Study Of Gas Sensing Properties Of Metal Oxide Gas Sensor Doped With Carbon Nanotubes

Tungsten Oxide (WO₃) and Tin Oxide (SnO₂) are both important functional materials. Due to their excellent gas-sensing properties, over the years increasing amount of research activities has been dedicated to these materials. However, as candidates for gas-sensing purposes both WO₃ and SnO₂ have some shortcomings that limit their applicability, such as low sensitivity, high working temperature, poor selectivity and high resistance. Carbon Nanotubes (CNTs) can be considered as nearly ideal one-dimensional conductors with high specific area and excellent adsorptive capability. In terms of electrical conductivity, single-wall carbon nanotubes (SWCNTs) can be either metallic or semiconducting. In this thesis CNTs are used as 'doping agent' in WO₃ and SnO₂ as an attempt to overcome the abovementioned problems in the pure metal oxides. The work focuses on optimizing the gas-sensing properties of CNT-SnO₂ and CNT-WO₃ by adjusting various synthesizing parameters.(1) The CNTs were modified and functionalized by various physical and chemical methods, including ball milling, ultrasonication and oxidation in mixed acid. SEM was used to evaluate the effects of different approaches. The optimal CNT samples were prepared using ball milling and ultrasonication methods.(2) WO₃ nano-powders were synthesized by sol-gel techniques. The gas sensor was prepared by mixing WO₃ with dispersed and functionalized CNTs. The results indicate that CNTs exist between the WO₃ grains, the average particle size of which is about 30-50nm, thus increasing the porosity of the material. CNT-WO₃ gas sensor is far more sensitive to acetone than its pure counterpart, and 0.4wt% is identified as the optimal 'doping' concentration for acetone sensors. The device is good for low-concentration detection, with good selectivity and fast response. CNT-WO₃ is ideal sensor for acetone.(3) SnO₂ nano-powders were synthesized by sol-gel techniques. The Stanmim Oxide (SnO₂) gas sensor was then prepared by adding 0.1wt% CNTs. The sample has low resistance, high sensitivity and fast response. (4) To understand the sensing mechanism of MO_x-CNTs gas sensors, a model was proposed that's based on the intergranular barrier model of metal oxide, with the theories of metal oxide and carbon nanotube taken into account.