Selective Hydrocarbon Gas Sensor based on Carbon Nanotubes Functionalized with Metal Oxide Nanocrystals

This work describes the theory and results for a new class of low-cost chemo-resistive gas sensors designed for selective hydrocarbon gas detection. The sensors utilize a multi-walled carbon nanotubes (MWCNTs) backbone functionalized with metal oxide nanocrystals (MONCs). Specifically, nanoparticles were grown on the surface of MWCNTs using Atomic Layer Deposition (ALD). The crystallinity of ZnO-MWCNTs heterostructure was examined by high-resolution Transmission Electron Microscope (HRTEM). The structure of ZnO/MWCNTs was analyzed using Scanning Electron Microscope (SEM) and Energy Dispersive X- ray (EDX). Hall Effect measurement shows p-type characteristics of Multi-Wall Carbon Nanotubes (MWCNTs), supporting the typical PN junction formation with n-type ZnO nanocrystals. The electron-donating ability of ZnO caused strong response to ppm levels of toluene at room temperature (25°C) and showed strong selectivity with other VOC gases such as benzene, methane and formaldehyde. Two different MOX nanocrystals (TiO2 and ZnO nanocrystals) are deposited on the functionalized MWCNTs. The results from these different MOX nanocrystals (MONCs) are compared. In addition, the different photocatalytic efficiencies of ZnO and TiO2 nanocrystals are analyzed. In addition, sensor response under two different levels of relative humidity (RH) were compared. The developed theory may enable a new generation of ubiquitous environmental sensors.