608 research outputs found
Magnetic nanocomposite aerogels
No full textAerogels are regarded as ideal candidates for the design of functional nanocomposites containing supported metal or metal oxide nanoparticles. The large specific surface area together with the open pore structure enables aerogels to effectively host finely dispersed nanoparticles up to the desired loading, to provide nanoparticle accessibility and/or to prevent nanoparticle agglomeration, as required to supply their specific functionalities. The preparation of highly porous nanocomposite aerogels containing magnetic metal, alloy or metal oxide nanoparticles dispersed into amorphous silica, with high purity and homogeneity, was successfully achieved by a novel sol-gel procedure involving urea-assisted co-gelation of the precursor phases. This method allows fast gelation, giving rise to aerogels with 97% porosity, and it is very versatile allowing to vary composition, loading and average size of the nanoparticles. The characterization of the morphological and structural features of the nanocomposite aerogels is carried out using different techniques, such as X-ray diffraction, Transmission Electron Microscopy and X-ray Absorption Spectroscopy. The characterization of the magnetic properties is carried out by SQUID magnetometry. © 2011 Materials Research Society
Evidence of a cubic iron sub-lattice in t-CuFe2O4 demonstrated by X-ray Absorption Fine Structure
No full textCopper ferrite, belonging to the wide and technologically relevant class of spinel ferrites, was grown in the form of t-CuFe2O4 nanocrystals within a porous matrix of silica in the form of either an aerogel or a xerogel, and compared to a bulk sample. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed the presence of two different sub-lattices within the crystal structure of t-CuFe2O4, one tetragonal and one cubic, defined by the Cu2+ and Fe3+ ions respectively. Our investigation provides evidence that the Jahn-Teller distortion, which occurs on the Cu2+ ions located in octahedral sites, does not affect the coordination geometry of the Fe3+ ions, regardless of their location in octahedral or tetrahedral sites
Scanning transmission electron microscopy study of the evolution of needle-like nanostructures in CoFe2O4 and nife 2O4 silica nanocomposite aerogels
No full textMagnetic nanocomposite materials consisting of 10 wt % CoFe 2O4 or NiFe2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol?gel method. A 100-kV aberration-corrected scanning transmission electron microscope (STEM) has been used to study these materials, and bright field and high angle annular dark field images show that after heat treatment at both 450 and 900 °C, they contain needle-like nanostructures Ì?1 nm in width and 10 nm in length. High resolution STEM images show that the needle-like nanostructures have a layered internal structure with typical interlayer spacings of 0.33 ± 0.02 nm. Electron energy loss spectroscopy using a 0.13-nm diameter probe gives information on the composition of these nanostructures. The results presented here for samples heat treated at 450 °C are consistent with needle-like nanostructures arising from Co and Ni silicate hydroxides which are separate from the also present Fe-containing phase of ferrihydrite nanoparticles. Samples heat treated at 900 °C have previously been shown to contain round ferrite nanoparticles ?8 nm in diameter. The results presented here are consistent with the needle-like nanostructures being transformed into ferrite-like phases after heat treatment at 900 °C, and the needle-like nanostructures are often found attached to round ferrite nanoparticles. © 2011 American Chemical Society
NiFe2O4 Nanoparticles Dispersed in an Aerogel Silica Matrix: An X-ray Absorption Study
No full textThe formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol?gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites
Bimetallic Fe/Mo–SiO2 aerogel catalysts for catalytic carbon vapour deposition production of carbon nanotubes
No full textAbstract Highly porous nanocomposite aerogels based on bimetallic Fe and Mo nanoparticles with a variable Fe:Mo weight ratio ranging from 5:1; 3:1; and 0.7:1 dispersed on amorphous silica were obtained. N2 physisorption, X-ray diffraction, and transmission electron microscopy indicate that the Fe/Mo–SiO2 nanocomposite aerogels as obtained by
a co-gelation sol–gel route followed by supercritical drying and reduction treatment under H2 exhibit Fe and Mo nanocrystals with size in the range 4–10 and 15 nm, respectively, supported on highly porous silica. The catalytic performance of the Fe/Mo–SiO2 aerogels for the synthesis of multi wall carbon nanotubes (MWCNT) by catalytic chemical vapour deposition (CCVD) was evaluated in terms of amount and quality of the produced CNTs as assessed by gravimetric results, thermal analysis, and TEM. The effect of catalyst composition and CCVD temperature was investigated, pointing out that high reaction temperatures (800 ?C) favor the formation of MWCNTs with high quality in elevated yield, the highest C uptake value being[400 %. Catalyst composition and CCVD temperature were also found to affect the homogeneity of CNT morphology, the best MWCNT quality (with outer diameter 23–25 nm) being achieved at 800 ?C with the catalyst having the largest Mo content
In situ TEM crystallization of amorphous iron particles
No full textEven though sub-micron and nano-sized iron particles generally display single or polycrystalline structures, a growing interest has also been dedicated to the class of amorphous ones, whose absence of a crystal structure is capable of modifying their physical properties. Among the several routes so far described to prepare amorphous iron particles, we report here about the crystallization of those prepared by chemical reduction of Fe3+ ions using NaBH4, with sizes ranging between 80 and 200 nm and showing a high stability against oxidation. Their crystallization was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and in situ heating transmission electron microscopy (TEM). The latter technique was performed by the combined use of electron diffraction of a selected sample area, and bright and dark field TEM imaging, and allowed determining that the crystallization turns the starting amorphous particles into polycrystalline α-Fe ones. Also, under the high vacuum of the TEM column, the crystallization temperature of the particles shifted to 550°C from the 465°C, previously observed by DSC and XRD under 105 Pa of Ar. This indicates the pivotal role of the external pressure in influencing the starting point of phase transition. Conversely, upon both the DSC/XRD pressure and the TEM vacuum conditions, the mean size of the crystal domains increases as a consequence of further thermal increase, even if with some pressure-related differences
Cation Distribution and Vacancies in Nickel Cobaltite
No full textSamples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles. Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state
Ni-based xero- and aerogels as catalysts for nitroxidation processes
No full textPorous nanocomposites made out of nickel dispersed on silica or alumina matrices were prepared as prospective catalysts for the nitroxidation of hydrocarbons in the form of aerogel or xerogel by adopting either a supercritical or a conventional gel drying procedure. The structural and textural features of the materials were investigated by X-ray diffraction, transmission electron microscopy and N 2 physisorption and combined to the acid/base and reducibility data as deduced by adsorption microcalorimetry and temperature programmed reduction (TPR) profiles. The alumina-based samples are made out of nanocrystalline nickel aluminate and are mesoporous, although the aerogel has larger pore volumes and surface area than the xerogel. On the other hand, in the silica-based samples nickel oxide nanocrystals are dispersed on amorphous silica, the size of the nanocrystals being around 5 nm in the microporous xerogel and 14 nm in the mainly mesoporous aerogel. TPR data point out that the aluminabased samples have similar reducibility, whereas significant differences were observed in the silica-supported composites, the NiO-SiO 2 aerogel exhibiting improved reducibility at low temperature. The NO-catalyst interaction was monitored by temperature programmed NO reaction coupled to mass spectrometry and preliminary tests on the use of the NiO-SiO 2 xerogel and aerogel nanocomposites for the catalytic nitroxidation of 1-methylnaphthalene to 1-naphthonitrile were obtained in a fixedbed continuous-flow reactor. The data indicate that the aerogel exhibits larger selectivity than the corresponding xerogel, pointing out the importance of tuning the sol-gel parameters in the design of porous composite materials for catalytic applications
Euphorbia characias latex: Micromorphology of rubber particles and rubber transferase activity
No full textWe have recently characterized a natural rubber in the latex of Euphorbia characias. Following that study,
we here investigated the rubber particles and rubber transferase in that Mediterranean shrub. Rubber
particles, observed by scanning electron microscopy, are spherical in shape with diameter ranging from
0.02 to 1.2 mm. Washed rubber particles exhibit rubber transferase activity with a rate of radiolabeled
[14C]IPP incorporation of 4.5 pmol min1 mg1.
Denaturing electrophoresis profile of washed rubber particles reveals a single protein band of 37 kDa
that is recognized in western blot analysis by antibodies raised against the synthetic peptide whose
sequence, DVVIRTSGETRLSNF, is included in one of the five regions conserved among cis-prenyl chain
elongation enzymes.
The cDNA nucleotide sequence of E. characias rubber transferase (GenBank JX564541) and the deduced
amino acid sequence appear to be highly homologous to the sequence of several plant cisprenyltransferases
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