Edge X-ray Absorption Research Articles

A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

ThTi2O6 derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U5+ or U6+, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd3+, Ca2+, Al3+, or Cr3+, on the Th or Ti site. U L3 edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M4 edge XANES evidenced U5+ as the major oxidation state in all compounds, with a minor fraction of U6+ (2–13%). The balance of X-ray and Raman spectroscopy data support uranate, rather than uranyl, as the dominant U6+ speciation in the reported brannerites. It is considered that the U6+ concentration was limited by unfavourable electrostatic repulsion arising from substitution in the octahedral Th or Ti sites, which share two or three edges, respectively, with neighbouring polyhedra in the brannerite structure.

Synthesis and comprehensive synchrotron-based structural analysis of Si-doped nanodiamond composite films deposited on cemented carbide

Si-doped nanodiamond composite (NDC) films were deposited on unheated WC − Co substrates by coaxial arc plasma deposition for advanced cutting tools applications. The nanoindentation test revealed changes in films hardness and Young's modulus attributed to various Si doping concentrations (0, 1, 5, and 20 at.%) and the catalytic effects of Co diffused from the substrate surface into the films. To reveal the physical origin behind these changes, the films' structure was examined by synchrotron-based Auger electron spectroscopy (AES), soft X-ray photoemission spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The AES and XPS analysis showed a consistent correlation between nanoindentation measurements and the estimated C sp3 fraction in the films. The NEXAFS spectra revealed intense C − C σ* resonance, consistent with the measured film hardness. The 1 at.% Si-doped film, deposited with an undoped NDC buffer layer to mitigate the Co catalytic effects, exhibited the highest hardness of 60 GPa, the largest C sp3 fraction, and the most intense CC σ* peak. Si doping resulted in the formation of C − Si sp3 bonds at the expense of C sp2 bonds, increasing the fraction of C sp3 bonds and enhancing film hardness. The synchrotron-based analysis effectively revealed the electronic states of NDC hard coatings, particularly after the removal of surface contaminants through mechanical polishing.

Network formation and differentiation of chitosan–acrylic acid hydrogels using X-ray absorption spectroscopy and multivariate analysis of Fourier transform infrared spectra

Chitosan–acrylic acid (Cs–AA) hydrogels were prepared by blending different Cs concentrations and AA:Cs ratios. The Cs–AA hydrogel samples were analyzed using synchrotron radiation-induced near edge X-ray absorption fine structure (NEXAFS) and Fourier transform infrared (FTIR) spectroscopies. The characteristics of Cs were observed through the C, O, and N K edges in NEXAFS. Significant shifts in the Cs characteristic features were induced by the addition of AA. The NEXAFS findings confirmed the new covalent and ionic bridges to form the network of the Cs–AA hydrogel. Raw FTIR spectra were used to investigate the different functional groups of each of the macromolecules. The study attempted to discriminate the Cs–AA hydrogels prepared with different Cs wt% concentration and AA:Cs ratio using multivariate approaches. The preprocessed dataset was evaluated using principal component analysis (PCA) and hierarchical cluster analysis (HCA). The PCA and HCA methods showed the possibility of discriminating the Cs–AA hydrogel samples from the control samples and with each other. The approaches further confirmed that the CO groups found in the range of 1700–1800 cm−1 can be used as an identifier for Cs–AA hydrogel samples prepared using different solution parameters. The use of FTIR coupled with multivariate approaches provided a simple and reliable way to inspect the possible discrimination of Cs–AA hydrogels.

Making chemical sense of phase in soft X-ray spectroptychography

Spectroptychography is being used to realize a significant improvement in the spatial resolution of x-ray spectromicroscopy, allowing chemical microanalysis at finer spatial scales. The chemical sensitivity of near edge X-ray absorption fine structure (NEXAFS) is familiar to most researchers who use x-ray spectromicroscopy for chemical microanalysis. However, the additional phase information available through ptychography provides additional and tantalizing data, and potentially additional chemical information. This paper explores the chemical information available in phase for a system of silicon dioxide nanospheres.

Adsorption structure of iron phthalocyanine and titanyl phthalocyanine on Cu(1 1 1)

The adsorption structure of iron phthalocyanine (FePc) and titanyl phthalocyanine (TiOPc) was studied by a combination of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and normal incidence X-ray standing waves (NIXSW) technique. The FePc results demonstrate that the molecule adsorbs with the Fe metal centre at an adsorption height of 2.44 ± 0.09 Å, its macrocycle plane mostly parallel with the underlying surface and a single adsorption configuration. However, a small distortion of the isoindole groups, with respect to one another, is required to rationalise the results. The TiOPc results similarly indicate that the macrocycle plane is mostly parallel with the underlying surface up to thick multilayer films, yet, in the monolayer regime, the molecule must adsorb in multiple configurations. These configurations are nominally assigned to a mixture of adsorption configurations with some Ti=O bonds pointing towards the surface, and some pointing away. We determine that, in both configurations, the Ti metal centre sits at a similar adsorption height above the surface of 3.00 ± 0.20 Å.

A library of calcium mineral reference spectra recorded by parallel imaging using NEXAFS spectromicroscopy

Calcium minerals are ubiquitous in geology and life chemistry. Understanding the phase and chemical state of calcium minerals is important for numerous processes including materials chemistry, hard tissue biogenesis and geological processes. Photoemission spectroscopies such as near edge X-ray absorption fine structure (NEXAFS) and scanning transmission X-ray microscopy have been instrumental in identifying and characterizing calcium minerals in all these areas. In this work, we have recorded reference spectra for a range of different calcium minerals including a series of calcium carbonates, calcium oxalates and calcium phosphates. While collections of reference spectra for several calcium minerals can be found in the literature, these spectra have been reported in different contexts using a variety of instruments. We, here, report a comprehensive list of references recorded in parallel in a single experiment by imaging an array of calcium minerals using a NEXAFS microscope. We present reference NEXAFS spectra at the calcium L-, carbon K- and oxygen K-edges.

3d and 5d electronic structures and orbital hybridization in Ba- and Ca-doped La2CoIrO6 double perovskites

Here we present a detailed investigation of the Co and Ir local electronic structures in ${\mathrm{La}}_{1.5}{A}_{0.5}{\mathrm{CoIrO}}_{6}$ ($A=\mathrm{Ba}$, Ca) compounds in order to unravel the orbital hybridization mechanism in these CoIr-based double perovskites. Our results of x-ray powder diffraction, ac and dc magnetization, Co and Ir ${L}_{2,3}$-edge and Co $K$-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism suggest a competition between magnetic interactions. A dominant antiferromagnetic coupling is found to be responsible for the ferrimagnetic behavior observed for $A=\mathrm{Ca}$ below $\ensuremath{\sim}96\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the competing magnetic phases, and the cationic disorder in this compound giving rise to a spin-glass state at low temperatures. For the $A=\mathrm{Ba}$, on the other hand, there is no evidence of long-range order down to its spin-glass transition temperature. The remarkably different magnetic properties observed between these two compounds are discussed in terms of the structural distortion that alters the strength of the Co-Ir couplings, with a relevant role played by the Co $3d\phantom{\rule{4pt}{0ex}}{e}_{g}--\mathrm{Ir} 5d\phantom{\rule{4pt}{0ex}}{j}_{\mathrm{eff}}=1/2$ hybridization.

Elucidating the local structure of V substitutes in In2S3 as potential intermediate band material by x-ray absorption spectroscopy and first principles calculations

Vanadium doped indium sulphide, In2S3:V, is studied as a potential absorber material for intermediate band (IB) solar cells. Based on electronic considerations, it is usually assumed that V occupies octahedrally coordinated In sites, although geometrical considerations would favour tetrahedral In sites. In this study, we therefore combined experimental x-ray diffraction and x-ray absorption spectroscopy with ab initio theoretical calculations of both and phase to elucidate the incorporation of V in In2S3:V thin films grown with different V content and different growth temperatures. Comparing shape and position of the measured and calculated x-ray absorption edge of V, comparing experimentally determined and calculated V–S bond lengths, and evaluating the calculated heat of solution of V on different lattice sites all indicate that V is incorporated on octahedral rather than tetrahedral sites in the In2S3 matrix. For this material system, the electronic benefit of octahedral coordination thus outweighs the mechanical stress of the associated lattice relaxation. Finally, we studied the electronic structure of V-substituted - using hybrid density functional calculations and find that for a concentration of 1.9 at %, V on octahedrally coordinated In sites forms an empty IB isolated from valence band and conduction band (CB). By increasing the V content to 3.8 at %, however, the gap between IB and CB closes, which results in a reduction of the band gap. This differs from the electronic structure calculated for -:V and clearly demonstrates that both crystal structure and V incorporation site affect the resulting electronic material properties.

Quantitative evaluation of perfluorinated alkanethiol molecular order on gold surfaces.

Self-assembled monolayers (SAMs) of perfluoroalkanethiols [CF3(CF2)xCH2CH2SH (x = 3, 5, 7, and 9)] on gold were characterized by x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), and static time-of-flight secondary ion mass spectrometry (ToF-SIMS). Perfluoroalkanethiols of several chain lengths were synthesized using a known hydride reduction method for transforming commercially available perfluoroalkyliodides to corresponding perfluoroalkanethiols. This strategy provides improved product yields compared to other known routes based on hydrolysis from the common thioacetyl perfluoroalkyl intermediate. Angle-dependent XPS analysis revealed that CF3(CF2)xCH2CH2SH (x = 5, 7, and 9; F6, F8, and F10, respectively) SAMs on gold exhibited significant enrichment of the terminal CF3 group at the outer monolayer surface with the sulfur present as a metal-bound thiolate located at the monolayer-gold interface. XPS of the CF3(CF2)3CH2CH2SH (F4) monolayer revealed a thin film with a significant (>50%) amount of hydrocarbon contamination consistent with poorly organized monolayers, while the longest thiol (F10) showed XPS signals attributed to substantial ordering and anisotropy. ToF-SIMS spectra from all four SAMs contained molecular ions representative of the particular perfluorinated thiol used to prepare the monolayer. NEXAFS methods were used to determine degrees of ordering and average tilt for molecules comprising monolayers. The SAMs prepared from the longest (F10) thiols exhibited the highest degree of ordering with the molecular axis nearly perpendicular to the gold surface. The degree of ordering decreased significantly with decreasing length of the perfluorocarbon tail.

Enhanced phosphorus fixation in red mud-amended acidic soil subjected to periodic flooding-drying and straw incorporation

Globally, red mud is a solid waste from the aluminum industry, which is rich in iron oxides. It is an effective soil amendment in agriculture that protects connected waters from legacy diffuse phosphorus (P) soil losses. However, other management practices such as flooding and drying and/or organic carbon inputs could potentially alter P fixation in these red mud-amended soils thereby releasing P to waters. The present study was designed and conducted to monitor the mobilization of P in a red mud-amended acidic soil subjected to periodic flooding-drying, straw incorporation, and a mix of both management practices. Sequential extraction and K edge X-ray absorption near-edge structure spectroscopy (k-XANES) were employed to distinguish P fractions/species and the Langmuir model was fitted to evaluate soil P sorption capacity. The content of labile P indicated by CaCl2–P was increased significantly by 101% and 28.7% in the straw incorporation and periodic flooding-drying treatments, while it decreased significantly by 22.3% in the combined periodic flooding-drying with straw incorporation treatment, compared with Control. The inherent phosphate contained in sorghum straw, and the enhanced iron (Fe) reduction and dissolution of Calcium (Ca)-bound P induced by straw addition contributed to mobilization of P in the straw incorporation treatment. In contrast, the increased poorly crystalline Al/Fe oxides-bound P and occluded Fe-bound P fraction in the combined periodic flooding-drying with straw incorporation treatment explains the decrease in CaCl2–P. Furthermore, the increased soil P sorption capacity and the decreased P desorption rate were also responsible for the reduced P loss risk in the treatment. The results of structural equation modelling (SEM) indicated that organically complexed Fe and Fe-bound P were directly affecting P mobilization in the amended soil. Overall, the present study shows that appropriate flooding-drying events coupled with straw incorporation could be a mitigation practice for stabilizing P in red mud-amended soil. However, before it can be applied on a wide scale, multi-point and field trials should be carried out to further evaluate actual environmental implications.

Spatially-resolved dynamic sampling of different phasic magnetic resonances of nanoparticle ensembles in a magnetotactic bacterium Magnetospirillum magnetotacticum

Nanoscaled magnetic particle ensembles are promising building blocks for realizing magnon based binary logic. Element-specific real-space monitoring of magnetic resonance modes with sampling rates in the GHz regime is imperative for the experimental verification of future complex magnonic devices. Here we present the observation of different phasic magnetic resonance modes using the element-specific technique of time-resolved scanning transmission x-ray microscopy within a chain of dipolarly coupled Fe3O4 nanoparticles (40–50 nm particle size) inside a single cell of a magnetotactic bacterium Magnetospirillum magnetotacticum. The particles are probed with 25 nm resolution at the Fe L3 x-ray absorption edge in response to a microwave excitation of 4.07 GHz. A plethora of resonance modes is observed within multiple particle segments oscillating in- and out-of-phase, well resembled by micromagnetic simulations.

Revealing the Role of CO during CO2 Hydrogenation on Cu Surfaces with In Situ Soft X-Ray Spectroscopy.

The reactions of H2, CO2, and CO gas mixtures on the surface of Cu at 200 °C, relevant for industrial methanol synthesis, are investigated using a combination of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and atmospheric-pressure near edge X-ray absorption fine structure (AtmP-NEXAFS) spectroscopy bridging pressures from 0.1 mbar to 1 bar. We find that the order of gas dosing can critically affect the catalyst chemical state, with the Cu catalyst maintained in a metallic state when H2 is introduced prior to the addition of CO2. Only on increasing the CO2 partial pressure is CuO formation observed that coexists with metallic Cu. When only CO2 is present, the surface oxidizes to Cu2O and CuO, and the subsequent addition of H2 partially reduces the surface to Cu2O without recovering metallic Cu, consistent with a high kinetic barrier to H2 dissociation on Cu2O. The addition of CO to the gas mixture is found to play a key role in removing adsorbed oxygen that otherwise passivates the Cu surface, making metallic Cu surface sites available for CO2 activation and subsequent conversion to CH3OH. These findings are corroborated by mass spectrometry measurements, which show increased H2O formation when H2 is dosed before rather than after CO2. The importance of maintaining metallic Cu sites during the methanol synthesis reaction is thereby highlighted, with the inclusion of CO in the gas feed helping to achieve this even in the absence of ZnO as the catalyst support.

Metallocene-Filled Single-Walled Carbon Nanotube Hybrids.

In this paper, the growth mechanism, structure, growth processes, growth kinetics, and optical, vibronic and electronic properties of metallocene-filled single-walled carbon nanotubes (SWCNTs) are considered. A description of the procedures used to fill the nanotubes is provided. An investigation of doping effects on metallicity-mixed SWCNTs filled with metallocenes by Raman spectroscopy, near edge X-ray absorption fine structure spectroscopy, photoemission spectroscopy, and optical absorption spectroscopy is described. The studies of doping effects on metallicity-sorted SWCNTs filled with metallocenes are discussed. Doping effects in metallicity-mixed and sorted SWCNTs upon the chemical transformation of encapsulated molecules are analyzed. A discussion of the modification of the electronic properties of filled SWCNTs is presented. Applications of metallocene-filled SWCNTs in electrochemistry, thermoelectric power generation, chemical sensors, and magnetic recording are discussed.

NEXAFS spectra of model sulfide chains: implications for sulfur networks obtained from inverse vulcanization.

Inverse vulcanization is a promising route to stabilize sulfur in lithium-sulfur batteries, but the resulting sulfur strand lengths in the materials are elusive. We address the strand length by characterization via sulfur near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Theoretical predictions of NEXAFS spectra for model molecules containing strands with up to three sulfur atoms are verified by experiment. The near perfect agreement between simulation and experiment on the absolute energy scale allows for the predictions for larger chain lengths also. Inspection and interpretation of NEXAFS spectra from real battery materials on this basis reveals the appearance of single connecting sulfur atoms for very low sulfur content, and of longer strands when the sulfur fraction increases.

Further evidence in support of new empirical relation for chemical shift and effective charge in the X-ray absorption edge shifts

When a compound is formed, re-arrangement of electrons takes place. Consequently, the energies of core levels as well as outer states are altered. As a result, the position of the absorption edge undergoes shift in its energy. This shift in energy, when measured with respect to the pure metal, is called chemical shift, represented by ΔE. The concept of effective charge is found to be useful to understand chemical shift in X-ray absorption spectra. Validity of new empirical relation ΔE=aebq, between the chemical shift ΔE and effective charge ‘q’, has been demonstrated successfully using its linearized form ln|ΔE|=ln|a|+bq for a variety of binary and ternary cobalt compounds. As the proposed equation is empirical in nature, it is always desirable to examine its validity to as many systems as possible. The results obtained validate the correctness of exponential relation for chemical shift ΔE and effective charge ‘q’ proposed earlier.

Kinetics, Electronic Properties of Filled Carbon Nanotubes Investigated with Spectroscopy for Applications.

The paper is dedicated to the discussion of kinetics of growth, and electronic properties of filled carbon nanotubes investigated by spectroscopy for applications. The paper starts with discussion of growth of carbon nanotubes inside metallocene-filled carbon nanotubes. Nickelocene, cobaltocene are considered for growth of carbon nanotubes. Then, the investigations of filled carbon nanotubes by four spectroscopic techniques are discussed. Among them are Raman spectroscopy, near edge X-ray absorption fine-structure spectroscopy, photoemission spectroscopy, optical absorption spectroscopy. It is discussed that metal halogenides, metal chalcogenides, metals lead to changes in electronic structure of nanotubes with n- or p-doping. The filling of carbon nanotubes with different organic and inorganic substances results in many promising applications. This review adds significant contribution to understanding of the kinetics and electronic properties of filled SWCNTs with considering new results of recent investigations. Challenges in various fields are analyzed and summarized, which shows the author's viewpoint of progress in the spectroscopy of filled SWCNTs. This is a valuable step toward applications of filled SWCNTs and transfer of existing ideas from lab to industrial scale.

Fe,Mg-Codoped Bismuth Tantalate Pyrochlores: Crystal Structure, Thermal Stability, Optical and Electrical Properties, XPS, NEXAFS, ESR, and 57Fe Mössbauer Spectroscopy Study

The effect of Fe and Mg-codoping on the crystal structure, optical and dielectric properties of bismuth tantalate-based pyrochlores has been studied. Samples of Bi2MgxFe1−xTa2O9.5−Δ (x ≤ 0.7) are characterized by a porous dendrite-like microstructure. Fe,Mg-codoped bismuth tantalate pyrochlores are thermally stable up to a temperature of 1140 °C (x = 1). The Bi2Mg0.5Fe0.5Ta2O9.5−Δ thermal expansion coefficient increases uniformly and weakly from 3.6 to 9.3 × 10−6 °C−1 (30–1050 °C). The unit cell parameter of solid solutions increases uniformly from 10.5009(1) Å (x = 0.3) up to 10.5225(7) Å (x = 0.7). The structural parameters of disordered pyrochlore are determined by the Rietveld method (sp. gr. Fd3¯m:2 (227), Z = 8). According to near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy data, ions in solid solutions are in the charge states Bi (+3), Mg (+2), Fe (+3), Ta (+5-δ). The Mössbauer spectrum is represented by a symmetric doublet with parameters IS = 0.365 ± 0.0020 mm/s, QS = 0.604 ± 0.034 mm/s, related to Fe3+ ions in regular axial octahedral positions. The samples exhibit the properties of dielectrics. The permittivity and the tangent of dielectric losses at 20 °C increases with the growth of iron content in the samples in the range of 28.5–30.5 and 0.001 (1 MHz). The width of the band gap of the obtained materials for direct allowed electronic transitions is in the range of 2.16(5)–2.41(5) eV. The studied samples satisfy the condition of efficient conversion of solar energy into an electrical one and are promising as catalysts and light-absorbing elements for solar panels.

Overlooked role associated with the active-site density in perovskite nickelates to the anisotropic catalytic activities for water splitting

The d-band correlated rare-earth nickelate (ReNiO3) is a typical quantum material that exhibits comparable reactivities to the noble metal oxide in oxygen evolution reactions (OER) for water splitting, apart from their well-known correlated electronic functionalities, such as metal to insulator transition. Nevertheless, the potential anisotropy in the catalyst reactivity of OER for ReNiO3 and its underneath mechanisms are yet under debate. Herein, we demonstrate the previously overlooked role associated with the surface atomic density of the Ni active-site that dominant in the anisotropic OER catalytic activities of ReNiO3. Despite its more localized electron configurations as indicated by the near edge x-ray absorption fine structure analysis and correlated transport, the OER catalytic activity was surprisingly observed to be higher for quasi-single crystalline NdNiO3 (001)/LaAlO3 (110), compared to that of NdNiO3(010)/LaAlO3 (001) and NdNiO3(1¯10)/LaAlO3 (111). This is attributed to the highest surface atomic density associated with the Ni active-site within NdNiO3 (001), compared to NdNiO3 (010) and NdNiO3 (1¯10), and this kinetically reduces the overpotential of OER and the charge transfer resistance of NdNiO3 (001). The anisotropic OER activity sheds a light on the crystal orientation in the optimization of the ReNiO3 catalyst for water splitting.

Microalgae-derived single-atom oxygen reduction catalysts for zinc-air batteries

The need to fight against global warming has motivated the development of new technologies for green production of various chemicals and materials. Microalgae can convert CO2 into valuable biofuels, thus playing important roles in carbon neutrality. However, appropriately utilization of microalgae biomass remains an important issue to address. Here we demonstrate that the microalgae biomass with abundant membrane-bound biomolecules and singly isolated metal atoms can be upgraded into single-atom site catalysts (SACs) with high oxygen reduction reaction (ORR) activity. Through hydrothermal and pyrolysis treatment of microalgae, Fe–N4 structured SACs (malg-SACs) are obtained, as confirmed by near edge X-ray absorption fine structure (NEXAFS), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). Under optimal conditions, the malg-SAC exhibits a remarkable ORR activity (E1/2 = 0.875 V vs. RHE), outstanding long-term stability, and good resistance to CO and CH3OH poisoning. When assembled in a primary zinc-air battery, the full device reaches a maximum power density of 220.7 mW cm−2 with negligible voltage decay over the 6 h test, far outperforming the commercial Pt/C (112.1 mW cm−2). Our work reports a sustainable pathway to convert CO2-capture biomass into high-performance SACs, simultaneously addressing environmental and energy issues.

Spectroscopic Manifestations and Implications for Catalysis of Quasi-d10 Configurations in Formal Gold(III) Complexes.

Several gold +I and +III complexes are investigated computationally and spectroscopically, focusing on the d-configuration and physical oxidation state of the metal center. Density functional theory calculations reveal the non-negligible electron-sharing covalent character of the metal-to-ligand σ-bonding framework. The bonding of gold(III) is shown to be isoelectronic to the formal CuIII complex [Cu(CF3 )4 ]1- , in which the metal center tries to populate its formally unoccupied 3dx2-y2 orbital via σ-bonding, leading to a reduced d10 CuI description. However, Au L3 -edge X-ray absorption spectroscopy reveals excitation into the d-orbital of the AuIII species is still possible, showing that a genuine d10 configuration is not achieved. We also find an increased electron-sharing nature of the σ-bonds in the AuI species, relative to their AgI and CuI analogues, due to the low-lying 6s orbital. We propose that gold +I and +III complexes form similar bonds with substrates, owing primarily to participation of the 5dx2-y2 or 6s orbital, respectively, in bonding, indicating why AuI and AuIII complexes often have similar reactivity.