Highly sensitive NO₂ sensors using lamellar-structured WO₃ particles prepared by an acidification method

Tungsten trioxide (WO₃) was prepared by acidification of Na₂WO₄ with acid solutions such as H₂SO₄, HCl, and HNO₃ (pH 0.5 to -0.8) and tested for its NO₂ sensing properties. Acidification with strong acid solutions (pH -0.5, -0.8) was found to produce lamellar-structured WO₃ particles, which consisted of nano-sized crystalline plates that were 100-350 nm in lateral size and 20-50 nm in thickness, as observed by XRD and SEM analyses. The sizes of the primary and secondary particles were decreased by decreasing the pH of the acid solution used. This was accompanied by an increase in the specific surface area. The NO₂ responses of the prepared WO₃ lamellae were dependent on their morphology. The device using smaller WO₃ lamellae prepared with a H₂SO₄ solution (pH -0.8) had the highest sensor response, exhibiting a high sensor response (S = 150-280), even to dilute NO₂ (50-1000 ppb) in air at 200 °C. The use of smaller lamellae resulted in a decrease in the electrical resistance of the device, probably due to intimate contact between smaller lamellar particles, which allowed the detection of NO₂ in a rather wide concentration range. In addition, the developed device showed high NO₂ selectivity without substantial interference from NO.