Improvement of conducting polymer gas sensors

Abstract

The use of gas sensors is rapidly increasing, along with the number of candidate materials for gas sensing applications. In order to improve their competitivity, conducting polymer gas sensors have to be constantly improved. This work on conducting polymer chemoresistive gas sensors consists in improving the quality regarding three aspects: the substrates, the sensitive layer itself and the sensing mechanism of these devices.A new array of micromachined devices was developed, comprising a new design (array of 4 discreet devices) and a new electrode material (platinum). This array was found to be more suitable for the requirements of our electronic nose applications. The electrochemistry of the electrode material was also employed to check the quality and to condition the substrates prior to every single electropolymerisation, which was reflected by an increase in quality.Poly(pyrrole) and poly(aniline) doped with alkyl sulfonate were investigated as well as new coatings doped with aromatic sulfonate or metal phthalocyanine sulfonate. Their sensitivity towards CO and NO2 was evaluated for possible applications in the automotive industry. The reproducibility from sensor to sensor was also addressed. Monitoring in-situ the resistance of the film during the electrodeposition, employing a technique specially designed for this purpose led to significant improvement in the reproducibility but also in the response time and in the stability over time.The conducting polymers gas sensors were also tested towards ethanol and water, both as coatings for chemoresistive devices and QCM sensors. The aim was to get a deeper insight into the sensing mechanism of these gas sensitive materials in order to be able to engineer a coating for a given vapour. Following conductivity, viscoelasticity and mass upon exposure demonstrated that a Langmuir adsorption isotherm model was not suitable. A new model was defined, a double-diffusion model, where the vapour diffuses rapidly in the pores of the polymer before penetrating in the material itself, explaining the large response time of the conducting polymer gas sensors.</p