Nitrogen doped graphene quantum dots modified polyaniline for room temperature alcohol sensing

Masemola, Clinton Michael
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The development of flexible, affordable and reliable sensors for the detection of volatile organic compounds (VOCs) has become increasingly important. This study focuses on the fabrication of a flexible room temperature VOCs sensor containing nitrogen-doped graphene quantum dots (NGQDs) and polyaniline (Pani) composite as the sensing layer. The synthesis of NGQDs was carried out via a microwave-assisted hydrothermal method where citric acid and urea were used as carbon and nitrogen sources respectively. Various reaction times were employed in order to determine the optimal time and the products were characterized using transmission electron microscopy, X-ray diffraction, Fourier transform infra-red spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, UV-Visible spectroscopy, thermal gravimetric analysis and X-ray photoelectron spectroscopy. Uniformly distributed and spherical NGQDs were achieved after a 4 minutes reaction time and these dots were further loaded onto Pani to produce NGQDs/Pani composites via in-situ polymerization. Various percentage loadings of NGQDs on Pani were studied which produced composites with different morphological outlines and porosity. The optimum composite was further processed by the electrospinning technique in order to achieve composite fibers with enhanced structural and surface properties. Optimized electrospun polymer composite fibers were characterized and later used for ethanol vapor sensing. Simple and room temperature operable sensors based on bare Pani, NGQDs/Pani and electrospun NGQDs/Pani/PAN were designed on gold-plated interdigitated electrodes (IDEs) embedded on a printed-circuit board (PCB) substrate. The sensors showed a good response towards 50 –150 ppm ethanol vapors at room temperatures and relative humidity of 45 %. The loading of NGQDs on Pani promoted a better response signal and sensitivity, and improved the baseline resistance of the composites which was attributed to the enhanced conductive pathways between Pani and NGQDs. The sensor reached the lowest response and recovery times of 122 s and 99 s towards 100 ppm of ethanol. The electrospun composite based sensor showed a sensitivity towards ethanol vapors that was significantly higher (by up to 6 folds) compared to other sensors. A short response time of 70 seconds was also recorded for the electrospun composite based sensor towards 100 ppm of ethanol vapor. All sensors exhibited a good reproducibility of the response signal towards 100 ppm of ethanol vapor and a sensing mechanism was proposed
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the award of the degree of Master of Science in Chemistry, 2021