Multi Walled Carbon Nanotube Based Chemoresistive Methane Sensors

Carbon nanotube (CNT) low power chemoresistive sensors have the potential to detect sub-ppm levels of methane (CH4), one of the most prominent greenhouse gases. Multi-walled carbon nanotube (MWCNT) surface was activated by O2-plasma or UV-O3 (UVO) treatment. This was followed by deposition of metal-oxide nanocrystals (MONCs) on to the surface of MWCNTs. The sensor surface has been functionalized for methane by this process. Surface-activated, MONCs-functionalized MWCNT chemoresistors were able to sense 10 ppm of methane in dry air at room temperature. The effects of O2-plasma and UVO activation of the MWCNT surface have been studied using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The morphology of the MONCs, deposited by atomic layer deposition (ALD) process on to the MWCNT surface, was characterized by scanning electron microscopy (SEM) and TEM; the chemical composition was characterized by energy dispersive x-ray spectroscopy (EDS) and Raman spectroscopy; the crystal quality was characterized by Raman spectroscopy and TEM. The photonic property of the functionalizing ZnO nanocrystals has been characterized by photoluminescence (PL) spectroscopy. Characterization results suggest reactive functional groups such as COOH, COH, and C=O are formed as a result of surface pre-activation of MWCNT, and are essential for the nucleation and binding of the high crystal quality MONCs on to the MWCNT surface. Surface pre-activated and MONC-functionalized MWCNT (MONC-MWCNT) chemoresistor methane sensors were tested at room temperature in presence of 10 ppm methane in dry air. The response of the sensor was 4 times higher at 10 ppm methane compared to other existing sensors. The improved performance of the sensor is a result of strong electron transfer to the MONCs from methane molecules and energetically favorable electron transport at the junction between these particular MONCs (i.e., ZnO or SnO2) and MWCNT. The surface pre-activation processes were essential for functionalization of MWCNT by the ZnO or SnO2 NCs and hence the reversible response of the sensor in presence or absence of 10 ppm methane in dry air. Compared to UVO technique the O2-plasma based surface pre-activation process was more effective in enhancing sensor response to 10 ppm methane in air mixture. After being exposed to the methane in air mixture the change in the resistance of the sensors was reversed by N2 purging, zero air purging, N2 purging along with UV irradiation or UV irradiation alone. UV irradiation was highly effective in reducing the sensor recovery time. As a result of the UV irradiation, the recovery time was reduced 2 orders of magnitude compared to previous result. Change in baseline resistance of the MONC-MWCNT sensor as a function of varying relative humidity was studied as well. The sensors showed strong dependence on RH change necessitating the fabrication of a differential resistive grid composed of selectively functionalized CNTs. The differential grid uses constructive and destructive interference of the responses from different grid elements at different RH levels, hence nullify the RH interference during methane sensing.