A. Cirera

Effects of Nb doping on the TiO2 anatase-to-rutile phase transition

We study the influence of Nb doping on the TiO2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO2 rutile nanoparticles.

Ultralow power consumption gas sensors based on self-heated individual nanowires

Dissipated power in metal oxide nanowires (rNW<45 nm) often causes important self-heating effects and as a result, undesired aging and failure of the devices. Nevertheless, this effect can be used to optimize the sensing conditions for the detection of various gaseous species, avoiding the requirement of external heaters. In this letter, the sensing capabilities of self-heated individual SnO2 nanowires toward NO2 are presented. These proof-of-concept systems exhibited responses nearly identical to those obtained with integrated microheaters, demonstrating the feasibility of taking advantage of self-heating in nanowires to develop ultralow power consumption integrated devices.

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.

Use of zeolite films to improve the selectivity of reactive gas sensors

Abstract Semiconductor (Pd-doped SnO2) gas sensors covered with zeolitic films (MFI or LTA) have been developed and used for gas phase sensing of different species (methane, propane, and ethanol) at different humidity levels. The dynamic responses obtained with these sensors were compared with the response of a reference sensor without a zeolitic layer. The results clearly indicate that a suitable zeolite layer strongly reduces, and in some cases suppresses, the response of the sensor to paraffins, thereby increasing the sensor selectivity to the alcohol, while the reference sensor could not discriminate between these molecules. This clearly shows the potential of zeolite-based sensors to achieve a higher selectivity/sensitivity in gas sensing applications.

Surface activation by Pt-nanoclusters on titania for gas sensing applications

Abstract Platinum supported on titania was prepared by two impregnation procedures using platinum chloride as a precursor. The catalytic precursor has been introduced before and after titania structural stabilisation, followed in both cases by a calcination process for chemical stabilisation. Anatase surface reactivity towards platinum is higher than rutile since better catalytic incorporation has been observed, by X-ray fluorescence (XRF) analyses, when introduced in anatase phase. Likewise, X-ray photoelectron spectroscopy (XPS) measurements show that the surface concentration of platinum nanoclusters is improved when adding the precursor in anatase phase. Depending on the additive concentration and the stabilisation temperature, platinum nanoclusters exhibit grain sizes between 2 and 4 nm, as shown by high resolution TEM (HRTEM) micrographs. However, concentrations higher than 2.5 at.% Pt/Ti, lead to a clustering phenomena to grain sizes up to 200 nm. Pt/TiO2 with 2.4 at.% metal content and calcined at 1100 °C shows maximum density of Pt nanoparticles and enhanced sensing behaviour. The response of thick-film Pt/TiO2 layers under gas exposures has been monitored showing that their suitability for in situ control of combustion processes is strongly related to the technological procedure used in sensor fabrication.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking

We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.

Microwave processing for the low cost, mass production of undoped and in situ catalytic doped nanosized SnO2 gas sensor powders

Abstract Nanosized tin oxide powders were obtained for application in thick film gas sensor technology. This requires an innovative technique, involving the use of microwave energy with a wavelength of 2.45 GHz, which produces doped or undoped powder precursors in just a few minutes. Further stabilisation treatments — conventional heating, OH-stimulated microwaves and combined treatments — were also considered. Reproducibility, low cost and suitability for mass production demonstrate the industrial and scientific feasibility of this new procedure. Material structural characterisation and electrical properties after gas exposure of improved sensors of Pt and Pd in situ doped, and undoped SnO 2 are introduced, showing the suitability of the material.

Optimization of tin dioxide nanosticks faceting for the improvement of palladium nanocluster epitaxy

Semispherical palladium nanoclusters have been epitaxed on {110} facets of tin dioxide nanosticks. The synthesis of tin dioxide nanoparticles has been optimized to obtain a crystallite shape with a maximum surface area lying on the rutile structure {110} planes, which are the most active for gas sensing. For this purpose, we describe a microwave method, which allowed us to obtain monocrystalline stick-like tin dioxide nanoparticles (so-called nanosticks) with rectangular prism shape. These nanosticks present long lateral {110} faces, squared cross-section 5–25 nm wide, and lengths of up to 0.5 μm.

CO–CH4 selectivity enhancement by in situ Pd-catalysed microwave SnO2 nanoparticles for gas detectors using active filter

Abstract Nanosized tin oxide powders were obtained using microwave procedure for their application on gas sensor technology. This technology allows taking advantage of low cost and high volume mass production for the easy in situ introduction of the palladium catalytic additive. The effects of a controlled high palladium catalysation, reaching active filter properties, are reported showing an enhancement of the selectivity for CO and CH 4 . In this work, electrical results of the sensor performances are discussed and correlated with their structural parameters and the used technological procedures. Especial emphasis has been devoted to the active filter efficiency deduced from CO consumption measurements. The applicability to the present state of the art in sensor technology of such active-filter catalysed SnO 2 is shown by means of its implementation in micromachined substrates operated in pulsed mode.

New method to obtain stable small-sized SnO2 powders for gas sensors

Abstract A new method based on the pyrolytic reaction of SnCl4⋅5(H2O) in the range of 400–900°C is reported to produce stable small grains of SnO2 from 6 to 34 nm. The characterisation of these nanocrystallites is discussed, giving important results about oxygen incorporation to the SnO2 lattice and grain growth. This allows the understanding of the crystalline features of the samples. The influence of this new method on grain growth topics (nucleation, Ostwald ripening, coalescence) is discussed. Examples of the CO and NO2 detection are reported, showing its relation with structural parameters.