Transparent conducting oxide electrodes for novel metal oxide gas sensors

We report fabrication and gas sensing properties of semiconducting metal oxide gas sensors using conducting oxide electrodes. Indium–tin oxide (ITO) and aluminum-doped zinc oxide (AZO) films are used to replace Pt electrodes in WO3 or SnO2 thin-film gas sensors. Before and after thermal annealing at 300 °C for 3200 min, the resistivity of the ITO film increases from 1.3 × 10−4 to 7.0 × 10−4 Ω cm, whereas the AZO film shows a significant increase in resistivity from 2.0 × 10−3 to 5.1 × 101 Ω cm due to the annihilation of oxygen vacancies in the film. Upon exposure to 50 ppm CO at 300 °C, WO3 or SnO2 thin-film sensors with ITO interdigitated electrodes (IDEs) on glass substrates display higher responses than sensors with Pt IDEs, attributed to the low-resistance ohmic contacts between the electrode (ITO) and the sensing material (WO3 or SnO2). The reproducible response, the concentration-dependent modulation in sensitivity, and a sub-ppm detection limit indicates the reliable operation of sensors made with ITO IDEs. The high transmittance, exceeding 75%, of the sensors at visible wavelengths holds promise for future applications to transparent gas sensors.