Anodically grown nanocrystalline titania thin film for hydrogen gas sensors – a comparative study of planar and MAIM configuration

Abstract

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.