Fabrication of WO3 nanodot-based microsensors highly sensitive to hydrogen

A novel approach that involves electrochemical anodising for fabricating WO3 nanodot-based microsensors is presented. The formation of gas sensing film is assisted by a nanoporous anodic alumina layer grown from a thin layer of aluminium sputtering-deposited over a thin layer of tungsten (Al/W bilayer). The anodic film annealed at 500 °C consists of a self-ordered array of WO3 nanodots (∼50 nm), each composed of few nanocrystallites (∼9 nm) having thermostable tetragonal structure. The processes for forming the WO3 nanodot active films are fully compatible with standard technology for fabrication of sensor chips on a micromachined silicon wafer. Characterization of the WO3 nanodot film was performed by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction analysis. A study of the ability of the WO3 nanodot-based microsensors to detect H2, CO, ethanol and relative humidity was presented for the first time. High sensitivity to hydrogen, with linear response characteristic was obtained in the range of low H2 concentrations (5–20 ppm). The gas sensing mechanism of the WO3 nanodot-based sensors was discussed.