Carbon nanotubes (CNTs) deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates have been investigated as resistive gas sensors for NO2. Upon exposure to NO2, the electrical resistance of the CNTs was found to decrease. The maximum variation of resistance to NO2 was found at an operating temperature of around 165 °C. The sensor exhibited high sensitivity to NO2 gas at concentrations as low as 10 ppb, fast response time, and good selectivity. A thermal treatment method, based on repeated heating and cooling of the films, adjusted the resistance of the sensor film and optimized the sensor response to NO2.

In this work a combined experimental and theoretical study on carbon nanotube (CNT) based system for gas sensing applications is reported. Carbon nanotubes thin films have been deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates provided with Pt electrodes. Microstructural features as determined by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy highlight the growth of defective tubular carbon structures. The electrical resistivity of the CNT film shows a semiconductinglike temperature dependence and a p-type response with decreasing electrical resistance upon exposure to NO2 gas (100 ppb). No response has been found by exposing the film to CO gas in the temperature range between 25 and 250 °C. In order to obtain a theoretical validation of the experimental results, the equilibrium position, charge transfer, and density of states are calculated from first principles for the CNT+CO and CNT+NO2 systems. Our spin-unrestricted density functional calculations show that NO2 retains its spin-polarized state upon adsorption. Both CO and NO2 molecules adsorb weakly on the tube wall, with essentially no charge transfer between the tube and molecules. The electronic properties of CNTs are sensitive to the adsorption of NO2, due to an acceptorlike peak close to the tube valence-band maximum, while they are insensitive to the CO adsorption. According to the experimental findings, our theoretical results suggest that gas-induced modification of the density of states close to the Fermi level might significantly affect the transport properties of nanotubes.

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First-principles calculations within the density functional theory have been performed in order to investigate ozone adsorption on carbon nanotubes. Particular emphasis is placed on the effects of Stone-Wales-like defects on the structural and electronic properties of (i) ideal tubes and (ii) tubes in the presence of ozone. Our results show that structural deformations induced on the pure carbon nanotubes by Stone-Wales defects are similar, as expected, to those induced on graphite; for the (10,0) tube, the semiconducting character is kept, though with a small reduction of the band gap. As for the ozone adsorption, the process on ideal nanotubes is most likely physisorption, though slightly stronger if compared to other previously studied molecules and consistent with the strong oxydizing nature of O(3). However, when ozone adsorbs on Stone-Wales defects, a strong chemisorption occurs, leading to relevant structural relaxations and to the formation of a CO covalent bond; this is consistent with experimental observations of CO functional groups, as well as of the liberation of CO gas phase and of the formation of C vacancies, thus explaining the consumption of the nanotube film upon ozone exposure.

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