Síntesis, caracterización y ensayos cinéticos del nanocomposite RuO2/TiO2/MWCNT/Pt en la producción fotocatalítica de combustibles solares bajo iluminación UV y VIS

Se prepararon partículas de nanocomposites basados en TiO2 y nanotubos de carbono multicapa platinizados para la obtención de combustibles solares. Se evaluó la actividad fotocatalítica del material en la producción de hidrógeno, en procesos de degradación de ácido fórmico, y en la obtención de hidrocarburos a partir de la reducción de CO2 en agua. Los nanocomposites fueron sintetizados por medio de la técnica sol-gel. Se estudió el efecto de la proporción y el diámetro de los nanotubos de carbono en la actividad del material bajo iluminación ultravioleta y visible. Se estudió el efecto de la adición de RuO2 (0,5% wt.) en la actividad bajo iluminación visible. Los materiales fueron caracterizados por ATR, XRD, BET, HRTEM y SEM. Se obtuvieron sólidos macroporosos, con contenido de fase anatasa superior al 99% y tamaño cristalino comprendido entre 15 y 21 nm. Los resultados cinéticos mostraron una producción óptima de hidrógeno para el composite TiO2/(5wt.%)MWCNT/Pt(60-80 nm), con eficiencia cuántica y eficiencia energética de 1,27% y 0,27%, respectivamente. En el caso de radiación visible, la producción de hidrógeno fue nula para los composites TiO2/MWCNT/Pt, mientras que para el sistema RuO2/TiO2/MWCNT/Pt se observó que la adicción de MWCNT inhibía la actividad fotocatalítica del composite RuO2/TiO2 en la región del visible. Por otra parte, en los ensayos de reducción de CO2 no se detectó ningún producto de reacción.

Nanostructuració d'òxids mitjançant mètodes de solució química avançats i nanolitografia tova

Un dels reptes tecnològics més importants del segle XXI és el desenvolupament i organització de materials funcionals a escala nanomètrica ja que permet modificar-ne les propietats fonamentals i generar-ne de noves. En el cas dels òxids complexos aquesta tecnologia ha generat grans perspectives en diferents àrees d’estudi perquè presenten propietats molt interessants com la magnetoresistència colossal, superconductivitat o multiferroicitat. En particular, en l’òxid complex superconductor YBa2Cu3O7 (YBCO) s’ha demostrat que la formació de capes superconductores sobre plantilles nanoestructurades o bé la formació de nanocompostos en una matriu superconductora permet millorar de manera espectacular les seves propietats (corrent crític). Aquests resultats introduiran canvis paradigmàtics en la tecnologia de l’energia elèctrica (cables, motors, generadors) i en totes aquelles aplicacions que requereixin camps magnètics intensos. Ara bé, cal aconseguir-ho mitjançant tècniques de baix cost i que permetin un fàcil escalat. Durant els 4 mesos que s’ha disfrutat de l’ajut BP s’han preparat amb èxit noves capes primes superconductores nanoestructurades mitjançant la introducció d’una fase secundària nanomètrica complexa, Ba2YTaO6 (BYTO), dins la matriu d’YBCO pel mètode de baix cost de deposició de solucions químiques. Aquesta nova composició ha donat lloc a un augment significatiu de les propietats superconductores comparat amb la fase tradicional d’YBCO i per tant tenen gran potencial per fabricar futures cintes superconductores.

Induced sputum versus exhaled nitric oxide for the evaluation of airway inflammation in allergic paediatric asthma patients treated with omalizumab

Our purpose is to determine the inflammatory changes in the airways of allergic paediatric asthma patients treated with omalizumab, measured by the percentage of eosinophils in induced sputum and exhaled nitric oxide (FENO). We observed a progressive and statistically significant decrease of eosinophil count in the induced sputum meanwhile FENO, although very sensible, was a less reproducible and thus a less reliable method to evaluate chronic airway inflammation in this population. Induced sputum seems to be a better method to monitor chronic inflammation and thus the response to chronic omalizumab treatment while FENO measurement would be more useful to monitor acute events preceding exacerbations.

Surface states in template synthesized tin oxide nanoparticles

Tin-oxide nanoparticles with controlled narrow size distributions are synthesized while physically encapsulated inside silica mesoporous templates. By means of ultraviolet-visible spectroscopy, a redshift of the optical absorbance edge is observed. Photoluminescence measurements corroborate the existence of an optical transition at 3.2 eV. The associated band of states in the semiconductor gap is present even on template-synthesized nanopowders calcined at 800°C, which contrasts with the evolution of the gap states measured on materials obtained by other methods. The gap states are thus considered to be surface localized, disappearing with surface faceting or being hidden by the surface-to-bulk ratio decrease.

Conduction mechanisms and charge storage in Si-nanocrystals metal-oxide-semiconductor memory devices studied with conducting atomic force microscopy

In this work, we demonstrate that conductive atomic force microscopy (C-AFM) is a very powerful tool to investigate, at the nanoscale, metal-oxide-semiconductor structures with silicon nanocrystals (Si-nc) embedded in the gate oxide as memory devices. The high lateral resolution of this technique allows us to study extremely small areas ( ~ 300nm2) and, therefore, the electrical properties of a reduced number of Si-nc. C-AFM experiments have demonstrated that Si-nc enhance the gate oxide electrical conduction due to trap-assisted tunneling. On the other hand, Si-nc can act as trapping centers. The amount of charge stored in Si-nc has been estimated through the change induced in the barrier height measured from the I-V characteristics. The results show that only ~ 20% of the Si-nc are charged, demonstrating that the electrical behavior at the nanoscale is consistent with the macroscopic characterization.

Strain-induced stabilization of new magnetic spinel structures in epitaxial oxide heterostructures.

We report here on the growth of NiFe2O4 epitaxial thin films of different thickness (3 nm ¿ t ¿ 32 nm) on single crystalline substrates having spinel (MgAl2O4) or perovskite (SrTiO3) structure. Ultrathin films, grown on any of those substrates, display a huge enhancement of the saturation magnetization: we will show that partial cationic inversion may account for this enhancement, although we will argue that suppression of antiparallel collinear spin alignment due to size-effects cannot be excluded. Besides, for thicker films, the magnetization of films on MAO is found to be similar to that of bulk ferrite; in contrast, the magnetization of films on STO is substantially lower than bulk. We discuss on the possible mechanisms leading to this remarkable difference of magnetization.

Metal-Nitride-oxide-semiconductor light-emitting devices for general lighting.

The potential for application of silicon nitride-based light sources to general lighting is reported. The mechanism of current injection and transport in silicon nitride layers and silicon oxide tunnel layers is determined by electro-optical characterization of both bi- and tri-layers. It is shown that red luminescence is due to bipolar injection by direct tunneling, whereas Poole-Frenkel ionization is responsible for blue-green emission. The emission appears warm white to the eye, and the technology has potential for large-area lighting devices. A photometric study, including color rendering, color quality and luminous efficacy of radiation, measured under various AC excitation conditions, is given for a spectrum deemed promising for lighting. A correlated color temperature of 4800K was obtained using a 35% duty cycle of the AC excitation signal. Under these conditions, values for general color rendering index of 93 and luminous efficacy of radiation of 112 lm/W are demonstrated. This proof of concept demonstrates that mature silicon technology, which is extendable to lowcost, large-area lamps, can be used for general lighting purposes. Once the external quantum efficiency is improved to exceed 10%, this technique could be competitive with other energy-efficient solid-state lighting options. ©2011 Optical Society of America OCIS codes: (230.2090) Electro-optical devices; (150.2950) Illumination.

Intense green-yellow electroluminescence from Tb+-implanted silicon-rich silicon nitride/oxide light emitting devices

High optical power density of 0.5 mW/cm2, external quantum efficiency of 0.1%, and population inversion of 7% are reported from Tb+-implanted silicon-rich silicon nitride/oxide light emitting devices. Electrical and electroluminescence mechanisms in these devices were investigated. The excitation cross section for the 543 nm Tb3+ emission was estimated under electrical pumping, resulting in a value of 8.2 × 10−14 cm2, which is one order of magnitude larger than one reported for Tb3+:SiO2 light emitting devices. These results demonstrate the potentiality of Tb+-implanted silicon nitride material for the development of integrated light sources compatible with Si technology.

Gas-sensing properties of sprayed films of (CdO)x(ZnO)1-x mixed oxide

A novel NO2 sensor based on (CdO)x(ZnO)1-x mixed-oxide thin films deposited by the spray pyrolysis technique is developed. The sensor response to 3-ppm NO2 is studied in the range 50°C-350°C for three different film compositions. The device is also tested for other harmful gases, such as CO (300 ppm) and CH4 (3000 ppm). The sensor response to these reducing gases is different at different temperatures varying from the response typical for the p-type semiconductor to that typical for the n-type semiconductor. Satisfactory response to NO2 and dynamic behavior at 230°C, as well as low resistivity, are observed for the mixed-oxide film with 30% Cd. The response to interfering gas is poor at working temperature (230°C). On the basis of this study, a possible sensing mechanism is proposed.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron¿hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron¿hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron¿hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Configurational statistical model for the damaged structure of silicon oxide after ion implantation

A configurational model for silicon oxide damaged after a high-dose ion implantation of a nonreactive species is presented. Based on statistics of silicon-centered tetrahedra, the model takes into account not only the closest environment of a given silicon atom, but also the second neighborhood, so it is specified whether the oxygen attached to one given silicon is bridging two tetrahedra or not. The frequencies and intensities of infrared vibrational bands have been calculated by averaging over the distributions and these results are in agreement with the ones obtained from infrared experimental spectra. Likewise, the chemical shifts obtained from x-ray photoelectron spectroscopy (XPS) analysis are similar to the reported values for the charge-transfer model of SiOx compounds.

Gas identification with tin oxide sensor array and self-organizing maps: adaptive correction of sensor drifts

Low-cost tin oxide gas sensors are inherently nonspecific. In addition, they have several undesirable characteristics such as slow response, nonlinearities, and long-term drifts. This paper shows that the combination of a gas-sensor array together with self-organizing maps (SOM's) permit success in gas classification problems. The system is able to determine the gas present in an atmosphere with error rates lower than 3%. Correction of the sensor's drift with an adaptive SOM has also been investigated

Synthesis, structure, and magnetic studies on self-assembled BiFeO3-CoFe2O4 nanocomposite thin films

Self-assembled (0.65)BiFeO3-(0.35)CoFe2O4 (BFO-CFO) nanostructures were deposited on SrTiO3 (001) and (111) substrates by pulsed laser deposition at various temperatures from 500 to 800°C. The crystal phases and the lattice strain for the two different substrate orientations have been determined and compared. The films grow epitaxial on both substrates but separation of the spinel and perovskite crystallites, without parasitic phases, is only obtained for growth temperatures of around 600-650°C. The BFO crystallites are out-of-plane expanded on STO(001), whereas they are almost relaxed on (111). In contrast, CFO crystallites grow out-of-plane compressed on both substrates. The asymmetric behavior of the cell parameters of CFO and BFO is discussed on the basis of the role of the epitaxial stress caused by the substrate and the spinel-perovskite interfacial stress. It is concluded that interfacial stress dominates the elastic properties of CFO crystallites and thus it may play a fundamental on the interface magnetoelectric coupling in these nanocomposites.

Characterization of nano-oxide layers fabricated by ion beam oxidation

In this paper, a remote O2 ion source is used for the formation of nano-oxide layers. The oxidation efficiency was measured in CoFe-oxide films, and a decrease of the oxide layer with the pan angle and the oxidation pressure is observed. For the same oxidation pressure, the oxidation efficiency depends on the O2 content in the Ar-O2 plasma. These results were applied in optimizing the fabrication of Al2O3 barrier for tunnel junctions. This method was also used to fabricate junctions with Fe-oxide layers inserted at the Al2O3-CoFe interface. TEM and magnetization data indicate that after anneal at 385°C, a homogeneous ferromagnetic Fe-oxide layer (Fe3O4?) is formed.