The characterisation of thin films of polyaniline for gas sensing

Abstract

This project is concerned with the preparation of high quality polyaniline thin films, which can be used as a gas sensor. A 5% solution of emeraldine-base polyaniline (the insulating form) was dissolved in N- methyl-2-pyrrolidinone (NMP) and left for 48 hours, before either being centrifuged three times (and decanted) or being homogenised. These methods ensured the removal of any lumps. The solution was then spun onto circular glass or sapphire substrates (which had been cleaned in isopropanol for one hour) and the spinning conditions could be altered on the programmable spinner. Thin films (the order of micrometers) were spun and the whole process was carried out in a clean room environment. Polyaniline-coated interdigitated electrodes measured a change in resistance upon exposure to NO(_x) gas. Certain parameters (i.e. gas concentration, response time, recovery time) were varied and the response of three sensors (of different thickness) were noted. The results suggested that the change upon exposure was due to a surface effect and the best sensor was found to be 0.19µm thick. Optical experiments carried out on the spun films (typically in the order of 0.2µm thick) found that at 2.7eV, the absorption coefficient of polyaniline was l.7xl0(^5)cm(^-l). Following on from this, certain energy levels could be measured accurately. The optical band-gap was found to be 1.47eV and also, other transitions were measured at 3.19eV and 5.56eV.