Abstract

Solid state chemical sensors are gaining popularity and finding extensive use in process control, environmental monitoring and residential safety. ZnO, a semiconducting metal oxide, and carbon nanotubes (CNTs) have attracted great interest over the years for their sensitivity to a variety of gases. Nanostructured sensing materials, such as nanowires, nanotubes and quantum dots offer an inherently high surface area, thus reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this work, ZnO nano-structures and 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 μm. 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 200oC. Values on the order of 60%, 42% and 29% for 315 PPM of CO, O2 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. Our future works entails correlating the sensitivity of the gas sensors to the material properties in addition to the device and the process parameters, with further development in methods for fabricating gas sensors and improvement in the selectivity of the sensor. For CNT based sensors, using as-grown multiwall carbon-nanotubes MWCNTs for gas sensor fabrication would be the next step in this research. In addition to developing standard fabrication techniques, further research is required for improving selectivity for different gases and organic solvents by decorating or filling CNTs with metal nano-particles or different groups of organic molecules. Also, future work will be focused to correlate sensitivity of HiPCo produced SWCNTs, Laser ablated SWCNTs and MWCNTs to their material properties.

Library of Congress Subject Headings

Chemical detectors--Materials; Chemical detectors--Design and construction; Zinc oxide; Carbon; Nanotubes; Nanostructured materials

Publication Date

2009

Document Type

Thesis

Department, Program, or Center

Microelectronic Engineering (KGCOE)

Advisor

Hubbard, Seth

Advisor/Committee Member

Fuller, Lynn

Advisor/Committee Member

Raffaelle, Ryne

Comments

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: TP159.C46 S35 2009

Campus

RIT – Main Campus

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