Edge Spectroscopy Research Articles

Post-synthetic modification of a MOF via Continuous Flow Methods for Gold E-Waste Recycling.

This study pioneers the use of continuous flow methods to modify the nanoporous metal-organic framework Fe-BTC (MIL-100(Fe)) with redox-active poly-p-phenylenediamine (PpPDA). The Fe-BTC/PpPDA composite, known for its gold extraction capabilities, was synthesized in continuous flow on a 250 g scale. Fe-BTC/PpPDA was then tested in eight industrial leachates (e.g., cyanide, thiourea, aqua regia) exhibiting varying effectiveness, pH, and gold speciation, which led to significant differences in the composite's performance for gold extraction. The composite performed best in solutions containing [AuCl4]- species. Structured into spherical beads using a novel continuous flow microdroplet technique, these adsorbents were tested for gold recovery from real e-waste solutions in a breakthrough epemriment. They achieved a capacity of ~600 mg of gold per gram before breakthrough and ~900 mg per gram at a 60% recovery rate. Selectivity ratios for Au/Ni, Au/Co, and Au/Fe were 972, 262, and 193, respectively. In situ X-ray absorption near edge spectroscopy (XANES) provided evidence of the reduction of Au³⁺ to Au⁰, facilitated by the redox-active oligomers. Outperforming several commercial resins, Fe-BTC/PpPDA shows great promise for scalable, selective metal recovery from waste streams. This study highlights the potential of MOF/polymer composites and continuous flow methods for large-scale production.

Operando XANES Reveals the Chemical State of Iron-Oxide Monolayers During Low-Temperature CO Oxidation.

We have used grazing incidence X-ray absorption near edge spectroscopy (XANES) to investigate the behavior of monolayer FeOxfilms on Pt(111) under near ambient pressure CO oxidation conditions with a total gas pressure of 1 bar. Spectra indicate reversible changes during oxidation and reduction by O2 and CO at 150ºC, attributed to a transformation between FeO bilayer and FeO2 trilayer phases. The trilayer phase is also reduced upon heating in CO+O2, consistent with a Mars-van-Krevelen type mechanism for CO oxidation. At higher temperatures, the monolayer film dewets the surface, resulting in a loss of the observed reducibility. A similar iron oxide film prepared on Au(111) shows little sign of reduction or oxidation under the same conditions. The results highlight the unique properties of monolayer FeO and the importance of the Pt support in this reaction. The study furthermore demonstrates the power of grazing-incidence XAFS for in situ studies of these model catalysts under realistic conditions.

Strong Magnetic Exchange Interactions and Delocalized Mn-O States Enable High-Voltage Capacity in the Na-Ion Cathode P2-Na0.67[Mg0.28Mn0.72]O2.

The increased capacity offered by oxygen-redox active cathode materials for rechargeable lithium- and sodium-ion batteries (LIBs and NIBs, respectively) offers a pathway to the next generation of high-gravimetric-capacity cathodes for use in devices, transportation and on the grid. Many of these materials, however, are plagued with voltage fade, voltage hysteresis and O2 loss, the origins of which can be traced back to changes in their electronic and chemical structures on cycling. Developing a detailed understanding of these changes is critical to mitigating these cathodes' poor performance. In this work, we present an analysis of the redox mechanism of P2-Na0.67[Mg0.28Mn0.72]O2, a layered NIB cathode whose high capacity has previously been attributed to trapped O2 molecules. We examine a variety of charge compensation scenarios, calculate their corresponding densities of states and spectroscopic properties, and systematically compare the results to experimental data: 25Mg and 17O nuclear magnetic resonance (NMR) spectroscopy, operando X-band and ex situ high-frequency electron paramagnetic resonance (EPR), ex situ magnetometry, and O and Mn K-edge X-ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Spectroscopy (XANES). Via a process of elimination, we suggest that the mechanism for O redox in this material is dominated by a process that involves the formation of strongly antiferromagnetic, delocalized Mn-O states which form after Mg2+ migration at high voltages. Our results primarily rely on noninvasive techniques that are vital to understanding the electronic structure of metastable cycled cathode samples.

Thermochemical Transformation of Calcium during Biomass Burning and the Effects on Postfire Aqueous Dissolution of Macronutrients.

Calcium is commonly the most abundant element in fire residues and its speciation largely determines the geochemical properties of fire residues and their effects on postfire soil chemistry. To explore the effects of biomass composition and fire conditions on ash Ca speciation, this study characterizes the speciation of Ca in charcoal and ash samples that were derived from different plant compartments and thermal conditions, using Ca K-edge X-ray absorption near edge spectroscopy. Results showed that biomass contains abundant organic Ca complexes, which were mineralized into fairchildite and calcite after heating at 450 to 600 °C and then CaO, as temperature increased to 750 °C. Apatite could be an abundant Ca species in fire residues if the Ca/P molar ratio of the biomass is small (<2). The mineralization of organic Ca to the identified Ca minerals during burning was negligibly affected by the oxygen level. Calcium speciation in prescribed fire residues resembled that of biomass ash burned at 550 °C with similar Ca/P molar ratios. Batch experiments showed that macronutrients (Ca, Mg, K, and P) were differentially released, as a result of different solubility of minerals in ashes and reprecipitation of minerals. The aqueous solubility of Ca, Mg, and P decreased as pH increased from 5 to 9, while K showed no pH dependency and was almost completely soluble. Results from this study improve our understanding of the chemistry of fire residues and their geochemical behaviors, which can help evaluate the impact of fire on postfire soil properties and macronutrient cycling.

Effect of glass content on the phase assemblage and processing requirements of zirconolite glass-ceramics for actinide immobilisation

Zirconolite is a candidate wasteform for actinides (U/Pu/Th), and glass incorporation to form glass-ceramics provides flexibility to accommodate heterogeneous wastes while simplifying processing requirements; however, the glass content required to optimise these benefits remains unestablished. This systematic study investigated the effect of glass content on zirconolite crystallisation, phase assemblage, phase composition, cerium valence states and partitioning. Cerium-bearing zirconolite glass-ceramics were fabricated by targeting zirconolite (Ca0.8Ce0.2ZrTi1.6Al0.4O7; Ce as actinide surrogate) with varying amounts (0–100 vol%) of glass addition (NaAl1.5B0.5Ca0.7Ti0.2Si2O8.6). Differential scanning calorimetry data revealed that glass addition lowered the zirconolite crystallisation temperature (1283°C to 1200°C). X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy analyses showed that glass addition lowered the temperature (1320°C to 1270°C) required to fabricate phase-pure zirconolite glass-ceramics and minimise CeO2/ZrO2 phases, with > 90 % of cerium preferentially partitioned into zirconolite. X-ray absorption near edge spectroscopy data showed predominantly Ce4+ within zirconolite as targeted.

High quantum yield luminescence and scintillation properties of high-Ce-doped MgF2–Al2O3–B2O3 glasses and their glass structure

Ce-doped inorganic single crystals are currently used in scintillators in various instruments because of their outstanding luminescence properties. However, these crystals are expensive to manufacture and are inefficient in terms of their energy consumption. In this study, xCeF3-doped 40MgF2–20Al2O3–40B2O3 glasses (x = 0−30 mol%) were prepared by a melt-quenching method in N2 atmosphere, and their photoluminescence, scintillation, and dosimetry properties were investigated. The X-ray absorption fine edge spectroscopy of the Ce L3-edge indicated that the doped Ce exists in the form of Ce3+ ions, and that Ce4+ does not form because of the low basicity of the glass. The glasses underwent photoluminescence at 380 nm due to the 5d-4f transition of Ce3+, with a very high quantum yield of up to ∼100 % and low concentration quenching. The peak shape of the X-ray-induced luminescence was similar to that of the PL with a decay time of 110−70 ns. The light yields for gamma rays (137Cs source) were obtained from the pulse-height spectra, and the highest light yield among the present samples was 1071 photons/MeV with the energy resolution of 17 %. The thermoluminescence glow curves had peaks at 365 and 460 K and a good linear relationship existed in the range of 0.5–1000 mGy, with the dose response being comparable to that of a commercial dosimeter. The origin of the high quantum yield was probed by analyzing the glass structure using molecular dynamics simulation based on a graph neural network. A strong tendency to selectively bind cations and anion pairs was found: Mg preferably binds to F, B preferably binds to O, and Al binds to O and F, which can be explained by soft and hard acid and base theories. Because of this selectivity the glass forms fluoride- and oxide-rich domains, which seem to be responsible for the more effective dispersal of the luminescent center. The results of this study are expected to contribute to the development of glasses with enhanced photoluminescence and scintillation luminescence properties.

Dual-Ion-Doped Hydrated Vanadium Oxides Nanowires As Cathode Materials for High-Rate Aqueous Zinc Ion Batteries

Due to environmental-friendliness, high safety and stability, high theoretical capacity as well as low cost, aqueous zinc ion batteries (AZIBs) are budding as attractive alternatives for large-scale energy storage applications. However, the lack of suitable positive electrode materials becomes the main issue for AZIBs to pursue high energy density and excellent rate capability. Herein, the dual-ion-doped hydrated vanadium oxide nanowires (VOH) are synthesized through hydrothermal method, which is suitable to be cathode materials for AZIBs. Through metal ions and crystal water pre-intercalation, the interlayer spacing of (001) becomes larger, which is proposed to have better capability to accommodate Zn2+. Moreover, the pre-intercalated metal ions can serve as structural pillars between layers, providing excellent stability during discharging/charging process. In this study, the novel dual-ion-doped VOH exhibits more than 420 mA h g−1 at 0.1 A g−1 as well as more than 260 mA h g−1 at 5 A g−1. In addition, it can retain more than 80% capacity after 500 cycles at 5 A g−1. The energy storage mechanism of dual-ion-doped VOH is revealed via operando x-ray absorption near edge spectroscopy. This study implies that this dual-ion-doped VOH nanowires can serve as suitable positive electrode materials for high-performance aqueous zinc ion batteries.

Speciation of toxic metals in metal finishing filter cake by X-ray absorption spectroscopy

The speciation of Cr, Zn, Cu and Pb in two metal finishing filter cakes (TX and ST) was investigated by X-ray absorption spectroscopy (XAS) complemented by X-ray fluorescence (XRF) and X-ray diffraction (XRD). XRF showed that concentrations of Cr, Zn, Cu and Pb were 1.4%, 0.19%, 0.20% and 0.01%, respectively, in TX, and 12.6%, 3.3%, 1.3% and 0.21% in ST. No crystalline phases were detected in TX by XRD whereas ST was dominated by calcite. Cr and Fe K edge XAS showed Cr to be trivalent and octahedrally coordinated, co-precipitated with Fe as CrxFe1-x-(oxy)hydroxides in both filter cakes. Zn, P and Ca K edge XAS showed that 2ZnCO3∙3Zn(OH)2 and Zn3(PO4)2 were the dominant zinc-containing phases, with combined tetrahedral and octahedral coordination; Zn phases were slightly more crystalline in TX than ST. Pb L3 edge X-ray absorption near edge spectroscopy (XANES) found that Pb was likely adsorbed on amorphous SiO2. Cu, Si and S K edge XAS showed that all Cu was divalent, and the dominant copper phases were found to be Cu2Cl(OH)3, Cu(OH)2 and CuSO4·5H2O for ST, whereas Cu appeared to adsorb to amorphous SiO2 for TX, which contained much less Pb. Cr is thus immobilized in the filter cakes in a phase with low solubility at environmentally feasible pH values, whereas Zn, Cu and Pb could be released when the pH decreases below 8 or above 11. These findings are significant for the development of waste management regulations and/or metal recovery methods (e.g., hydro/pyrometallurgy).

Co-Ni co-sputter deposited bimetallic thin film catalysts for alkaline hydrogen and oxygen evolution reactions

Co-Ni bi-metallic thin films with different stoichiometries have been deposited by magnetron co-sputtering technique on Ni foam (NF), Si and carbon paper substrates. The structural and morphological characterisations of these films have been done by X-ray diffraction, X-ray absorption spectroscopy and Field Emission Scanning Electron Microscope measurements. The performances of these films for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been evaluated through electrochemical measurements like Linear Sweep Voltammetry, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy measurements. Operando X ray absorption near edge spectroscopy measurements have also been carried out during HER and OER to decipher the redox reaction mechanisms. It has been observed that the HER and OER performance of all the Co Ni bi-metallic thin films on NF are found to be better than that of bare NF. The CoNi sample with composition of 58.7 % Co and 41.3 % Ni shows the best catalytic activity and stability towards HER and OER reactions. Though the HER activity is not the best but is comparable to many results reported in the literature, however the catalyst shows excellent OER activity with overpotential of 214 mV for 10 mA/cm2 current density. This is much better than that of the OER benchmark catalyst RuO2 and most of those reported in the literature. The OER is kinetically slower than HER and requires more overpotential, hence a significant improvement in the catalytic OER activity will definitely enhance the overall water splitting activity and hydrogen production efficiency. Thus the Co-Ni bi-metallic catalyst prepared by an easily scalable and highly reproducible magnetron co-sputtering technique has significant potential to emerge out as a technologically viable water splitting catalyst.

Electronic structures of skyrmionic polycrystalline MnSi thin film studied by resonance photoemission and x-ray near edge spectroscopy

Magnetic nanometric skyrmions are small complex vortex-like topological defects, mainly found in non-centrosymmetric crystals such as MnSi. They have potential applications for future spintronic devices. In this article, the structural, electronic, and magnetic states of the Mn atoms in a polycrystalline MnSi thin film facing a c-sapphire substrate were studied using x-ray diffraction, x-ray photo-emission spectroscopy, resonance photoemission spectroscopy (RPES), and extended x-ray absorption fine structure (EXAFS). The valence band spectra indicate the metallic nature of the film. The RPES study reveals the presence of major itinerant Mn 3d states near EF and also the mixed Mn 3d and Si 3s–3p states from 5.3 to 11.3 eV. The EXAFS spectrum does not show the existence of oxygen vacancies in the system, and the obtained magnetic moment in the non-stoichiometric MnSi thin film is a combination of the partially itinerant and partially localized Mn 3d states.

High valency of charge compensator (Mo6+) to substitute Ti site in REE doped zirconolite (REE=Nd, Sm, Gd, Ho and Yb): Solid solubility, phase evolution and structural analysis

Zirconolite ceramics have been proven to be a promising matrix for high-level waste immobilization, especially for minor actinides. In this study, a series of CaZr1-2xREE2xTi2-xMoxO7 (REE = Nd, Sm, Gd, Ho, Yb) samples were prepared to investigate the formation of zirconolite REE as surrogates of minor actinides and Mo6+ as charge compensator. Synchrotron and in-house powder X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) were used to study their solid solubility, phase evolution, and structural details. XRD results show that the formation of zirconolite-2M can be up to x = 0.15 in the CaZr1-2xNd2xTi2-xMoxO7 samples when co-existence of 2 M and 4 M was observed at x = 0.20 and 0.25. The “yellow phase” appears when x exceeds 0.4. The phase evolutions of CaZr1-2xSm2xTi2-xMoxO7, CaZr1-2xGd2xTi2-xMoxO7, CaZr1-2xHo2xTi2-xMoxO7 and CaZr1-2xYb2xTi2-xMoxO7 solid solutions are similar to each other: (1) zirconolite-2M in x = 0.05 and 0.10; (2) co-existence of 2 M and 4 M and then formation of 4 M phase from x = 0.15 to 0.25; (3) formation of “yellow phase” and pyrochlore when x ≥ 0.30. Interestingly, perovskite exits in all the ceramic samples, and its lattice parameters slightly change when increasing the dopants. Rietveld refinement results reveal that Sm/Ho mainly incorporates into 8f (Wyckoff position) Zr-sites while Mo replaces the 8f (Wyckoff position) Ti-sites. XANES results further confirm the occupation of Mo in TiO5 coordination environments. Furthermore, there is a considerable amount of Sm/Ho incorporated into 8f Ca-sites, demonstrating a different substitution mechanism from the preliminary design.

Saltwater intrusion increases phosphorus abundance and alters availability in coastal soils with implications for future sea level rise

Sea level rise (SLR) promotes saltwater intrusion (SWI) into coastal soils globally at an increasing rate, impacting phosphorus (P) dynamics and adjacent water quality. However, how SWI influences P molecular speciation and availability in coastal soils remains poorly understood. By using a space-for-time substitution strategy, we evaluated the SWI impacts on P transformation along a SWI gradient at the Rehoboth Inland Bay, which consists of five sampling locations along a transect representing different SWI degrees. Soils were analyzed at the macro- and micro-scale using X-ray absorption near edge spectroscopy (XANES) and the modified Hedley fractionation. With increasing distance from the Bay, soil salinity (29.3–0.07 mmhos cm−1), the proportion of Fe3+ to total Fe, and P concentrations decreased. The fractionation showed that recalcitrant P was dominant (86.9–89.5% of total P). With increasing SWI, labile P increased gradually, reached a plateau, and then decreased sharply. Bulk XANES spectroscopy showed that soil P was likely dominated by iron and aluminum-associated P (Fe/Al-P), regardless of the SWI degree. Hence, with increasing SWI, P increasingly accumulated in a recalcitrant pool, mainly as Fe/Al-P. μ-XANES spectroscopy revealed that calcium-associated P (Ca-P) existed in P-rich spots of the greatest SWI soil while Al-P occurred in P-rich spots of the low SWI soil, consistent with the greater HCl-P (presumably Ca-P) in the former soil. Overall, results demonstrate that SWI impacts P availability and environmental risk in coastal soils depending on the degree of SWI. These findings have important implications for understanding soil P cycling and availability in SLR-impacted coastal areas.

Anisotropy of optical transitions in β-Ga2O3 investigated by polarized photoluminescence excitation spectroscopy

The monoclinic beta-phase of gallium oxide possesses an ultra-wide bandgap that surpasses other wide bandgap materials such as SiC and GaN, making it a promising candidate for power electronic device technologies. We investigate the first fundamental optical transitions in this material, which exhibit a strong directional dependence. To determine the energies and orientations of these transitions, temperature-dependent and angular resolved photoluminescence excitation spectroscopy is applied. We observe a distinct excitation channel located energetically between those of the first two optical transitions Γ1−1 and Γ1−2. While previous absorption edge and reflectance spectroscopy studies have assigned a transition in this spectral range to either the Γ1−1 or the Γ1−2 transition, our findings demonstrate no pronounced polarization dependence of this excitation channel within the (010) plane, an observation not reflected in calculations of the band-to-band transitions in β-Ga2O3.

In situ arsenic immobilization by natural iron (oxyhydr)oxide precipitates in As–contaminated groundwater irrigation canals

Arsenic-contaminated groundwater is widely used in agriculture. To meet the increasing demand for safe water in agriculture, an efficient and cost-effective method for As removal from groundwater is urgently needed. We hypothesized that Fe (oxyhydr)oxide (FeOOH) minerals precipitated in situ from indigenous Fe in groundwater may immobilize As, providing a solution for safely using As-contaminated groundwater in irrigation. To confirm this hypothesis and identify the controlling mechanisms, we comprehensively evaluated the transport, speciation changes, and immobilization of As and Fe in agricultural canals irrigated using As-contaminated groundwater. The efficiently removed As and Fe in the canals accumulated in shallow sediment rather than subsurface sediment. Linear combination fitting (LCF) analysis of X–ray absorption near edge spectroscopy (XANES) indicated that As(V) was the dominant As species, followed by As(III), and there was no FeAsO4 precipitate. Sequential extraction revealed higher contents of amorphous FeOOH and associated As in shallower sediment than in the subsurface layer. Stoichiometric molar ratio calculations, SEM‒EDS, FTIR, and fluorescence spectroscopy collectively demonstrated that the microbial reductive dissolution of amorphous FeOOH proceeded via reactive dissolved organic matter (DOM) consumption in subsurface anoxic porewater environment facilitating high labile As, whereas in surface sediment, the in situ-generated amorphous FeOOH was stable and strongly inhibited As release via adsorption. In summary, groundwater Fe2+ can efficiently precipitate in benthic surface sediment as abundant amorphous FeOOH, which immobilizes most of the dissolved As, protecting agricultural soil from contamination. This field research supports the critical roles of the phase and reactivity of in situ-generated FeOOH in As immobilization and provides new insight into the sustainable use of contaminated water.

Density Functional Theory Study of Lithiation and Sodiation in Sb2S3 and Sb2Se3 Anode Accompanying by S/Se Redox

Density functional theory simulation has been performed to illuminate the mechanism of lithiation and sodiation in Sb2S3 and Sb2Se3 anodes which is accompanied by anionic S/Se redox. The lithiation and sodiation of Sb2S3 and Sb2Se3 is comprised of two steps, (a) conversion and (b) alloying -dealloying. During conversion Sb and alkaline (Li/Na) chalcogenides are formed. Voltages during the conversion reaction of lithiation and sodiation were ∼1.6 and ∼1.25 V, respectively, for both Sb2S3 and Sb2Se3. Comparison of X-ray absorption near edge spectroscopy imaging of S/Se as present in pristine chalcogenides and A2S/Se with A = Li/Na reflects the presence of S/Se redox, which is further confirmed by electronic charge density analysis. Sb acts as an active center for the second step alloying-dealloying reaction. The formation of alloy mainly occurs via formation of Li3Sb and Na3Sb, which exhibits redox peaks at 1.025 V for lithiation and 0.686 V for sodiation. As reported in earlier reports, the redox peak, at 0.95 V is found to appear due to the formation of alloy NaSb.

Electronic and thermal properties of Nb2CCl2 MXenes

The molten salt synthesis has recently opened up new opportunities to functionalize two-dimensional MXenes with uniform halogen terminations. In this work, we synthesize Nb2CCl2 MXenes by etching the Al layer from the Nb2AlC precursor in the melt of CuCl2 Lewis acid. In Cl-terminated MXenes, intercalation of water is negligible, so that MXenes get closer to ideal, ordered van der Waals solids. We identify the preferential atomic stacking arrangement, which is of the AB type, as opposed to the AA stack assumed in previous works. We combine X-ray absorption near edge spectroscopy with density functional theory to clarify the local chemical environment of Cl atoms. Using vibrating sample magnetometry and specific heat measurements, we confirm that Nb2CCl2 MXenes are superconducting with a transition temperature of 6.0 K and find a Debye temperature of 320 K. We further show that the transition temperature slightly decreases with applied isostatic pressure.

Local electronic structures of annealing induced phases in cobalt doped TiO2 nanoparticles: a combined study of transmission electron microscopy and x-ray absorption fine structure spectroscopy

The evolution of the nanostructures and electronic properties of 5% cobalt-doped TiO2 nanoparticles (NPs) annealed at 400 °C, 600 °C, and 800 °C have been investigated to understand the structural phase transformations through chemical co-precipitation synthesis. A detailed analysis of the X-ray Diffractogram confirms that the sample annealed at 400 °C is anatase, at 600 °C, the mixed phase of anatase and rutile evolves, and at 800 °C, the sample is of rutile structure. A detailed morphological study by scanning transmission electron microscope provides the particle size, lattice spacing, and variation in polycrystalline grain growth at different phases. Electron Energy Loss Spectroscopy analysis indicates from the O K, Co, and Ti L 2,3-edges that Ti4+ ions are primarily in an octahedral symmetry with the oxygen ligands changing their structural phases from anatase to mixed phase and then stable rutile phase with increasing temperature of annealing. X-ray Absorption Near Edge Spectroscopy (XANES) extracts information about the varying oxidation states and 3-dimensional geometry of Ti-ions. The unresolved issues of the structural details at the atomic-scale picture with the local environment of the cation with a few nearest neighbour shells are derived from Extended X-ray Absorption Fine Structure (EXAFS) and pre-edge parts of the absorption spectra. The limits of EXAFS in this situation of asymmetric bond length disorder, which is typical for mixed-valence oxides, are generated to reconcile the two data and highlight the value of pre-edge XANES analysis for identifying local heterogeneities in structural and compositional motifs. TiO2 possesses unique properties depending upon its structural phase. The Ti L 2,3-edge spectrum indicates that there is an octahedron connectivity of the Oxygen atoms at the anatase state which transforms to a higher energetic tetrahedral correspondence as it proceeds towards the rutile phase. The driving force behind such interest is to modulate the properties of TiO2 NPs to better photocatalytic material and to integrate its application as a versatile energy storage device.

Dislocation density as a factor compensating the polarization doping effect in graded p-AlGaN contact layers

One of the approaches to improve p-doping properties in (Al)GaN-based materials is using a method called polarization doping. In this study, the graded p-AlGaN contact layers were deposited using plasma-assisted molecular beam epitaxy with different III/N ratios. To study the influence of Ga flux on the structural and electrical properties of the grown graded p-AlGaN structures, the samples were investigated using reflection high-energy electron diffraction, atomic force microscopy, secondary ion mass spectrometry, X-ray diffraction, and Hall effect measurements. The electronic structure of the samples was investigated by X-ray absorption near edge spectroscopy, while the polarity of the layers was analyzed by means of transmission electron microscopy. Our study reveals that growing with a higher Ga flux increases Mg incorporation and changes its distribution in the graded layer. However, it was shown that the main factor drastically affecting the magnitude and the type of conductivity in the graded p-AlGaN structures is the dislocation density. The highest hole concentration of ∼3.0×1018 cm−3 was observed for sample grown with a low Ga flux.

Trace Element Distribution and Arsenic Speciation in Toenails as Affected by External Contamination and Evaluation of a Cleaning Protocol.

Trace element concentrations in toenail clippings have increasingly been used to measure trace element exposure in epidemeological research. Conventional methods such as inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography ICP-MS (HPLC-ICP-MS) are commonly used to measure trace elements and their speciation in toenails. However, the impact of the removal of external contamination on trace element quantification has not been thoroughly studied. In this work, the microdistribution of trace elements (As, Ca, Co, Cu, Fe, K, Mn, Ni, Rb, S, Sr, Ti, and Zn) in dirty and washed toenails and the speciation of As in situ in toenails were investigated using synchrotron X-ray fluorescence microscopy (XFM) and laterally resolved X-ray absorption near edge spectroscopy (XANES). XFM showed different distribution patterns for each trace element, consistent with their binding properties and nail structure. External (terrestrial) contamination was identified and distinguished from the endogenous accumulation of trace elements in toenails─contaminated areas were characterized by the co-occurrence of Co, Fe, and Mn with elements such as Ti and Rb (i.e., indicators of terrestrial contamination). The XANES spectra showed the presence of one As species in washed toenails, corresponding to As bound to sulfhydryl groups. In dirty specimens, a mixed speciation was found in localized areas, containing AsIII-S species and AsV species. ArsenicV is thought to be associated with surface contamination and exogenous As. These findings provide new insights into the speciation of arsenic in toenails, the microdistribution of trace elements, and the effectiveness of a cleaning protocol in removing external contamination.

Light‐Driven C−C Coupling for Targeted Synthesis of CH3COOH with Nearly 100 % Selectivity from CO2

AbstractTargeted synthesis of acetic acid (CH3COOH) from CO2 photoreduction under mild conditions mainly limits by the kinetic challenge of the C−C coupling. Herein, we utilized doping engineering to build charge‐asymmetrical metal pair sites for boosted C−C coupling, enhancing the activity and selectivity of CO2 photoreduction towards CH3COOH. As a prototype, the Pd doped Co3O4 atomic layers are synthesized, where the established charge‐asymmetrical cobalt pair sites are verified by X‐ray photoelectron spectroscopy and X‐ray absorption near edge spectroscopy spectra. Theoretical calculations not only reveal the charge‐asymmetrical cobalt pair sites caused by Pd atom doping, but also manifest the promoted C−C coupling of double *COOH intermediates through shortening of the coupled C−C bond distance from 1.54 to 1.52 Å and lowering their formation energy barrier from 0.77 to 0.33 eV. Importantly, the decreased reaction energy barrier from the protonation of two*COOH into *CO intermediates for the Pd‐Co3O4 atomic layer slab is 0.49 eV, higher than that of the Co3O4 atomic layer slab (0.41 eV). Therefore, the Pd‐Co3O4 atomic layers exhibit the CH3COOH evolution rate of ca. 13.8 μmol g−1 h−1 with near 100% selectivity, both of which outperform all previously reported single photocatalysts for CO2 photoreduction towards CH3COOH under similar conditions.