Polyaniline Based Gas Sensors Understanding of Sensing Mechanisms and Performance Enhancement with Nanocomposites and Device Structure

Abstract

Gas sensors have emerged as novel tools in healthcare diagnosis, agriculture, food packaging and safety, automobiles, etc. Metal oxide based conductometric gas sensors operate at high temperatures (250-400 °C), which consume extra power, and limit portability as well as wearable applications. Polyaniline (PANI), a conjugated polymer, has shown gas sensing capabilities at room temperature (~20 °C), however, the sensor characteristics such as sensitivity, response time, and selectivity have scope for improvement. The sensor characteristics can be improved by either complex formations with metal oxides and metal nanocomposites, or with device structure such as field effect transistors (FET). In the first study, the effects of the environment, molecular weight, and chain conformation of PANI on H2S gas sensing characteristics were studied. The sensor response was found to be influenced by the moisture in the environment because of the moisture dependent chemical behaviour of H2S gas. Molecular weight of PANI controlled the chain conformation, which affected the specific surface area, pore size, surface heterogeneity, concentration of active sites, energy level gap of the material. In the second study, SnO2 nanoparticles were embedded in PANI thin film matrix, and a p-n junction with a depletion layer was formed at the PANI-SnO2 interface. Upon exposure to NH3, the polarons in PANI were trapped by NH3 leading to change in the width of the conducting path due to rearrangement of charge carriers along the depletion region. The change in the conduction path along with the trapped polarons enhanced the sensor response. The response for NH3 increased with temperature due to charge carrier redistribution and change in the depletion region; however, the response decreased for higher temperatures due to excessive number of charge carriers in the PANI matrix. Therefore, the maximum response was observed at 35 °C. 1 wt% SnO2 with respect to aniline precursor, has shown the best response of 0.35 for 100 ppm of NH3,