Electrospun Metal Oxide Nanofibers for High Temperature Gas Sensing

Sensors for monitoring gas composition in high temperature environments are of paramount importance to improve combustion efficiency and control emissions. There is an urgent demand to develop miniaturized, robust and cost-effective high temperature gas sensors with good thermal stability, high sensitivity and selectivity. This dissertation aims at developing thermal stable nanostructure enabled resistor-configured gas sensors for in-situ and real-time gas detection at high temperature above 800 oC with good sensitivity and selectivity. Electrospinning technique was used to fabricate metal oxide nanofibers due to its efficient and facile process to generate one-dimensional nanostructure in large scale. Perovskite La0.67Sr0.33MnO3 (LSMO) and CeO2 nanofibers were first fabricated by electrospinning followed by a calcination process. Both of LSMO and CeO2 nanofibers exhibited excellent thermal stability in terms of both morphology and chemical composition at 1000 oC. These two materials were first employed as sensing materials in resistive sensors to in-situ and real-time detect O2 at high temperature (800 – 1000 oC), which showed good sensitivity, recoverability and reproducibility. In addition, CeO2 nanofibers-based sensor also demonstrated its excellent sensitivity towards CO. To improve the selectivity of resistor-configured gas sensors for reducing gas detection, three different approaches were exploited. First, impedance spectroscopy was employed to investigate the impedance profile changes of Pt-CeO2 nanofibers in different gas atmosphere (N2, O2, CO, CO2, SO2, NO, C3H8) at 800 oC. Equivalent circuit analysis was conducted to investigate the sensing mechanisms. By operating the sensor at a high frequency (e.g., 100 kHz), the sensor can selectively detect strong reducing gas (CO and C3H8) and eliminate the interference from all oxidizing gases and weak reducing gases. To further improve the selectivity within the reducing gas group, p-LSMO NFs/n-CeO2 NFs heterojunction composites were prepared by sonication and systematically investigated. The optimal LSMO-CeO2 NFs composite with CeO2 NFs content of 80% showed the good sensitivity as well as the improved selectivity to C3H8 over other reducing gases such as CO and CH4 at high operation temperature of 800 oC. Lastly, Ce-Ni-O composite nanofibers were fabricated by electrospinning and subsequent calcination and showed an excellent sensitivity and selectivity towards C3H8 and negligible response to CO and CH4, due to the fast reaction kinetics between Ce-Ni-O nanofibers and propane. The sensing mechanism was also proposed. By designing novel materials and employing impedancemetric technique, nanostructure enabled gas sensors with good stability, high sensitivity and selectivity for reducing gas detection were successfully demonstrated. This dissertation opens an avenue in the design of high temperature gas sensor with high performance