MICROMACHINED HYDROGEN SENSORS WITH NANOSTRUCTURED TUNGSTEN OXIDE ACTIVE LAYERS: NANODOTS VS NANOTUBES. A COMPARATIVE STUDY

Nanostructured metal oxides are of growing importance for application as active layers in gas micro-sensors. Tungsten trioxide is the most promising candidate for selectively detecting hydrogen, which is an important issue in hydrogen-energy-based systems for safety use. In this work, two types of sensors employing identical silicon micro-hotplate transducer geometry but with different morphological structures of the active film are prepared and evaluated. Type 1 sensor utilizes a smooth thin film composed of self-ordered tungsten oxide nanodots synthesized by anodizing Al/W/Ti metal layers, whereas Type 2 sensor employs an array of tungsten oxide nanotubes prepared by sputter-deposition over porous anodic alumina templates. While Type 1 sensor is considered as mainly two-dimensional limiting the response kinetics to the sensitive film surface, Type 2 sensor is three-dimensional since the diffusion of chemical species within the nanotubes and their interaction with the nanotube walls play an important role in the detection mechanism. The dataset for each micro-sensor comprised hydrogen dosed with various concentrations at several operating temperatures. The results show, for example, that the sensitivity of Type 2 sensor is tenfold higher whereas Type 1 sensor is 5 times faster, both being best responsive to hydrogen gas at as low temperature as 150 degrees C. Although the two sensing films were successfully integrated in the micro-hotplate transducers, the approach to forming Type 1 sensor is more facile, cost-effective, better reproducible and reduces the number of processes, materials and production time.