X-ray Absorption Spectroscopy Methods Research Articles

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis.

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Demonstration of Sensitivity of the Total-Electron-Yield Extended X-ray Absorption Fine Structure Method on Plastic Deformation of the Surface Layer

X-ray Absorption Fine Structure Spectroscopy (XAFS) has proven instrumental for the study of atomic-scale structures across diverse materials. This study conducts a meticulous comparative analysis between total electron yield (TEY) and absorption coefficients at the K absorption edge of polycrystalline Fe and Zr60Cu20Fe20 alloy. Our findings not only highlight differences between TEY and transmission XAFS measurements but also demonstrate the capabilities and limitations inherent in these measurement modes within the context of XAFS. This article provides an experimental exploration of widely used X-ray absorption spectroscopy methods, shedding light on the nuances of TEY and transmission XAFS. Through presenting experimental results, we aim to offer insights crucial to the material science community, guiding experimentalists in optimizing measurements while raising awareness about potential misinterpretations.

Electronic and Crystal Structure of New CrxZrSe2 Intercalation Compounds.

New intercalation compounds CrxZrSe2 were synthesized and thoroughly studied. Cr atoms were found to occupy the positions both tetrahedrally and octahedrally coordinated by the Se atoms in the interlayer gap. The magnetic properties and electrical resistivity were studied in the temperature ranges of 2.4-300 K and 80-340 K, respectively. The compounds change their behavior from semiconducting (x = 0.1) to metallic (x > 0.1). The magnetic interaction strongly depends on the Cr content and temperature. The spin-glass state with antiferromagnetic interaction exists at T < Tcrit for CrxZrSe2 with x ≤ 0.2, while at x ≥ 0.3 ferromagnetic contribution arises as well. The single crystals of CrxZrSe2 were grown to study the electronic structure of the materials. A combination of the X-ray photoelectron spectroscopy (including that across Cr 2p-3d and Zr 3p-4d resonance) and X-ray absorption spectroscopy methods allowed to propose the location of the Cr 3d-states in the Se 3p-Zr 4d energy gap.

Copper(II) coordination compounds based on bis-hydrazones of 2,6-diacetylpyridine: synthesis, structure, and cytotoxic activity.

By reacting a series of 2,6-diacetylpyridine bis-hydrazones containing pyrimidine (H2L1), benzimidazole (H2L2) and phthalazine (H2L3) heterocyclic fragments with copper(II) chloride and bromide, a variety of pentacoordinated complexes of the composition [Cu(H2L1)X]X, [Cu(HL2)X] and [Cu(HL3)X], where X = Cl-, Br-, are formed. The properties and structure of the compounds were studied by means of NMR, IR, UV-vis, ESR, and X-ray absorption spectroscopy, cyclic voltammetry and X-Ray single crystal diffraction methods. It was shown that complexes of the cationic type [Cu(H2L1)X]X have an asymmetric structure with a distorted square-pyramidal geometry of the coordination unit. The coordination polyhedron of metal chelates [Cu(HL2)X] and [Cu(HL3)X] is an almost ideal square pyramid. Investigations of the cytotoxic activity of the obtained compounds in vitro on human hepatocellular carcinoma (HepG2) and non-tumor human lung fibroblast (MRC-5) cell lines demonstrated that complexes show higher activity compared with the well-known anticancer agent cisplatin. In addition, metal chelates [Cu(H2L1)Cl]Cl, [Cu(HL2)Cl], [Cu(HL2)Br] and [Cu(HL3)Cl] were less toxic to non-tumor cells MRC-5. A study of the binding of complexes to bovine serum albumin (BSA) protein using fluorescence spectroscopy showed that copper complexes are strongly bound to BSA. To study the mechanism of interaction of the complexes with the DNA of cancer cells, molecular dynamics simulation of the compound [Cu(HL3)Cl] was carried out. It was shown that the complex enters into π-stacking interactions predominantly with adenine and thymine bases.

N, O and F K-edges XAS and DFT combination for the exploration of linkage isomers of coordinated nitric oxide in a ruthenium nitrosyl complex

We report on the experimental investigation of single crystals of trans-[RuNOPy4F](ClO4)2 (1) in its ground state (GS). The X-ray absorption spectroscopy (XAS) spectra measured at the N, O and F K-edges were compared to TDDFT calculations to identify and assign the absorption peaks, and to elucidate the structures of coordinated nitric oxide (NO). Based on a reasonable match of experimental and calculated spectra of GS, the N, O and F K-edges XAS spectra of Ru-ON (MS1) and Ru-η2-(NO) (MS2) isomers of 1 were calculated. According to the calculations, the energy or/and intensity of the 1 s→LUMO, LUMO+ 1 peaks of N, O or F K-edge changes significantly after GS isomerization to both MS1 and MS2. Current theoretical modeling of the NO linkage isomer in 1 is a background for the future investigation of isomerization process of NO by XAS methods. Since the investigated isomerization occurs in a variety of different nitrosyl complexes, obtained results can be extrapolated to a large family of transition metal nitrosyl compounds.

From Molecular Oxo-Hydroxo Ce Clusters to Crystalline CeO2

Many studies report the synthesis and characterizations of CeO2 nanoparticles (NPs) due to their applications in catalysis, energy storage, or biomedical fields. In this study, we report a comprehensive interpretation of X-ray measurements on a series of oxo-hydroxo polynuclear Ce complexes whose atomic structures are resolved. The set of investigated samples represent the formation of CeO2 small nanoparticles from a few Ce ions. Such clusters might serve as good building blocks in nanoscale architectures, and many clusters together may perform as highly active small particles. Therefore, the set of Ce clusters with growing size (from 0.6 to 1.2 nm) and nuclearity (from 6 to 38 Ce atoms) were synthesized and characterized by single-crystal X-ray diffraction (XRD), high-energy X-ray scattering (HEXS), and high-energy-resolution fluorescence detection (HERFD) X-ray absorption spectroscopy (XAS) methods and compared to larger CeO2 NPs and bulk CeO2. Methods reveal consistent trends as the size of the system grows from molecular Ce-{n} 2, 6, 24, 38, and 40 clusters to bulk CeO2. HEXS reveals a broadening in distribution for the short Ce–O bonds for the small clusters. Concomitantly, the HERFD-XAS performed at the Ce LIII edge indicates a gradual splitting of the Ce 5d states as the core of the complexes becomes more CeO2-like. The experimental observations have been supported by electronic structure calculations, based on the crystallographic determination of the cluster structures. Theoretical simulations allow us to isolate the structural and electronic properties of individual Ce sites within clusters and mark the great difference between surface and core Ce atoms. It also shows how a combination of simulations from different sites results in the accurate reproduction of the corresponding experimental data. This approach based on the experimentally determined atomic coordinates of clusters was then successfully extended to model Ce LIII edge HERFD-XAS spectra for larger CeO2 NPs. By linking the atomic and electronic structures of Ce polynuclear complexes, CeO2 nanoparticles, and bulk CeO2, this work extends the fundamental knowledge of Ce oxide nanomaterials and supports a better understanding and predictability of their electronic structure.

Copper coordination compounds based on bis-quinolylhydrazone of 2,6-diacetylpyridine: Synthesis, structure and cytotoxic activity

Bis-hetarylhydrazone H2L (1), a condensation product of 2,6-diacetylpyridine with 2-hydrazinoquinoline, has been synthesized. By reaction of 1 with Cu(II) nitrate and perchlorate binuclear mixed-valence coordination compounds [CuICuII(H2L)2](NO3)3⋅2H2O (2) and [CuICuII(H2L)2](ClO4)3 (3) were obtained. Cu(I) Ion in the compounds 2, 3 is in the distorted tetrahedral N-donor environment, while Cu(II) center is in the distorted octahedral one. Cu(II) Chloride and bromide form with 1 asymmetric mononuclear compounds with composition [Cu(H2L)Cl]Cl (4) and [Cu(H2L)Br]Br (5) with tetragonal–pyramidal coordination polyhedron. The structures of the compounds were studied by means of NMR, IR, ESR, UV–vis, X-ray absorption spectroscopy and X-ray single crystal diffraction methods. The effect of compounds on viability of the Hep-2 and HepG2 cell lines was investigated in vitro. The study showed that 1 is non-toxic at tested concentrations (1–100 μM), while all the complexes exhibit significant dose-dependent cytotoxic effect. It is shown that compounds 2–5 demonstrate higher cytotoxicity towards considered cancer cell cultures than widely used commercial anticancer pharmaceuticals – cisplatin and carboplatin. Quantum-chemical modeling of the structure of the ligand 1 and its complexes was performed using density functional theory at B3LYP/6-311G(d) level of theory. Molecular electrostatic potential (MEP) and average local ionization energy (ALIE) surfaces were calculated to rationalize the reactive properties of the compounds.

Nanometer-scale depth-resolved hard x-ray absorption spectroscopy based on the detection of energy-loss Auger electrons with low energies

X-ray absorption spectroscopy (XAS) provides information on the chemical state and atomic structure of a target element even without long-range periodicity. A depth-resolved surface-sensitive XAS method for K-edge using hard x rays is proposed herein. The principle of this method is based on the selective detection of low-energy electrons using an electron analyzer. The detected electrons originate from the LMM Auger electrons that cascade from the KLL Auger process caused by K-shell absorption and lose the energy corresponding to their travel distance in a solid. The analysis depth was confirmed to be in the nanometer range using a GaN substrate with a thin oxide film of the defined thickness. In addition, depth-resolved information regarding the local atomic structure, including the interatomic distance and crystallinity, was obtained via the Fourier transformation of the spectral oscillations. Because the proposed technique does not require a change in the detection angle, it is also expected to be used for particles and samples with uneven surfaces.

Correlating wavelength dependence in LiMn2O4 cathode photo-accelerated fast charging with deformations in local structure

The growth in electrified transportation has benefited from the massive worldwide research efforts used to discover and improve electrode materials and electrolytes. Nevertheless, lithium-ion batteries still suffer from a slow-charging limitation. Recently, it has been demonstrated that white light illumination of LiMn2O4 provokes faster charging, improving the kinetics of delithiation without the use of nanostructured active materials. In this work, we probe the mechanism of photo-accelerated fast charging and show that Mn d-d electronic transitions occurring under red light illumination are largely responsible for the increased charging rate. It is further demonstrated through X-ray absorption spectroscopy methods that LiMn2O4 Mn–Mn bond distances shorten after d-electron excitation. The shrinkage in the crystal volume beneficially contributes to delithiation kinetics by lowering the resistance to lithium-ion conduction. Advanced materials that can absorb light to modulate their structure may provide us with a new mechanistic pathway to pursue for increasing charge transfer rates.

Local structure study of the Fe ions in mixed-valence iron(II)-iron(III) metal formate frameworks

X-ray Absorption Spectroscopy (XAS) methods have been used to study the mixed-valence iron(II)-iron(III) metal–organic frameworks (MOFs) that undergo order–disorder phase transitions. In particular, this paper reports X-ray Absorption Fine Structure (XAFS) study of the electronic and local structure at the Fe K-edge (7 keV) for three mixed-valence iron(II)-iron(III) formate frameworks, namely dietylammonium ((C2H5)2NH2[FeIIIFeII(HCOO)6], DEtAFeFe), dimethylammonium ((CH3)2NH2[FeIIIFeII(HCOO)6], DMAFeFe) and N-etylmethylammonium (CH3NH2C2H5[FeIIIFeII(HCOO)6], EtMeAFeFe) analogues. DMAFeFe (DEtAFeFe) was studied at 140 K and 155 K (240 K and 250 K) in detail to obtain information on the structural changes during the phase transitions. The near-edge x-ray absorption fine structure (XANES) analysis confirms the octahedrally coordinated Fe in the studied samples and the electron transfer from divalent to trivalent iron ions. A slight variation was found in the Fe-O bond lengths and quantified the order in the bond length distribution measured by the Debye-Waller factor (σ2) around the phase transition temperature. The Fe2+/Fe3+ ions are homogeneously distributed in the order–disorder phase transition samples. The slight variation in the local structure defines the nature of the phase transition.

Aliovalent Doping of CeO2 Improves the Stability of Atomically Dispersed Pt.

Atomically dispersed supported catalysts hold considerable promise as catalytic materials. The ability to employ and stabilize them against aggregation in complex process environments remains a key challenge to the elusive goal of 100% atom utilization in catalysis. Herein, using a Gd-doped ceria support for atomically dispersed surface Pt atoms, we establish how the combined effects of aliovalent doping and oxygen vacancy generation provide dynamic mechanisms that serve to enhance the stability of supported single-atom configurations. Using correlated, in situ X-ray absorption, photoelectron, and vibrational spectroscopy methods for the analysis of samples on the two types of support (with and without Gd doping), we establish that the Pt atoms are located proximal to Gd dopants, forming a speciation that serves to enhance the thermal stability of Pt atoms against aggregation.

Real-Time Observation of Surface Chemical Reactions Proceeding in the Depth Direction by Wavelength-Dispersive Soft X-ray Absorption Spectroscopy.

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous time- and depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O2 gas at the surface and the interlayer reaction by oxygen migration.

Iodine local environment in high pressure borosilicate glasses: An X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigation

• Iodine is dissolved as iodide (I − ) in high pressure borosilicate glasses. • Both alkali and alkaline-earth cations act as a charge compensator to I − species. • The absence of iodate (I 5+ ) suggests that there is a complex interplay between I − species and oxygen species within the glass structure. The 129 I radioactive isotope is a by-product of nuclear plants activity. Owing to its strong volatility, there is currently no ideal protocol to immobilize 129 I in nuclear waste borosilicate glasses. Recently, we have proposed the use of high-pressure syntheses to dissolve iodine in various glass compositions; however, I speciation and dissolution mechanism could not be determined. We have adopted an approach combining X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) methods to determine I speciation and molecular environment in glasses containing from 0.5 to 2.5 mol.% I. The XPS spectra reveal that I is mostly dissolved as iodide (>85% I − ) with a small proportion of elemental iodine (<15% I 0 ) and the absence of iodate species (I 5+ ). For borosilicate glasses, the XAS results and subsequent spectrum simulations suggested that Na and Ca are involved in the I − vicinity with averaged derived coordination number (CN) of 3.6 and 2.0 and bond length to the nearest neighbour (r X-I ) 2.98 and 2.85 Å, respectively. These results suggest that the coexistence of both I − and I 5+ species is not requested for electric neutrality but instead, we explain the I speciation by the possible interplay with oxygen species from the borosilicate matrix. In addition, the results imply that the borosilicate network is affected by the I dissolution.

Elucidating the structure-dependent selectivity of CuZn towards methane and ethanol in CO2 electroreduction using tailored Cu/ZnO precatalysts.

Understanding the catalyst compositional and structural features that control selectivity is of uttermost importance to target desired products in chemical reactions. In this joint experimental–computational work, we leverage tailored Cu/ZnO precatalysts as a material platform to identify the intrinsic features of methane-producing and ethanol-producing CuZn catalysts in the electrochemical CO2 reduction reaction (CO2RR). Specifically, we find that Cu@ZnO nanocrystals, where a central Cu domain is decorated with ZnO domains, and ZnO@Cu nanocrystals, where a central ZnO domain is decorated with Cu domains, evolve into Cu@CuZn core@shell catalysts that are selective for methane (∼52%) and ethanol (∼39%), respectively. Operando X-ray absorption spectroscopy and various microscopy methods evidence that a higher degree of surface alloying along with a higher concentration of metallic Zn improve the ethanol selectivity. Density functional theory explains that the combination of electronic and tandem effects accounts for such selectivity. These findings mark a step ahead towards understanding structure–property relationships in bimetallic catalysts for the CO2RR and their rational tuning to increase selectivity towards target products, especially alcohols.

Exploring the Charge Compensation Mechanism of P2-Type Na0.6Mg0.3Mn0.7O2 Cathode Materials for Advanced Sodium-Ion Batteries

P2-type sodium layered transition metal oxides have been intensively investigated as promising cathode materials for sodium-ion batteries (SIBs) by virtue of their high specific capacity and high operating voltage. However, they suffer from problems of voltage decay, capacity fading, and structural deterioration, which hinder their practical application. Therefore, a mechanistic understanding of the cationic/anionic redox activity and capacity fading is indispensable for the further improvement of electrochemical performance. Here, a prototype cathode material of P2-type Na0.6Mg0.3Mn0.7O2 is comprehensively investigated, which presents both cationic and anionic redox behaviors during the cycling process. By a combination of soft X-ray absorption spectroscopy and electroanalytical methods, we unambiguously reveal that only oxygen redox reaction is involved in the initial charge process, then both oxygen and manganese participate in the charge compensation in the following discharge process. In addition, a gradient distribution of Mn valence state from surface to bulk is disclosed, which could be mainly related to the irreversible oxygen activity during the charge process. Furthermore, we find that the average oxidation state of Mn is reduced upon extended cycles, leading to the noticeable capacity fading. Our results provide deeper insights into the intrinsic cationic/anionic redox mechanism of P2-type materials, which is vital for the rational design and optimization of advanced cathode materials for SIBs.

P.191 The impact of physical activity and fitness on resilience to modern-life stress and the role of brain-derived neurotrophic factor (BDNF)

Although zinc is ubiquitous in ambient particulate matter (PM), exposure to size fractionated residual oil fly ash (ROFA) PM is of particular concern due to several studies implicating particle zinc in mortality, adverse respiratory and cardiac effects. The lack of zinc speciation data is an impediment to our understanding of the biological mechanism of Zn induced toxicity. ROFA PM2.5+ samples (i.e. particle with aerodynamic diameter >2.5 μm) are prepared in a small 732 kW practical fire tube boiler by combusting one No. 5 and three No. 6 residual oil(s) of varying sulfur and ash contents. The combined X-ray absorption spectroscopy and selective extraction method is used to determine the Zn speciation in PM2.5+ samples. The Zn speciation in ROFA PM2.5 (i.e. particle with aerodynamic diameter <2.5 μm) samples, reported earlier, is included here for comparison purposes. The data show that zinc sulfate is predominant in both fractions. In addition, an appreciable amount of zinc phosphate was detected in the PM2.5 fraction. The insoluble zinc forms in both the fractions are identified as Zn-sorbed-ferrihydrite and zinc sulphide. The variation of Zn speciation across different size fractions has implication on the bioavailability and toxicity of Zn. Such source specific speciation data are needed for developing source inventories, and for amending existing regulations/framing new regulations for different size fractions.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy.

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst’s interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

QEXAFS method implementation using adaptive X-ray optical elements

A modification of the of high-speed high-resolution X-ray absorption spectroscopy (Quick Extended X-ray Absorption Fine Structure, QEXAFS) method based on an adaptive bending element of X-ray optics is described. This technique was implemented at the Langmuir station of the Kurchatov Synchrotron Radiation Source (KISI-Kurchatov). The absorption spectrum of a model sample, bromine in a sodium bromide compound, was recorded. A recording speed up to 5 seconds per spectrum was achieved in the continuously recording regime. The method is applicable for obtaining information about both the structure and dynamics of a wide range of objects, ordered and disordered, in any aggregate state.

Elucidating the Redox Mechanism of Battery Cathode Materials Made from Earth-Abundant Elements

Sodium ion batteries (NIBs) are a promising technology for grid storage due to the natural abundance, low cost, and suitable redox potential (-2.71 V vs. SHE) of Na. Exploration of NIB cathode materials that include abundant and inexpensive elements, has led to the investigation of sodium iron manganese oxides. However, these materials suffer from low electrochemical reversibility during cycling, and their reaction mechanisms are unresolved, presenting an obstacle for their further optimization. In literature, three different reaction mechanisms have been reported for sodium iron manganese cathodes including 1) Fe 2+/3+, 2) Fe 3+/4+, and 3) oxygen redox. Additionally, the reported redox measurements are ex situ, which leaves uncertainty if the oxidation state of these materials changes between electrochemical cycling and redox measurement. Herein, for the first time we combine operando x-ray absorption spectroscopy (XAS) and computation method to report the redox mechanisms of P2-Na2/3FexMn1-xO2 (x=1/4, 1/3, 1/2). The redox mechanism and capacity retention of these cathode materials is found to be dependent on the Fe content. When x=1/4, 1/3 the redox mechanism is Fe 2+/3+ and Mn 3+/4+, and the material exhibits high capacity retention. When x=1/2, the redox mechanism is Fe 3+/4+ and Mn 3+/ 4+, and the material shows poor capacity retention. These results indicate that improved reversibility of sodium iron manganese oxides can be obtained by limiting the amount of the Fe 3+/4+ redox through reducing the Fe content. These results will contribute to the rational design of long lifetime cathode materials made from earth-abundant elements. Figure 1

X-ray absorption spectral analysis of heavy metals with Si-PIN detectors

An X-ray absorption spectroscopy method for the detection of heavy metals at the K-absorption edges is implemented using the designed Si-PIN detector. The limits of detection of heavy metals and uncertainty of measurement are estimated by the results of measurements of the thickness of gold and lead foil.