Elements of a Model Function for the Atomic Absorption Background in EXAFS Spectra

From the study of multielectron photoexcitations in atoms, a specific ansatz for the EXAFS atomic background is derived, to improve or replace the conventional spline approximation. The elements of the ansatz describe multielectron resonances, edges and shake-off features at proper theoretical energies. The approach is tested on the atomic background of Rb ion and on EXAFS-free absorption spectra of rubidium vapor and krypton

Determination of Valence States of Chromium in Calcium Chromates by Using X-ray Absorption Near-Edge Structure (XANES) Spectroscopy

Calcium chromates with the empirical formulas Ca10Cr6−O25, Ca3Cr2O8, and Ca5Cr3O12, which form at temperatures >900°C at CaO:Cr2O3 molar ratios of geqslant R: gt-or-equal, slanted3 in an oxidation atmosphere have been synthesized in the pure state. X-ray absorption near-edge structure (XANES) spectroscopy has been used to determine the average valence state of chromium in the samples. The presence of unusual chromium valence states, 4+ and 5+, which was proposed via X-ray diffractometry studies, is strongly supported

Structure and Population of Complex Ionic Species in FeCl2 Aqueous Solution by X-ray Absorption Spectroscopy

Technologies for mass production require cheap and abundant materials such as ferrouschloride (FeCl$_2$). The literature survey shows the lack of experimental studies to validate theoreticalconclusions related to the population of ionic Fe-species in the aqueous FeCl$_2$ solution. Here, wepresent an in situ X-ray absorption study of the structure of the ionic species in the FeCl$_2$ aqueoussolution at different concentrations (1–4 molL$^{-1}$) and temperatures (25–80 C). We found that atlow temperature and low FeCl$_2$ concentration, the octahedral first coordination sphere around Feis occupied by one Cl ion at a distance of 2.33 (0.02) Å and five water molecules at a distanceof 2.095 (0.005) Å. The structure of the ionic complex gradually changes with an increase intemperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yielda neutral Fe[Cl$_2$(H$_2$O)$_4$]$^0$. The observed substitutional mechanism is facilitated by the presence of theintramolecular hydrogen bonds as well as entropic reasons. The transition from the single chargedFe[Cl(H$_2$O)$_5$]$^+$ to the neutral $^0

Structural analysis of sunlight efficient Cu and Zr modified TiO2\mathrm{TiO_{2}} photocatalyst

Solving of the renewable energy as well as environmental sustainability issues is on the top ofconsideration nowadays. Using abundant, inexpensive, nontoxic and efficient material,photocatalysis can be a great alternative and promising approach for many important processes [1].The most known (due to its availability and appropriate properties) photocatalyst is TiO2. However,it is only activated by irradiation at wavelengths shorter than 400 nm. To effectively utilize lightwith longer wavelength (visible spectrum) metal and non-metal doping were explored [2].Nevertheless, quantum yield of electronic process under visible light in doped materials is muchlower than under UV, due to crystal defects and generation of recombination centers. On the otherhand the surface modification process is very attractive because the visible-light activity can beinduced without changes in crystal structure. Most probably, visible light sensitivity can beachieved by the interfacial charge transfer process of excited TiO2 electrons to the modifier [3].Evonik Degussa P25 is one of the most efficient photocatalyst, showing photocatalytic activity evenunder the visible light [4]. The surface modification of TiO2 with two different elementssimultaneously finds its place among different approaches. The improvement in the activity of TiO2modified with appropriate combination of two or more elements is assigned to the existence ofsynergistic effect. The promising combination for sunlight driven TiO2 photocatalysis is acombination of Cu and Zr for TiO2 surface modification [5].The objective of our research is to improve photocatalytic properties of P25 by facile surfacemodification method with Cu and Zr, to find the correlation between catalytic and structuralproperties of modified P25 materials, and to identify the synergistic role of the two elements. Wereport on the results of Cu and Zr K-edge EXAFS (Extended X-ray Absorption Fine Structure)analysis of Cu and Zr chemical state and local environment in the surface modified P25photocatalyst

Micro-XANES analysis of metal accumulation in plants on sub-cellular level

There is a growing need to develop powerful analytical tools for monitoring concentrations and chemical state of trace element in the biosphere and its abiotic environment, due to pollution and degradation of ecosystems worldwide. Increased metal concentrations present in the environment pose a threat to all living organisms from microorganisms and plants to animals and humans, because they interfere with vital biological processes. The goal is to efficiently assess metal bioavailability and toxicity, and gain more knowledge on the mechanisms of metal uptake, accumulation and detoxification in living organisms [1]. In this work we demonstrate that a combination of micro-XRF imaging and micro-XANES and EXAFS analysis represents a powerful and indispensable tool for characterization of metal pollutants on subcellular level. The methodological approaches for efficient micro-XAS experiments are presented, the limitations and sources of potential systematic errors in XANES and EXAFS analysis (especially at low energies) due to self-absorption effects and strong energy dependent penetration depth of X-ray beam in the sample are discussed. Some typical examples of such combined micro-spectroscopy analysis are selected from the following research fields: Cd/Zn hyper-accumulating plants, that can be used for phytoremediation of Cd/Zn polluted and degraded ecosystems, including investigation of the role of externally supplied sulphur compounds in the nutrient solution of the Cd/Zn hyperaccumulator Thlaspi praecox, that may alter leaf Cd distribution and Cd ligand environment [2]; biofortification, which aims to increase essential elements (Fe) concentrations in the edible plant parts [3]; microbial regulation of metal (Fe, Pd) uptake and formation of Fe-oxide and Pd nanoparticles, encapsulated in exopolysaccharide to avoid iron toxicity under anaerobic conditions, discovered on a strain of Klebsiella oxytoca, isolated from acid pyrite-mine drainage[4]

XAS studies of Sn modified TiO2\mathrm{TiO_{2}} coatings

Coatings based on $\mathrm{TiO_{2}}$ are extensively investigated material for the preparation of surfaces which are referred as self-cleaning. Under solar illumination these coatings catalyze pollutants degradation and enhance their removal from the surface due to photoinduced superhydrophilicity [1]. There are two main drawbacks of using pure $\mathrm{TiO_{2}}$ (anatase) as a photocatalyst; i) its band gap lies in the UVA region so it can exploit only a part of the sunlight spectrum, and ii) it has a high degree of recombination between photo generated electrons and holes on the surface. There are various strategies to improve the photocatalytic efficiency of $\mathrm{TiO_{2}}$, one of them is the modification with transition metals. Based on our previous experiences with sol-gel synthesis of low-temperature $\mathrm{TiO_{2}}$ thin films [2], we prepared a series of Sn modified $\mathrm{TiO_{2}}$ photocatalysts. As a starting material only organic (Ti and Sn alkoxide) precursors were used. The loadings of Sn cations were varied in the range of 0.05 to 20 mol.%. The coatings, deposited on glass substrates by dip-coating technique, were dried at 150 oC. In addition, another set of photocatalyst coatings was prepared by further calcination at 500 °C in air. The comparison of photocatalytic activities of Sn modified $\mathrm{TiO_{2}}$ to unmodified $\mathrm{TiO_{2}}$ showed that Sn loadings in the range of 1-10 mol.% improved photocatalytic activity up to 8 times. At lower loadings of Sn, the photocatalytic activity was improved only by 30 %. After the coatings are calcined, their photocatalytic activity was significantly reduced.The objective of the research was to examine the mechanism responsible for photocatalytic properties of Sn modified $\mathrm{TiO_{2}}$ and to to clarify the role of Sn cations in the $\mathrm{TiO_{2}}$ photocatalytic process. For this purpose, Sn and Ti K-edge XANES and EXAFS analysis was used to precisely determine the local structure and the site of incorporation of Sn cations on titania nanoparticles in the coatings. We examined the hypothesis that the solid-solid interface was a crucial structural feature that facilitates charge separation and enhances photocatalytic efficiency of titania

A structural study of amorphous alkoxide-derived lead titanium complexes

We studied amorphous lead titanium alkoxide-derived heterometallic complexes, prepared from lead acetate and titanium n-propoxide or n-butoxide in parent alcohol medium. According to gas-liquid chromatography (GLC) and thermogravimetric analysis with mass spectrometry of evolved species (TGA/EGA) analyses, the type of alkoxide group influences oxo or acetate bridging, as well as the amounts of hydroxyl and organic groups bound to the metal network. From XANES and EXAFS analysis, local environments of lead and titanium atoms were determined within the analyzed range of 3.4 Å. Local environments depend weakly on the type of alkoxide used. Titanium atoms are pentacoordinated. A Pb–Ti correlation is established with lead atoms bound to titanium atoms by oxygen linkages