Multi-edge X-ray Absorption Spectroscopy Research Articles

Examining the Effect of Cu and Mn Dopants on the Structure of Zinc Blende ZnS Nanopowders.

It is known that doping zinc sulfide (ZnS) nanoparticles with Mn or Cu ions significantly affects their luminescent properties. Herein, we investigated how dopant atoms are incorporated into the structure of ZnS using X-ray diffraction and multi-edge X-ray absorption spectroscopy. The observed broadening of the X-ray diffraction patterns indicates an average crystallite size of about 6 nm. By analyzing the Zn, Mn, and Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra using the reverse Monte Carlo method, we were able to determine the relaxations of the local environments around the dopants. Our findings suggested that upon the substitution of Zn by Mn or Cu ions, there is a shortening of the Cu-S bonds by 0.08 Å, whereas the Mn-S bonds exhibited lengthening by 0.07 Å. These experimental results were further confirmed by first-principles density functional theory calculations, which explained the increase in the Mn-S bond lengths due to the high-spin state of Mn2+ ions.

Local structure and magnetic properties of a nanocrystalline Mn-rich Cantor alloy thin film down to the atomic scale

The huge atomic heterogeneity of high-entropy materials along with a possibility to unravel the behavior of individual components at the atomic scale suggests a great promise in designing new compositionally complex systems with the desired multi-functionality. Herein, we apply multi-edge X-ray absorption spectroscopy (extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and X-ray magnetic circular dichroism (XMCD)) to probe the structural, electronic, and magnetic properties of all individual constituents in the single-phase face-centered cubic (fcc)-structured nanocrystalline thin film of Cr20Mn26Fe18Co19Ni17 (at.%) high-entropy alloy on the local scale. The local crystallographic ordering and component-dependent lattice displacements were explored within the reverse Monte Carlo approach applied to EXAFS spectra collected at the K absorption edges of several constituents at room temperature. A homogeneous short-range fcc atomic environment around the absorbers of each type with very similar statistically averaged interatomic distances (2.54–2.55 Å) to their nearest-neighbors and enlarged structural relaxations of Cr atoms were revealed. XANES and XMCD spectra collected at the L2,3 absorption edges of all principal components at low temperature from the oxidized and in situ cleaned surfaces were used to probe the oxidation states, the changes in the electronic structure, and magnetic behavior of all constituents at the surface and in the sub-surface volume of the film. The spin and orbital magnetic moments of Fe, Co, and Ni components were quantitatively evaluated. The presence of magnetic phase transitions and the co-existence of different magnetic phases were uncovered by conventional magnetometry in a broad temperature range.

The more the better: on the formation of single-phase high entropy alloy nanoparticles as catalysts for the oxygen reduction reaction.

High entropy alloys (HEAs) are an important new material class with significant application potential in catalysis and electrocatalysis. The entropy-driven formation of HEA materials requires high temperatures and controlled cooling rates. However, catalysts in general also require highly dispersed materials, i.e., nanoparticles. Only then a favorable utilization of the expensive raw materials can be achieved. Several recently reported HEA nanoparticle synthesis strategies, therefore, avoid the high-temperature regime to prevent particle growth. In our work, we investigate a system of five noble metal single-source precursors with superior catalytic activity for the oxygen reduction reaction. Combining in situ X-ray powder diffraction with multi-edge X-ray absorption spectroscopy, we address the fundamental question of how single-phase HEA nanoparticles can form at low temperatures. It is demonstrated that the formation of HEA nanoparticles is governed by stochastic principles and the inhibition of precursor mobility during the formation process favors the formation of a single phase. The proposed formation principle is supported by simulations of the nanoparticle formation in a randomized process, rationalizing the experimentally found differences between two-element and multi-element metal precursor mixtures.

Inner relaxations in equiatomic single-phase high-entropy cantor alloy

The superior properties of high-entropy multi-functional materials are strongly connected with their atomic heterogeneity through many different local atomic interactions. The detailed element-specific studies on a local scale can provide insight into the primary arrangements of atoms in multicomponent systems and benefit to unravel the role of individual components in certain macroscopic properties of complex compounds. Herein, multi-edge X-ray absorption spectroscopy combined with reverse Monte Carlo simulations was used to explore a homogeneity of the local crystallographic ordering and specific structure relaxations of each constituent in the equiatomic single-phase face-centered cubic CrMnFeCoNi high-entropy alloy at room temperature. Within the considered fitting approach, all five elements of the alloy were found to be distributed at the nodes of the fcc lattice without any signatures of the additional phases at the atomic scale and exhibit very close statistically averaged interatomic distances (2.54-2.55 \r{A}) with their nearest-neighbors. Enlarged structural displacements were found solely for Cr atoms. The macroscopic magnetic properties probed by conventional magnetometry demonstrate no opening of the hysteresis loops at 5 K and illustrate a complex character of the long-range magnetic order after field-assisted cooling in $\pm$5 T. The observed magnetic behavior is assigned to effects related to structural relaxations of Cr. Besides, the advantages and limitations of the reverse Monte Carlo approach to studies of multicomponent systems like high-entropy alloys are highlighted.

Revealing the local structure of CuMo[formula omitted]W[formula omitted]O[formula omitted] solid solutions by multi-edge X-ray absorption spectroscopy

The effect of tungsten substitution with molybdenum on the structure of CuMo1−xWxO4 (x=0.20, 0.30, 0.50, 0.75) solid solutions was studied by multi-edge X-ray absorption spectroscopy. The simultaneous analysis of EXAFS spectra measured at several (Cu K-edge, Mo K-edge and W L3-edge) absorption edges was performed by the reverse Monte Carlo method taking into account multiple-scattering effects. The degree of distortion of the coordination shells and its dependence on the composition were estimated from partial radial distribution functions (RDFs) g(r) and bond angle distribution functions (BADFs) f(φ). The analysis of partial RDFs suggests that the structure of solid solutions is mainly determined by the tungsten-related sublattice, while molybdenum atoms adapt to a locally distorted environment. As a result, the coordination of both tungsten and molybdenum atoms remains octahedral as in CuWO4 for all the studied compositions.

Structural dynamics of an iron molybdate catalyst under redox cycling conditions studied with in situ multi edge XAS and XRD.

The structural dynamics and phase transformations of an iron molybdate catalyst with excess molybdenum trioxide (Mo/Fe = 2.0) were studied during redox cycling of the catalyst using in situ multi-edge X-ray absorption spectroscopy (XAS) at the Mo K-edge (transmission mode) and Fe K-edge (fluorescence mode) in combination with X-ray diffraction (XRD). X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis showed that heating under reducing conditions with methanol up to 400 °C produced MoO2 and FeMoO4. Linear combination fitting (LCF) analysis showed that iron was reduced completely, while molybdenum remained partly oxidized (60% as Mo(vi)). Complementary in situ XRD also supported the phase transformation due to reduction of Fe2(MoO4)3 and MoO3 to FeMoO4 and MoO2. Subsequent heating under oxidative conditions from 200 to 500 °C transformed the catalyst into its initial state via Fe2O3 and extra MoO3 as intermediate phases. This underlines the segregation and iron enrichment during redox cycling. MoO3 volatilization, observed under industrial reaction conditions of a methanol and oxygen containing atmosphere, causes this segregation to be permanent. Complete regeneration could only be achieved at 500 °C, which is significantly higher than industrial reaction temperatures. Overall, multi edge in situ XAS along with complementary XRD was found to be an ideal tool for tracing the different amorphous and crystalline phases present during redox cycling of the catalyst.

The Myth of d8 Copper(III).

Seventeen Cu complexes with formal oxidation states ranging from CuI to CuIII are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally CuIII species is extremely covalent, resulting in Cu K-edge and L2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L2,3-edge data reveals that all formally CuIII species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d8 description for these formally CuIII species is inaccurate. This study provides an alternative explanation for the competence of formally CuIII species in transformations that are traditionally described as metal-centered, 2-electron CuI/CuIII redox processes.

Chalcogen Impact on Covalency within Molecular [Cu3(μ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study.

Reaction of the tricopper(I)-dinitrogen tris(β-diketiminate) cyclophane, Cu3(N2)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu3(μ3-O)L (2) or the corresponding Cu3(μ3-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L2,3-edge X-ray absorption data quantify 49.5% Cu 3d character in the lowest unoccupied molecular orbital of 2, with Cu 3d participation decreasing to 33.0% in 4 and 40.8% in the related sulfide cluster Cu3(μ3-S)L (3). Multiedge XAS and UV/visible/near-IR spectra are employed to benchmark density functional theory calculations, which describe the copper-chalcogen interactions as highly covalent across the series of [Cu3(μ-E)]3+ clusters. This result highlights that the metal-ligand covalency is not reserved for more formally oxidized metal centers (i.e., CuIII + O2- vs CuII + O-) but rather is a significant contributor even at more typical ligand-field cases (i.e., Cu3II/II/I + E2-). This bonding is reminiscent of that observed in p-block elements rather than in early-transition-metal complexes.

Multi-edge X-ray absorption spectroscopy study of road dust samples from a traffic area of Venice using stoichiometric and environmental references

The appropriate selection of representative pure compounds to be used as reference is a crucial step for successful analysis of X-ray absorption near edge spectroscopy (XANES) data, and it is often not a trivial task. This is particularly true when complex environmental matrices are investigated, being their elemental speciation a priori unknown. In this paper, an investigation on the speciation of Cu, Zn, and Sb based on the use of conventional (stoichiometric compounds) and non-conventional (environmental samples or relevant certified materials) references is explored. This method can be useful in when the effectiveness of XANES analysis is limited because of the difficulty in obtaining a set of references sufficiently representative of the investigated samples. Road dust samples collected along the bridge connecting Venice to the mainland were used to show the potentialities and the limits of this approach.

Distorted tetrahedral nickel-nitrosyl complexes: spectroscopic characterization and electronic structure.

The linear nickel-nitrosyl complex [Ni(NO)(L3)] supported by a highly hindered tridentate nitrogen-based ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3), was prepared by the reaction of the potassium salt of the ligand with the nickel-nitrosyl precursor [Ni(NO)(Br)(PPh 3 ) 2 ]. The obtained nitrosyl complexes as well as the corresponding chlorido complexes [Ni(NO)(Cl)(PPh 3 ) 2 ] and [Ni(Cl)(L3)] were characterized by X-ray crystallography and different spectroscopic methods including IR/far-IR, UV-Vis, NMR, and multi-edge X-ray absorption spectroscopy at the Ni K-, Ni L-, Cl K-, and P K-edges. For comparative electronic structure analysis we also performed DFT calculations to further elucidate the electronic structure of [Ni(NO)(L3)]. These results provide the nickel oxidation state and the character of the Ni-NO bond. The complex [Ni(NO)(L3)] is best described as [Ni (II) (NO (-) )(L3)], and the spectroscopic results indicate that the phosphane complexes have a similar [Ni (II) (NO (-) )(X)(PPh 3 ) 2 ] ground state.

Multi-edge X-ray absorption spectroscopy of thorium, neptunium and plutonium hexacyanoferrate compounds

Although transition metal cyano bimetallic compounds have been well known for decades for their interesting optical and magnetic properties, reports on actinide hexacyanoferrate compounds are scarce. For instance, a thorough structural description is still lacking. Another question is the possible covalency or charge transfer effects in these materials that are known to foster electron delocalization with a large variety of transition metal cations. In this paper, new members of the actinide(IV) hexacyanoferrates have been synthesized with Th, Np and Pu. This is the first review of thorium to plutonium hexacyanoferrate compounds since the early investigations during the Manhattan Project some 70 years ago. We have carried out an extensive structural characterization using powder X-ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS) and X-ray microscopy for the plutonium adduct. The crystallographic space group of microcrystalline Th, Np and Pu hexacyanoferrate compounds appears to be very similar to that of the early lanthanide adducts, suggesting that the tetravalent actinides are arranged in a tricapped trigonal prismatic polyhedron of coordination number 9, in which the actinide atom is bonded to six nitrogen atoms and to three water molecules. Further combined analysis of the iron K-edge and actinide LIII-edge EXAFS data and XRD data provided the basis for a three-dimensional molecular model. Structural data in terms of actinide–ligand bond lengths have been compared to those reported for the parent lanthanide(III) compounds, confirming the structural similarities. In addition, two new structures with the thorium cation have been obtained and described using single-crystal XRD: (H5O2)[Th(DMF)5(H2O)]2[Fe(CN)6]3 and [Th(DMF)4(H2O)3][Fe(CN)6](NO3)·2H2O. This structural description of the Th, Np and Pu hexacyanoferrate system will be followed by a semi-quantitative electronic description of the actinide–cyano bond using NEXAFS data analysis in a coming paper.

Multi-edge X-ray Absorption Spectroscopy. 1. X-ray Absorption near-Edge Structure Analysis of a Biomimetic Model of FeFe-Hydrogenase

In this work, we demonstrate the potential of multi-edge X-ray absorption near-edge structure (XANES) analysis in completely defining the ground state electronic structure of a prototypical biomimetic complex of the [2Fe]-subcluster of the catalytic H-cluster of FeFe-hydrogenase. The spectral features at the ionization thresholds for Fe, S, C, and O 1s (K-edge) and Fe 2p (L-edge) core electrons were considered simultaneously to obtain the atomic compositions of the unoccupied frontier molecular orbitals. A systematic error analysis was carried out at the most informative S K-edge for spectra collected by multiple detection methods, at various data collection temperatures, and different sample preparation protocols. As expected for the difference in bonding between bridging and terminal Fe-S(thiolate) coordination, the Fe-S bond is more covalent in the [2Fe]-biomimetic complex with formally iron(I) centers (36 ± 2% S character per Fe-S bond) than in the previously described [2Fe-2S] clusters (25 ± 3% S character per Fe-S bond) with formally iron(III) centers. An electron hole-based analysis of the pre-edge features at Fe K-, Fe L-, and S K-edges experimentally defines the composition of the first three frontier unoccupied molecular orbitals to contain 4% Fe 4p, 44% Fe 3d, and 24% S 3p contributions per electron hole, respectively. The complementary CO ligand contribution thus can be defined as 28% per electron hole. These experimental orbital covalency values are important in rationalizing redox properties, electrophilicity of the metals, or nucleophilicity of the ligands, and critically evaluating the absolute accuracy of electronic structure calculations.

Selenide Retention by Mackinawite

The isotope (79)Se may be of great concern with regard to the safe disposal of nuclear wastes in deep geological repositories due to its long half-life and potential mobility in the geosphere. The Se mobility is controlled by the oxidation state: the oxidized species (Se(IV)) and (Se(VI)) are highly mobile, whereas the reduced species (Se(0) and Se(-II)) form low soluble solids. The mobility of this trace pollutant can be greatly reduced by interacting with the various barriers of the repository. Numerous studies report on the oxidized species retention by mineral phases, but only very scarce studies report on the selenide (Se(-II)) retention. In the present study, the selenide retention by coprecipitation with and by adsorption on mackinawite (FeS) was investigated. XRD and SEM analyses of the samples reveal no significant influence of Se on the mackinawite precipitate morphology and structure. Samples from coprecipitation and from adsorption are characterized at the molecular scale by a multi-edge X-ray absorption spectroscopy (XAS) investigation. In the coprecipitation experiment, all elements (S, Fe, and Se) are in a low ionic oxidation state and the EXAFS data strongly point to selenium located in a mackinawite-like sulfide environment. By contacting selenide ions with FeS in suspension, part of Se is located in an environment similar to that found in the coprecipitation experiment. The explanation is a dynamical dissolution-recrystallization mechanism of the highly reactive mackinawite. This is the first experimental study to report on selenide incorporation in iron monosulfide by a multi-edge XAS approach.

Influence of Sequential Thiolate Oxidation on a Nitrile Hydratase Mimic Probed by Multiedge X-ray Absorption Spectroscopy

Nitrile hydratases (NHases) are Fe(III)- and Co(III)-containing hydrolytic enzymes that convert nitriles into amides. The metal-center is contained within an N(2)S(3) coordination motif with two post-translationally modified cysteinates contained in a cis arrangement, which have been converted into a sulfinate (R-SO(2)(-)) and a sulfenate (R-SO(-)) group. Herein, we utilize Ru L-edge and ligand (N-, S-, and P-) K-edge X-ray absorption spectroscopies to probe the influence that these modifications have on the electronic structure of a series of sequentially oxidized thiolate-coordinated Ru(II) complexes ((bmmp-TASN)RuPPh(3), (bmmp-O(2)-TASN)RuPPh(3), and (bmmp-O(3)-TASN)RuPPh(3)). Included is the use of N K-edge spectroscopy, which was used for the first time to extract N-metal covalency parameters. We find that upon oxygenation of the bis-thiolate compound (bmmp-TASN)RuPPh(3) to the sulfenato species (bmmp-O(2)-TASN)RuPPh(3) and then to the mixed sulfenato/sulfinato speices (bmmp-O(3)-TASN)RuPPh(3) the complexes become progressively more ionic, and hence the Ru(II) center becomes a harder Lewis acid. These findings are reinforced by hybrid DFT calculations (B(38HF)P86) using a large quadruple-ζ basis set. The biological implications of these findings in relation to the NHase catalytic cycle are discussed in terms of the creation of a harder Lewis acid, which aids in nitrile hydrolysis.

High-resolution x-ray absorption spectroscopy of BaTiO3: Experiment and first-principles calculations

High resolution x-ray absorption spectroscopy (XAS) affords new insight into the microscopic properties of perovskite transition metal oxides. Interpretation of XAS spectra in transition metal oxides requires theoretical tools capable of describing relativistic and many-body effects. In this work, full relativistic (SPR-KKR) and multiplet calculations (CTM4XAS) are carried out and compared to experimental multiedge XAS spectra of BaTiO${}_{3}$ single crystals. The impact of relativistic and many-body effects on the calculated density of states and x-ray absorption near edge structure spectra are individually considered.

Structural Characterization of Nanocrystalline Sb-Doped SnO2 Xerogels by Multiedge X-ray Absorption Spectroscopy

Multiedge XAS data are presented for Sb-doped SnO2 xerogels dried at 200 °C and fired at 550 °C, aiming to determine the location of antimony doping in the host matrix and then to understand the ro...

Three-Coordinate Copper(I) Amido and Aminyl Radical Complexes

A three-coordinate Cu-NR(2) system (R = p-tolyl) supported by the anionic bis(phosphino)borate ligand [Ph(2)B(CH(2)P(t)Bu(2))(2)](-) has been isolated and structurally characterized in both its anionic Cu(I) and neutral (formally) Cu(II) oxidation states. A large rate constant for the self-exchange electron-transfer reaction (k(S) >or= 10(7) M(-1) s(-1)) makes this system a functional model for the type-1 active sites in blue copper proteins. Multiedge X-ray absorption spectroscopy, multifrequency electron paramagnetic resonance, and density functional theory analyses collectively indicate that the oxidized form is best regarded as a Cu(I)-aminyl radical complex rather than a Cu(II)-amido species, with about 70% localization of the unpaired electron on the NR(2) unit. Hydrogen-atom transfer and C-C coupling reactions are presented as examples of chemical reactivity manifested by this unusual electronic structure.

A multi-edge X-ray absorption spectroscopy study of the reactivity of zinc di-alkyl-di-thiophosphates anti-wear additives: 2. In situ studies of steel/oil interfaces

Multi-edge X-ray absorption spectroscopy, using synchrotron radiation, was used to examine changes in the structure and chemical composition of steel/oil interfaces when mediated by the presence of the anti-wear additive zinc di-alkyl-di-thiophosphate (ZDDP). The experiments were carried out at the Zn, Fe, O and C K-edges and the Zn and Fe L II/III-edges. Examination of the steel substrate, prior to reaction, revealed that the metal surface was partly oxidised, with the oxide structure mostly involving the iron atoms having a tetrahedral coordination to oxygen. Distinct changes to the oxide structure were observed following its reaction associated with the formation of its interface with ZDDP formulated in lubrication oil. Temperature programmed studies on the interfacial structure were consistent with a model involving ion exchange between Zn atoms in ZDDP and Fe(II) cations in the oxide in a reaction that appeared to be reversible below 75°C. Grazing incidence absorption spectroscopy measurements provided evidence for the breakdown of the ZDDP molecule following its adsorption onto the steel substrate surface. At higher temperatures chemical reaction between the additive and the surface oxide layer led to the formation of an amorphous anti-wear film. The structure of this film was consistent with a Zn–O bond (1.98 Å) typical of partly oxidised sulphur atoms associated with the formation of a random network glass structure with corner sharing thiophosphate and sulphate anions containing network-modifying Zn cations. A reaction scheme consistent with the proposed structure is discussed.

A multi-edge X-ray absorption spectroscopy study of the reactivity of zinc di-alkyl-di-thiophosphates (ZDDPS) anti-wear additives:: 1. An examination of representative model compounds

The potential of X-ray absorption spectroscopy in the analysis of tribologically important interfaces is demonstrated. The application of the technique to the characterisation of representative model compounds is presented. The results showed the reliability of this method to investigate the local environment around a selected atom providing useful `finger prints' for individual compounds. In a companion paper [1]this forms the basis for the examination of the structural role played by zinc di-alkyl-di-thiophosphates (ZDDPs) formulation additives in the anti-wear mediation of steel/oil interfaces.