Showing papers on "Extended X-ray absorption fine structure published in 2013"

Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

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In situ X-ray absorption spectroscopy investigation of a bifunctional manganese oxide catalyst with high activity for electrochemical water oxidation and oxygen reduction.

Abstract: In situ X-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochemical systems, with the ability to elucidate the chemical nature of electrocatalysts under reaction conditions. In this study, we perform in situ XAS measurements on a bifunctional manganese oxide (MnOx) catalyst with high electrochemical activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), we find that exposure to an ORR-relevant potential of 0.7 V vs RHE produces a disordered Mn3II,III,IIIO4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs RHE, relevant to the OER, we observe an oxidation of approximately 80% of the catalytic thin film to form a mixed MnIII,IV oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged Mn3II,III,IIIO4. XAS...

Temperature dependent absorption cross-sections of O2–O2 collision pairs between 340 and 630 nm and at atmospherically relevant pressure

Abstract: The collisions between two oxygen molecules give rise to O4 absorption in the Earth atmosphere. O4 absorption is relevant to atmospheric transmission and Earth's radiation budget. O4 is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O4 absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203–295 K), and at 820 mbar pressure. We characterize the temperature dependent O4 line shape and provide high precision O4 absorption cross section reference spectra that are suitable for atmospheric O4 measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within <3%, the uncertainty of our measurements). The enthalpy of formation is determined to be ΔH250 = −0.12 ± 0.12 kJ mol−1, which is essentially zero, and supports previous assignments of O4 as collision induced absorption (CIA). At 203 K, van der Waals complexes (O2-dimer) contribute less than 0.14% to the O4 absorption in air. We conclude that O2-dimer is not observable in the Earth atmosphere, and as a consequence the atmospheric O4 distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10−3 from knowledge of the oxygen concentration profile.

Adsorption of Uranium(VI) to Manganese Oxides: X-ray Absorption Spectroscopy and Surface Complexation Modeling

Abstract: The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO2, surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO2 appears to ...

Catalysis and In Situ Studies of Rh1/Co3O4 Nanorods in Reduction of NO with H2

Abstract: Efficient use of precious metal atoms in heterogeneous catalysis is important in chemical transformation and environmental remediation. Co3O4 with singly dispersed Rh atoms, Rh1/Co3O4, was synthesized for reduction of nitric oxide with hydrogen. Studies using extended X-ray absorption fine structure (EXAFS) showed that the singly dispersed Rh atoms are bonded to surface oxygen atoms before catalysis. In situ studies using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), EXAFS, and X-ray Absorption Near Edge Structure (XANES) suggested that the surface of Rh1/Co3O4 with singly dispersed Rh atoms is restructured into a new geometry at 220 °C in the mixture of reactant gases (NO and H2). It forms RhCon nanoclusters singly dispersed in the surface layer of Co3O4. The restructured catalyst, RhCon/Co3O4 exhibits a much better catalytic performance in contrast to Rh1/Co3O4 without a restructuring. RhCon/Co3O4 is highly active for reduction of nitric oxide with hydrogen. Selectivity to the production o...

Atmospheric absorption of terahertz radiation and water vapor continuum effects

Abstract: The water vapor continuum absorption spectrum was investigated using Fourier Transform Spectroscopy. The transmission of broadband terahertz radiation from 0.300 to 1.500 THz was recorded for multiple path lengths and relative humidity levels. The absorption coefficient as a function of frequency was determined and compared with theoretical predictions and available water vapor absorption data. The prediction code is able to separately model the different parts of atmospheric absorption for a range of experimental conditions. A variety of conditions were accurately modeled using this code including both self and foreign gas broadening for low and high water vapor pressures for many different measurement techniques. The intensity and location of the observed absorption lines were also in good agreement with spectral databases. However, there was a discrepancy between the resonant line spectrum simulation and the observed absorption spectrum in the atmospheric transmission windows caused by the continuum absorption. A small discrepancy remained even after using the best available data from the literature to account for the continuum absorption. From the experimental and resonant line simulation spectra the air-broadening continuum parameter was calculated and compared with values available in the literature.

Low-dimensional systems investigated by x-ray absorption spectroscopy: a selection of 2D, 1D and 0D cases

Abstract: Over the last three decades low-dimensional systems have attracted increasing interest both from the fundamental and technological points of view due to their unique physical and chemical properties. X-ray absorption spectroscopy (XAS) is a powerful tool for the characterization of such kinds of systems, owing to its chemical selectivity and high sensitivity in interatomic distance determination. Moreover, XAS does not require long-range ordering, that is usually absent in low-dimensional systems. Finally, this technique can simultaneously provide information on electronic and local structural properties of the nanomaterials, significantly contributing to clarify the relation between their atomic structure and their peculiar physical properties. This review provides a general introduction to XAS, discussing the basic theory of the technique, the most used detection modes, the related experimental setups and some complementary relevant characterization techniques (diffraction anomalous fine structure, extended energy-loss fine structure, pair distribution function, x-ray emission spectroscopy, high-energy resolution fluorescence detected XAS and x-ray Raman scattering). Subsequently, a selection of significant applications of XAS to two-, one- and zero-dimensional systems will be presented. The selected low-dimensional systems include IV and III–V semiconductor films, quantum wells, quantum wires and quantum dots; carbon-based nanomaterials (epitaxial graphene and carbon nanotubes); metal oxide films, nanowires, nanorods and nanocrystals; metal nanoparticles. Finally, the future perspectives for the application of XAS to nanostructures are discussed.

Defining Crystalline/Amorphous Phases of Nanoparticles through X-ray Absorption Spectroscopy and X-ray Diffraction: The Case of Nickel Phosphide

Abstract: In this study we elucidate the structural distinctions between amorphous and crystalline Ni2P nanoparticles synthesized using tri-n-octylphosphine (TOP), through X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and inductively coupled plasma (ICP). We determine the differences in their chemical and atomic structure, which have not been previously reported, yet are essential for understanding their potential as nanocatalysts. These structural characteristics are related to the corresponding nanoparticle magnetic properties analyzed by performing magnetic measurements. XAS results reveal a significant P concentration in the amorphous nanoparticle sample – placing the stoichiometry close to Ni2P – despite XRD results that show only fcc Ni contributions. By comparing the long-range structural order from XRD to the short-range radial structure from EXAFS we conclude that both techniques are necessary to obtain a complete structural picture of amorphous and crystalline nanoparticle phases due to th...

Understanding the impact of aluminum oxide binder on Ni/HZSM-5 for phenol hydrodeoxygenation

Abstract: The properties of supported Ni particles on HZMS-5 and Al2O3-HZSM-5 were comparably investigated by diverse characteristic techniques. Ni/Al2O3-HZSM-5 had at least three times higher concentrations of accessible Ni atoms (average diameter Ni-0: 8.8 nm) compared to Ni/HZSM-5 (average diameter Ni-0 : 35 run), which are consistently evidenced by TEM and XRD as well as H-2 chemisorption and IR spectra of adsorbed CO. The Ni nanoparticles interacted strongly with the binder through the interaction between NiO and Al2O3, explored by the combined extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and H-2 temperature-programmed reduction (TPR) techniques. The Bronsted acid sites on two supports probed by IR of adsorbed pyridine were similar, but Lewis acid sites contributed by the gamma-Al2O3 were more abundant on Al2O3-HZSM-5. The acid sites of the two catalysts responded differently to metal incorporation and subsequent treatments, reflecting changes in Al environments illuminated by Al-27 MAS NMR. In situ IR spectra of adsorbed species demonstrates that Al2O3-HZSM-5 has higher adsorption capacity for phenol, cyclohexanone, and cyclohexanol due to stronger adsorption of these compounds on the gamma-Al2O3 binder. (c) 2012 Elsevier B.V. All rights reserved.

An experimental and theoretical investigation of the inversion of pd@pt core@shell dendrimer-encapsulated nanoparticles.

Abstract: Bimetallic PdPt dendrimer-encapsulated nanoparticles (DENs) having sizes of about 2 nm were synthesized by a homogeneous route that involved (1) formation of a Pd core, (2) deposition of a Cu shell onto the Pd core in the presence of H2 gas, and (3) galvanic exchange of Pt for the Cu shell. Under these conditions, a Pd@Pt core@shell DEN is anticipated, but detailed characterization by in-situ extended X-ray absorption fine structure (EXAFS) spectroscopy and other analytical methods indicate that the metals invert to yield a Pt-rich core with primarily Pd in the shell. The experimental findings correlate well with density functional theoretical (DFT) calculations. Theory suggests that the increased disorder associated with <∼2 nm diameter nanoparticles, along with the relatively large number of edge and corner sites, drives the structural rearrangement. This type of rearrangement is not observed on larger nanoparticles or in bulk metals.

Labile Cu(I) Catalyst/Spectator Cu(II) Species in Copper-Catalyzed C–C Coupling Reaction: Operando IR, in Situ XANES/EXAFS Evidence and Kinetic Investigations

Abstract: Insights toward the Cu-catalyzed C–C coupling reaction were investigated through operando IR and in situ X-ray absorption near-edge structure/extended X-ray absorption fine structure. It was found that the Cu(I) complex formed from the reaction of CuI with β-diketone nucleophile was liable under the cross-coupling conditions, which is usually considered as active catalytic species. This labile Cu(I) complex could rapidly disproportionate to the spectator Cu(II) and Cu(0) species under the reaction conditions, which was an off-cycle process. In this copper-catalyzed C–C coupling reaction, β-diketone might act both as the substrate and the ligand.

Investigating the molecule-substrate interaction of prototypic tetrapyrrole compounds: Adsorption and self-metalation of porphine on Cu(111)

Abstract: We report on the adsorption and self-metalation of a prototypic tetrapyrrole compound, the free-base porphine (2H-P), on the Cu(111) surface. Our multitechnique study combines scanning tunneling microscopy (STM) results with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) data whose interpretation is supported by density functional theory calculations. In the first layer in contact with the copper substrate the molecules adsorb coplanar with the surface as shown by angle-resolved NEXAFS measurements. The quenching of the first resonance in the magic angle spectra of both carbon and nitrogen regions indicates a substantial electron transfer from the substrate to the LUMO of the molecule. The stepwise annealing of a bilayer of 2H-P molecules sequentially transforms the XP and NEXAFS signatures of the nitrogen regions into those indicative of the coordinated nitrogen species of the metalated copper porphine (Cu-P), i.e., we observe a temperature-induced self-metalation of the system. Pre- and post-metalation species are clearly discriminable by STM, corroborating the spectroscopic results. Similar to the free-base porphine, the Cu-P adsorbs flat in the first layer without distortion of the macrocycle. Additionally, the electron transfer from the copper surface to the molecule is preserved upon metalation. This behavior contrasts the self-metalation of tetraphenylporphyrin (2H-TPP) on Cu(111), where both the molecular conformation and the interaction with the substrate are strongly affected by the metalation process.

Periodic behavior of lanthanide coordination within reverse micelles.

Abstract: Trends in lanthanide(III) (Ln(III)) coordination were investigated within nanoconfined solvation environments. Ln(III) ions were incorporated into the cores of reverse micelles (RMs) formed with malonamide amphiphiles in n-heptane by contact with aqueous phases containing nitrate and Ln(III); both insert into pre-organized RM units built up of DMDOHEMA (N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide) that are either relatively large and hydrated or small and dry, depending on whether the organic phase is acidic or neutral, respectively. Structural aspects of the Ln(III) complex formation and the RM morphology were obtained by use of XAS (X-ray absorption spectroscopy) and SAXS (small-angle X-ray scattering). The Ln(III) coordination environments were determined through use of L(3)-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure), which provide metrical insights into the chemistry across the period. Hydration numbers for the Eu species were measured using TRLIFS (time-resolved laser-induced fluorescence spectroscopy). The picture that emerges from a system-wide perspective of the Ln-O interatomic distances and number of coordinating oxygen atoms for the extracted complexes of Ln(III) in the first half of the series (i.e., Nd, Eu) is that they are different from those in the second half of the series (i.e., Tb, Yb): the number of coordinating oxygen atoms decrease from 9O for early lanthanides to 8O for the late ones--a trend that is consistent with the effect of the lanthanide contraction. The environment within the RM, altered by either the presence or absence of acid, also had a pronounced influence on the nitrate coordination mode; for example, the larger, more hydrated, acidic RM core favors monodentate coordination, whereas the small, dry, neutral core favors bidentate coordination to Ln(III). These findings show that the coordination chemistry of lanthanides within nanoconfined environments is neither equivalent to the solid nor bulk solution behaviors. Herein we address atomic- and mesoscale phenomena in the under-explored field of lanthanide coordination and periodic behavior within RMs, providing a consilience of fundamental insights into the chemistry of growing importance in technologies as diverse as nanosynthesis and separations science.

X-ray Absorption Spectroscopy of Ground and Excited Rhenium–Carbonyl–Diimine Complexes: Evidence for a Two-Center Electron Transfer

Abstract: Steady-state and picosecond time-resolved X-ray absorption spectroscopy is used to study the ground and lowest triplet states of [ReX(CO)3(bpy)]n+, X = Etpy (n = 1), Cl, or Br (n = 0). We demonstrate that the transient spectra at both the Re L3- and Br K-edges show the emergence of a pre-edge feature, absent in the ground-state spectrum, which is associated with the electron hole created in the highest occupied molecular orbital following photoexcitation. Importantly, these features have the same dynamics, confirming previous predictions that the low-lying excited states of these complexes involve a two-center charge transfer from both the Re and the ligand, X. We also demonstrate that the DFT optimized ground and excited structures allow us to reproduce the experimental XANES and EXAFS spectra. The ground-state structural refinement shows that the Br atom contributes very little to the latter, whereas the Re–C–O scattering paths are dominant due to the so-called focusing effect. For the excited-state spe...

Review of LiFePO4 Phase Transition Mechanisms and New Observations from X-ray Absorption Spectroscopy

Abstract: The high rate capability and reversibility of lithium iron phosphate battery cathodes is attributed to the highly reversible transition between its LiFePO4 and FePO4 phases. Conflicting models exist for the phase transition mechanism, such as the shrinking or expanding core models, surface reaction limited model, and the single-phase kinetic model. A literature review suggests that the subscribed theories depend upon the experimental methods and also the geometry and size of the LiFePO4 particles. We study the electronic structure and disorder during the two-phase transition of commercial oval-shaped LiFePO4 particles using in situ XAS (X-ray absorption spectroscopy) with the �μ XANES (X-ray absorption near edge structure) difference technique along with traditional EXAFS (extended X-ray absorption fine structure) to track the Debye-Waller (DW) factor. The �μ XANES magnitude, |�μ|, tracks changes in the electronic structure which does not follow a synchronous path with lithium content. The magnitude of the DW shows the degree of structural disorder reaches a maximum near the middle of the charge/discharge cycle. The combined |�μ| and EXAFS results suggests the LiFePO4/FePO4 transition occurs through a “collective mosaic” or an unrelaxed “sequential” single-phase kinetic model.

Optical band gap of NpO2 and PuO2 from optical absorbance of epitaxial films

Abstract: We report a solution based synthesis of epitaxial thin films of neptunium oxide and plutonium oxide. Actinides represent a challenge to first principle calculations due to features that arise from f orbital interactions. Conventional semi-local density functional theory predicts NpO2 and PuO2 to be metallic, when they are well known insulators. Improvements in theory are dependent on comparison with accurate measurements of material properties, which in turn demand high-quality samples. The high melting point of actinide oxides and their inherent radioactivity makes single crystal and epitaxial film formation challenging. We report on the preparation of high quality epitaxial actinide films. The films have been characterized through a combination of X-ray diffraction and X-ray absorption fine structure (XANES and EXAFS) measurements. We report band gaps of 2.80 ± 0.1 eV and 2.85 ± 0.1 eV at room temperature for PuO2 and NpO2, respectively, and compare our measurements with state-of-the-art calculations.

Short range order in bimetallic nanoalloys: An extended X-ray absorption fine structure study

Abstract: Partial coordination numbers measured by extended X-ray absorption fine structure (EXAFS) spectroscopy have been used for decades to resolve between different compositional motifs in bulk and nanoscale bimetallic alloys. Due to the ensemble-averaging nature of EXAFS, the values of the coordination numbers in nanoparticles cannot be simply interpreted in terms of the degree of alloying or segregation if the compositional distribution is broad. We demonstrate that a Cowley short range order parameter is an objective measure of either the segregation tendency (e.g., a core-shell type) or the degree of randomness (in homogeneous nanoalloys). This criterion can be used even in the case when the clusters are random but have broad compositional distributions. All cases are illustrated using the analyses of EXAFS data obtained in three different nanoscale bimetallic systems: Pt(core)-Pd(shell), Pd(core)-Pt(shell), and Pt-Pd random alloy.

Lead is not off center in PbTe: the importance of r-space phase information in extended x-ray absorption fine structure spectroscopy.

Abstract: PbTe is a well-known thermoelectric material. Recent x-ray total scattering studies suggest that Pb moves off center along 100 in PbTe, by $\ensuremath{\sim}0.2\text{ }\text{ }\AA{}$ at 300 K, producing a split Pb-Te pair distribution. We present an extended x-ray absorption fine structure spectroscopy (EXAFS) study of PbTe (and Tl doped PbTe) to determine if Pb or Te is off center. EXAFS provides sensitive $r$- or $k$-space phase information which can differentiate between a split peak for the Pb-Te distribution (indicative of off-center Pb) and a thermally broadened peak. We find no evidence for a split peak for Pb-Te or Te-Pb. At 300 K, the vibration amplitude for Pb-Te (or Te-Pb) is large; this thermally induced disorder is indicative of weak bonds, and the large disorder is consistent with the low thermal conductivity at 300 K. We also find evidence of an anharmonic potential for the nearest Pb-Te bonds, consistent with the overall anharmonicity found for the phonon modes. This effect is modeled by a ``skew'' factor ($C3$) which significantly improves the fit of the Pb-Te and Te-Pb peaks for the high temperature EXAFS data; $C3$ becomes significant above approximately 150--200 K. The consequences of these results will be discussed.

Characterization of Pt-doped SnO2 catalyst for a high-performance micro gas sensor

Abstract: The atomic scale structure and its dependence on Pt concentration of a Pt-doped SnO2 (Pt–SnO2) thin film produced by a sputter-deposition method was investigated, which showed high-performance as a methane gas sensor. Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) analyses showed that Pt–SnO2 has a rutile structure similar to SnO2 crystals at less than 10 at% Pt where the Pt ion was located at the Sn position in the rutile structure. There was no evidence that Pt metal clusters were formed in the Pt–SnO2 films. The Pt–SnO2 structure became amorphous at greater than 11 at% Pt. We found a good correlation between the methane activity and local structure of Pt.

Osteopontin: A Uranium Phosphorylated Binding‐Site Characterization

Abstract: Herein, we describe the structural investigation of one possible uranyl binding site inside a nonstructured protein. This approach couples spectroscopy, thermodynamics, and theoretical calculations (DFT) and studies the interaction of uranyl ions with a phosphopeptide, thus mimicking a possible osteopontin (OPN) hydroxyapatite growth-inhibition site. Although thermodynamical aspects were investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) and isothermal titration calorimetry (ITC), structural characterization was performed by extended X-ray absorption fine structure (EXAFS) at the U LIII -edge combined with attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. From the vibrational and fluorescence spectra, several structural models of a UO2 (2+) /peptide complex were developed and subsequently refined by using theoretical calculations to fit the experimental EXAFS obtained. The structural effect of the pH value was also considered under acidic to moderately acidic conditions (pH 1.5-5.5). Most importantly, the uranyl/peptide coordination environment was similar to that of the native protein.

Effect of geometry on the magnetic properties of CoFe2O4–PbTiO3 multiferroic composites

Abstract: In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element- and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4–PbTiO3 (CFO–PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A- and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L3,2-edge XMCD measurements of the CFO–PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO–PTO composites.

Effect of Al3+ co-doping on the dopant local structure, optical properties, and exciton dynamics in Cu+-doped ZnSe nanocrystals.

Abstract: The dopant local structure and optical properties of Cu-doped ZnSe (ZnSe:Cu) and Cu and Al co-doped ZnSe (ZnSe:Cu,Al) nanocrystals (NCs) were studied with an emphasis on understanding the impact of introducing Al as a co-dopant. Quantum-confined NCs with zinc blende crystal structure and particle size of 6 ± 0.6 A were synthesized using a wet chemical route. The local structure of the Cu dopant, studied by extended X-ray absorption fine structure, indicated that Cu in ZnSe:Cu NCs occupies a site that is neither substitutional nor interstitial and is adjacent to a Se vacancy. Additionally, we estimated that approximately 25 ± 8% of Cu was located on the surface of the NC. Al3+ co-doping aids in Cu doping by accounting for the charge imbalance originated by Cu+ doping and consequently reduces surface Cu doping. The Cu ions remain distorted from the center of the tetrahedron to one of the triangular faces. The lifetime of the dopant-related photoluminescence was found to increase from 550 ± 60 to 700 ± 60 ns...

In situ structural characterization of ferric iron dimers in aqueous solutions: identification of μ-oxo species.

Abstract: The structure of ferric iron (Fe3+) dimers in aqueous solutions has long been debated. In this work, we have determined the dimer structure in situ in aqueous solutions using extended X-ray absorption fine structure (EXAFS) spectroscopy. An Fe K-edge EXAFS analysis of 0.2 M ferric nitrate solutions at pH 1.28–1.81 identified a Fe–Fe distance at ∼3.6 A, strongly indicating that the dimers take the μ-oxo form. The EXAFS analysis also indicates two short Fe–O bonds at ∼1.80 A and ten long Fe–O bonds at ∼2.08 A, consistent with the μ-oxo dimer structure. The scattering from the Fe–Fe paths interferes destructively with that from paths belonging to Fe(OH2)63+ monomers that coexist with the dimers, leading to a less apparent Fe shell in the EXAFS Fourier transform. This might be a reason why the characteristic Fe–Fe distance was not detected in previous EXAFS studies. The existence of μ-oxo dimers is further confirmed by Mossbauer analyses of analogous quick frozen solutions. This work also explores the electro...

An X-ray Absorption Spectroscopy Study of the Inversion Degree in Zinc Ferrite Nanocrystals Dispersed on a Highly Porous Silica Aerogel Matrix

Abstract: The structural properties of zinc ferrite nanoparticles with spinel structure dispersed in a highly porous SiO2 aerogel matrix were compared with a bulk zinc ferrite sample. In particular, the details of the cation distribution between the octahedral (B) and tetrahedral (A) sites of the spinel structure were determined using X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the zinc ferrite spinel structures varies with particle size. In particular, in the bulk microcrystalline sample, Zn2+ ions are at the tetrahedral sites and trivalent Fe3+ ions occupy octahedral sites (normal spinel). When particle size decreases, Zn2+ ions are transferred to octahedral sites and the degree of inversion is found to increase as the nanoparticle size decreases. This is the first time that a variation of the degree of inversion with particle size is observed in ferrite nanoparticles grown within an aerogel matrix.