Ab initio calculations of amplitude and phase functions for extended x-ray absorption fine structure spectroscopy

Unlocking Catalytic Potential: Exploring the Impact of Thermal Treatment on Enhanced Electrocatalysis of Nanomaterials

In the evolving field of electrocatalysis, thermal treatment of nano-electrocatalysts has become an essential strategy for performance enhancement. This review systematically investigates the impact of various thermal treatments on the catalytic potential of nano-electrocatalysts. The focus encompasses an in-depth analysis of the changes induced in structural, morphological, and compositional properties, as well as alterations in electro-active surface area, surface chemistry, and crystal defects. By providing a comprehensive comparison of commonly used thermal techniques, such as annealing, calcination, sintering, pyrolysis, hydrothermal, and solvothermal methods, this review serves as a scientific guide for selecting the right thermal technique and favorable temperature to tailor the nano-electrocatalysts for optimal electrocatalysis. The resultant modifications in catalytic activity are explored across key electrochemical reactions such as electrochemical (bio)sensing, catalytic degradation, oxygen reduction reaction, hydrogen evolution reaction, overall water splitting, fuel cells, and carbon dioxide reduction reaction. Through a detailed examination of the underlying mechanisms and synergistic effects, this review contributes to a fundamental understanding of the role of thermal treatments in enhancing electrocatalytic properties. The insights provided offer a roadmap for future research aimed at optimizing the electrocatalytic performance of nanomaterials, fostering the development of next-generation sensors and energy conversion technologies.

Structural origins of the unusual thermal stability of amorphous CuxGe50-xTe50(0⩽x⩽33.3)

We have investigated the local atomic structures of several compositions of the amorphous phase of the system Cu_xGe50-xTe₅₀(0⩽x⩽33.3), based on extended x-ray absorption fine-structure as well as anomalous x-ray scattering experiments, and discuss the unusual trend regarding their thermal stability as a function of the Cu content. At low concentrations (x⩽15), Cu atoms tend to agglomerate in flat nanoclusters reminiscent of the crystalline phase of metallic Cu, leading to a more and more Ge-deficient Ge-Te host network structure with growing Cu content and an increasing thermal stability. At higher Cu concentrations (x⩾25), Cu is incorporated into the network, leading to an overall weaker bonding situation which is associated with a decreasing thermal stability.

Revisiting the K-edge X-ray absorption fine structure of Si, Ge-Si alloys, and the isoelectronic series: CuBr, ZnSe, GaAs, and Ge

Extended X-ray absorption fine structure (EXAFS) has evolved into an unprecedented local-structure technique that is routinely used to study materials' problems in the biological, chemical, and physical sciences. Like many other experimental techniques, EXAFS also requires that several key atomic parameters must be known a priori before structural information can be quantitatively determined. Utilizing current analytical methods, we revisit the isoelectronic series CuBr, ZnSe, GaAs, and Ge originally studied by Stern et al. during the early development of EXAFS [E. A. Stern et al., Phys. Rev. B: Condens. Matter Mater. Phys., 1980, 21, 5521; B. A. Bunker and E. A. Stern, Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 27, 1017]. We demonstrate that the ab initio EXAFS code FEFF accurately predicts the atomic phase shifts and backscattering amplitudes that are primarily functions of the sum of atomic numbers Z along an EXAFS scattering path. We also investigate quantitative fitting and first- and second-shell phase transferability together with problems that arise if a backscattering atom is identified incorrectly in an EXAFS fitting model. Features in the near-edge region, on the other hand, are shown to require a comprehensive treatment of the band structure and density-of-states, including effects of the screened Coulomb interaction between the photoelectron and core hole. We demonstrate that the Bethe-Salpeter equation (BSE) accurately captures the NEXAFS (or XANES) portion of the spectrum for the isoelectronic series in addition to Si and Ge-Si alloys, including within a few eV of the absorption edge, where band structure and excitonic effects are most important.

Unravelling the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ mixed ionic electronic conductors

The crystal structures of non-substituted and Mo-substituted neodymium tungstates are described in detail through neutron diffraction and high-resolution X-ray diffraction. Combined X-ray and neutron diffraction refinements and electron probe micro-analysis were employed to locate Mo atoms in the crystal structure of Nd_6−yW_1−zMo_zO_12−δ (z = 0, 0.25), while X-ray absorption spectroscopy in the near-edge regions confirmed no changes in the oxidation states of Nd and W.

Mixed ionic electronic conducting ceramics Nd_6−yWO_12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO₂, CO, H₂, H₂O or H₂S. Due to this chemical stability, they are promising and cost-efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La_6−yWO_12−δ, the ionic/electronic transport mechanism in Nd_6−yWO_12−δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd_5.8WO_12−δ and molybdenum-substituted Nd_5.7W_0.75Mo_0.25O_12−δ prepared by the citrate complexation route. High-resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd_5.8WO_12−δ and Nd_5.7W_0.75Mo_0.25O_12−δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm3m and doubled unit-cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La_6−yWO_12−δ. X-ray absorption spectroscopy as a function of partial pressure pO₂ in the near-edge regions excludes oxidation state changes of Nd (Nd³⁺) and W (W⁶⁺) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd_6−yWO_12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.

Single-Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside

Single-atom catalysts (SACs) have recently emerged as an exciting system in heterogeneous catalysis showing outstanding performance in many catalytic reactions. Single-atom catalytic sites alone are not stable and thus require stabilization from substrates. Crystalline porous materials such as zeolites and metal-organic frameworks (MOFs) are excellent substrates for SACs, offering high stability with the potential to further enhance their performance due to synergistic effects. This review features recent work on the structure, electronic, and catalytic properties of zeolite and MOF-protected SACs, offering atomic-scale views from the "inside" thanks to the subatomic resolution of synchrotron X-ray absorption spectroscopy (XAS). The extended X-ray absorption fine structure and associated methods will be shown to be powerful tools in identifying the single-atom site and can provide details into the coordination environment and bonding disorder of SACs. The X-ray absorption near-edge structure will be demonstrated as a valuable method in probing the electronic properties of SACs by analyzing the white line intensity, absorption edge shift, and pre-/postedge features. Emphasis is also placed on in situ/operando XAS using state-of-the-art equipment, which can unveil the changes in structure and properties of SACs during the dynamic catalytic processes in a highly sensitive and time-resolved manner.

Temperature-Dependent EXELFS of Chemically Ordered and Disordered Fe3Al

A parallel detection system makes possible quantitative studies of extended electron energy-loss fine-stucture (EXELFS). We are studying the short-range chemical order and local thermal vibrations of chemically ordered and disordered Fe3Al foils using transmission EXELFS. The average number of Fe atoms in the first nearest-neighbor (1nn) shell surrounding the Al atoms in chemically ordered and chemically disordered materials are determined by comparing our data with ab initio EXAFS calculations. The mean-squared relative thermal displacements between the Al atoms and their 1nn shell are roughly determined by our temperature-dependent data.

Chemically disordered Fe-27at%Al and Fe-25at%Al samples were prepared by piston-anvil quenching. Some samples were annealed at 300 °C for over a day to develop DO3 order. Aluminum K edge spectra were collected with a Gatan 666 parallel detection magnetic prism spectrometer attached to a Philips EM 430 electron microscope. Room temperature spectra were collected from submicron regions of the Fe-27Al samples with 300 keV electrons and a beam current of 10 nA.

γ-Alumina-supported Pt17 cluster: controlled loading, geometrical structure, and size-specific catalytic activity for carbon monoxide and propylene oxidation

Although Pt is extensively used as a catalyst to purify automotive exhaust gas, it is desirable to reduce Pt consumption through size reduction because Pt is a rare element and an expensive noble metal. In this study, we successfully loaded a Pt17 cluster on γ-alumina (γ-Al2O3) (Pt17/γ-Al2O3) using [Pt17(CO)12(PPh3)8]Cl n (n = 1, 2) as a precursor. In addition, we demonstrated that Pt is not present in the form of an oxide in Pt17/γ-Al2O3 but instead has a framework structure as a metal cluster. Moreover, we revealed that Pt17/γ-Al2O3 exhibits higher catalytic activity for carbon monoxide and propylene oxidation than γ-Al2O3-supported larger Pt nanoparticles (PtNP/γ-Al2O3) prepared using the conventional impregnation method. Recently, our group discovered a simple method for synthesizing the precursor [Pt17(CO)12(PPh3)8]Cl n . Furthermore, Pt17 is a Pt cluster within the size range associated with high catalytic activity. By combining our established synthesis and loading methods, other groups can conduct further research on Pt17/γ-Al2O3 to explore its catalytic activities in greater depth.

Energy-Dependent Relative Cross Sections in Carbon 1s Photoionization: Separation of Direct Shake and Inelastic Scattering Effects in Single Molecules

We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range allows us to study and disentangle shake processes and intramolecular inelastic scattering effects. In this gas-phase study, relative intensities of the carbon 1s photoelectron lines from chemically inequivalent carbon atoms in the same molecule have been measured as a function of the incident photon energy in the range of 300-6000 eV. We present relative cross sections for the chemically shifted carbon 1s lines in the photoelectron spectra of ethyl trifluoroacetate (the "ESCA" molecule). The results are compared with those of methyl trifluoroacetate and S-ethyl trifluorothioacetate as well as a series of chloro-substituted ethanes and 2-butyne. In the soft X-ray energy range, the cross sections show an extended X-ray absorption fine structure type of wiggles, as was previously observed for a series of chloroethanes. The oscillations are damped in the hard X-ray energy range, but deviations of cross-section ratios from stoichiometry persist, even at high energies. The current findings are supported by theoretical calculations based on a multiple scattering model. The use of soft and tender X-rays provides a more complete picture of the dominant processes accompanying photoionization. Such processes reduce the main photoelectron line intensities by 20-60%. Using both energy ranges enabled us to discern the process of intramolecular inelastic scattering of the outgoing electron, whose significance is otherwise difficult to assess for isolated molecules. This effect relates to the notion of the inelastic mean free path commonly used in photoemission studies of clusters and condensed matter.

TiO2-supported Pt single atoms by surface organometallic chemistry for photocatalytic hydrogen evolution

Platinum single atoms are grafted by SOMC on morphology-controlled TiO₂. Their structure is characterized by EXAFS and other techniques, and their activity and stability in HER and backwards reaction are studied and compared to Pt nanoparticles.

Probing a Device's Active Atoms

Materials science and device studies have, when implemented jointly as "operando" studies, better revealed the causal link between the properties of the device's materials and its operation, with applications ranging from gas sensing to information and energy technologies. Here, as a further step that maximizes this causal link, the paper focuses on the electronic properties of those atoms that drive a device's operation by using it to read out the materials property. It is demonstrated how this method can reveal insight into the operation of a macroscale, industrial-grade microelectronic device on the atomic level. A magnetic tunnel junction's (MTJ's) current, which involves charge transport across different atomic species and interfaces, is measured while these atoms absorb soft X-rays with synchrotron-grade brilliance. X-ray absorption is found to affect magnetotransport when the photon energy and linear polarization are tuned to excite FeO bonds parallel to the MTJ's interfaces. This explicit link between the device's spintronic performance and these FeO bonds, although predicted, challenges conventional wisdom on their detrimental spintronic impact. The technique opens interdisciplinary possibilities to directly probe the role of different atomic species on device operation, and shall considerably simplify the materials science iterations within device research.

X-ray absorption spectroscopy of dinuclear metallohydrolases

In this mini-review, we briefly discuss the physical origin of x-ray absorption spectroscopy (XAS) before illustrating its application using dinuclear metallohydrolases as exemplary systems. The systems we have selected for illustrative purposes present a challenging problem for XAS, one that is ideal to demonstrate the potential of this methodology for structure/function studies of metalloenzymes in general. When the metal ion is redox active, XAS provides a sensitive measure of oxidation-state-dependent differences. When the metal ion is zinc, XAS is the only spectroscopic method that will provide easily accessible structural information in solution. In the case of heterodimetallic sites, XAS has the unique ability to interrogate each metal site independently in the same sample. One of the strongest advantages of XAS is its ability to examine metal ion site structures with crystallographic precision, without the need for a crystal. This is key for studying flexible metal ion sites, such as those described in the selected examples, because it allows one to monitor structural changes that occur during substrate turnover.

Characterization of MOFs. 2. Long and Local Range Order Structural Determination of MOFs by Combining EXAFS and Diffraction Techniques

This chapter provides an elementary introduction to X‐ray and neutron scattering theory, written with a didactic perspective. At the beginning, the scattering process is introduced in a general way and then a differentiation between crystalline samples and amorphous samples is made, leading to the Bragg equation or to the Debye equation and to the Pair Distribution Function (PDF) approach, respectively. Advantages and disadvantages of the use of X‐rays or neutrons for scattering experiments are underlined. The basics of Extended X‐ray Absorption Fine Structure (EXAFS) spectroscopy are also reported. Starting from these basics, five examples have been selected from the recent literature where the concepts described in the first didactic part have been applied to the understanding of the structure of different MOFs materials.

Stereochemical analysis of ferrocene and the uncertainty of fluorescence XAFS data

Methods for the quantification of statistically valid measures of the uncertainties associated with X-ray absorption fine structure (XAFS) data obtained from dilute solutions using fluorescence measurements are developed. Experimental data obtained from 10 mM solutions of the organometallic compound ferrocene, Fe(C(5)H(5))(2), are analysed within this framework and, following correction for various electronic and geometrical factors, give robust estimates of the standard errors of the individual measurements. The reliability of the refinement statistics of standard current XAFS structure approaches that do not include propagation of experimental uncertainties to assess subtle structural distortions is assessed in terms of refinements obtained for the staggered and eclipsed conformations of the C(5)H(5) rings of ferrocene. Standard approaches (XFIT, IFEFFIT) give refinement statistics that appear to show strong, but opposite, preferences for the different conformations. Incorporation of experimental uncertainties into an IFEFFIT-like analysis yield refinement statistics for the staggered and eclipsed forms of ferrocene which show a far more realistic preference for the eclipsed form which accurately reflects the reliability of the analysis. Moreover, the more strongly founded estimates of the refined parameter uncertainties allow more direct comparison with those obtained by other techniques. These XAFS-based estimates of the bond distances have accuracies comparable with those obtained using single-crystal diffraction techniques and are superior in terms of their use in comparisons of experimental and computed structures.

An Investigation of Uranium L-Edges of Metamict and Annealed Betafite

The uranium site in naturally occurring metamict minerals of the pyrochlore group (A1–2B2O6Y0–1) has been investigated using x-ray absorption spectroscopy (XAS). Pyrochlore structures are common phases in proposed polycrystalline waste forms. Betafite, a member of the pyrochlore group (B = 2Ti ≥ Nb+Ta), exhibits U-O bond lengths of 1.94 and 2.37 A for the metamict state, and 2.03 and 2.51 A for the crystalline (annealed) state. The U-O bond lengths decrease (∼0.1 A) and there is a disruption in the second nearest neighbor periodicidy as material is converted from the crystalline to the metamict state.

X-Ray Absorption Spectroscopic Studies of Catalytic Materials

X-ray absorption spectra (XAS) contain information in the LIII near-edge region on filling of the absorber d-band, and in the extended fine-structure region on the physical environment of the absorber. We report here an evaluation of the effect on platinum LIII edges of preparation in clusters with a high fraction of Pt atoms at the surface. We also report the effects on platinum and rhenium LIII edges from addition of copper. These effects are surprisingly small.

We have also re-evaluated extended x-ray absorption fine-structure spectra (EXAFS) of platinum and rhenium in alumina-supported platinum-rhenium bimetallic catalysts. A novel feature of this new analysis was the requirement that interatomic distances, coordination numbers, and Debye-Waller type factors maintain certain physically necessary relationships among themselves. This procedure decreased the number of free variables and increased the amount of information returned by the analysis.

EXAFS Spectroscopy – Fundamentals, Measurement Techniques, Data Evaluation and Applications in the Field of Phthalocyanines

EXAFS spectroscopy is a useful method for determining the local structure around a specific atom in disordered systems. This technique provides information about the coordination number, the nature of the scattering atoms surrounding a particular absorbing atom, the interatomic distance between the absorbing atom and the backscattering atoms, and Debye–Waller factor. The measurements are done with high energy X-rays, which are normally generated by synchrotron radiation sources. The data analysis is facilitated by specially developed program packages suitable for evaluation purposes. EXAFS spectroscopy is employed in several fields for a variety of applications. Here the structural characterization of a series of amorphous μ-oxo-bridged metallophthalocyanine dimers is presented. It is found that the phthalocyanine macrocycle has significant influence in the spectra and the results obtained are in agreement with the well-known structure of phthalocyanine complexes.

Partial cation-order and early-stage, phase separation in phase W, Li x Co 1− x O: 0.075≤ x ≤0.24−0.31

We report the characterization using X-ray and neutron powder diffraction, transmission electron microscopy and extended X-ray absorption fine structure of a new, partially ordered rock-salt-like solid solution phase Li x Co 1− x O: 0.075≤ x ≤0.24−0.31. The cation stacking sequence along [111] consists of alternating planes of Co and Co/Li. Nano-sized domains of this cation-ordered phase appear alongside disordered regions; domain size increases from 2 to 8 nm with increasing Li content. Compositions of ordered and disordered regions are Li- and Co-rich, respectively, and, therefore, the phase exhibits frozen-in, incipient phase separation. This microstructure could be considered as a precursor to precipitation of fully ordered, rhombohedral LiCoO 2 .

EXAFS Characterization of Dendrimer−Pt Nanocomposites Used for the Preparation of Pt/γ-Al2O3Catalysts

Pt/gamma-Al2O3 catalysts were prepared using hydroxyl-terminated generation four (G4OH) PAMAM dendrimers as the templating agents and the various steps of the preparation process were monitored by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS results indicate that, upon hydrolysis, chlorine ligands in the H(2)PtCl(6) and K(2)PtCl(4) precursors were partially replaced by aquo ligands to form [PtCl3(H2O)3]+ and [PtCl2(H2O)2] species, respectively. The results further suggest that, after interaction of such species with the dendrimer molecules, chlorine ligands from the first coordination shell of Pt were replaced by nitrogen atoms from the dendrimer interior, indicating that complexation took place. This process was accompanied by a substantial transfer of electron density from the dendrimer to platinum, indicating that the dendrimer plays the role of a ligand. Following treatment of the H(2)PtCl(6)/G4OH and K(2)PtCl(4)/G4OH complexes with NaBH4, no substantial changes were observed in the electronic or coordination environment of platinum, indicating that metal nanoparticles were not formed during this step under our experimental conditions. However, when the reduction treatment was performed with H2, the formation of extremely small platinum clusters, incorporating no more than four Pt atoms was observed. The nuclearity of these clusters depends on the length of the hydrogen treatment. These Pt species remained strongly bonded to the dendrimer. Formation of larger platinum nanoparticles, with an average diameter of approximately 10 A, was finally observed after the deposition and drying of the H(2)PtCl(6)/G4OH nanocomposites on a gamma-Al(2)O(3) surface, suggesting that the formation of such nanoparticles may be related to the collapse of the dendrimer structure. The platinum nanoparticles formed appear to have high mobility because subsequent thermal treatment in O2/H2, used to remove the dendrimer component, led to further sintering.