A. Cornet

Crystalline structure, defects and gas sensor response to NO2 and H2S of tungsten trioxide nanopowders

AbstractStructural and NO 2 and H 2 S gas-sensing properties of nanocrystalline WO 3 powders are analysed in this work. Sensor response of thick-film gas sensors was studied in dry and humid air. Interesting differences were found on the sensor response between sensors based on 400 and700 8C-annealed WO 3 , what motivated a structural study of these materials. Crystalline structure and defects were characterised by X-raydiffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). Experimental results showed that both triclinic andmonoclinic structures are present in the analysed materials, although their amount depends on the annealing treatment. Crystalline shearplanes, which are defects associated to oxygen deficient tungsten trioxide, were found in 400 8C-annealed WO 3 and their influence on XRDspectra was analysed by XRD simulations. Moreover, XRD and Raman spectra were also acquired at normal metal oxide-based gas sensorworking temperatures in order to relate both crystalline structure and sensor response.# 2003 Elsevier Science B.V. All rights reserved.

Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)

We present tunneling experiments on Fe(001)/MgO(20 A)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

NH3 interaction with chromium-doped WO3 nanocrystalline powders for gas sensing applications

Ammonia interaction with chromium-doped WO3 nanocrystalline powders was investigated. Chromium centres were characterised by EELS, Raman, XPS, EPR and UV-visible spectroscopy. Essentially, these studies revealed that chromium was well distributed on WO3, forming Cr(III) species and chromyl groups [CrO]3+. Test of thick-film gas sensors based on chromium-doped WO3 revealed that chromium addition boosted sensor response to NH3. Besides, it also avoided unsatisfactory dynamic behaviour previously found in gas sensors based on pure WO3. DRIFT spectroscopy and TPD were used to investigate the surface chemistry of ammonia over pure and chromium-doped WO3 nanocrystalline powders. It was concluded that chromium favours the combustion of ammonia into molecular nitrogen and nitrous oxide on chromium centres, which contributes to the improvement in the sensing properties of the material.

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.

Mesoporous Catalytic Filters for Semiconductor Gas Sensors

Abstract An effective way to improve sensor selectivity and stability is the use of catalytic filters to block interfering and poisoning gas molecules from reaching the sensor surface. Mesoporous silica with high resistivity and hugh surface areas are ideally suited as a base material for this application. When impregnated with proper catalysts, mesoporous silica has a great potential to eliminate responses to undesired gases even of thin-film and/or micro-machined sensors. In this paper, we report our initial results on thick film SnO 2 -based gas sensors covered with a catalytic filter consisting of Pd and Pt loaded mesoporous silica. Results indicate that selective oxidation of CO in the catalytic filter leads to the elimination of CO interference to a CH 4 sensor with no perceptible deterioration in sensing performance.

A new CO2 gas sensing material

Abstract A new material for CO 2 sensing based on resistive changes is described. Using hydrated LaCl 3 as precursor and through two different synthesis routes—simple oxidation and a sol–gel-derived method—LaOCl powders are obtained. The main sensing characteristics of these powders are analysed, with special emphasis on (a) their response to CO 2 at a wide range of relative humidities and (b) their cross-sensitivity with CO. Compared with other metal oxide-based materials, lanthanum oxychloride offers a low working temperature and an improved sensor response in both dry and humid atmospheres.

Study of the influence of Nb content and sintering temperature on TiO2 sensing films

Nb-doped TiO2 nanopowders have been synthesized with a wide range of Nb contents (0–10 at.%) and of calcination temperatures (600–900 °C). The materials have been structurally characterized by means of X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The niobium introduction retards the anatase-to-rutile transformation and hinders grain growth mechanisms. The electrical behavior studied for CO and ethanol gases showed that the response to ethanol is slightly diminished by the incorporation of Nb atoms and not affected by the structure modification. However, the response to CO is modified by structure changes such as the anatase/rutile transformation, grain size and Nb segregation. For the samples treated at 700 °C, the best response to CO is achieved for 4 at.% of Nb.

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.