K-edge Absorption Spectra Research Articles

Comprehensive studies of structural, electronic and magnetic properties of Zn0.95Co0.05O nanopowders

X-ray absorption (XANES, EXAFS, XMCD) and photoelectron (XPS) spectroscopic techniques were employed to study local structural, electronic and magnetic properties of Zn0.95Co0.05O nanopowders. The substitutional Co2+ ions are incorporated in ZnO lattice at regular Zn sites and the sample is characterized by high structural order. There was no sign of ferromagnetic ordering of Co magnetic moments and the sample is in paramagnetic state at all temperatures down to 5K. The possible connection of the structural defects with the absence of ferromagnetism is discussed on the basis of theoretical calculations of the O K-edge absorption spectra.

Fluorination-dependent molecular orbital occupancy in ring-shaped perfluorocarbons.

Perfluorocarbons are a family of molecules consisting mainly of carbon and fluorine atoms. They have interesting chemical properties and have diverse applications in biomedicine, physical chemistry and polymer science. In this work, carbon K-edge absorption and emission spectra of liquid decalin are presented and compared to perfluorodecalin. A comprehensive picture of the electronic structure of decalin is provided based on soft X-ray absorption and emission spectroscopies. Experimental data are compared to theoretical time-dependent density functional theory for the hydrocarbon, the perfluorocarbon and the stepwise fluorinated derivatives. We observed a molecular orbital change from unoccupied to occupied orbitals for perfluorodecalin, which was induced through the fluorination process.

Polarized X-ray Absorption Spectroscopy Observation of Electronic and Structural Changes of Chemical Vapor Deposition Graphene in Contact with Water

The interaction of chemical vapor deposition (CVD)-grown graphene films with water was studied by means of in situ angle-dependent X-ray absorption spectroscopy (XAS). We found that when the graphene layer is in contact with water there is a reduction in the π* peak intensity in the carbon K-edge absorption spectra, accompanied by an extension of the σ* peak to lower energies, which are indicative of chemical modifications of the graphene and a reduction in the number of unsaturated carbon bonds due to the covalent attachment of contaminant species. In addition to the chemical changes a decrease in the dichroic ratio measured by polarized XAS measurements was observed, which indicates an increase on the nanometer scale corrugation. These changes can strongly influence the electronic properties, mechanical robustness, and resistance to sloughing, as well as graphene reactivity.

Microstructure evolution and oxidation states of Co in perovskite-type oxide Ba10Co0.7Fe0.2Nb0.1O3–δ annealed in CO2 atmosphere

Ba1.0Co0.7Fe0.2Nb0.1O3–δ (BCFN) oxide with perovskite cubic structure was synthesized by solid state reaction method. CO2 corrosion of BCFN membrane was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFT) and X-ray absorption fine structure spectroscopy (XAFS). Cobalt (Co) K-edge absorption spectra of BCFN annealed in CO2 reveal that the oxidation states of Co in all the samples were larger than +3 and they decreased with the increase of calcination time. At 800 °C, 1% CO2 introduced into He could speed up the reduction of Co cations in comparison with pure He. In addition, sulfate ions in the bulk of BCFN membrane preferred to migrate to the surface under CO2 calcination and form monoclinic Ba(CO3)0.9(SO4)0.1 besides orthorhombic witherite. Moreover, SEM results indicate that the nucleation and growth of carbonates grains started at the grain boundary of the membrane.

VO2(110) film formation on TiO2(110) through post-reduction of ALD grown vanadium oxide

Abstract Epitaxial VO 2 film was produced through post-reduction of vanadium oxide film grown on rutile TiO 2 (1 1 0) by atomic layer deposition (ALD) method. The ALD grown film was transformed to VO 2 after annealing at 350 °C under 2% hydrogen gas flow. Metal–insulator phase transition of the transformed VO 2 (1 1 0) film has been characterized with resonance photoemission and soft X-ray absorption spectroscopy. Formation of high quality film is evident from clear metal–insulator transition features in oxygen K-edge absorption spectra and metallic peaks in the Fermi level in V 2p-3d resonance photoemission map taken at 127 °C. The VO 2 film formation through the post-reduction of ALD grown vanadium oxide at considerably low temperature could provide route to incorporate VO 2 conformal layer into an integrated device utilizing the metal–insulator transition.

Scalable synthesis and electrochemical investigations of fluorine-doped lithium manganese spinel oxide

A scalable synthesis route for the preparation of fluorine-doped lithium manganese spinel was developed. The effect of 2.5% fluorine doping on the properties of aluminum-doped lithium manganese spinel was investigated by examining the electrochemical performance and structural changes. The electrochemical study indicates that the initial discharge capacity increases by about 5% while other properties, like the capacity retention and rate performance, remain almost unchanged. The structural studies indicate that no impurity phase was formed, along with only little morphological changes and a slight increase in the lattice parameter. The Mn K-edge absorption spectra clearly show that the average manganese oxidation state decreases as a result of fluorine doping and Mn LII,III -edge spectra indicate that the electronic structure of the fluorine-doped spinel changes significantly, which may alleviate the intrinsic structural instability of this material.

Energy levels of the Ce activator relative to the YAP(Ce) scintillator host

The electronic structure of the cerium-activated yttrium aluminum perovskite [YAP(Ce)] scintillator has been studied by core level x-ray spectroscopy and first-principles calculations. X-ray absorption and emission spectra at the oxygen K-edge of YAP(Ce) and CeO2 have been measured and compared with the calculated partial density of states. With the known band gap of CeO2, the measured oxygen K-edge absorption and emission spectra are used to construct a fixed relation between the valence and conduction bands of YAP and CeO2. This allows us to determine the fundamental band gap of YAP to be 8.1 ± 0.3 eV. A comparison between the cerium M4,5-edges x-ray absorption spectra of the YAP(Ce) and Ce model compounds (CeO2, CeF3, and Ce foils) then shows that the Ce activator is in the desired Ce3+, with a small fraction of Ce4+ due to oxidization at the surface. Finally, we determine that the ground state 4f1 energy level of the Ce3+ activator lies 1.8 ± 0.5 eV above the top of the valence band of the host YAP.

Molecular mixing in donor and acceptor domains as investigated by scanning transmission X-ray microscopy

The nanolevel molecular structure of a bulk heterojunction (BHJ) with a donor (D) polymer and acceptor (A) fullerene derivative is indispensable for true comprehension of highly efficient organic photovoltaic devices. Here, we performed scanning transmission X-ray microscopy of a poly(9,9-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend film with periodic nanostructure. The spatially resolved carbon K-edge absorption spectra revealed that the nanostructure consists of two types of domains with considerable molecular mixing. The fullerene mass fraction is 71 ± 1 and 33 ± 2 wt % for the PC71BM- and F8T2-rich domains, respectively.

Analysis of the near-edge X-ray-absorption fine-structure of anthracene: A combined theoretical and experimental study

The near-edge fine structure of the carbon K-edge absorption spectrum of anthracene was measured and theoretically analyzed by density functional theory calculations implemented in the StoBe code. It is demonstrated that the consideration of electronic relaxation of excited states around localized core holes yields a significant improvement of the calculated excitation energies and reproduces the experimentally observed fine structure well. The detailed analysis of excitation spectra calculated for each symmetry inequivalent excitation center allows in particular to examine the influence of chemical shifts and core hole effects on the excitation energies. Moreover, the visualization of final states explains the large variations in the oscillator strength of various transitions as well as the nature of Rydberg-states that exhibit a notable density of states below the ionization potentials.

In Situ X-Ray Absorption Spectroscopy Study of Sn Underpotential Deposition on Ni from Perchloric Acid

In situ X-ray absorption spectroscopy (XAS) was applied to investigate the Sn underpotential deposition (UPD) on Ni surface from 0.2 M HClO4 solution containing 10−3 M Sn2+ with relation to the inhibition effect of Sn on aqueous corrosion of Ni. The periodical emersion method under potentiostatic polarization, using the surface-roughened Ni plate (surface roughness Sr = 78.3) as a working electrode was employed to detect sensitively the sub-monolayer coverage of Sn on Ni. The Sn K-edge absorption spectra in a scanning XAS mode were measured by monitoring the Sn Kα1fluorescence line. The Sn K-edge absorption near-edge structure (XANES) in the Sn-UPD potential region has revealed that the Sn-UPD layer on Ni is oxygenated. The extended X-ray absorption fine structure (EXAFS) analysis was performed with a two shell fit of the nearest neighbor Sn-Ni and Sn-O interactions, assuming that the uppermost Ni surface exposed to the solution is mainly oriented to the (111) plane. The results have indicated that Sn atoms are substituted like a surface alloy at face-center-cubic sites in the first Ni layer and further bonded with oxygen atoms. The strong inhibition effect of Sn on aqueous corrosion of Ni is ascribed to the bond between Sn and O atoms in addition to the bond between Sn and Ni atoms.

Femtosecond x-ray absorption spectroscopy with hard x-ray free electron laser

We have developed a method of dispersive x-ray absorption spectroscopy with a hard x-ray free electron laser (XFEL), generated by a self-amplified spontaneous emission (SASE) mechanism. A transmission grating was utilized for splitting SASE-XFEL light, which has a relatively large bandwidth (ΔE/E ∼ 5 × 10−3), into several branches. Two primary split beams were introduced into a dispersive spectrometer for measuring signal and reference spectra simultaneously. After normalization, we obtained a Zn K-edge absorption spectrum with a photon-energy range of 210 eV, which is in excellent agreement with that measured by a conventional wavelength-scanning method. From the analysis of the difference spectra, the noise ratio was evaluated to be ∼3 × 10−3, which is sufficiently small to trace minute changes in transient spectra induced by an ultrafast optical laser. This scheme enables us to perform single-shot, high-accuracy x-ray absorption spectroscopy with femtosecond time resolution.

Elucidating the Nature of Pseudo Jahn–Teller Distortions in LixMnPO4: Combining Density Functional Theory with Soft and Hard X-ray Spectroscopy

A combination of soft and hard synchrotron-based spectroscopy with first-principles density functional theory within the GGA + U framework is used to investigate the distortion of the Mn local environment of LixMnPO4 as a function of electrochemical delithiation (x = 1.0, 0.75, 0.5, 0.25) and its effect on the electron and hole polaron formation. Analysis of the soft X-ray absorption spectroscopy (XAS) of the Mn L3,2-edges confirmed the evolution from the Mn2+ to the Mn3+ charge state as a two-phase reaction upon delithiation; the corresponding Mn K-edge extended X-ray fine structure measurements clearly revealed a splitting of the Mn–O nearest-neighbor distances with increasing Mn3+ character. In addition, the O K-edge absorption and emission spectra confirmed the corresponding orbital lifting of degeneracy accompanying the distortion of the MnO6 octahedra in the Mn3+ state. Our GGA + U calculations show that the distortion is not a strict Jahn–Teller distortion but is instead a preferential elongation of two of the equatorial Mn–O bonds (edge-sharing with the PO4), which results in a Mn–O–P induction driven hybridization of the unoccupied states (i.e., a pseudo Jahn–Teller distortion). Excellent agreement between the calculated electronic structure and our soft X-ray measurements of the electrochemically delithiated LixMnPO4 nanoparticles verifies the link between the preferential structural distortion and the resultant hybridization of the unoccupied 3d dxz and dx2–y2 orbitals. Our analysis of the corresponding calculated electron and hole polaron supports claims that the elongation of the equatorial bonds (edge-sharing with the PO4) in the Mn3+ charge state (i.e., the pseudo Jahn–Teller distortion) is responsible for increasing the activation energy for polaron migration and the formation energy of the electron (hole) lithium ion (vacancy) complex of the Mn olivine compared to the Fe olivine.

Structure and reducibility of a Fe/Al2O3 catalyst for selective catalytic reduction studied by Fe K-edge XAFS spectroscopy

EXAFS and pre-edge information from the Fe K-edge absorption spectra is used in this study to characterise the local environment and geometry of Fe-centres in a 1% Fe/Al2O3 model catalyst. The EXAFS results reveal clusters of 2-3 Fe oxo-moieties dispersed on the Al2O3-support. The Fe3+ centres are coordinated by 6 O-atoms in a strongly distorted octahedral geometry. This is supported by the pre-edge peak, which is far more intense than in α-Fe2O3 absorption data acquired for comparison. For preliminary investigations, catalytic tests for selective catalytic reduction (SCR) of NOx by ammonia and in situ XANES studies of Fe/Al2O3 are compared with previously published data for Fe/zeolite systems. The low SCR activity of Fe/Al2O3 (compared to Fe/zeolite catalysts) in which the Fe3+ species do not change their electronic state under SCR-relevant conditions (reference temperature 250°C in our case) correlates well to the significantly higher temperatures required to reduce these species to Fe2+. The difference in reducibility (and consequently in the SCR-activity) between the two systems probably results from differences in the structure and the electronic interaction of the Fe oxo-moieties and the particular catalyst support.

Local structure of Titanium in natural glasses probed by X-ray absorption fine structure

Synchrotron radiation has been used to collect titanium K-edge absorption spectra of a suite of natural glasses (tektites, impact glasses, fault rocks and volcanic glasses). XANES and XAFS analysis provided the qualitative and quantitative information of Ti oxidation state, Ti-O distance and site geometry. Tektites possess four-, five-, six-coordinated Ti, whereas fault rock-pseudotachylite, volcanic glasses and impact glass only presented five- and six-coordinated Ti. This study indicated that different petrogenesis of natural glasses has different local structures of titanium.

Electronic structure and characteristics of Fe 3d valence states of Fe1.01Se superconductors under pressure probed by x-ray absorption spectroscopy and resonant x-ray emission spectroscopy

The electronic structure and characteristics of Fe 3d valence states of iron-chalcogenide Fe(1.01)Se superconductors under pressure were probed with x-ray absorption spectroscopy and resonant x-ray emission spectroscopy (RXES). The intensity of the pre-edge peak at ~7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe(1.01)Se decreases for pressure from 0.5 GPa increased to 6.9 GPa. The satellite line Kβ' was reduced in intensity upon applying pressure and became absent for pressure 52 GPa. Fe(1.01)Se shows a small net magnetic moment of Fe(2+), likely arising from strong Fe-Fe spin fluctuations. The 1s3p-RXES spectra of Fe(1.01)Se at pressures 0.5, 6.9, and 52 GPa recorded at the Fe K-edge reveal that unoccupied Fe 3d states exhibit a delocalized character, stemming from hybridization of Fe 3d and 4p orbitals arising from a local distortion around the Fe atom in a tetrahedral site. Application of pressure causes suppression of this on-site Fe 3d-Fe 4p hybridization, and thereby decreases the intensity of the pre-edge feature in the Fe K-edge absorption spectrum of Fe(1.01)Se. Compression enhances spin fluctuations at Fe sites in Fe(1.01)Se and increases the corresponding T(c), through a competition between nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic superexchange interactions. This result aids our understanding of the physics underlying iron-based superconductors.

DMSO-Water Clustering in Solution Observed in Soft X-ray Spectra.

The significant deviation from the ideality of dimethyl sulfoxide (DMSO)/water mixtures can be addressed based on the change of the local molecular orbitals of each solvent upon mixing. Oxygen K-edge absorption and emission spectra of DMSO/water solutions were measured using the liquid microjet technique. The spectra demonstrate that the hydrogen bond network in liquid water is already influenced at small DMSO concentrations, and at the molar fraction xDMSO = 0.43 we find strong evidence of DMSO-water clustering reflected by the influence on the occupied molecular orbitals.

On the Interfacial Electronic Structure Origin of Efficiency Enhancement in Hematite Photoanodes

The electronic structure origins of interfacial losses in hematite photoanodes and the cause of the literature-reported order-of-magnitude photocurrent increase upon short high-temperature annealing are investigated. Synchrotron-based soft X-ray absorption spectroscopy is used to probe the unoccupied states at and near the interface between hematite (α-Fe2O3) and fluorine-doped tin oxide (FTO). Oxygen K-edge and iron L-edge absorption spectra indicate that the interfacial interaction reduces the degree of p–d hybridization and alters the crystal field in α-Fe2O3. The interface is found to be associated with a distribution of unoccupied oxygen p-hybridized states located below the lowest unoccupied iron 3d states in α-Fe2O3 (just below the conduction band minimum), which are eliminated with high-temperature processing. These data facilitate future efforts to engineer favorable interfacial compositions and associated electrochemical potential gradients within photoanodes, which are required to efficiently separate charge carriers in operating photoelectrochemical systems such as solar cells and photocatalytic devices.

X-ray Absorption Study of the Solvation Structure of Cu2+ in Methanol and Dimethyl Sulfoxide

The solvation structure of Cu(2+) in methanol (MeOH) and dimethyl sulfoxide (DMSO) has been determined by studying both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions of the K-edge absorption spectra. The EXAFS technique has been found to provide a very accurate determination of the next-neighbor coordination distances, but it is inconclusive in the determination of the coordination numbers and polyhedral environment. Conversely, quantitative analysis of the XANES spectra unambiguously shows the presence of an average 5-fold coordination in both the MeOH and DMSO solution, ruling out the usually proposed octahedral Jahn-Teller distorted geometry. The EXAFS and XANES techniques provide coherent values of the Cu-O first-shell distances that are coincident in the two solvents. This investigation shows that the combined analysis of the EXAFS and XANES data allows a reliable determination of the structural properties of electrolyte solutions, which is very difficult to achieve with other experimental techniques.

Mechanisms of near edge fine structure formation in K-edge absorption spectra of oxygen and metal atoms in the perovskite-structure titanates

Full multiple scattering in the cluster approximation with semiempirical MT potential was used for thorough investigation of the near-edge fine structure of Ti and O K spectra from PbTiO3, BaTiO3, CaTiO3 and SrTiO3 crystals. The calculated spectra were shown to be in good agreement with the experiment. The mechanisms underlying the emergence of fine structure elements of the spectra were revealed. Near-edge structure of titanium K spectra was examined in detail. In the general case, this structure was found to have five features of various nature. The quantitative description of near-edge structure requires a model of incomplete shielding of the 1s hole. Substantiation of this model was proposed.

Pressure dependence of the electronic structure and spin state in Fe1.01Se superconductors probed by x-ray absorption and x-ray emission spectroscopy

Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe${}_{1.01}$Se superconductors up to \ensuremath{\sim}66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of \ensuremath{\sim}7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe${}_{1.01}$Se decreases progressively with pressure up to \ensuremath{\sim}10 GPa. A new prepeak at energy of \ensuremath{\sim}7113.7 eV develops for pressure above \ensuremath{\sim}13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe${}_{1.01}$Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe${}_{1.01}$Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe${}_{1.01}$Se. A discontinuous variation of the integrated absolute difference values of the K\ensuremath{\beta} emission line was observed, originating from a phase transition of Fe${}_{1.01}$Se for a pressure >12 GPa. Fe${}_{1.01}$Se shows a small net magnetic moment of Fe${}^{2+}$ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K\ensuremath{\beta}\ensuremath{'} was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe${}_{1.01}$Se superconductors. The experimental results provide insight into the spin state of Fe${}_{1.01}$Se superconductors under pressure.