CO Gas Sensing with ZnO Nanowire Mat

In the past decade, significant advances have been made in the synthesis of ZnO nanostructures. Here, we report the electrical properties of a gas sensor constructed from mats of ZnO nanowires grown on sapphire substrate that showed reversible response to CO exposure. The sensor is a two-terminal design, where the terminals consist of two 25 $\mu $m diameter gold wires laid parallel on the nanowire mat about $\sim $2 mm apart. The nanowires had an average diameter of 50 nm with lengths of about 10 $\mu $m. The mat was about 20 $\mu $m thick and extended over area of about 1 cm$^{2}$. When exposed to Ar, CO$_{2}$, H$_{2}$ no significant changes in the current-voltage (I-V) behavior of the mat were observed. But CO exposure resulted in a dramatic increase in electrical conductivity, with the current increasing by about a factor of four. The response was reversible after evacuation. I-V measurement of the substrate showed near zero current (I $\le $ 100 fA) under vacuum, indicating that all of the current was through the nanowires. The I-V characteristics were acquired with a source-measure unit and the bias voltage was swept over the range of -5 V to +5 V with a typical step size of 50 mV. Typical currents when exposed to CO were in the range of 40 nA.