Multi-walled carbon nanotubes as high temperature carbon monoxide sensors

Heat treatment of acid-treated multi-walled carbon nanotubes (MWCNTs) produces profound changes in their electrical properties. Below 600 °C the resistance of a thick film of MWCNT (∼100 μm) was below 200 Ω while at 700 °C, the resistance increased to over 20 MΩ. This process was irreversible. TEM showed that for a heat treatment ≤600 °C, the tube nature prevailed, but above 600 °C, nanoparticles of carbon materials with graphitic layers as well as tubes are present. The resistance changes upon interaction with carbon monoxide were monitored for materials heated at 600 and 700 °C. For materials heated at temperatures ≤600 °C, the largest changes in resistances (p-type) were observed at 400 °C with CO, with no measureable resistance changes at 100 and 600 °C. For materials heated to 700 °C, p-type resistance changes were observed for both CO and O2 between 600 and 700 °C, with no changes at 800 °C, and background resistances approaching 95 MΩ at 500 °C. MWCNTs are demonstrated as potential materials for carbon monoxide sensing over the temperature range of 600–700 °C, but not very suitable for sensing between 100 and 400 °C, primarily because of the drift in the background.