Room temperature NO₂-sensing properties of hexagonal tungsten oxide nanorods

Hexagonal WO₃ nanorods were synthesized through a facile hydrothermal method. The nanorods properties were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The NO₂-sensing performances in terms of sensor response, response/recovery times and repeatability at room temperature were optimized by varying the heat treatment temperature of WO₃ nanorods. The optimized NO₂ sensor (400-℃-annealed WO₃ nanorods) showed an ultra-high sensor response of 3.2 and short response time of 1 s to 5-ppm NO₂. In addition, the 400-℃-annealed sample exhibited more stable repeatability. Furthermore, dynamic responses measurements of annealed samples showed that all the annealed WO₃ nanorods sensors presented p-type behaviors. We suppose the p-type behavior of the WO₃ nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor is exposed to NO₂ at room temperature.Therefore, the 400-℃-annealed WO₃ nanorods sensor is one of the most energy conservation candidates to detect NO₂ at room temperature.