Sérgio R. Teixeira

Nanoscale Ru(0) particles: arene hydrogenation catalysts in imidazolium ionic liquids.

The reduction of [Ru(COD)(2-methylallyl) 2] (COD = 1,5-cyclooctadiene) dispersed in various room-temperature ionic liquids (ILs), namely, 1- n-butyl-3-methylimidazolium (BMI) and 1- n-decyl-3-methylimidazolium (DMI), associated with the N-bis(trifluoromethanesulfonyl)imidates (NTf 2) and the corresponding tetrafluoroborates (BF 4) with hydrogen gas (4 bar) at 50 degrees C leads to well-dispersed immobilized nanoparticles. Transmission electron microscopy (TEM) analysis of the particles dispersed in the ionic liquid shows the presence of [Ru(0)] n nanoparticles (Ru-NPs) of 2.1-3.5 nm in diameter. Nanoparticles with a smaller mean diameter were obtained in the ILs containing the less coordinating anion (NTf 2) than that in the tetrafluoroborate analogues. The ruthenium nanoparticles in ionic liquids were used for liquid-liquid biphasic hydrogenation of arenes under mild reaction conditions (50-90 degrees C and 4 bar). The apparent activation energy of E A = 42.0 kJ mol (-1) was estimated for the hydrogenation of toluene in the biphasic liquid-liquid system with Ru-NPs/BMI.NTf 2. TEM analysis of the ionic liquid material after the hydrogenation reactions shows no significant agglomeration of the [Ru(0)] n nanoparticles. The catalyst ionic liquid phase can be reused several times without a significant loss in catalytic activity.

Self-Organized TiO2 Nanotube Arrays: Synthesis by Anodization in an Ionic Liquid and Assessment of Photocatalytic Properties

Self-organized TiO(2) nanotube (NT) arrays were produced by anodization in ethylene glycol (EG) electrolytes containing 1-n-butyl-3-methyl-imidazolium tetrafluoroborate (BMI.BF(4)) ionic liquid and water. The morphology of the as-formed NTs was considerably affected by changing the anodization time, voltage, and water and ionic liquid electrolyte concentrations. In general, a nanoporous layer was formed on the top surface of the TiO(2) NTs, except for anodization at 100 V with 1 vol % of BMI.BF(4), where the NTs mouth was revealed. The length and bottom diameter of the NTs as well as the pore diameter of the top layer showed a linear relationship with increased anodization voltage. These TiO(2) NTs were tested as photocatalysts for methyl orange photodegradation and hydrogen evolution from water/methanol solutions by UV light irradiation. The results show that the TiO(2) NTs obtained by anodization in EG/H(2)O/BMI.BF(4) electrolytes are active and efficient for both applications.

Ruthenium dioxide nanoparticles in ionic liquids: synthesis, characterization and catalytic properties in hydrogenation of olefins and arenes

A reacao de NaBH 4 com RuCl 3 dissolvido no liquido ionico 1-n-butil-3-metilimidazolio hexafluorfosfato (BMI.PF 6 ) e um metodo simples e reprodutivel para a sintese de nanoparticulas de RuO 2 estaveis com distribuicao estreita e diâmetro da particula entre 2-3 nm. As nanoparticulas de RuO 2 foram caracterizadas por XRD, XPS, EDS e TEM. Estas nanoparticulas mostraram alta atividade catalitica tanto na catalise heterogenea quanto na hidrogenacao bifasica liquido-liquido de olefinas e arenos sob condicoes moderadas de reacao. Experimentos de envenenamento com Hg(0) e CS 2 , e analises de XRD e TEM de particulas isoladas apos a catalise indicaram a formacao de nanoparticulas de Ru(0). As nanoparticulas podem ser reutilizadas em condicoes de catalise heterogenea ate 10 vezes na hidrogenacao de 1-hexeno rendendo um numero total de ciclos cataliticos de 175.000 para atomos de Ru expostos. The reaction of NaBH 4 with RuCl 3 dissolved in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF 6 ) ionic liquid is a simple and reproducible method for the synthesis of stable RuO 2 nanoparticles with a narrow size distribution within 2-3 nm. RuO 2 nanoparticles were characterized by XRD, XPS, EDS and TEM. These nanoparticles showed high catalytic activity either in the solventless or liquid-liquid biphasic hydrogenation of olefins and arenes under mild reaction conditions. Hg(0) and CS 2 poisoning experiments and XRD and TEM analysis of particles isolated after catalysis indicated the formation of Ru(0) nanoparticles. The nanoparticles could be re-used in solventless conditions up to 10 times in the hydrogenation of 1-hexene yielding a total turnover number for exposed Ru atoms of 175,000.

Structural, magnetic and transport properties of NiFeχAg(1−χ) heterogeneous alloys

Abstract We carried out a comprehensive study of structural, magnetic and electrotransport properties of as-deposited and annealed (Ni 80 Fe 20 ) χ Ag( 1−χ ) heterogenous alloys prepared by sputtering. The NiFe atomic concentration was varied between 15% and 40%. These alloys consist of small magnetic particles (Ni 80 Fe 20 ) embedded in a nonmagnetic matrix (Ag). The structures of these alloys were investigated by X-ray diffraction, scanning electron microscopy and high-resolution cross-section transmission electron microscopy. The magnetic measurements were made using SQUID magnetometry and ferromagnetic resonance. Magnetoresistance was measured with a conventional four-point probe between 1.5 K and room temperature in field range 0–6T. Three contributions to the magnetoresistance of these granular alloys have been clearly identified: the spin-valve (or giant) magnetoresistance as in multilayers, scattering on magnetic fluctuations (as in any ferromagnetic metal around its magnetic ordering temperature), and anisotropic magnetoresistance. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between Ni 80 Fe 20 and Ag, and on temperature. We discuss the different shapes and amplitudes of magnetoresistance versus Ni 80 Fe 20 concentration or temperature and their evolution upon annealing in terms of the relative roles of these three contributions. The magnetoresistance in multilayers (current in-plane or perpendicular to the plane) and granular alloys are also compared.

Integrity of hafnium silicate/silicon dioxide ultrathin films on Si

Rapid thermal annealing at 1000 °C of (HfO2)1−x(SiO2)x pseudobinary alloy films deposited on Si were performed in N2 or O2 atmospheres. The effects on the atomic transport, structure, and composition were investigated using isotopic substitution of oxygen, high-resolution transmission electron microscopy, nuclear reaction analyses, narrow nuclear reaction resonance profiling, and grazing angle x-ray reflection.

Giant magnetoresistance in melt-spun metallic ribbons

Abstract We report the observation of giant magnetoresistance in as-quenched and annealed metallic ribbons prepared by melt-spinning in an argon atmosphere. The samples consist of heterogeneous Co x Cu 1− x alloys with x varying between 5 and 30%. A magnetoresistance of the order of 20% in 50 kOe at 5 K is obtained in these samples after optimized annealing. The amplitude of this magnetoresistance is similar to that obtained in sputtered samples of the same composition. In contrast to techniques of thin-film growth (sputtering, UHV evaporation, MBE), this method of preparation allows for the production of a large quantity of material exhibiting giant magnetoresistance in the form of ribbons or wires which are a few tens of microns thick, a few millimeters wide and tens of meters long.

Insights into the active surface species formed on Ta2O5 nanotubes in the catalytic oxidation of CO

Freestanding Ta2O5 nanotubes were prepared by an anodizing method. As-anodized amorphous nanotubes were calcined at high temperature to obtain a crystalline phase. All materials were studied by means of BET analysis, XRD, TEM, SEM, XPS, and FTIR and were evaluated in the catalytic oxidation of CO. An XPS study confirmed the formation of different tantalum surface species after high temperature treatment of amorphous Ta2O5 nanotubes. Calcination at 800 °C generated Ta(4+) while higher temperature (1000 °C) treatment led to the formation of Ta(3+) species. These materials also showed significant differences in catalytic activity. Higher activity was observed for samples calcined at 800 °C than at 1000 °C, suggesting that Ta(4+) species are active sites for CO oxidation.

Photocatalytic hydrogen production of Co(OH)2 nanoparticle-coated α-Fe2O3 nanorings

The production of hydrogen from water using only a catalyst and solar energy is one of the most challenging and promising outlets for the generation of clean and renewable energy. Semiconductor photocatalysts for solar hydrogen production by water photolysis must employ stable, non-toxic, abundant and inexpensive visible-light absorbers capable of harvesting light photons with adequate potential to reduce water. Here, we show that α-Fe₂O₃ can meet these requirements by means of using hydrothermally prepared nanorings. These iron oxide nanoring photocatalysts proved capable of producing hydrogen efficiently without application of an external bias. In addition, Co(OH)₂ nanoparticles were shown to be efficient co-catalysts on the nanoring surface by improving the efficiency of hydrogen generation. Both nanoparticle-coated and uncoated nanorings displayed superior photocatalytic activity for hydrogen evolution when compared with TiO₂ nanoparticles, showing themselves to be promising materials for water-splitting using only solar light.

In vitro study of the proliferation and growth of human bone marrow cells on apatite-wollastonite-2M glass ceramics

This study concerns the preparation and in vitro characterization of an apatite-wollastonite-2M bioactive glass ceramic which is intended to be used for the regeneration of hard tissue (i.e. in dental and craniomaxillofacial surgery). This bioglass ceramic has been obtained by appropriate thermal treatment through the devitrification (crystallization) of a glass with a stoichiometric eutectic composition within the Ca(3)(PO(4))(2)-CaSiO(3) binary system. Crack-free specimens of the bioglass ceramic were immersed in human bone marrow cell cultures for 3, 7, 14 and 21days, in order to study biocompatibility. Cell morphology, proliferation and colonization were assessed by scanning electron microscopy and confocal laser scanning microscopy. A total protein content assay was used to evaluate the viability and proliferation of cultured bone marrow cells. The results showed that the cells were able to adhere and proliferate on the designed material due to the essentiality of silicon and calcium as accessory factors for cell activity stimulation.

“Imprinting” Catalytically Active Pd Nanoparticles onto Ionic-Liquid-Modified Al2O3 Supports

A new sputtering chamber that allows the constant mixing of the solid support during sputtering by using an electro-magnetic oscillator was developed for the generation of metal nanoparticles (top down approach) uniformly distributed over the solid supports. By using this new chamber, small and welldistributed Pd nanoparticles (Pd-NPs) of 2.6 or 4.3 nm were produced over Al2O3 and new imidazolium ionic liquids covalently supported on Al2O3 by simple sputtering from a Pd foil. The Pd-NPs uniformly distributed over the solid supports display comparable catalytic performances in the hydrogenation of 1,3-cyclohexadiene and 1,3-cyclooctadiene to that achieved by using a catalyst prepared by conventional chemical methods (bottom up approach). Mechanistic and labelling studies show that the hydrogenation of 1,3-cyclohexadiene catalysed by Pd-NPs occurs via meta-stable p-allyl intermediates, characteristic of homogeneous-like catalytically active sites, and disproportionation through the outer-sphere mechanism. The ratio of hydrogenation to disproportionation is dependent on the Pd-NP size, and the disproportionation products are more pronounced with small NPs because of the higher affinity of dienes for this size of particle. Therefore, the surface structural features of small Pd-NPs facilitate the arrangement of diene molecules in the correct geometry for the transfer of H from the donor to the acceptor sites, typical of poly-metallic catalytically active sites. It is proposed that the reaction of 1,3-cyclohexadiene under H2 can be used to probe the homogeneous/ heterogeneous nature of supported metal NPs. It is demonstrated for the first time that highly active and selective nanocatalysts were obtained by using the sputtering-deposition technique (top down approach) and this opens a new window of opportunity for the preparation of size-controlled metal NPs with clean surfaces. Introduction