X-ray Absorption Spectroscopy Study Research Articles

(Invited) In-Situ/Operando X-Ray Absorption Spectroscopy: Methods and Challenges in Electrocatalysts Characterization

During the last decades X-ray absorption spectroscopy (XAS) has proved to be a fundamental tool for understanding the nature of active catalytic species, especially in the form of highly dispersed, amorphous phases, and in the case of single atom catalysts. [1-4] The efforts in developing this technique under working conditions are justified by the extraordinary information that can be obtained on the identity of active species, reaction intermediates and activation/degradation phenomena. Clearly, the challenges embrace many fields: 1) catalysts can be very complex materials, comprising different, even disordered, phases; 2) it is a technique that uses synchrotron radiation, and the design of an appropriate experimental set-up requires know-how, but also synergy with a beamline team; 3) a rigorous interpretation of the XAS data requires high level of expertise to avoid easy misinterpretations, and the continuous development of new data analysis methods.This talk aims to discuss in-situ/operando XAS studies to investigate the activity and stability of novel catalysts during electrochemical reactions in proton exchange membrane fuel cell (PEMFC) devices and during CO2 reduction. In most studies with operando XAS, electrochemical studies have been performed in liquid electrolyte cells. While instructive, the performance and stability observed in liquid electrolyte at room temperature is however never directly transferable to the performance in a PEMFC device, operating at high temperature, high current density, and with a solid-polymer electrolyte interface. This current limitation can be overcome by the design of an appropriate cell able to isolate and extract the signal coming from the area where the reactions take place.Part of the talk will also be devoted to some time-resolved structural probing techniques to follow a reaction pathway under high temperature or pressure conditions. M. Elmaalouf,M. Odziomek, S. Duran, M. Gayrard, M. Bahri, C. Tard, A. Zitolo, B. Lassalle-Kaiser, J.Y. Piquemal, O. Ersen, C. Boissière, C. Sanchez, M. Giraud, M. Faustini and J. Peron Nat Commun 2021, 12 , 3935 D. Karapinar, N.H. Tran, N.R. Sahraie, J. Li, D. Wakerley, N. Touati, S. Zanna, D. Taverna, L.H. Galvão Tizei, A. Zitolo, F. Jaouen, V. Mougel, M. Fontecave, Angewandte Chemie International Edition 2019, 58(42), 15098-15103 Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F., Nat Commun 2017, 8 (1), 957 A. Zitolo, V. Goellner, V. Armel, M. T. Sougrati, T. Mineva, L. Stievano, E. Fonda, F. Jaouen, Nature Materials 2015, 14, 937-942

(Invited) Excited States and Reaction Mechanisms of Catalysts with Redox Active Ligands Investigated with X-Ray Spectroscopy

Metal dithiolenes and related molecular catalysts known for their redox-active ligands and their efficient and robust electro- and photo-catalysis of the hydrogen evolution reaction are investigated. To understand how the redox-active ligands of these catalysts influence their electronic excited states and participate in their chemical reactions, we need element-specific probes of charge and bonding at both metal and ligand atomic sites. X-ray absorption spectroscopy (XAS) affords us this opportunity and is used to study the electro- and photo-catalytic reaction mechanisms of these complexes, with a specific focus on the role of metal versus ligand atomic sites. Steady-state and time-resolved XAS studies at the Ni and S K-edges will be presented to identify the redox activity of metal vs. ligand sites on timescales spanning from the ultrafast excited state relaxation and photochemical activation processes to the steady-state creation of important intermediates in the multi-step hydrogen evolution reaction.

Effect of Cr-substitution on vanadium dioxide thin films studied by soft X-ray magnetic circular dichroism

The tunability of magnetic properties of VO 2 has fascinated substantial attention in the first generation of spintronics-related applications such as read head sensors for hard disk drives and magnetic random-access memory. In this work, we focus on Cr doped VO 2 , V 1− x Cr x O 2 (0 ≤ x ≤ 0 . 3) (VCO) thin films and study the effect of Cr-substitution on the structural, electronic and magnetic properties. The VCO films were deposited on R-sapphire substrates using a pulse laser deposition method. Soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD) studies of VCO thin films were performed to reveal and understand the origin of maTablegnetization and their enhancement with Cr-substitution. The XAS spectra suggest that the valences of V ions are in tetra and pentavalent states and Cr ions in the trivalent state. The XMCD spectra imply that the V and Cr ions are ferromagnetic at room temperature. These results also indicate that the VCO films are in the ferromagnetic insulating state and charge-ordered at the V and Cr sites. These results are consistent with the first principle density functional theory calculations. • XRD results suggest that a gradual structural change from the monoclinic M1 phase (0 ≤ x < 0.05) to triclinic T phase (0.05 ≤ x ≤ 0.20 to monoclinic M2 phase ( x > 0.20). • First Study to understand the origin of magnetization. • The VCO films are in the ferromagnetic insulating state and charge-ordered at the V and Cr sites, consistent with DFT. • The XMCD spectra indicate that the V and Cr ions are ferromagnetic at room temperature.

Strong electronic coupling between single Ru atoms and cobalt-vanadium layered double hydroxide harness efficient water splitting

Atomic coordination modulation and electronic structure engineering are appealing routes to develop versatile electrocatalysts targeting high-performance water electrolysis. Herein, atomically dispersed Ru sites are successfully anchored on the surface of CoV layered double hydroxide (LDH), affording a vertically aligned and interconnected nanosheet array architecture. Benefitting from the strong electronic coupling, fast charge transfer capability and well-defined morphology of as-prepared catalyst, ultralow overpotentials for hydrogen evolution reaction (HER, η10 = 28 mV) and oxygen evolution reaction (OER, η25 = 263 mV) are required. The two-electrode configuration cell only requires a cell voltage of 1.52 V to reach 10 mA cm−2, which is lower than that of commercialized Pt/C||RuO2 couple. Synchrotron X-ray absorption spectroscopy studies in combination with density functional theory calculations reveal that the strong electronic coupling between monatomic Ru with CoV LDH induces spatial charge redistribution and a distorted coordination environment around V atoms, thereby accelerating the hydrogen release for HER and reducing the rate-determining step (O* → OOH*) free energy for OER.

Lanthanum captured in montmorillonite: Evidence of inner-sphere complexes from X-ray Absorption Spectroscopy investigations

An X-Ray Absorption Spectroscopy (XAS) study of the local structural environment of La ions hosted in montmorillonite is presented. The structural location of La ions in the host phases and whether they are incorporated in the structure or adsorbed at the surface has been assessed. Two clays from the Clay Mineral Society repository (STx and SWy) in pristine form and polymer-modified with penta-ethylen-hexamine (C10H28N6) have been studied. Ca and Na-rich montmorillonite have been contacted with 19 and 200 mM La solutions. XAS results yielded LaO distances ranging between 2.57 and 2.61 Å, compatible with literature data for La ions coordinated by water molecules. We provide evidence for the presence of a further La-Si/Al distance suggesting that La may also be bonded to the oxygen atoms on the outer surfaces of the TOT layer. Such a feature may play an important role in limiting the release/efficiency of the process. This paper is a further step in the research for REEs recovery from Electric and Electronic wastes (WEE) via a solid-liquid process, and their re-use as secondary raw materials. The knowledge of the ions-sorbent interaction mechanism, such as its nature and strength, is a fundamental information to tailor sorbents and procedures in practical applications.

Structural and photocatalytic properties and X-ray absorption spectroscopic study of BiVO4 nanoparticles incorporated with Fe synthesized by sonochemical method

In this work, Fe-incorporated BiVO4 nanoparticles with different Fe-loading contents (0–4%) were synthesized via one-step sonochemical process. Crystal structure of all samples was investigated by X-ray diffraction technique (XRD). XRD patterns obviously show the main structure of monoclinic BiVO4 structure. The secondary phase is found in the form of Fe-based oxide as a hematite Fe2O3 phase at for Fe-loading contents ≥ 2%. Relevant chemical bonding of as-synthesized samples was carried out by Raman spectroscopy indicating the fundamental vibration with various vibration modes of VO43− tetrahedron and V–O band, respectively. Morphological structure of pure BiVO4 shows rod-like structure while 1–4%Fe-incorporated BiVO4 display different morphologies. The chemical compositions and oxidation numbers of all elements of the samples were carried out via X-ray photoelectron spectroscopy (XPS). XPS spectra indicate the existence of all elements on their surface and the oxidation states of all elements are clearly scrutinized. Local structure of all samples was investigated to interrogate the local atomic site of Fe atoms by X-ray absorption spectroscopy (XAS). The normalized Fe K-edge XANES spectra of all samples indicate that the local atomic site of Fe atoms would not replace in local sites of either Bi or V sites in BiVO4 crystal verified by simulated XANES spectra. However, the specific features of measured XANES spectra of all samples corresponds to the Fe K-edge XANES spectra of Fe2O3 and BiFeO3 structure suggesting that local structure of all samples are formed to Fe-based oxide between Fe2O3 and BiFeO3 structure. Fitting EXAFS spectra of 1–4%Fe-incorporated were practically conducted by artemis program with Fe2O3 and BiFeO3 used as structural models to identify local atomic environment of Fe atoms. Results show agreeable fitting with their structural models and reveal pertinent information of localization of Fe atoms. Optical properties of the samples were analyzed by UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS). DRS results exhibit the absorption edge in visible range of all samples. Meanwhile, influence of Fe loading contents into pure BiVO4 displays to confirm the red-shift on the absorption edge in visible range to higher wavelength, which suggests the lower optical band gap of pure BiVO4. The optimized photocatalytic degradation of RhB was performed by 4%Fe–BiVO4 with 82% decolorization under visible-light irradiation within 10 min and exhibited rate constant at 0.150 min−1.

Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

Palladium is an increasingly investigated electrocatalyst for the electrochemical reduction of carbon dioxide due to its unique ability to yield carbon monoxide or formate with large selectivities at high vs low overpotentials (i.e., ∼−0.5 to ∼−1.0 vs ∼−0.1 to ∼−0.4 V vs the reversible hydrogen electrode), respectively. While this behavior has been described multiple times on different Pd-electrocatalysts, previous studies disagree with regard to palladium’s ability to form a hydride phase (PdHx) under CO2 reduction reaction (CO2RR) conditions, as well as on the influence of this PdHx on its CO2RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO2RR with adsorbed CO, whose precise influence on the formation of PdHx and corresponding effects on the CO2RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdHx formation at CO2RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO2 and bicarbonate) and as a function of the applied potential and duration of the potential hold. The results of these electrochemical measurements unveiled the strong influence of surface-adsorbed CO on the formation rate of Pd-hydride, for which the final stoichiometry was nevertheless found to be independent of the presence of CO2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of β-phase PdHx (with x ≈ 0.6) at all applied potentials under CO2RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies.

CO2 laser heating system for in situ radial x-ray absorption at 16-BM-D at the Advanced Photon Source.

We present a portable CO2 laser heating system for in situ x-ray absorption spectroscopy (XAS) studies at 16-BM-D (High Pressure Collaborative Access Team, Advanced Photon Source, Argonne National Laboratory). Back scattering optical measurements are made possible by the implementation of a Ge beamsplitter. Optical pyrometry is conducted in the near-infrared, and our temperature measurements are free of chromatic aberration due to the implementation of the peak-scaling method [A. Kavner and W. R. Panero, Phys. Earth Planet. Inter. 143-144, 527-539 (2004) and A. Kavner and C. Nugent, Rev. Sci. Instrum. 79, 024902 (2008)] and mode scrambling of the input signal. Laser power stabilization is established using electronic feedback, providing a steady power over second timescales [Childs etal., Rev. Sci. Instrum. 91, 103003 (2020)]-crucial for longer XAS collections. Examples of in situ high pressure-temperature extended x-ray absorption fine structure measurements of ZrO2 are presented to demonstrate this new capability.

Functionalized polyvinyl alcohol aerogel for efficient and selective removal of arsenite from aqueous matrices

The removal of arsenic (As) from wastewater and groundwater has become a critical environmental issue. In this study, a novel sorbent, dithiocarbamate-functionalized cross-linked polyvinyl alcohol (DFCPVA) aerogel was successfully developed to address this issue. The aerogel was analyzed for its ability to remove arsenite (AsIII) from aqueous matrices, and it showed excellent sorption performance for AsIII under both acidic and neutral conditions. The AsIII sorption efficiency of DFCPVA was not significantly hindered by competing metal ions present in the solution. The pseudo-second-order kinetic model described the sorption well, and the Langmuir model fit the equilibrium sorption isotherm more closely than the Freundlich model. It was anticipated that the sorption rate of AsIII by DFCPVA would be rapid, as equilibrium was established in 15 min. The maximum sorption capacity of DFCPVA was 535 µmol g−1, as determined by the isotherm study. Sorption experiments, Fourier transform infrared (FTIR) spectroscopy, and X-ray absorption spectroscopy (XAS) studies demonstrated that the sorption of AsIII by DFCPVA followed a chelation mechanism. The sorption capacity of DFCPVA was slightly decreased when stored at 40 ℃ for one month; however, the capacity was not reduced when it was stored in a refrigerator (i.e., cold condition).

Effects of oxygen on the adsorption/oxidation of aqueous Sb(III) by Fe-loaded biochar: An X-ray absorption spectroscopy study

Fe-loaded biochar (FeBC) has been considered for Sb(III) adsorption, but the effects of oxygen (O2) on the adsorption need further investigation. Liquid-/solid-phase analyses were conducted to investigate the role of O2 in the Sb(III) adsorption by FeBC. The adsorption was best described by the pseudo-second-order (PSO) model for kinetic results and by the Langmuir model for thermodynamic results. More than 96.8 % of Sb(III) was adsorbed by FeBC, and available O2 increased the liquid-phase Sb(III) oxidation efficiency by 2.1–7.5 times. The peak changes at ~1640 and 3450 cm−1 in FTIR spectra indicated the occurrence of inner-sphere complexation between Sb(III)/Sb(V) and hydroxyl (–OH)/carboxyl (–COOH) groups in FeBC under aerobic and anaerobic conditions. Fe/Sb X-ray absorption spectroscopy (XAS) analysis results showed aqueous Sb(III) complexed to the edge-sharing Fe(III)-O-Fe(III) in FeBC. Regardless of whether O2 was available or not, solid-phase edge-sharing Fe(III)-O-Sb(V) complexes (~3.05 Å), which had lower toxicity and migration ability than aqueous Sb(III), formed through a ligand-to-metal charge-transfer (LMCT) process. More than 91 % of adsorbed Sb(III) was oxidized to edge-sharing Fe(III)-O-Sb(V) complexes in 3 h. Additionally, the Sb(V) from liquid-phase oxidation could also directly complex to the Fe(III)-O-Fe(III) and form edge-sharing Fe(III)-O-Sb(V) complexes. These results provide evidence to inform further FeBC application for the Sb-contaminated water treatment.

(Digital Presentation) Time-Resolved Operando XAS of FexNi100-XOy Electrocatalysts for the Oxygen Evolution Reaction Reveals Temporal Shift in Ni K-Edge during Ni2+/3+ Redox Reaction

FexNi100-xOy electrocatalysts have become a focus for alkaline water electrolysis and the oxidative half reaction of oxygen evolution. Under alkaline conditions, the oxygen evolution reaction (OER) can be promoted by non-precious metal oxide and hydroxide electrocatalysts. In particular, electrocatalyst compositions from first row late transition metals such as iron, nickel, manganese, and cobalt have emerged as some of the most active catalysts for the OER, and the role of iron has been identified as key within multi-metallic compositions. In our research, we have focused specifically on the iron-nickel bimetallic composition and have developed synthesis methods to be able to control the bimetallic composition, surface chemistry, and three-dimensional morphology of a suite of FexNi100-xOy nanoparticle electrocatalysts. With this subset of highly-active FexNi100-xOy nanoparticle electrocatalysts, we have recently been focused on operando x-ray absorption spectroscopy (XAS) studies to understand how the chemistry of the iron and nickel species within these nanocatalysts changes as a result of exposure to the electrochemical environment. This talk will focus on a set of operando data obtained with a quick scan detector at beamline 9-3 at SSRL, where we were able to capture temporal, dynamic changes to the nickel chemistry as a function of applied voltage in the Faradaic region of the OER. Specifically, we have captured the change in the Ni K-edge that occurs as a result of the voltage stepping through the Ni redox reaction, where the Ni species in our electrocatalyst oxidizes from a 2+ to a 3+/4+ oxidation state. This redox reaction is associated with a phase transition from Ni(OH)2 to NiOOH. Our measurements were able to obtain 90 s scans during a 15 min time period of measurement, where we find that the Ni redox reaction and phase transition occurs during the first 10 minutes of measurement. Modeling of the extended x-ray absorption fine structure (EXAFS) region suggests that both Ni – O and Ni – M (M = metal) scattering path atomic pairs undergo a compression in distance. Further, the time scale of the phase transition suggests that the Ni redox reaction is mass transport limited. In this talk, I will discuss these results and present our analysis of the time-resolved x-ray absorption spectroscopy data.

Advanced Catalysts for PEM Water Electrolyzers, Fabricated By Reactive Spray Deposition Technology: Study of the Degradation Mechanisms in the Catalysts’ Layers By in-Situ and Ex-Situ Synchrotron X-Ray Absorption Spectroscopy

Development of novel and improved catalysts and membrane electrode assemblies (MEAs) for proton exchange membrane (PEM) based energy conversion devices is of crucial importance for widespread application of the PEM fuel cells (FCs) and water electrolyzers (WEs). These PEM based devices are critical for the Hydrogen Economy (HE) implementation. The HE is the economy of the near future and is the only viable alternative to the current fossil fuel-based economy. This future green economy will eliminate the greenhouse gas emissions and stop the imminent global warming and climate change. Currently, the main challenges that the state-of-the-art MEAs for PEMWEs are facing are: (i) high cost because of the high platinum group metals (PGM) loadings in their catalysts layers and time consuming and expensive multi-step fabrication processes; and (ii) poor durability caused by the instability of the catalysts and the materials [1, 2].The reactive spray deposition technology (RSDT), developed at UConn, is a novel methodology for fabrication of advanced MEAs for PEMFCs and PEMWEs [3-5]. The RSDT is a flame assisted method [4, 5] that combines the catalysts synthesis and deposition directly on the PEM membrane in one-step, which results in fast and facile fabrication of large scale (up to 1000 cm2) MEAs [4, 5]. In addition, this technology allows precise control of the composition, morphology, and particle size distribution of wide range of nanoparticles supported and unsupported on carbon, and thus ensures fine tuning of the catalysts’ activity and durability. The RSDT fabricated MEAs have improved activity and durability performance with one order of magnitude lower catalyst loadings in their electrodes in comparison to the state‐of‐the‐art commercial MEAs for PEM water electrolyzers [6]. In this work we report the results from the in‐situ and ex‐situ synchrotron X-ray Absorption Spectroscopy (XAS) studies of the Ir/IrOx (anode) and Pt/C (cathode) catalysts, fabricated by the RSDT. The experiments were performed at beamline 7-BM at NSLS II at Brookhaven National Laboratory. The electronic and atomic structures of the Ir and Pt in the catalyst layers after their fabrication, during operation at different cell voltages, and after long term steady‐state operation were studied by XANES and XAFS, respectively. The results obtained, provide better understanding of the improved activity of the catalysts, as well as of the degradation mechanisms that govern the MEAs’ failure. References https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf https://www.energy.gov/sites/prod/files/2015/06/f23/fcto_myrdd_production.pdf Kim, S., Myles, Maric, R., et al. Electrochimica Acta, 177, 190-200 (2015).Yu, H., Baricci, A., Bisello, A., Bonville, L., Maric, R., et al. Electrochimica Acta, 247, 1155-1168 (2017).Roller, J., Maric, R., 24(8) December 2015 pp. 1529-1541 (2015).Mirshekari, G., Ouimet, R., Zeng, Z, Yu, H., Bliznakov, S., Bonville, L., Niedzwiecki, A., Errico, S., Capuano, C., Mani, P., Ayers, K., Maric, R. International Journal for Hydrogen Energy, 46(2), 2021, pp. 1526-1539 (2021).

Following a Silent Metal Ion: A Combined X-ray Absorption and Nuclear Magnetic Resonance Spectroscopic Study of the Zn2+ Cation Dissipative Translocation between Two Different Ligands.

The dissipative translocation of the Zn2+ ion between two prototypical coordination complexes has been investigated by combining X-ray absorption and 1H NMR spectroscopy. An integrated experimental and theoretical approach, based on state-of-the-art Multivariate Curve Resolution and DFT based theoretical analyses, is presented as a means to understand the concentration time evolution of all relevant Zn and organic species in the investigated processes, and accurately characterize the solution structures of the key metal coordination complexes. Specifically, we investigate the dissipative translocation of the Zn2+ cation from hexaaza-18-crown-6 to two terpyridine moieties and back again to hexaaza-18-crown-6 using 2-cyano-2-phenylpropanoic acid and its para-chloro derivative as fuels. Our interdisciplinary approach has been proven to be a valuable tool to shed light on reactive systems containing metal ions that are silent to other spectroscopic methods. These combined experimental approaches will enable future applications to chemical and biological systems in a predictive manner.

Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal-Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study.

The porous Hoffman-type 3D lattice Fe(pz)[NiII(CN)4] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

Design of Ru-Ni diatomic sites for efficient alkaline hydrogen oxidation.

Anion exchange membrane fuel cells are limited by the slow kinetics of alkaline hydrogen oxidation reaction (HOR). Here, we establish HOR catalytic activities of single-atom and diatomic sites as a function of *H and *OH binding energies to screen the optimal active sites for the HOR. As a result, the Ru-Ni diatomic one is identified as the best active center. Guided by the theoretical finding, we subsequently synthesize a catalyst with Ru-Ni diatomic sites supported on N-doped porous carbon, which exhibits excellent catalytic activity, CO tolerance, and stability for alkaline HOR and is also superior to single-site counterparts. In situ scanning electrochemical microscopy study validates the HOR activity resulting from the Ru-Ni diatomic sites. Furthermore, in situ x-ray absorption spectroscopy and computational studies unveil a synergistic interaction between Ru and Ni to promote the molecular H2 dissociation and strengthen OH adsorption at the diatomic sites, and thus enhance the kinetics of HOR.

X-ray absorption spectroscopy study of Mn reference compounds for Mn speciation in terrestrial surface environments

Abstract X-ray absorption spectroscopy (XAS) offers great potential to identify and quantify Mn species in surface environments by means of linear combination fit (LCF), fingerprint, and shell-fit analyses of bulk Mn XAS spectra. However, these approaches are complicated by the lack of a comprehensive and accessible spectrum library. Additionally, molecular-level information on Mn coordination in some potentially important Mn species occurring in soils and sediments is missing. Therefore, we investigated a suite of 32 natural and synthetic Mn reference compounds, including Mn oxide, oxyhydroxide, carbonate, phosphate, and silicate minerals, as well as organic and adsorbed Mn species, by Mn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The ability of XAS to infer the average oxidation state (AOS) of Mn was assessed by comparing XANES-derived AOS with the AOS obtained from redox titrations. All reference compounds were studied for their local (&amp;lt;5 Å) Mn coordination environment using EXAFS shell-fit analysis. Statistical analyses were employed to clarify how well and to what extent individual Mn species (groups) can be distinguished by XAS based on spectral uniqueness. Our results show that LCF analysis of normalized XANES spectra can reliably quantify the Mn AOS within ~0.1 v.u. in the range +2 to +4. These spectra are diagnostic for most Mn species investigated, but unsuitable to identify and quantify members of the manganate and Mn(III)-oxyhydroxide groups. First-derivative XANES fingerprinting allows the unique identification of pyrolusite, ramsdellite, and potentially lithiophorite within the manganate group. However, XANES spectra of individual Mn compounds can vary significantly depending on chemical composition and/or crystallinity, which limits the accuracy of XANES-based speciation analyses. In contrast, EXAFS spectra provide a much better discriminatory power to identify and quantify Mn species. Principal component and cluster analyses of k2-weighted EXAFS spectra of Mn reference compounds implied that EXAFS LCF analysis of environmental samples can identify and quantify at least the following primary Mn species groups: (1) Phyllo- and tectomanganates with large tunnel sizes (2 × 2 and larger; hollandite sensu stricto, romanèchite, todorokite); (2) tectomanganates with small tunnel sizes (2 × 2 and smaller; cryptomelane, pyrolusite, ramsdellite); (3) Mn(III)-dominated species (nesosilicates, oxyhydroxides, organic compounds, spinels); (4) Mn(II) species (carbonate, phosphate, and phyllosilicate minerals, adsorbed and organic species); and (5) manganosite. All Mn compounds, except for members of the manganate group (excluding pyrolusite) and adsorbed Mn(II) species, exhibit unique EXAFS spectra that would allow their identification and quantification in mixtures. Therefore, our results highlight the potential of Mn K-edge EXAFS spectroscopy to assess bulk Mn speciation in soils and sediments. A complete XAS-based speciation analysis of bulk Mn in environmental samples should preferably include the determination of Mn valences following the “Combo” method of Manceau et al. (2012), EXAFS LCF analyses based on principal component and target transformation results, as well as EXAFS shell-fit analyses for the validation of LCF results. For this purpose, all 32 XAS reference spectra are provided in the Online Materials1 for further use by the scientific community.

Biological Samples Preparation for Speciation at Cryogenic Temperature using High-Resolution X-Ray Absorption Spectroscopy.

The study of elements with X-ray absorption spectroscopy (XAS) is of particular interest when studying the role of metals in biological systems. Sample preparation is a key and often complex procedure, particularly for biological samples. Although X-ray speciation techniques are widely used, no detailed protocol has been yet disseminated for users of the technique. Further, chemical state modification is of concern, and cryo-based techniques are recommended to analyze the biological samples in their near-native hydrated state to provide the maximum preservation of chemical integrity of the cells or tissues. Here, we propose a cellular preparation protocol based on cryo-preserved samples. It is demonstrated in a high energy resolution fluorescence detected X-ray absorption spectroscopy study of selenium in cancer cells and a study of iron in phytoplankton. This protocol can be used with other biological samples and other X-ray techniques that can be damaged by irradiation.

Local environment of iodine dissolved as iodate in high-pressure aluminoborosilicate glasses: A I K-edge x-ray absorption spectroscopic study.

The use of high-pressure synthesis conditions to produce I-bearing aluminoborosilicate represents a promising issue for the immobilization of 129I radioisotope. Furthermore, iodine appears to be more solubilized in glasses under its iodate (I5+) form rather than its iodide (I-) form. Currently, the local atomic environment for iodine is poorly constrained for I- and virtually unknown for I5+ or I7+. We used I K-edge x-ray absorption spectroscopy conducted at 20K for determining the local atomic environment of iodine dissolved as I-, I5+, and I7+ in a series of aluminoborosilicate glasses. We determined that I- is surrounded by either Na+ or Ca2+ in agreement with previous studies. The signal collected from EXAFS reveals that I5+ is surrounded invariably by three oxygen atoms forming an IO3 - cluster charge compensated by Na+ and/or Ca2+. The I-O distance in iodate dissolved in glass is comparable to the I-O distance in crystalline compounds at ∼1.8 Å. The distance to the second nearest neighbor (Na+ or Ca2+) is also constant at ∼3.2 Å. This derived distance is identical to the distance between I- and Na+ or Ca2+ in the case of iodide local environment. For one sample containing iodate and periodate, the distinction between the local environment of I5+ and I7+ could not be made, suggesting that both environments have comparable EXAFS signals.

Self-standing Co2.4Sn0.6O4 nano rods as high performance anode materials for sodium-ion battery and investigation on its reaction mechanism

The self-standing nanorod Co2.4Sn0.6O4 is synthesized as a high-performance anode material in search of high capacity and stable anode materials for sodium-ion batteries. The Co2.4Sn0.6O4 nanorod exhibits a high reversible capacity of 576 mAh g−1 at a current density of 80 mA g−1 and shows excellent high-rate capability. The X-ray absorption spectroscopy study reveals the mechanisms of charge storage reaction and improved cycling performance of Co2.4Sn0.6O4. A partially limited conversion reaction of Co– and Sn-oxide during the cycling effectively regulate the irreversible capacity loss over the cycling that is commonly observed from the conversion and alloying reaction-based anode materials. Furthermore, Co2.4Sn0.6O4 also exhibits superior sodium-ion full cell performance when coupled with a NaNi2/3Bi1/3O2 cathode, demonstrating an energy density of 262 Wh kg−1.

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.