Vertical Mode Gas Sensing Performance of TiO2 Nanotube Array by Tuning of Surface Area and Carrier Transport Length

1-D oxide nanostructures-based metal-insulator-metal structures represent potential gas sensor devices, owing to their vertical electron transport feature. In this paper, we demonstrate that for achieving optimum gas sensing by a TiO₂ nanotube (NT) array in vertical mode, tuning of NTs (1-D) surface area as well as carrier transport path length by tailoring the NT length can be a valuable approach. For anodization times of 1, 4, 8, 12, and 16 h, the corresponding NT length was found to be 280-320, 506-514, 1730-1790, 2000-2200, and 2380-2420 nm, respectively, with almost no variation in tube diameters or separations. The carrier concentrations of the NTs were found to be decreasing with increasing tube length. The vertical device structure, employing NT arrays of different lengths as the sensing layer, was investigated in the temperature range of 50 °C-250 °C for sensing acetone, as a test gas/vapor, in the concentration range of 10-1000 ppm. The response magnitude of the sensor was increased with increased NT length, possibly owing to the availability of larger amounts of gas interaction sites due to higher surface area at increased length. The response time and recovery time of the developed sensor also increased with increasing tube length and became excessively sluggish after the critical tube length exceeded 2250 nm owing to slower adsorption/desorption and diffusion kinetics.