Narrow Results

High resolution XANES measurements have been made at the CuK-absorption edge on two La_2-xSr_xCuO_4-y single crystals; one under-doped (x = 0.13) and the other over-doped (x = 0.19). These measurements have been made in E//ab and E//c directions at several temperatures to compare the doping, orientation and temperature dependence in their behaviour. Broadly speaking, their XANES spectra resemble the typical one from a crystal with square-planar geometry and the various fine structure features assigned to the possible transitions they arise due to, particularly in terms of the fact that the 4p band in Cu splits into 4p(π) and 4p(σ) components in these square-planar systems. However, the temperature dependence of the various features in the spectra from the two crystals appear to be opposite in nature and exhibit some characteristic temperatures at which the trend seems to reverse. All the experimental results and their implication are discussed and a possibility of phase transition of these systems at lower temperatures is hinted at.

A crystal-chemical study of trioctahedral micas previously characterized by single-crystal XRD has been performed by XANES spectroscopy at the Si and Al K edges. XANES, being a local structural probe, can investigate distortion and modification of the tetrahedral sheet with increasing Fe for Mg substitution in the octahedral sheet. Comparison of XANES spectra allows determining the size of the tetrahedral site occupied by either Si or Al. The Si-O distance remains essentially unchanged whereas the Al-O distance appears to increase. The behavior may be interpreted as a tilt of the tetrahedra, initially rotated to match the ideal mica geometry, with increasing Fe substitution in the octahedral sheet.

Valence state of the doped Ru at the Mn sites in Pr_0.5Sr_0.5MnO₃ has been studied using Ru K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In comparison with XANES of reference ruthenates, it is found that the Mn-site doped Ru is dominantly in tetravalent state. This result suggests that the change of charge carrier density by Ru doping is not sufficient to understand drastic enhancement of ferromagnetism observed in the transport properties of Ru-doped Pr_0.5Sr_0.5MnO₃. The prime role of Ru in Mn³⁺–O–Mn⁴⁺ networks is discussed in terms of the valency effect and the magnetic interactions between doped Ru and Mn ions.

X-ray absorption measurements have been made at the CuL₃ and O K-edge on (Hg_0.5Bi_0.5)Sr₂(Ca_1-xPr_x)Cu₂O_7-δ (x=0.35,0.50) and (Hg_0.5Bi_0.5)Sr₂(Ca_0.65Nd_0.35)-Cu₂O_7-δ superconducting systems. Our results clearly indicate that hole density decreases with increase in Pr content. The values of T_c for Pr and Nd (x=0.35) systems are the same but the itinerant hole density is found to be larger in the latter case. In view of the fact that the density of holes depends more upon diamagnetic fraction than the T_c in these superconducting systems, we can conclude that superconducting properties are different for these two cases.

The 3d transition-metal diborides TiB₂, VB₂ and CrB₂ have been characterized by X-ray absorption near-edge-structure (XANES) spectroscopy at the metal K edges. XANES spectra at the Ti, V and CrK-edges of TiB₂, VB₂ and CrB₂, respectively, show similar spectral features up to an energy of about 70 eV above threshold. These features can be correlated to their specific atomic arrangement and electronic structure via multiple-scattering calculations as a function of the cluster size. Actually the second pre-edge feature is assigned to a transition from the metal 1s core state to the central metal 4p state hybridized with 3d band state of the higher-shell metal atoms. The first weak feature on the rising part of the edge is due to higher-shells multiple scattering contributions, suggesting that the transition features at the metal K-edge XANES of 3d transition-metal diborides are strongly affected by medium or long-range order contributions.

XANES measurements at the Fe-K edge on natural South African sapphire single crystal (corundum) and an irradiated sample with fluence 1 × 10¹²Ni⁶⁺ions/cm² are reported. Some decrease in intensity of pre-edge features (1s → 3d) and increase in intensity of 1s → 4p transition in Fe is observed with Ni fluence. Structural changes and modification on surface of irradiated sapphire with Ni⁶⁺ ion have been observed by the atomic force microscopy technique and discussed in the term of defects.

Active-ilmenite powder derived from natural ilmenite sand was prepared by the ball milling process with acid-solution and Deionized (DI) water. Morphology and particle size of active-ilmenite product in acid/DI-assisted ball milling process were monitored by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). Surface atomic component and chemical bonding were investigated by X-ray photoelectron spectroscopy (XPS). Meanwhile, bulk chemical oxidation and fine structure of active-ilmenite were studied by X-ray absorption near edge structure (XANES) and extended X-ray absorption ne-structure spectroscopy (EXAFS) to confirm the oxidation state and local active species structure at surface. Active-ilmenite by acid-assisted ball milling process is a distinctive method for the preparation of active-ilmenite product with high active surface. Moreover, the distortion of TiO₆ and FeO₆ octahedral cluster on the sample surface was detected in all milled samples with acid-assisted ball milling process. The presence of Fe${}^{2+}$, Fe${}^{3+}$ ions and miniature sulfate was also detected on the sample surface by milled product with acid-assisted method.

In this report, micro-emulsion method was used to obtain La, Mn-doped Ba₃Co₂${\text{Fe}}_{24}$${\text{O}}_{41}$ single-phase Z-type hexa-nanoferrites by maintaining annealing temperature at 1200^∘C. X-ray diffraction (XRD) analysis shows lattice parameters, crystallite size, and volume are significantly enhanced by introducing La and Mn ions instead of Ba and Fe, respectively. Moreover, La and Mn ion doping improved the preferred orientation of atoms through the basal plane of Z-type hexaferrite. Field emission scanning electron microscopy (FESEM) study reveals that the surface morphology of La- and Mn-doped Ba₃Co₂${\text{Fe}}_{24}$${\text{O}}_{41}$, Z-type ferrites show ultrafine and small particle size. The analysis of X-Ray absorption near edge spectroscopy (XANES) suggests that transition in ${\text{La}}^{+3}$ has occurred from 3d core level to empty electronic state above the Fermi level. Furthermore, it is observed that in the Z-type ferrites Co exist as divalent ion and Fe as trivalent ion by analyzing XANES. Vibrating samples magnetometer (VSM) indicated that the samples are ferromagnetic as they have narrow hysteresis loops. The decrease in coercivity is due to paramagnetic dopant element Manganese (Mn).

The electrochemical insertion–deinsertion of lithium into CuO electrode was examined by Cu K-edge X-ray absorption near edge structure (XANES) during the first electrochemical cycle. The XANES spectra in the Li_xCuO (x: lithium content) system reveal that the initial insertion of lithium leads to the reduction of the Cu²⁺ in the pristine CuO to form the reduced nanosized Cu metal. In the successive deinsertion of lithium, the reduced Cu particles changed partially to the phase of Cu₂O.

Transformation of organofluorine compounds by oxidation pathways is rare in chemistry and biology since C-F bonds formed by the most electronegative element should react with electron-deficient oxidizing species. Recently, we have shown that $\mu$-nitrido diiron phthalocyanine complexes efficiently catalyze oxidative defluorination of poly- and perfluoroaromatics by H₂O₂. Herein, we studied their more biologically relevant porphyrin counterpart, $\mu$-nitrido diiron(III,IV) tetraphenylporphyrin complex (TPP)Fe${}^{\text{III}}(\mu$N)Fe${}^{\text{IV}}$(TPP) in stoichiometric and catalytic reactions with a series of fluorinated aromatic compounds under oxidation conditions. The addition of hexafluorobenzene to (TPP)Fe${}^{\text{III}}(\mu$N)Fe${}^{\text{IV}}$(TPP) in the presence of $t$-butylhydroperoxide led to the formation of high-valent $\mu$-nitrido diiron(IV,IV) porphyrin cation radical complex [TPP)Fe${}^{\text{IV}}(\mu$ -N)Fe${}^{\text{IV}}$(TPP${}^{+•}$]F₂. This complex was isolated and its structural and electronic properties were investigated by spectroscopic methods (EXAFS, XANES, EPR, UV-vis). Replacement of ${}^t$BuOOH with H₂O₂ oxidant resulted in the catalytic defluorination of selected heavily fluorinated aromatic compounds with high conversions (25–84%), TON (1768–3535), and defluorination degrees (71–84%). The scope of oxidative defluorination with (TPP)Fe${}^{\text{III}}(\mu$N)Fe${}^{\text{IV}}$(TPP) was extended to perfluorinated olefins exemplified by perfluoroallylbenzene. The perfluorinated double bond was more reactive compared with perfluorinated aromatic moiety providing C₆F₅CF₂COOH and C₆F₅COOH products. The properties of $\mu$-nitrido diiron tetraphenylporphyrin in homogeneous and heterogeneous catalytic defluorination were compared with those of its phthalocyanine counterpart.

The chemical reduction of nitrate or nitrite species by zero-valent iron nanoparticle (ZVIN) in aqueous solution and related reaction kinetics or mechanisms using fine structure characterization were investigated. Experimentally, ZVIN of this study was prepared by borohydride reduction method at room temperature. The morphology of as-synthesized ZVIN shows that the nearly ball and ultrafine particles ranged of 20–50 nm were observed with FE-SEM analysis. The kinetic model of nitrites or nitrates reductive reaction by ZVIN is proposed as a pseudo-first-order kinetic equation. The nitrite and nitrate removal efficiencies using ZVIN were found 65–83% and 51–68%, respectively, based on three different initial concentrations. By using XRD patterns, the quantitative relationship between nitrite and Fe(III) or Fe(II) becomes similar to the one between nitrate and Fe(III) in the ZVIN study. The possible reason is linked with a faster nitrite reduction by ZVIN. In fact, the occurrence of the relative faster nitrite reductive reaction suggested that the passivation of the ZVIN have a significant contribution to iron corrosion. The XANES spectra show that the nitrites or nitrates reduce to N₂ while oxidizing the ZVIN to Fe₂O₃ or Fe₃O₄ electrochemically. It is also very clear that decontamination of nitrate or nitrite species in groundwater via the in-situ remediation with a ZVIN permeable reactive barrier would be environmentally attractive.

Mn K-edge absorption measurements were carried out on α-Mn₂O₃ and Mn₃O₄ nanocrystals supported on a mesoporous silica, SBA-15. The X-ray absorption near edge structure (XANES) spectra demonstrate the existence of the oxidation states of Mn (2+ and 3+) in Mn₃O₄ and Mn (3+) in Mn₂O₃, those ions were present in different octahedral environments. Meanwhile, XANES data demonstrate that some Mn atoms that are bonding to the inner wall of the channels as isolated species, may exist as Mn⁴⁺ in Mn₂O₃/SBA-15. In addition, the structure, texture, and electronic properties of nanocomposites were also studied using various characterization techniques including X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). The formation of the hausmannite Mn₃O₄ and bixbyite Mn₂O₃ structures has been confirmed clearly by XRD. The prepared nanocomposites of MnO_x showed significant catalytic activity towards CO oxidation below 523 K.

Speciation of copper and zinc (1:1) in MCM-41 has been studied by X-ray absorption near edge structural (XANES) and X-ray absorption fine structural (EXAFS) spectroscopies in the present work. The least-square fitted XANES spectra show that Cu(II) and Zn(II) are the major copper and zinc species, respectively in the channels of MCM-41 during calcination at 573–1173 K. The EXAFS data indicate that copper in MCM-41 possesses a Cu–O bond distance of 1.97 Å with a coordination number (CN) of 3.4. During calcination at 573–1173 K, about 3.8–4.2 nearest oxygen atoms are bonded to the central copper atoms with Cu–O bond distances of 1.95–1.96 Å in MCM-41. A small amount of Cu–Zn alloy may be formed during calcination. Existence of Zn–O)–Si (3.05–3.11 Å) with CNs of 1.7–2.3 is also observed by EXAFS, suggesting an interaction between zinc and the framework SiO₂ of MCM-41 during calcination.

X-ray absorption spectroscopy (XAS) is an extremely valuable tool for the characterization of the electronic and geometric properties of nanoparticles. However, there are drawbacks when it comes, for example, to time-resolved in situ measurements of wet-chemical synthetic reactions or when X-ray absorption near edge structure (XANES) spectra are used for characterizing atoms that occupy different sites in a nanoparticle. In this paper, we report results of test experiments using resonant inelastic X-ray scattering (RIXS) for obtaining high-resolution spectra that allow in principle site- and/or valency-specific XANES measurements. For a detailed analysis of wet-chemical reactions, a microreactor system is used in which time resolution is obtained by spatial resolution, i.e., by measuring spectra at various points along the microreactor. This system provides significantly better resolution (in the order of milliseconds) compared to conventional techniques.

The following sections are included:

• XEOL Emission Spectra and Applications
  • Introduction
  • Applications Ranging from Analytical Chemistry to X-ray Detection
    • Rare Earths in Inorganic Host Matrices
    • Low Temperature Organic Shpol’skii Matrices and Glassy Systems
    • X-ray Detection Using Spontaneous or Stimulated Luminescence
  • Advances Stimulated by Synchrotron Radiation Sources
    • Optical Activity in Emission Spectra
    • Time Resolved XEOL Decays
    • Imaging and Spectromicroscopy

• Near Edge and Extended Structures in XEOL Excitation Spectra
  • Review of Published Literature
    • Positive and/or Negative Edge Jumps
    • Site-Selectivity and Implicit Restrictions
    • Luminescence of Porous Silicon
  • Theoretical Bases for the Analysis of Excitation Spectra
    • Energy Conversion and Thermalization
    • Energy Transfer Mechanisms
      • Photoconductivity
      • Radiative Transfer and Self-Absorption
      • Forster-Dexter Mechanisms, and Exciton Diffusion
      • Luminescence Time Decays and Diffusion Lengths
      • Surface Recombination Allowed by Diffusion
    • Luminescence Yield and Nonlinear Luminescence

• Conclusion and New Perspectives

• Acknowledgments

• References

The following sections are included:

• Introduction
  • X-ray Absorption Spectroscopy Using Synchrotron Radiation
  • Detection Methods
  • Comparison of Laboratory XPS and XANES

• Antiwear Films Generated from Zinc Dialkyldithiophosphates (ZDDPs)

• XANES Characterization of Model Compounds
  • Polyphosphate Chain Length Effect on P K and L Edge XANES
  • Cation Effect P K and L Edge XANES
  • O K Edge XANES Spectra of Model Compounds
  • S K Edge XANES of Model Compounds

• Characterization of Tribochemical Films Using XAS
  • EXAFS Characterization of Antiwear Films and Wear Particles Generated from ZDDPs
  • XANES Characterization of Antiwear Films Generated from ZDDPs

• ZDDP in Combination with Other Additives
  • ZDDP + Detergents and Dispersants
  • ZDDP + Molybdenum Dithiocarbamate (MoDTC)

• Polyphosphate Film Thickness Using P K Edge XANES Spectroscopy

• XANES Studies of Phosphate Coatings Generated from ZDDPs

• X-ray Spectromicroscopy
  • X-ray Spectromicroscopy of Alkyl and Aryl ZDDP Derived Antiwear Films
  • Neutral and Overbased Ca Sulphonate in Combination with and without ZDDP

• Conclusions and Future Work

• Acknowledgments

• References

The following sections are included:

• Introduction

• Unified MS XAFS and NEXAFS Calculations
  • Curved Wave Scattering Theory
  • Path Filters
  • Scattering Potentials
  • Self-energy and Mean Free Path
  • Thermal and Configurational Disorder

• Applications to XAFS and XANES
  • Information Content in XAFS
  • Example of MS XAFS Calculation
  • NEXAFS and Shape Resonances
  • Application to XANES

• Conclusions

• Acknowledgments

• References

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ZnO–Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (> 50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹+ species (∼ 60%). Reduced Cu/ZnO–Al₂O₃ catalysts show the presence of metallic Cu (upto 20 %) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase (∼ 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.

The electrochemical insertion–deinsertion of lithium into CuO electrode was examined by Cu K-edge X-ray absorption near edge structure (XANES) during the first electrochemical cycle. The XANES spectra in the Li_xCuO (x: lithium content) system reveal that the initial insertion of lithium leads to the reduction of the Cu²⁺ in the pristine CuO to form the reduced nanosized Cu metal. In the successive deinsertion of lithium, the reduced Cu particles changed partially to the phase of Cu₂O.

X-ray absorption fine structure (XAFS) spectroscopy has been widely used for decades in a wide range of scientific fields, including physics, chemistry, biology, materials, environmental sciences, and so on. In this chapter, we introduce the XAFS principles, including its basic theory, data analysis and experiment, from the view point of practical use. To show its strength as a local structure probe, applications of XAFS in various functional materials are introduced, covering nanoparticles and catalysts, magnetic semiconductors, thin film materials, complex compounds, in situ probing of the nucleation and growth processes of nanomaterials, as well as operando study of catalysts under working conditions.

In addition, we also briefly introduce some relatively new XAFS-related techniques, such as time-resolved and space-resolved XAFS techniques.