Monte‐Carlo simulation of x‐ray photoelectron emission from silver

Silver 3d x‐ray photoelectron spectroscopy (XPS) spectra were simulated with the Monte‐Carlo method using an effective energy‐loss function that was derived from a reflected electron energy‐loss spectroscopy (REELS) analysis based on an extended Landau approach. After confirming that Monte‐Carlo simulation based on the use of the effective energy‐loss function can successfully describe the experimental REELS spectrum and Ag 3d XPS spectrum, we applied Monte‐Carlo simulation to predict the angular distribution of Ag 3d x‐ray photoelectrons for different x‐ray incidence angles and different photoelectron take‐off angles. The experimental photoelectron emission microscope that we are constructing was confirmed as being close to the optimum configuration, in which the x‐ray incident angle as measured from the surface normal direction is 74° and the photoelectron take‐off angle is set normal to the surface. The depth distribution functions of the Ag 3d X‐ray photoelectrons for different energy windows suggest that the photoelectron emission microscope will exhibit greater surface sensitivity for narrower photoelectron energy windows. Copyright © 2003 John Wiley & Sons, Ltd.     

Ti‐Based Catalysts and Photocatalysts: Characterization and Modeling

This perspective article aims to underline how cutting‐edge synchrotron radiation spectroscopies such as extended X‐ray absorption spectroscopy (EXAFS), X‐ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X‐ray emission spectroscopy (XES) and resonant inelastic X‐ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti‐based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV‐Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band‐gap and shape engineered TiO₂ materials and microporous titanosilicates (ETS‐10, TS‐1 and Ti?AlPO‐5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.     

An X‐ray absorption spectroscopic study of carbon monoxide‐adsorbed Cu surface

Adsorption/interaction of Carbon monoxide (CO) on a catalytic surface is the key step in electrochemical conversion of CO2 for environmental consideration. Copper (Cu) is known to be the most efficient catalyst for this purpose. Thus, this paper investigates effects of CO adsorption on the electronic/atomic state of polycrystalline Cu surface by using x‐ray absorption spectroscopy (XAS). X‐ray absorption near‐edge structure (XANES) tells that the Cu K‐edge shift +0.2 eV on adsorbing CO. Extended x‐ray absorption fine structure (EXAFS) analysis informs that CO adsorption disturbs Cu surface, i.e. increase of Cu‐Cu bonding distance and decrease of the coordination number of the first nearest neighbor. Both the results of XANES and EXAFS imply decrease of d‐electron density of Cu on the adsorption. Demonstrated is that XAS is very useful in studying the surface phenomena of a catalyst but requires further efforts. Copyright © 2014 John Wiley & Sons, Ltd.     

Consequences of x‐ray absorption fine structure for quantitative core‐level photoelectron spectroscopy

The dependence of x‐ray absorption fine structure (XAFS) on the chemical environment implies a corresponding environmental dependence of photoionization cross‐sections. The practical consequences of the XAFS for quantitative core‐level photoelectron spectroscopy have been studied for a number of materials with the help of simulations based on the ab initio FEFF code. The XAFS effects are predicted to be significant only for photoelectron kinetic energies below 250–300 eV. These effects take the form of systematic sensitivity factor variations between different chemical structural environments (e.g. compound materials quantified with reference to elemental standards). Apart from a few exceptions (e.g. the 2p core levels of Zn, Ga, Ge and As in metallic environments), the XAFS should introduce no significant errors when conventional Mg Kα or Al Kα sources are used for excitation. For core‐level spectra excited with synchrotron radiation or He II radiation sources, the quantification errors can readily approach or exceed 10% at room temperature (and are expected to increase at lower temperatures). Copyright © 2003 John Wiley & Sons, Ltd.     

Application of anomalous small‐angle X‐ray scattering to spherical polyelectrolyte brushes

The analysis of spherical polyelectrolyte brushes by anomalous small‐angle X‐ray scattering (ASAXS) is considered. The particles under consideration consist of a solid poly(styrene) core onto which chains of poly(acrylic acid) are grafted. If Rubidium ions are chosen as counterions, ASAXS can be applied to the study of these systems because the absorption edge of Rb (15199.6eV) can be reached by synchrotron radiation. Here we discuss the results to be obtained by the application of ASAXS to spherical polyelectrolyte brushes.     

X‐Ray Near‐Edge Absorption Study of Temperature‐Induced Low‐Spin‐to‐High‐Spin Change in Metallo‐Supramolecular Assemblies

X‐ray absorption near the iron K edge (XANES) was used to investigate the characteristics of temperature‐induced low‐spin‐to‐high‐spin change (SC) in metallo‐supramolecular polyelectrolyte amphiphile complexes (PAC) containing FeN₆ octahedra attached to two or six amphiphilic molecules. Compared to the typical spin‐crossover material Fe(phen)₂(NCS)₂ XANES spectra of PAC show fingerprint features restricted to the near‐edge region which mainly measures multiple scattering (MS) events. The changes of the XANES profiles during SC are thus attributed to the structure changes due to different MS path lengths. Our results can be interpreted by a uniaxial deformation of FeN₆ octahedra in PAC. This is in agreement with the prediction that SC is originated by a structural phase transition in the amphiphilic matrix of PAC, but in contrast to Fe(phen)₂(NCS)₂, showing the typical spin crossover being associated with shortening of all the metal–ligand distances.     

High‐energy monochromated Cu Kα1 x‐ray source for electron spectroscopy of materials: initial results

The construction of an x‐ray source for high‐energy XPS studies, based on Cu Kα₁ radiation (hν = 8047.8 eV), is described. This source has been fitted to a Scienta ESCA300 electron spectrometer and initial results for pure iron, chromium and stainless steel are presented. The Fe 1 s and Cr 1 s core levels (at ～7112 and 5989 eV binding energy, respectively) are readily observed at good resolution along with their KLL Auger series. It is concluded that the new source shows much promise for investigation of the electronic structure of ferrous and other alloys of scientific and technological importance. Copyright © 2004 John Wiley & Sons, Ltd.     

Site‐selective X‐ray absorption spectroscopy of cobalt nanoparticles

An extensive study of fluorescence‐detected X‐ray absorption near‐edge structure (XANES) spectroscopy, with the purpose of realizing site‐selectivity, applied to smoothly oxidized cobalt nanoparticles is presented. For this, resonant inelastic X‐ray scattering (RIXS) planes of the nanoparticles were recorded using high‐resolution detection of Kβ emission. The Kβ line represents a superposition of emission lines that correspond to different homovalent compounds inherent in the nanoparticle and that are energetically different. Therefore Co K‐edge XANES spectra, extracted from distinct emission energies of the RIXS planes, show partial valence‐selectivity, which by assuming a simple core–shell model for our nanoparticle, turns over to partial site‐selectivity. The pure site‐specific XANES spectra, for the core and shell respectively, have been obtained by means of a numerical procedure. The influences of the lifetime broadening related to site‐selectivity and XANES were considered and have been accounted for in the final solutions. A metallic Co core exhibiting the crystallographic β‐manganese and hexagonal‐close‐packed phases as well as a Co‐O/C shell of valence 2 showing a wurtzite‐type contribution to the standard rocksalt structure are recognized as the final site‐specific candidates for our system. The separate determination of the physical properties of atoms on different sites in nanoparticles provides for the first time some information about the interaction between the nanoparticle and the corresponding coating that determines at least partly the properties of the particle. This offers new opportunities for tailoring the properties of nanoparticles using suitable surfactants or coatings. Copyright © 2011 John Wiley & Sons, Ltd.     

Structure determination of phase II of the antifungal drug griseofulvin by powder X‐ray diffraction

Two new crystalline polymorphs of the widely used antifungal drug griseofulvin (phases II and III), which originate from the crystallization of the melt, have been detected recently. The crystal structure of phase II of griseofulvin {systematic name: (2S,6′R)‐7‐chloro‐2′,4,6‐trimethoxy‐6′‐methyl‐3H,4′H‐spiro[1‐benzofuran‐2,1′‐cyclohex‐2‐ene]‐3,4′‐dione}, C₁₇H₁₇ClO₆, has been solved by powder X‐ray diffraction (PXRD). The PXRD pattern of this new phase was recorded at room temperature using synchrotron radiation. The starting structural model was generated by a Monte Carlo simulated annealing method. The final structure was obtained through Rietveld refinement with soft restraints for interatomic bond lengths and angles, except for the aromatic ring, where a rigid‐body constraint was applied. The symmetry is orthorhombic (space group P2₁2₁2₁) and the asymmetric unit contains two molecules.     

Structure determination of a new cocrystal of carbamazepine and dl‐tartaric acid by synchrotron powder X‐ray diffraction

The crystal structure of a new cocrystal of carbamazepine (systematic name: 5H‐dibenzo[b,f]azepine‐5‐carboxamide, C₁₅H₁₂N₂O) and dl ‐tartaric acid (C₄H₆O₆), obtained by liquid‐assisted grinding, was solved by powder X‐ray diffraction (PXRD). The high‐resolution PXRD pattern of this new phase was recorded at room temperature thanks to synchrotron experiments at the European Synchrotron Radiation Facility (Grenoble, France). The starting structural model was generated by a Monte‐Carlo simulated annealing method. The final structure was obtained through Rietveld refinement and an energy minimization simulation was used to estimate the H‐atom positions. The stability of the proposed structure as a function of temperature was also assessed from molecular dynamics simulations. The symmetry is monoclinic (space group P2₁/c) and contains eight molecules per unit cell, namely, four dl ‐tartaric acid and four carbamazepine molecules.     

Monte‐Carlo simulation of spatial distribution of x‐rays in multi‐film targets. II: Spatial distribution of continuous x‐rays and temperature elevation in W/Al film targets

A Monte‐Carlo simulation written in C++ has been developed to describe spatial distributions of energy dissipation and x‐rays generated by penetrating kilovolt electrons in a multi‐film target. To evaluate the x‐ray source, especially its size and intensity, the use of brightness is proposed as a figure of merit. The Monte‐Carlo simulation approach was applied to a W (5 µm)/Al (200 µm) film target, which has been in practical use as an x‐ray source of small size with high emissivity. The result demonstrates that the experiment had considerable success. The optimum operating condition for the W (5 µm)/Al (200 µm) target in practical use has also been proposed by considering temperature elevation, spot size of electron beam, x‐ray source size and x‐ray intensity. Copyright © 2005 John Wiley & Sons, Ltd.     

Local Structure Study of Lanthanide Elements by X‐Ray Absorption Near Edge Structure Spectroscopy

This paper describes a systematic study of the spectra and local structures of lanthanide (Ln) L‐edge XANES. We found that Ln L₁ and L₃‐edge XANES spectra exhibit characteristic features correlated to their local symmetry through experimental and theoretical simulations. We also propose a simple local structure index criterion for a combination of XANES study and theoretical simulation. Possible solutions of intrinsic problems such as low resolution of characteristic features in the Ln L‐edge XANES and site distributions are also discussed.     

X-ray absorption near edge structures (xanes) of Cu/ZnO/Al2O3 Co shift catalysts

X-ray absorption near edge structures (XANES) of some Cu/ZnO/Al₂O₃CO shift catalysts have been measured at high resolution using synchrotron radiation, and compared to reference compounds Prior to reduction. CuO and ZnO are present, after reduction ZnO and highly dispersed Cu, no indication was found for Cu₂O or the presence of spinels. The results are qualitatively confirmed by preliminary analysis of EXAFS spectra.     

X‐ray Absorption Near‐Edge Structure and Nuclear Magnetic Resonance Study of the Lithium–Sulfur Battery and its Components

Understanding the mechanism(s) of polysulfide formation and knowledge about the interactions of sulfur and polysulfides with a host matrix and electrolyte are essential for the development of long‐cycle‐life lithium–sulfur (Li–S) batteries. To achieve this goal, new analytical tools need to be developed. Herein, sulfur K‐edge X‐ray absorption near‐edge structure (XANES) and ^6,7Li magic‐angle spinning (MAS) NMR studies on a Li–S battery and its sulfur components are reported. The characterization of different stoichiometric mixtures of sulfur and lithium compounds (polysulfides), synthesized through a chemical route with all‐sulfur‐based components in the Li–S battery (sulfur and electrolyte), enables the understanding of changes in the batteries measured in postmortem mode and in operando mode. A detailed XANES analysis is performed on different battery components (cathode composite and separator). The relative amounts of each sulfur compound in the cathode and separator are determined precisely, according to the linear combination fit of the XANES spectra, by using reference compounds. Complementary information about the lithium species within the cathode are obtained by using ⁷Li MAS NMR spectroscopy. The setup for the in operando XANES measurements can be viewed as a valuable analytical tool that can aid the understanding of the sulfur environment in Li–S batteries.     

Semiconductor Nanocrystal Engineering by Applying Thiol‐ and Solvent‐Coordinated Cation Exchange Kinetics

Thiol‐ and solvent‐coordinated cation exchange kinetics have been applied to engineer the composition and crystallinity of novel nanocrystals. The detailed thermodynamics and kinetics of the reactions were explored by NMR spectroscopy, time‐dependent photoluminescence (PL) characterizations and theoretical simulations. The fine structure of the colloidal semiconductor nanocrystals (CSNCs) was investigated by X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS). In this way, high‐quality p‐type Ag‐doped ZnS quantum dots (QDs) and Au@ZnS hetero‐nanocrystals with a cubic phase ZnS shell were synthesized successfully.The unprecedented dominant Ag⁺‐dopant‐induced fluorescence and p‐type conductivity in the zinc‐blende ZnS are reported.     

In‐situ XAFS Studies of Mn12 Molecular‐Cluster Batteries: Super‐Reduced Mn12 Clusters in Solid‐State Electrochemistry

In situ X‐ray absorption fine structure (XAFS) analyses were performed on rechargeable molecular cluster batteries (MCBs), which were formed by a lithium anode and cathode‐active material, [Mn₁₂O₁₂(CH₃CH₂C(CH₃)₂COO)₁₆(H₂O)₄] with tert‐pentyl carboxylate ligand (abbreviated as Mn12tPe), and with eight Mn³⁺ and four Mn⁴⁺ centers. This mixed valence cluster compound is used in an effort to develop a reusable in situ battery cell that is suitable for such long‐term performance tests. The Mn12tPe MCBs exhibit a large capacity of approximately 210 Ah kg⁻¹ in the voltage range V=4.0–2.0 V. The X‐ray absorption near‐edge structure (XANES) spectra exhibit a systematic change during the charging/discharging with an isosbestic point at 6555 eV, which strongly suggests that only either the Mn³⁺ or Mn⁴⁺ ions in the Mn12 skeleton are involved in this battery reaction. The averaged manganese valence, determined from the absorption‐edge energy, decreased monotonically from 3.3 to 2.5 in the first half of the discharging (4.0>V>2.8 V), but changed little in the second half (2.8>V>2.0 V). The former valence change indicates a reduction of the initial [Mn12]⁰ state by approximately ten electrons, which corresponds well with the half value of the observed capacity. Therefore, the large capacity of the Mn12 MCBs can be understood as being due to a combination of the redox change of the manganese ions and presumably a capacitance effect. The extended X‐ray absorption fine structure (EXAFS) indicates a gradual increase of the Mn²⁺ sites in the first half of the discharging, which is consistent with the XANES spectra. It can be concluded that the Mn12tPe MCBs would include a solid‐state electrochemical reaction, mainly between the neutral state [Mn12]⁰ and the super‐reduced state [Mn12]⁸⁻ that is obtained by a local reduction of the eight Mn³⁺ ions in Mn12 toward Mn²⁺ ions.     

XANES Spectroscopic Studies of the Phase Transition in Gd2Zr2O7

The structural phase transition from fluorite to pyrochlore and the strength of the coordination bond of Zr–O in Gd₂Zr₂O₇ were investigated by XANES spectra of both O and Zr K‐edge. The energy difference of the O K‐edge absorption spectra at 532 and 536 eV was assigned to the crystal field splitting energy of the 4d orbital (ΔE_4d, t_2g and e_g) of the Zr ion. Also, in the samples prepared at higher temperatures, the 536 eV peak moves slightly to higher energy, whereas the absorption energy of 532 eV peak does not shift. A correlation between ΔE_4d and the strength of interaction between Zr (4d) and O (2p) orbitals has been found. Furthermore, two Zr K‐edge absorptions at 18020 and 18030 eV of Gd₂Zr₂O₇ have been observed; the splitting energy (ΔE), peak intensity ratio (I₁₈₀₃₀/I₁₈₀₂₀), and FWHM of the first derivative of the absorption curve depend on the preparation temperatures. The effect of crystal symmetry and Zr‐O bonding character on the XANES spectral profile was discussed.     

Deactivation of Cu‐Exchanged Automotive‐Emission NH3‐SCR Catalysts Elucidated with Nanoscale Resolution Using Scanning Transmission X‐ray Microscopy

To gain insight into the underlying mechanisms of catalyst durability for the selective catalytic reduction (SCR) of NO_x with an ammonia reductant, we employed scanning transmission X‐ray microscopy (STXM) to study Cu‐exchanged zeolites with the CHA and MFI framework structures before and after simulated 135 000‐mile aging. X‐ray absorption near‐edge structure (XANES) measurements were performed at the Al K‐ and Cu L‐edges. The local environment of framework Al, the oxidation state of Cu, and geometric changes were analyzed, showing a multi‐factor‐induced catalytic deactivation. In Cu‐exchanged MFI, a transformation of Cu^II to Cu^I and Cu_xO_y was observed. We also found a spatial correlation between extra‐framework Al and deactivated Cu species near the surface of the zeolite as well as a weak positive correlation between the amount of Cu^I and tri‐coordinated Al. By inspecting both Al and Cu in fresh and aged Cu‐exchanged zeolites, we conclude that the importance of the preservation of isolated Cu^II sites trumps that of Brønsted acid sites for NH₃‐SCR activity.