Ammonia sensing properties of multiwalled carbon nanotubes embedded in porous alumina templates

Highly ordered anodic aluminum oxide templates were fabricated by a two step anodization process. Vertically aligned multiwalled carbon nanotube (MWCNT) arrays were grown in these templates through xylene pyrolysis, without the use of a catalyst and were integrated into a resistive sensor design. An equivalent circuit model was developed to understand the operation and to propose design changes for increased sensitivity. During the xylene pyrolysis, a thin layer of amorphous carbon (5–50 nm), forms on both sides of the template as a by-product of the CVD process. This a-C layer plays a crucial role in determining baseline resistance and the sensitivity, since it affects the current path. A study was undertaken to elucidate the dependence of sensitivity on the thickness of amorphous carbon layers, by subjecting the device to post-processing step of plasma oxidation.