Effect of composition and coating on the interparticle interactions and magnetic hardness of MFe2O4 (M = Fe, Co, Zn) nanoparticles.

Single domain superparamagnetic ferrite nanoparticles with the composition MFe2O4 (M = Fe, Co, Zn) have been prepared by thermal decomposition of metal acetylacetonates in diphenyl ether or dibenzyl ether, using oleic acid in the presence of oleylamine as a stabilizing agent. The Fe, Co and Zn ferrite nanoparticles are monodisperse with diameters of 4.9, 4.4 and 4.7 nm, respectively. The TG and IR results indicate that four or six carboxylate groups per nm2 are bonded at the surface of the particles acting as chelating and/or bridging bidentate ligands depending on the composition. The oleate groups minimize the interparticle interactions in Fe and Zn ferrite samples, while in the Co ferrite sample dipolar interactions produce broad maxima in the ZFC and energy barriers distribution curves. The inversion degree has been estimated from the Raman spectra and the obtained x values have been used to calculate the saturation magnetization and compare them with the experimental MS values. Compared to bulk materials, the magnetization value is higher for the Zn ferrite sample due to its mixed spinel cation distribution. For the Co ferrite sample, and probably for the Fe one, the low value of saturation magnetization seems to be due to the surface disordered layer of canted spins. Compared to non-coated nanoparticles with the same composition and similar size, the oleate groups, covalently bonded to the superficial cations, increase the anisotropy field and decrease the magnetization.

Chlorination and solvothermal treatment of Zr(C5H5)2Cl2: a synthetic combination to produce nanometric tetragonal ZrO2.

A novel synthetic strategy based on the combination of the chlorination of an organometallic precursor followed by solvothermal treatment is found to be successful in the synthesis of tetragonal nano-ZrO(2) or nano-ZrO(2), embedded in an amorphous carbon matrix, depending on the solvent employed in the solvothermal step. The chemical and structural features (chemical composition, size and surface defects) of the intermediate and final materials have been determined experimentally mainly by high resolution transmission electron microscopy, electron energy loss spectroscopy, and Z-contrast images. These local techniques reveal that the nanoparticles consist of tetragonal ZrO(2) with an average size of 1.7 ± 0.4 and 6.2 ± 0.9 nm for the embedded in carbon and the free nano-ZrO(2), respectively.

Nickelocene as precursor of microporous organometallic-derived carbon and nickel oxide-carbon nanocomposite.

Microporous flower-like and spherical carbon particles, made of graphene-like layers, have been obtained via chlorination of nickelocene (Ni(C5H5)2). Their mechanism of formation, in terms of morphology and micro-nanostructure, has been followed from 200 to 900°C. Conventional transmission electron microscopy and high-resolution-TEM observations allow determining that their structure is made of highly disordered graphene-like layers. The Raman spectrum of the high temperature sample exhibits the characteristics D and G bands. The peak positions, the ratio of their intensities (ID/IG) and full width at half maximum suggest a high degree of disorder in the nanostructures. The calculated in-plane correlation length of these graphene-like layers is 1.15nm. In all the carbon particles, electron energy-loss spectroscopy shows sp2 carbon bonding content higher than 95% and mass density in the range of 1.0-1.6g/cm3. Textural studies show Type I adsorption isotherms with surface area of 922m2/g for the sample produced at 900°C. In addition, the basic hydrothermal treatment of the sample chlorinated at 600°C yields a composite material with NiO nanoparticles well dispersed within the carbon matrix.

On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3.

Hexagonal (W,V)O3-x oxides of high thermal stability have been synthesized hydrothermally through the intermediate products Nax(W,V)O3·zH2O and (NH4)0.33-x(W,V)O3-y. The obtained crystals show nanostructured surface via the formation of a dense population of polyhedral nanocavities self-distributed along particular crystallographic directions. Nanocavities present a regular size that ranges from 5 to 10 nm in both length and width. The synthesis process involves a significant topotactic relationship between the as-synthesized product and the desired final product and this relationship is suggested as the origin of the observed surface nanostructure. The comparison of our results with observations in different solids has allowed us to suggest that the formation of nanocavities is an extensive spontaneous process when materials are obtained by the chemical reactions of solids leading to products with defined crystallographic orientation with respect to the original compound. The characterization provides evidence regarding the potential relevance of nanocavities in the functional properties of the resulting solids.