Investigation of nanostructured conducting polymers and graphene/polyaniline nanocomposite based thin films for hydrogen gas sensing
Creators: name: Al-Mashat, Laith
Publisher: name: RMIT University
Year: 2024

description: Novel conductometric and layered Surface Acoustic Wave (SAW) hydrogen gas sensors based on nanostructured conducting polymers and graphene/polyaniline nanocomposite are reported in this PhD dissertation. Template-free electropolymerization and/or chemical polymerization methods were employed during the synthesis of the nanostructured polythiophene, polypyrrole, polyaniline, polyanisidine, polyethylaniline and graphene/polyaniline nanocomposite which were investigated for their hydrogen gas sensing characteristics. The nanostructured gas sensitive films' physical and chemical properties were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, Ultraviolet-visible spectroscopy (UV-Vis), and X-ray photoemission spectroscopy (XPS). A programmable gas calibration and data acquisition system was utilized to measure the sensors' responses towards several concentrations of hydrogen gas at room temperature.<br><br>A comparative study on the performance of conductometric hydrogen gas sensors based on electropolymerized polythiophene nanostructured films was conducted for the first time in this thesis according to the best of the author's knowledge. Polythiophene gas sensitive films featuring nanofibers with diameters of 10-40 nm were successfully electrodeposited on conductometric transducers. Electropolymerization parameters such as counterion's type, the concentration of the electrolyte, the electropolymerization potential and the deposition time were shown to affect the morphology of the gas sensitive film and ultimately its response towards hydrogen gas.<br><br>Electropolymerized polypyrrole nanowires with diameters of 40-90 nm and chemically polymerized polypyrrole nanofibers of 18 nm in diameter were employed for hydrogen gas sensing for the first time herein. The effect of polypyrrole's counterion type on the resultant gas sensor electrical characteristics was investigated herein. Via a comprehensive investigation, it was found that chemically synthesized chloride-doped polypyrrole nanofibers to be more sensitive to hydrogen gas than the perchlorate-doped electropolymerized polypyrrole nanowires due to the smaller molecular size of the incorporated counterion into the polymer matrix. <br><br>Novel layered SAW gas sensors based on polythiophene nanofibers, polypyrrole nanofibers, nanoporous polyaniline, polyanisidine nanofibers and polyethylaniline nanofibers were developed and tested at room temperature. The highest response was observed for the polyanisidine nanofibers/ZnO/36° YX LiTaO<sub>3</sub> SAW gas sensor with a 294 kHz frequency shift from the centre frequency upon exposure to hydrogen gas with the concentration of 1% in ambient air. Morphological analysis of the deposited polyanisidine nanofibers based thin film revealed that the nanofibers, ~55 nm in diameter, were not densely packed that allows deep and efficient penetration of target gas molecules into the sensitive film and makes gas sensing possible over the entire length of a nanofiber into a mesh.<br><br>To the best knowledge of the author of this dissertation, the first ever reported hydrogen gas sensor based on graphene/polyaniline nanocomposite was developed and characterized for the first time in this PhD program. This sensor outperformed hydrogen gas sensors based on polyaniline nanofibers at room temperature. After analysing the nanocomposite's characterization results, the author of this thesis suggested that the observed high response is attributed to the graphene/polyaniline nanocomposite's high surface area compared to that of the pure polyaniline nanofibers due to the growth of polyaniline nanofibers in the order of 25-50 nm in diameter on the graphene nanosheets' surfaces.<br>