2. Light Scattering by Molecules.- 1.3. Characteristics of Surface Raman Scattering.- 1.4. The SERS Experiment.- 1.5. Active Sites and the Quenching of SERS.- 1.6. Metal-Molecule Complex.- 1.7. Theoretical Considerations.- 2. Experimental Methods.- 2.1. Introduction.- 2.2. Intensity of Detected Scattered Light.- 2.3. Laser Radiation Sources.- 2.4. Optical Setup and Depolarization Ratio Measurements.- 2.5. Electrochemical Cell, Instrumentation, and Pretreatment.- 2.6. The Monochromator and Detection System.- 3. Theory of the Electromagnetic Enhancement in SERS.- 3.1. The Electromagnetic Enhancement for Spherical Particles.- 3.1.1. Electrostatic Boundary Value Problem for a Metal Sphere.- 3.1.2. Enhancement Factors for a Spherical Geometry.- 3.2. The Electromagnetic Enhancement for a Prolate Metal Spheroid.- 3.2.1. Electrostatic Boundary Problem for a Prolate Metal Spheroid.- 3.2.2. Enhancement Factors for Prolate Spheroidal Geometry.- 3.3. Electrodynamic Effects.- 4. The Chemical Enhancement in SERS.- 4.1. Normal Raman Scattering.- 4.2. Resonance Raman Scattering.- 4.3. Herzberg-Teller Corrections.- 4.4. Surface-Enhanced Raman Spectroscopy: A Charge Transfer Theory.- 5. Overall Enhancement Equations for Surface Raman Scattering.- 5.1. Effect of Concentration in a Pure EM Surface Effect.- 5.2. Overall Enhancement Equation for SERS.- 5.3. Enhanced Scattering in a Surface-Enhanced Resonance Raman Process.- 6. Symmetry Considerations for SERS.- 6.1. Vibrational Selection Rules for SERS.- 6.2. Surface Selection Rules in SERS.- 7. Effects of Electrode Potential in SERS.- 7.1. Effect of Electrode Potential on SERS Intensities.- 7.1.1. Charge Transfer Resonance Dependence on Potential and Excitation Frequency.- 7.1.2. Electric Field Effects.- 7.2. SERS Intensities as a Function of Potential in the Presence of an Electrode Reaction.- 8. Application of SERS to Chemical Systems.- 8.1. Neutral Nitrogen-Containing Molecules on Ag and Cu Electrodes.- 8.2. Anions and the Effect of Supporting Electrolyte at Ag Electrodes.- 8.3. Cationic Species at Ag Electrodes.- 8.4. Hydrocarbons at Ag Films and Au Electrodes.- 8.5. SERS under Nonstandard Conditions and in Nonaqueous Media.- References.- 7. ESR Spectroscopy of Electrode Processes.- 1. Introduction.- 1.1. External Generation Methods.- 1.2. Internal Generation Methods.- 2. Theory.- 2.1. Introductory Remarks.- 2.2. The g-Value.- 2.3. Hyperfine Splitting.- 2.4. Linewidths.- 2.5. The ESR Spectrometer.- 3. Practice.- 3.1. The Allendoerfer Cell.- 3.2. The Compton-Coles Cell.- 3.3. The Compton-Waller Cell.- 3.4 Some Practical Hints.- 4. Applications.- 4.1. Radical Identification.- 4.2. Spin Trapping.- 4.3. The Kinetics and Mechanisms of Electrode Reactions.- 4.4. Dynamic Processes and ESR Lineshapes.- 4.5. Adsorbed Radicals.- References.- 8. Mossbauer Spectroscopy.- 1. Introduction.- 2. Theoretical Aspects.- 2.1. Recoil Energy, Resonance, and Doppler Effect.- 2.2. Phonons, Mossbauer Effect, and Recoilless Fraction.- 2.3. Electric Hyperfine Interactions.- 2.3.1. Isomer Shift.- 2.3.2. Quadrupole Splitting.- 2.4. Magnetic Hyperfine Interaction.- 3. Experimental Aspects.- 3.1. Instrumentation and Modes of Operation.- 3.2. Sources, Data Acquisition, and Data Analysis.- 3.3. In Situ Mossbauer Spectroscopy.- 3.4. Quasi In Situ Mossbauer Spectroscopy.- 3.4.1. Quasi In Situ Conversion Electron Mossbauer Spectroscopy.- 3.4.2. Low-Temperature Quenching.- 3.5. Limitations of the Technique.- 4. Model Systems.- 4.1. Electrochemical Properties of Iron and Its Oxides.- 4.1.1. The Iron Oxyhydroxide System.- 4.1.2. The Passive Film of Iron.- 4.2. Mixed Ni-Fe Oxyhydroxides as Electrocatalysts for Oxygen Evolution.- 4.3. Prussian Blue.- 4.4. Transition Metal Macrocycles as Catalysts for the Electrochemical Reduction of Dioxygen.- 4.5. Tin.- 4.6. In Situ Emission Mossbauer.- References.

Structural determination of a highly stable metal-organic framework with possible application to interim radioactive waste scavenging: Hf-UiO-66

Abstract: High-resolution synchrotron radiation x-ray powder diffraction (HR-XRPD) combined with Hf $L$3-edge extended x-ray absorption fine structure allowed us to determine the structure of a Hf-UiO-66 metal-organic framework (MOF) showing that it is isoreticular to Zr-UiO-66 MOF [Cavka et al., J. Am. Chem. Soc. 130, 13850 (2008).]. Thermal gravimetric measurements (coupled with mass spectroscopy) and temperature-dependent synchrotron radiation XRPD proved the high thermal stability of the Hf-UiO-66 MOF. The Langmuir surface area (849 m${}^{2}/$g) combined with the high stability of the UiO-66 framework and with the high neutron absorption cross section of Hf suggest that among all microporous crystalline materials the Hf-UiO-66 MOF possesses the physical and chemical requirements for the interim storage of radioactive waste in a much safer way than is currently available. The first results proving the synthesis of a MOF material with UiO-66 topology realized by a B-containing linker are also reported, allowing a further improvement of the neutron shielding power of this class of materials.

Cluster Coordination and Photoluminescence Properties of alpha-Ag2WO4 Microcrystals

Abstract: In this paper, we report our initial research to obtain hexagonal rod-like elongated silver tungstate (α-Ag2WO4) microcrystals by different methods [sonochemistry (SC), coprecipitation (CP), and conventional hydrothermal (CH)] and to study their cluster coordination and optical properties. These microcrystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier transform infrared (FT-IR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The shape and average size of these α-Ag2WO4 microcrystals were observed by field-emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were investigated by ultraviolet–visible (UV–vis) spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data confirmed that α-Ag2WO4 microcrystals have an orthorhombic structure. FT-IR spectra exhibited four IR-active modes in a range from 250 to 1000 cm–1. XANES...

Self-metalation of 2H-tetraphenylporphyrin on Cu(111): An x-ray spectroscopy study

Abstract: The bonding and the temperature-driven metalation of 2H-tetraphenylporphyrin (2H-TPP) on the Cu(111) surface under ultrahigh vacuum conditions were investigated by a combination of x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy with density functional theory calculations. Thin films were prepared by organic molecular beam epitaxy and subsequent annealing. Our systematic study provides an understanding of the changes of the spectroscopic signature during adsorption and metalation. Specifically, we achieved a detailed peak assignment of the 2H-TPP multilayer data of the C1s and the N1s region. After annealing to 420 K both XPS and NEXAFS show the signatures of a metalloporphyrin, which indicates self-metalation at the porphyrin-substrate interface, resulting in Cu-TPP. Furthermore, for 2H-TPP monolayer samples we show how the strong influence of the copper surface is reflected in the spectroscopic signatures. Adsorption results in a strongly deformed macrocycle and a quenching of the first NEXAFS resonance in the nitrogen edge suggesting electron transfer into the LUMO. For Cu-TPP the spectroscopic data indicate a reduced interaction of first-layer molecules with the substrate as demonstrated by the relaxed macrocycle geometry.

X-ray-induced photo-chemistry and X-ray absorption spectroscopy of biological samples.

Abstract: As synchrotron light sources and optics deliver greater photon flux on samples, X-ray-induced photo-chemistry is increasingly encountered in X-ray absorption spectroscopy (XAS) experiments. The resulting problems are particularly pronounced for biological XAS experiments. This is because biological samples are very often quite dilute and therefore require signal averaging to achieve adequate signal-to-noise ratios, with correspondingly greater exposures to the X-ray beam. This paper reviews the origins of photo-reduction and photo-oxidation, the impact that they can have on active site structure, and the methods that can be used to provide relief from X-ray-induced photo-chemical artifacts.

Highly Dispersed TiO6 Units in a Layered Double Hydroxide for Water Splitting

Abstract: A family of photocatalysts for water splitting into hydrogen was prepared by distributing TiO6 units in an MTi-layered double hydroxide matrix (M=Ni, Zn, Mg) that displays largely enhanced photocatalytic activity with an H2-production rate of 31.4 mu mol?h-1 as well as excellent recyclable performance. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping and XPS measurement reveal that a high dispersion of TiO6 octahedra in the layered doubled hydroxide (LDH) matrix was obtained by the formation of an M2+-O-Ti network, rather different from the aggregation state of TiO6 in the inorganic layered material K2Ti4O9. Both transient absorption and photoluminescence spectra demonstrate that the electronhole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which is attributed to the abundant surface defects that serve as trapping sites for photogenerated electrons verified by positron annihilation and extended X-ray absorption fine structure (EXAFS) techniques. In addition, a theoretical study on the basis of DFT calculations demonstrates that the electronic structure of the TiO6 units was modified by the adjacent MO6 octahedron by means of covalent interactions, with a much decreased bandgap of 2.1 eV, which accounts for its superior water-splitting behavior. Therefore, the dispersion strategy for TiO6 units within a 2D inorganic matrix can be extended to fabricate other oxide or hydroxide catalysts with greatly enhanced performance in photocatalysis and energy conversion.

Intraparticle reduction of arsenite (As(III)) by nanoscale zerovalent iron (nZVI) investigated with In Situ X-ray absorption spectroscopy.

Abstract: While a high efficiency of contaminant removal by nanoscale zerovalent iron (nZVI) has often been reported for several contaminants of great concern, including aqueous arsenic species, the transformations and translocation of contaminants at and within the nanoparticles are not clearly understood. By analysis using in situ time-dependent X-ray absorption spectroscopy (XAS) of the arsenic core level for nZVI in anoxic As(III) solutions, we have observed that As(III) species underwent two stages of transformation upon adsorption at the nZVI surface. The first stage corresponds to breaking of As-O bonds at the particle surface, and the second stage involves further reduction and diffusion of arsenic across the thin oxide layer enclosing the nanoparticles, which results in arsenic forming an intermetallic phase with the Fe(0) core. Extended X-ray absorption fine-structure (EXAFS) data from experiments conducted at different iron/arsenic ratios indicate that the reduced arsenic species tend to be enriched at the surface of the Fe(0) core region and had limited mobility into the interior of the metal core within the experimental time frame (up to 22 h). Therefore, there was an accumulation of partially reduced arsenic at the Fe(0)/oxide interface when a relatively large arsenic content was present in the solid phase. These results illuminate the role of intraparticle diffusion and reduction in affecting the chemical state and spatial distribution of arsenic in nZVI materials.

Dopant Location, Local Structure, and Electronic Properties of Au24Pt(SR)18 Nanoclusters

Abstract: We report element-specific X-ray spectroscopy results on the structure and bonding of Au24Pt, a thiolate-protected bimetallic nanocluster. Platinum L3-edge extended X-ray absorption fine-structure (EXAFS) data, in association with X-ray photoelectron spectroscopy (XPS) compositional analysis, was used to identify the location of the Pt dopant to be in the center of the icosahedron Au13 core. Comparison of Au24Pt with the structure of Au25 by gold L3-edge EXAFS clearly shows contraction of both metal–thiolate and metal–metal bond distances, caused by Pt doping. The doping effect on the electronic properties of Au24Pt was further evaluated by high-resolution Au 4f core-level XPS and ab initio calculations, which elucidate the importance of bimetallic (Pt–Au) bonding and bond contraction effects on the properties of Pt-doped thiolate-gold nanoclusters.

Performance and status of beamline BL8 at SLRI for X-ray absorption spectroscopy.

Abstract: Beamline BL8 of the Synchrotron Light Research Institute (Thailand) is routinely operated for X-ray absorption spectroscopy (XAS) in an intermediate photon energy range (1.25–10 keV). The photon energy is scanned by using a double-crystal monochromator, the crystal pair of which can be interchanged among KTP(011), InSb(111), Si(111) and Ge(220). The experimental set-up conveniently facilitates XAS measurements in transmission and fluorescence-yield modes at several K-edges of elements ranging from magnesium to zinc. Instrumentation and specification of the beamline and the XAS station are described, together with the determination of the available photon flux [0.1–6 × 1010 photon s−1 (100 mA)−1], energy resolution (1–5 × 10−4) and stability of photon energy calibration (0.07 eV), representing the beamline performance. Data quality and accuracy of XANES and EXAFS measured at BL8 are compared with those of other well established beamlines. A noted distinction of BL8 is its relatively high sensitivity for studying phosphorous, sulfur and chlorine in diluted systems and its maximum beam size of 14 mm (width) × 1 mm (height), which is suitable for bulk characterization.

Molecular-Scale Structure of Uranium(VI) Immobilized with Goethite and Phosphate

Abstract: The molecular-scale immobilization mechanisms of uranium uptake in the presence of phosphate and goethite were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Wet chemistry data from U(VI)-equilibrated goethite suspensions at pH 4–7 in the presence of ∼100 μM total phosphate indicated changes in U(VI) uptake mechanisms from adsorption to precipitation with increasing total uranium concentrations and with increasing pH. EXAFS analysis revealed that the precipitated U(VI) had a structure consistent with the meta-autunite group of solids. The adsorbed U(VI), in the absence of phosphate at pH 4–7, formed bidentate edge-sharing, ≡Fe(OH)2UO2, and bidentate corner-sharing, (≡FeOH)2UO2, surface complexes with respective U–Fe coordination distances of ∼3.45 and ∼4.3 A. In the presence of phosphate and goethite, the relative amounts of precipitated and adsorbed U(VI) were quantified using linear combinations of the EXAFS spectra of precipitated U(VI) and phosphate-free adsorbed U(VI). A U...

In situ investigations of a Li-rich Mn–Ni layered oxide for Li-ion batteries

Abstract: A Li-rich layered oxide with the formula Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 was successfully synthesised and characterised using several in situ characterisation techniques. The electronic state and structural evolution of the material upon cycling were investigated using in situ XRD, EXAFS and XANES measurements. XANES and SQUID magnetic measurements showed that the initial material contains a certain amount of Mn3+ in a low spin configuration (average Mn oxidation state: +3.75). In situ measurements showed that the first part of the charge (up to 4.4 V vs. Li+/Li) corresponds to oxidation of the Mn3+ fraction, and that the oxidation of nickel occurs only later, on the main charge plateau at 4.5 V. Electrochemical and structural results tend to show that the main first-charge plateau is a two-phase process where a new phase is created. This new phase is structurally very close to the starting one, and could be an oxygen-deficient spinel with a = 8.25 A. This process is non-reversible, and further cycling occurs in the new phase formed in situ.

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy.

Abstract: Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono...

Atomic structure relaxation in nanocrystalline NiO studied by EXAFS spectroscopy: Role of nickel vacancies

Abstract: Nanocrystalline NiO samples have been studied using the Ni K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy and recently developed modeling technique, combining classical molecular dynamics with ab initio multiple-scattering EXAFS calculations (MD-EXAFS). Conventional analysis of the EXAFS signals from the first two coordination shells of nickel revealed that (i) the second shell average distance R(Ni–Ni2) expands in nanocrystalline NiO compared to microcrystalline NiO, in agreement with overall unit cell volume expansion observed by x-ray diffraction; (ii) on the contrary, the first shell average distance R(Ni–O1) in nanocrystalline NiO shrinks compared to microcrystalline NiO; (iii) the thermal contribution into the meansquare relative displacement σ 2 is close in both microcrystalline and nanocrystalline NiO and can be described by the Debye model; (iv) the static disorder is additionally present in nanocrystalline NiO in both the first Ni–O1 and second Ni–Ni2 shells due to nanocrystal structure relaxation. Within the MD-EXAFS method, the force-field potential models have been developed for nanosized NiO using as a criterion the agreement between the experimental and theoretical EXAFS spectra. The best solutions have been obtained for the 3D cubic-shaped nanoparticle models with nonzero Ni vacancy concentration Cvac: Cvac ≈ 0.4–1.2% for NiO nanoparticles having the cube size of L ≈ 3.6–4.2 nm and Cvac ≈ 1.6–2.0% for NiO thin film composed of cubic nanograins with a size of L ≈ 1.3–2.1 nm. Thus our results show that the Ni vacancies in nanosized NiO play important role in its atomic structure relaxation along with the size reduction effect.

X-ray absorption spectroscopy of biological samples. A tutorial

Abstract: X-ray absorption spectroscopy (XAS) is an element specific spectroscopy sensitive to the local chemical and structural order of the absorber element. XAS is nowadays increasingly used for the speciation analysis of chemical elements owing to the development of new synchrotron radiation facilities worldwide. XAS can be divided into X-ray absorption near edge structure (XANES), which provides information primarily about the geometry and oxidation state, and extended X-ray absorption fine structure (EXAFS), which provides information about metal site ligation. The main advantages of the XAS method are its subatomic (angstrom) resolution, the ability to analyze almost any type of samples including amorphous (non-crystalline) materials, the possibility to analyze such materials in situ requiring minor or no sample preparation. The main limitations of XAS are its sensitivity in the mM (or μg g−1) range, the difficulty to deconvolute the bulk data when the sample is composed of a mixture of structures of the absorber element, and the limited chemical selectivity of ligands to within one row of the periodic table. This tutorial will discuss the strengths and limitations of XAS and compare them to those of alternative or complementary methods such as X-ray diffraction and X-ray photoelectron spectroscopy. The tutorial will also present and discuss the specific needs in terms of sample preparation and preservation all along the process of storage and analysis, and discuss the importance of the use of cryogenic methods when XAS is applied to biological samples. Applications in life sciences are reviewed, not exhaustively, with a special emphasis on some characteristic examples. The article ends with some perspectives on future trends of XAS: micro- and nano-XAS, time-resolved XAS, and high energy resolution XAS.

The irreversible formation of palladium carbide during hydrogenation of 1-pentyne over silica-supported palladium nanoparticles: in situ Pd K and L3 edge XAS.

Abstract: The catalytically active phase of silica-supported palladium catalysts in the selective and non-selective hydrogenation of 1-pentyne was determined using in situ X-ray absorption spectroscopy at the Pd K and L3 edges. Upon exposure to alkyne, a palladium carbide-like phase rapidly forms, which prevents hydrogen to diffuse into the bulk of the nano-sized particles. Both selective and non-selective hydrogenation occur over carbided particles. The palladium carbide-like phase is stable under reaction conditions and only partially decomposes under high hydrogen partial pressure. Non-selective hydrogenation to pentane is not indicative of hydride formation. The palladium carbide phase was detected in the EXAFS analysis and the K edge XANES showed representative features.

Structural characterization of electrodeposited copper hexacyanoferrate films by using a spectroscopic multi-technique approach.

Abstract: A deep structural investigation predominantly by X-ray spectroscopic techniques is conducted on films of copper hexacyanoferrate (CuHCF) deposited under different conditions, aimed at establishing structure–properties relationships. We show that the potentiodynamic electrosynthesis of CuHCF on carbon-based surfaces produces a highly disordered material, with a variable amount of Prussian Blue (PB). The subsequent Cu2+ intercalation induces the partial conversion of PB into CuHCF, which explains the improved electrocatalytic properties after the intercalation process. Both Cu and Fe K-edge data have been recorded. For the sample with the lower amount of PB, we could perform a multiple edge data analysis to determine the local atomic environment around both metal centres using the same set of structural parameters. The presence of high multiplicity Cu–N–C–Fe linear chains has allowed us to determine accurately the local environment of Fe while fitting the Cu K-edge data only. Using this approach we have retrieved structural information around Fe for those samples in which the concomitant presence of PB would have made impossible the analysis of the Fe K-edge. The Fe–C, C–N and Cu–N bond distances have been found in agreement with those of the bulk structures, but higher values of [Fe(CN)6] vacancies for the building blocks have been evidenced, reaching a value of ∼45% in one sample. XANES, Raman and SEM data agree with the model proposed for each studied electrode.

Quantitative Analysis of Modulated Excitation X-ray Absorption Spectra: Enhanced Precision of EXAFS Fitting

Abstract: The sensitivity of X-ray absorption spectroscopy (XAS) can be increased by using the modulated excitation approach, where the sample is excited with a periodic external stimulation (e.g., changes in temperature, gas/liquid concentration, etc.) and the corresponding spectra are filtered with the frequency of the excitation. The resulting demodulated spectra contain similar spectral features as difference spectra but to much higher k values because of the isolation of the structural change that occurs with the same frequency as that of the excitation. Furthermore, multiple intermediate species that occur with a phase delay can uniquely be differentiated. We present a robust approach for the quantitative analysis of demodulated extended X-ray absorption fine structure (EXAFS) spectra. Simulations show that the proposed fitting approach resolves small changes in the sample’s structure with greatly enhanced precision. Experimentally, this is demonstrated by the formation of ruthenium oxide species upon partial...

Aqueous Uranium(VI) Complexes with Acetic and Succinic Acid: Speciation and Structure Revisited

Abstract: We employed density functional theory (DFT) calculations, and ultraviolet–visible (UV–vis), extended X-ray absorption fine-structure (EXAFS), and attenuated total reflection Fourier-transform infrared (IR) spectroscopy analyzed with iterative transformation factor analysis (ITFA) to determine the structures and the pH-speciation of aqueous acetate (ac) and succinate (suc) U(VI) complexes. In the acetate system, all spectroscopies confirm the thermodynamically predicted pH-speciation by Ahrland (1951), with the hydrated uranyl ion and a 1:1, a 1:2 and a 1:3 U(VI)-ac complex. In the succinate system, we identified a new 1:3 U(VI)-suc complex, in addition to the previously known 1:1 and 1:2 U(VI)-suc complexes, and determined the pH-speciation for all complexes. The IR spectra show absorption bands of the antisymmetric stretching mode of the uranyl mojety (υ3(UO2)) at 949, 939, 924 cm–1 and at 950, 938, 925 cm–1 for the 1:1, 1:2 and 1:3 U(VI)-ac and U(VI)-suc complexes, respectively. IR absorption bands at 1...