The synthesis, characterization of oxidized multi-walled carbon nanotubes, and application to surface acoustic wave quartz crystal gas sensor

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 Mg28-Ni68-Mo4 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.