Atomic scale structure and bond stretching force constants in stoichiometric and off-stoichiometric kesterites.

The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu-Se and Zn-Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells.

Interlayer Coordination of Pd-Pd Units in Exfoliated Black Phosphorus.

The chemical functionalization of 2D exfoliated black phosphorus (2D BP) continues to attract great interest, although a satisfactory structural characterization of the functionalized material has seldom been achieved. Herein, we provide the first complete structural characterization of 2D BP functionalized with rare discrete Pd2 units, obtained through a mild decomposition of the organometallic dimeric precursor [Pd(η3-C3H5)Cl]2. A multitechnique approach, including HAADF-STEM, solid-state NMR, XPS, and XAS, was used to study in detail the morphology of the palladated nanosheets (Pd2/BP) and to unravel the coordination of Pd2 units to phosphorus atoms of 2D BP. In particular, XAS, backed up by DFT modeling, revealed the existence of unprecedented interlayer Pd-Pd units, sandwiched between stacked BP layers. The preliminary application of Pd2/BP as a catalyst for the hydrogen evolution reaction (HER) in acidic medium highlighted an activity increase due to the presence of Pd2 units.

Copper Oxide Nanomaterial Fate in Plant Tissue: Nanoscale Impacts on Reproductive Tissues.

A thorough understanding of the implications of chronic low-dose exposure to engineered nanomaterials through the food chain is lacking. The present study aimed to characterize such a response in Cucurbita pepo L. (zucchini) upon exposure to a potential nanoscale fertilizer: copper oxide (CuO) nanoparticles. Zucchini was grown in soil amended with nano-CuO, bulk CuO (100 mg Kg-1), and CuSO4 (320 mg Kg-1) from germination to flowering (60 days). Nano-CuO treatment had no impact on plant morphology or growth nor pollen formation and viability. The uptake of Cu was comparable in the plant tissues under all treatments. RNA-seq analyses on vegetative and reproductive tissues highlighted common and nanoscale-specific components of the response. Mitochondrial and chloroplast functions were uniquely modulated in response to nanomaterial exposure as compared with conventional bulk and salt forms. X-ray absorption spectroscopy showed that the Cu local structure changed upon nano-CuO internalization, suggesting potential nanoparticle biotransformation within the plant tissues. These findings demonstrate the potential positive physiological, cellular, and molecular response related to nano-CuO application as a plant fertilizer, highlighting the differential mechanisms involved in the exposure to Cu in nanoscale, bulk, or salt forms. Nano-CuO uniquely stimulates plant response in a way that can minimize agrochemical inputs to the environment and therefore could be an important strategy in nanoenabled agriculture.

The LISA beamline at ESRF.

This contribution provides a description of LISA, the new Italian Collaborating Research Group beamline operative at the European Synchrotron Radiation Facility. A presentation of the instruments available and optical devices is given as well as the main X-ray parameters (flux, energy resolution, focal spot dimensions, etc.) and comparison with theoretical calculations. The beamline has been open to users since April 2018 and will be ready at the opening of the Extremely Brilliant Source in late-2020.

Multi-scale theoretical approach to X-ray absorption spectra in disordered systems: an application to the study of Zn(ii) in water.

We develop a multi-scale theoretical approach aimed at calculating from first principles X-ray absorption spectra of liquid solutions and disordered systems. We test the method by considering the paradigmatic case of Zn(ii) in water which, besides being relevant in itself, is also of interest for biology. With the help of classical molecular dynamics simulations we start by producing bunches of configurations differing for the Zn(ii)-water coordination mode. Different coordination modes are obtained by making use of the so-called dummy atoms method. From the collected molecular dynamics trajectories, snapshots of a more manageable subsystem encompassing the metal site and two solvation layers are cut out. Density functional theory is used to optimize and relax these reduced system configurations employing a uniform dielectric to mimic the surrounding bulk liquid water. On the resulting structures, fully quantum mechanical X-ray absorption spectra calculations are performed by including core-hole effects and core-level shifts. The proposed approach does not rely on any guessing or fitting of the force field or of the atomic positions of the system. The comparison of the theoretically computed spectrum with the experimental Zn K-edge XANES data unambiguously demonstrates that among the different a priori possible geometries, Zn(ii) in water lives in an octahedral coordination mode.

Observation of charge transfer cascades in α-Fe2O3/IrOx photoanodes by operando X-ray absorption spectroscopy.

Electrochemical devices for energy conversion and storage are central for a sustainable economy. The performance of electrodes is driven by charge transfer across different layer materials and an understanding of the mechanistics is pivotal to gain improved efficiency. Here, we directly observe the transfer of photogenerated charge carriers in a photoanode made of hematite (α-Fe2O3) and a hydrous iridium oxide (IrOx) overlayer, which plays a key role in photoelectrochemical water oxidation. Through the use of operando X-ray absorption spectroscopy (XAS), we probe the change in occupancy of the Ir 5d levels during optical band gap excitation of α-Fe2O3. At potentials where no photocurrent is observed, electrons flow from the α-Fe2O3 photoanode to the IrOx overlayer. In contrast, when the composite electrode produces a sustained photocurrent (i.e., 1.4 V vs. RHE), a significant transfer of holes from the illuminated α-Fe2O3 to the IrOx layer is clearly demonstrated. The analysis of the operando XAS spectra further suggests that oxygen evolution actually occurs both at the α-Fe2O3/electrolyte and α-Fe2O3/IrOx interfaces. These findings represent an important outcome for a better understanding of composite photoelectrodes and their use in photoelectrochemical systems, such as hydrogen generation or CO2 reduction from sunlight.

A Pd/C-CeO2 Anode Catalyst for High-Performance Platinum-Free Anion Exchange Membrane Fuel Cells.

One of the biggest obstacles to the dissemination of fuel cells is their cost, a large part of which is due to platinum (Pt) electrocatalysts. Complete removal of Pt is a difficult if not impossible task for proton exchange membrane fuel cells (PEM-FCs). The anion exchange membrane fuel cell (AEM-FC) has long been proposed as a solution as non-Pt metals may be employed. Despite this, few examples of Pt-free AEM-FCs have been demonstrated with modest power output. The main obstacle preventing the realization of a high power density Pt-free AEM-FC is sluggish hydrogen oxidation (HOR) kinetics of the anode catalyst. Here we describe a Pt-free AEM-FC that employs a mixed carbon-CeO2 supported palladium (Pd) anode catalyst that exhibits enhanced kinetics for the HOR. AEM-FC tests run on dry H2 and pure air show peak power densities of more than 500 mW cm(-2) .

Insights into the effect of iron and cobalt doping on the structure of nanosized ZnO.

Here we report an in-depth structural characterization of transition metal-doped zinc oxide nanoparticles that have recently been used as anode materials for Li-ion batteries. Structural refinement of powder X-ray diffraction (XRD) data allowed the determination of small though reproducible changes in the unit cell dimensions of four ZnO samples (wurtzite structure) prepared with different dopants or different synthesis conditions. Moreover, large variations of the full width at half-maximum of the XRD reflections indicate that the crystallinity of the samples decreases in the order ZnO, Zn0.9Co0.1O, Zn0.9Fe0.1O/C, and Zn0.9Fe0.1O (the crystallite sizes as determined by Williamson-Hall plots are 42, 29, 15, and 13 nm, respectively). X-ray absorption spectroscopy data indicate that Co is divalent, whereas Fe is purely trivalent in Zn0.9Fe0.1O and 95% trivalent (Fe(3+)/(Fe(3+) + Fe(2+)) ratio = 0.95) in Zn0.9Fe0.1O/C. The aliovalent substitution of Fe(3+) for Zn(2+) implies the formation of local defects around Fe(3+) such as cationic vacancies or interstitial oxygen for charge balance. The EXAFS (extended X-ray absorption fine structure) data, besides providing local Fe-O and Co-O bond distances, are consistent with a large amount of charge-compensating defects. The Co-doped sample displays similar EXAFS features to those of pure ZnO, suggesting the absence of a large concentration of defects as found in the Fe-doped samples. These results are of substantial importance for understanding and elucidating the modified electrochemical lithiation mechanism by introducing transition metal dopants into the ZnO structure for the application as lithium-ion anode material.

Studying the surface reaction between NiO and Al2O3 via total reflection EXAFS (ReflEXAFS).

The reaction between NiO and (0001)- and (1102)-oriented Al2O3 single crystals has been investigated on model experimental systems by using the ReflEXAFS technique. Depth-sensitive information is obtained by collecting data above and below the critical angle for total reflection. A systematic protocol for data analysis, based on the recently developed CARD code, was implemented, and a detailed description of the reactive systems was obtained. In particular, for (1102)-oriented Al2O3, the reaction with NiO is almost complete after heating for 6 h at 1273 K, and an almost uniform layer of spinel is found below a mixed (NiO + spinel) layer at the very upmost part of the sample. In the case of the (0001)-oriented Al2O3, for the same temperature and heating time, the reaction shows a lower advancement degree and a residual fraction of at least 30% NiO is detected in the ReflEXAFS spectra.

Fixed energy X-ray absorption voltammetry.

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV represents a novel in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials and consists of recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the absorption coefficient. Although the information given by FEXRAV obviously does not supply the detailed information of X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS), it allows to quickly map the oxidation states of the element under consideration within the selected potential windows. This leads to the rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window) and is preliminary to more deep X-ray absorption spectroscopy (XAS) characterizations, like XANES or EXAFS. In addition, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis.

An EXAFS study of the binding of Cd and Pb ions to lipid films.

Extended X-ray Absorption Fine Structure (EXAFS) measurements performed on Langmuir-Blodgett (LB) films containing cadmium and lead ions reveal the different coordination structures of the two cations in lipid membranes. We describe the local atomic environment of cadmium and lead in LB films prepared with stearic and 1,2 distearoyl-Lα-phosphatidic acids. The measurements have been performed on films of two different thicknesses, one and seven molecular layers, and at two different values of relative humidity. The local atomic environment of Cd ions in stearate films is consistent with unidentate coordination in which a Cd ion binds two stearate molecules, while that of Pb ions is consistent with a bidentate coordination in which a Pb ion binds one stearate molecule. Furthermore, in lead stearate films, there is Pb-Pb coordination as already observed in Langmuir films. In films of Pb-phosphatidic acid, oxygen atoms of the organic phosphate and oxygen atoms of bound water form two distinct shells.

Gas phase deposition of well-defined bimetallic gold-silver clusters for photocatalytic applications.

Cluster beam deposition is employed for fabricating well-defined bimetallic plasmonic photocatalysts to enhance their activity while facilitating a more fundamental understanding of their properties. AuxAg1-x clusters with compositions (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1) spanning the metals' miscibility range were produced in the gas-phase and soft-landed on TiO2 P25-coated silicon wafers with an optimal coverage of 4 atomic monolayer equivalents. Electron microscopy images show that at this coverage most clusters remain well dispersed whereas EXAFS data are in agreement with the finding that the deposited clusters have an average size of ca. 5 nm and feature the same composition as the ablated alloy targets. A composition-dependant electron transfer from Au to Ag that is likely to impart chemical stability to the bimetallic clusters and protect Ag atoms against oxidation is additionally evidenced by XPS and XANES. Under simulated solar light, AuxAg1-x clusters show a remarkable composition-dependent volcano-type enhancement of their photocatalytic activity towards degradation of stearic acid, a model compound for organic fouling on surfaces. The Formal Quantum Efficiency (FQE) is peaking at the Au0.3Ag0.7 composition with a value that is twice as high as that of the pristine TiO2 P25 under solar simulator. Under UV the FQE of all compositions remains similar to that of pristine TiO2. A classical electromagnetic simulation study confirms that among all compositions Au0.3Ag0.7 features the largest near-field enhancement in the wavelength range of maximal solar light intensity, as well as sufficient individual photon energy resulting in a better photocatalytic self-cleaning activity. This allows ascribing the mechanism for photocatalysis mostly to the plasmonic effect of the bimetallic clusters through direct electron injection and near-field enhancement from the resonant cluster towards the conduction band of TiO2. These results not only demonstrate the added value of using well-defined bimetallic nanocatalysts to enhance their photocatalytic activity but also highlights the potential of the cluster beam deposition to design tailored noble metal modified photocatalytic surfaces with controlled compositions and sizes without involving potentially hazardous chemical agents.

Giant uniaxial negative thermal expansion in FeZr2 alloy over a wide temperature range.

Negative thermal expansion (NTE) alloys possess great practical merit as thermal offsets for positive thermal expansion due to its metallic properties. However, achieving a large NTE with a wide temperature range remains a great challenge. Herein, a metallic framework-like material FeZr2 is found to exhibit a giant uniaxial (1D) NTE with a wide temperature range (93-1078 K, [Formula: see text]). Such uniaxial NTE is the strongest in all metal-based NTE materials. The direct experimental evidence and DFT calculations reveal that the origin of giant NTE is the couple with phonons, flexible framework-like structure, and soft bonds. Interestingly, the present metallic FeZr2 excites giant 1D NTE mainly driven by high-frequency optical branches. It is unlike the NTE in traditional framework materials, which are generally dominated by low energy acoustic branches. In the present study, a giant uniaxial NTE alloy is reported, and the complex mechanism has been revealed. It is of great significance for understanding the nature of thermal expansion and guiding the regulation of thermal expansion.

Surface science approach to the solid-liquid interface: surface-dependent precipitation of Ni(OH)2 on α-Al2O3 surfaces.

Surface-dependent precipitation: The adsorption of Ni(II) complexes in aqueous solution on (0001) and (1102) α-Al(2)O(3) single-crystal surfaces has been studied (see the X-ray absorption spectra obtained for parallel and perpendicular polarization directions). The use of planar model systems emphasizes the crucial role of the Al(2)O(3) orientation for Ni dispersion with practical implications in catalyst preparation procedures.

µ-EXAFS, µ-XRF, and µ-PL characterization of a multi-quantum-well electroabsorption modulated laser realized via selective area growth.

In the past few years, strong efforts have been devoted to improving the frequency of optical-fiber communications. In particular, the use of a special kind of integrated optoelectronic device called an electroabsorption modulated laser (EML) allows communication at 10 Gb s(-1) or higher over long propagation spans (up to 80 km). Such devices are realized using the selective area growth (SAG) technique and are based on a multiple quantum well (MQW) distributed-feedback laser (DFB) monolithically integrated with a MQW electroabsorption modulator (EAM). Since the variation in the chemical composition between these two structures takes place on the micrometer scale, in order to study the spatial variation of the relevant parameters of the MQW EML structures, the X-ray microbeam available at the ESRF ID22 beamline is used. The effectiveness of the SAG technique in modulating the chemical composition of the quaternary alloy is proven by a micrometer-resolved X-ray fluorescence (µ-XRF) map. Here, reported micrometer-resolved extended X-ray absorption fine structure (µ-EXAFS) spectra represent the state of the art of µ-EXAFS achievable at third-generation synchrotron radiation sources. The results are in qualitative agreement with X-ray diffraction (XRD) and micrometer-resolved photoluminescence (µ-PL) data, but a technical improvement is still crucial in order to make µ-EXAFS really quantitative on such complex heterostructures.

Cation Disorder and Local Structural Distortions in AgxBi1-xS2 Nanoparticles.

By combining X-ray absorption fine structure and X-ray diffraction measurements with density functional and molecular dynamics simulations, we study the structure of a set of AgxBi1-xS2 nanoparticles, a materials system of considerable current interest for photovoltaics. An apparent contradiction between the evidence provided by X-ray absorption and diffraction measurements is solved by means of the simulations. We find that disorder in the cation sublattice induces strong local distortions, leading to the appearance of short Ag-S bonds, the overall lattice symmetry remaining close to hexagonal.

Unravelling the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ mixed ionic electronic conductors.

Mixed ionic electronic conducting ceramics Nd6-y WO12-δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or H2S. Due to this chemical stability, they are promising and cost-efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6-y WO12-δ, the ionic/electronic transport mechanism in Nd6-y WO12-δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12-δ and molybdenum-substituted Nd5.7W0.75Mo0.25O12-δ prepared by the citrate complexation route. High-resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm 3 m and doubled unit-cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La6-y WO12-δ. X-ray absorption spectroscopy as a function of partial pressure pO2 in the near-edge regions excludes oxidation state changes of Nd (Nd3+) and W (W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd6-y WO12-δ is therefore explained by the higher Mo reducibility and the creation of additional - disordered - oxygen vacancies.

Toward ultimate nonvolatile resistive memories: The mechanism behind ovonic threshold switching revealed.

Fifty years after its discovery, the ovonic threshold switching (OTS) phenomenon, a unique nonlinear conductivity behavior observed in some chalcogenide glasses, has been recently the source of a real technological breakthrough in the field of data storage memories. This breakthrough was achieved because of the successful 3D integration of so-called OTS selector devices with innovative phase-change memories, both based on chalcogenide materials. This paves the way for storage class memories as well as neuromorphic circuits. We elucidate the mechanism behind OTS switching by new state-of-the-art materials using electrical, optical, and x-ray absorption experiments, as well as ab initio molecular dynamics simulations. The model explaining the switching mechanism occurring in amorphous OTS materials under electric field involves the metastable formation of newly introduced metavalent bonds. This model opens the way for design of improved OTS materials and for future types of applications such as brain-inspired computing.

Black Phosphorus/Palladium Nanohybrid: Unraveling the Nature of P-Pd Interaction and Application in Selective Hydrogenation.

The burgeoning interest in two-dimensional (2D) black phosphorus (bP) contributes to the expansion of its applications in numerous fields. In the present study, 2D bP is used as a support for homogeneously dispersed palladium nanoparticles directly grown on it by a wet chemical process. Electron energy loss spectroscopy-scanning transmission electron microscopy analysis evidences a strong interaction between palladium and P atoms of the bP nanosheets. A quantitative evaluation of this interaction comes from the X-ray absorption spectroscopy measurements that show a very short Pd-P distance of 2.26 Å, proving for the first time the existence of an unprecedented Pd-P coordination bond of a covalent nature. Additionally, the average Pd-P coordination number of about 1.7 reveals that bP acts as a polydentate phosphine ligand toward the surface of the Pd atoms of the nanoparticles, thus preventing their agglomeration and inferring with structural stability. These unique properties result in a superior performance in the catalytic hydrogenation of chloronitroarenes to chloroanilines, where a higher chemoselectivity in comparison to other heterogeneous catalyst based on palladium has been observed.

Chemical variability of artificial stone powders in relation to their health effects.

The occurrence of highly severe silica-related diseases among the resin- and silica-based artificial stone workers was claimed, associated to an extremely short latency. High levels of exposure and intrinsic properties of AS are thought to modulate the development of silicosis and auto-immune diseases. This study compares parent materials and processed dusts, to shed light on changes of AS occurring in the manufacturing process, through an XRF, EPR and XAS investigation. We point out the extremely wide variability of the materials, the occurrence of chemical signatures impressed by the processing techniques, and the unprecedented generation of stable radicals associated to the lysis of the Si-O chemical bond inside the resin coated respirable crystalline silica. These results suggest that the AS processing in industrial stone workshops can create respirable dusts with peculiar physical and chemical properties, to be correlated to the observed clinical evidences.