GRAIN-SIZE EFFECTS IN TUNGSTEN OXIDE-BASED SENSOR FOR NITROGEN-OXIDES

The effects of the grain size of WO3 on WO3-based sensor for NO(x) were investigated. By hydrolyzing ammonium paratungstate with hot nitric acid in solution and calcining the resulting precipitate in air at prescribed temperatures in the range of 300 to 600-degrees-C, the mean crystallite sizes (D) of WO3 were varied in the range of 16 to 57 nm. The sensitivities of the WO3 sensor elements to NO2 (10 ppm) as well as to NO (200 ppm) remained independent of D down to D = 33 nm, below which the sensitivities increased steeply with a decrease in D, reaching at D = 25 nm the sensitivity values three to four times as large as those for D > 33 nm.