Engineered metal oxide nanostructures for highly sensitive and selective toxic gas sensing

Abstract

This thesis presents a comprehensive study on the effects of surfactant-assisted synthesis and doping on transition metal oxides (TMOs), and their influence on toxic gas sensing performance, aimed at the development of highly selective and sensitive gas sensors. The optimization and characterization of the fabricated TMOs are specifically focused on enhancing sensing performance in terms of sensitivity, selectivity, response and recovery times, and long-term stability. Among various metal oxide sensing materials, cobalt oxide (Co3O4) has emerged as a promising candidate due to its excellent catalytic activity, chemical stability, and favourable electronic properties. Co3O4 is a p-type semiconductor that exhibits notable gas sensing behaviour toward different target gases. In this thesis, CTAB-assisted molybdenum (Mo)-doped Co3O4 nanostructures have been synthesized using a hydrothermal method and investigated as a sensing material for the detection of carbon monoxide (CO) gas. Carbon monoxide is a highly toxic and colourless gas that poses serious health hazards even at low concentrations, making its accurate detection extremely important. To optimize and further improve the sensing performance, different concentrations of Mo dopant were incorporated into the Co3O4 structure, and a surfactant (CTAB) was introduced during the synthesis process to assist in controlling the nanostructure morphology. The sensing characteristics were systematically investigated, and the results indicate that the sensor performance strongly depends on the synergistic effect of Mo doping and CTAB-assisted nanostructure formation. Among the synthesized samples, the CTAB-assisted 2.5 mol% Mo-doped Co3O4 sensor exhibited the best sensing performance toward CO gas, showing an exceptionally high response of 4188% at a CO concentration of 100 ppm. Furthermore, the fabricated sensor demonstrated a remarkably low limit of detection of 4 ppb, indicating its capability to detect trace levels of CO gas. The sensor also exhibited excellent repeatability and long-term stability. In addition, the sensing performance remained reliable under humid environmental conditions and the sensor displayed relatively short response and recovery times.