Fabrication of a Methane Sensor by Zinc Oxide Functionalization of Carbon Nanotubes

The leakage of coal gas could be prevented through the detection of methane, which is its main chemical component. Our goal was to design a highly reactive methane sensor using carbon nanotubes on a silicon substrate. The carbon nanotubes were functionalized with zinc oxide by atomic layer deposition at 175°C, 200°C and 225°C. Atomic Layer Deposition is used to fabricate thin, conformal films in a layer-by-layer fashion onto a substrate and uses self-limiting surface reactions to control film growth. Due the inert nature of carbon nanotubes, the carbon nanotubes were activated with either oxygen plasma for 5 or 10 minutes or ultraviolet O₃ for 10, 20, 30, or 60 minutes before functionalization. In order to measure sensor response, methane in air calibration mixtures were introduced to the samples inside a test chamber and the concentration was observed using a reference methane detector. From this investigation, it was concluded that as the atomic layer deposition process temperature increases, the crystalline quality of the substrates improves.  Carbon nanotubes activated with UV O₃ were not reactive to methane at all while carbon nanotubes activated with oxygen plasma were highly sensitive. Currently, our zinc oxide-carbon nanotube sensor detects a 2 ppm concentration of methane at room temperature, one of the best reported sensitivities to this date. This sensor was prepared with 10 minute oxygen plasma treatment and 225°C atomic layer deposition, concluding that this combination of activation and functionalization provides for the most effective methane sensor.