WO3-based Gas Sensors Research Articles

A dynamic response model for the WO3-based ozone sensors

In this paper, a dynamic model based on the Wolkenstein adsorption theory is developed for temperatures equal to or less than 250 °C and non-dissociative adsorption. We assumed that the ozone detection mechanism of WO3-based gas sensors in dry air is essentially due to adsorption of species O2, O2−, O and O− at the surface of the grains. The simulation results are compared to the experimental ones. The correlation between simulated and experimental results is rather good. The model is then extended to the dissociative adsorption and higher temperatures. At temperature higher than 300 °C, an increase in the recovery time appears on experimental results and cannot be explained by a simple adsorption/desorption mechanism.

A WO 3-based gas sensor array with linear temperature gradient for wine quality monitoring

In this work we describe the design implementation, validated by experimental results, of an innovative gas sensor array for wine quality monitoring. The main innovation of this integrated array deals with the simultaneous outputs, from a single chip on custom SMD socket, of eight different signals coming from a WO 3 thin film structure heated in a linear temperature gradient mode, allowing an overall evaluation of gas sensing properties of the material in a 100 °C-wide window, typically from 300 to 400 °C. The implemented sensitive layer is a WO 3 film deposed by RF-sputtering. Preliminary tests of gas sensing showed good responses to the target analytes for the specific application (1-heptanol, 3-methylbutanol, benzaldehyde and ethyl-hexanoate).

WO3 films modified with functionalised multi-wall carbon nanotubes: Morphological, compositional and gas response studies

Active layers for gas sensing applications have been obtained by adding oxygen plasma functionalised multi-walled carbon nanotubes (MWCNTs) to WO3 and using the drop-coating deposition method. Scanning electron micrographs showed that MWCNTs are well dispersed on the WO3 matrix. X-ray photoelectron analysis showed that by adding MWCNTs to the WO3 films the contribution of the doublet related with surface defects to the total W 4f signal increased. This gives evidence that the surface area of WO3 films increases due to the addition of MWCNTs to the film. Two different ratios of MWCNTs in WO3 (1/100 and 1/1000) were considered and the response of these sensors towards toxic gases such as nitrogen dioxide, carbon monoxide and ammonia was compared with that of WO3 and MWCNTs gas sensors. The results indicate that the detection at ambient temperature of these toxic gases using WO3-based gas sensors can be highly improved by dispersing an adequate quantity of MWCNTs into the WO3 matrix. A possible mechanism explaining the behaviour of hybrid sensors is introduced.

Optimised temperature modulation of metal oxide micro-hotplate gas sensors through multilevel pseudo random sequences

We show how it is possible to optimise a multi-frequency signal to be used in the modulation of the operating temperature of an integrated gas sensor microarray. In the first step, a multilevel pseudo random sequence (ML-PRS), which allows for modulating the operating temperature of the sensors in a wide frequency range, is used to obtain an estimate of the impulse response of each microsensor-gas system. ML-PRS are interesting because they help to reduce the effects of noise and non-linearity as experienced with gas sensors. In the second step, by analysing the spectral components of the impulse response estimates, the modulating frequencies that better help in discriminating and quantifying the gases and gas mixtures considered are found. Finally, by selecting a subset of the best modulating frequencies, an optimised multi-frequency temperature-modulating signal can be synthesised. The method is illustrated by analysing different concentrations of NH 3, NO 2 and their mixtures using a microarray of WO 3-based gas sensors, but it can be further applied to any given gas analysis problem.

Nanomaterials for environmental applications: novel WO3-based gas sensors made by advanced gas deposition

Nanomaterials for Environmental Applications: Novel WO3-based Gas Sensors made by Advanced Gas Deposition

Effects of NiO addition in WO 3-based gas sensors prepared by thick film process

NiO-doped WO 3 thick films were prepared by the screen-printing technique. The microstructure and the electrical properties were investigated as functions of the amount of NiO, partial pressure of oxygen, concentration of NO 2 and temperature. The grain growth is inhibited by the addition of NiO. The electrical conductance of undoped WO 3 is high at low partial pressure of oxygen. The electrical conductance of NiO-doped WO 3 in air increases with NiO content of up to 1.0 mol%, and then it decreases. The sensitivity for NO 2 gas increases for samples with 1.0 mol% of NiO, and then it decreases for 10 mol% NiO samples.

Influence of binders on the sensing and electrical characteristics of WO 3-based gas sensors

The effects of various binders added in the solidification process, such as polyvinyl alcohol (PVA), silica sol and Al 2O 3, on the sensing and electrical characteristics of WO 3-based n-type semiconductor gas sensors are investigated. Grain sizes show a slight variation according to the employed binders. In the case of WO 3 films fabricated with silica sol or Al 2O 3 binder, some residue of binders at grain boundary and agglomerates of WO 3 grains are observed. The electron concentration of WO 3 film around room temperature and the temperature dependence of film resistance in normal air do not show any systematic dependence on the employed binders. In NO x gas, however, the optimum operation temperature and the sensitivity of WO 3 sensors at that temperature are observed to depend greatly on the employed binders. The resistance of the WO 3 films shows an exponential temperature dependence in NO x gas in the temperature range of 110–375°C, and the increase of film resistance in NO x gas is observed to depend greatly on the binders added in WO 3 films. Sensitivity to various ambient gases does not show any systematic variation. All these results mean that the binders affect the sensing characteristics of WO 3-based gas sensors mainly through the modification of chemical conditions at grain boundary rather than the modification of grain size and electron concentration.

NO x gas sensing characteristics of WO 3 thin films activated by noble metals (Pd, Pt, Au) layers

WO 3-based gas sensor devices have been fabricated by reactive rf sputtering on glass substrates. Palladium (Pd), platinum (Pt), gold (Au) noble metals (100–600 Å) were evaporated as activator layers onto WO 3 thin films and Al layers (1500 Å) were sputtered on top of them as upper electrodes for sensor output. The described sensing element was found to possess excellent sensitivity towards NO and NO 2 gases, at low temperatures (100–300°C). The optimum operating temperature was 150 and 200°C for Pt, Pd and Au, respectively. The resistance of the thin-film gas sensor increases reversibly in the presence of low concentrations of NO (0–440 ppm) and NO 2 (0–10 ppm) gases in air. The activator layers have an promotional effect on the speed of response to NO x at low temperatures and on selectivity enhanced with respect to other reducing gases (CO, CH 4, H 2, SO 2, H 2S, NH 3). The temperature dependence of response and recovery times has been examined. The influence of the thickness of the noble metals on NO x gases sensitivity has been investigated. The NO and NO 2 gases calibration curves have been recorded for the optimum performances of each fabricated sensor-device.