Noble metal catalytic contacts to sol–gel nanocrystalline zinc oxide thin films for sensing methane

Undoped nanocrystalline n-ZnO thin films were deposited on SiO2-coated p-Si substrates by a sol–gel method to fabricate ZnO-based resistive sensors for methane detection. The sensor performance was studied with three different catalytic noble metal contacts to ZnO, i.e. palladium–silver (26%), rhodium and platinum. The response magnitude, response time and recovery time were studied in the temperature range of 50–350 °C and at methane concentrations of 0.1%, 0.5% and 1%. Pd–Ag and Rh contacts were found to give lower optimum operating temperature (250 °C) than Pt contact (300 °C). Although Rh contact showed a higher response (83.6%) than Pd–Ag (74.3%) and Pt (69.3%) contacts, Pd–Ag contact was found to exhibit the shortest response and recovery time amongst the three catalytic contacts to ZnO.