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

气敏技术在环境污染监测中发挥着重要作用。暴露于超过环境标准水平的污染物对人体健康有害并破坏环境，因此，必须准确地监测气体浓度并快速确定危险水平。然而，对于监测这样的气体污染物，传统的气体传感器如金属氧化物薄膜传感器由于其差的灵敏度和慢的响应而具有严重的局限性。由于单壁碳纳米管（SWNTs）具有大的有效表面积和半导体量子线特性，有望作为传感器头材料，本课题旨在基于表面科学的原理，开发利用单壁碳纳米管薄膜的超灵敏气体检测技术。值得特别提及的新发现如下。(1)采用热化学气相沉积（CVD）法在传统的传感器衬底上直接生长单壁碳纳米管薄膜，成功地制备了气敏传感器。SWNT薄膜气体传感器具有对NO_2等氧化性气体的高灵敏度（ppp量级）、室温工作、易于大规模测量、响应速度快、恢复速度快等优良的传感性能。此外，人们发现，单壁碳纳米管薄膜功能化的Pt纳米粒子作为超灵敏的CO检测器下降到1 ppm。(2)为了探讨单壁碳纳米管对NO_2的吸附机理，研究了真空条件下NO_2浓度与传感器响应的关系。结果表明，NO_2的吸附符合Langmuir等温式。在真空中的灵敏度也被发现是约1.4倍，高于在空气中。在真空条件下，NO_2的检测限可达0.1ppb量级。高灵敏度归因于在高真空下具有清洁表面的单壁碳纳米管的固有特性。

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.