A porous tungsten oxide (WO₃) film based NO₂ sensor was developed by a one-step flame based process called Reactive Spray Deposition Technology (RSDT). The RSDT process allows the flexibility to create monoclinic gamma-WO₃ with control of the particle size, porosity and thickness of the film. This nano-crystalline WO₃ film was deposited directly on gold interdigitated electrodes. The sensor response was measured at the parts per million (ppm) level, (0.17-5 ppm in air at 100% relative humidity) at different operating temperatures (250-350°C) to determine the optimum working temperature. The sensitivity was  found to be maximum at 300 °C. The sensors showed a relatively fast response time (∼7s) and recovery time (∼5 min), respectively. The stability of the sensor was evaluated for 300 h in 0.5 ppm NO₂ in air at 100% relative humidity (2000 response-recovery cycles). The sensor was stable up to 6 days (~150 h) of continuous operation and degraded between 150 to 300 h. In order to check the selectivity, the sensor was also tested for 10 ppm acetone, 100 ppm ethanol, 10-100 ppm H₂, and 10 ppm isoprene. Interference was negligible with 10 ppm acetone, 100 ppm ethanol, 10 ppm H₂ and humidity; however, it was significant with 10 ppm isoprene and 100 ppm H_2. We have also compared the test data of this gamma WO₃ based NO₂ sensor prepared by RSDT with the results reported in the literature. The morphology and surface properties of the WO₃ film were investigated with XRD, Raman spectroscopy, BET, SEM, TEM, and HRTEM. 

Here we will provide a brief description of the synthesis, fabrication and testing procedure of the NO₂ sensor. The microstructure of the tungsten oxide films, and the effect of the film structure, grain size, and the sensor response to the ppm level concentration of NO₂ will be described. The sensing mechanism with emphasis to adsorption and desorption will be explained in details.