Mechanism of NH3 gas sensing by SnO2/PANI nanocomposites: charge transport and temperature dependence study

Abstract

Metal oxide-Polyaniline (PANI) nanocomposites have shown improved gas sensing characteristics that can be attributed to the formation of a p–n junction between the n-type metal oxide and the p-type PANI. The charge transport, grain boundary depletion region, and intragrain resistance are studied to understand the gas sensing mechanism of pristine metal oxide gas sensors. However, gas sensing mechanisms for metal-oxide/PANI nanocomposites have not been studied extensively. In this work, we have studied the gas sensing mechanism of SnO2/PANI nanocomposites using electrochemical impedance spectroscopy, and temperature dependent gas sensing experiments. Well-defined SnO2 nanoclusters were observed in the PANI matrix. The n-type SnO2 was covered by p-type PANI, and a depletion region was formed at the interface. The presence of the p–n junction depletion region was confirmed by impedance spectroscopy. The polarons in PANI were trapped by NH3 leading to a 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. For higher loadings of SnO2, the depletion region was deformed, and the sensor response decreased due to non-uniform boundaries. 1 wt% SnO2 with respect to aniline precursor in in situ synthesis showed the best response of 37.8% for 100 ppm NH3 at 35 °C. The response was stable for low humidity levels up to 51%RH. The response increased for higher humidity levels. The sensor response increased from 0.17 to 2.99 upon bending 1000 times at 7.8 mm diameter due to cracks in the surface. The sensor showed only 10% variation in response after 9 months.