Functional characterization of carbon nanotube networked films functionalized with tuned loading of Au nanoclusters for gas sensing applications

We investigate the impact of the tailored load of gold (Au) nanoclusters functionalizing the sidewalls of the carbon nanotubes (CNTs) networks on gas sensing performance of a chemiresistor, operating at a working temperature in the range of 20–250 °C. CNTs networked films have been grown by radiofrequency plasma enhanced chemical vapour deposition (RF-PECVD) technology onto low-cost alumina substrate, provided with 6 nm nominally thick cobalt (Co) growth-catalyst. Nanoclusters of Au have been deposited by sputtering onto CNTs networks with a controlled loading of equivalent thickness of 2.5, 5 and 10 nm. Microstructure and morphology of the CNTs have been characterized by FE-SEM and TEM with diameter of the bundles of nanotubules of 10–40 nm. CNTs and Au-modified CNTs exhibit a p-type response with a decrease in electrical resistance upon exposure to oxidizing NO2 gas and an increase in resistance upon exposure to reducing gases (NH3, CO, N2O, H2S, SO2). Negligible response has been found for CNTs and Au-modified CNTs sensors exposed to CO, N2O, SO2. In the contrast, significantly enhanced gas response of NO2, H2S and NH3, up to a low limit of sub-ppm level, has been measured for Au-functionalized CNTs-chemiresistors. Highest gas sensitivity to NO2, H2S and NH3 has been found by CNTs functionalized with Au loading of 5 nm, at 200 °C. An optimal operating temperature for each Au-modified CNTs-sensor exposed to NO2 gas has been recorded. Good repeatability of the electrical response to 200 ppb NO2 is also reported, at 200 °C. These results demonstrate the efficiency of the CNTs-chemiresistors functionalized with Au nanoclusters for selective air-pollutants environmental monitoring applications.