Scientific Papers in SCI

Abstract

MCM-22 zeolites with Si/Al in the 9–46 range were synthesised in rotating autoclave and characterised by X-ray diffraction, 1H, 29Si and 27Al magic angle spinning nuclear magnetic resonance, scanning electron microscopy and nitrogen physisorption. For the Si/Al = 21, 30 and 46 samples both X-ray diffraction and scanning electron microscopy revealed the crystallisation of pure MCM-22. Besides the latter, crystals of ferrierite also formed during the synthesis of the Si/Al = 9 sample. Based on the 1H MAS NMR spectra of dehydrated samples, the different proton species present on the MCM-22 samples were determined and quantified. Information about the incorporation of Al ions into the zeolite framework, as well as on the preferential crystallographic sites occupied in dependence on the Si/Al ratio of the sample, was obtained by 27Al MAS NMR spectroscopy. From 29Si MAS NMR spectra, differences in the degree of crystallinity of the samples were assessed, the results being in agreement with the diffraction data. Nitrogen physisorption runs revealed the microporous nature of the adsorbents, with a supermicropore to ultramicropore volume ratio in good agreement, for the best crystallised samples, with the porous structure with supercages and sinusoidal channels of the ideal MCM-22 crystal.

Abstract

Herein we present a detailed analysis of the effect of the spin-coating protocol over the optical properties of nanoparticle-based one-dimensional photonic crystals. Based on these results, we provide a reliable synthetic route to attain high-quality porous multilayers in which the effect of imperfections is minimized and whose Bragg diffraction can be precisely tuned over the entire visible and near-infrared spectrum. We present a systematic study of the effect of the acceleration ramp and final rotation speed over the structural and optical quality of these materials. This allows us to relate the structural variations observed with the different relative importance of fluid flow and solvent evaporation on the thinning of each layer in the stack for the different deposition conditions employed.

Abstract

Crystalline porous zirconium membranes were obtained by physical vapor deposition on AAO templates at room temperature. These membranes were found to have similar hexagonal nanohole arrays as the template and high crystallinity. The pore size of the synthesized metallic membranes could be controlled during the synthesis through appropriate parameters in the experimental procedure.

Abstract

The performance of protective thin films is clearly influenced by their microstructure. The objective of this work is to study the influence of the structure of TiC/a-C nanocomposite coatings with a-C contents ranging from ~ 0% to 100% on their mechanical and tribological properties measured by ultramicroindentation and pin-on-disks tests at ambient air, respectively. The microstructure evolves from a polycrystalline columnar structure consisting of TiC crystals to an amorphous and dense TiC/a-C nanocomposite structure when the amount of a-C is increased. The former samples show high hardness, moderate friction and high wear rates, while the latter ones show a decrease in hardness but an improvement in tribological performance. No apparent direct correlation is found between hardness and wear rate, which is controlled by the friction coefficient. These results are compared to the literature and explained according to the different film microstructures and chemical bonding nature. The film stress has also been measured at the macro and micro levels by the curvature and Williamson–Hall methods respectively. Other mechanical properties of the coating such as resilience and toughness were evaluated by estimating the H3/E⁎2 and H/E⁎ ratios and the percentage of elastic work (We). None of these parameters showed a tendency that could explain the observed tribological results, indicating that for self-lubricant nanocomposite systems this correlation is not so simple and that the assembly of different factors must be taken into account.

Abstract

Herein, we report an experimental analysis of the photogenerated current of very thin and uniform dye-sensitized nanocrytalline titanium oxide (nc-TiO2) electrodes coupled to high-quality one-dimensional photonic crystals. The effect of well-defined optical absorption resonances are detected both in optical spectroscopy and photogenerated current experiments, a clear correspondence between them being established. Our study demonstrates that light trapping within absorbing electrodes is responsible for the absorption enhancement that has previously been reported and unveils the mechanism behind it. We prove that this effect improves significantly the power conversion efficiency of very thin electrodes.

Abstract

A systematic study of the different hydrolyzed species derived from the hydrated Po(IV) in water, [Po(H2O)n(OH)m](4−m) for 1 m 4, and 4 m + n 9, has been carried out by means of quantum mechanical computations. The effects of outer solvation shells have been included using a polarizable continuum dielectric model. For a fixed number of hydroxyl groups, the preferred hydration number for the Po(IV) can be determined in terms of Gibbs energy. It is shown that the hydration number (n) systematically decreases with the increase in the number of hydroxyl groups (m) in such a way the total coordination number (n + m) becomes smaller, being 9 in the aquocomplex and 4 in the neutral hydroxo-complex. Free energies for the hydrolysis processes involving Po(IV) complexes and a different number of hydroxyl groups have been computed, revealing the strong tendency toward hydrolysis exhibited by these complexes. The predominant species of Po(IV) in aqueous solutions are ruled by a dynamical equilibrium involving aggregates containing in the first coordination shell OH− groups and water molecules. Although there is not experimental information to check the theoretical predictions, theoretical computations in solution seem to suggest that the most likely clusters are [Po(H2O)5(OH)2]2+ and [Po(H2O)4(OH)2]2+. The geometry of the different clusters is ruled by the trend of hydroxyl groups to be mutually orthogonal and to promote a strong perturbation of the water molecule in trans-position by lengthening the Po−H2O distances and tilting the corresponding bond angle. A general thermodynamic cycle is defined to compute the Gibbs free energy associated to the formation of the different hydrolyzed forms in solution. From it, the estimates of pKa values associated to the different protolytic equilibria are provided and discussed. Comparison of the relative values of pKa along a hydrolysis series with the experimental values for other tetravalent cations supports its consistency.

Abstract

Titanium carbide (TiC) and nitride (TiN) are two of the most used materials in the field of protective coatings, due to their optimal mechanical and tribological properties. The addition of the second phase can provide extra benefits to the coating, like improved hardness, reduced friction and/or oxidation resistance. In this work, we present two series of coatings in which hard crystalline TiC and TiN phases are mixed at the nanometric level with a soft lubricant phase like amorphous carbon (a-C). Both series of TiC/a-C and TiN/a-C nanocomposite coatings were prepared by double magnetron sputtering of C and Ti(N) targets in a Ar atmosphere (P = 5 x 10(-3) Torr) by changing the power ratio applied to each magnetron. The chemical composition has been measured by electron energy loss spectroscopy, and the phase composition changes gradually from pure C to pure TiC or TiN through nanocomposite structures with variable phase contents. These structures are confirmed by transmission electron microscopy and diffraction techniques. like X-ray diffraction and electron diffraction. The mechanical and tribological properties are found to be mainly controlled by the hard/soft phase ratio present in the coating. The changes in hardness values follow similar trends in both types of nanocomposite samples. Introducing a small amount of TiN or TiC into a-C matrix causes a hardness reduction, but further addition of crystalline phase makes increase the hardness. The best tribological properties are found for nanocomposite coatings (both TiN/a-C and TiC/a-C) with high amount of a-C (> 65%), showing low friction values (f similar to 0.1) and high wear resistance (k about 10(-7) mm(3) N-1 m(-1)). However, coatings with 50-60% a-C show a good compromise between tribological and mechanical properties.

Abstract

Gold nanoparticles with an average particle size below 3 nm have been synthesized and stabilized with different thiol-derivatized molecules for studying the influence of the capping molecule on the magnetic behavior. Thiolated-alkane chains with different lengths as well as a thiol-containing biomolecule (tiopronin) have been selected as protecting shells for the synthesized NPs. Magnetic characterization indicates that the appearance of a ferromagnetic-like behavior is related not only with the formation of Au-S bonds linking the protective molecules to the nanoparticle surface but also with the formation of the nanoparticle itself as well as with the geometry of the capping molecule. The later seems to determine whether the protective monolayer shell is ordered or not. The simultaneous presence of Au-Au and Au-S bonds together with a reduced particle diameter, and the formation of an ordered monolayer protective shell, have been proved to be key parameters for the ferromagnetic-like behavior exhibited by thiol-functionalized gold NPs.

Abstract

Carbon nanotubes (CNTs) have been grown both on a sample of stainless steel (317-2R) and on the same steel coated with cobalt colloid nanoparticles. Both materials are suitable supports for the growth of vertically aligned carbon nanotubes, although a more sparse growth of significantly thicker carbon nanotubes is observed in the case of the bare steel. We find that carbon nanotubes grown directly on the stainless steel support show very poor tribological behaviour whereas the support using nanoparticles for carbon nanotube growth displayed interesting tribological properties with friction coefficients of approximately 0.1-0.2. The modified CNT material (studied by Raman spectroscopy) adheres to both mating surfaces avoiding direct contact between asperities and plough so the friction and wear processes decrease greatly.

Abstract

In the present work we synthesized inorganic oxide nanoparticle carriers of platinum compounds and tested their therapeutic effect on animal models in which C6 glioma cells have been inoculated. TiO2-containing Pt(NH3)(4)Cl-2 complexes were synthesized using sol-gel methods. The platinum species are chemically bonded to the TiO2 carrier, as shown by Fourier transform infrared spectroscopy of probe molecules. Treatment with TiO2-Pt nanoparticles reduces tumour growth rate by up to 56%, showing that a synergistic effect exists between the TiO2 carrier and the platinum drug.

Abstract

TiCxN1-x-based powdered cermets were synthesized by a one step mechanically induced self-sustaining reaction (MSR) process from mixtures of elemental powders, and subsequently sintered by a pressureless method. The composition and microstructure of the ceramic and binder phases before and after the sintering process were studied by X-ray diffraction, scanning and transmission electron microscopy, and electron diffraction. The powdered cermets showed excellent binder dispersion and a nanometer character for the ceramic and binder particles. The TiCxN1-x stoichiometry was consistently richer in carbon than expected from the raw powder composition. An important amount of titanium was present in the binder after MSR synthesis, and intermetallic Ti-Ni or Ti-Co phases were obtained in some cases. After sintering, the binder phase was always constituted by intermetallic compounds. The morphology of the ceramic phase in the final bodies was dependent on the C/N ratio of TiCxN1-x and its growth primarily occurred through a coalescence process. The presence of titanium in the binder reduced hard particle solubility in the melted binder and its grain growth. 

Abstract

Directionally solidified Al2O3-based eutectics are in situ composites grown from the melt. The directional nature of the solidification process makes these materials highly anisotropic and therefore the measurement and quantification of their crystallographic texture is necessary to understand their physical properties. We studied the texture of Al2O3-ZrO2 (12 mol% Y2O3) eutectic rods by means of X-ray and electron backscattering diffraction. The phases grow according to the relationship (10 (1) over bar2}(Al2O3) //{110}(ZrO2) and Al2O3 is oriented with its c-axis approximately parallel to the growth direction. We observed that the c-axis orientation is not constant throughout the sample, but instead changes according to the distance from the growth axis. The c-axis orientation spread was found to be 10 degrees. This spread is a consequence of the curvature of the liquid-solid interface during solidification, whose shape we reconstructed using EBSD data.

Abstract

A very simple route for the synthesis of spherical down-conversion (DC) and up-conversion (UC) NaYF4-based nanophosphors is described which consists of a precipitation reaction at rather low temperature (max. 120 degrees C) from solutions containing sodium fluoride and appropriate Ln precursors in a common solvent (ethylene glycol/water mixtures), without the use of any other additive (complexing agent or surfactant). The role played by the nature of the yttrium precursor and the solvent on the morphological characteristics of the nanoparticles is discussed. The size of the nanospheres, which crystallized in the cubic alpha-NaYF4 phase, could be tuned within the 45-155 nm range by adjusting the reaction parameters (temperature and ethylene glycol/water ratio). The applicability of this method is illustrated for the synthesis of Eu-III-doped (DC), Tb-III-doped (DC) and Yb-III/Er-III codoped (UC) NaYF4 nanophosphors, whose luminescent properties were also analysed.

Abstract

A metallic monolith coated with 1% Au/CeO2 Catalyst has been calcined at 300 degrees C and tested in the CO oxidation reaction both in "dry" and "wet" conditions. The light-off curves show a positive effect of the presence of water in the reactive feed on the catalytic activity. With the aim to explain these observations, a FTIR CO adsorption study at liquid nitrogen temperature was performed over a similar powder catalyst. At this low temperature the oxygen mobility from the bulk to the surface is minimized and then surface phenomena are evidenced. Both, the effect of different pre-treatments of the catalysts and the presence of pre-adsorbed water on the surface have been examined. The studies reveal that the previous treatment of the sample deeply affects the surface species and the gold particle size. The water addition provokes oxidation of the surface and improves the CO oxidation activity.

Abstract

Nanocrystalline Mg powders of different particle size were obtained by inert gas evaporation and studied during electrochemical and gas-phase hydrogen cycling processes. The samples were compared to dehydrided samples obtained by mechanical milling of MgH2 with and without 2 mol% Nb2O5 as catalyst. The hydrogen overpotential of the pure Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced with smaller particle sizes reaching values comparable to samples with Nb2O5 additive. On the other hand gas-phase charging experiments showed the capacity loss with smaller particle sizes due to oxidation effects. These oxidation effects are different depending on the synthesis method used and showed a major influence on the hydrogen sorption kinetics.

Abstract

ZnO nanoparticles have been prepared by amine template assisted sol-gel precipitation and further hydrothermal treatment. We have investigated the effect of different pH values achieved by means of triethylamine (TEA) addition in the final surface and structural properties. Two sets of samples were obtained after thermal treatment, one with no hydrothermal pre-treatment and a second hydrothermally pre-treated. Surprisingly the precipitate obtained after the amine addition also exhibits good photocatalytic properties. It has been stated that calcination treatment leads in both sets of samples to a significant improvement in the photocatalytic properties of the studied systems. Therefore, interesting comparison has been performed between hydrothermal pre-treated and direct thermal treated samples. Surface and morphological features notably differ from ZnO prepared using different synthetic route. Wide Surface and structural characterization of the samples have been carried out, and correlations with precipitation pH are pointed out from this characterization. In all cases. the amine templated ZnO obtained exhibit high conversion values for phenol photo-oxidation reaction. Further calcination treatment in all of the studied samples clearly leads to photocatalytic conversions higher than that exhibited by TiO2 Degussa P25. This fact is even more significant if we consider that hydrothermally and calcined ZnO exhibit almost null surface area values. leading to a startling intrinsic photoactivity. The structural excellence (crystallinity, lack of defects, crystallite size, etc.) of such systems is clearly responsible for their high photoactivity values.

Abstract

Emanation thermal analysis (ETA), thermogravimetry and high temperature XRD were used to characterize the thermal behavior during dehydration of natural Na montmorillonite (Upton Wyoming, USA) and homoionic montmorillonite (MMT) samples saturated with different cations, i.e. Li+, Cs+, NH4+, Mg2+ and Al3+. ETA results characterized radon mobility and microstructure changes that accompanied the mass loss of the samples due to dehydration on heating in air. A collapse of interlayer space between the silicate sheets after water release from the MMT samples was characterized by a decrease of the radon release rate, Delta E. Decreases in c-axis basal spacing (d(001)) values determined from XRD patterns for the different montmorillonite samples follow the sequence:

Mg-MMT>AI-MMT>Li-MMT>Na-MMT>NH4-MMT>Cs-MMT

The decrease of the radon release rate (Delta E) determined by ETA that characterized microstructure changes due to collapse of interlayer space corresponded well to differences in the c-axis basal spacing (Delta d(001)) values determined from the XRD patterns before and after samples dehydration.

Abstract

The purpose of this Comment is to show that we find that the conclusions presented in a paper by Nair and Vijaya [Phys. Rev. A 76, 053805 (2007)], concerning the perfect periodic ordering of self-assembled photonic crystals, are not supported and contradict previous studies of this matter.

Abstract

An analysis of the diffracted beams emerging from three-dimensional photonic crystals is herein presented. The wave vectors of nonspecular beams are calculated for a triangular two-dimensional lattice and the change in their directions as a function of the wavelength is confirmed experimentally for the case of face-centered-cubic colloidal crystals illuminated under normal incidence. A fluctuating behavior of beam intensity as a function of the wavelength of the incident light is predicted for perfectly ordered lattices. As it is the case for specularly reflected and ballistically transmitted beams, this modulation arises from multipole resonances of the sphere ensemble that are smoothed out via the diffuse light scattering produced by imperfections in the crystalline structure. When optical extinction is introduced in order to model the effect of imperfections, it is possible to accurately reproduce experimental observations.

Abstract

A new type of superhydrophobic material consisting of a surface with supported Ag@TiO2 core-shell nanofibers has been prepared at low temperature by plasma-enhanced chemical vapor deposition (PECVD). The fibers are formed by an inner nanocrystalline silver thread which is covered by a TiO2 overlayer. Water contact angles depend on the width of the fibers and on their surface concentration, reaching a maximum wetting angle close to 180 degrees for a surface concentration of similar to 15 fibers mu m(-2) and a thickness of 200 nm. When irradiated with UV light, these surfaces become superhydrophilic (i.e., 0 degrees contact angle). The decrease rate of the contact angle depends on both the crystalline state of the titania and on the size of the individual TiO2 domains covering the fibers. To the best of our knowledge, this is one of the few examples existing in the literature where a superhydrophobic surface transforms reversibly into a superhydrophilic one as an effect of light irradiation.

Abstract

Herein we report an analysis of the variation of the optical properties of different nanoparticle-based one-dimensional photonic crystal architectures versus changes in the ambient vapor pressure. Gradual shift of the optical response provides us with information on the sorption properties of these structures and allow us to measure precise adsorption isotherms of these porous multilayers. The potential of nanoparticle-based one-dimensional photonic crystals as base materials for optical sensing devices is demonstrated in this way.

Abstract

This work reports on the reactivity toward carbon of a La0.5Sr0.5CoO3-delta perovskite prepared by spray pyrolysis. It is shown that this perovskite presents a moderate activity for the thermal oxidation of carbon, producing a decrease in its temperature of combustion of ca. 150 degrees C and a significant increase in the selectivity toward CO2. Different experiments were carried out with electron energy loss spectroscopy (EELS) and X-ray photoemission spectroscopy (XPS) for the perovskite and its physical mixture with a high-surface area carbon material. In the physical mixture, the cobalt at the surface was partially reduced to Co2+ even at room temperature. XPS demonstrated a species of oxygen with low electron density at the catalyst surface. This species seemed to play a significant role in the oxidation processes at the perovskite surface. A model is proposed to account for the changes exhibited by the catalyst during its reaction with carbon.

Abstract

A series of Au/TiO2 samples with gold loadings ranging from 0.11% to 1.26% have been prepared by deposition-precipitation, characterised by means of XRD, S-BET, XRF, TEM, XPS and DR UV-Vis techniques and tested in the gaseous CO oxidation and photocatalytic degradation of phenol in aqueous media. The catalytic performances of the solids on both reactions depend on the gold content. Besides this, the gold particle size plays a determinant role in the catalytic activity for the CO reaction, but apparently its influence on the photocatalytic activity appears to be negligible and only very small gold particles seem to participate on the photocatalytic process. On the other hand, the electronic properties of the solids, measured by its band gap energy, are a key factor in the photochemical activity but do not have a clear influence in the CO oxidation reaction.

Abstract

The state of the nickel in a Ni/CeO2 catalyst during hydrogen reduction, steam and dry reforming of methane, have been studied by means of in situ XAS spectroscopy. The catalyst was prepared by a combustion method and, after calcination, very small (about 10 nm) and homogeneous cubic particles of NiO are formed. Once reduced in hydrogen at high temperature (750 °C), the catalyst is active in both steam and dry reforming reactions of methane, but much more stable for the dry reforming reaction. In situ XAS analysis reveals that under reaction conditions at high temperature, the nickel remains completely reduced to Ni(0). However, under strongly reducing conditions (hydrogen or dry reforming at 750 °C), the Ni K-edge X-ray absorption spectrum undergoes unexpected modifications that, according to the parameters obtained by fitting analysis, come from changes in the size and morphology of nickel particles, which are now flattened and strongly stabilized on the partially reduced ceria surface. These morphology changes reflect a kind of strong metal–support interaction (SMSI), and could account for the higher stability observed for the dry reforming reaction. The flattened particles are not stable after cooling down to room temperature in hydrogen, while the coke deposited during the dry reforming reaction seems to block the particles, which partially remain even after cooling down to room temperature.

Abstract

An electron reaching the detector after being backscattered from a solid surface in a reflection electron energy loss spectroscopy (REELS) experiment follows a so-called V-type trajectory if it is reasonable to consider that it has only one large elastic scattering event along its total path length traveled inside the solid. V-type trajectories are explicitly assumed in the dielectric model developed by Yubero [Phys. Rev. B 53, 9728 (1996)] for quantification of electron energy losses in REELS experiments. However, the condition under which this approximation is valid has not previously been investigated explicitly quantitatively. Here, we have studied to what extent these REELS electrons can be considered to follow near V-type trajectories. To this end, we have made Monte Carlo simulations of trajectories for electrons traveling at different energies in different experimental geometries in solids with different elastic scattering properties. Path lengths up to three to four times the corresponding inelastic mean free paths have been considered to account for 80-90% of the total electrons having one single inelastic scattering event. On this basis, we have made detailed and systematic studies of the correlation between the distribution of path lengths, the maximum depth reached, and the fraction of all electrons that have experienced near V-type trajectories. These investigations show that the assumption of V-type trajectories for the relevant path lengths is, in general, a good approximation. In the rare cases, when the detection angle corresponds to a scattering angle with a deep minimum in the cross section, very few electrons have experienced true V-type trajectories. However, even in these extreme cases, a large fraction of the relevant electrons have near V-type trajectories.

Abstract

The tribological behaviour of nanocomposite coatings made of nanocrystalline metal carbides and amorphous carbon (a-C) prepared by PVD/CVD techniques is found to be very dependant on the film deposition technique, synthesis conditions and testing parameters. Focusing in the TiC/amorphous carbon-based nanostructured system, this paper is devoted to an assessment of the factors governing the tribological performance of this family of nanocomposites using a series of TiC/a-C films prepared by magnetron sputtering technique varying the power applied to each target (titanium or graphite) as model system to establish correlations between film microstructure and chemical compositions and tribological properties measured by a pin-on-disk tribometer. The film microstructure goes from a quasi -polycrystalline TiC to a nanocomposite formed by nanocrystals of TiC embedded in an amorphous carbon matrix as observed by transmission electron microscopy (TEM). The nanocrystalline/amorphous ratio appears to be the key-parameter to control the tribological properties and its quantification has been done by electron energy-loss spectroscopy (EELS). A significant change in the tribological performance is observed for nanocomposites with amorphous carbon phase contents above 60-65%. The friction coefficient decreases from 0.3 to 0.1 and the film wear rates by a factor of 10. Examination of the wear scars on ball and film surfaces by laser micro-Raman spectroscopy has allowed to determine the presence of metallic oxides and carbonaceous compounds responsible of the observed friction behaviour. The revision of the literature results in view of the conclusions obtained enabled to explain their apparent dispersion in the tribological performance.