Available online at www.sciencedirect.com Materials Chemistry and Physics 109 (2008) 148–155 The synthesis, characterization of oxidized multi-walled carbon nanotubes, and application to surface acoustic wave quartz crystal gas sensor Hao-Lin Hsu a , Jih-Mirn Jehng a,∗ , Yuh Sung b , Li-Chun Wang b , Sang-Ren Yang b a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC b Physical Chemistry Section, Chung-Shan Institute of Science & Technology Lung-Tan, Tao-Yuan 325, Taiwan, ROC Received 5 July 2007; received in revised form 5 November 2007; accepted 8 November 2007 Abstract Intermetallic alloy catalysts had been prepared by the polyol method, and used for the growth of the individual- or bundle-shaped multi-walled carbon nanotubes (MWCNTs) by thermal chemical vapor deposition method. The purified MWCNTs catalyzed by Mg 28 -Ni 68 -Mo 4 alloy catalyst were oxidized with the nitric acid/hydrogen peroxide solution (volume ratio = 2/1) to generate carboxylic acid groups. The oxidized MWCNTs (MWCNT-COOH) were further modified with different molecular weights of polyethylene glycols and chloromethyl styrene, respectively, to verify carboxylic acid groups and achieve higher hydrophobic property. Contents of organic functional groups grafted on MWCNTs were estimated with thermogravimetric analysis experiments. In addition, the MWCNT-COOH and poly(n,n-dimethylamino propylsilsesquioxane) (SXNR) were mixed in the THF solvent, and sprayed onto the surface of SAW crystal gas sensor. The MWCNT-COOH were employed to the 156 MHz surface acoustic wave (SAW) quartz crystal sensor for the adsorption of ethanol vapor. The SAW quartz crystal gas sensor coated with the MWCNT-COOH/SXNR was exhibited a high response for ethanol vapor efficiently. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Oxidation; Gas sensor 1. Introduction Quasi one-dimensional carbon nanotubes (CNTs), which are the graphite sheet rolled-over into the cylinder tube material, have become an attractive material for its specific mechani- cal, physical, chemical, electronic, and field-emission properties [1–4]. For example, CNTs are thermally stable up to 2800 ◦ C in vacuum, the thermal conductivity is about twice as high as diamond, and the electric-current-carrying capacity is 1000 times higher than the copper wires [5]. Owing to mechani- cal properties of the high Young’s modules (∼1 Tpa) and high tensile strength (∼100 Gpa), CNTs are extraordinary strong and quite stiff than the steel wire. These outstanding proper- ties of CNTs have been investigated and applied for various devices such as scanning probe microscopy tips [6], field- emission displays [7] and micro-electronic devices [8,9]. The disadvantage of the CNTs is difficult to dissolve or disperse in ∗ Corresponding author. Tel.: +886 4 22852439; fax: +886 4 22854734. E-mail address: jmjehng@dragon.nchu.edu.tw (J.-M. Jehng). common organic solvents. The surface modification and func- tionalization of CNTs can improve the surface properties of the CNTs and result in a better dispersion in the organic sol- vents. Functionalization and solubilization of the single-walled carbon nanotubes (SWCNTs) and the multi-walled carbon nan- otubes (MWCNTs) have been extensively studied by many investigators [10–19]. Most of the previous reports are based on the utilization of carboxylic acid (COOH) groups, which can provide reactive sites for interacting with different reac- tive compounds, at the ends and side-walls of the CNTs. In the earliest study, Haddon and his co-workers [10,11] have reported that the shortened SWCNTs with the COOH groups were proceeded the amidation reaction with octadecylamine or 4-tetradecylaniline by using of thionyl chloride (SOCl 2 ). However, SOCl 2 is hazardous to be used because it is highly active to react with hydrogen oxide to generate toxic com- pounds. In order to remove the impurities, such as amorphous carbon and metal catalyst on outer surfaces of CNTs as syn- thesized by thermal chemical vapor deposition method (CVD), strong acids (sulfuric acid and nitric acid) [10,11] have been 0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.006