Structure and Surface Relaxation of CeO2 Nanoparticles Unveiled by Combining Real and Reciprocal Space Total Scattering Analysis.

We present a combined real and reciprocal space structural and microstructural characterization of CeO2 nanoparticles (NPs) exhibiting different crystallite sizes; ~3 nm CeO2 NPs were produced by an inverse micellae wet synthetic path and then annealed at different temperatures. X-ray total scattering data were analyzed by combining real-space-based Pair Distribution Function analysis and the reciprocal-space-based Debye Scattering Equation method with atomistic models. Subtle atomic-scale relaxations occur at the nanocrystal surface. The structural analysis was corroborated by ab initio DFT and force field calculations; micro-Raman and electron spin resonance added important insights to the NPs' defective structure. The combination of the above techniques suggests a core-shell like structure of ultrasmall NPs. These exhibit an expanded outer shell having a defective fluorite structure, while the inner shell is similar to the bulk structure. The presence of partially reduced O2-δ species testifies to the high surface activity of the NPs. On increasing the annealing temperature, the particle dimensions increase, limiting disorder as a consequence of the progressive surface-to-volume ratio reduction.

Local Structure and Magnetism of Fe2O3 Maghemite Nanocrystals: The Role of Crystal Dimension.

Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.

Percolating hierarchical defect structures drive phase transformation in Ce1-x Gd x O2-x/2: a total scattering study.

A new hierarchical approach is presented for elucidating the structural disorder in Ce1-x Gd x O2-x/2 solid solutions on different scale lengths. The primary goal of this investigation is to shed light on the relations between the short-range and the average structure of these materials via an analysis of disorder on the mesocopic scale. Real-space (pair distribution function) and reciprocal-space (Rietveld refinement and microstructure probing) analysis of X-ray powder diffraction data and electron spin resonance (ESR) investigations were carried out following this approach. On the local scale, Gd- and Ce-rich droplets (i.e. small regions a few ångströms wide) form, exhibiting either a distorted fluorite (CeO2) or a C-type (Gd2O3) structure in the whole compositional range. These droplets can then form C-type nanodomains which, for Gd concentrations x Gd ≤ 0.25, are embedded in the fluorite matrix. At the site percolation threshold p C for a cubic lattice (x Gd = p C ≃ 0.311), C-type nanodomains percolate inside each crystallite and a structural phase transformation is observed. When this occurs, the peak-to-peak ESR line width ΔH pp shows a step-like behaviour, which can be associated with the increase in Gd-Gd dipolar interactions. A general crystallographic rationale is presented to explain the fluorite-to-C-type phase transformation. The approach shown here could be adopted more generally in the analysis of disorder in other highly doped materials.

Klebsiella sp. strain C2A isolated from olive oil mill waste is able to tolerate and degrade tannic acid in very high concentrations.

Four bacterial strains capable of growing in the presence of tannic acid as sole carbon and energy source were isolated from olive mill waste mixtures. 16S rRNA gene sequencing assigned them to the genus Klebsiella. The most efficient strain, Klebsiella sp. strain C2A, was able to degrade 3.5 g L(-1) tannic acid within 35 h with synthesizing gallic acid as main product. The capability of Klebsiella sp. strain C2A to produce tannase was evidenced at high concentrations of tannic acid up to 50 g L(-1) . The bacteria adapted to the toxicity of tannic acids by an increase in the membrane lipid fatty acids degree of saturation, especially in the presence of concentrations higher than 20 g L(-1) . The highly tolerant and adaptable bacterial strain characterized in this study could be used in bioremediation processes of wastes rich in polyphenols such as those derived from olive mills, winery or tanneries.

Effect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles.

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO(2) nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO(2) nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.

Melting of orbital ordering in KMgxCu1-xF3 solid solution.

The long-range and short-range structures of KMgxCu1-xF3 (0 < x < 1) have been investigated by means of XRPD and EPR. Two different solid solutions are present, based on the structure of KMgF3 (for x > 0.42) and of KCuF3 (for x < 0.26), respectively, and they are separated by a biphasic zone. Positional disorder is induced by doping due to the different Cu and Mg environments. In fact, the EPR measurements have shown that the Cu environment is isotropic for x > 0.8. It shows axial symmetry for 0.45 < x < 0.70 and orthorhombic symmetry for x = 0.43. For x > 0.42, the crystallographic structure is cubic, and in absence of local disorder, a fully isotropic octahedral undistorted environment is expected for Cu. In the tetragonal structure, collective magnetic interactions arise, and a progressive EPR signal symmetrization is observed due to anisotropic exchange and to Dzialoshinsky-Moriya antisymmetric exchange processes. The mixing of triplet and singlet states induced by the above exchange mechanisms leads to the conclusion that the orbital order is melt in the x = 0.1 sample, for which the cooperative Jahn-Teller distortion is still active and the 3D magnetic order is still antiferromagnetic, as in KCuF3.