Structural and electronic transformations of GeSe2 glass under high pressures studied by X-ray absorption spectroscopy.

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above [Formula: see text] 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure.

A novel electrochemical flow-cell for operando XAS investigations in X-ray opaque supports.

Improvement of electrochemical technologies is one of the most popular topics in the field of renewable energy. However, this process requires a deep understanding of the electrode-electrolyte interface behavior under operando conditions. X-ray absorption spectroscopy (XAS) is widely employed to characterize electrode materials, providing element-selective oxidation state and local structure. Several existing cells allow studies as close as possible to realistic operating conditions, but most of them rely on the deposition of the electrodes on conductive and X-ray transparent materials, from where the radiation impinges the sample. In this work, we present a new electrochemical flow-cell for operando XAS that can be used with X-ray opaque substrates, since the signal is effectively detected from the electrode surface, as the radiation passes through a thin layer of electrolyte (∼17 µm). The electrolyte can flow over the electrode, reducing bubble formation and avoiding strong reactant concentration gradients. We show that high-quality data can be obtained under operando conditions, thanks to the high efficiency of the cell from the hard X-ray regime down to ∼4 keV. We report as a case study the operando XAS investigation at the Fe and Ni K-edges on Ni-doped γ-Fe2O3 films, epitaxially grown on Pt substrates. The effect of the Ni content on the catalytic performances for the oxygen evolution reaction is discussed.

Oxide Coating Role on the Bulk Structural Stability of Active LiMn2O4 Cathodes.

The protective coating of the electrode materials is a known source of improvement of the cycling performances in battery devices. In the case of the LiMn2O4 cathodes, the coating with a thin alumina layer has been proven to show performance efficiency. However, the precise mechanism of its effect on the performance improvement of the electrodes is still not clear. In this work we investigate alumina-coating-induced effects on the structural dynamics of the active materials in correlation to the modified solid electrolyte interface dynamics. The local structures of coated and uncoated samples at different galvanostatic points are studied by both soft X-ray absorption measurements at the Mn L-edges and O K-edge (in total electron yield mode) and hard X-ray absorption at the Mn K-edge (in transmission mode). The different probing depths of the employed techniques allowed us to study the structural dynamics both at the surface and within the bulk of the active material. We demonstrate that the coating successfully hinders the Mn3+ disproportionation and, hence, the degradation of the active material. Side products (layered Li2MnO3 and MnO) and changes in the local crystal symmetry with formation of Li2Mn2O4 are observed in uncoated electrodes. The role of alumina coating on the stability of the passivation layer and its consequent effect on the structural stability of the bulk active materials is discussed.

EXAFS investigations on the pressure induced local structural changes of GeSe2glass under different hydrostatic conditions.

Pressure-induced transformations in glassy GeSe2have been studied using the x-ray absorption spectroscopy. Experiments have been carried out at the scanning-energy beamline BM23 (European Synchrotron Radiation Facility) providing a micrometric x-ray focal spot up to pressures of about 45 GPa in a diamond anvil cell. Both Se and Ge K-edge experiments were performed under different hydrostatic conditions identifying the metallization onsets by accurate determinations of the edge shifts. The semiconductor-metal transition was observed to be completed around 20 GPa when neon was used as a pressure transmitting medium (PTM), while this transition was slightly shifted to lower pressures when no PTM was used. Accurate double-edge extended x-ray absorption fine structure (EXAFS) refinements were carried out using advanced data-analysis methods. EXAFS data-analysis confirmed the trend shown by the edge shifts for this disordered material, showing that the transition from tetrahedral to octahedral coordination for Ge sites is not fully achieved at 45 GPa. Results of present high pressure EXAFS experiments have shown the absence of significant neon incorporation into the glass within the pressure range up to 45 GPa.

Advances in modelling X-ray absorption spectroscopy data using reverse Monte Carlo.

Modern extended X-ray absorption fine structure (EXAFS) analysis is based on multiple-scattering calculations. Those calculations are carried out for fixed atomic configurations and proper account of the thermal and static disorder, corresponding to well-defined pair and higher-order distribution functions, can be obtained using different methods. The application of the Reverse Monte Carlo (RMC) method is able to provide tridimensional models of the atomic structure compatible with a given set of experimental data, producing useful and consistent structural models. This method has been proposed and applied also to EXAFS data by several authors in the last 25 years and has been fully implemented in the framework of the RMC-GnXAS method for EXAFS data-analysis. Here we present the extension and application of this method to multiple-edge studies of molecules, crystalline solids and liquids, including the long-range constraints provided by other techniques (e.g. diffraction). The potential and possible weaknesses of the RMC method are discussed, as well as the importance of accounting for the effect of noise levels in XAFS data. Results of RMC refinements are reported for several exemplary cases including Br2 and GeI4 molecular gases, crystalline Ge and AgBr, amorphous Ge and liquid AgBr. Those applications show the general interest for this method, and the importance of combining multiple set of data for improving the accuracy of the structural refinement both at short and long range.

Emerging oxidized and defective phases in low-dimensional CrCl3.

Two-dimensional (2D) magnets such as chromium trihalides CrX3 (X = I, Br, Cl) represent a frontier for spintronics applications and, in particular, CrCl3 has attracted research interest due its relative stability under ambient conditions without rapid degradation, as opposed to CrI3. Herein, mechanically exfoliated CrCl3 flakes are characterized at the atomic scale and the electronic structures of pristine, oxidized, and defective monolayer CrCl3 phases are investigated employing density functional theory (DFT) calculations, scanning tunneling spectroscopy (STS), core level X-ray photoemission spectroscopy (XPS), and valence band XPS and ultraviolet photoemission spectroscopy (UPS). As revealed by atomically resolved transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis, the CrCl3 flakes show spontaneous surface oxidation upon air exposure with an extrinsic long-range ordered oxidized O-CrCl3 structure and amorphous chromium oxide formation on the edges of the flakes. XPS proves that CrCl3 is thermally stable up to 200 °C having intrinsically Cl vacancy-defects whose concentration is tunable via thermal annealing up to 400 °C. DFT calculations, supported by experimental valence band analysis, indicate that pure monolayer (ML) CrCl3 is an insulator with a band gap of 2.6 eV, while the electronic structures of oxidized and Cl defective phases of ML CrCl3, extrinsically emerging in exfoliated CrCl3 flakes, show in-gap spin-polarized states and relevant modifications of the electronic band structures.

A new internally heated diamond anvil cell system for time-resolved optical and x-ray measurements.

We have developed a new internally heated diamond anvil cell (DAC) system for in situ high-pressure and high-temperature x-ray and optical experiments. We have adopted a self-heating W/Re gasket design allowing for both sample confinement and heating. This solution has been seldom used in the past but proved to be very efficient to reduce the size of the heating spot near the sample region, improving heating and cooling rates as compared to other resistive heating strategies. The system has been widely tested under high-temperature conditions by performing several thermal emission measurements. A robust relationship between electric power and average sample temperature inside the DAC has been established up to about 1500 K by a measurement campaign on different simple substances. A micro-Raman spectrometer was used for various in situ optical measurements and allowed us to map the temperature distribution of the sample. The distribution resulted to be uniform within the typical uncertainty of these measurements (5% at 1000 K). The high-temperature performances of the DAC were also verified in a series of XAS (x-ray absorption spectroscopy) experiments using both nano-polycrystalline and single-crystal diamond anvils. XAS measurements of germanium at 3.5 GPa were obtained in the 300 K-1300 K range, studying the melting transition and nucleation to the crystal phase. The achievable heating and cooling rates of the DAC were studied exploiting a XAS dispersive setup, collecting series of near-edge XAS spectra with sub-second time resolution. An original XAS-based dynamical temperature calibration procedure was developed and used to monitor the sample and diamond temperatures during the application of constant power cycles, indicating that heating and cooling rates in the 100 K/s range can be easily achieved using this device.

Band Gap Implications on Nano-TiO2 Surface Modification with Ascorbic Acid for Visible Light-Active Polypropylene Coated Photocatalyst.

The effect of surface modification using ascorbic acid as a surface modifier of nano-TiO2 heterogeneous photocatalyst was studied. The preparation of supported photocatalyst was made by a specific paste containing ascorbic acid modified TiO2 nanoparticles used to cover Polypropylene as a support material. The obtained heterogeneous photocatalyst was thoroughly characterized (scanning electron microscope (SEM), RAMAN, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Diffuse Reflectance Spectra (DRS) and successfully applied in the visible light photodegradation of Alizarin Red S in water solutions. In particular, this new supported TiO2 photocatalyst showed a change in the adsorption mechanism of dye with respect to that of only TiO2 due to the surface properties. In addition, an improvement of photocatalytic performances in the visible light photodegration was obtained, showing a strict correlation between efficiency and energy band gap values, evidencing the favorable surface modification of TiO2 nanoparticles.

Structure of Water in Zn2+ Aqueous Solutions from Ambient Conditions up to the Gigapascal Pressure Range: A XANES and Molecular Dynamics Study.

The structural modifications induced on a 0.5 M Zn2+ aqueous solution by increasing the pressure to 6.4 GPa were investigated using a combination of X-ray absorption near edge structure (XANES) spectroscopy and molecular dynamics (MD) simulations. The Zn K-edge XANES experimental spectra show two different trends depending on the pressure and temperature conditions of the system. On the one hand, when the pressure is increased to 1.0 GPa while keeping the temperature at 300 K, the highly structured nature of Zn2+ second hydration shell is preserved. On the other hand, when the Zn2+ aqueous solution is simultaneously pressurized and heated to follow the melting curve above 1.0 GPa, the Zn2+ second shell loses its high degree of structuring and becomes much more disordered and unstructured. These results are confirmed by the analysis of MD simulations of Zn2+ aqueous solutions under high pressure. By combining distance and angular distribution functions it is possible to highlight the loss of water structuring in the Zn2+ second coordination shell that takes place upon pressurization and heating. A progressive crowding of the Zn2+ second shell is observed with increasing pressure; the water structure becomes remarkably different from that found at ambient conditions, and for pressure values higher than 1.0 GPa the tetrahedral arrangements of water molecules is highly distorted. Moreover, MD simulations of Zn2+ aqueous solutions performed at 1.0 GPa and at increasing temperature values have shown that the loss of water structuring in the Zn2+ second coordination shell observed by simultaneously pressurizing and heating is due to a combined effect of pressure and temperature, both producing an increase of the Zn2+ second-shell disorder.

EIS: the scattering beamline at FERMI.

The Elastic and Inelastic Scattering (EIS) beamline at the free-electron laser FERMI is presented. It consists of two separate end-stations: EIS-TIMEX, dedicated to ultrafast time-resolved studies of matter under extreme and metastable conditions, and EIS-TIMER, dedicated to time-resolved spectroscopy of mesoscopic dynamics in condensed matter. The scientific objectives are discussed and the instrument layout illustrated, together with the results from first exemplifying experiments.

Hydration properties of the Zn2+ ion in water at high pressure.

The structure and dynamics of water in ionic solutions at high pressure have been investigated using a combined approach based on extended X-ray absorption fine structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. Modification of the hydration properties of the Zn(2+) ion induced by a pressure increase from ambient condition up to ∼6.4 GPa has been revealed and accurately analyzed. With increasing pressure the first hydration shell of the Zn(2+) ion has been found to retain an octahedral symmetry with a shortening of the Zn-O distance up to 0.09 Šand an increased width associated with thermal motion, as compared to the ambient condition hydration complex. A very interesting picture of the dynamic behavior of the first hydration shell has emerged from the analysis of the simulations: up to 2.5 GPa no exchange events between first and second shell water molecules occurred, while above this pressure value several exchange events take place in the solution following an associative interchange mechanism. This result can be explained by the very high compression and packing of the solvent which force second shell water molecules to enter the Zn(2+) first hydration shell. MD simulations indicate a strong pressure effect also on the structure of the second coordination shell which is compressed and becomes more disordered and less structured with increasing pressure. The water mobility and the ion diffusion coefficient have been found to increase in the high density conditions, as a consequence of the rupture of the hydrogen bond network caused by pressure.

An XAS experimental approach to study low Pt content electrocatalysts operating in PEM fuel cells.

We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt loading 0.1 mg cm(-2)) operating at the cathode of a proton exchange membrane fuel cell (PEMFC). This catalyst is based on the use of a mesoporous inorganic matrix as a support for the catalyst Pt nanoparticles. Due to the high Pt dilution, in situ measurements of its structural properties by XAS are challenging and suitable experimental strategies must be devised for this purpose. In particular, we show that accurate XAS in situ fluorescence measurements can be obtained using an optimized fuel cell, suitable protocols for alignment of a focused X-ray beam and an appropriate filter for the background signal of the other atomic species contained in the electrodes. Details, advantages and limitations of the XAS technique for in situ measurements are discussed. Analysis of the near-edge XAS and EXAFS (extended X-ray absorption fine structure) data, corroborated by a HRTEM (high-resolution transmission electron microscopy) study, shows that the Pt particles have a local structure compatible with that of bulk Pt (fcc) and coordination numbers match those expected for particles with typical sizes in the 1.5-2.0 nm range. Substantial changes in the oxidation state and in local atomic arrangement of the Pt particles are found for different applied potentials. The catalyst support, containing W atoms, exhibits a partial reduction upon PEMFC activation, thus mimicking the catalyst behavior. This indicates a possible role of the mesoporous matrix in favouring the oxygen reduction reaction (ORR) and stimulates further research on active catalyst supports.

Nearest-neighbor oxygen distances in liquid water and ice observed by x-ray Raman based extended x-ray absorption fine structure.

We report the nearest-neighbor oxygen-oxygen radial distribution function (NN O-O RDF) of room temperature liquid water and polycrystalline ice Ih (-16.8 degrees C) obtained by x-ray Raman based extended x-ray absorption fine structure (EXAFS) spectroscopy. The spectra of the two systems were taken under identical experimental conditions using the same procedures to obtain the NN O-O RDFs. This protocol ensured a measurement of the relative distance distribution with very small systematic errors. The NN O-O RDF of water is found to be more asymmetric (tail extending to longer distances) with longer average distance (2.81 A for water and 2.76 A for ice) but a slightly shorter peak position (2.70 A for water and 2.71 A for ice). The refinement also showed a small but significant contribution from the linear O-H-O multiple scattering signal. The high sensitivity to short range distances of the EXAFS probe will set further restrictions to the range of possible models of liquid water.

Performance of a fuel cell optimized for in situX-ray absorption experiments.

A commercial fuel cell has been successfully modified to carry out X-ray absorption spectroscopy (XAS) measurements under optimized in operando conditions. The design is conceived for the performance of XAS experiments in transmission mode over a wide range of X-ray energies above 6 keV, owing to the reduced absorption of the cell. The wide angular aperture allows the collection of XAS in fluorescence mode and of X-ray diffraction patterns when needed. Details of the design of the cell and its performances are given. The quality of the extended X-ray absorption fine-structure spectra under working conditions has been verified at the ESRF and ELETTRA synchrotron radiation facilities, showing that relatively fast and low-noise transmission measurements on electrodes over a wide range of catalyst concentrations and energies are feasible.

Is there icosahedral ordering in liquid and undercooled metals?

The local structure of simple liquids is significantly different from that of corresponding crystalline systems. Signatures of fivefold local ordering have been previously found, but current knowledge is limited to pair distribution, leaving considerable uncertainty in the determination of the geometrical structure. New x-ray absorption experimental results on liquid and undercooled liquid copper, interpreted using an advanced data-analysis method based on multiple-scattering simulations, are shown to contain direct information on triplet correlations making feasible a reliable determination of the bond-angle distribution and fraction of nearly icosahedral configurations in liquids.

Local structure in molecular complexes probed by multiple-scattering XAS.

The GNXAS (n-body distribution function X-ray absorption spectroscopy) method for multiple-scattering (MS) data analysis of EXAFS (extended X-ray absorption fine structure) data and the results recently obtained on molecular complexes relevant to biological matter are briefly reviewed and discussed. Practical MS calculations for important molecular fragments like Fe-O-O and Fe-C-N-Cu are presented in detail showing the potential of the techniques for measuring bond-angle distributions. The optimal conditions for obtaining accurate structural refinements using EXAFS measurements and modern data-analysis schemes are discussed as well as the current perspectives in the exploitation of the technique.

Multiple-Edge XAS Studies of Synthetic Iron-Copper Bridged Molecular Assemblies Relevant to Cytochrome c Oxidase. Structure Determination Using Multiple-Scattering Analysis with Statistical Evaluation of Errors.

An X-ray absorption spectroscopy study has been carried out at the Fe and Cu K-edges for two bridged molecular assemblies, both of which contain an Fe-X-Cu (X = O(2)(-), OH(-)) bridge unit, some of whose features are relevant to the binuclear site of cytochrome c oxidase. The two complexes [(OEP)Fe-O-Cu(Me(6)tren)](1+) and [(OEP)Fe-(OH)-Cu(Me(5)tren)(OClO(3))](1+) have similar structural fragments around the metal centers except that they differ significantly in the bridge structure (the former contains a linear oxo bridge while the latter has a bent hydroxo bridge). We report a comparative study of these complexes using multiple-scattering (MS) EXAFS analysis and the program package GNXAS. It is found that there is a dramatic increase in the amplitude of the Fe-X-Cu MS pathway as the bridge unit approaches linearity. Full EXAFS MS analysis enables accurate quantitation of bridge metrical details and geometry for both complexes. These studies were done with an expanded version of GNXAS, which allows for simultaneous multiple-edge fitting. Such multiple-edge analysis (using both Fe and Cu edge data) allows common pathways (in this case involving the Fe-X-Cu bridge) to be constrained to be the same, thus improving the observation/variable ratio and enhancing sensitivity for determination of the bridge structure. The accuracy of the structural determination for the bridge units is evaluated by a statistical analysis methodology in which correlations among fitting parameters are identified and contour plots are used to determine random error. The overall error in the EXAFS structural determination is found by establishing the variance with the crystallographically determined values: for the EXAFS-determined parameters at distances below 4 Å, distances and angles deviated on average from crystallographic values by 0.014 Å and 1.5 degrees, respectively. It is also established that structural features in the Fe absorption preedge are diagnostic of oxo vs hydroxo ligation. The relevance of this study to the structural definition of binuclear bridged sites in cytochrome c oxidase and other metalloenzymes is considered.