P. Nelli

Wo3 Sputtered Thin-Films For Nox Monitoring

We present in this paper preliminary results concerning the preparation of tungsten trioxide thin films by reactive sputtering, the characterization either of their structural properties by means of XRD measurements or of the film morphology with the AFM microscope and the electrical response of the film towards toxic and pollutant gases. WO3 thin films showed a good sensitivity towards low NOx concentrations (1–10 ppm) in the temperature interval 200–500 °C and they were also sensitive to 10 ppm NH3. These films were also selective to Nox with respect to interfering gases like CH4, CO and SO2 in the same temperature range.

A novel method for the preparation of NH3 sensors based on ZnO-In thin films

Abstract This paper reports the preparation of ZnO-In thin films that are capable of detecting low ammonia concentrations in air. Indium and zinc layers are subsequently deposited by thermal evaporation at 70 °C onto an alumina substrate; the metal-semiconductor phase transformation is obtained on thermal cycling of the films in a synthetic air flow at a maximum temperature of 500 °C. XRD analysis shows that these films are polycrystalline and are slightly oriented in the (002) direction. SEM micrographs show that the surface of the metal-oxide film presents a needle-shaped structure; the needle size ranges between 1 and 3 μm. ZnO-In thin films can detect ammonia concentrations in the range 1–10 ppm by operating between 200 and 500 °C and are selective towards reducing gases like CO and CH 4 ; a good response towards a few ppm NO x is also observed in the same temperature range. This sensor seems to be suitable for detecting some ppm of ammonia in air.

MoO3-based sputtered thin films for fast NO2 detection

Abstract The authors report about preparation and characterization of thin films of MoO3 as a material for gas-sensing applications. Structural investigation of the films is carried out by electron microscopy and X-ray diffraction techniques. The sensing behavior of the MoO3 films was tested to NO2, showing capability to detect a few ppm of NO2 with considerably short response time.

Reactively sputtered indium tin oxide polycrystalline thin films as NO and NO2 gas sensors

Abstract Indium tin oxide polycrystalline thin films (90%In 2 O 3 + 10%SnO 2 (by weigth)) have been grown by r.f. reactive sputtering from a target of the same compound. The films exhibit columnar structure with a preferential (400) orientation with a dispersion of 8° around the normal of the film plane, as obtained by rocking curve measurements. These films exhibit an increase in resistivity when exposed to small concentrations of NO or NO 2 in air. At an operating temperature of 590K they have a sensitivity S NO,NO 2 , defined as S NO,NO 2 = ( I air − I NO,NO 2 )/ I NO,NO 2 for NO and NO 2 chemisorption, of 100 and 30 respectively for gases at room temperature and atmospheric pressure at a concentration of 1000 ppm. Four zones for oxygen ion adsorption and desorption can be distinguished in the plot of conductivity activation energy vs. temperature. The absence of long-term drift and a satisfactory reproducibility even at high NO and/or NO 2 concentrations are attributed to the excellent crystallographic properties of these films and should allow the development of detectors for nitrogen oxides at concentrations in the range 1–2000 ppm.

Characterization of a molybdenum oxide sputtered thin film as a gas sensor

We report about preparation and characterization of sputtered MoO3 thin film as sensing layer for gas detection. We show its capability to detect CO concentrations lower than 10 ppm in wet air, a feature that allows direct usage of this material for environmental monitoring. The research also highlights some criteria for selecting a suitable material and shows which features are important for a thin film to be a good candidate as a chemical sensor.

Characterization of a nanosized TiO2 gas sensor

Abstract Thin films were obtained by r.f. reactive sputtering from a Ti.1W.9 target onto a Si substrate followed by annealing in air at 800 °C. The thermal treatment results in a nanosized TiO2 thin film with high surface-to-volume ratio. The nanosized structure, its stability, together with the ease of preparation, make this material suitable as a gas sensor. The sensing layer proved capable to detect 20 ppm of NO2 at a temperature suitable for monitoring of exhaust gases of engines. Its high sensitivity suggests use of this sensor for environmental purposes.

An electronic nose for the recognition of the vineyard of a red wine

Sensor arrays have been tentatively utilized in the past as tools for the odours characterisation of particular classes of foods and beverages. In this paper a technique aiming to discriminate among similar wines is described. A sensor array of metal-oxide based gas sensors was employed for the recognition of two wines, having the same denomination (Groppello red wine) but coming from different vineyards. The performance of the array was compared with that of the standard chemical analytical approach adopted by the wines authority.

Methods for the preparation of NO, NO2 and H2 sensors based on tin oxide thin films, grown by means of the r.f. magnetron sputtering technique

Abstract Thin films based on SnO 2 (SnO 2 , SnO 2 Pd, SnO 2 (Bi 2 O 3 ), SnO 2 (In) and ITO (90% In 2 O 3 , 10% SnO 2 )) have been grown by means of the r.f. magnetron sputtering technique in order to be used as gas sensors. Sensors based on SnO 2 with additives show a high sensitivity to nitrogen oxides at temperatures close to 573 K, while they show a great sensitivity to the H 2 present in dry air in the range 623–723 K, except for the ITO thin films. The sensor with the highest response to nitrogen oxides is the SnO 2 (In) thin film, which presents a sensitivity of 20 in 20 ppm of NO at an operating temperature of 573 K. The sensors with the highest sensitivity to H 2 is the SnO 2 Pd thin film grown by the RGTO (rheotaxial growth and thermal oxidation) technique; it has a sensitivity equal to 50 in 50 ppm of H 2 at an operating temperature of 623 K. Each kind of gas sensor is characterized by XRD and SEM analysis and by its response curves to various gases; the height of the eV s barrier is also obtained for SnO 2 thin films. The mechanisms of interaction between the thin-film surface and the various gases have also been investigated.

The aging effect on SnO2-Au thin film sensors : electrical and structural characterization

Abstract There is an increasing demand for semiconducting gas sensors for several monitoring applications that have sensitivity, selectivity and reliability on a long-term scale. In this paper, we have described the preparation of SnO2–Au thin film sensors by the RGTO technique; these sensors have proved to be capable of sensitive and selective detection of CO. The TEM and AES analysis showed that 6 months of sensor aging at 400°C did not produce any valuable rearrangement of gold atoms onto a tin dioxide surface. Most of the sensor resistance variation was observed during the first 20 days, and a limited drift was observed in the remaining period.

Sub-ppm NO2 sensors based on nanosized thin films of titanium-tungsten oxides

Abstract We describe the preparation of thin films of TiWO 3 obtained by means of r.f. sputtering deposition followed by a thermal oxidation. The sensing characteristics of these thin films were obtained by measuring the response towards NO 2 in the interval 0.5–20 ppm and to other interfering gases like ethyl alcohol, CH 4 and CO; the influence of water vapour to the response towards NO 2 was also investigated. The material seems to be a promising detector of NO 2 for environmental monitoring.