Exploring Mechanism of Gas Adsorption on Carbon Nanotubes and its Application to Ultrasensitive Gas Sensors

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Gas sensing technology plays an important role in monitoring environmental pollution. The exposure to pollutants at a level exceeding environmental standards is harmful to human health and damages the environment, thus, gas concentrations must be monitored accurately and a hazardous level must be determined rapidly. However, for monitoring such gas pollutants, conventional gas sensors such as metal oxide thin-film sensors have serious limitations due to their poor sensitivity and slow response. Due to large effective surface area and semiconducting quantum wire properties of single-walled carbon nanotubes (SWNTs), it has been expected as a sensor head material.In this project, it is an aim to develop ultrasensitive gas detecting technique using SWNT thin films based on surface science. The new findings worthy of special mention are as follows.(1) A gas sensor was successfully fabricated by growing a SWNT thin film directly on a conventional sensor substrate using thermal chemical vapor deposition (CVD). The SWNT thin-film gas sensor exhibits excellent sensing performance such as high sensitivity (ppp order) for oxidizing gases such as NO_2, room-temperature operation, simplicity in large-scale, fast response, and quick recovery. Moreover, it was found that SWNT thin film functionalized with Pt nanoparticles acts as ultrasensitive CO detector down to 1 ppm.(2) To explore the adsorption mechanism on the SWNT, the relationship between NO_2 concentration and sensor response in vacuum was investigated. It was found that the feature of NO_2 adsorption can be described based on the Langmuir isotherm. The sensitivity in vacuum was also found to be approximately 1.4 times higher than that in air. In vacuum, NO_2 detection limit of 0.1 ppb order was achieved. The high sensitivity is attributed to the intrinsic properties of SWNTs with a clean surface under high vacuum.