Polypyrrole-Coated Carbon Nanotubes for Low Temperature Gas Sensor

Simple, low cost and energy efficient gas sensors for toxic gas measurements are in great demand for applications in industries and for environmental analysis. And, almost half a century ago, it has been already realized that semiconducting metal oxides have great potential as gas sensing materials, owing to the fact that chemical interaction of gas molecules with the semiconductor’s surface leads to changes in the electrical conductivity. Especially, SnO₂ has been the most frequently used n-type semiconducting material for gas sensing. However, most of those inorganic sensors inherently suffer from some problems such as low selectivity, reduced life time, and relatively high operable temperature leading to increased power consumption which limit their versatility.

Recently, the conducting polymers have shown great promise in sensory applications. Delocalized electronic states combined with the restriction on the extent of delocalization leads to the formation of a large energy gap making most conductive polymers behave like p-type semiconductors. Advantages of using conducting polymers as sensing materials are ease of synthesis, fast response, low cost, and room temperature response.

In this study, polypyrrole-coated carbon nanotube(CNT) was prepared by chemical oxidation polymerization of pyrrole on the surface-functionalized carbon nanotube in the presence of FeCl₃. As increasing the concentration of pyrrole, the thickness of polypyrrole layer on the surface of CNT was gradually increased. The obtained material was mixed with a solution containing gold(Au), platinum(Pt), silver(Ag) ion and simultaneously reduced by adding NaBH₄ solution. The detection of carbon monoxide, toluene, and formaldehyde was systematically examined. Based on the gas sensing results, the thickness of polypyrrole, operating temperature, and concentration of metal catalyst were roughly optimized. Finally, the polypyrrole-coated CNT showed considerably promising gas sensing characteristics as compared to conventional SnO₂ powder.