Fundamental Investigation Of Resistive Gas Sensors Based On MWCNTs And RGO

Carbon nanomaterials has been promising for developing the new generation of miniaturised, low-power, ubiquitous sensors due to their unique electrical, optical and mechanical properties. In the particular case of gas sensing, some low-dimensional carbon nanomaterials such as carbon nanotubes and graphene have most of their atoms exposed to the environment and, therefore, offer high specific surface area, which is advantageous for achieving high sensitivity. High quality crystal lattices, high carrier mobility and low noise are other major advantages for application in gas sensing. Besides, the sensitivity and selectivity of carbon nanomaterials can be engineered by employing different techniques both to create defects and graft functional groups to their surface in a controlled way. Additionally, their mechanical properties make them suitable to become integrated in flexible electronic devices. As is known to all, the electronic signal transduction(e.g. a resistance change) of chemical environmental analytes is advantageous over other transduction methods, owing to lower cost, increased device simplicity, higher sample throughput and better portability.This dissertation was based on resistive devices with planar interdigital electrodes(IDEs), and carried out a series of analysis and research on sensing characteristics of fabricated gas sensor with carbon nanomaterials such as MWCNTs or graphene as gas sensitive materials to specified gases or vapors. An insight into the relevant sensing mechanisms was under investigation combined with some analytical methods. The main content includes the following points:1. A hybrid sensing mechanism, including electron-hole exchange theory and inter-sheet distance variation, was proposed, which has enriched the applicable transduction mechanisms for RGO. Different quantity of RGO material was deposited onto IDEs to study the effect of total flow rate on sensing response and electric resistance of devices. When the total flow rate of nitrogen or dry air was increased, a compressive stress was created onto the RGO film, which reduced the inter-sheet distance, and made an electric resistance decrease, and vice versa. It was found that the sensing response of the sensor exposed to the same concentration of diluted carbon dioxide increased as the total flow rate increased. The reason is that more carbon dioxide gas molecules penetrated into the lower layer of the RGO film, and were adsorbed on the sorption sites at larger total flow rate. Larger electric resistance variation and smaller electric resistance gave rise to a larger sensing response. The results also showed that devices with more deposited RGO solution would have an electric resistance increase on exposure to NO2 or H2 S gas; however, an increasement at H2 S atmosphere and a decrease at NO2 atmosphere was shown for devices with less RGO solution deposited. A hybrid sensing mechanism, including electron-hole exchange theory and inter-sheet distance variation, was proposed to account for the phenomena. Both processes happened simultaneously. It seemed that the initial electric resistance of the sensor decided the corresponding predominant mechanism. That is to say, according to the sensing performances of all sensors, a threshold electric resistance of RGO sensitive films should lie between 16.2 k? and 46.4 k? to make an explicit distinction between both sensing mechanisms.2. Porous ZnO film(insulating at room temperature) was utilized as supporting material in RGO-ZnO sensitive film, and RGO-ZnO film and traditional single-layer RGO film were studied on sensing characteristics for comparison mainly for carbon dioxide detection at room temperature. The results indicated an improvement in the sensing response, sensitivity, and response-recovery time for RGO-ZnO film compared with traditional RGO film. Meanwhile, this improved film structure alleaviated the restrictions of sensors performance to the film thickness. The conventional RGO sensing film was that RGO material was deposited directly in contact with IDEs, and few gaps existed between the RGO film and IDEs. However, in the case of formation of a supporting material, the ZnO film, on IDEs, the most part of RGO was supported by the uneven and porous ZnO film, and only a small fraction of RGO was in touch with IDEs through the cracks between ZnO granules. It could be concluded that more gaps would exist between the RGO film and the rough ZnO film in the two-step film structure. The surface of the two-step film would be rougher than the counterpart using only RGO. Therefore, on exposure to the target gas at a fixed concentration level, more adsorption sites exist in the two-step film structure and more gas molecules can access the adsorption sites, leading to larger sensing response and smaller response-recovery time. For the conventional thin film sensors, their sensitivities become saturated and the response-recovery time increased after the film thickness increases to some point. The two-step film structure may alleviate this restriction because of the above analysis.3. RGO-PEI sensitive films were airbrushed onto IDEs, and the relevant sensing characteristics study for CO2 gas were taken the lead. The results showed a smaller baseline drift and a more stable sensing performance for RGO-PEI bilayer film compared with RGO-PEI composite film. In addition, RGO film and RGO-PEI bilayer film were investigated on sensing response, long-term stability, repeatability, lowest concentration detection, response-recovery time, and sensing response difference under two kinds of carrier gases to carbon dioxide gas with relevant explanations. The results exhibited that RGO-PEI bilayer film performed a better repeatability but a longer response-recovery time due to chemisorptions between PEI and CO2 on exposure to 3667 ppm CO2. Both sensors could detect as low as 20 ppm CO2, showed a larger sensing response in air than that in nitrogen, and underwent 50% decay of sensing response after four months storage.4. The sensing characteristics differences of MWCNTs-PEO bilayer and composite films to toluene vapor were detailedly analyzed, and a modest selectivity for toluene vapor was taken on for MWCNTs-PEO bilayer film. MWCNTs-PEO bilayer and composite films were studied for comparison in this dissertation, and the experimental results indicated that baseline drift existed for both films, and the sensitivity was almost two times larger for bilayer film than composite film. Due to these, the bilayer film was further studied. The subsequent experiments showed that as the quantity of PEO solution increased, the initial electric resistance, sensing response and sensitivity of the corresponding devices became larger. It was concluded that there were two main reasons for the initial electric resistance variation. The more polymer solution was deposited, the more contact and junction resistance was emerged. The other one might be referred to the tunnel effect between MWCNTs and PEO. The conductivity of PEO was negligible compared with that of MWCNTs. PEO covered the entangled MWCNTs bundles by van der Waals forces and a MWCNTs-PEO-MWCNTs structure was formed. As the quantity of PEO solution was increased, the insulating film became thicker resulting in decrease of tunnel effect. Poor conductivity of PEO matrix resulted in a difficult transport of charged carriers between the adjacent MWCNTs. Hence, the charged carriers transport became harder, leading to a larger initial electric resistance. For sensing response and sensitivity increase, the reason was that more adsorption sites were emerged as more quantity of PEO solution was airbrushed, which ensured more contact with toluene molecules and more complete adsorption of toluene molecules. It could be concluded that, compared with MWCNTs single layer film, bilayer film had a modest selectivity to toluene vapor compared to other tested vapors, due to the usage of sensitive materials of MWCNTs and PEO.