Multiwalled carbon nanotube ͑MWCNT͒ films have been deposited by using plasma enhanced chemical vapor deposition ͑PECVD͒ system onto Cr-Au patterned alumina substrates for NO 2 and NH 3 gas sensing applications, at operating temperature of 200°C. Nanoclusters of different MWCNT-growth catalysts ͑Fe and Co͒ have been predeposited onto substrates to promote the growth of the carbon nanotube films with different structural properties. It is demonstrated that the gas sensitivity of the MWCNT films depends on catalyst used for their growth with highest gas sensitivity achieved for Co-grown MWCNT films. The chemiresistor gas response is attributed to the p-type conductivity in semiconducting MWCNTs and the electrical charge transfer is found to be the major sensing mechanism. The results obtained demonstrate that the MWCNT-based sensors exhibit high gas sensitivity, fast response and reversibility, good repeatability and reproducibility, and sub-ppm range detection limit with the gas sensing properties of the MWCNT films tailored by catalyst employed in the PECVD growth process.Carbon nanotubes 1,2 have attracted considerable attention in recent years, most notably because of their potential as active building blocks for novel nanoscale electronic devices, 3,4 expecially for chemical gas nanosensors. 5-10 Sensing devices based on carbon nanotubes are emerging as a powerful class of ultrasensitive and high-performance sensors for detection of chemical 5-12 and biological 13,14 species.Nanostructures 15-17 based on semiconducting metal oxides and noble metals, such as nanobelts, nanowires, nanorods, nanohorns, nanoparticles, nanoclusters, nanocrystals, and nanotubes, have been proposed as interesting nanomaterials for gas sensing applications.The carbon-derived nanostructures, such as carbon nanotubes, are widely perceived as promising nanomaterials for the development of novel chemical nanosensors to generate significant impact in nanoscience and nanotechnology. There are two main types of carbon nanotubes: the single-walled carbon nanotube structure which consists of single graphene sheets seamlessly wrapped into cylindrical tubes with caps normally closed and multiwalled carbon nanotube ͑MWCNT͒ structure which comprises an array of nanotubes that are concentrically nested like rings of a tree trunk. These carbon nanotubes offer many opportunities for the assembly of nanosensors and nanoarrays by the bottom-up paradigm.The rational control in the carbon nanotube growth process of key nanomaterial parameters, including chemical composition, size, structure, morphology, purity, doping, and functionalization, is critical for depositing films of carbon nanotubes with desired and predictable properties to realizing nanosensors with optimal functions. Previously, it has been suggested 18,19 that the type and the size of the growth catalyst used in the CVD growth process can define the diameter of as-grown carbon nanotubes. Consequently, the structural properties of carbon nanotubes can be controlled and thus the possibility ...
