Gas sensing characteristics of pure and Fe-doped tungsten oxide thin films

In this study gas sensing characteristics of pure and 10at% Fe-doped nanostructured tungsten oxide thin films of various thicknesses (100-500 nm) prepared by electron beam evaporation are reported. The sensing performances of the films towards various gasses (H₂, NH₃, NO₂, N₂O) at different operating temperatures (150-280°C) have been investigated. Very high sensing response of both types of films towards NO₂ compared to the other target gasses was observed. This indicated that the WO₃ based sensors are highly selective to NO₂ exhibiting much lower response to the other target gasses. The pure WO₃ sensor of 300 nm film thickness has shown the highest response amplitude at lower temperature (150°C). Doping of the tungsten oxide film with Fe significantly increases the base conductance of the pure film but also decreases the gas sensing response. The amount of Fe additives (10 at%) has not been optimized which can be one factor for the decrease of sensing response. A film thickness of 400 nm has shown the optimum sensing responses among the WO₃:Fe films. However, change of operating temperature between 200-250°C has shown little variation to the sensing response of the different films (100-500 nm). The high sensing performance of the WO₃ based sensors to NO₂ can be attributed due to the nanostructural nature of the films obtained through e-beam evaporation and subsequent annealing of the films at 300°C for 1 hour in air.