Very small superparamagnetic iron
oxide nanoparticles were characterized
by innovative synchrotron X-ray total scattering methods and Debye
function analysis. Using the information from both Bragg and diffuse
scattering, size-dependent core–shell magnetite–maghemite
compositions and full size (number- and mass-based) distributions
were derived within a coherent approach. The magnetite core radii
in 10 nm sized NPs well match the magnetic domain sizes and show a
clear correlation to the saturation magnetization values, while the
oxidized shells seem to be magnetically silent. Very broad superstructure
peaks likely produced by the polycrystalline nature of the surface
layers were experimentally detected in room temperature oxidized samples.
Effective magnetic anisotropy constants, derived by taking the knowledge
of the full size-distributions into account, show an inverse dependence
on the NPs size, witnessing a major surface contribution. Finally,
an additional amorphous component was uncovered within the diffuse
scattering of the “ordered” magnetite–maghemite
NPs. Under the hypothesis that this material may form an external
dead layer, an additional thickness varying between 0.3 and 1.0 nm
should be added to the overall core–shell NPs size.
Nanocrystalline TiO(2) samples, prepared for smart textiles applications by the sol-gel technique in acidic or basic media, have been characterized by synchrotron X-ray powder diffraction and total scattering methods based on a fast implementation of the Debye function and original algorithms for sampling interatomic distances. Compared to the popular and widely used Rietveld-based approaches, our method is able to simultaneously model both Bragg and diffuse contributions and to quantitatively extract either sizes and size distribution information from the experimental data. The photocatalytic activity of the investigated samples is here systematically correlated to the average sizes and size distributions of anisotropically shaped coherent domains, modeled according to bivariate populations of nanocrystals grown along two normal directions.
The size-driven expansion and oxidation-driven contraction phenomena of nonstoichiometric magnetite-maghemite core-shell nanoparticles have been investigated by the total scattering Debye function approach. Results from a large set of samples are discussed in terms of significant effects on the sample average lattice parameter and on the possibility of deriving the sample average oxidation level from accurate, diffraction-based, cell values. Controlling subtle experimental effects affecting the measurement of diffraction angles and correcting for extra-sample scattering contributions to the pattern intensity are crucial issues for accurately estimating lattice parameters and cation vacancies. The average nanoparticle stoichiometry appears to be controlled mainly by iron depletion of octahedral sites. A simple law with a single adjustable parameter, well correlating lattice parameter, stoichiometry and size effects of all the nanoparticles present in the whole set of samples used in this study, is proposed. research papers
Come together! A combined real‐ and reciprocal‐space total scattering approach was realized by applying Debye and radial distribution functions to nanocrystalline and amorphous fractions within the same experimental pattern. The method allows for the quantitative description of microstructural features induced by the amorphous–crystalline interplay in silica–titania nanocomposites (see picture).
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