| dc.description.abstract | Due to the demands of the current age gas sensing devices to satisfy humanity and environmental requirements, sensors with arrangement of excellent sensitivity and selectivity, prompt response-recovery times, low operating temperature, reproducibility, insignificant interference from ambient humidity, and long-term stability are desired. This is motivated by the global gas sensor market estimated at USD 2.19 billion in 2019 and a compound annual growth rate of 8.3% increase expected form 2020-2027. Moreover, according to studies, more than 5 million deaths related to air pollution are reported annually. In South Africa alone, more than 20000 deaths are reported yearly. Therefore, this study investigates the low and room temperature (RT) gas sensing properties of pure TiO2 nanostructures such as nanotubes, nanowires and hierarchical spheres and that doped with various Mn concentrations towards detection of toxic gases and volatile organic compounds for indoor and outdoor air quality monitoring for health and security purposes. Our findings show that designing and tuning of nanostructure morphology has significant impact on the structural, optical and sensing properties of TiO2. Thus, herein through a simple hydrothermal method and followed by washing with various concentrations of Hydrochloric acid (HCl) and distilled water (DW) we achieved a notable surface area of 1375 m2/g with a change in morphology from nanoparticles to nanotubes. The gas sensing characteristics, such as response, sensitivity and selectivity conducted towards CH4, NH3, CO and NO2 gases at different operating temperatures, demonstrated higher selectivity towards CH4 gas at 23 °C. This was behaviour was attributed to the high surface area and crystallinity of displayed by the multi dimensional nanotubes acting as nanochannels for gas diffusion. Furthermore, through annealing we tuned the selectivity of TiO2 towards NO2, the sensor displayed an enhanced sensitivity of 29.44 ppm-1 and admirable selectivity towards NO2, among other interfering gases, ensuring adequate safety in monitoring NO2 in automobiles and households. This study has further afforded a breakthrough to design for the first time the dual-functionality sensor for detection of toluene (C7H8) and xylene (C8H10) from TiO2 nanowires operating at room and low temperature, for low power consumption purpose and such sensor is a striking platform for economic and indoor air quality monitoring. To the best of our knowledge, a temperature-dependent selectivity dual sensor operating a room temperature and 125 °C has never been reported. By testing the sensor in various relative humidity, the findings validated that the current sensor can be used for detection of C7H8 and C8H10 in a vastly sensitive and selective way with insignificant interference from ambient humidity. Besides, through annealing, we further showed that the selectivity of the nanostructures can be tuned, due to exposed facets containing plentiful active oxygen species which are more activity for adsorption. Thus, the superior sensitivity towards toluene and ethylbenzene among other aromatic hydrocarbon VOCs, was exhibited by thermally treated sea urchin like TiO2 hierarchical spheres. The T5 sensor (i.e. annealed at 500 °C) displayed temperature-dependent selectivity towards ethylbenzene at 75 ºC. The T7 sensor (i.e. annealed at 700 °C) revealed the highest response of 13 towards toluene at 150 ºC, which could be justified by exposed high surface energy {001} facets. The sensor illustrated a robust stability towards relative humidity (10-90%), which is very vital for practical and real-time applications in ambient conditions. In addition, by utilizing a simple synthesis strategy that permits easy and reproducible results, we prepared manganese (Mn) doped TiO2 hierarchical structure having recorded a highest surface area reported. This process indicated that the improvement of the surface area with an increase in Mn doping concentration shows a clear dependency on the doping concentration, resulting to a record surface area of 434714 m2/g and pore volume of 27.85 cm3/g. A gas sensor based on optimized Mn doped TiO2 established a poor response towards reducing gases (CO and CH4), volatile organic compounds (C7H8, C3H7OH, C6H6, E-C6H6 and C8H8), while tested to C2H5OH vapour, unprecedented response, improved sensitivity and selectivity was witnessed at 100 °C. This was due to a remarkable surface and abundant porosity allowing more gas adsorption. The Mn4 sensor showed a strong repeatability over 10 cycles at 100 ppm C2H5OH after 20 days, without showing any drift, confirming its robust stability, which could be useful for detection of C2H5OH in real applications. Finally, this work has been adopted for intellectual property and an innovation disclosure has been granted (Disclosure number: T18/19-00029), while a United States patent is currently being filed. | en_ZA |
