C. Cantalini

Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films

Carbon nanotubes (CNTs) deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates have been investigated as resistive gas sensors for NO2. Upon exposure to NO2, the electrical resistance of the CNTs was found to decrease. The maximum variation of resistance to NO2 was found at an operating temperature of around 165 °C. The sensor exhibited high sensitivity to NO2 gas at concentrations as low as 10 ppb, fast response time, and good selectivity. A thermal treatment method, based on repeated heating and cooling of the films, adjusted the resistance of the sensor film and optimized the sensor response to NO2.

NO2 sensitivity of WO3 thin film obtained by high vacuum thermal evaporation

Abstract The gas sensitivity, selectivity and stability properties of WO 3 thin films for the detection of NO 2 gas in the concentration range 0.2–5 ppm, have been evaluated and discussed in the light of the preparation conditions and working temperature. Thin films were obtained by evaporating high purity WO 3 powder by an electrically heated crucible at about 5 × 10 −4 Pa on sapphire substrates provided with Pt interdigital type sputtered electrodes and annealed for 1 h at 400, 500 and 600°C. The film morphology, crystalline phase and chemical composition were characterised through AFM, low angle XRD and XPS. The electrical response was measured by means of DC current mode. The annealed films showed crystallographic orientation belonging to the triclinic structure of WO 3 , while the as-deposited films were found to be amorphous. The binding energies of O 1s and W 4f confirmed the existence of the WO 3 phase, with a stoichiometric ratio close to the theoretical one. All the films showed the highest sensitivity to NO 2 at a working temperature of 200°C. The 500°C annealed film was found to be the most sensitive to NO 2 gas, compared to those annealed at 400 and 600°C. No cross sensitivity effects were found by exposing the sensors to CO, CH 4 . WO 3 films showed strong sensitivity to C 2 H 5 OH and H 2 O. Long term stability test at a working temperature of 350°C, performed by cycling the films in dry air and 5 ppm NO 2 revealed no substantial change in the electrical properties in terms of drift and sensitivity.

Investigation on the O3 sensitivity properties of WO3 thin films prepared by sol–gel, thermal evaporation and r.f. sputtering techniques

WO3 thin films have been deposited on alumina substrates provided with platinum interdigital electrodes by sol–gel (SG), r.f. sputtering (RFS), and vacuum thermal evaporation (VTE) techniques and annealed at temperatures between 500°C and 600°C for 1 to 30 h in static air. The morphology, crystalline phase and chemical composition of the films have been characterised using SEM, glancing XRD and XPS techniques. The electrical response has been measured exposing the films to O3 (10–180 ppb), NO2 (0.2–1 ppm), NOx (27 ppm NO and 1 ppm NO2) at different operating temperatures ranging between 200 and 400°C and humid air at 50% R.H. SG prepared films have shown bigger responses (S=IAir/Igas) with respect to VTE and RFS for all the investigated gases and operating temperatures. RFS prepared has resulted to be less sensitive, but faster in the response and more stable in terms of signal reproducibility. The response to O3 has been found to be at maximum at 400°C. At this temperature the response to 80 ppb of ozone has been: S=35 (SG), S=18 (VTE) and S=5 (RFS). The NO2 and NOx response reached the maximum at 200°C and becomes negligible at 400°C. Improvements on the O3 gas sensitivity and selectivity can be achieved by fixing the operating temperature of the films at 400°C.

NO2 gas sensitivity of carbon nanotubes obtained by plasma enhanced chemical vapor deposition

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition on Si/Si3N4 substrates provided with platinum interdigital electrodes have been investigated as resistive gas sensors towards NO2 oxidizing gas.The electrical response has been measured exposing the films to sub-ppm NO2 concentrations (10‐100 ppb in dry air) at different operating temperatures ranging between 25 and 215 8C. The response to NO2 has been found to be at maximum at around 165 8C. Upon exposure to NO2 the electrical resistance of randomly oriented CNTs is found to decrease. Gas sensitivity, response time and reproducibility of the electrical response resulted to be dependant from the preparation conditions and film thickness. The prepared films show fast dynamic of the electrical response and high reproducibility of the electrical properties. The resistance decrease of the CNTs when exposed to NO2 gas and the sensor response to concentrations as low as 10 ppb NO2, suggest the possibility to utilize CNTs as new sensors for air-quality monitoring. # 2003 Elsevier Science B.V. All rights reserved.

Cross sensitivity and stability of NO2 sensors from WO3 thin film

The H2O, C2H5OH, CO, CH4, NO and SO2 cross sensitivity to NO2 gas, as well as the long term stability of the electrical response of WO3 thin films have been evaluated and discussed in the light of different preparation conditions and working temperatures. Thin films have been obtained by evaporating high purity WO3 powder at 5 × 10−4 Pa on sapphire substrates provided with Pt interdigital sputtered electrodes and annealed at 500°C for 6, 12 and 24 h. The film morphology, crystalline phase and chemical composition have been characterized through AFM, glancing angle XRD and XPS. The as-deposited film is amorphous with WO3 stoichiometry on the surface, after annealing at 500°C the films are well crystallised but with preferential orientation of WO3 along the (200) plane. The increasing of the annealing time shows a positive effect on the crystallite and grain size of the film, while the mean roughness and surface area difference slightly decrease. The binding energies of the annealed films are close to that of WO3 and small downshifts from the characteristic binding energy of W 4f72 reflects the formation of oxygen vacancies on the longer time annealed films. All the films show the highest sensitivity to NO2 gas (0.7–5 ppm concentration range), at 250°C working temperature. At this temperature and 1.7 ppm NO2 the calculated sensitivities yield S = 12, S = 43 and S = 45 for 6, 12 and 24 h annealed films, respectively. No cross sensitivity has been found by exposing the WO3 films to CO and CH4. Negligible H2O cross to NO2 has resulted for the 24 h annealed film in the 40–80% relative humidity range, as well as to 300 ppm SO2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 measure. The increase in the annealing time had positive effects on the sensitivity, cross sensitivity and long term stability properties. The 45-fold increase in the resistance of the 24 h annealed on exposure to 1.7 ppm of NO2, as well as the good long term stability properties of its electrical response, suggest the possibility of utilising the sensor for air-quality monitoring.

Microstructural effect on NO2 sensitivity of WO3 thin film gas sensors Part 1. Thin film devices, sensors and actuators

Abstract Microstructures of thermally evaporated WO3 thin films on sapphire substrates are investigated by wide-angle X-ray diffraction, atomic force microscope and X-ray photoelectron spectrum. The as-deposited film is amorphous and crack-free with WO3 stoichiometry on the surface. After annealing at above 400 °C, the film is crystallized. Compared to the monoclinic phase of the starting WO3 powder, fewer peaks are evident at room temperature for the annealed film. This highlights that the film grown on the sapphire has a preferential orientation of WO3 (200), probably because of the atomic arrangement similar to the sapphire. The crystallite sizes are estimated from the major peak to be 17.5–23.9 nm according to the Scherrer equation. The increasing of annealing temperature exhibits positive effects on the surface roughness or fractal dimension, surface area and grain size of the film. However, abnormal increments in the topographical parameters occur in case of annealing at 600 °C. The binding energy of the annealed film is close to that of WO3, and a small downshift of 0.1 eV reflecs the formation of oxygen vacancies on the surface. The heterogeneity parameter of the film is introduced into the Schottky barrier equation. The highest sensitivity of the 500 °C-annealed sensor is explained in terms of the annealing temperature effect on the geometrical and chemical heterogeneities.

Effects of oxygen annealing on gas sensing properties of carbon nanotube thin films

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition have been investigated as resistive gas sensors towards NO2 oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes’ electrical resistance as determined by volt-amperometric measurements. In particular the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. The electrical response was then measured exposing the films to sub-ppm NO2 concentrations (100 ppb in air) at 165 °C. Upon exposure to NO2, the electrical resistance of CNTs was found to decrease. The obtained results demonstrate that nanotubes could find use as a sensitive chemical gas sensor for (a) the fast response accompanied by a high sensitivity to sub-ppm NO2 exposure, and (b) the precise recover of the base resistance value in absence of NO2 at a fixed operating temperature, likewise indicating that intrinsic properties measured on as prepared nanotubes may be severely changed by extrinsic oxidative treatment effects.

Carbon monoxide response of molybdenum oxide thin films deposited by different techniques

Abstract MoO3 thin films have been deposited on alumina substrates by radio frequency (RF) sputtering from a metallic molybdenum target in a reactive atmosphere and by the sol–gel (SG) technique using molybdenum ethoxide solutions. The as-deposited RF films have been annealed at 500°C for 1 h, while the SG films have been annealed at temperature range between 400°C and 700°C for 1 h. The formation of a well-developed nanoparticle structure for the RF films with respect to the SG ones was suggested by scanning electron microscopy (SEM) characterisation. X-ray diffraction (XRD) has confirmed the formation of crystalline orthorhombic MoO3 structures (JCPDS 5-508) for both the RF and SG films after annealing. The gas sensing properties towards CO have been examined. MoO3-based gas sensors developed are capable of CO down to few ppm with a very fast response.

The influence of air and vacuum thermal treatments on the NO2 gas sensitivity of WO3 thin films prepared by thermal evaporation

Abstract WO 3 films with thickness of 80 nm have been thermally evaporated onto Si 3 N 4 /Si substrates. The films have been initially treated in oxygen by a 24-h-long annealing at 300°C and 500°C. XPS measurements, to follow W 4f, O 1s peaks and the valence band, have been performed on these samples as prepared and after successive ultra high vacuum (UHV) thermal treatments. We observed that the UHV annealing procedure produces a lack of oxygen at the surface of the as deposited and 300°C annealed samples strongly modifying the W 4f peak and producing the increase of metallic states at the Fermi edge, while very few modifications have been observed in the 500°C sample. The films submitted to UHV thermal treatments have been also investigated as resistive gas sensors towards NO 2 . We observed a lowering of the base resistance and a decrease of the sensitivity properties with respect to the corresponding non-vacuum treated samples.

NO2 response of In2O3 thin film gas sensors prepared by sol-gel and vacuum thermal evaporation techniques

In2O3 thin films have been prepared by high vacuum thermal evaporation (HVTE) and by sol–gel (SG) techniques. The deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. After annealing at 500°C, the films are highly crystalline cubic In2O3. XPS characterization has revealed the formation of stoichiometric In2O3 (HVTE) and nearly stoichiometric In2O3−x (SG). SEM characterization has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO2 gas (0.7–7 ppm concentration range), at 250°C working temperature. Negligible H2O cross has resulted in the 40–80% relative humidity range. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 response.