Anna Corrias

Formation and cation distribution in supported manganese ferrite nanoparticles: an X-ray absorption study

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe(2)O(4) nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900 degrees C). Up to 450 degrees C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900 degrees C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe(2)O(4) (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900 degrees C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn(2+) to Mn(3+) in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions.

Synthesis and microstructure of manganese ferrite colloidal nanocrystals

The atomic level structure of a series of monodisperse single crystalline nanoparticles with a magnetic core of manganese ferrite was studied using X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) techniques at both the Fe and Mn K-edges, and conventional and high resolution transmission electron microscopy (TEM and HRTEM). In particular, insights on the non-stoichiometry and on the inversion degree of manganese ferrite nanocrystals of different size were obtained by the use of complementary structural and spectroscopic characterization techniques. The inversion degree of the ferrite nanocrystals, i.e. the cation distribution between the octahedral and tetrahedral sites in the spinel structure, was found to be much higher (around 0.6) than the literature values reported for bulk stoichiometric manganese ferrite (around 0.2). The high inversion degree of the nanoparticles is ascribed to the partial oxidation of Mn(2+) to Mn(3+) which was evidenced by XANES, leading to non-stoichiometric manganese ferrite.

4-Methylpentan-2-ol dehydration over zirconia catalysts prepared by sol-gel

Zirconia samples have been prepared from xerogels and aerogels obtained using zirconium n-propoxide as precursor. Structure and texture have been investigated by X-ray diffraction, thermal analysis, transmission electron microscopy, nitrogen adsorption/desorption. Surface acidity and basicity have been assessed by adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. 4-Methylpentan-2-ol dehydration has been tested at atmospheric pressure in a fixed-bed flow microreactor. The xerogel gives tetragonal zirconia upon calcination, during which a mesoporous system is formed. The crystal phase depends on the presence of oxygen during the cooling step in the case of the aerogel, whose texture is partially retained upon calcination. Both kinds of catalysts have well-balanced concentrations of acid and base sites, but the acid sites are weaker in comparison with the basic ones. At 603 K the initial conversion of 4-methylpentan-2-ol over the calcined xerogel and aerogel is 45 and 63%, respectively; the selectivity to 4-methylpent-1-ene is 77% for both. The occurrence of an E2-like mechanism with the activated complex having a marked carbanionic character seems probable. The aerogel catalyst is quite stable during operation, whereas changes in activity and selectivity are observed for the xerogel catalyst.

Fe-Co-B amorphous alloy powder by chemical reduction

Fine boride powders, prepared by reactions between sodium or potassium borohydride and aqueous (or alcoholic) solutions of transition metal salts have long been known to be good catalysts for hydrogenation reactions. New interest around them has recently grown, because they were found to exhibit amorphous structure. Classical chemical reactions may thus offer a simple route to prepare amorphous metal powders, paralleling more sophisticated methods so far used to reach the same goal. To explore the potential of these reactions, we considered the system Fe-Co-B with the aim of examining the characteristics of the product obtained in the easiest way and without any particular precautions. The structure of the amorphous alloy appears to be different from that of materials with a similar Me/B ratio prepared via melt spinning.

Structural characterization study of FeCo alloy nanoparticles in a highly porous aerogel silica matrix

A series of FeCo-SiO(2) nanocomposite aerogels having different FeCo loadings of 3, 5, and 8 wt % were prepared using a novel urea-assisted sol-gel route. The size of the nanoparticles, which was estimated using Scherrer analysis of the main peak of the x-ray diffraction pattern, varies from 3 to 8 nm. X-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) techniques at both Fe and Co K edges were used to investigate the structure of the FeCo nanoparticles. EXAFS and XANES show that FeCo nanoparticles have the typical bcc structure. Evidence of oxidation was observed in low FeCo content aerogels. Spatially resolved electron energy loss spectroscopy analysis suggests the formation of a passivation layer of predominantly iron oxide.

Iron oxide–silica aerogel and xerogel nanocomposite materials

Abstract The sol–gel method was used to prepare iron oxide–silica nanocomposite materials. Different drying conditions were used in order to obtain aerogel and xerogel materials. The samples were characterized by thermal analysis, X-ray diffraction (XRD), N 2 -physisorption and transmission electron microscopy (TEM) techniques. Aerogel samples have a much higher surface area than the xerogel samples; moreover, different supercritical drying conditions give rise to a different porous structure. The stabilization of the maghemite phase in the xerogel nanocomposites depends on the iron content and it is more difficult in the aerogel samples. The early sol–gel preparation stages also play an important role.

Design of water-based ferrofluids as contrast agents for magnetic resonance imaging

We report the synthesis, characterization and relaxometric study of ferrofluids based on iron oxide, with potential for use as magnetic resonance imaging (MRI) contrast agents (CAs). The effect of different cost-effective, water-based surface modification approaches which can be easily scaled-up for the large scale synthesis of the ferrofluids has been investigated. Surface modification was achieved by silanization, and/or coating with non-toxic commercial dispersants (a lauric polysorbate and a block copolymer with pigment affinic groups, namely Tween 20 and Disperbyk 190) which were added after or during iron oxide nanoparticle synthesis. It was observed that all the materials synthesized functioned as negative contrast agents at physiological temperature and at frequencies covered by clinical imagers. The relaxometric properties of the magnetic nanoparticles were significantly improved after surface coating with stabilizers compared to the original iron oxide nanoparticles, with particular reference to the silica-coated magnetic nanoparticles. The results indicate that the optimization of the preparation of colloidal magnetic ferrofluids by surface modification is effective in the design of novel contrast agents for MRI by enabling better or more effective interaction between the coated iron oxide nanoparticles and protons present in their aqueous environment.

An X-ray absorption spectroscopy study of the inversion degree in zinc ferrite nanocrystals dispersed on a highly porous silica aerogel matrix

The structural properties of zinc ferrite nanoparticles with spinel structure dispersed in a highly porous SiO(2) aerogel matrix were compared with a bulk zinc ferrite sample. In particular, the details of the cation distribution between the octahedral (B) and tetrahedral (A) sites of the spinel structure were determined using X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the zinc ferrite spinel structures varies with particle size. In particular, in the bulk microcrystalline sample, Zn(2+) ions are at the tetrahedral sites and trivalent Fe(3+) ions occupy octahedral sites (normal spinel). When particle size decreases, Zn(2+) ions are transferred to octahedral sites and the degree of inversion is found to increase as the nanoparticle size decreases. This is the first time that a variation of the degree of inversion with particle size is observed in ferrite nanoparticles grown within an aerogel matrix.

The synthesis of nanocrystalline nickel boride powders by ball milling of elemental components

Solid state reaction induced by ball milling the elemental components has been investigated on Ni60B40 and Ni80B20 mixtures by means of transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. The time evolution of the reaction is similar in the two samples, although slower in Ni80B20. Reaction begins with the formation of o-Ni3B and of a disordered phase. The final products are nanocrystalline o-Ni3B and unreacted nickel in Ni80B20 and nanocrystalline t-Ni2B in Ni60B40. Different milling conditions, tested on the Ni60B40 sample, do not modify the path but affect the rate of the solid state reaction.

ZnFe2O4 nanoparticles dispersed in a highly porous silica aerogel matrix: a magnetic study

We report the detailed structural characterization and magnetic investigation of nanocrystalline zinc ferrite nanoparticles supported on a silica aerogel porous matrix which differ in size (in the range 4-11 nm) and the inversion degree (from 0.4 to 0.2) as compared to bulk zinc ferrite which has a normal spinel structure. The samples were investigated by zero-field-cooling-field-cooling, thermo-remnant DC magnetization measurements, AC magnetization investigation and Mössbauer spectroscopy. The nanocomposites are superparamagnetic at room temperature; the temperature of the superparamagnetic transition in the samples decreases with the particle size and therefore it is mainly determined by the inversion degree rather than by the particle size, which would give an opposite effect on the blocking temperature. The contribution of particle interaction to the magnetic behavior of the nanocomposites decreases significantly in the sample with the largest particle size. The values of the anisotropy constant give evidence that the anisotropy constant decreases upon increasing the particle size of the samples. All these results clearly indicate that, even when dispersed with low concentration in a non-magnetic and highly porous and insulating matrix, the zinc ferrite nanoparticles show a magnetic behavior similar to that displayed when they are unsupported or dispersed in a similar but denser matrix, and with higher loading. The effective anisotropy measured for our samples appears to be systematically higher than that measured for supported zinc ferrite nanoparticles of similar size, indicating that this effect probably occurs as a consequence of the high inversion degree.