Fast response methane sensor using nanocrystalline zinc oxide thin films derived by sol–gel method

Undoped nanocrystalline n-ZnO thin films were deposited on SiO2-coated p-Si substrates by sol–gel technique to fabricate ZnO-based resistive sensors for methane detection. The resistance change was studied at different temperatures (50, 150, 200, 250, 300 and 350 °C) with two different metallic contacts to ZnO, e.g. gold and palladium-silver, in air and in the presence of three different concentrations (0.1, 0.5 and 1%) of methane in N2 carrier gas. The response, response time and recovery time were studied in detail. A significant improvement of the sensor performance was observed with a Pd-Ag (26%) contact. The maximum response of 74.3% and the lowest response time of 16.3 s were obtained at 250 °C using the Pd-Ag (26%) catalytic metal contact to ZnO. The mechanism of sensing by ZnO was suggested similar to that of the SnO2 sensors reported by Yamazoe.