K-edge X-ray Absorption Near-edge Structure Spectra Research Articles

X-ray absorption spectroscopy and theoretical investigations of the effect of extended ligands in potassium organic matter interaction.

Potassium (K) is an essential nutrient for plant growth, and despite its abundance in soil, most of the K is structurally bound in minerals, limiting its bioavailability and making this soil K reservoir largely inaccessible to plants. Microbial biochemical weathering has been shown to be a promising pathway to sustainably increase plant available K. However, the mechanisms underpinning microbial K uptake, transformation, storage, and sharing are poorly resolved. To better understand the controls on microbial K transformations, we performed K K-edge x-ray absorption near-edge structure (XANES) spectroscopy on K-organic salts, including acetate, citrate, nitrate, oxalate, and tartrate, which are frequently observed as low molecular weight organic acids secreted by soil microbes, as well as humic acid, which acts as a proxy for higher molecular weight organic acids. The organic salts display feature-rich K XANES spectra, each demonstrating numerous unique features spanning ∼13eV range across the absorption edge. In contrast, the spectra for humic acid have one broad, wide feature across the same energy range. We used a combination of time-dependent density functional theory and the Bethe-Salpeter equation based approach within the OCEAN code to simulate the experimental spectra for K-nitrate (KNO3) and K-citrate [K3(C6H5O7)·H2O] to identify the electronic transitions that give rise to some of the outlying and unique spectral features in the organic salts. KNO3 has both the lowest and highest lying energy features, and K3(C6H5O7)·H2O is produced by several soil microbes and is effective at mineral weathering. Our results analyze the K-organic salt bonding in detail to elucidate why the spectral shapes differ and indicate that the K K-edge XANES spectra are associated with the entire ligand despite similar first-shell bonding environments around the K center. The improved understanding of K bonding environments with organic ligands and their use for interpretation of the K-XANES spectra provides an important toolkit to understand how K is transformed by microbial processes and made bioavailable for plant uptake.

Self-Organization of 1-Propanol at H-ZSM-5 Brønsted Acid Sites.

In situ Al K-edge X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) spectroscopy in conjunction with ab initio molecular dynamics (AIMD) simulations show that adsorption of 1-propanol alters the structure of the Brønsted acid site through changes in the associated aluminum-oxygen tetrahedron in zeolite H-MFI. The decreasing intensity of the pre-edge signal of the in situ Al K-edge XANES spectra with increasing 1-propanol coverage shows that Al T-sites become more symmetric as the sorbed alcohol molecules form monomers, dimers, and trimers. The adsorption of monomeric 1-propanol on Brønsted acid sites reduces the distortion of the associated Al T-site, shortens the Al-O distance, and causes the formation of a Zundel-like structure. With dimeric and trimeric alcohol clusters, the zeolite proton is fully transferred to the alcohols and the aluminum-oxygen tetrahedron becomes fully symmetric. The subtle changes in Al-K-edge XANES in the presence of sorbate structures, with the use of theory, are used to probe the local zeolite structures and provide a basis to predict the population and chemical state of the sorbed species.

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N 5 Site for Enhanced Electrocatalytic Reaction

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N ₅ Site for Enhanced Electrocatalytic Reaction

Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne

Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne

Accurate, affordable, and generalizable machine learning simulations of transition metal x-ray absorption spectra using the XANESNET deep neural network.

The affordable, accurate, and generalizable prediction of spectroscopic observables plays a key role in the analysis of increasingly complex experiments. In this article, we develop and deploy a deep neural network-XANESNET-for predicting the lineshape of first-row transition metal K-edge x-ray absorption near-edge structure (XANES) spectra. XANESNET predicts the spectral intensities using only information about the local coordination geometry of the transition metal complexes encoded in a feature vector of weighted atom-centered symmetry functions. We address in detail the calibration of the feature vector for the particularities of the problem at hand, and we explore the individual feature importance to reveal the physical insight that XANESNET obtains at the Fe K-edge. XANESNET relies on only a few judiciously selected features-radial information on the first and second coordination shells suffices along with angular information sufficient to separate satisfactorily key coordination geometries. The feature importance is found to reflect the XANES spectral window under consideration and is consistent with the expected underlying physics. We subsequently apply XANESNET at nine first-row transition metal (Ti-Zn) K-edges. It can be optimized in as little as a minute, predicts instantaneously, and provides K-edge XANES spectra with an average accuracy of ∼±2%-4% in which the positions of prominent peaks are matched with a >90% hit rate to sub-eV (∼0.8eV) error.

Development of electrically conductive ZrO2-CaO-Fe2O3-V2O5 glass and glass-ceramics as a new cathode active material for Na-ion batteries with high performance

Glass-ceramics xZrO2•10Fe2O3•(90-x)V2O5 with ‘x’ between 0 and 30 mol% and yZrO2･(20-y)CaO･10Fe2O3･70 V2O5 glass with ‘y’ between 0 and 20 mol%, respectively abbreviated as xZFV and yZCFV, before and after heat treatment at 500 °C for 100 min, were evaluated as potential cathode-active materials for sodium-ion batteries (SIBs). Relationships between physical properties and local structure of xZFV and yZCFV glass-ceramics were investigated by 57Fe-Mössbauer spectroscopy, V K-edge X-ray absorption near edge structure (XANES), X-ray diffractometry (XRD), DC four-probe method and differential thermal analysis (DTA). SIBs containing heat-treated xZFV glass-ceramics showed the highest discharge capacity of 153 mA h g−1 under a current density of 50 mA•g−1, which exhibited a high electrical conductivity of 1.8 × 10−2 Scm−1. Precipitation of V0.05Zr0.95O2 and Fe2V4O13 nanoparticles were confirmed from the XRD pattern of the heat-treated 20ZFV glass, consistent with the lower energy of the pre-edge peak at 5467 eV in the V K-edge XANES spectrum. This result is associated with the reduction of vanadium ions from VV to VIV. It is concluded that the precipitation of stable vanadium bronze phases with high electrical conductivity and structural stability effectively enable the high SIB capacity of these materials.

Electronic properties, bonding structure and mechanical behaviours of a-CNx: Si (: B) thin films

The electronic properties, bonding structure and mechanical behaviours of amorphous carbon nitride (a-CNx), silicon doped a-CNx (a-CNx:Si) and boron doped a-CNx (a-CNx:B) thin films alloys were studied by the C K-edge, N K-edge x-ray absorption near edge structure (XANES) and valence band photoelectron spectroscopy (VB-PES). The a-CNx:Si and a-CNx:B shows higher Young’s modulus compare to a-CNx thin film that are due to increase of carbon sp3-hybridized cluster in C and N K-edge XANES spectra. We have observed from the C and N K-edge XANES spectra that the formation of different carbon, nitrogen, silicon and boron bonding with carbon in the a-CNx:Si(:B) film structure. VB-PES spectra showed that the p-σ band became more intense than the p-π band upon the addition of silicon and boron in a-CNx film structure, which confirmed the role played by the σ bonds in controlling the electronic structure of a-CNx films and hence their mechanical behaviour. These films could be very useful in the surface coating technology.

Water electrolysis system based on a lithium cation-exchange membrane

The electronic properties, bonding structure and mechanical behaviours of amorphous carbon nitride (a-CNx), silicon doped a-CNx (a-CNx:Si) and boron doped a-CNx (a-CNx:B) thin films alloys were studied by the C K-edge, N K-edge x-ray absorption near edge structure (XANES) and valence band photoelectron spectroscopy (VB-PES). The a-CNx:Si and a-CNx:B shows higher Young’s modulus compare to a-CNx thin film that are due to increase of carbon sp3-hybridized cluster in C and N K-edge XANES spectra. We have observed from the C and N K-edge XANES spectra that the formation of different carbon, nitrogen, silicon and boron bonding with carbon in the a-CNx:Si(:B) film structure. VB-PES spectra showed that the p-σ band became more intense than the p-π band upon the addition of silicon and boron in a-CNx film structure, which confirmed the role played by the σ bonds in controlling the electronic structure of a-CNx films and hence their mechanical behaviour. These films could be very useful in the surface coating technology.

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Summary Atomically dispersed Fe–N–C catalyst has shown great performance in the CO2-to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe–N–C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FECO) above 90% over a broad window from −0.3 to −0.8 V (versus RHE) (in particular, FECO > 97% at −0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H2 generation but also promote CO2 activation and protonation with the assistance of co-adsorbed H2O on the “atomic interface” of “FeN4/graphitic N.”

Structural and electronic structure investigations on sonication based synthesized graphene oxide and reduced-graphene oxide nano-sheets

In the present study, we applied a sonication based method to achieve graphene oxide (GO) and reduced graphene oxide (r-GO) nano-structures. X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM) and x-ray absorption near edge structure (XANES) spectroscopy measurements were applied to characterize the samples. XRD, TEM and Raman results demonstrate significant crystalline nature of GO nano sheets. On the other hand, the r-GO sample shows poor crystallinity with a large number of lattice defects. The FTIR spectrum conveys the attachment of significant oxygen-containing functional groups in GO sample. The detachment of oxygen function groups from the r-GO sample, under the reduction process, is also confirmed by the FTIR spectra. Local electronic structure investigations, from the C K-edge XANES of GO and r-GO nano sheets, convey the lowering of 1s → π* and 1s → σ* transitions and lesser sp2 character of the carbon ring. The O K-edge XANES spectra have shown the accumulation of C = O σ*, C–O σ* and O–H σ* states at the basal plane of GO and elimination of such functional groups in the r-GO samples and convey controlled tailoring of the functionalities of GO and r-GO nano sheets.

Synchrotron X-ray absorption spectroscopy and cation distribution studies of NiAl2O4, CuAl2O4, and ZnAl2O4 nanoparticles synthesized by sol-gel auto combustion method

An array of modern approaches to X-ray including X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were employed to examine the local atomic structure, cation distribution, and degree of inversion of nanocrystalline metal aluminate MAl2O4 (M = Ni, Cu, Zn) powders produced using the sol-gel auto combustion technique. All samples were revealed to display single-phase cubic spinel structure with Fd-3m space group, as established by the XRD results. The cation distribution within these aluminate spinel structures were completely unique as suggested by the divergent disparity in comparative intensity between (220), (400), (422), and (440) diffraction plans for each XRD pattern. Rietveld refinement examination of the XRD data validates the ZnAl2O4 samples as having normal spinel structure, while CuAl2O4 and NiAl2O4 samples comprise partially-inverse spinel structures with low and high degree of inversion, respectively. The investigative findings gained from experimental Ni, Cu, Zn, and Al K-edge XANES spectra accommodated with the hypothetical computation of absorption spectra using the FEFF9.7 code reinforce this. Four-fold synchronization with oxygen atoms was demonstrated by all Zn ions discovered in the ZnAl2O4 samples. Conversely, the Ni and Cu ions located in the NiAl2O4 and CuAl2O4 samples tend to be synchronized in both tetrahedral and octahedral fashion by four or six oxygen atoms, respectively. Additionally, M2+ altering from Zn2+ ions to be Cu2+ and Ni2+ ions caused a substantial translocation of divalent metal (M2+) ions from the tetrahedral (A) sites to octahedral (B) sites, as clearly demonstrated by the EXAFS spectra. Specifically, the degree of inversion in these metal aluminates is completely dissimilar and identified as 0.83 for NiAl2O4, 0.42 for CuAl2O4 and 0 for ZnAl2O4, as indicated by the curve-fitting analysis of Ni, Cu, and Zn K-edge EXAFS spectra. These values comprise vital data for comprehension of numerous pertinent properties.

Probing local structure of ZnO-modified Pb(Mg1/3Nb2/3)0.9Ti0.1O3 ceramic with synchrotron X-ray absorption spectroscopy technique

ABSTRACTThis work aims to study the effects of ZnO modifications on local structures of Pb(Mg1/3Nb2/3)0.9Ti0.1O3 (PMNT) ceramics. Different contents of ZnO (0, 0.4, 2.0, 4.0 and 11.0 mol%) were added into PMNT ceramics. The local structures of the samples were characterized by synchrotron X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of both Zn and Ti K-edges were displayed. The Zn K-edge energy values of all samples were in the range of 9660–9666 eV. The Zn ions incorporated in all ceramics were in the form of Zn2+. The value of Ti K-edge energy, which was equal to 4979 eV, did not change with ZnO modifications. Comparing the experimental Zn K-edge XANES spectra to the spectra simulated via FTFF program confirmed the substitution of the Zn ions for B-site ions. The intensity and the area under curve of the pre-edge peak of Ti K-edge gradually increased with increasing ZnO content up to 4.0 mol%. This suggested a gradual change in the displacement out from octahedral centers of Ti ions. This could be an evidence of the enhancement in ferroelectric order of ZnO-modified PMNT ceramics.

Structural characterization of Zr-doped ZnO films deposited on quartz substrates by reactive radio frequency magnetron co-sputtering

Zr-doped ZnO (Zn1−xZrxO, 0 ≤ x ≤ 0.067; ZZO) thin films were grown at room temperature by reactive radio frequency co-sputtering on quartz substrates The ZZO films have a strong preferred orientation toward the c-axis but this orientation weakened as x increased. The size of crystallites of the ZZO films decreased and the surfaces of the ZZO films smoothened as x increased. The Zn atom were divalent and slightly affected only in the c-axis. Zn K-edge X-ray absorption near edge structure (XANES) spectra of the ZZO films show that the Zn atom were divalent and slightly affected only in the c-axis. The Zr atoms were tetravalent and their K-edge XANES spectra were slight different from that of ZrO2. Polarization-dependent Zn K-edge extended x-ray absorption fine-structure (EXAFS) spectra revealed that the local structural variation was stronger along the c-axis than along other axes. The Zn atom site was well replaced by Zr atom regardless of x. Average optical transmittance in the visible region was ≥90%, and as x increased, optical band gap increased and blue-shifted.

RETRACTED ARTICLE: Investigation of Co-Doped ZnO Nanowires by X-ray Absorption Spectroscopy and Ab Initio Simulation

The local structure of single room- and high-temperature Co-implanted ZnO nanowires with subsequent thermal annealing has been studied using hard-x-ray techniques in combination with ab initio Zn K-edge x-ray absorption near-edge structure (XANES) simulations. X-ray fluorescence data reveal a homogeneous distribution of Co atoms/ions with concentration of about 0.1 at.% to 0.3 at.% in the nanowires. XANES data indicate substitutional incorporation of Co2+ ions at Zn sites in both types of nanowire. Improved structural order around Co atoms is obtained in nanowires with high-temperature ion implantation followed by thermal annealing. The ab initio Zn K-edge simulations not only confirm recovery of implantation-induced damage in the ZnO host lattice by the thermal annealing process, but also assist in studying the effect of oxygen vacancies in the Zn K-edge XANES spectra. Microphotoluminescence data certify that high-temperature ion implantation with subsequent thermal annealing is an effective approach to achieve the strongest optical activation of Co ions and good energy transfer to Co ions from the ZnO host matrix.

Effect of electric field on local structure of PZT single crystal studied by X-ray absorption spectroscopy technique

ABSTRACTThe aim of this work is to investigate the local structure of PbZr1−xTixO3 (PZT) single crystal with x = 0.44 under the application of electric field employing Synchrotron X-ray Absorption Near-Edge Structure (XANES) technique. In this experiment, PZT single crystal was subjected to 0.35 kV/mm and 0.70 kV/mm electric fields. The Ti K-edge XANES spectra reflect the significant change of the local structure of PZT unit cell before and after static electrical loading. From the XANES results, it was clearly seen that XANES spectral intensity gradually changed when the electrical load was applied. This can be caused by the shift of Ti atoms in the unit cell. The clear agreement between the measured and simulated XANES spectra was the supporting evidence of Ti off-center displacement in the PZT unit cell.

Mechanistic insights on the electronic properties and electronic/atomic structure aspects in orthorhombic SrVO3 thin films: XANES-EXAFS study.

Correlations among the B-O6 octahedra distortions, existing polymorphous phases, band structures and electronic conductivities of ABO3 perovskites are matters for debate and require a deep understanding of their local atomic/electronic structures and diverse assets. In this study, to illustrate the distortion in V-O6 octahedra and its implication on the band structure and electronic properties, spectroscopic investigations on the RF-sputtering grown insulating SrVO3 thin films were employed using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). V K-edge and V L3,2-edge XANES, along with atomic multiplet calculations, have confirmed the 4+ oxidation state of V ions in the pristine and annealed SrVO3 thin films. Lower t2g/eg peak intensity ratio and smaller energy separation between t2g and eg peaks in the O K-edge XANES spectra, compared to the VO2 reference sample, have confirmed a larger V-O6 distortion in the orthorhombic SrVO3 thin films. Moreover, from the EXAFS data analysis, the local orthorhombic structure has been identified in the pristine and annealed SrVO3 thin films, compelling significant distortion in the V-O6 octahedra. Dimerization in the vanadium chains and V-V twisting, caused by V-O6 octahedra distortion, manifests a miscellaneous ligand field interaction between O 2p and V 3d orbitals and facilitates (i) a larger separation between the bonding and antibonding d‖ orbitals and (ii) an upward shift of the π* band in the band structure, leading to larger band gaps in the insulating SrVO3 thin films. Our spectroscopy results may open up new avenues for the mechanism of insulating/conducting character in other complicated perovskite materials using XANES-EXAFS.

Identification of Second Shell Coordination in Transition Metal Species Using Theoretical XANES: Example of Ti-O-(C, Si, Ge) Complexes.

X-ray absorption near-edge structure (XANES) is a common technique for elucidating oxidation state and first shell coordination geometry in transition metal complexes, among many other materials. However, the structural information obtained from XANES is often limited to the first coordination sphere. In this study, we show how XANES can be used to differentiate between C, Si, and Ge in the second coordination shell of Ti-O-(C, Si, Ge) molecular complexes based on differences in their Ti K-edge XANES spectra. Experimental spectra were compared with theoretical spectra calculated using density functional theory structural optimization and ab initio XANES calculations. The unique features for second shell C, Si, and Ge present in the Ti K pre-edge XANES are attributed to the interaction between the Ti center and the O-X (X = C, Si, or Ge) antibonding orbitals.

Standard Protocol and Quality Assessment of Soil Phosphorus Speciation by P K-Edge XANES Spectroscopy.

Phosphorus (P) in soils is most often bound as phosphate to one or more of the following four elements or compounds: calcium, aluminum, iron, and soil organic matter. A promising method for direct P speciation in soils is synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy at the K-edge of P. However, the quality of this method is debated controversially, partly because a standard protocol for reproducible spectrum deconvolution is lacking and minor modifications of the applied deconvolution procedure can lead to considerable changes in the P speciation results. On the basis of the observation that appropriate baseline correction and edge-step normalization are crucial for correct linear combination (LC) fitting results, we established a standard protocol for the deconvolution and LC fitting of P K-edge XANES spectra. We evaluated the quality of LC fits obtained according to this standard protocol with 16 defined dilute (2 mg P g(-1)) ternary mixtures of aluminum phosphate, iron phosphate, hydroxyapatite, and phytic acid in a quartz matrix. The LC fitting results were compared with the contribution of the different P compounds to total P in the various mixtures. Compared to using a traditional LC fitting procedure, our standard protocol reduced the fitting error by 6% (absolute). However, P portions smaller than 5% should be confirmed with other methods or excluded from the P speciation results. A publicly available database of P K-edge XANES reference spectra was initiated.

XANES measurements probing the local order and electronic structure of Pb1−xBaxZr0.40Ti0.60O3 ferroelectric materials

In this study, the electronic and local structures of Pb1−xBaxZr0.40Ti0.60O3 ferroelectric ceramic samples were characterized using X-ray absorption near-edge structure (XANES) measurements. The analysis of XANES spectra collected at the Ti K- and L-edges showed that the substitution of Pb by Ba leads to a decrease in the local distortion around the Ti atoms in the TiO6 octahedron. The analysis of O K-edge XANES spectra and density of states ab initio calculations showed that the hybridization between the O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization.