Identifier: TDX:1914
Authors: Espinosa Piragua, Edwin Herberth
Abstract:
The world-wide concern for environmental safety demands the monitoring of the emission of hazardous gases into the atmosphere. In this context, metal oxide-based sensors could be a key device, provided their power consumption could be dramatically reduced. Semiconductor oxides that can be produced by mature thin or thick-film technologies have been widely studied and employed as low-cost sensing elements for the detection and monitoring of hazardous gases. They show high sensitivity to a wide spectrum of gases, fast response time and fair long-term stability.Among the existing metal oxides, TiO2, SnO2 and WO3 have proved to be promising for gas sensing applications. Several studies have shown that it can be used for the detection of nitrogen oxides (NO and NO2), hydrogen sulphide, carbon monoxide, ammonia vapours, hydrocarbons and oxygen. Carbon nanotubes (CNTs)-based gas sensors have proved to work well at room temperature, which reduces the power consumption of the device and enables the safer detection of flammable gases.The objective of this thesis has consisted in the design and fabrication of gas sensors for detecting gas traces (like NO2, CO, NH3 y O2 amongst others) operating the sensors at temperatures between 25 and 150°C. Carbon nanotubes, depending of their chirality and diameter, show an electronic structure that can be either metallic or semiconducting. In practice it is difficult to obtain only semiconducting nanotubes from as-grown samples, which are typically mixtures of both metallic and semiconducting CNTs.Functionalisation of nanotubes with other chemical groups on the sidewalls is attempted to modify the properties required for an application in hand. For example, chemical modification of the sidewalls may improve the adhesion characteristics of nanotubes in a substrate and make functional composites. Uniform functionalisations either with oxygen or hydrogen were applied to the as-provided CNTs in order to graft functional groups at their surface and to improve their dispersion and surface reactivity. In this thesis, we study and compare the performance in gas sensing of hybrid materials consisting of functionalized multiwall carbon nanotubes dispersed in a metal oxide matrix. Three different metal oxides namely SnO2, WO3 and TiO2 were considered. An adequate mixture of the components was obtained by dissolving them in glycerol (employed as organic vehicle). The sensing films were dried in order to burn out the organic vehicle and after that annealed in ambient atmosphere.Plasma treatment was found to improve the sensing potential of MWCNT. Oxygen plasma functionalisation of MWCNT proved to play an important role in their responsiveness to ammonia and nitrogen dioxide. The presence of oxygen at the surface of MWCNT was found to increase their sensitivity to NO2, and CO (But did not exceed 20%). Nevertheless, nanotubes show a slow recovery process (especially when they work at room temperature), which can prevent their use in several situations.It is well known that metal oxide sensors are usually operated at temperatures well above 250°C. In this thesis, we have shown that the addition of a suitable quantity of MWCNTs in a metal oxide film can lower the sensor operating temperature. Sensors based on SnO2/MWCNTs hybrid films operated at room temperature showed the higher sensitivity towards NO2 in the ppb range, among the different materials studied. The response mechanism is fully reversible, since the sensors can recover their baseline resistance after each exposure to pollutant gases. Our results suggest that there is an optimum amount of carbon nanotubes to be added to each particular metal oxide in order to enhance responsiveness.
Repositori URV