Anodically grown nanocrystalline titania thin film for hydrogen gas sensors—A comparative study of planar and MAIM device configurations

Nanocrystalline titanium dioxide (TiO2) thin film was grown by UV assisted anodization of 0.25 mm thick titanium foil (99.7% pure). Room temperature electrochemical oxidation and photo etching were carried out in 0.1 M dilute H2SO4 electrolyte and at 10 V potentiostatic bias without and with 400 W UV light illumination respectively. While 2D-XRD confirmed the anatase crystalline feature AFM study revealed a rough morphology. A band gap of 3.35 eV was determined by optical study. Palladium (Pd) contacts were deposited laterally on the oxide surface as catalytic metal electrodes to fabricate a planar sensor configuration and a vertical metal-active insulator-metal (MAIM) structure was configured using palladium and titanium-silver alloy as top and bottom electrode respectively. The hydrogen sensor study was carried out at different temperatures (100–175 °C) and with different gas concentrations (0.1–1%) for both planar and MAIM structures in nitrogen as well as in air. A detailed hydrogen gas response characteristic was studied for these two types of sensor structures. Both the sensor structures were found suitable to sense 1% hydrogen in nitrogen ambient and at the optimum temperature of 150 °C with a pretty fast response time of 1.1 s for planar sensor and 1.4 s for MAIM sensor. However, the corresponding recovery time of 102.5 s and 92.3 s were quite long. The subsequent studies in air in the identical conditions recorded the response time and recovery time of 0.49 s and 28.8 s for planar and 1.5 s and 44.6 s for MAIM configurations respectively. The selectivity and the long term stability were investigated. A comparative sensor study with the two devices was performed and the results have been explained.