Effect of chemical oxidation on the gas sensing properties of multi-walled carbon nanotubes

The effect of chemical oxidation on the gas sensing properties of multi-walled carbon nanotubes (MWNT) is presented in this paper.The MWNT were chemically oxidised in a mixture of concentrated sulphuric and nitric acids. The oxidation time was varied from 4 h to 12 h. X-ray photoelectron spectroscopy was used to study the effect on the chemical composition of the nanotube surface. The results show that 8 h oxidation may be the favourite condition to introduce enough functional groups with inherent properties of MWNT being retained as much as possible. The different MWNT samples were deposited onto gas sensor substrates by drop coating method. The gas sensing properties of samples with different oxidation time for NO2 and NH3 were investigated at different concentration and temperature. The MWNT oxidised for 8 h showed the best gas sensing performance. As obtained MWNT did not show responsiveness below 30 ppm NO2 while chemically oxidised MWNT can credibly detect NO2 as low as 5 ppm at room temperature. Meanwhile, there was almost no response when as obtained MWNT in presence of 1000 ppm NH3, while the oxidised MWNT showed quite good responsiveness at room temperature. Temperature dependant experiments showed that with the operation temperature increasing,the sensitivity increased obviously and the recovery process speeded up as well.