2 research outputs found

    Insights into the Effect of Iron and Cobalt Doping on the Structure of Nanosized ZnO

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    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, Zn<sub>0.9</sub>Co<sub>0.1</sub>O, Zn<sub>0.9</sub>Fe<sub>0.1</sub>O/C, and Zn<sub>0.9</sub>Fe<sub>0.1</sub>O (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 Zn<sub>0.9</sub>Fe<sub>0.1</sub>O and 95% trivalent (Fe<sup>3+</sup>/(Fe<sup>3+</sup> + Fe<sup>2+</sup>) ratio = 0.95) in Zn<sub>0.9</sub>Fe<sub>0.1</sub>O/C. The aliovalent substitution of Fe<sup>3+</sup> for Zn<sup>2+</sup> implies the formation of local defects around Fe<sup>3+</sup> 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

    Dioxygen Oxidation Cu(II) → Cu(III) in the Copper Complex of <i>cyclo</i>(Lys‑dHis-βAla-His): A Case Study by EXAFS and XANES Approach

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    A former spectroscopic study of Cu­(II) coordination by the 13-membered ring cyclic tetrapeptide <i>c</i>(Lys-dHis-βAla-His) (DK13), revealed the presence, at alkaline pH, of a stable peptide/Cu­(III) complex formed in solution by atmospheric dioxygen oxidation. To understand the nature of this coordination compound and to investigate the role of the His residues in the Cu­(III) species formation, Cu K-edge XANES, and EXAFS spectra have been collected for DK13 and two other 13-membered cyclo-peptides: the diastereoisomer <i>c</i>(Lys-His-βAla-His) (LK13), and <i>c</i>(Gly-βAla-Gly-Lys) (GK13), devoid of His residues. Comparison of pre-edge peak features with those of Cu model compounds, allowed us to get information on copper oxidation state in two of the three peptides, DK13 and GK13: DK13 contains only Cu­(III) ions in the experimental conditions, while GK13 binds only with Cu­(II). For LK13/Cu complex, EXAFS spectrum suggested and UV–vis analysis confirmed the presence of a mixture of Cu­(II) and Cu­(III) coordinated species. Theoretical XANES spectra have been calculated by means of the MXAN code. The good agreement between theoretical and experimental XANES data collected for DK13, suggests that the refined structure, at least in the first coordination shell around Cu, is a good approximation of the DK13/Cu­(III) coordination species present at strongly alkaline pH. All the data are consistent with a slightly distorted pyramidal CuN<sub>4</sub> unit, coming from the peptide bonds. Surprisingly, the His side-chains seemed not involved in the final, stable, Cu­(III) scaffold
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