XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

X-ray Absorption Spectroscopy Studies of the Local Atomic and Electronic Structure of Iron Incorporated into Electrodeposited Hydrous Nickel Oxide Films†

Abstract: The authors have utilized X-ray absorption fine structure (XAFS) spectroscopy to investigate the local atomic and electronic structure of iron incorporated into electrodeposited nickel hydroxide films. It was found that cathodic codeposition from a solution containing Fe(II) and Ni(II) ions results in iron occupying Ni lattice sites in {alpha}-Ni(OH){sub 2}. The X-ray absorption near edge structure (XANES) shows that Fe is present as Fe(III) ions in the cathodically codeposited film. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that Fe is coordinated to oxygen at {approximately}2.00 {angstrom} and to Ni at {approximately}3.11 {angstrom}. This Fe-O bond length is smaller than the Fe(II)-O bond distance found in Fe(OH){sub 2} ({approximately}2.10 {angstrom}) but is in good agreement with the average Fe(III)-O bond distance found in FeOOH ({alpha}, {gamma}). The Fe-Ni bond distance is in agreement with that of the Ni(II)-Ni(II) bond distance found in {alpha}-Ni(OH){sub 2}. Moreover, the radial structure function (RSF) around Fe shows a distinct peak at {approximately}5.8 {angstrom}, which is a fingerprint of the brucite ({alpha}-Ni(OH){sub 2}) structure. On anodic oxidation of the codeposited film in KOH, the workers found that the Fe ions occupied Ni lattice sites in {gamma}-NiOOH. The XANES shows that the Fe edgemore » shifts to higher energy values, indicating an increase in the oxidation state of Fe on charging. Analysis of the EXAFS data shows that Fe is coordinated to oxygen at {approximately}1.94 {angstrom} and to Ni at {approximately}2.84 {angstrom}. The latter value is in good agreement with the Ni(IV)-Ni(IV) bond length found in {gamma}-NiOOH. The RSF around Fe in the oxidized film shows a distinct peak at {approximately}5.4 {angstrom}, just as in the RSF of Ni in {gamma}-NiOOH. The Fe-O bond distance of {approximately}1.94 {angstrom} is in good agreement with the Fe(IV)-O bond distance found in SrFeO{sub 3}. The results strongly suggest that the Fe ions in the oxidized film are nominally tetravalent but with the Fe-O bond possessing a high degree of covalency.« less

An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research

Abstract: X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper, we report the design and performance of an improved XAFS and XES spectrometer based on the general conceptual design of Seidler et al. [Rev. Sci. Instrum. 85, 113906 (2014)]. New developments include reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorption fine structure (EXAFS) measurement and analysis for the first time with this type of modern laboratory XAFS configuration. This instrument enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES), EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery electrode materials, EXAFS of Ni with full modeling of results to validate monochromator performance, valence-to-core XES for 3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of analytical chemistry.

X-ray Absorption Spectroscopic Study on Valence State and Local Atomic Structure of Transition Metal Oxides Doped in MgH2

Abstract: A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH2 doped with metal oxides (Nb2O5, V2O5, and TiO2nano) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti2O3, respectively, the additives are reduced by MgH2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal−oxygen and metal−metal bonds, being the same as the references of NbO, VO, and Ti2O3. The local structure formed after ball milling or dehydrog...

Geometry and Hydration Structure of Pt(II) Square Planar Complexes [Pt(H2O)4]2+ and [PtCl4]2- as Studied by X-ray Absorption Spectroscopies and Quantum-Mechanical Computations

Abstract: The geometrical structure of the tetraaquo and tetrachloro Pt(II) complexes in aqueous solutions has been studied by means of X-ray absorption spectroscopies (EXAFS and XANES), combined with quantum-mechanical computations. The latter were carried out to supply independent information about the arrangement of water molecules around the complexes. To this aim the [PtCl4]2-•(H2O)2 and [Pt(H2O)4]2+•(H2O)8 structures were optimized and the XANES spectra computed using this theoretical structural information were compared with the experimental spectra. From this comparison it was deduced that the hydration shell of the tetraaquo complex was responsible for a small feature of the XANES spectrum above the white line. Pt−Cl distance in [PtCl4]2- units, both in the crystalline compound, K2PtCl4, and in aqueous solution, was found to be 2.30 A. Pt−O distance in [Pt(H2O)4]2+ species was 2.02 A. No evidence of stable axial water molecules was found in the aquo complex case. Quantum-mechanical optimization of [PtCl4]2...

Ambient-ageing processes in amine self-assembled monolayers on microarray slides as studied by ToF-SIMS with principal component analysis, XPS, and NEXAFS spectroscopy

Abstract: We investigated the ageing of amine-terminated self-assembled monolayers (amine-SAMs) on different silica substrates due to exposure to different ambient gases, pressures, and/or temperatures using time-of-flight secondary ion mass spectrometry (ToF-SIMS) with principal component analysis and complementary methods of surface analysis as X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). The goal of this study is to examine the durability of primary amine groups of amine-SAMs stored in a user laboratory prior to being used as supports for biomolecule immobilization and other applications. We prepared amine-SAMs on the native oxides of silicon wafers and glass slides using 3-aminopropyl triethoxysilane, by using optimized conditions such as anhydrous organic solvent and reaction time scale of hours to avoid multilayer growth. Selected commercial amine-SAM slides have been investigated, too. When the amine-SAMs are exposed to air, oxygen incorporation occurs, followed by formation of amide groups. The formation of oxygen species due to ageing was proved by ToF-SIMS, XPS, and NEXAFS findings such as CNO(-) secondary ion emission at m/z 42, observation of the N 1s HNC=O component peak at 400.2-400.3 eV in XPS, and, last but not least, by formation of a π*(HNC=O) resonance at 401 eV in the N K-edge X-ray absorption spectrum. It is concluded that the used multi-method approach comprising complementary ToF-SIMS, XPS, and NEXAFS analyses is well suited for a thorough study of chemical aspects of ageing phenomena of amine-SAM surfaces.