X-ray Absorption Spectroscopy Research Articles

Packing Engineering of Zirconium Metal-Organic Cages in Mixed Matrix Membranes for CO2/CH4 Separation.

Metal-organic cages (MOCs) have been considered as emerging zero-dimensional (0D) porous fillers to generate molecularly homogeneous MOC-based membrane materials. However, the discontinuous pore connectivity and low filler concentrations limit the improvement of membrane separation performance. Herein, we propose the dimension augmentation of MOCs in membranes using three-dimensional (3D) supramolecular MOC networks as filler materials in mixed matrix membranes (MMMs). We further explore the packing engineering of MOC networks to produce distinct polymorphs (α and β phases) for tailoring membrane performance. Synchrotron X-ray absorption and positron annihilation lifetime spectroscopy were employed to differentiate distinct MOC polymorphous networks within membranes. Gas permeation tests revealed that the corresponding MMMs showed superior CO2/CH4 separation performance, exceeding the Robeson upper bound. Our proposed approach is expected to enrich the repertoire of reticular chemistry pertaining to molecular-based networks.

Solar-driven production of renewable chemicals via biomass hydrogenation with green methanol

Solar-driven, selective biomass hydrogenation is recognized as a promising route to renewable chemicals production, but remains challenging. Here, we report a TiO2 supported Cu single-atom catalyst with a four-coordinated Cu1−O4 structure, which can be universally applied for solar-driven production of various renewable chemicals from lignocellulosic biomass-derived platform molecules with good yields using green methanol as a hydrogen donor, to address this challenge. It is significant that the biomass upgrading driven by natural sunlight on a gram scale demonstrates the great practical potential. By combining in situ soft X-ray absorption spectroscopy with theoretical calculations, we successfully identify the dynamic evolution of Cu sites along with the biomass hydrogenation and methanol oxidation, where the tandem process is enabled by the photogenerated electrons and holes to complete a chemical cycle. The concept of solar-driven biomass hydrogenation proposed here provides an efficient and sustainable methodology for the sustainable production of renewable chemicals.

Investigation of magnetic properties in Y3Fe5O12/Cr2O3/Pt tri-layers

Abstract Artificial multiferroics combining ferroelectric and magnetic materials exhibits a sizable magnetoelectric coupling and further shows a great potential for realizing low power consumption information processing. In this work, hybrid structures consisting of the ferrimagnetic yttrium iron garnet (Y3Fe5O12, (YIG)) layer and anti-ferromagnetic chromium oxide (Cr2O3) layer are deposited directly onto (110)-oriented single-crystal Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) ferroelectric substrates by means of radio frequency magnetron sputtering and post-annealing procedures. The dynamical properties of YIG films are investigated via ferromagnetic resonance measurements and a low damping constant of 0.00258 is obtained. Combining the element-specific soft x-ray absorption spectroscopy, x-ray magnetic circular dichroism and x-ray magnetic linear dichroism, the magnetic order in YIG film and the Néel order in Cr2O3 layers are further characterized. By capping a platinum (Pt) layer on PMN-PT/YIG/Cr2O3, a visible spin-charge converted signal has been obtained via both spin pumping and spin Seebeck measurements. These results indicate that the artificial multiferroic structures PMN-PT/YIG/Cr2O3/Pt serve as a promising material platform for realizing an efficient manipulation of magnetism via electrical means.

Tailoring Water-in-DMSO Electrolyte for Ultra-stable Rechargeable Zinc Batteries.

Rechargeable zinc batteries (RZBs) are hindered by two primary challenges: instability of Zn anode and deterioration of the cathode structure in traditional aqueous electrolytes, largely attributable to the decomposition of active H2O. Here, we design and synthesize a non-flammable water-in-dimethyl sulfoxide electrolyte to address these issues. X-ray absorption spectroscopy, in situ techniques and computational simulations demonstrate that the activity of H2O in this electrolyte is extremely compressed, which not only suppresses the side reactions and increases the reversibility of Zn anode, but also diminishes the cathode dissolution and proton intercalation. The hybrid solid-electrolyte interface (SEI), formed in situ, helps Zn-Zn symmetric cell a prolonged lifespan exceeding 10000 hat 0.5 mA cm-2 and 600 h at a 60% discharge depth. The versatility of this electrolyte endows the Zn-VO2 full batteries ultra-stable cycling performance. This work provides insights into electrolyte structure-property relationships, and facilitates the design of high-performance RZBs.

Chemical speciation and availability of molybdenum in soils to wheat uptake.

Molybdenum (Mo) is an essential micronutrient for plants, yet it also poses potential environmental risks when present in excess. This study investigated the Mo speciation in soils with varying properties and their influences on Mo uptake by wheat (Triticum aestivum L.), a staple crop with significant implications for global food security. Mo K-edge X-ray absorption spectroscopy (XAS), combined with a sequential extraction method, was employed to analyze the chemical speciation and fractionation of Mo in the soils before and after wheat cultivation. The predominant Mo species identified were sorbed molybdate (Mo(VI)) and Ca- and Fe-Mo(VI) precipitates. After wheat cultivation, sorbed Mo(VI) and Ca-Mo(VI) decreased while Fe-Mo(VI) increased, with the most notable changes observed in the alkaline soil. These changes indicated that the desorption of sorbed Mo(VI) and dissolution of Ca-Mo(VI) contributed to soil Mo availability, while Fe-Mo(VI) precipitation restricted it. Bioaccumulation and translocation factors revealed efficient Mo uptake and transport within wheat plants, with shoots being the primary site of Mo accumulation. Elevated Mo concentrations in wheat grains raise potential human health concerns due to dietary exposure. These findings underscore the critical role of soil Mo speciation in controlling Mo dynamics in soil-wheat systems, providing valuable insights for managing Mo in agricultural soils to balance its nutritional benefits with the risks of excessive crop accumulation.

Swift heavy ion induced electronic structure modifications in ZnO thin films: X-ray absorption spectroscopy study

Abstract The ZnO is a potential material for electronic devices application due to its versatile semiconducting properties with a direct band gap of 3.3 eV.
This research represents a modification in the electronic structure of ZnO thin films (350 nm) irradiated with swift heavy ions (SHI) at different fluence. ZnO thin films were grown on silicon substrate (100) using radio-frequency (RF) magnetron sputtering which are highly oriented in single phase/plane (002). These deposited thin films were subjected to SHI of 100 MeV O-ion with fluence of 3$\times$10$^{12}$ ions/cm$^2$, 45 MeV Li-ion beams with fluence of 3$\times$10$^{12}$ ions/cm$^2$, and 120 MeV Au-ion with varying fluence from 1$\times$10$^{11}$ to 5$\times$10$^{12}$ ions/cm$^2$. {SRIM/TRIM software was utilized to studied theoretical effect of SHI like ion ranges, phonon energy, etc.} Modifications in the structure and the electrical properties of ZnO thin films were investigated using the high-resolution X-ray diffraction (HRXRD), {Atomic Force Microscopy (AFM)} and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the O K- and Zn L$_{3,2}$-edges. HRXRD revealed the structure, crystallinity and phase of the system whereas NEXAFS revealed the local atom bonding symmetry via the molecular orbital hybridization between Zn and O. Further, Density functional theory (DFT) based first principle FEFF9.6 software has been discussed for the understanding of the various parameter and cards for the good fitting of NEXAFS. Simulations were utilized to support the NEXAFS experimental data for the determination of various physical entity and their interpretations for diverse applications. NEXAFS measurements provide the deeper understanding of the effects of SHI for the promising applications through controlled fluence rate of ion beam irradiations.

Resolving the Nanostructure of Carbon Nitride-Supported Single-Atom Catalysts.

Single-atom catalysts (SACs) are gathering significant attention in chemistry due to their unique properties, offering uniform active site distribution and enhanced selectivity. However, their precise structure often remains unclear, with multiple models proposed in the literature. Understanding the coordination environment of the active site at the atomic level is crucial for explaining catalytic activity. Here, a comprehensive study of SACs made of carbon nitride (CNx) containing isolated nickel atoms is presented. Using a combination of synthesis techniques and characterization methods including Fourier-transform infrared spectroscopy, X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations, the local environment of nickel active centers in CNx-supported SACs is investigated. These results challenge conventional structural models and propose a new architecture that better aligns with current experimental evidence. This new structure serves as a foundational step toward a rational approach to catalyst development and can facilitate more precise design and application of these innovative catalysts.

Toward a Machine Learning Approach to Interpreting X-ray Spectra of Trace Impurities by Converting XANES to EXAFS.

The fact that the photoabsorption spectrum of a material contains information about the atomic structure, commonly understood in terms of multiple scattering theory, is the basis of the popular extended X-ray absorption spectroscopy (EXAFS) technique. How much of the same structural information is present in other complementary spectroscopic signals is not obvious. Here we use a machine learning approach to demonstrate that within theoretical models that accurately predict the EXAFS signal, the extended near-edge region does indeed contain the EXAFS-accessible structural information. We do this by exhibiting deep operator neural networks (DeepONets) that have learned the relationship between the extended and near edge portions of the X-ray absorption spectrum to predict the former from the latter. We find that we can accurately predict the EXAFS spectrum between 6 and 14 Å-1 from the first 6 Å-1 (≈100 eV) of the absorption spectrum of Cu2+ substitutional defects in the Fe3+ mineral hematite (α-Fe2O3). This surprising finding implies that theoretical analyses of X-ray absorption spectra could be implemented that extract the same conclusions as high-quality EXAFS studies from spectra collected over a much smaller range of photon energies. This relaxes a host of experimental limitations related to the X-ray source and measurement sample, including collection time, minimum dopant concentration, source brilliance, and energy range. We describe the theoretical data sets and DeepONet construction and show that the resulting DeepONets produce EXAFS that recovers linear combination fits to experimental data with accuracy approaching the original ab initio calculations. We discuss the implications of our findings for minor constituent characterization and for understanding the information content of spectroscopic data more broadly, including how this approach might be applied to measured experimental spectra. To encourage similar efforts, the simulated X-ray spectra, machine learning, and fitting code are publicly available.

Boosting the Performance of Alkaline Anion Exchange Membrane Water Electrolyzer with Vanadium-Doped NiFe2O4.

Developing efficient, economical, and stable catalysts for the oxygen evolution reaction is pivotal for producing large-scale green hydrogen in the future. Herein, a vanadium-doped nickel-iron oxide supported on nickel foam (V-NiFe2O4/NF) is introduced, and synthesized via a facile hydrothermal method as a highly efficient electrocatalyst for water electrolysis. X-ray photoelectron and absorption spectroscopies reveal a synergistic interaction between the vanadium dopant and nickel/iron in the host material, which tunes the electronic structure of NiFe2O4 to increase the number of electrochemically active sites. The V-NiFe2O4/NF electrode exhibited superior electrochemical performance, with a low overpotential of 186mV at a current density of 10mAcm-2, a Tafel slope value of 54.45mV dec-1, and minimal charge transfer resistance. Employing the V-NiFe2O4/NF electrode as an anode in an alkaline anion exchange membrane water electrolyzer single-cell, a cell voltage of 1.711V is required to achieve a high current density of 1.0 Acm-2. Remarkably, the cell delivered an energy conversion efficiency of 73.30% with enduring stability, making it a promising candidate for industrial applications.

Ligand-Induced Activation of Single-Atom Palladium Heterogeneous Catalysts for Cross-Coupling Reactions.

Single-atom heterogeneous catalysts (SACs) are potential, recoverable alternatives to soluble organometallic complexes for cross-coupling reactions in fine-chemical synthesis. When developing SACs for these applications, it is often expected that the need for ligands, which are essential for organometallic catalysts, can be bypassed. Contrary to that, ligands remain almost always required for palladium atoms stabilized on commonly used functionalized carbon and carbon nitride supports, as the catalysts otherwise show limited activity. Despite this, ligand optimization has received little attention, and their role in activating SACs is poorly understood. Here, we explore the impact of structurally diverse phosphine ligands on the performance of nitrogen-doped carbon supported single-atoms (Pd1@NC) in the Sonogashira-Hagihara (SH) cross-coupling reaction, using X-ray absorption spectroscopy and density functional theory simulations to rationalize the observed trends. Compared to the ligand-free SAC, SH activity is enhanced in almost all ligand-assisted systems, with reactivity varying by up to 8 orders of magnitude depending on the ligand choice. Distinct trends emerge based on the free ligand volume and ligand class. Unlike molecular systems, the electronic effects of phosphine ligands are less significant in SACs due to the modulating influence of the support. Instead, the performance of SAC-ligand systems is governed by a balance between the ligand deformation energy during coordination with metal centers, and their resulting accessibility to cross-coupling reagents. These findings offer key insights into optimizing Pd-SACs by leveraging phosphine ligands to activate metal centers and tailor the 3D environment.

A new conservation material for gold in heritage wall paintings: polymer-stabilized nanogold gels (NGGs).

Gilded wall paintings such as those in Petra-Jordan undergo deterioration processes such as delamination and loss of the gold layer. The aim of this work is to produce a functioning long-lasting adhesive that compensates for binder and gold loss while stabilising the gold layer. Polymer-stabilised gold nanoparticles (AuNPs) as a conservation material for gilded heritage paintings (Nano Gold Gel (NGG)) were synthesised using two facile and affordable synthesis approaches. AuNPs enhance the stability of the adhesive polymer over time and introduce mass conservation to the gold layer. Two natural polymers and one synthetic polymer, frequently used in conservation as adhesives, were used as reducing agents and stabilisers for the nanoparticles. The chemical alteration of the polymers and the Au-polymer interaction at the molecular level were investigated with FTIR spectroscopy, while the chemical environment of gold was investigated with X-ray absorption spectroscopy (XANES/EXAFS). The synthesized NGG was applied on the replica samples to reattach the gold layer to its support. Characterisation results indicate that the formation of AuNPs stabilised by the three polymers did not alter the chemical structure of the polymers. The applied NGG successfully achieved re-adhesion and exhibited appropriate optical and chemical properties for use as a conservation material.

Rocksalt-type heavy rare earth monoxides TbO, DyO, and ErO exhibiting metallic electronic states and ferromagnetism.

Solid-phase rare earth monoxides have been recently synthesized via thin film epitaxy. However, it has been difficult to synthesize heavy rare earth monoxides owing to their severe chemical instability. In this study, rocksalt-type heavy rare earth monoxides REOs (RE = Tb, Dy, Er) were synthesized for the first time, as single-phase epitaxial thin films. The REOs were characterized by hard X-ray photoemission spectroscopy, X-ray absorption spectroscopy, resonant photoemission spectroscopy, and magnetization measurements. These REOs showed metallic electronic states with the almost localized 4f states, indicating the 4fn5d1 electronic configurations of Tb, Dy, and Er ions. X-ray magnetic circular dichroism measurements of TbO evidenced the magnetic ordering of the 4f spins below the Curie temperature (TC). The TC was evaluated to be 233 K for TbO, 142 K for DyO, and 88 K for ErO, where the TC decreased with the 4f electron number, approximately proportional to the de Gennes factor.

Implementation of simultaneous ultraviolet/visible and x-ray absorption spectroscopy with microfluidics.

X-ray spectroscopies are uniquely poised to describe the geometric and electronic structure of metalloenzyme active sites under a wide variety of sample conditions. UV/Vis (ultraviolet/visible) spectroscopy is a similarly well-established technique that can identify and quantify catalytic intermediates. The work described here reports the first simultaneous collection of full in situ UV/Vis and high-energy resolution fluorescence detected x-ray absorption spectra. Implementation of a fiber optic UV/Vis spectrometer and parabolic mirror setup inside the dual array valence emission spectrometer allowing for simultaneous measurement of microfluidic flow and mixing samples at the Photon-In Photon-Out X-ray Spectroscopy beamline is described, and initial results on ferricyanide and a dilute iron protein are presented. In conjunction with advanced microfluidic mixing techniques, this will allow for the measurement and quantification of highly reactive catalytic intermediates at reaction-relevant temperatures on the millisecond timescale while avoiding potential complications induced by freeze quenching samples.

Operando X-ray absorption spectroscopic flow cell for electrochemical CO2 reduction: new insight into the role of copper species

We report a novel operando X-ray absorption spectroscopic (XAS) flow cell that enables to probe Cu-GDE thoroughly from surface to bulk during electrochemical CO2 reduction, providing information on copper oxidation and coordination states.

Acetylene semi-hydrogenation catalyzed by Pd single atoms sandwiched in zeolitic imidazolate frameworks via hydrogen activation and spillover.

The semi-hydrogenation of alkynes into alkenes rather than alkanes is of great importance in the chemical industry, and palladium-based metallic catalysts are currently employed. Unfortunately, a fairly high cost and uncontrollable over-hydrogenation impeded the application of Pd-based catalysts on a large scale. Herein, a sandwich structure single atom Pd catalyst, Z@Pd@Z, was prepared via impregnation exchange and epitaxial growth methods (Z stands for ZIF-8), in which Pd single atoms were stabilized by pyrrolic N in a zeolitic imidazolate framework (ZIF-8). Semi-hydrogenation of acetylene was performed and Z@Pd@Z achieved 100% acetylene conversion at 120 °C with an ethylene selectivity of more than 98.3% at an extra low Pd concentration. Z@Pd@Z exhibited a specific activity of 1872.69 mLC2H4 mgPd-1 h-1, surpassing most of the reported Pd-based catalysts. The existence of Pd single atoms coordinated by nitrogen (Pd-N4) was verified by XAS (synchrotron X-ray absorption spectroscopy), which provided active sites for H2 dissociation and the dissociated hydrogen quickly spilled over the surface of the outer ZIF layer to hydrogenate alkyne to ethene; besides, the catalytic activity could be controlled by adjusting the thickness of the outer ZIF layer. The confinement of the ZIF on Pd single-atom sites and the high energy barrier of ethylene hydrogenation were found to be responsible for the superior C2H2 semi-hydrogenation activity. This work opens up valuable insights into the design of ZIF-derived single-atom catalysts for efficient acetylene selective hydrogenation.

Elucidation of the Co4+ state with strong charge-transfer effects in charged LiCoO2 by resonant soft X-ray emission spectroscopy at the Co L3 edge.

To understand the electronic-structure change of LiCoO2, a widely used cathode material in Li-ion batteries, during charge-discharge, we performed ex situ soft X-ray absorption spectroscopy (XAS) and resonant soft X-ray emission spectroscopy (RXES) of the Co L3 edge in combination with charge-transfer multiplet calculations. The RXES profile significantly changed for the charged state at 4.2 V vs. Li/Li+, corresponding to the oxidation reaction from a Co3+ low-spin state for the initial state, while the XAS profile exhibited small changes. For the 4.2-V charged state, we confirmed that approximately half of the initial Co3+ ions were oxidized to Co4+ ions. The multiplet calculation of the RXES results revealed that the Co4+ state has a negative charge-transfer energy and the d6L̲ state (L̲ is a ligand hole) is the most stable. Therefore, the O 2p hole created by the strong charge-transfer effect plays a major role in the redox reaction of LiCoO2.

Synergistic effects of heteroatom engineering in N-doped TiO2 films probed by X-ray absorption and photoelectron spectroscopy

Synergistic effects of heteroatom engineering in N-doped TiO2 films probed by X-ray absorption and photoelectron spectroscopy

Structural, magnetic and x-ray absorption spectroscopy studies of new Cr-based low, medium and high-entropy spinel oxides

Structural, magnetic and x-ray absorption spectroscopy studies of new Cr-based low, medium and high-entropy spinel oxides

Combining sequential extractions with bulk and micro X-ray spectroscopy to elucidate iron and phosphorus speciation in sediments of an iron-treated peat lake.

In shallow lakes, mobilization of legacy phosphorus (P) from the sediments can be the main cause for persisting eutrophication after reduction of external P input. In-lake remediation measures can be applied to reduce internal P loading and to achieve ecosystem recovery. The eutrophic shallow peat lake Terra Nova (The Netherlands) was treated with iron (Fe) to enhance P retention in the sediment. This treatment, however, intensified seasonal internal P loading. An earlier study suggested that Fe addition led to increased P binding by easily-reducible Fe(III) associated with organic matter (OM), which readily releases P when bottom waters turn hypoxic. In this complementary study, bulk and micro Fe K-edge and P K-edge X-ray absorption spectroscopy and micro-focused X-ray fluorescence spectroscopy were applied to characterize the P hosting Fe(III) pool. Combined with sequential extraction data, the synchrotron X-ray analyses revealed that a continuum of co-precipitates of Fe(III) with calcium, phosphate, manganese and organic carbon within the OM matrix constitutes the reducible Fe(III) pool. The complementary analyses also shed new light on the interpretation of sequential extraction results, demonstrating that pyrite was not quantitatively extracted by nitric acid (HNO3) and that most of the Fe(II) extracted by hydrochloric acid (HCl) originated from phyllosilicate minerals. Formation of an amorphous inorganic-organic co-precipitate upon Fe addition constitutes an effective P sink in the studied peaty sediments. However, the high intrinsic reactivity of this nanoscale co-precipitate and its fine distribution in the OM matrix makes it very susceptible to reductive dissolution, leading to P remobilization under reducing conditions.

Zero-strain high entropy spinel oxide (FeNiCuCrMn)3O4@rGO as high-performance anode for sodium ion battery

High entropy spinel oxides (HEOs), as a new type of anode material with at least five transition metal elements and high theoretical specific capacity, have enormous promise in sodium ion batteries (SIBs). However, the influence of different transition metal elements on the structure and electrochemical performances of HEOs is not clear. In this study, the addition of Mn to HEO can increase the surface oxygen vacancy (Ov) concentration, shorten the band gap, and enhance the electrochemical performance of (FeNiCuCrMn)3O4@rGO. XPS results show (FeNiCuCrMn)3O4@rGO has the highest surface Ov concentration. Ab initio calculations demonstrate that the indirect band gap of (FeNiCuCrMn)3O4@rGO is 0.068 eV. In situ X-ray diffraction and synchrotron X-ray absorption spectroscopy reveal that (FeNiCuCrMn)3O4@rGO exhibits a zero-strain characteristic with an inconsequential unit cell volume change of 0.2 %. Specifically, (FeNiCuCrMn)3O4@rGO as anode materials for SIBs show good cyclic stability (with a capacity retention of 84 % at 500 mA g−1 after 3000 cycles). Overall, this work provides a new direction for optimizing transition metal elements in HEOs.