Room temperature polymeric gas sensor for the detection of toxic gases

Abstract

In this thesis, polyaniline nanofiber based flexible gas sensors have been developed for the detection of toxic environmental gases. Several substrates such as rigid silicon dioxide, polyimide and paper have been fabricated as sensor platforms. Gold interdigitated microelectrodes have been fabricated on silicon dioxide and polyimide substrates by photolithography and e-beam evaporation technique (lift-off process). Polyimide film with 15 micron thickness has been fabricated by spin coating. Simple filter papers have been used and silver and carbon electrodes have been fabricated on the paper by screen printing method. Electrospinning has been used as the principal method of synthesizing the sensing material (polyaniline nanofibers). Polyaniline Emeraldine Base (EB) doped by camphor-sulfonic acid (HCSA) was used as the precursor. Two types of polymeric binders were used, polystyrene (PS) and polyethylene oxide (PEO). The effect of using a secondary solvent, tetrahydrofuran, along with the primary solvent, the chloroform, has been investigated by UV-Visible spectroscopy. The sensors were fabricated by a simple and low-cost procedure. The sensor platforms were directly placed in front of the syringe needle of the electrospinning setup and high quality nanofibers were deposited directly on the interdigitated electrodes that were fabricated on the substrates beforehand. After electrospinning and post-processing, the sensors were taken to the gas sensing test chamber to test their performance in sensing the target gases. The target gases were common toxic environmental gases, hydrogen sulfide ($H_2S$), nitrogen dioxide ($NO_2$) and ace-tone. The sensors have shown good sensitivity, low detection limit and very good recovery time compared with the previously proposed sensors based on PAni. Several characterization techniques have been undertaken to analyze the morphology and properties of the synthesized nanofibers. The electrical resistance of the sensors has been measured by a semiconductor parameter analyzer. Several SEM images were taken from each sample and these images have been used to investigate the effects of processing parameters (such as binder's concentration) and environmental parameters (such as humidity) on the morphology of the nanofibers and their gas sensing performance. Energy Dispersive (EDS) and Fourier Transform Infra-Red (FT-IR) spectra of the nanofibers were taken in order to determine the elements and components existing in the nanofibers web. Several complementary tests have been done to examine the dependence of the performance of the sensor to the environmental humidity and the mechanical robustness and flexibility of the sensors. Stability of the sensor’s response over time has been tested and sensor’s detection limit was determined. Simplicity of the synthesizing method, together with the cheap and flexible substrates provide a low-cost and room temperature gas sensor that is unique in terms of sensitivity, detection limit and recovery time.