Highly Selective Ozone Sensors Based on Functionalized Carbon Nanotubes

Carbon nanotubes (CNTs) have several advantages as sensor elements, including large surface area for electrical conductivity, low cost, low-temperature operation and the ability to be functionalized with a variety of polymers for enhanced sensitivity. Using CNTs several researchers have demonstrated laboratory detectors of common environmental gases such as ammonia, nitrogen and carbon dioxide. Most of these sensors are based on DC operation and do not offer the sensitivity and selectivity to be useful in ambient sensing of environmental pollutants. In this paper, the design and performance of ozone sensors comprising pristine CNTs functionalized by covalent modification with octadecylamine (ODA) groups is investigated. These CNTs are dispersed and sonicated into an ink formulation spray-coated onto interdigitated electrodes on alumina substrate. The complex AC (1 kHz) impedance of the sensor, measured under ozone exposure between 200 to 500 ppb, reveals impedance change $>50\%$ at 75°C and nearly total sensor retractability upon removal of gas exposure. The selectivity is demonstrated by cross-sensitivity determination for various gases such as methane, carbon dioxide, ammonia and water vapor.