2 research outputs found
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, 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
A former spectroscopic study of CuÂ(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/CuÂ(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 CuÂ(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
CuÂ(III) ions in the experimental conditions, while GK13 binds only
with CuÂ(II). For LK13/Cu complex, EXAFS spectrum suggested and UV–vis
analysis confirmed the presence of a mixture of CuÂ(II) and CuÂ(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/CuÂ(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, CuÂ(III) scaffold