Oxygen K-edge Research Articles

Investigating local degradation and thermal stability of charged nickel-based cathode materials through real-time electron microscopy.

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li(x)Ni(0.8)Co(0.15)Al(0.05)O2 (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O2 gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to x = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system.

Direct observation of ferroelectric field effect and vacancy-controlled screening at the BiFeO3/LaxSr1-xMnO3 interface.

The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.

Charging-Discharging Phenomena on P2-Na2/3Co2/3Mn1/3O2 Observed by X-ray Absorption Spectroscopy

Charging-discharging phenomena of P2-Na2/3Co2/3Mn1/3O2 prepared by a co-precipitation method were investigated by ex situ and in situ X-ray absorption spectroscopy [1, 2]. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and Mn edges, suggesting that the redox pairs, i.e. Co3+/Co2+ and Mn4+/Mn3+, are both involved in the reaction.[1] J.-H. Cheng, C.-J. Pan, J.-F. Lee, J.-M. Chen, M. Guignard, C. Delmas, D. Carlier, and B.-J. Hwang, Chem. Mater., 26 (2014 ) 1219–1225.[2] D. Carlier, J. H. Cheng, R. Berthelot, M. Guignard, M. Yoncheva, R. Stoyanova, B. J. Hwang and C. Delmas, Dalton Trans.,40 ( 2011), 9306.

X-ray Raman scattering spectroscopy

For elements with low atomic number, or shallow absorption edges falling in the energy range below ~1 keV, x-ray absorption studies are often limited by surface sensitivity and the necessity of a vacuum environment, making bulk-sensitive measurements and for example studies of liquids difficult. An exciting alternative is provided by X-ray Raman scattering (XRS) spectroscopy. It is used to measure a photon-in-photon-out process, where a hard x-ray photon loses only part of its energy creating an excitation of an inner core electron. As such, it is the x-ray analogue of electron energy loss spectroscopy. The advantage of XRS is that the incident photon energy can be chosen freely and thus low-energy absorption edges can be studied with high-energy X-rays. Thus XRS is becoming increasingly popular since it allows for bulk-sensitive measurements of inner core spectra where the corresponding x-ray absorption threshold falls into the soft x-ray regime. This lifts all constraints on the sample environment inherent to soft x-ray studies, and offers access to bulk-sensitive information on solids, liquids and gases as well as systems in enclosed sample environments such as high-pressure cells. For example the microscopic structure of water within the supercritical regime has been recently studied using the oxygen K-edge excitation spectra measured by XRS, yielding new information on the hydrogen-bond network of water in extreme conditions [1]. Another important feature of XRS is that it allows for other than dipole transitions to be studied, thanks to an practically unlimited range of momentum transfer offered by hard x-rays. These higher order multipole excitations can yield novel information on the electronic structure, not accessible by many other spectroscopies [2]. The availability of XRS instruments at third-generation synchrotron radiation sources has made highly accurate XRS measurements possible. XRS can be even used as a contrast mechanism in three-dimensional X-ray imaging [3]. In this contribution, the capabilities of XRS and recent examples of novel studies allowed by it will be reviewed.

A Stable, Magnetic, and Metallic Li3O4 Compound as a Discharge Product in a Li-Air Battery.

The Li-air battery with the specific energy exceeding that of a Li ion battery has been aimed as the next-generation battery. The improvement of the performance of the Li-air battery needs a full resolution of the actual discharge products. Li2O2 has been long recognized as the main discharge product, with which, however, there are obvious failures on the understanding of various experimental observations (e.g., magnetism, oxygen K-edge spectrum, etc.) on discharge products. There is a possibility of the existence of other Li-O compounds unknown thus far. Here, a hitherto unknown Li3O4 compound as a discharge product of the Li-air battery was predicted through first-principles swarm structure searching calculations. The new compound has a unique structure featuring the mixture of superoxide O2(-) and peroxide O2(2-), the first such example in the Li-O system. The existence of superoxide O2(-) creates magnetism and hole-doped metallicity. Findings of Li3O4 gave rise to direct explanations of the unresolved experimental magnetism, triple peaks of oxygen K-edge spectra, and the Raman peak at 1125 cm(-1) of the discharge products. Our work enables an opportunity for the performance of capacity, charge overpotential, and round-trip efficiency of the Li-air battery.

Effect of the hydrophobic alcohol chain length on the hydrogen-bond network of water.

The microscopic structure of the hydrogen-bond network of water-alcohol mixtures was studied using X-ray Raman scattering (XRS). To systematically examine how the hydrogen-bond network of water is affected by an increasing size of the hydrophobic group, small linear alcohols (methanol, ethanol, and propanol) in constant mole fractions were studied. The oxygen K-edge spectra were not altered upon hydration of the alcohols beyond a simple superposition of signals from alcohol and water. The experiment thus indicates that alcohols do not have a substantial effect on the structure of the hydrogen-bond network of water. In particular, no apparent breaking or forming of the hydrogen bonds is observed to take place in the overall structure. In addition, there is no indication of changes in the tetrahedrality of the hydrogen-bond network of water in the vicinity of alcohol molecules.

Oxygen spectroscopy and polarization-dependent imaging contrast (PIC)-mapping of calcium carbonate minerals and biominerals.

X-ray absorption near-edge structure (XANES) spectroscopy and spectromicroscopy have been extensively used to characterize biominerals. Using either Ca or C spectra, unique information has been obtained regarding amorphous biominerals and nanocrystal orientations. Building on these results, we demonstrate that recording XANES spectra of calcium carbonate at the oxygen K-edge enables polarization-dependent imaging contrast (PIC) mapping with unprecedented contrast, signal-to-noise ratio, and magnification. O and Ca spectra are presented for six calcium carbonate minerals: aragonite, calcite, vaterite, monohydrocalcite, and both hydrated and anhydrous amorphous calcium carbonate. The crystalline minerals reveal excellent agreement of the extent and direction of polarization dependences in simulated and experimental XANES spectra due to X-ray linear dichroism. This effect is particularly strong for aragonite, calcite, and vaterite. In natural biominerals, oxygen PIC-mapping generated high-magnification maps of unprecedented clarity from nacre and prismatic structures and their interface in Mytilus californianus shells. These maps revealed blocky aragonite crystals at the nacre-prismatic boundary and the narrowest calcite needle-prisms. In the tunic spicules of Herdmania momus, O PIC-mapping revealed the size and arrangement of some of the largest vaterite single crystals known. O spectroscopy therefore enables the simultaneous measurement of chemical and orientational information in CaCO3 biominerals and is thus a powerful means for analyzing these and other complex materials. As described here, PIC-mapping and spectroscopy at the O K-edge are methods for gathering valuable data that can be carried out using spectromicroscopy beamlines at most synchrotrons without the expense of additional equipment.

The nature of the nonmetal–metal transition in LixCoO2 oxide

In this paper we analyze the nature of the nonmetal–metal transition in the LixCoO2 oxide (0.53 ≤ x ≤ 1) on the basis on the investigations of the structural, transport properties, oxygen K-edge NEXAFS spectroscopy and calculated density of states (DOS). Our investigations show that the stoichiometric Li1CoO2 oxide is a semiconductor, but extraction of the lithium ions leads to the metallic properties of the system. Besides, KKR-CPA showed that the distortion of the oxygen octahedron significantly modifies top valence states. We postulate that the nonmetal–metal transition can be interpreted on the basis on the Anderson type transition.

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.

Polarization Dependence of Aragonite Calcium L-Edge XANES Spectrum IndicatescandbAxes Orientation

Calcium carbonate minerals are frequently found in biomineral structures and are the predominant mineral in invertebrates. While there are several calcium carbonate polymorphs, aragonite and calcite are the two most commonly found in biogenic systems. Currently, calcium L-edge X-ray absorption near-edge structure (XANES) spectra are used to distinguish between different calcium carbonate polymorphs, including calcite and aragonite, while oxygen and carbon K-edge XANES spectra are often used to determine the c axis orientation of a given calcium carbonate crystal. By doing a full analysis of the calcite and aragonite calcium L-edge XANES spectrum for both geologic and biogenic systems, we were able to show that aragonite has a polarization-dependent peak while calcite does not. Analysis based on both multiplet models and density functional calculations show how the polarization dependence arises from directional bonds between the calcium and oxygen atoms within aragonite. These data not only enable an interpretation of the aragonite calcium L-edge XANES spectrum but also the ability to determine the orientation of the c and b axes of aragonite crystals within a biomineral sample.

Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications.

The effect of monovalent cations (Li+, K+, NH4+, Na+) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.

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.

DNA damage by soft X-ray exposure at oxygen K-edge

In order to obtain detailed insights into the physicochemical mechanism of DNA damage induction in terms of photoabsorption modes, we have prepared thin DNA films of closed circular plasmid (pUC18) on a cover slip without any additives. Using this film, we have performed preliminary experiments by exposing to soft X-rays with energies around oxygen K-shell ionization threshold. The DNA damage yields of strand breaks and base lesions or AP sites were quantified by biochemical treatments. We confirmed that the DNA film can work as a specimen irradiation. The DNA damage yields induced by π* excitation of a K-shell electron of oxygen atoms in DNA were significantly larger those for oxygen K-ionization.

Bond cleavages of adenosine 5'-triphosphate induced by monochromatic soft X-rays

To investigate which type of bond is likely to be cleaved by soft X-ray exposure to an adenosine 5'-triphosphate (ATP), we observed spectral changes in X-ray absorption near edge structure (XANES) around nitrogen and oxygen K-edge of an ATP film by soft X-ray irradiation. Experiments were performed at a synchrotron soft X-ray beamline at SPring-8, Japan. The XANES spectra around the nitrogen and oxygen .K-edge slightly varied by exposure to 560 eV soft X-rays. These changes are originated from the cleavage of C-N bonds between a sugar and a nucleobase site and of C-O, P-O or O-H bond of sugar and phosphate site. From the comparison between the change in XANES intensity of σ* peak at nitrogen and that at oxygen K-edges, it is inferred that the C-O, P-O or O-H bond of sugar and phosphate is much efficiently cleaved than the C-N of N-glycoside bond by the exposure of 560 eV soft X-ray to ATP film.

Near-Edge X-ray Absorption Fine Structure Spectra and Site-Selective Dissociation of Phenol

A time-of-flight mass spectrometer with orthogonal acceleration and soft X-rays from synchrotron radiation were utilized to measure near-edge X-ray absorption fine structure (NEXAFS) spectra of carbon and oxygen in phenol and the corresponding ionic fragments following core excitation. The photon energies were in the range of 284-298 eV for the carbon K-edge and 529.5-554.5 eV for the oxygen K-edge. The total ion yield, ion intensity for each ionic fragment, and ion intensity ratio, defined as ion intensity divided by total ion yield, were measured as a function of photon energy. Possible mechanisms of dissociation are proposed and enhancements of specific products of dissociation are reported. In general, the enhancement of these specific products is small in the carbon K-edge region but is clear for some products at the oxygen K-edge. In particular, elimination of the H atom from the hydroxyl group was observed only at the oxygen K-edge. One remarkable result is that an excitation of a core-level electron of oxygen greatly enhanced the cleavage of specific C-C bonds.

Characterization of tribofilms derived from zinc dialkyl dithiophosphate and serpentine by X-ray absorption spectroscopy

The tribological performance of serpentine in combination with ZDDP as additive for base oil was investigated by a Plint high frequency friction tester at room temperature and 100°C. The tribofilms formed by serpentine and ZDDP were analyzed using the scanning electron microscopy technique equipped with energy dispersive X-ray spectroscopy. X-ray absorption spectroscopy at phosphorus K- and L3,2-edges, sulfur K- and L3,2-edges, silicon K-edge, magnesium K-edge, oxygen K-edge, and zinc L3,2-edge were recorded to determine the chemistry of the tribofilms. It is found that a combination of serpentine with ZDDP helps reduce the friction of oil blend and exhibits better antiwear properties than base oil.

Simultaneous Reduction of Co3+ and Mn4+ in P2-Na2/3Co2/3Mn1/3O2 As Evidenced by X-ray Absorption Spectroscopy during Electrochemical Sodium Intercalation

Sodium intercalation in P2-Na2/3Co2/3Mn1/3O2 (obtained by a coprecipitation method) was investigated by ex situ and in situ X-ray absorption spectroscopy. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and the Mn edges, suggesting that the redox pairs, that is, Co3+/Co2+ and Mn4+/Mn3+, are both involved in the reaction.

An EELS study of near edge structures of the oxygen K-edge in spinels

Spectra of seven aluminate, ferrite and chromate spinels were collected at the oxygen K-edge in order to examine the effect of octahedral ion composition on the near edge structures and explore whether these may shed some light on previously reported major variations in the optical absorption coefficient (e) for the spin-allowed d–d band transitions due to tetrahedrally coordinated Fe2+ in optical spectra of oxide spinels. Interpretation of the energy loss near edge structure (ELNES) spectra was aided using the multiple scattering code FEFF8.40. For the chromate samples spin polarised calculations were included. Gold’s iterative deconvolution method was utilized to improve on energy resolution, resulting in spectra equal in merit to those recorded by technically superior instruments. Results include absolute energy positions and interpretation of ELNES features in terms of transitions to available states. We conclude that the major causes of differences in the optical absorption coefficient mentioned above are hybridization between oxygen p- and octahedral metal orbitals. Our study does not support the idea presented by other authors that there exists a local antiferromagnetic ordering in MgCr2O4 at room temperature.

Oxygen K-edge electron energy loss spectra of hydrous and anhydrous compounds

First-principles calculations have been employed to examine the possible use of electron energy loss spectroscopy (EELS) as a tool for determining the presence of OH groups and hence hydrogen content in compounds. Our density functional theory (DFT) based calculations describe accurately the experimental EELS results for forsterite (Mg2SiO4), hambergite (Be2BO3(OH)), brucite (Mg(OH)2) and diaspore (α-AlOOH). DFT calculations were complemented by an experimental time resolved study of the oxygen K-edge in diaspore. The results show unambiguously that there is no connection between a pre-edge feature in the oxygen K-edge spectrum of diaspore and the presence of OH groups in the structure. Instead, the experimental study shows that the pre-edge feature in diaspore is transient. It can be explained by the presence of molecular O2, which is produced as a result of the electron irradiation.

Nature of the Chemical Bond of Aqueous Fe2+ Probed by Soft X-ray Spectroscopies and ab Initio Calculations

Aqueous iron(II) chloride is studied by soft X-ray absorption, emission, and resonant inelastic Raman scattering techniques on the Fe L-edge and O K-edge using the liquid-jet technique. Soft X-ray spectroscopies allow in situ and atom-specific probing of the electronic structure of the aqueous complex and thus open the door for the investigation of chemical bonding and molecular orbital mixing. In this work, we combine theoretical ab initio restricted active space self-consistent field and local atomic multiplet calculations with experimental soft X-ray spectroscopic methods for a description of the local electronic structure of the aqueous ferrous ion complex. We demonstrate that the atomic iron valence final states dominate the resonant inelastic X-ray scattering spectra of the complex over the ligand-to-metal charge transfer transitions, which indicates a weak interaction of Fe(2+) ion with surrounding water molecules. Moreover, the oxygen K-edge also shows only minor changes due to the presence of Fe(2+) implying a small influence on the hydrogen-bond network of water.