High-temperature NO sensing performance of WO₃ deposited by spray coating

Nitric oxide (NO) selective sensors capable of sensing in a hot-gas environment are increasingly required for monitoring combustion and processes yielding high temperature gas containing NO. This work reports the fabrication of sensors by a facile deposition of water-based ink blended commercial WO₃ powders via spray coating on sensor platforms fitted with Au-interdigitated electrodes (IDEs) and the characterization of their sensing performances under hot NO-containing air at temperatures exceeding 500 °C. After deposition and heat treatment of the sensing material on the substrate fitted with Au-IDE at 700 °C, the composition and morphology of the active material were analyzed and the presence of a single phase, fine particulates of WO₃, has been confirmed by XRD and SEM, respectively. The investigation of the sensing properties revealed that, contrary to the previous reports, this WO₃ sensor can detect NO with a good sensitivity (∼22% for 200 ppm NO) and selectivity at 700 °C under humidity. The effect of relative humidity on sensing performance was also investigated. Also, under humidity values as high as 10% RH and at gas temperatures as high as 700 °C, a reasonably good sensor performance has been observed. It is likely that the improved response towards NO at moderately elevated temperatures resulted from the humidity related water molecules which are adsorbed on the surfaces of WO₃ particles, providing high affinity hydrogen bonds between NO and OH.