NH₃ Gas Sensors Based on Single-Walled Carbon Nanotubes Interlocked with Metal-Tethered Tetragonal Nanobrackets

Although carbon nanotubes (CNTs) are promising candidates for gas-sensing materials owing to their high gas sensitivity and room-temperature operation, they suffer from poor selectivity to specific gas species. Noncovalent functionalization of CNTs with metal complex molecules is an effective and facile way to overcome this problem to enhance their sensitivity. This article reports on sensitive and selective gas sensors to ammonia (NH3) based on single-walled carbon nanotubes (SWNTs) interlocked with metal-tethered tetragonal nanobrackets (M-NBs, M = Cu(II) and Co(II)). The SWNT sensors with Cu- and Co-NBs showed 2.31 ± 0.11 and 2.24 ± 0.04%/ppm in sensitivities to NH3, which are more than 10 times higher than those of the pristine SWNT sensors prepared by removing M-NBs from the interlocked SWNTs. In addition, the detection limits of the SWNT sensors with Cu- and Co-NBs were estimated to be 16 ± 7 and 44 ± 5 ppb, respectively. Such high sensitivities are observed only for NH3, indicating that these sensors are highly selective for NH3 relative to other analyte gases/vapors (H2, CH4, ethanol, and acetone). The transfer characteristics of the field-effect transistor-type sensors and the binding energy to NH3 by density functional theory revealed that the metal ions of the M-NBs provided adsorption sites, resulting in greater charge transfer from NH3. This study suggests that both Cu- and Co-NBs stably and rigidly immobilized on the SWNT surface effectively enhance the sensitivity and selectivity of the gas sensors to NH3.