Alcohol sensing performance of ZnO hexagonal nanotubes at low temperatures: A qualitative understanding

ZnO hexagonal nanotube array was synthesized on fluorine doped tin-oxide (FTO) coated glass substrate (thickness: 1.1 mm, surface Resistivity: similar to 10 Omega/sq), by a two-step process consisting of electrodeposition and electrochemical etching. Aqueous solution of equi-molar zinc nitrate hexahydrate and hexamethylenetetramine was used as the electrolyte for synthesis of ZnO hexagonal nanorods by electrodeposition technique with-1.8V potential at 80 degrees C for 40 min. Grown hexagonal nanorods were then electrochemically etched to form the nanotube array by using ethylenediamine at 75 degrees C for 2 hours with-0.06V bias. After detailed structural characterizations, resistive mode alcohol sensing (10-700 ppm) was carried out in the temperature range of 27 degrees C-150 degrees C. 75 degrees C was found to be the optimum operating temperature for ethanol, methanol and 2-propanol detection. However, at even room temperature (27 degrees C), the sensor offered promising response characteristics towards alcohols. At this temperature, for similar to 10 ppm of ethanol, methanol and 2-propanol, response magnitude was observed to be similar to 30%,similar to 18% and similar to 10%, respectively; while for 700 ppm the corresponding response magnitude was similar to 64%,similar to 51%,similar to 48%, respectively. Under the influence of humidity, the baseline resistance decreased with increased humidity while the corresponding change in response magnitude (compared to dry air) was found to be insignificant. A qualitative model has been demonstrated correlating the surface to volume ratio of the nanotubes with the response characteristics. (C) 2016 Elsevier B.V. All rights reserved.