Simple point contact WO₃ sensor for NO₂ sensing and relevant impedance analysis

A simple and new point contact tungsten trioxide (WO₃) sensor, which can be prepared by the oxidation of tungsten filaments via in-situ induction heating, likely detects low concentration (ppm level) environmental pollutants such as NO₂. X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) were applied to characterize the phase and the microstructure of the samples, respectively. It was found that the synthesized WO₃ films exhibited a monoclinic phase and were composed of hierarchical microcrystals and nanocrystals. The point contact WO₃ sensor (W-WO₃-W) showed rectifying characteristics and an ideal sensing performance of about 110℃. A single semicircle in Nyquist plots was recorded by electrochemical impedance spectroscopy (EIS) at a relatively low temperature of 150℃ but faded away above 200℃, which revealed that the sensing process was governed by a determining factor, i.e., grain boundaries at the contact site.