Site-selective X-ray Absorption Research Articles

Bifunctional NiCo-CuO Nanostructures: A Promising Catalyst for Energy Conversion and Storage.

This investigation explores the potential of co-incorporating nickel (Ni) and cobalt (Co)into copper oxide (CuO)nanostructures for bifunctional electrochemical charge storage and oxygen evolution reactions (OER). A facile wet chemical synthesis method is employed to co-incorporate Ni and Co into CuO, yielding diverse nanostructured morphologies, including rods, spheres, and flake. The X-ray diffraction (XRD) and Raman analyses confirmed the formation of NiCo-CuO nanostructure, with minor phases of nickel oxide (NiO)and cobalt tetraoxide (Co3O4). High-resolution Transmission Electron Microscope (HRTEM) also confirms the diverse morphologies and the minor phases of oxides. Synchrotron X-ray absorption spectroscopy revealed higher charge states of Cu, Ni, and Co in the NiCo-CuO nanostructure, enhancing its charge storage and OER. Site-selective X-ray absorption near edge structure analysis elucidated the spatial distribution of Cu, Ni, and Co in the nanostructure. Furthermore, extended X-ray absorption fine structure spectroscopy provided insights into the local atomic structures, revealing increased coordination numbers and interatomic distances in the NiCo-CuO nanostructure. In situ Raman analysis discloses the transformation of Co3O4 into cobalt hydroxide (Co(OH)2) and cobalt oxide (CoO) into cobalt oxyhydroxide (CoOOH) The NiCo-CuO nanostructures exhibited superior specific capacitance, favorable Tafel behavior, and low overpotential positioning as promising bifunctional materials for energy storage and conversion applications. This work contributes to the development of efficient CuO nanocatalysts.

Site-selective soft X-ray absorption as a tool to study protonation and electronic structure of gas-phase DNA.

The conformation and the electronic structure of gas-phase oligonucleotides depends strongly on the protonation site. 5'-d(FUAG) can either be protonated at the A-N1 or at the G-N7 position. We have stored protonated 5'-d(FUAG) cations in a cryogenic ion trap held at about 20 K. To identify the protonation site and the corresponding electronic structure, we have employed soft X-ray absorption spectroscopy at the nitrogen K-edge. The obtained spectra were interpreted by comparison to time-dependent density functional theory calculations using a short-range exchange correlation functional. Despite the fact that guanine has a significantly higher proton affinity than adenine, the agreement between experiment and theory is better for the A-N1 protonated system. Furthermore, an inverse site sensitivity is observed in which the yield of the nucleobase fragments that contain the absorption site appears substantially reduced, which could be explained by non-statistical fragmentation processes, localized on the photoabsorbing nucleobase.

Defects and oxygen vacancies tailored structural, optical and electronic structure properties of Co-doped ZnO nanoparticle samples probed using soft X-ray absorption spectroscopy

The structural, optical and electronic structure properties of all the Zn1-xCoxO nanoparticles [x = 0.01, 0.03, and 0.05], synthesized using co-precipitation method have been studied using X-ray diffraction (XRD), Surface Enhanced Raman Spectroscopy (SERS), UV–Vis–NIR spectroscopy and X-ray Absorption Spectroscopy measurements. The XRD and SERS analysis have confirmed the formation of hexagonal wurtzite structure (space group C46v). UV–Vis–NIR spectra indicate the blue shifting of the absorbance edge and the band gap is found to decrease with doping. The electronic structure of undoped and Co-doped ZnO nanoparticles have been investigated using site selective and element sensitive X-ray absorption spectroscopy. From the XAS spectra at Zn L3,2, Co L3,2 and O K-edges we have found that Zn ions are in +2 valence state in undoped and Co-doped ZnO nanoparticles. From the XAS analysis it can be seen that proper incorporation of Co-ion in the host lattice take place in t2geg state Co+2 (d7) under tetrahedral symmetry and maintain the crystal symmetry with lattice distortion. The Co-ion doping in the ZnO nano-matrix leads to creation of various defects & oxygen vacancies, which results in the blue shift in band-gap energy makes them useful for various applications e.g. photocatalytic water splitting, hydrogen generation, spintronics and optoelectronics.

Hydrogen and oxygen trapping at the H-cluster of [FeFe]-hydrogenase revealed by site-selective spectroscopy and QM/MM calculations

[FeFe]-hydrogenases are superior hydrogen conversion catalysts. They bind a cofactor (H-cluster) comprising a four-iron and a diiron unit with three carbon monoxide (CO) and two cyanide (CN−) ligands. Hydrogen (H2) and oxygen (O2) binding at the H-cluster was studied in the C169A variant of [FeFe]-hydrogenase HYDA1, in comparison to the active oxidized (Hox) and CO-inhibited (Hox-CO) species in wildtype enzyme. 57Fe labeling of the diiron site was achieved by in vitro maturation with a synthetic cofactor analogue. Site-selective X-ray absorption, emission, and nuclear inelastic/forward scattering methods and infrared spectroscopy were combined with quantum chemical calculations to determine the molecular and electronic structure and vibrational dynamics of detected cofactor species. Hox reveals an apical vacancy at Fed in a [4Fe4S-2Fe]3− complex with the net spin on Fed whereas Hox-CO shows an apical CN− at Fed in a [4Fe4S-2Fe(CO)]3− complex with net spin sharing among Fep and Fed (proximal or distal iron ions in [2Fe]). At ambient O2 pressure, a novel H-cluster species (Hox-O2) accumulated in C169A, assigned to a [4Fe4S-2Fe(O2)]3− complex with an apical superoxide (O2−) carrying the net spin bound at Fed. H2 exposure populated the two-electron reduced Hhyd species in C169A, assigned as a [(H)4Fe4S-2Fe(H)]3− complex with the net spin on the reduced cubane, an apical hydride at Fed, and a proton at a cysteine ligand. Hox-O2 and Hhyd are stabilized by impaired O2– protonation or proton release after H2 cleavage due to interruption of the proton path towards and out of the active site.

Investigation of the metal-insulator transition in NdNiO3 films by site-selective X-ray absorption spectroscopy.

In this work, multifunctional oxide NdNiO3 (NNO) thin films grown on a SrTiO3 (STO) substrate using pulsed-laser deposition are studied. Temperature dependent resistivity measurements revealed that NNO/STO samples exhibit a sharp thickness dependent metal-insulator transition (MIT) over a range of 150-200 K. It is known that the electronic properties of correlated oxides are extremely complex and sensitive to changes in orbital occupancy. To evaluate the changes in the electronic and/or crystallographic structure responsible for the MIT, a site-selective (O, Ni and Nd) X-ray absorption near edge structure (XANES) analysis is performed above and below the transition temperature. Analysis of XANES spectra suggests that: (i) in NNO films nominally trivalent Ni ions exhibit multiple valency (bond disproportionation), (ii) intermetallic hybridization plays an important role, (iii) the presence of strong O 2p-O 2p hole correlation at low temperature results in the opening of the p-p gap and (iv) the valency of Nd ions matches well with that of Nd3+. For NNO films exhibiting a sharp MIT, Ni 3d electron localization and concurrent existence of Ni 3d8 and Ni 3d8L[combining low line]2 states are responsible for the observed transition. At temperatures below the MIT the O 2p-O 2p hole correlation is strong enough to split the O 2p band stabilizing insulating phase. Temperature and thickness dependent differences observed in the site-selective XANES data are discussed in terms of possible mechanisms for the MIT (negative charge-transfer type).

表面化学反応の実時間・実空間分解分光への挑戦： 不可逆かつ不均一な反応の追跡を目指して

A novel experimental technique, time-resolved wavelength-dispersive soft X-ray imaging spectroscopy, is proposed in order to achieve real-time and real-space resolved spectroscopy for the observation of irreversible and inhomogeneous surface chemical reactions. By combining the wavelength-dispersed soft X rays, in which the X-ray wavelength (photon energy) changes as a function of position on the sample, with the photoelectron emission microscope, the soft X-ray absorption spectra are separately obtained at different positions on the sample without scanning the X-ray monochromator. Therefore, the real-time resolved measurement of site-selective soft X-ray absorption spectroscopy is realized in one event without repeating the chemical reaction. It is expected that the spatial distribution of different chemical species is traced during the surface chemical reaction, which is essential to understand the reaction mechanism.

Hydride binding to the active site of [FeFe]-hydrogenase.

[FeFe]-hydrogenase from green algae (HydA1) is the most efficient hydrogen (H2) producing enzyme in nature and of prime interest for (bio)technology. Its active site is a unique six-iron center (H-cluster) composed of a cubane cluster, [4Fe4S]H, cysteine-linked to a diiron unit, [2Fe]H, which carries unusual carbon monoxide (CO) and cyanide ligands and a bridging azadithiolate group. We have probed the molecular and electronic configurations of the H-cluster in functional oxidized, reduced, and super-reduced or CO-inhibited HydA1 protein, in particular searching for intermediates with iron-hydride bonds. Site-selective X-ray absorption and emission spectroscopy were used to distinguish between low- and high-spin iron sites in the two subcomplexes of the H-cluster. The experimental methods and spectral simulations were calibrated using synthetic model complexes with ligand variations and bound hydride species. Distinct X-ray spectroscopic signatures of electronic excitation or decay transitions in [4Fe4S]H and [2Fe]H were obtained, which were quantitatively reproduced by density functional theory calculations, thereby leading to specific H-cluster model structures. We show that iron-hydride bonds are absent in the reduced state, whereas only in the super-reduced state, ligand rotation facilitates hydride binding presumably to the Fe-Fe bridging position at [2Fe]H. These results are in agreement with a catalytic cycle involving three main intermediates and at least two protonation and electron transfer steps prior to the H2 formation chemistry in [FeFe]-hydrogenases.

Site-selective high-resolution X-ray absorption spectroscopy and high-resolution X-ray emission spectroscopy of cobalt nanoparticles.

The special (macroscopic) properties of nanoparticles are mainly due to their large surface-to-volume ratio. Thus, the separate characterization of geometric and electronic properties of surface and bulk would be favorable for a better understanding of the properties of nanoparticles. Because of the chemical sensitivity of X-ray fluorescence lines, in particular those involving higher lying electronic states, high-resolution fluorescence-detected X-ray absorption spectra (HRFD-XAS) offer these opportunities. In this study, three types of wet-chemically synthesized Co nanoparticles, ∼6 nm in diameter with varying thicknesses of a protective shell, were investigated at the ID26 beamline of the European Synchrotron Radiation Facility. HRFD-XAS spectra at the Co K-edge, that is, X-ray absorption near-edge structure (HRFD-XANES) and extended X-ray absorption fine structure (HRFD-EXAFS) spectra, were recorded via detection of the Kβ1,3 fluorescence at specific energies. As these spectra are only partly site-selective due to a strong overlap of the emission lines, a numerical procedure was applied based on a least-squares fitting procedure, realized by singular value decomposition. The detailed analysis of the obtained site-selective spectra, regarding chemical composition and crystallographic phase, using measured and simulated FEFF9-based reference spectra, showed that the metallic core had mainly hexagonal close-packed structure with lattice constants matching bulk Co; the spectra for the shell could be satisfactorily fitted by a mixture of CoO and CoCO3; however, with an obvious need for at least a third compound. To obtain additional information about ligands attached to Co, valence-to-core X-ray emission spectra (VTC-XES) using the Kβ2,5 and the satellite structure Kβ″ and VTC-XANES spectra thereof were also recorded, by which the former results are confirmed. Further on, FEFF simulations indicate that a Co-N compound is a very likely candidate for the third component. The presented results clearly show that VTC-XES and HRFD-XAS are suitable tools for the detailed specification of the core and the surface of nanoparticles, in particular upon realizing "real" site-selectivity for XANES and EXAFS with a general strategy applicable to a wide range of systems.

Atomic-scale distortion of optically activated Sm dopants identified with site-selective X-ray absorption spectroscopy

The local structure of luminescent Sm dopants was investigated using an X-ray absorption fine-structure technique with X-ray-excited optical luminescence. Because this technique evaluates X-ray absorption from luminescence, only optically active sites are analyzed. The Sm L3 near-edge spectrum contains split 5d states and a shake-up transition that are specific to luminescent Sm. Theoretical calculations using cluster models identified an atomic-scale distortion that can reproduce the split 5d states. The model with C4v local symmetry and compressive bond length of Sm–O of a six-fold oxygen (SmO6) cluster is most consistent with the experimental results.

Internal charge transfer in metallicity sorted ferrocene filled carbon nanotube hybrids

Site selective high resolution photoemission and X-ray absorption have revealed the complex interplay between ferrocene and single-walled carbon nanotubes (SWCNTs) upon filling. The use of ultraclean and metallicity sorted nanotubes as starting material and a subsequent ferrocene filling, yields metallicity sorted hybrids, where the details of the internal charge transfer from Fe to the SWCNTs have been distinctly unravelled. An n-type doping of the SWCNTs has been identified, with the peculiarity that purely metallic tubes are prone up to 30% higher doping than their semiconducting counterparts. Concomitantly, the average Fe valency changes from 2 in ferrocene to 2.3 in the ferrocene/semiconducting SWCNT hybrids and to 2.4 in the ferrocene/metallic SWCNT hybrids. The origin of this extra charge transfer has been revealed to be ionic by resonant photoemission, which excludes a finite hybridization between the ferrocene filler and the SWCNTs. This does not only substantiate that the internal charge transfer determines the resulting electronic transport properties in the filled 1D carbon hybrids, but that the metallic or semiconducting character of the encapsulating tubes is crucial towards tailoring and regulating tunable 1D electronic transport properties in these materials that are highly desirable in nanoelectronics.

Site-selective investigation of site symmetry and site occupation of iron in Fe-doped lithium niobate crystals

Lithium niobate (LN) crystals, in particular after strong oxidation, are of great relevance for applications in photorefraction and nonlinear optics. Crystals doped with 2 or 4 wt % of iron are studied by extended x-ray absorption fine structure spectroscopy. The Fe atoms are observed to be sixfold coordinated by oxygen atoms and incorporated into the Li site. No evidence for formation of iron oxide clusters is found. In situ x-ray absorption spectroscopy measurements of the thermoelectric oxidation of a LN:Fe 2 wt % crystal and the analyses of the partially site-selective x-ray absorption near edge structure spectra of a LN:Fe 4 wt % crystal provide site-selective structural information. We found that the Fe2+ and Fe3+ sites have the same site symmetries and positions in the LN matrix, i.e., the Fe2+ and Fe3+ atoms are both octahedrally coordinated by six oxygen atoms and both occupy the Li site.

Capacitance XAFS method: a new site-selective and microscopic x-ray absorption spectroscopy

For local structure and electronic state analyses of electron trapping centers, we developed a site-selective x-ray absorption fine structure XAFS measurement, i.e., the capacitance XAFS method. The concept of the capacitance XAFS measurement is based on the fact that the x-ray absorption of trapping centers, not the bulk, can be evaluated from the capacitance change due to x-ray induced photoemission of a localized electron. With this method, micro spectroscopy can be achieved by scanning probe detection of the capacitance. Moreover, trapping centers with a specific eigen-energy can be selectively observed by controlling the sample bias voltage. We tested these advantages in detail for two typical metal and semiconductor systems. (1) The capacitance XAFS spectrum of Cu indicated twodimensional (2D) electronic states at a hetero interface between native oxide (Cu2O) and bulk Cu; the Cu 4pπ states of the electron trap were confined into a 2D plane. (2) The capacitance XAFS spectrum of GaAs depends on the sample bias voltage and on selectively indicated electronic states of surface trapping center due to Ga oxide, which captures electrons at a positive bias voltage.

Selective X-ray absorption spectroscopy of self-assembled atom in InAs quantum dot

To discuss the local structure of an atom in the self-organized InAs quantum dot (QD), we perform a site-selective X-ray absorption fine structure (XAFS) measurement, employing the capacitance XAFS method. Since the X-ray-induced photoemission of confined electrons in a QD via inner-shell absorption can be detected by capacitor, the photon energy dependence of the capacitance provides the XAFS spectrum of the atom in the QD. It is found that when the bias voltage applied to the capacitor aligned the Fermi energy with the quantum level, the site-selectivity is enhanced. The peak energy shift of ∼1.5 eV is observed in the site-selective XAFS spectrum of an As atom in QD. The molecular orbital calculation indicates that the energy shift originates from the point defect at the boundary between an InAs QD and a GaAs barrier layer.

Capacitance XAFS method: X-ray absorption spectroscopy of low-dimensional structures.

For local structure analyses of low-dimensional structures in semiconductors, such as defects, interfaces and surfaces, a new site-selective x-ray absorption fine structure (XAFS) measurement, 'capacitance XAFS method', is presented. This method measures the x-ray photon energy dependence of the capacitance involved in diode structures. Since the capacitance is changed by an x-ray induced photoemission of localized electrons in the low dimensional structures, a site-selective analysis can be made. Although macroscopic x-ray absorption can be measured by conventional XAFS analyses, low-dimensional x-ray absorption coefficients are observed by the capacitance XAFS measurements.

X-ray-excited optical luminescence of erbium-doped semiconductor: site-selective X-ray absorption spectroscopy of an optically active atom

The X-ray-excited optical luminescence (XEOL) of erbium-doped silicon (Si:Er) thin film is observed for the site-selective X-ray absorption spectroscopy (XAS) of an optically active Er atom. In order to achieve the site-selectivity at the atomic level, intense X-ray from an undulator is used for the excitation of Er. Indirect excitation caused by X-ray absorption at Si around Er is investigated, and X-ray induced luminescence independent of the inner-shell excitation of Er is observed. A possible mechanism is proposed, in which the formation of an electron-hole pair in Si and the subsequent transfer of recombination energy excite the valence electron of Er. According to this model, energy back-transfer with phonon absorption is used to suppress the indirect excitation, resulting in the site-selective XAS of Er atom.

X-ray-excited optical luminescence of impurity atom in semiconductor.

We observed the x-ray-excited optical luminescence (XEOL) of erbium-doped silicon (Si:Er) thin films to make a site-selective x-ray absorption fine structure (XAFS) measurement of an optically active Er atom. The undulator beam was used for the increment of the electron population in the excited state, and following XEOL at an infrared wavelength of 1.54 microm with minimum absorption loss in the host Si was detected. The edge-jump and XAFS oscillation were successfully obtained at the Er L(III)-edge. This spectrum originated from inner-shell excitation and relaxation of only the optically active Er atom, indicating that site-selectivity at an atomic level was achieved.

Site-selective x-ray absorption fine structure analysis of an optically active center in Er-doped semiconductor thin film using x-ray-excited optical luminescence

In order to discuss the local structure of an optically active center in Er-doped Si thin film, site-selective x-ray absorption fine structure (XAFS) analysis using x-ray-excited optical luminescence was performed. The XAFS spectrum at the Er LIII edge was obtained from the x-ray photon energy dependence of the peak intensity of infrared luminescence due to Er intra-4f transition. Although conventional XAFS measurement analyzes the average structure of all of the Er, this method intrinsically provides structural information for only optically active Er. A broad 2p–5d resonant peak in the site-selective XAFS spectrum is reproduced by a density-of-state calculation of a distorted ErO6 cluster, assuming an Er transformation from an octahedral center of 0.25 Å.

X-ray induced photoemission of a localized electron and its application to site-selective x-ray absorption fine structure measurement

For local structure analyses of carrier trap centers in semiconductors, site-selective x-ray absorption fine structure (XAFS) measurement, the “capacitance XAFS” method, is proposed. The concept of capacitance XAFS measurement is based on the fact that the amount of x-ray absorption of trap centers, not the bulk, may be evaluated from the capacitance change due to x-ray induced photoemission of a localized electron. In order to verify this model, characteristics of x-ray induced photoemission from defects are investigated by capacitance–voltage measurement. The temperature dependence of the photocarrier concentration in a semiconductor corresponds to that of the capacitance XAFS signal amplitude. On the other hand, no influence of the thermal excitation of defects on the capacitance XAFS signal amplitude is observed. These results indicate that the capacitance XAFS signal originates from localized electron emission via the inner-shell excitation of defect atoms, resulting in site selectivity to the trap centers.

Site-Selective X-Ray Absorption Fine Structure (XAFS) Spectroscopy. (1) Design of Fluorescence Spectrometer and Emission Spectra

Abstract A Rowland-type fluorescence spectrometer was designed to measure site-selective XAFS spectra by utilizing the chemical shift of emission peak for each site. The positions of Johansson-type cylindrically-bent Ge(111) crystal and scintillation counter were controlled independently by five-channel stepping motors, thereby satisfying precise Rowland condition. Thus, measurements of many fluorescence lines over wide ranges of Bragg angle 55.6—83.9° and Rowland radius 127.7—240.9 mm are made feasible. The Cu Kα peak shifted by +1.6 eV on going from Cu to CuCl and by -0.6 eV on going from CuCl to CuCl2. The Cr Kβ1 peak shifted by +0.8 eV on going from Cr to Cr2O3 and by -1.6 eV on going from Cr2O3 to K2CrO4. The FWHM of Cu Kα peaks was as small as 3.0 eV. The major factor to control the energy resolution of this spectrometer was found to be geometrical angle width (slit size). The effects of Rowland radius and X-ray penetration depth into the bent crystal were smaller in the range of slit size in this paper [slit length (horizontal) in front of I0 ion chamber was 0.5—2.0 mm and two slit lengths (vertical) in Rowland circle were 0.5—5.5 mm].

Site-Selective X-Ray Absorption Fine Structure (XAFS) Spectroscopy (2). XAFS Spectra Tuned to Surface Active Sites of Cu/ZnO and Cr/SiO2 Catalysts

Abstract Site-selective XAFS spectra were measured by tuning the Rowland-type fluorescence spectrometer to each site of Cu/ZnO and Cr/SiO2 catalysts. The chemical shifts between Cu0 and CuI sites and between CrIII and CrVI sites were relatively large for model compounds (ΔE = 1.6 eV), and were utilized as the tune energies of site-selective XAFS. Site-selective XANES tuned to Cu0, CuI, and CrIII sites were successfully obtained. Based on the correlation between the chemical shift and the peak width (energy resolution of the fluorescence spectrometer), site-selective XANES spectra were analyzed. The population ratio of Cu metal, Cu2O, and CuI atomically dispersed on ZnO was estimated to be 70(±23) : 22(±14) : 8(±5). The contribution of the CrIII site to site-selective XANES tuned to the CrIII site was 94(±3)%.