High-resolution X-ray Absorption Spectroscopy Research Articles

Local structural investigations of graphitic ZnO and reduced graphene oxide composite

Reduced graphene oxide (rGO) assisted two-dimensional (2D) graphitic ZnO (gZnO) have been prepared and characterized by Raman, FTIR, Cross-sectional high resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) techniques comprising of X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Raman spectroscopy and FTIR measurements suggest formation of ZnO and the same has been corroborated by XANES measurements. HRTEM and polarization dependent XANES measurements with two different sample orientations confirm the formation of gZnO/rGO sandwiched layers. Density Functional Theory simulated relaxed initial structure of bilayers of gZnO sandwiched between successive rGO layers fits the EXAFS data reasonably well, supported by formation energy calculations which indicate better stability for samples having bilayers to a few layers of gZnO between rGO layers than with monolayer. These relaxed structures also help to understand the electronic structure contributing in XANES spectrum, with the help of ligand field theory.

Gold speciation in hydrothermal fluids revealed by in situ high energy resolution X-ray absorption spectroscopy

Abstract Gold mobilization, transfer, and concentration in the Earth’s crust are controlled by hydrothermal sulfur- and chloride-bearing fluids. Yet the exact chemical identity, structure, and stability of Au-bearing species and, in particular, the respective contributions of the sulfide (HS−) and trisulfur ion (S3⋅−) ligands to Au transport lack direct in situ evidence. Here we employed high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/S3⋅−-bearing solutions at typical hydrothermal temperatures and pressures (T = 350 °C, P = 600 bar) to reveal differences in dissolved Au spectral signatures indicative of contrasting fluid-phase Au speciation as a function of acidity and redox conditions. Combined with in situ Au solubility measurements and quantum-chemical and thermodynamic modeling, our spectroscopic data provide direct evidence for the Au(HS)S3− and Au(HS)2− complexes predominant at acidic-to-neutral and alkaline conditions, respectively. Our findings thus directly confirm a recent speciation scheme for Au in aqueous S-bearing fluids established using less direct methods, and highlight an important role of the trisulfur ion in gold mobilization and concentration in hydrothermal-magmatic deposits associated with subduction zones. More generally, our results show that HERFD-XAS enables the identification of structural and coordination features in metal complexes virtually unresolvable using classical XAS techniques. By avoiding limitations of less direct techniques, our integrated high-resolution spectroscopic approach opens perspectives for studies of the speciation and solubility of gold and other metals in high T-P fluids, and potentially silicate melts, inaccessible to direct observation in nature.

Ligand size effects in two-dimensional hybrid copper halide perovskites crystals

Intensive effort to tailor photophysics of lead-free perovskites is appealing in recent years. However, their combined electronic and optical property elucidations remain elusive. Here, we report spectroscopic observations of the coexistence Zhang-Rice singlet state and exotic electronic transitions in two-dimensional copper-based perovskites. Herein, several perovskites with different alkylammonium spacers are investigated to unravel their correlated electronic systems and optical responses. Namely, methylammonium, ethylammonium, phenylmethylammonium and phenethylammonium. Using temperature dependent high-resolution X-ray absorption spectroscopy, we observe distinct electronic features highlighting the impact of short spacer chains compared to long-conjugated ligands, demonstrating a pronounced 3d9 and 3d9L signature linewidth variation. Corroborated by density functional theory calculations, the transient dynamics evolution of copper-based hybrid perovskites is influenced by the strong interplay of electron-phonon interactions and geometric constrictions. This finding sheds light on tuning the electronic and optical properties of hybrid perovskites towards efficient photoactive-based devices.

Electronic and crystal structures of LnFeAsO1−x H x (Ln = La, Sm) studied by x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction: II pressure dependence

We examine electronic and crystal structures of iron-based superconductors LnFeAsO1−x H x (Ln = La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe-K absorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES for Ln = La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when the T c approaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected for Ln = Sm. We reveal that the superconductivity is closely related to the lower spin state for Ln = La unlike Sm case. We observed that As height from the Fe basal plane and As–Fe–As angle on the FeAs4 tetrahedron for Ln = La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa for Ln = Sm.

Site‐Selective Real‐Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L‐Edge X‐Ray Absorption Spectroscopy**

Time‐resolved X‐ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X‐ray transitions with insights from high‐level ab initio calculations. The specific target has been a heteroleptic [IrIII(ppy)2(bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high‐resolution Ir L‐edge X‐ray absorption spectroscopy on the picosecond time scale and restricted active space self‐consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.

High-resolution X-ray absorption spectroscopy of alkanethiolate self-assembled monolayers on Au(111) and Ag(111)

High-resolution C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of non-substituted alkanethiolate self-assembled monolayers (SAMs) on Au(111) and Ag(111) were studied, with a particular emphasis on their fine structure and temperature dependence. Depending on the orientation of the light polarization with respect to the surface normal, the spectra were either dominated by the 'C−H' band, comprised of several σ*C−H / Rydberg resonances or exhibited several usually "hidden" features, including a sharp pre-edge resonance at ∼286.8 eV, related to the terminal methyl group of the SAM forming molecules and accompanied by a vibronic feature. The spectra exhibit pronounced reversible temperature dependence at going from cryogenic (60−70 K) to room temperature, including a shift of the 'C−H' band to lower energy by 250−300 meV, broadening and decrease in intensity of nearly all resonances, and disappearance of several "fine structure" features. This behavior was attributed to the nuclear motion effects, with the major impact of progressing orientational and conformational disorder in the SAMs, and correlated with the length of the of molecular backbone and specific character of the SAM-ambient interface.

Effect of germanium auto-diffusion on the bond lengths of Ga and P atoms in GaP/Ge(111) investigated by using X-ray absorption spectroscopy.

The germanium auto-diffusion effects on the inter-atomic distance between the nearest neighbors of the Ga atom in GaP epilayers are investigated using high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy. The GaP layers grown on Ge (111) are structurally coherent and relaxed but they show the presence of residual strain which is attributed to the auto-diffusion of Ge from the results of secondary ion mass spectrometry and electrochemical capacitance voltage measurements. Subsequently, the inter-atomic distances between the nearest neighbors of Ga atom in GaP are determined from X-ray absorption fine-structure spectra performed at the Ga K-edge. The estimated local bond lengths of Ga with its first and second nearest neighbors show asymmetric variation for the in-plane and out-of-plane direction of GaP/Ge(111). The magnitude and direction of in-plane and out-of-plane microscopic residual strain present in the GaP/Ge are calculated from the difference in bond lengths which explains the presence of macroscopic residual tensile strain estimated from HRXRD. Modified nearest neighbor configurations of Ga in the auto-diffused GaP epilayer are proposed for new possibilities within the GaP/Ge hetero-structure, such as the conversion from indirect to direct band structures and engineering the tensile strain quantum dot structures on (111) surfaces.

Electronic structures of Bi2Se3 and AgxBi2Se3 under pressure studied by high-resolution x-ray absorption spectroscopy and density functional theory calculations

The pressure-induced change in the electronic structures of the superconductors ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Ag}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ has been measured with high-resolution x-ray absorption spectroscopy. As a common feature for these compounds, we find that pressure causes the broadening of the Se $4p$ band and an energy shift of the Bi $6s$ band above the Fermi level up to the pressure of the first structural transition. These results, corroborated by density functional theory calculations, correlate with an increase of the carrier density, the disappearance of the band gap, and the emergence of superconductivity. The electronic structure changes significantly at the pressure of the first structural transition, which may be a trigger of the emergence of superconductivity, while above the pressure of the first phase transition it does not change much even around the second phase transition pressure, corresponding to the nearly constant ${T}_{\mathrm{c}}$ above the pressure of the second structural transition.

Tuning magnetic anisotropy by interfacial engineering in SrFeO2.5/La2/3Ba1/3MnO3/SrFeO2.5 trilayers

Exploring emergent phenomena in complex oxide heterostructures by interfacial engineering is the frontier of the oxide electronics. The heterointerface formed by oxides with different structures is particularly interesting since symmetry mismatch may produce considerable interface reconstruction and unexpected emergent phenomena. Here, we demonstrate the abnormal magnetic anisotropy in SrFeO2.5/La2/3Ba1/3MnO3/SrFeO2.5 trilayers that are consisted of the perovskite/brownmillerite heterostructures. The compressively strained La2/3Ba1/3MnO3 layer sandwiched between the two SrFeO2.5 layers exhibits in-plane magnetic anisotropy, while the La2/3Ba1/3MnO3 bare film with the same strain state shows perpendicular magnetic anisotropy at low temperature. The high-resolution scanning transmission electron microscope and x-ray absorption spectroscopy analysis reveal the off-center displacement of the Mn ions at interfaces. This would cause a strong orbital reconstruction of Mn ions at the interface and thus the in-plane magnetic anisotropy. This work shows the great potential to explore novel phenomena in artificially designed multilayers by interfacial engineering.

QEXAFS method implementation using adaptive X-ray optical elements

A modification of the of high-speed high-resolution X-ray absorption spectroscopy (Quick Extended X-ray Absorption Fine Structure, QEXAFS) method based on an adaptive bending element of X-ray optics is described. This technique was implemented at the Langmuir station of the Kurchatov Synchrotron Radiation Source (KISI-Kurchatov). The absorption spectrum of a model sample, bromine in a sodium bromide compound, was recorded. A recording speed up to 5 seconds per spectrum was achieved in the continuously recording regime. The method is applicable for obtaining information about both the structure and dynamics of a wide range of objects, ordered and disordered, in any aggregate state.

Enhanced ambient stability of exfoliated black phosphorus by passivation with nickel nanoparticles

Since its discovery, the environmental instability of exfoliated black phosphorus (2D bP) has emerged as a challenge that hampers its wide application in chemistry, physics, and materials science. Many studies have been carried out to overcome this drawback. Here we show a relevant enhancement of ambient stability in few-layer bP decorated with nickel nanoparticles as compared to pristine bP. In detail, the behavior of the Ni-functionalized material exposed to ambient conditions in the dark is accurately studied by Transmission Electron Microscopy (TEM), Raman Spectroscopy, and high resolution x-ray Photoemission and Absorption Spectroscopy. These techniques provide a morphological and quantitative insight of the oxidation process taking place at the surface of the bP flakes. In the presence of Ni nanoparticles (NPs), the decay time of 2D bP to phosphorus oxides is more than three time slower compared to pristine bP, demonstrating an improved structural stability within 20 months of observation.

In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts.

The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al2O3. Following the reductive activation, it was found that the NiCoO/γ-Al2O3 catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al2O3 catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions.

Measurement of f orbital hybridization in rare earths through electric dipole-octupole interference in x-ray absorption spectroscopy

Hybridization between $f$ and $p$ orbitals is relevant in many applications of rare-earth materials (catalysis, magnetism, nuclear industry) and in fundamental research (Kondo and heavy fermion systems). The authors investigated the ground state mixing between $f$ and $p$ orbitals in rare earths using high resolution x-ray absorption spectroscopy with linearly polarized x-rays, through the interference between electric dipole (\ensuremath{\Delta}L=1) and octupole (\ensuremath{\Delta}L=3) transitions. The $f\ensuremath{-}p$ hybridization was quantified by an hexadecapole moment using a sum rule. This approach has been put to the test on Gadolinium Gallium Garnet where $4f$ states are expected to be localized.

Baltic Fucus vesiculosus as potential bio-sorbent for Zn removal: Mechanism insight

This research aimed to find the best phenotype of the brown algae Fucus vesiculosus (kelp) which has the greater potential to become a sorption byproduct for Zn removal from contaminated waters. Thus, the Zn uptake capacity and sorption mechanisms of the kelp collected from the Baltic Sea shore was, for the first time, investigated under various conditions, and compared to the phenotype habiting on the Irish Sea shore. Sorption studies were performed investigating the effect of algal dosage, Zn sources as well as algal harvesting time of the year on Zn uptake capacity. The results suggested that the Baltic algae is a better bio-sorbent for Zn uptake. Sorption mechanisms were studied by employing various indirect and direct approaches, more importantly, including high resolution synchrotron X-Ray Fluorescence and X-Ray Absorption Spectroscopy (XAS) and molecular modelling (MM). The results revealed that alginate and cellulose are among the main polysaccharide bonding Zn at algal surface, via coordination with O atoms from carboxyl and hydroxyl groups. XAS results giving direct measurements of Zn bonding environment on algal surface are supported by MM outputs and suggested that Zn is surrounded by ca. 5 O atoms at interatomic distances varying from 1.94 to 2.02 Å. The results contribute to understanding sorption mechanisms which can further lead to finding the best eluent for Zn desorption from the used biomass, bio sorbent reconditioning and reuse in multiple sorption desorption cycles as well as process optimization before industrial scaling up.

Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge High Resolution X-ray Absorption Spectroscopy.

The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe–Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron–sulfur clusters. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco.

Speciation Change of Uranyl in Lithium Borate Glasses

Determining the uranyl(VI) UO22+ reactivity in crystalline and amorphous oxides is necessary to control its mobility. The intrinsic versatility of borate structural units containing both triangular BO3 and tetrahedral BO4 makes them original and rich hosts for uranyl. As part of the effort to determine the uranium stability in borate oxides, we have determined the speciation of uranium(VI) in two lithium borate glasses containing, respectively, 10 mol % and 30 mol % Li2O using a combined structural and spectroscopic approach based on X-ray absorption spectroscopy (XAS). M4- and L3-edge high-resolution XAS demonstrates the speciation of U(VI) as uranyl in both glasses. Comparison of uranyl bond distances obtained by EXAFS with distances found in borate crystals reveals that in the low alkali borate glass, uranyl is present as hexagonal bipyramids with six equatorial oxygen ligands. This local environment was never observed in any other oxide glass. We show that the increase of the lithium content induces the decrease of the equatorial coordination number. The associated uranyl bond elongation suggests the influence of the alkali cations in relation with drastic changes in the structure of the borate network. The spectroscopic evidence of this speciation change is discussed in terms of change in the uranyl electronic structure and covalency.

Electrochemical Adsorption on Pt Nanoparticles in Alkaline Solution Observed Using In Situ High Energy Resolution X-ray Absorption Spectroscopy

The oxygen reduction reaction (ORR) on Pt/C in alkaline solution was studied by in situ high energy resolution X-ray absorption spectroscopy. To discuss the X-ray absorption near-edge structure (XANES), this paper introduced the rate of change of the Δμ (RCD), which is an analysis method that is sensitive to surface adsorption. The surface adsorptions as hydrogen (below 0.34 V), superoxide anion (from 0.34 V to 0.74 V), hydroxyl species (from 0.44 V to 0.74 V), atomic oxygen (above 0.74 V), and α-PtO2 (above 0.94 V) were distinguished. It is clarified that the catalytic activity in an alkaline solution is enhanced by the stability of atomic oxygen and the low stability of superoxide anion/peroxide adsorption on the platinum surface.

New aspects in the study of carbon-hydrogen interaction in hydrogenated carbon nanotubes for energy storage applications

The paper presents the results of the combined investigation of changes in the carbon nanotubes structure caused by hydrogenation. Hydrogen adsorption by bundles of single-wall carbon nanotubes (SWNTs) was simulated by the molecular mechanics and DFT-LDA methods and applied to interpret the results of the experimental study carried out by using high-resolution X-ray absorption and photoelectron spectroscopy. As a result, new aspects of physical and chemical adsorption of hydrogen atoms on SWNTs were revealed, as well as preferable ways for SWNTs hydrogenation. Due to van-der-Waals interactions, location of hydrogen molecules inside the channels is energetically more favorable than that outside the channels. Chemical covalent bonds between hydrogen atoms and nanotube walls arise with formation of the C4H and C2H phases. However, not the entire surface of the SWNTs located inside the bundles is accessible for hydrogenation but only segments adjacent to triangular pores.

Effect of γ-manganite particle size on Zn2+ coordination environment during adsorption and desorption

The nanostructured manganese oxyhydroxides are great importance in controlling the environmental pollution. Here we investigated the properties of γ-manganite (γ-MnOOH) having two different particle sizes and their effects on Zn2+ coordination during adsorption and desorption, using a combination of X-ray diffraction analysis, thermal gravimetry, pore structure analysis, high-resolution transmission electron microscopy, acid-base titration and X-ray absorption spectroscopy. The small-particle-sized (SPS) γ-MnOOH with short aging time has a low degree of crystallinity, a large specific surface area, a low point of zero change and a low structural stability relative to the big-particle-sized (BPS) one. Comparison of natural sample with synthetic γ-MnOOH by atom distances indicates that the growth force along [11-1] direction is larger than that along [101] direction, although the particle size along the former direction is much smaller. Regarding the adsorption and desorption of Zn2+, the SPS γ-MnOOH has a higher adsorption density and lower desorption percentage than the BPS one under the same condition. Based on the desorption of Zn2+ adsorbed in corner-sharing linkage on γ-MnOOH surface, the linear combination fitting results indicate that, Zn2+ is mainly desorbed as Zn2+ four-fold coordination (ZnO4) complexes for low initial concentration of Zn2+ ([Zn]initial) and as Zn2+ six-fold coordination (ZnO6) complexes for high [Zn]initial; SPS γ-MnOOH has a higher ZnO4 percentage than the BPS one. The compensating charge capacity of ZnO6 and ZnO4 complexes in γ-MnOOH may increase in the linkage order: ZnO6-corner-sharing < ZnO4-corner-sharing < ZnO6-edge-sharing < ZnO4-edge-sharing. These results indicate that the SPS γ-MnOOH can better capture and fix Zn2+ during adsorption and desorption process.

High-Resolution Extended X-ray Absorption Fine Structure Analysis Provides Evidence for a Longer Fe···Fe Distance in the Q Intermediate of Methane Monooxygenase.

Despite decades of intense research, the core structure of the methane C-H bond breaking diiron(IV) intermediate, Q, of soluble methane monooxygenase remains controversial, with conflicting reports supporting either a "diamond" diiron core structure or an open core structure. Early extended X-ray absorption fine structure (EXAFS) data assigned a short 2.46 Å Fe-Fe distance to Q (Shu et al. Science 1997, 275, 515 ) that is inconsistent with several theoretical studies and in conflict with our recent high-resolution Fe K-edge X-ray absorption spectroscopy (XAS) studies (Castillo et al. J. Am. Chem. Soc. 2017, 139, 18024 ). Herein, we revisit the EXAFS of Q using high-energy resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) studies. The present data show no evidence for a short Fe-Fe distance, but rather a long 3.4 Å diiron distance, as observed in open core synthetic model complexes. The previously reported 2.46 Å feature plausibly arises from a background metallic iron contribution from the experimental setup, which is eliminated in HERFD-EXAFS due to the increased selectivity. Herein, we explore the origin of the short diiron feature in partial-fluorescent yield EXAFS measurements and discuss the diagnostic features of background metallic scattering contribution to the EXAFS of dilute biological samples. Lastly, differences in sample preparation and resultant sample inhomogeneity in rapid-freeze quenched samples for EXAFS analysis are discussed. The presented approaches have broad implications for EXAFS studies of all dilute iron-containing samples.