High-temperature resistive hydrogen sensor based on thin nanoporous rutile TiO₂ film on anodic aluminum oxide

Thin titanium oxide (TiO₂) films were prepared through electron-beam evaporation of titanium metal on substrates followed by oxidizing and sintering at 600 °C in flowing oxygen. The thicknesses of the as-deposit metal films were 25 nm, 50 nm and 100 nm. The phase of the TiO₂ converted from the thermal oxidation was detected to be rutile. Nanoporous anodized aluminum oxide (AAO) and plain thermal silicon oxide on top of a commercial silicon wafer were used as substrates to support the TiO₂ thin layer. A pair of interdigit platinum electrodes with a spacing of 5 μm was fabricated on the TiO₂ thin films by photolithography. At 500 °C, the samples showed different sensing behaviors to hydrogen concentration levels ranging from 5 ppm to 500 ppm, with nitrogen as the background gas. It was found that the sensitivity was significantly enhanced by the increased specific surface area of the TiO₂ thin film due to the shaping of the porous AAO substrate. The performance of the sensor based on the TiO₂ film converted from 25-nm-thick Ti on the porous AAO substrate, which has the largest specific surface area among all the other samples, was featured by a conductance change of 25-90 times and considerable resolution for 5 ppm to 500 ppm H₂, as well as very fast response and recovery (the time delay to reach or retreat to half of the maximum stable signal, t_50%, was always no more than 10 s). The rutile-phased thin TiO₂ film on AAO is proven to be a promising high-temperature hydrogen sensor with satisfactory performance, excellent durability, and ideal compatibility to micro-miniaturization.