Sensors and Actuators B 150 (2010) 641–648 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Application of plasma modified multi-wall carbon nanotubes to ethanol vapor detection Chun-Kuo Liu a , Jyh-Ming Wu b , Han C. Shih a,c,∗ a Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan b Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan c Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 11114, Taiwan article info Article history: Received 23 May 2010 Received in revised form 1 August 2010 Accepted 19 August 2010 Available online 26 August 2010 Keywords: Multi-wall carbon nanotubes (MWCNTs) Chemical vapor deposition (CVD) Microwave plasma enhanced chemical vapor deposition (MPECVD) Surface modification Ethanol Gas sensor abstract The purified multi-wall carbon nanotubes (MWCNTs) obtained by a nitric acid (4N) treatment after a chemical vapor deposition were treated by oxygen or fluorine plasma for surface modification. The plasma modified MWCNTs were manufactured by microwave plasma enhanced chemical vapor deposition, and were developed as novel gas sensor materials. In gas-sensing tests, the MWCNT-based gas sensors have shown a p-type response with resistance enhancement upon exposure to 50–500 ppm ethanol at room temperature. Oxygen plasma modification can increase the sensor response from 1.03 to 1.16 on process duration of 30 s due to the apparent elimination of amorphous carbon, as demonstrated by Raman results. However, oxygen plasma modification has no effective assistance in decreasing the response and recovery time. By applying fluorine plasma modification, the sensor response increases from 1.03 to 1.13 on process duration of 60 s, but the response and recovery time can decrease apparently from 225 to 95 s and 452 to 227 s due to the existence of numerous fluorine-included functional groups, as demonstrated by the results of X-ray photoelectron spectroscopy. The sensitivity increases three more times (from 0.0003 to 0.0011) and the linear range of measurement can also extend. Therefore, the plasma modified MWCNTs can elevate the sensitivity and reactivity for room temperature ethanol sensing, especially fluorinated MWCNTs. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The discovery of carbon nanotubes (CNTs) was in 1991 [1], and their outstanding physical, chemical, mechanical and electri- cal properties have been gradually researched due to their unique structure [2–4]. Many important applications have been demon- strated, and both CNT-based [5–13] and CNT-doped [14–20] gas sensors were received considerable attention because of their nanometer hollow geometry, high specific surface area, high elec- tron mobility, surface modification, and functionalization. Ethanol sensors have wide applications in traffic safety, food- stuffs, fermentation processes, and alcoholic beverage production processes. Several different sensing technologies have been used to detect ethanol concentrations. Among them, the resistive method [5–20] has the advantages of simplicity of construction, low cost, and popular applications. ∗ Corresponding author at: Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan. Tel.: +886 3 5715131x33845; fax: +886 3 5710290. E-mail address: hcshih@mx.nthu.edu.tw (H.C. Shih). Semiconductor metal oxide (e.g., SnO 2 [20,21] and TiO 2 [19,20]) is often used as an ethanol sensor, but generally it requires a high working temperature exceeding 300 ◦ C. Conducting polymer (e.g. polypyrrole [18] and polyvinyl alcohol [22]) is also used for room temperature ethanol sensing, but it does not possess good thermal stability. CNT-based ethanol sensors may work at room tempera- ture and have a quite low detection limit; however, they exhibit a lower sensitivity due to fewer percentages of semiconducting nanotubes that can be modulated by gas molecules. Besides, the recovery time of CNT-based ethanol sensors is actually long because of the strong adsorption between CNTs and gas molecules [8,9]. Some kinds of surface modifications, including acid treatment [23], annealing [23], and oxygen plasma treatment [24,25] have been reported for enhancing the sensing sensitivity. Therefore, how to enhance the reactivity of CNT-based ethanol sensors is our major purpose. 2. Experimental 2.1. Materials The MWCNTs used in this work were synthesized by chem- ical vapor deposition (CVD) in a tube furnace system [24]. The 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.08.026