Showing papers on "XANES published in 2006"

Near edge X-ray absorption fine structure spectroscopy as a tool to probe electronic and structural properties of thin organic films and liquids

Abstract: Synchrotron-based spectroscopic techniques have contributed significantly to a better understanding of the properties of materials on the macroscopic and microscopic scale over the last decades. They can be applied to samples from a diversity of fields, including Biology, Life Sciences, Chemistry and Materials. One of these techniques is Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy, revealing electronic structure and information on the orientation of adsorbed molecules. The present article describes the basics of the technique and the progress it has made over the last three decades, and summarizes some of its more recent developments and applications. This tutorial review article should be accessible for novices to the field from Physics, Chemistry, Biology, Materials, and the Life Sciences, interested in thin organic films and liquid systems.

Near-edge X-ray absorption fine structure spectroscopy as a tool for investigating nanomaterials.

Abstract: We have demonstrated near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a particularly useful and effective technique for simultaneously probing the surface chemistry, surface molecular orientation, degree of order, and electronic structure of carbon nanotubes and related nanomaterials. Specifically, we employ NEXAFS in the study of single-walled carbon nanotube and multi-walled carbon nanotube powders, films, and arrays, as well as of boron nitride nanotubes. We have focused on the advantages of NEXAFS as an exciting, complementary tool to conventional microscopy and spectroscopy for providing chemical and structural information about nanoscale samples.

Picosecond X-ray Absorption Spectroscopy of a Photoinduced Iron(II) Spin Crossover Reaction in Solution

Abstract: In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron−nitrogen (Fe−N) bond is lengthened by 0.21 ± 0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change i...

Microfluidic Synthesis of Cobalt Nanoparticles

Abstract: Co nanoparticles with three different crystal structures were synthesized in a microfluidic reactor through manipulation of reaction times, flow rates, and quenching procedures. Cobalt nanoparticles of face-centered cubic (β) phase were obtained from a high flow rate of the reactants followed by in situ quenching of the reaction. hcp and e-cobalt nanoparticles were obtained at a low flow rate of the reactants followed by in situ quenching and delayed quenching, respectively. The crystal structures were characterized using Co K-edge X-ray absorption near edge structure (XANES) spectroscopy, X-ray diffraction (XRD), and selected area electron diffraction (SAED). In situ XANES measurements on Co nanoparticles coming out of the outlet of the microfluidic reactor at different flow rates seem to indicate that the difference in flow rate influences the nucleation process in a critical way and that particle growth occurs mainly outside the reactor. The magnetic properties of the cobalt nanoparticles, measured usi...

Electronic and molecular structure of photoexcited [Ru-II(bpy)(3)](2+) probed by picosecond X-ray absorption spectroscopy

Abstract: L(2,3) X-ray absorption spectra of aqueous [Ru(II)(bpy)3]2+ have been recorded in its ground and excited states, 50 ps after short pulse laser excitation. Significant changes in both the XANES (X-ray Near-Edge Absorption Structure) and the EXAFS (Extended X-ray Absorption Fine Structure) regions of the excited state complex are detected. The XANES line shapes have been quantitatively simulated using a crystal field multiplet code in trigonal symmetry. In addition, spectral changes in the EXAFS region of both ground and excited states are analyzed in order to extract structural parameters of their corresponding molecular structures. We obtain a Ru-N bond contraction by approximately 0.03 angstroms in the excited-state complex, as compared to the ground-state compound. This contraction results from electrostatic and polarization contributions, limited by steric constraints on the bpy ligands.

2D-mapping of the catalyst structure inside a catalytic microreactor at work: partial oxidation of methane over Rh/Al2O3.

Abstract: Tremendous changes of the structure of Rh particles occurred during partial methane oxidation to hydrogen and carbon monoxide over a 2.5 wt % Rh/Al2O3 catalyst upon ignition of the catalytic reaction. Furthermore, near the ignition temperature a variation in the Rh-valence state along the catalyst bed was observed. By combining hard X-ray absorption spectroscopy (X-ray absorption near edge structure, XANES) with a charged coupled device (CCD) camera and using a suitable spectroscopic cell with gas supply and on-line mass spectrometry, we demonstrate that 2D-mapping of the Rh-oxidation state in a catalyst bed can be achieved during the catalytic reaction. For this purpose, X-ray images were recorded with the CCD camera at each energy around the Rh K-edge with and without the spectroscopic cell. This resulted effectively in the transmitted and incident intensity at each energy and at each pixel of the spectroscopic cell. Reconstruction of the full Rh K-edge XANES spectra at each pixel revealed the local dis...

XANES study of the susceptibility of nano-sized cobalt crystallites to oxidation during realistic Fischer–Tropsch synthesis

Abstract: The oxidation of cobalt during Fischer–Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study, wax coated samples of a Co/Pt/Al2O3 catalyst were taken from a 100-barrel/day slurry bubble column reactor operated at commercially relevant FTS conditions, i.e. 230 °C, 20 bar, (H2 + CO) conversion between 50 and 70%, feed gas composition of ca. 50 vol.% H2 and 25 vol.% CO, P H 2 O / P H 2 = 1 – 1.5 , P H 2 O = 4 – 6 bar and quantitatively characterised by X-ray absorption near edge spectroscopy (XANES). The cobalt catalyst samples, carefully removed from the reactor during the course of Fischer–Tropsch synthesis, were protected from air by the FTS wax. It is clear from the XANES measurements that during realistic FTS conditions cobalt crystallites of 6 nm supported on alumina were stable against oxidation to CoO/CoAl2O4 and a gradual reduction of residual cobalt oxide (i.e. following activation in pure hydrogen) was observed. This result is in line with recent thermodynamic analysis of the oxidation and re-reduction of nano-sized cobalt crystallites in water/hydrogen mixtures.

Unusual Non-Bulk Properties in Nanoscale Materials: Thermal Metal−Metal Bond Contraction of γ-Alumina-Supported Pt Catalysts

Abstract: Variable-temperature X-ray absorption spectroscopy measurements of sub-nanometer Pt nanoparticles on a high-surface-area γ-alumina support reveal that the Pt−Pt bonds exhibit contraction upon heating [i.e., negative thermal expansion (NTE)]. Bare clusters under a He environment show an average linear expansion coefficient of −2.5 × 10-5 K-1 over the temperature range studied. Adsorption of hydrogen results in an overall bond relaxation and less dramatic Pt−Pt thermal bond-length contraction. From the temperature effect on bond length, disorder parameters, and the X-ray absorption near edge structure (XANES) spectra, temperature-dependent charge transfer between the support and the Pt clusters was concluded to be responsible for the observed behavior.

Structural Feature and Catalytic Performance of Cu Species Distributed over TiO2 Nanotubes

Abstract: Copper oxide was deposited on tubular TiO2 via Cu2+ introduction into a titanate nanotube aggregate followed by calcination. The titanate has a layered structure allowing Cu intercalation and can readily transform into anatase TiO2 via calcination for condensation of the constituting layers. The activity of the tubular catalysts, with a Cu content of 2 wt %, in selective NO reduction with NH3 was compared with those of other 2 wt % Cu/TiO2 catalysts using TiO2 nanoparticles as the support. The Cu species supported on the nanotubes showed a higher activity than those supported on the nanoparticles. X-ray absorption near-edge structure (XANES) analysis showed that the Cu species on all the TiO2 supports are in the +2 state. Extended X-ray absorption fine structure (EXAFS) investigations of these catalysts reflected higher degrees of CuO dispersion and Cu2+ dissolution into the TiO2 lattice for the tubular Cu/TiO2 catalysts. Absence of CuO bulk detection by a temperature-programmed reduction analysis for the...

Synthesis and characterization of LiFePO4 and LiTi0.01Fe0.99PO4 cathode materials

Abstract: Nanocrystalline LiFePO 4 and doped LiTi 0 . 0 1 Fe 0 . 9 9 PO 4 powders were synthesized via a sol-gel preparation route. High-resolution tunneling electron microscopy observation and energy dispersive spectroscopy, mappingshow the homogeneous distribution of dopant Ti cations in the crystals. Fe and O K-edge X-ray absorption near-edge structure (XANES) measurements show that Ti 4 + doping induces an increased unoccupied d-state in LiFePO 4 , resulting in an enhanced p-type semiconductivity. In situ Fe K-edge XANES measurements of Ti-doped and undoped LiFePO 4 electrodes have been performed to determine the change of Fe valence during the lithium intercalation and de-intercalation processes. Both LiFePO 4 and doped LiTi 0 . 0 1 Fe 0 . 9 9 PO 4 cathodes demonstrate good electrochemical performance.

Influence of N-doping on the structure and electronic properties of titania nanoparticle photocatalysts.

Abstract: N-containing TiO(2)-based nanostructured materials (average particle size approximately 10 nm) with an anatase-type structure were investigated using oxygen (O) K-edge and titanium (Ti) K- and L-edge X-ray absorption near-edge spectroscopy (XANES). The Ti K pre-edge features indicate that samples predominantly contain ([6])Ti with some ([5])Ti, and there is no evidence for ([4])Ti. We observed that those samples with a larger fraction of Ti in a fivefold coordination, that is, with a significant number of oxygen vacancies, also present a modified Ti environment at the medium-range scale. The presence of these defects drastically modifies the electronic structure of the conduction band, as evidenced by the O K XANES spectra, but does not result in the presence of reduced Ti(3+) states. We discuss the influence of N-doping on titania nanoparticles and their structure, electronics and photocatalytic activity.

Local Fe structure and ferromagnetism in Fe-doped ZnO films

Abstract: The local Fe structure and corresponding ferromagnetism are different for various concentrations of Fe-doped ZnO (Zn1?xFexO, x = 0?0.07) films, which are prepared on LiNbO3(104) substrates by reactive magnetron sputtering. X-ray photoelectron spectroscopy and x-ray absorption near-edge structure (XANES) reveal that, when x?0.04, Fe is in the 2+ state and is incorporated into the wurtzite lattice of ZnO, and as x increases further, a second phase Fe3O4 is induced. Furthermore, full multiple-scattering substitution ab?initio calculation of Fe?K-edge XANES is used to confirm the local structure of Fe in films with different x. The single-phase Fe-doped ZnO films (x?0.04) exhibit ferromagnetism above room temperature and the mechanism of bound magnetic polarons (BMPs) is proposed to discuss the magnetic properties. The presence of the second phase is responsible for the strong ferromagnetism for higher Fe concentration.

Radiation effects in water ice: a near-edge x-ray absorption fine structure study.

Abstract: The changes in the structure and composition of vapor-deposited ice films irradiated at 20K with soft x-ray photons (3–900eV) and their subsequent evolution with temperatures between 20 and 150K have been investigated by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K edge. We observe the hydroxyl OH, the atomic oxygen O, and the hydroperoxyl HO2 radicals, as well as the oxygen O2 and hydrogen peroxide H2O2 molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (Icryst) ice films. The evolution of their concentrations with the temperature indicates that HO2, O2, and H2O2 result from a simple step reaction fuelled by OH, where O2 is a product of HO2 and HO2 a product of H2O2. The local order of ice is also modified, whatever the initial structure is. The crystalline ice Icryst becomes amorphous. The high-density amorphous phase (Iah) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amo...

Characterization of As-Doped p-Type ZnO by X-ray Absorption Near-Edge Structure Spectroscopy: Theory

Abstract: Vaithianathan et al. [Appl. Phys. Lett. 88, 112103 (2006)] measured x-ray absorption near-edge structure (XANES) of As-doped ZnO and analyzed it as evidence for AsO acceptors. However, upon carrying out first principles calculations, we found that the simulated XANES spectrum for AsO is very different from that observed. Instead, the simulated spectrum for AsZn–2VZn defect complex, which is predicted to be an acceptor [S. Limpijumnong et al., Phys. Rev. Lett. 92, 155504 (2004)], is far more consistent with the XANES data. The combination of our study, with the XANES of Vaithianathan et al. might be, until now, the strongest support for the AsZn–2VZn model.

Probing the Ordering of Semiconducting Fluorene−Thiophene Copolymer Surfaces on Rubbed Polyimide Substrates by Near-Edge X-ray Absorption Fine Structure

Abstract: The temperature-dependent alignment of semiconducting liquid crystalline fluorene−thiophene copolymer (F8T2) thin film surfaces was investigated using the near-edge X-ray absorption fine structure (NEXAFS) technique. Partial electron yield spectra were recorded over a range of temperatures in order to observe directly the surface orientation as the polymer is heated and cooled through glass, crystal, and liquid crystal phases. In addition, samples annealed under varying processing conditions and quenched to room temperature were analyzed. The NEXAFS data show that (a) in thin F8T2 films at all temperatures the polymer backbone lies in the plane of the substrate, (b) the fluorene and thiophene rings are rotated randomly about the molecular axis, (c) orientation of the polymer backbone can be controlled using a rubbed polyimide alignment layer as a template for liquid crystal orientation, and (d) under proper annealing conditions there is strong temperature-dependent alignment of the copolymer main-chain ax...

Redox and speciation micromapping using dispersive X‐ray absorption spectroscopy: Application to iron in chlorite mineral of a metamorphic rock thin section

Abstract: [1] The first dispersive μ-XANES mapping is presented here. The experiments have been conducted at the iron K-edge, on the “Dispersive-EXAFS” beamline of the European Synchrotron Radiation Facility (France). The mapping has been performed on a polished thin section of 30 μm of a natural metamorphic rock including different types of minerals. Because of the high X-ray absorption due to the thickness of the glass sample holder (∼1 mm), the data have been collected in the fluorescence mode using the so-called “Turbo-XAFS” design. The effective spot size was approximately 10 × 10 microns, and an area 390 × 180 microns in size was mapped. Improvements of the acquisition process allowed collection of each XANES spectrum in 1.5 s and with a step size of 5 microns so that 2808 spectra with full XANES information were collected. Then, automatic procedures for data reduction and mapping reconstruction were developed using Matlab®. The results are maps of iron content, oxidation state, and speciation, with a 5 μm spatial resolution after two-dimensional deconvolution. Subsequent analyses of the reconstructed images provide some quantitative calibrations.

An experimental determination of the effect of pressure on the Fe3+/∑Fe ratio of an anhydrous silicate melt to 3.0 GPa

Abstract: The effect of pressure on the Fe 3+ /∑Fe ratio of an anhydrous andesitic melt was determined from 04 to 30 GPa at 1400 °C with oxygen fugacity controlled internally by the Ru + RuO 2 buffer Values of Fe 3+ /∑Fe were determined by Mossbauer spectroscopy on quenched glasses with a precision of ±001, one standard deviation This precision was verified independently by XANES spectroscopy of the same samples The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s → 3d transition with increasing pressure The results to 20 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 30 GPa has higher Fe 3+ /∑Fe than predicted from the trend established by the lower-pressure data This might be due to a coordination change in Fe 3+ at high pressure; although there is no evidence for this in the Mossbauer spectra, such a change could account for the change in intensity of the 1s → 3d transition in the XANES spectra with pressure

Kinetically controlled vapor-diffusion synthesis of novel nanostructured metal hydroxide and phosphate films using no organic reagents

Abstract: Nanostructured Co5(OH)8Cl2·3H2O, Co5(OH)8(NO3)2·2H2O, Co5(OH)8SO4·2H2O, Zn5(OH)8(NO3)2·2H2O, Cu2(OH)3(NO3) and Mn3(PO4)2·7H2O thin films have been prepared using a kinetically controlled vapor-diffusion method. Vectorial control by diffusion of ammonia as a base catalyst into an aqueous metal salt solution yields large area (2 cm2) metal hydroxide and metal phosphate films with unique structures. No supporting substrate for growth of the films is necessary in this approach. The films were characterized using X-ray powder diffraction and scanning electron microscopy. The cobalt containing films were studied in more detail using transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption near edge structure and various chemical analysis techniques. For the first time the electronic properties and crystal structure of these materials could be studied in thin films not influenced by the presence of an underlying substrate. For Co5(OH)8(NO3)2·2H2O films, which crystallize in a layered hydrotalcite-like structure that is homogeneous from the nanoscale to the macroscale, unprecedented photoconductivity properties were observed. Resistivity measurements show that this material is a p-type semiconductor with an unusually long minority carrier lifetime and high carrier density.

Molecular and electronic structure in NaCl electrolytes of varying concentration: Identification of spectral fingerprints

Abstract: Near edge x-ray absorption spectra at the Na K edge in aqueous NaCl electrolytes are presented as a function of concentration. The spectra are modeled by electronic structure calculations. We find and identify the orbital origin of two spectral fingerprints with sensitivity to the Na+–H2O distance and the Na+–Cl− distance in the electrolyte. Interionic interaction is found to be crucial for the description of the electrolytes at high concentrations.