A parametric study of gas sensing response of zinc oxide nanostructures and carbon nanotubes

In this work, ZnO nano-structures and carbon nanotubes (CNTs) have been tested as chemical sensors and a detailed study on the effect of different process parameters such as temperature, carrier gas flow, inter-electrode spacing, gas concentration and material properties on gas sensitivity is presented.;Initial ZnO nanoparticles were prepared by a simple solution chemical process and characterized by Secondary Electron Microscopy (SEM) and Brunauer, Emmet and Teller (BET) Sorptometer to demonstrate the morphology and surface area respectively. The gas sensor platforms consisted of Pt inter-digitated fingers with a spacing of 10 &mgr;m. The sensor platform was dip-coated with ZnO nano-platelets suspended in terpineol to form a uniform film. Sensing was performed in a closed quartz chamber where, high purity N2 and dry industrial air were used as carrier and recovery gas respectively. Sensitivity of nano-platelets and porous films was measured for different concentrations of the analyte (H2). High response was observed at room temperature for H2 gas with sensitivities in excess 80% for 60ppm and about 55% for 80ppm of H2 gas at room temperature was observed for the nano-platelets and the porous films respectively. High sensitivity of the sensor at low temperatures is attributed to both the increased surface area of the porous ZnO nano-platelets and the presence of a Pt catalyst. Measurements at higher temperatures (150 °C) show even higher sensitivities, near 96% for a 20 ppm H2 concentration. Sensitivity with different gases and organic solvents was also measured at operating temperatures of 200°C. Values on the order of 60%, 42% and 29% for 315 PPM of CO, O 2 and NH3 whereas sensitivity values of 77.76%, 70.26% and 38.43% for C2H5OH, CH3OH and H2O were recorded for concentration values approximating 500 PPM. The sensors depict incomplete recovery of resistance at room temperature. This effect is possibly due to the traces of elemental Zn in the material, which were not oxidized at the time of recovery. However, this effect was not observed at higher temperatures.;Designed experiments conducted to understand effects of various device and process parameters show negative dependence of spacing on sensitivity with maximum effect of applied bias for lower concentration values. The sensitivity of the sensor was also recorded to increase with the increase in the number of electrodes. Higher sensitivity values nearing 70% were achieved with 30 IDEs for 60 PPM H2 when compared to 60% for 60 PPM of H2 with 20 IDEs. Interaction effects were observed and implemented to understand and model the behavior of the gas sensor.;Sensitivity of arc produced CNTs was measured to various gases and organic solvents. Values on the order of 24% were observed at 80 PPM CO as compared to values of sensitivity about 15% for O2 and 3% for H2. Also, sensitivity value of 15% was measured for as low as 4 PPM of DMA which suggests the capability of PPB levels of DMA using CNTs. A brief comparison of sensitivity values achieved for ZnO nano-platelets and CNTs with similar analytes was also presented.;Sensitivity to different analytes was measured using impedance spectroscopy for HiPCo produced SWCNT network. For experiments conducted with varying exposure time, sensitivity values nearing 6% for 0.01% (100 PPM) DMA for an exposure time of 25 minutes were recorded. Sensitivity values recorded for other solvents were 16.74%, 10.98%, 7.97%, 6.96% and 4.28% for concentration levels of 2.04%, 4.02%, 2.04%, 14% and 6.05% of NH3, IPA, CO, CH3OH and C2H5OH respectively. For experiments with varying concentration values of different analytes, higher response was observed for gaseous analytes. Results on the order of 15.27% and 3.82% were recorded for as low as 0.18% of both NH3 and CO. For the organic solvents, values approximating 2.64%, 2.36% and 0.10% for concentration levels of 0.29%, 0.92% and 0.42% of IPA, CH3OH and C2H5OH respectively. Results obtained with HiPCo produced SWCNT network at room temperature were comparable to the values of sensitivity shown by other researchers. (Abstract shortened by UMI.)...