Fluorescence Yield X-ray Absorption Spectroscopy Research Articles

Iridium Single Atoms to Nanoparticles: Nurturing the Local Synergy with Cobalt-Oxide Supported Palladium Nanoparticles for Oxygen Reduction Reaction.

A ternary catalyst comprising Iridium (Ir) single-atoms (SA)s decorated on the Co-oxide supported palladium (Pd)nanoparticles (denoted as CPI-SA) is developed in this work. The CPI-SA with 1wt.% of Ir exhibits unprecedented high mass activity (MA) of 7173 and 770mA mgIr -1, respectively, at 0.85 and 0.90V versus RHE in alkaline ORR (0.1m KOH), outperforming the commercial Johnson Matthey Pt catalyst (J.M.-Pt/C; 20wt.% Pt) by 107-folds. More importantly, the high structural reliability of the Ir single-atoms endows the CPI-SA with outstanding durability, where it shows progressively increasing MA of 13 342 and 1372mA mgIr -1, respectively, at 0.85 and 0.90V versus RHE up to 69 000 cycles (3 months) in the accelerated degradation test (ADT). Evidence from the in situ partial fluorescence yield X-ray absorption spectroscopy (PFY-XAS) and the electrochemical analysis indicate that the Ir single-atoms and adjacent Pd domains synergistically promote the O2 splitting and subsequent desorption of hydroxide ions (OH-), respectively. Whereas the Co-atoms underneath serve as electron injectors to boost the ORR activity of the Ir single-atoms. Besides, a progressive and sharp drop in the ORR performance is observed when Ir-clusters and Ir nanoparticles are decorated on the Co-oxide-supported Pd nanoparticles.

Real-time and operando observation of intermediates on TiO2 photoelectrocatalysis by soft X-ray absorption spectroscopy

The oxygen evolution reaction (OER) on the TiO2 surface at the solid–liquid interface was observed in real-time and operando using fluorescence-yield X-ray absorption spectroscopy (XAS) in the soft X-ray region. The OER was observed during a potential sweep with UV light on or off, focusing on the oxygen K-edge XAS. Although conventional XAS measurements require long measurement times, the spectra during the reaction were obtained every 3 s in the present experiment via wavelength-dispersive XAS technique. An increase in the X-ray absorption intensity at approximately 533.8 eV was observed during the potential sweep only with UV light on. The observed spectral change is discussed in relation to the reaction mechanism of the OER. The present technique can be applied to a wide range of analyses of (photo-) electrocatalysis and electrochemical reactions at solid–liquid interfaces to observe their products and intermediates during the reaction.

<i>Operando</i> Observation of the Electrochemical Oxygen Evolution Reaction with a Co Oxide Catalyst Using Fluorescence-yield Wavelength-Dispersive Soft X-ray Absorption Spectroscopy

Herein, an electrochemical oxygen evolution reaction (OER) at the solid–liquid interface was investigated in real time using fluorescence-yield X-ray absorption spectroscopy (XAS) in the soft X-ray region, which is a newly developed technique. A dedicated electrochemical cell was used, and the OER on the surface of a Co/Pt thin film working electrode in an alkaline solution (NaOH, 0.1 M) was investigated. When the electrode potential was shifted to a positive value, the peak of the oxygen K-edge was successfully detected using this technique, and the peak intensity increased in correlation with the electrode potential. In addition, it was indicated that the oxygen K-edge absorption peak was observed, before the electrode potential reaching the onset potential, i.e., from around 1.0 V vs. relative hydrogen electrode of the electrode potential. It was confirmed that the electrochemical reactions at the solid–liquid interface of metal electrodes like the OER catalyst are able to be observed in real-time.

Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb2Ni12(P,As)7.

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses TFL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.

Electronic transitions in CePd2Si2studied by resonant x-ray emission spectroscopy at high pressures and low temperatures

Temperature and pressure dependences of the electronic structure of the heavy-fermion system CePd${}_{2}$Si${}_{2}$ have been investigated using partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy at the Ce ${L}_{3}$ edge. The temperature dependence has also been measured for CeRh${}_{2}$Si${}_{2}$ for comparison. In both compounds Ce is in a weakly mixed valence state at ambient pressure, mostly ${f}^{1}$ with a small contribution from the ${f}^{0}$ component. No temperature dependence of the Ce valence is observed at temperatures as low as 8 K. In CePd${}_{2}$Si${}_{2}$ at 19 K, however, the Ce valence shows a continuous increase with pressure, indicating pressure-induced delocalization of the $4f$ states. Theoretical calculations based on the single impurity Anderson model reproduce the experimental results well. Pressure dependence of the difference between the ground state valence and the measured valence including the final state effect is also discussed.

Pressure and Temperature Dependences of the Electronic Structure of CeIrSi3 Probed by Resonant X-ray Emission Spectroscopy

Pressure and temperature dependences of the electronic structure of the heavy-fermion superconductor CeIrSi 3 have been investigated using partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy at the Ce L 3 edge. Ce is in a weakly mixed valence state at ambient pressure, mostly f 1 with a small contribution from the f 0 component. Pressure-induced increase of the Ce valence becomes apparent above 4 GPa, concomitantly with the disappearance of the superconductivity. No temperature dependence of the Ce valence is observed within the measured temperature range down to 24 K.

Fluorescence yield X-ray absorption spectroscopy for O-Kα with elimination of N-Kα background using superconducting tunnel junction detectors

Fluorescent yield X-ray absorption fine structure (XAFS) spectroscopy is widely used for measuring chemical or physical states of particular elements in materials. A superconducting tunnel junction (STJ) detector is promising for X-ray absorption spectroscopy, especially in a soft X-ray region below 1keV, because of an excellent energy resolution of 10–20eV, which is crucial for resolving the characteristic X-ray lines from dilute light elements in materials, and a high photon-counting-rate. We constructed an X-ray absorption spectrometer using a 100-pixel STJ array detector mounted on a cryogen-free 3He cryostat and a parallel data acquisition system. It has been confirmed that the STJ detector produces a single peak for the O-Kα line (525eV) with energy resolution of 22eV in full width at half maximum. An XAFS spectrum only for the oxygen atoms from a SiN/SiO2 thin film sample was successfully obtained by using the clear separation between the O-Kα line and the N-Kα line (392eV), eliminating the effect of the N-Kα background.

Fabrication of superconducting tunnel junctions for soft X-ray spectroscopy

Superconducting tunnel junction (STJ) array detectors with a new design, which has a minimum junction edge coverage of an SiO2 insulation, passivation layer and an asymmetric tunnel junction layer structure, have been fabricated for a soft X-ray region between 100 eV and 1 keV. The sensitive area was patterned by removing the SiO2 deposition layer by a lift-off technique that ensured no contamination layer on the top Nb electrode surface. The width of the passivation rim was as narrow as 0.5 µm at the junction edge. The clean Nb surface and the narrow SiO2 rim resulted in almost no artifact photon events in a low-energy region. The asymmetric layer design is effective in solving a problem of double peak response to monochromatic X-rays, which is commonly observed in STJ detectors. The performance of a 100 pixel array detector was investigated by the fluorescent X-ray analysis of oxides and nitrides: an energy resolution of about 30 eV for the total absorption of the Kα lines of oxygen and nitrogen. We plan to realize an energy resolution of better than 20 eV and a counting rate of over 1 Mcps for fluorescence-yield X-ray absorption spectroscopy for light trace elements. Copyright © 2011 John Wiley & Sons, Ltd.

Hybridization and suppression of superconductivity inCeFeAsO1−y: Pressure and temperature dependence of the electronic structure

Pressure and temperature dependence of the electronic structure of superconducting (SC) ${\text{CeFeAsO}}_{1\ensuremath{-}y}$ and non-SC ${\text{CeFeAsO}}_{1\ensuremath{-}y}$ have been investigated using two complementary hard x-ray spectroscopic probes at the $\text{Ce}\text{ }{L}_{3}$ edge, partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy. With increasing pressure, the ratio between the intensity of the peak related to the ${f}^{0}$ $({\text{Ce}}^{4+})$ state and that of the ${f}^{1}$ $({\text{Ce}}^{3+})$ state, $I({f}^{0})/I({f}^{1})$, is found to increase continuously for both compounds, indicating a continuous increase in the Ce valence. The valence of non-SC ${\text{CeFeAsO}}_{1\ensuremath{-}y}$ is found to be slightly higher than that of SC ${\text{CeFeAsO}}_{1\ensuremath{-}y}$ in the entire pressure and temperature ranges of this study. The valence of ${\text{CeFeAsO}}_{1\ensuremath{-}y}$ around 6 GPa, where the superconductivity breaks down, is estimated to be $\ensuremath{\sim}3.0$, but no change in the valence is observed upon cooling. The dependence of the interatomic distances on the concentration of oxygen vacancies is studied via extended absorption fine structure spectroscopy.

Multiple-scattering x-ray absorption investigation of the local structure around substitutional Ge impurities in Ag

We report on a low-temperature study of the local environment of Ge impurity atoms (1.5% concentration) in Ag by means of fluorescence yield x-ray absorption spectroscopy at the Ge K edge. A multiple-scattering analysis was used to obtain precise structural data up to the fifth-neighbor shell. Evidence for expanded Ag-Ag bond lengths in the neighborhood of the impurity is obtained.

Sorption and speciation of heavy metals on hydrous Fe and Mn oxides. From microscopic to macroscopic

The mobility, biodisponibility and toxicity of heavy metals are largely controlled by chemical reactions which take place at the hydrous oxide/water interface. The recent development of the synchrotron-based fluorescence yield X-ray absorption spectroscopy permits elucidation of these chemical reactions, at a molecular level. Six case studies are presented: the sorption mechanism of SeO 3 2−, UO 2 2+, Pb(II), Cr(III) on hydrous Fe oxide, Pb(II) on hydrous Mn oxide and the surface oxidation mechanism of the Cr(III) oxidation to Cr(VI) at the MnO 2/water interface. For all these examples the nature of the active site involved in the sorption process is described. On hydrous Fe oxide, cations [except Cr(III)] and anions adsorb on different surface sites: the former share edges and the latter double corners with Fe(O,OH) 6 octahedra. The sizes of these two surface sites are quite identical so that steric factors cannot account for the difference of sorption processes between cations and anions. The high reactivity of hydrous oxides results from the fact that hydroxyl groups of the two active sites form an ideal “template” for bridging heavy metals. Indeed, OH-OH distances match well with the geometry of the coordination polyhedra of heavy metals. As an application it is shown how the knowledge of sorption mechanisms, at a microscopic level, may help understanding aqueous chemical experiments conducted at a macroscopic level. More generally, this level of investigation is a key issue in evaluating the environmental impact of long term application of heavy metals to land.

In Situ Characterization of Heavy Metal Surface Reactions: The Chromium Case

Abstract Mobility and toxicity of heavy metals are largely controlled by chemical reactions which take place at the oxide/water interface. Insights in the mechanisms involved in these reactions can be obtained from the recently developed synchrotron-based fluorescence-yield X-ray absorption spectroscopy. The phenomena investigated in the present study are: the precipitation of Cr(HI), its coprecipitation with Fe(III), its sorption onto goethite (αFeOOH), and its oxidation at the surface of a Mn(IV) oxide, birnessite. These examples are used to show how a combination of macroscopic and spectroscopic information provides a mechanistic picture of the mobilization and immobilization pathways of heavy metals in natural systems.