82 research outputs found
Operando X-ray Absorption Spectroscopy for the study of charge transfer in electrodes for photo-electrochemical water oxidation.
Lecture for students of the doctorate in chemical sciences, Perugia University (cycles XXXV-XXXVI- XXXVII) hold on march 17, 2022
Operando X-ray Absorption Spectroscopy for the study of charge transfer in electrodes for photo-electrochemical water oxidation.
Lecture for students of the doctorate in chemical sciences, Perugia University (cycles XXXV-XXXVI- XXXVII) hold on march 17, 2022.PGP Key: 6A7C CB93 F409 EEE1 86B7 E3F9 98D1 318C 0436 39E
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.Comment: 8 pages, 3 figure
Response to W. K. Warburton's Comments on Treatment of EXAFS data taken in the fluorescence mode in non-linear conditions
Role of CaO addition in the local order around Erbium in SiO2 –GeO2 –P2O5 fiber preforms
International audienceThe development of materials for optical signal processing represents a major issue in present technology. In this contribution we present a study on Er-doped fiber preforms where particular attention is devoted on how the addition of CaO in the glass modifies the local environment of the rare earth. The results from photoluminescence and Extended X-ray Absorption Fine Structure (EXAFS) are compared and a clear link between the width of the emission line at 1.5 µm and the amorphous/crystalline local structure around the Er3+ ion is evidenced
Iron Speciation of Natural and Anthropogenic Dust by Spectroscopic and Chemical Methods
In this work, we have characterized the iron local structure in samples of two different types of atmospheric dust using X-ray absorption spectroscopy and selective leaching experiments. Specifically, we have investigated samples of long-range transported Saharan dust and freshly emitted steel plant fumes with the aim of individuating possible fingerprints of iron in the two cases. Findings include (1) prevalence of octahedral coordinated Fe 3 + for all samples; (2) presence of 6-fold coordinated Fe 3 + , aluminosilicates and iron oxy(hydr)oxides in Saharan dust and (3) of Fe-bearing spinel-like structures in the industrial fumes; (4) general predominance of the residual insoluble fraction with a notable difference: 69% for Saharan dust and 93% for steel production emissions, associated with aluminosilicates and non-reducible iron oxy(hydr)oxides, and Fe spinels, respectively. The remarkable differences between the X-ray absorption spectroscopy (XAS) spectra and leaching test results for the two sample types suggest the possibility to exploit the present approach in more complex cases. To this aim, two additional case studies of mixed aerosol samples are presented and discussed
Response to W. K. Warburton's Comments on Treatment of EXAFS data taken in the fluorescence mode in non-linear conditions by G. Ciatto et al. (2004). J. Synchrotron Rad. 11, 278–283
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Interface-driven phase separation in multifunctional materials: the case of GeMn ferromagnetic semiconductor
We use extensive first principle simulations to show the major role played by
interfaces in the mechanism of phase separation observed in semiconductor
multifunctional materials. We make an analogy with the precipitation sequence
observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones
in this new context. A new class of materials, the phases, is proposed
to understand the formation of the coherent precipitates observed in the GeMn
system. The interplay between formation and interface energies is analyzed for
these phases and for the structures usually considered in the literature. The
existence of the alpha phases is assessed with both theoretical and
experimental arguments
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
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