Application of plasma modified multi-wall carbon nanotubes to ethanol vapor detection

The purified multi-wall carbon nanotubes (MWCNTs) obtained by a nitric acid (4N) treatment after a chemical vapor deposition were treated by oxygen or fluorine plasma for surface modification. The plasma modified MWCNTs were manufactured by microwave plasma enhanced chemical vapor deposition, and were developed as novel gas sensor materials. In gas-sensing tests, the MWCNT-based gas sensors have shown a p-type response with resistance enhancement upon exposure to 50–500 ppm ethanol at room temperature. Oxygen plasma modification can increase the sensor response from 1.03 to 1.16 on process duration of 30 s due to the apparent elimination of amorphous carbon, as demonstrated by Raman results. However, oxygen plasma modification has no effective assistance in decreasing the response and recovery time. By applying fluorine plasma modification, the sensor response increases from 1.03 to 1.13 on process duration of 60 s, but the response and recovery time can decrease apparently from 225 to 95 s and 452 to 227 s due to the existence of numerous fluorine-included functional groups, as demonstrated by the results of X-ray photoelectron spectroscopy. The sensitivity increases three more times (from 0.0003 to 0.0011) and the linear range of measurement can also extend. Therefore, the plasma modified MWCNTs can elevate the sensitivity and reactivity for room temperature ethanol sensing, especially fluorinated MWCNTs.