X-ray Absorption Near-edge Spectroscopy Research Articles

Uncovering Electrochemical Cation-Storage Mechanisms in Defective Vanadium Ferrites Using Synchrotron-Quality, in-Lab X-Ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) is a critical tool for investigating new materials for electrochemical energy storage, providing important information on metal oxidation state and element-specific coordination. Historically, XAS measurements had required the energy specificity and brilliance of a synchrotron facility, but recent advances in detectors and optics are bringing XAS capabilities to the laboratory setting with multiple commercial instruments available. At the Naval Research Laboratory, we use laboratory-based XAS to study a class of disordered vanadium ferrite (VFe2Ox) aerogels that exhibit promising performance for electrochemical energy-storage applications such as rechargeable lithium-ion batteries.1,2 The structure and composition of these materials are readily varied via modifications to the epoxide-promoted sol–gel reaction of iron chloride and vanadium isopropoxide (e.g., substitution with other cations such as Al3+),2 as well as post-synthesis thermal treatments that render disordered, defective, or nanocrystalline forms of a given composition. The resulting series of VFe2Ox materials are evaluated by XAS in both ex situ and in situ configurations, including as powder-composite cathodes versus lithium metal in pouch cells with conventional nonaqueous lithium-ion electrolyte. X-ray Absorption Near-edge Spectroscopy (XANES) at the V K-edge and Fe K-edge is used to track V and Fe oxidation state, respectively, permitting the assignment of metal-centered redox across the broad potential range over which these materials are electrochemically active (2–3.4 V vs Li/Li+). Extended X-ray Absorption Fine Structure (EXAFS) analysis provides information on V- or Fe-specific coordination as a function of composition, structure, and state-of-charge. Parallel computation efforts using Density-Functional Theory offer a complementary feedback loop with experimental XANES and EXAFS to achieve a sophisticated description of these complex battery materials.1. C. N. Chervin, J. S. Ko, B. W. Miller, L. Dudek, A. N. Mansour, M. D. Donakowski, T.Brintlinger, P. Gogotsi, S. Chattopadhyay, T. Shibata, J. F. Parker, B. P. Hahn, D. R.Rolison, and J. W. Long, J. Mater. Chem. A 3, 12059 (2015).2. C. N. Chervin, R. H. DeBlock, J. F. Parker, B. M. Hudak, N. L. Skeele, J. S. Ko, D. R.Rolison, and J. W. Long, RSC Adv. 11, 14495 (2021).

(Invited) Understanding the Role of Dopants (Al, Zn, Zr) in Altering Charge-Storage Capacity and Cycling Profiles of VFe2Ox

Vanadium ferrite (VFe2Ox) is a defective spinel system that can incorporate substantial Li+ and exhibits a high charge–discharge rate, particularly when structured as a nanoscale aerogel.1 Cations such as Zr, Zn and Al,2 can readily enter the structure substitutionally and have strong, but differing effects on the charge-storage capacity of the material. These earth abundant, cost- effective constituent elements give this class of materials strong potential as future Li-ion battery cathodes but optimizing the stoichiometry for maximum capacity and stability will require understanding the redox sequence of the host cations (Fe, V) and role of intentional dopants.We use density functional theory calculations in concert with in situ and operando X-ray absorption near-edge spectroscopy (XANES) spectra obtained using an in-lab X-ray absorption spectrometer to uncover the quantum mechanical-level effects that underpin relevant energy- storage behaviors of doped and undoped VFe2Ox. Our experimental V K-edge and Fe K-edge spectra indicate reduction of both species during discharge but cannot distinguish between tetrahedral and octahedral Fe redox sites or fully resolve the valency of each element as a function of state of charge. Using a hybrid form of density functional theory that accounts for the strong correlation present in 3d elements, we show that both V and Fe are indeed reduced, but that only tetrahedral Fe is redox active until fully converted to Fe2+. Furthermore, both octahedral and tetrahedral Fe3+ are high spin configuration, but the 5μB moments in the fully filled majority spin channel at each symmetry site are anti-aligned. This arrangement allows for easy exchange of electrons and facilitates conduction. We also calculate XANES spectra based on first principles calculations to be compared directly to those measured in the lab. This cross- check allows us to understand the effect of Al, Zr, and Zn dopants on the redox sequence and relate these results to site preference and capacity. Our combined experimental and calculational investigation sheds light on how these complex materials store Li ions and points toward future alterations that may further improve their properties.

Pb speciation and elemental distribution in leeks by micro X-ray fluorescence and X-ray absorption near-edge structure.

Vegetables are crucial to a human diet as they supply the body with essential vitamins, minerals, etc. Heavy metals that accumulate in plants consequently enter the food chain and endanger people's health. Studying the spatial distribution and chemical forms of elements in plant/vegetable tissues is vital to comprehending the potential interactions between elements and detoxification mechanisms. In this study, leek plants and soil from vegetable gardens near lead-zinc mines were collected and cultivated with 500 mg L-1 PbNO3 solutions for three weeks. Micro X-ray fluorescence was used to map the distribution of Pb and other chemical elements in leek roots, and X-ray absorption near-edge spectroscopy was used to assess the Pb speciation in leek roots and leaves. These findings demonstrated that Pb, Cu, Mn, Cr, Ti and Fe were detected in the outer rings of the root's cross section, and high-intensity points were observed in the epidermis. Zn, K and Ca, on the other hand, were distributed throughout the root's cross section. Leek root and leaf contained significant quantities of lead phosphate and basic lead carbonate at more than 80%, followed by lead sulfide (19%) and lead stearate (11.1%). The capacity of leek roots to convert ambient lead into precipitated lead and fix it on the root epidermis and other inner surfaces is a key mechanism for reducing the toxic effects of Pb.

Negative magnetization, shielding current effect and divalent manganese in CaMn1-xTaxO3 manganites

Crystal and electronic structures, and magnetic properties of polycrystalline tantalum substituted manganites CaMn1-xTaxO3 (x = 0.00, 0.03, 0.06, 0.09 and 0.12) were studied by x-ray diffraction, x-ray absorption near edge spectroscopy (XANES) at Mn K-edge, and by magnetization measurements. Large changes of cell volume and magnetic transition temperature, in addition to shift of Mn K-edge spectra and change of shape of XANES spectra, indicate the existence of Mn3+ as well as Mn2+ species induced by the substitution of Mn4+ by Ta5+. Cell parameters and cell volume increase with increasing x because the Mn3+, Mn2+ and Ta5+ ions are larger that substituted Mn4+ ion. According to XANES results, at least the x = 0.00 sample is oxygen deficient. The negative magnetization was observed in the CaMnO3-δ sample, and a possible explanation of the phenomenon is given considering two canted (almost antiferro)magnetic sublattices and a shielding current, associated with the itinerant eg electrons, induced by the oxygen vacancies.

A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

ThTi2O6 derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U5+ or U6+, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd3+, Ca2+, Al3+, or Cr3+, on the Th or Ti site. U L3 edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M4 edge XANES evidenced U5+ as the major oxidation state in all compounds, with a minor fraction of U6+ (2–13%). The balance of X-ray and Raman spectroscopy data support uranate, rather than uranyl, as the dominant U6+ speciation in the reported brannerites. It is considered that the U6+ concentration was limited by unfavourable electrostatic repulsion arising from substitution in the octahedral Th or Ti sites, which share two or three edges, respectively, with neighbouring polyhedra in the brannerite structure.

Elemental sulfur redox bioconversion for selective recovery of phosphorus from Fe/Al-bound phosphate-rich anaerobically digested sludge: Sulfur oxidation or sulfur reduction?

The biological oxidation of elemental sulfur (S0) to sulfate and the reduction of S0 to sulfide provide a potential route for extracting and reclaiming phosphorus (P) from anaerobically digested sludge (ADS). However, the treatment performance, stability, and cost-effectiveness of the two opposing bioprocesses based on S° for selective P recovery from ADS remain unclear. This study aimed to compare the roles of S0-oxidizing bacteria (S0OB) and S0-reducing bacteria (S0RB) in liberating insoluble P from ADS through single-batch and consecutive multibatch experiments. Changes in P speciation in the sludge during the biological extraction processes were analyzed by using complementary sequential extraction and P X-ray absorption near-edge spectroscopy. Results showed that S0OB treatment extracted more phosphate from the sludge compared with S0RB treatment, but it also released a considerable amount of metal cations (e.g., heavy metals, Mg2+, Al3+, Ca2+) and negatively affected sludge dewaterability due to intense sludge acidification and cell lysis. At pH 1.2, the S0OB treatment released 92.9% of P from the sludge, with the dissolution of HAP, Fe-PO4, Mg3(PO4)2, and P-fehrrihy contributing 26.8%, 22.1%, 12.8%, and 10.5%, respectively. The S0RB treatment released 63.6% of P from the sludge at pH 7.0, with negligible dissolution of metal cations, thereby avoiding costly purification of the extract and alkali neutralization for pH adjustment. This treatment involved the replacement of phosphates bounded with Fe-PO4 (FePO4 and P-fehrrihy) and Al–PO4 (P-Alumina and AlPO4) with biogenic sulfides, with contributions of 72.7%, and 20.9%, respectively. Consecutive bioprocesses for P extraction were achieved by recirculating the treated sludge. Both S0OB and S0RB treatments did not affect the extent of sludge dewatering but considerably weakened the dewatering rate. The S0OB-treated sludge exhibited prolonged filtration time (from 3010 s to 9150 s) and expressing time (from 795 s to 4690 s) during compression dewatering. After removing metal cations using cation exchange resin (CER) and neutralizing using NaOH, a vivianite product Fe3(PO4)2·8H2O (purity: 84%) was harvested from the S0OB-treated extract through precipitation with FeSO4·7H2O. By contrast, a vivianite product Fe3(PO4)2·8H2O (purity: 81%) was directly obtained from the S0RB-treated extract through precipitation with FeSO4·7H2O. Ultimately, 79.8 and 57.9wt% of P were recovered from ADS through S0OB extraction–CER purification–alkali neutralization–vivianite crystallization, and S0RB extraction–vivianite crystallization, respectively. Collectively, biological S0 reduction is more applicable than biological S0 oxidation for selectively reclaiming P from Fe/Al-associated phosphate-rich ADS due to better cost-effectiveness and process simplicity. These findings are of significance for developing sludge management strategies to improve P reclamation with minimal process inputs.

Enhancement of Photocatalytic Rhodamine B Degradation over Magnesium–Manganese Baring Extracted Iron Oxalate from Converter Slag

In this work, iron oxalate from converter slag (FeOX-Slag) was produced by extraction of iron from converter slag using oxalic acid, followed by photo-reduction. The FeOX-Slag sample was subjected to various characterization techniques, including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), ultraviolet–visible diffuse reflectance spectroscopy (UV-DRS), photoluminescence spectroscopy (PL), X-ray absorption near-edge structure spectroscopy (XANES), and X-ray photoelectron spectroscopy (XPS), in order to gain insights into its physicochemical properties. Also, to compare the photocatalytic activity of the FeOX-Slag, commercial iron oxide (Fe2O3) was used as a precursor to produce normal iron oxalate (FeOX-Fe2O3). The obtained FeOX-Slag was applied to the photocatalytic degradation of rhodamine B (RhB), a model organic contaminant in wastewater, compared with the FeOX-Fe2O3. Using the produced FeOX-Slag, we were able to degrade RhB more than 98% within 90 min at a reaction rate constant of about 3.6 times faster than FeOX-Fe2O3. Photoluminescence results confirmed the less recombination of the electron–hole pairs in FeOX-Slag, compared to FeOX-Fe2O3, which may be due to the defect structure of iron oxalate by guest metal impurities. The higher separation and transportation of photogenerated electron–hole pairs cause the enhancement of the degradation photocatalytic RhB degradation activity of the FeOX-Slag. In addition, The FeOX-Slag showed higher light absorption ability than FeOX-Fe2O3, resulting in the enhancement of the RhB degradation performance. Thus, the optical properties and the results from the activity tests led to the proposal that FeOX-Slag may be used in a photocatalytic degradation process for RhB under light irradiation.

Geochemical enrichment, speciation and mobilization of arsenic and antimony in black shales (southern China): Evidence from sequential fractionation and XANES spectroscopy

The high geogenic levels of trace metal(loid)s preserved within black shale can be released during natural weathering, causing global concern. Currently, the accumulation, speciation and potential mobilization of two highly toxic metal(loid)s, arsenic (As) and antimony (Sb), from black shale to soils remain unresolved, which hinders the evaluation of their potential risk to surrounding environments. Here, we demonstrate the geochemical enrichment, species transformation, and leaching behavior of As and Sb by applying weathering-induced alteration, sequential extraction and synchrotron-based X-ray absorption near edge spectroscopy (XANES) in the analysis of a range of black shale and weathered samples from two profiles collected from typical Cambrian sedimentary strata of the Pearl River Delta, southern China. The results showed significant accumulation of trace metal(loid)s, including As (43.8–458.0 mg/kg), Sb (7.2–34.5 mg/kg), Cu (8.8–451.0 mg/kg) and Ni (22.2–108.0 mg/kg), in the black shale and weathered products, and high enrichment factors for As (22.0–266.8) and Sb (1.2–6.4) were observed, consistent with the very high degrees of weathering (chemical index of alteration: 83–91). Arsenic in the shale was dominated by arsenate (46–85%), as identified using XANES, and this was resulted from extensive weathering, which facilitated the oxidation of arsenite and arsenopyrite species. Accordingly, significant decreases in sulfide-associated As and Sb were observed in highly weathered products. As a result, As and Sb were primarily in the immobilizable fractions associated with amorphous and well-crystallized Fe/Mn oxides, largely due to the increased formation of secondary Fe (oxyhydr)oxides, including goethite-like minerals, which mainly derived from the oxidative weathering of chlorite-like minerals. Nevertheless, high concentrations of As (<64 μg/L), Sb (14–70 μg/L), and Ni (3–92 μg/L) still leached from black shale samples exposed to synthetic acid rain in the 18-h batch leaching experiment. Our combined characterization results indicated that a small percentage of the high contents of geogenic As and Sb could potentially be released. However, soluble trace metal(loid)s in shale leachate could still endanger surrounding ecosystems, and continual monitoring during long-term natural weathering processes is merited for global black shale regions.

Intercalation of Fe-montmorillonite for developing nacre-inspired flexible all-solid-state supercapacitor with circular economy approach

The concept of circular economy has gained attention for developing environmentally friendly and sustainable next-generation flexible supercapacitors. Montmorillonite (MMT) has been used to intercalate Fe ions (Fe-MMT) in designing an asymmetric flexible all-solid-state supercapacitor (FASC). The negative electrode was fabricated using PEDOT:PSS, and the positive nanocomposite electrode was made of Fe-MMT, PEDOT:PSS, and PVA, along with a gel electrolyte. Fe-MMT exhibited redox properties, allowing for the reaction of Fe2+/3+/Fe3+ during the charge-discharge cycle, as measured through in-situ X-ray absorption near-edge spectroscopy combined with cyclic-voltammetric measurements. The Fe-MMT nanocomposite exhibited a unique hierarchical nacre-like superstructure, characterized by synchrotron-based transmission X-ray imaging and X-ray diffraction, providing outstanding stability of the charge-discharge cycle under dynamic twisting and high-throughput channels to enhance charge diffusion. The maximum power density achieved was 2181 W kg−1 at 0.83 W h kg−1, and the maximum energy density was 3.27 W h kg−1 at 549 W kg−1. The active cations of the MMT-based electrodes could be further acquired from wastewater recycling, making this an eco-supercapacitor according to the concept of the circular economy. The use of MMT as a cation-exchange material demonstrates its potential for environmentally sustainable energy storage applications.

5D Analysis of Capacity Degradation in Battery Electrodes Enabled by Operando CT-XANES.

For devices encountering long-term stability challenges, a precise evaluation of degradation is of paramount importance. However, methods for comprehensively elucidating the degradation mechanisms in devices, particularly those undergoing dynamic chemical and mechanical changes during operation, such as batteries, are limited. Here, a method is presented using operando computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES) that can directly track the evolution of the 3D distribution of the local capacity loss in battery electrodes during (dis)charge cycles, thereby enabling a five-dimensional (the 3D spatial coordinates, time, and chemical state) analysis of the degradation. This paper demonstrates that the method can quantify the spatiotemporal dynamics of the local capacity degradation within an electrode during cycling, which has been truncated by existing bulk techniques, and correlate it with the overall electrode performance degradation. Furthermore, the method demonstrates its capability to uncover the correlation among observed local capacity degradation within electrodes, reaction history during past (dis)charge cycles, and electrode microstructure. The method thus provides critical insights into the identification of degradation factors that are not available through existing methods, and therefore, will contribute to the development of batteries with long-term stability.

Toward Understanding the Environmental Risks of Combined Microplastics/Nanomaterials Exposures: Unveiling ZnO Transformations after Adsorption onto Polystyrene Microplastics in Environmental Solutions.

Over recent decades, there has been a dramatic increase in the manufacture of engineered nanomaterials, which has inevitably led to their environmental release. Zinc oxide (ZnO) is among the more abundant nanomaterial manufactured due to its advantageous properties, used for piezoelectric, semiconducting, and antibacterial purposes. Plastic waste is ubiquitous and may break down or delaminate into smaller microplastics, leaving open the question of whether these small polymers may alter the fate of ZnO through adsorption within aquatic media (tap-water and seawater). Here, scanning electron microscopy analysis confirms the effective Zn nano/microstructures adsorption onto polystyrene surfaces after only 24-h incubation in the aquatic media. After pre-aging the nanomaterials for 7-days in different environmental media, nanoprobe X-ray absorption near-edge spectroscopy analysis reveals significant ZnO transformation toward Zn-sulfide and Zn-phosphate. The interaction between a commercial ZnO-based sunscreen with polystyrene and a cleanser consumer containing microbeads with ZnO nanomaterials is also studied, revealing the adsorption of transformed Zn-species in the microplastics surfaces, highlighting the environmental relevancy of this work. Understanding the structural and functional impacts of the microplastics/ZnO complexes, and how they evolve, will provide insights into their chemical nature, stability, transformations, and fate, which is key to predicting their bioreactivity in the environment.

Stacking order effects on the energetics and electronic properties of [formula omitted]-doped graphene/h-BN van der Waals heterostructures on SiC(0001)

Heterostructures based on the stacking of two-dimensional materials with different electronic properties have been the subject of several studies addressing the development of novel devices with multiple functionalities. Graphene (G)/hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively investigated, including studies on the large-scale synthesis of h-BN on graphene/SiC(0001) substrate. This work is a theoretical study using first-principles calculations of bilayer (G/h-BN) and trilayer (G/h-BN/G) heterostructures on SiC(0001) covered by a carbon buffer layer (SiC). The results show an energetic preference for h-BN encapsulation below a single layer of graphene on SiC, viz.: G/h-BN/SiC in the case of bilayer systems and G/h-BN/G/SiC in the case of trilayer systems. Indeed, the electronic structure calculations reveal the preservation of graphene linear energy band dispersion in the bilayer systems. The trilayer systems’ electronic structure depends on the stacking order, with the emergence of parabolic bands in energetically less stable systems. Structural characterizations of these bilayers and trilayers on SiC were carried out based on simulations of C-1s core-level-shift (CLS) and carbon K-edge X-ray absorption near edge spectroscopy (XANES) with the goal of assisting future experimental spectroscopy in these graphene/h-BN vdW systems.

Tracking the Dynamics of a Ag-MnOx Oxygen Reduction Catalyst Using In Situ and Operando X-ray Absorption Near-Edge Spectroscopy

Sustainable electricity generation via hydrogen fuel cells requires the development of efficient oxygen reduction reaction (ORR) catalysts. In situ/operando experiments are necessary to uncover the extent of dynamic material changes during catalysis. Herein, we use in situ/operando X-ray absorption near-edge spectroscopy to track Mn valence changes of a promising, ultrathin, porous MnOx layer on a Ag thin film. Mn–K-edge measurements as a function of electrochemical environment and ORR conditions reveal that, interestingly, when driving the ORR at 0.8 VRHE, the Mn is distinctively more reduced and the MnOx redox is nonreversible in contrast to measurements in N2-saturated electrolyte. Ex situ inductively coupled plasma mass spectrometry, atomic force microscopy, and X-ray photoelectron spectroscopy indicate that these phenomena do not correlate to metal dissolution but might be associated with morphological surface reconstruction related to Ag valency. This study highlights how the microenvironment and catalysis play a key role in the in situ/operando surface structure and chemical state.

Catalytic upgrading of bio-oil model mixtures in the presence of microporous HZSM-5 and γ-Al2O3 based Ni, Ta and Zr catalysts

Bio-fuel production from bio-oil was investigated in the presence of microporous HZSM-5 and Al2O3 (MA) based catalysts. This study focused on the catalytic upgrading of a model bio-oil compound with ethanol for bio-fuel production. The model compound containing hydroxypropanol/furfural/formic acid: 4/6.5/5 by volume was used as a bio-oil mixture. Activity tests were performed under atmospheric pressure with a volumetric feed ratio of 30/70 bio-oil mixture/ethanol using a 0.5 g catalyst. In the reaction studies performed on the commercial microporous HZSM-5 catalyst, the optimum co-cracking temperature was determined as 400 °C. Mesoporous γ-Al2O3 support was synthesized by the evaporation-induced self-assembly (EISA) synthesis method, and Ni, Ta, and Zr metals were incorporated into the structure of the catalyst by the impregnation method to improve the surface acidity. The synthesized catalysts were characterized by X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), N2 adsorption–desorption, scanning electron microscopy energy dispersive spectroscopy (SEM/EDS), X-ray fluorescence (XRF), Fourier-transformed infrared spectroscopy (FTIR), pyridine adsorbed diffuse reflectance FTIR spectroscopy (DRIFTS), and thermogravimetric/differential thermal analysis (TGA-DTA). Monometallic and pure alumina catalysts indicated an ordered mesoporous structure with surface area values 106–389 m2/g. Lewis and Brønsted acid sites associated with mild acidity were observed in the DRIFTS results of the 10 wt% Zr-containing alumina catalyst (10Zr@MA). The highest oil phase selectivity and isoparaffin selectivity in oil phase values were obtained in the presence of 10Zr@MA as 17.22% and 80.96%, respectively. 10Zr@MA catalyst showed the best catalytic activity compared to other catalysts, with 9.54% coke formation.

A Combined Wave Function and Density Functional Approach for K-Edge X-ray Absorption Near-Edge Spectroscopy: A Case Study of Hydrated First-Row Transition Metal Ions.

The prediction of X-ray absorption spectra (XAS) of transition metal complexes has important and broad application areas in chemistry and biology. In this letter, we have investigated the predictive ability of multiconfiguration pair-density functional theory (MC-PDFT) for X-ray absorption spectra by calculating the metal K pre-edge features of aquated 3d transition metal ions in common oxidation states. MC-PDFT results were compared with experimentally measured spectra as well as analyzed against results from restricted active-space second-order perturbation theory (RASPT2) and time-dependent density functional theory (TDDFT). As expected, TDDFT performs well for excited states that can be accurately represented by singly excited configurations but fails for excited states where higher order excitations become important. On the other hand, both RASPT2 and MC-PDFT provide quantitatively accurate results for all excited states irrespective of their character. While core-level spectroscopy with RASPT2 is accurate, it is computationally expensive. Our results show that MC-PDFT performs equally well with significantly lower computational cost and is an encouraging alternate approach for X-ray spectroscopies.

Long-and short-range structure of SnO2 nanoparticles: Synthesis and photo(electro)catalytic activity

We report herein a detailed study on the influence of hydrothermal treatment temperature on both long- and short-range structures of SnO2 nanoparticles (NPs) applied as photocatalysts for the discoloration of organic pollutants and as photoanodes for water splitting. Synchrotron X-ray diffraction and X-ray absorption near-edge spectroscopy measurements confirmed the enhancement of the structural order of SnO2 NPs as a function of hydrothermal temperature. Fourier transform infrared spectroscopy revealed that the hydrothermal treatment increased the amount of hydroxyl groups on the SnO2 NPs surface. Regarding the photocatalytic activity, the NPs were able to promote the discoloration of different dyes that can act as potential organic pollutants. The photoelectrocatalytic performance of the samples depended on the hydrothermal treatment temperature, with the degree of crystallinity and surface hydroxyl groups playing a significant role in their performance as photoanodes. In particular, the NPs treated at a higher temperature presented a better degree of crystallinity, in addition to many hydroxyls on their surface, leading to increased mobility of the photogenerated charge carriers and improving the interaction between the molecules degraded and the material surface. The results demonstrated that the hydroxyls adsorbed on the SnO2 surface favor the formation of hydroxyl radicals, a species that indirectly participate in the photocatalytic oxidation of rhodamine B dye. The photoelectrocatalytic tests showed that the NPs treated at 200 °C increased oxygen evolution reaction performance.

Pressure-induced Yb valence transition and magnetism in YbMn6Ge6−xSnx

We investigate the external pressure dependence of the crystal structure, Yb valence, and local magnetization in ${\mathrm{YbMn}}_{6}{\mathrm{Ge}}_{6\ensuremath{-}x}{\mathrm{Sn}}_{x}$ using x-ray diffraction, x-ray absorption near edge spectroscopy (XANES), and x-ray magnetic circular dichroism (XMCD). The application of physical pressure results in a weakly anisotropic unit cell volume contraction without modifying the crystal structure. The external pressure allows crossing the Yb magnetic instability, which occurs for strongly hybridized Yb (valence $\ensuremath{\approx}2.76$), with a concomitant slope change in the valence-pressure curve. In the Ge-rich compounds, Yb local magnetism vanishes at very high pressure ($P \ensuremath{\ge}39.8$ GPa) likely because of the simultaneous collapse of the Mn magnetism. The results underline similarities between external and chemical pressure increase, which favors trivalent and magnetic Yb. Some differences are observed in the Sn-rich compounds highlighting potential local pressure and/or electronic effects.

Comparison of sedimentary iron speciation obtained by sequential extraction and X-ray absorption spectroscopy

Iron (Fe) speciation in marine sediments and suspended particulate matter (SPM) are widely used to investigate the role of Fe minerals in marine biogeochemical cycling and to reconstruct paleo-redox conditions. A traditional method for the determination of Fe speciation are sequential extractions, in which operationally defined Fe pools are dissolved by specific extraction solutions. More recently, many studies make use of synchrotron radiation-based X-ray absorption near edge spectroscopy (XANES), especially if little sample material is available or a high spatial resolution is required. However, few studies have systematically compared Fe speciation obtained by the two methods. Here, we report Fe speciation for marine sediments (both wet and freeze-dried) and SPM from contrasting marine environments (western Baltic Sea and Amazon shelf) obtained by sequential extraction and XANES. The two methods yield comparable results regarding the approximate proportion of reactive Fe ((oxyhydr)oxide, carbonate and sulfide minerals) in the total Fe pool and the extent to which reactive Fe minerals have been converted to pyrite. However, methodological issues associated with both approaches complicate a direct comparison of specific mineral groups. Pyrite concentrations obtained by XANES are lower than those obtained by extraction, which is attributed to an underestimation by XANES due to self-absorption effects and an overestimation by the chromium reduction method due to dissolution of reduced sulfur phases that are not associated with Fe. As substantial amounts of pyrite are oxidized during freeze-drying and sample storage, we recommend to analyze wet sediments in modern marine environments. The XANES spectra of Fe (oxyhydr)oxide and Fe(III)-containing (ferric) clay minerals are similar, which is why these components cannot be differentiated by XANES. Similarly, sequential extraction schemes are known to dissolve a variable proportion of ferric clay minerals along with Fe carbonates and Fe (oxyhydr)oxides. Further investigating the role of ferric clay minerals in the low-temperature marine Fe cycle may therefore require the use of additional methods. Our findings may help to assess the transferability of XANES-based Fe speciation for small sediment and SPM samples to those reported in more traditional studies on Fe biogeochemical cycling and paleo-redox conditions.

The effect of Ge concentration on the Ni diffusion across AuGe metal interface of GaAs/AlGaAs heterostructure

X-ray absorption near edge spectroscopy (XANES) has been employed to investigate the effect of Ge concentration on the diffusion of Ni in AuGe/Ni/AuGe stack, which is an Ohmic contact to the GaAs/AlGaAs heterostructure. Four different AuGe weight ratios are chosen- one eutectic (AuGe 75:25) and three offeutectic (AuGe 88:12, 95:05 and 100:0). The AuGe/Ni/AuGe stacks are annealed at 100˚C and 300˚C temperatures for 2hrs to facilitate the diffusion of Ni in to AuGe matrix. XANES measurements were carried out at 45˚ incidence angle and total reflection geometry. Metallic Ni, NiO and Ni3Ge were used as reference samples for XANES analysis. The detailed XANES analysis confirmed that the high concentration of Ge in AuGe sandwich layers restricts the diffusion of Ni in to AuGe matrix. A possible mechanism behind the Ni diffusion into AuGe matrix is propounded based on the XANES result.

Experimental Evidence for Alloying Effects in Au–Pt-Catalyzed Low-Temperature CH4Activation with NO

The activation of methane with nitric oxide at low temperatures (300–400 °C) and atmospheric pressure was studied on Au–Pt nanoparticles with different Au/Pt ratios supported on alumina. The reaction forms HCN, a valuable chemical intermediate, via concurrent or successive C–H bond cleavage, NO dissociation, and C–N coupling reactions. The catalysts with different Au/Pt ratios had similar particle morphology and bulk properties, as evidenced by scanning transmission electron microscopy imaging and X-ray absorption near-edge structure spectroscopy, respectively. However, they exhibited different electronic and geometric surface properties, according to in situ diffuse reflectance infrared Fourier transform spectra of preadsorbed CO, a likely consequence of Au surface enrichment. These differences had catalytic implications, where the relative proportion of electron-deficient Pt sites on the surface of the catalysts was found to track with activity, expressed as turnover frequency for the total CH4 conversion and HCN formation. Based on the activity trend, kinetic comparison of reaction orders, temperature-programmed desorption experiments, and comparisons to the literature, it is proposed that the CH4–NO catalysis occurred mainly on contiguous Pt sites, that the higher C–H activation activity of electron-deficient Pt species may be related to weaker adsorption of CH4, and that the higher NO dissociation activity of metallic Pt species may have contributed to the complete oxidation of CH4 to CO2. Nonetheless, contact time and in situ spectroscopic studies at isothermal conditions revealed only subtle differences in the reaction scheme by surface modifications, indicating that Au affected the reactivity indirectly.