Early stages of solid state reactions: insights from micro-XRD and XAS

The mechanism of a solid state reaction in its early stages can be explored by investigating the time evolution of a model reactive system made of a thin layer of one reagent deposited onto a single crystal slab of the other reagent. Insights can be retrieved by comparing results at both local and long length scales obtained with films of different thicknesses and deposited onto different crystal orientations. In particular, reaction between ZnO and Al2O3has been chosen, as the spinel-forming reactions have been and still remain a model experimental system for investigating solid state reactions and because in the ZnO/Al2O3phase diagram, spinel is the only stable compound. The reaction initial steps have been investigated by using synchrotron X-ray diffraction, atomic force microscopy and X-ray absorption spectroscopy at the Zn-K edge starting from zincite films deposited onto (110)-, (012)-, (001)-oriented corundum single crystals [1,2]. The reaction eventually yields ZnAl2O4spinel but via a complex mechanism involving side and intermediate non-equilibrium compounds that do not appear in the equilibrium phase diagram of the pseudo-binary system. Spinel, when occurs, is polycrystalline at the end but initially forms with a few preferred orientations. Intermediate phases form before and in parallel with the growth of the spinel. Their number, composition, structure and kinetic role strongly depend on substrate orientation and film thickness. A more detailed understanding of the reactivity can be inferred by comparing EXAFS results to those of grazing incidence diffraction experiments of the films deposited on the (001) face of Al2O3and heat-treated at 10000C for different lengths of time. Information on the structure of the intermediate phases is given and results are discussed by comparing different films thickness to clarify the role of interfacial free energy and crystallographic orientation

Results on MOVPE SiGeSn deposition for the monolithic integration of III-V and IV elements in multi-junction solar cells

Abstract In order to produce a step forward towards the monolithic integration of III-V and IV compounds in multijunction solar cells, a first assessment of SiGeSn deposition in a metal organic vapour phase epitaxy (MOVPE) chamber also used for III-V growth has been carried out. The study brings insights on several aspects of the MOVPE SiGeSn growth in order to get a better control of SiGeSn composition and to obtain epitaxial layers with improved morphology. In particular, it is shown that the gas source Si2H6 is more influenced by the growth temperature compared to GeH4 and SnCl4, moreover, its competition with SnCl4 makes it difficult to incorporate Si in SiGeSn, as SnCl4 partial pressure is increased. SiGeSn morphology is shown to be strongly dependent on temperature, As carry-over and growth rate. A new growth model is introduced in order to explain the importance of the adatom bond lengths in inhibiting tin segregation when SiGeSn is grown at relatively high growth temperatures (>480 °C). In order to investigate the photovoltaic behaviour of SiGeSn, a single-junction GaAs/InGaP/SiGeSn/Ge functional device has been manufactured and characterized by external quantum efficiency (EQE) and current-voltage measurements. The experimental and the simulated EQE show the higher absorption coefficient of SiGeSn with respect to Ge, which allows using SiGeSn layers with a thickness three times lower than Ge to produce the same photovoltaic current

The Mitogenome Relationships and Phylogeography of Barn Swallows (Hirundo rustica)

The barn swallow (Hirundo rustica) poses a number of fascinating scientific questions, including the taxonomic status of postulated subspecies. Here, we obtained and assessed the sequence variation of 411 complete mitogenomes, mainly from the European H. r. rustica, but other subspecies as well. In almost every case, we observed subspecies-specific haplogroups, which we employed together with estimated radiation times to postulate a model for the geographical and temporal worldwide spread of the species. The female barn swallow carrying the Hirundo rustica ancestral mitogenome left Africa (or its vicinity) around 280 thousand years ago (kya), and her descendants expanded first into Eurasia and then, at least 51 kya, into the Americas, from where a relatively recent (<20 kya) back migration to Asia took place. The exception to the haplogroup subspecies specificity is represented by the sedentary Levantine H. r. transitiva that extensively shares haplogroup A with the migratory European H. r. rustica and, to a lesser extent, haplogroup B with the Egyptian H. r. savignii. Our data indicate that rustica and transitiva most likely derive from a sedentary Levantine population source that split at the end of the Younger Dryas (YD) (11.7 kya). Since then, however, transitiva received genetic inputs from and admixed with both the closely related rustica and the adjacent savignii. Demographic analyses confirm this species' strong link with climate fluctuations and human activities making it an excellent indicator for monitoring and assessing the impact of current global changes on wildlife

Mechanism of electrocatalytic and photo-electrocatalytic reactions by in operando X-Ray absorption spectroscopy

The purpose of my PhD research work is the investigation of the mechanism and the kinetics of electrocatalytic and photo-electrocatalytic reactions by means of in operando X-ray Absorption Spectroscopy. The attention has been focused on five electrochemical systems which can be divided into two groups on the basis of their catalytic properties. The first group essentially deals with the catalysis of the water splitting reaction: belonging to this group I firstly considered electrodes constituted of IrOx, which represents one of the major electrocatalysts in this field. Coupling IrOx electrocatalyst to a semiconductor like hematite it became possible to obtain and investigate bilayer electrodic systems for light driven water splitting. Afterward, I considered bilayer architectures composed of Ni(OH)2 and hematite, which represent cheaper and more sustainable alternatives to iridium based systems. Finally, some research work was devoted to Cu2O material, also very promising for photovoltaic applications. The experiments carried out for this materials were aimed at studying the structure and stability of one of the main precursors: copper(II) lactate in alkaline solution. The second group of electrodes plays a key role in the environmental field since comprises materials constituted of silver nanoparticles, which show great potentialities in the electrochemical de-halogenation of organic pollutants. Since the study of electrocatalytic processes involves the presence of systems in their real working conditions, adequate experimental setups and electrochemical devices were required in order to control the physic-chemical conditions of the sample. For each electrodic system the research work consisted of four steps: (i) experimental planning and construction of adequate spectro-electrochemical devices according to the subsequent experimental needs (these can be either manually-built or 3D printed); (ii) preparation of the electrodic materials (in collaboration with the University of Milan); (iii) In Situ and In Operando investigation by means of XAS Spectroscopy techniques at the ESRF (European Synchrotron Radiation Facility, Grenoble, France); and (iv) data analysis and fitting procedure

In Situ Dispersive EXAFS in Electrocatalysis: The Investigation of the Local Structure of IrOx in Chronoamperometric Conditions as a Case Study

An in situ study with dispersive EXAFS (Extended X-Ray Absorption Spectroscopy) at the Ir-L-III edge is performed to characterize Electrodeposited Iridium Oxide Films (EIROF) under chronoamperometric conditions. The technique monitors the local chemical environment and electronic structure of iridium during the oxidation of Ir(III) to Ir(IV) with a time resolution of milliseconds. The study is performed in both acidic and basic media. The Fourier transforms of the time-resolved EXAFS signals clearly show that the short-range structure of Ir is similar to that of rutile-type IrO2 and is maintained during the reaction, thus accounting for the flexibility of the structure of the electrode material in accommodating different oxidation states. From a more general point of view, the work demonstrates the capabilities of in situ experiments based on state-of-the-art dispersive EXAFS in clarifying the mechanistic aspects of electrochemical processes

Fixed Energy X-ray Absorption Voltammetry and Extended X-ray Absorption fine Structure of Ag nanoparticle electrodes

FEXRAV (Fixed Energy X-ray Absorption Voltammetry) has been recently introduced as a powerful tool in electrochemistry. It consists in measuring the X-ray absorption coefficient at a fixed X-ray energy while the electrochemical potential of the material under investigation is varied at will in a conventional three-electrode electrochemical cell. In the case of Ag electrodes, the usefulness of FEXRAV might appear limited: first of all, Ag cannot assume an extended variety of oxidation state values as other transition metals. In addition, only the Ag-K edge is accessible in the hard X-ray region that is needed for operando experiments. In turn, the final states are of p character, and therefore any charge state variation bound to the d orbitals (as happens in the electrochemistry of Ag) is difficult to observe in the}CANES (X-ray Absorption Near Edge Spectroscopy) region. However, Ag has plentiful applications in electrochemistry, in particular for what concerns the reduction of halogenated organic compounds. In this work we evidence the capabilities of FEXRAV in the case of Ag electrodes under working conditions in the presence of organic and inorganic halides. More specifically and for the first time, the adsorption of aqueous trichloromethane at a potential preceding the electron transfer reaction is observed. This paper therefore represents a significant advance for FEXRAV, and a fundamental contribution to the elucidation of the exceptional electrocatalytic activity of silver toward hydrodehalogenation reactions. (C) 2016 Elsevier B.V. All rights reserved

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

The heart rate response to exercise and circulating catecholamines in heart transplant recipients

The plasma concentration of noradrenaline ([NA]) is higher than that of adrenaline ([A]) both in normal subjects and in heart transplant recipients (HTR). Since in both groups the myocardial density of beta1-adrenergenic receptors is much greater than that of beta2-adrenergenic receptors, the chronotropic response of a denervated heart to changes in plasma [NA] and [A] in the absence of reinnervation should be similar to that of agonist stimulation of beta1-receptors. To test this hypothesis, 17 HTR and 9 healthy subjects (CTL) performed incremental exercise on a cycle ergometer to voluntary exhaustion. Heart rate (HR) was recorded by electrocardiography. [NA] and [A] were measured by high-pressure liquid chromatography at rest and at increasing workloads (w). In both groups, HR and [NA+A] increased with w, and HR with [NA+A]. Normalized HR values, plotted against the logarithm of [NA+A], fitted significantly logistic curves. The affinity constants were different, i.e. 2599+/-350 and 487+/-37 ng.l(-1), for HTR and CTL, respectively. The chronotropic effect of changes in [NA+A] in HTR was similar to that of combined beta1- and beta2-adrenergic activation evoked by applying isoprenaline to isolated heart myocytes (Brodde OE, Pharmacol Ther 60:405-430, 1993). These findings suggest that over time sympathetic reinnervation and the modulation of beta-receptors may take place in HTR, ruling out the hypothesis of persistent heart denervation

An efficient CuxOCu_xO photocathode for hydrogen production at neutral pH : new insights from combined spectroscopy and electrochemistry

Light-driven water splitting is one of the most promising approaches for using solar energy in light of more sustainable development. In this paper, a highly efficient p-type copper(II) oxide photocathode is studied. The material, prepared by thermal treatment of CuI nanoparticles, is initially partially reduced upon working conditions and soon reaches a stable form. Upon visible-light illumination, the material yields a photocurrent of 1.3 mA cm-2 at a potential of 0.2 V vs a reversible hydrogen electrode at mild pH under illumination by AM 1.5 G and retains 30% of its photoactivity after 6 h. This represents an unprecedented result for a nonprotected Cu oxide photocathode at neutral pH. The photocurrent efficiency as a function of the applied potential was determined using scanning electrochemical microscopy. The material was characterized in terms of photoelectrochemical features; X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, fixed-energy X-ray absorption voltammetry, and extended X-ray absorption fine structure analyses were carried out on pristine and used samples, which were used to explain the photoelectrochemical behavior. The optical features of the oxide are evidenced by direct reflectance spectroscopy and fluorescence spectroscopy, and Mott-Schottky analysis at different pH values explains the exceptional activity at neutral pH