Reversible oxidation effects on carbon nanotubes thin films for gas sensing applications

Carbon nanotubes (CNTs) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) have been investigated as resistive gas sensors towards NO2 oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes' electrical resistance as determined by volt-amperometric measurements. In particular, the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. After oxygen annealing, X-ray photoelectron spectroscopy (XPS) proves the increase of oxygen linked to nickel located at the nanotube's cap and a no appreciable variation of oxygen physisorbed on the carbon nanotubes. Tangential mode Raman lines from metallic oxidized CNTs was found to depend sensitively on adsorbed oxidizing molecules exhibiting different line shapes than the as-deposited nanotubes. However, the line shapes became identical after thermal annealing in vacuum at 250 deg. C, which is attributed to degassing of doping adsorbates. The electrical resistance measured by exposing the films to sub-ppm NO2 concentrations (100 ppb in air) at 165 deg. C was found to decrease. The obtained results demonstrate that nanotubes could find use as sensitive chemical gas sensor for the fast response accompanied by a high sensitivity to sub-ppm NO2 exposure; the precise recover of the base resistance value in absence of NO2 at a fixed operating temperature likewise indicate that intrinsic properties measured on as-prepared nanotubes may be severely changed by extrinsic oxidative treatment effects