Design of gas sensors using carbon nanotubes

Abstract

This thesis is concerned with the structural, chemical and physical properties of carbon nanotubes (CNTs) and composites of CNTs with conductive polymers and DNA. The application of these composites in electronic sensors for volatile organic compounds VOCs, carbon monoxide and ozone gas was also investigated. CNTs are promising materials for a gas sensor because of their unique properties; small size, large specific surface area and high aspect ratio. However, the conductance of bare CNTs gives a small response to many analytes and therefore the formation of CNT composites with other polymeric materials to enhance the sensing performance was explored. The first part of the study is based on coating CNTs by polypyrrole and using these composites to detect volatile organic compounds (VOCs: methanol, ethanol, acetone and chloroform). Polypyrrole (Ppy)-coated CNTs were prepared by an in situ oxidative polymerization method with FeCl3:6H2O as the oxidant. TEM and AFM images showed a significant change in the diameter of the nanotubes upon polymerization from the mean value of 10 nm for multi walled carbon nanotubes (MWCNTs) to 60 nm after coating and from 5 nm to 50 nm for single-walled carbon nanotubes (SWCNTs). FTIR and Raman spectra indicated successful coupling between CNTs and polypyrrole. In addition, I-V characterization of two terminal nanotube devices and impedance spectroscopy demonstrated the change in the electrical properties of drop-cast CNT films after coating with polymer. The electrical current decreased after coating for both MWCNTs and SWCNTs (8 mA to 0.027 μA) and (10 mA to 88 μA) at an applied voltage of 2 V. Uncoated CNTs had a small analytical sensitivity (S), where S is ii defined as the percentage change in resistance upon exposure to analyte. For 12.9 kPa of methanol vapour, typically S < 1% for bare CNTs, while the sensitivity of the nanocomposites was typically S > 50% for 12.9 kPa of MeOH at room temperature. The sensing mechanism was found to be reversible and the temperature dependence could be analyzed using a simple extension of the Van’t Hoff equation. This suggests that the temperature dependence of the sensitivity is controlled by the enthalpy of adsorption on the composite. The second part of this study used CNTs/boron nitride nanotube (BNNTs) composites as an ozone gas sensor. Ozone is a powerful oxidant and polymer additives are not sufficiently robust for this application. CNT/BNNT films were prepared by drop-casting from equimolar solutions of BNNTs/methanol and CNTs/methanol. The electrical properties of drop-cast CNTs were changed after adding the insulating BNNTs; the electrical current decreased from 8 mA to 1 mA at applied voltage of 2 V. The sensitivity was improved from 18% to 50% for 80 ppm of ozone. However, the problem with the CNT ozone sensor was a long recovery time which can be 25 min or more, depending on the gas concentration. For CNTs/BNNTs the recovery time was shorter, but still lies between (2-17) min at room temperature. The third part of the study was related to detection of CO gas by CNTs/Ppy at room temperature. It has been shown that the sensitivity of CNTs is enhanced after polypyrrole coating: > 20% for SWCNTs/Ppy and < 2% for SWCNTs at 1923 ppm CO in air. Again, the sensitivity of these nanotube composites decreased with increased temperature according to an adsorption equilibrium model.The last part of this study evaluated DNA@CNT composites as a VOCs sensor. Three samples of CNTs/DNA were prepared with three different amounts of -DNA 2μL, 5 μL and 10 μL of (500 μg mL-1 ) which were added to 50μL (0.001 mg mL-1 ) of an aqueous dispersion of CNTs. DNA@CNT Films were drop cast across microelectrodes and from I-V measurements, it was found that the current (at a bias of 2 V) decreased after coating with increasing amounts of DNA from 8 mA (bare CNTs) to 4.5 mA to 2 mA and finally to 1 mA for the 50:10 sample. AFM and TEM images showed the DNA coats the CNTs and this suggests that tunnel junctions are introduced between CNTs which account for the drop in conductance. These junctions are also suggested to be the origin of the improved sensing response: DNA@CNT composites have good sensitivity for VOCs (MeOH, EtOH, C3H6O and CHCl3) and are more sensitive to methanol vapour than other VOCs. Further, DNA/CNTs films show a larger response to chloroform vapour at 21.08 kPa than CNTs/Ppy films at room temperature. Interestingly, the sensitivity of CNTs/DNA films increased as the temperature was raised; this suggests that another mechanism apart from adsorption/desorption is involved in their response. Although CNTs have been suggested as transducers in various gas sensors, they show a poor sensitivity (fractional change in resistance upon exposure to analyte). However by preparing composites of CNTs and less conductive materials, the analytical sensitivity can be greatly increased even though the conductivity of the composite is usually much less than of the bare CNTs.