Ammonia Sensors Based on Composites of Carbon Nanotubes and Titanium Dioxide

Design of composite materials for ammonia (NH3) sensing is important because of two main reasons: (1) NH3 is the most common substitute for chlorofluorocarbons (CFCs) in cooling systems, and (2) most sensors show long recovery times at room temperature due to the tendency of ammonia to strongly interact with many substrates. Multiwalled carbon nanotubes (MWCNTs) have been used to sense polar molecules like carbon monoxide (CO), carbon dioxide (CO2), NH3, water (H2O), and ethanol (C2H5OH) (Ong et al., 2002; Valentini et al., 2004; Varghese et al., 2001), as well as non polar gases like helium (He) and nitrogen (N2) (Adu et al., 2001). For carbon nanotubes, studies have shown that O2 molecules are electron acceptors with substantial adsorption energies and charge transference, while NH3, N2, CO2, methane (CH4), H2O, hydrogen (H2) and argon (Ar) are electron donors (Zhao et al., 2002). Adsorption in CNTs is determined by adsorption energy and availability of sites, with typically four different adsorption sites: external surface, grooves between CNTs on the bundle outside, pores inside CNTs, and interstitial channel between adjacent tubes inside the bundle (Stan & Cole, 1998; Williams & Eklund, 2000). Additionally, theoretical studies carried out by Jhi et al. about the electronic and magnetic properties of oxidized CNTs indicated their high potential as gas sensors. These studies demonstrated that the sensing mechanism of as prepared CNTs is more related to oxygen doping than to intrinsic properties, and depends on the structural defects caused by the synthesis methods (Jhi et al., 2000). With the aim of increasing the response of CNTs to certain gases, some studies about substitutional functionalization of CNTs have been reported (Peng & Cho, 2003). Although boron (B) and nitrogen (N) doping improve the response to gases like CO and H2O, BxCy nanotubes show stronger chemisorption, in contrast with the substitutional functionalization with nitrogen (Villalpando-Paez et al., 2004). Oxidation of CNTs in acid solutions to graft oxygen functional groups [i.e., carboxyl (COOH), hydroxyl (OH), and carbonyl (CO)] on CNT walls have been widely used to diversify the sensing options. Another strategy has been the fabrication of compound materials based on CNTs and metallic oxides. Espinoza et al. reported the use of metallic oxides-CNT composites based on commercial tin (SnO2) and tungsten (WO3) oxide powders, as well as sol gel TiO2 materials, in the sensing of NO2 and CO at room temperature and 150°C. Their results indicated a better performance for CNT/SnO2 and CNT/WO3 than for CNT/TiO2, for the titania composite the sensor response was barely sizable (Espinoza et al., 2007).