Humidity-assisted selective reactivity between NO₂ and SO ₂ gas on carbon nanotubes

In spite of the technical importance of detecting environmental SO _x and NO_x gases, a selective detection has not been realized because of their similar chemical properties. In this report, adsorption and desorption of SO₂ and NO₂ gas on carbon nanotubes are investigated in terms of different humidity levels at room temperature. A random-network single walled carbon nanotube (SWCNT) resistor is constructed by a dip-pen method using a SWCNT/dichloroethane (DCE) solution. In the case of SO₂ gas adsorption, the resistance increases at high humidity level (92%) and shows no obvious change at low humidity levels. On the other hand, in the case of NO₂ gas adsorption, the resistance always decreases independent of moisture levels. Our density functional theory (DFT) calculations show that this selective behavior originates from cooperative charge compensation between the SO₂-nH₂O complex and the p-type CNT resistor. The change of response time and recovery time with different moisture levels is further investigated. This humidity-assisted gas reaction provides a simple route to detect these two gases selectively.