Gas sensing selectivity of hexagonal and monoclinic WO 3 to H 2S

Abstract

Hexagonal and monoclinic tungsten oxide (h- and m-WO 3) samples were produced by annealing hexagonal ammonium tungsten bronze, (NH 4) 0.07(NH 3) 0.04(H 2O) 0.09WO 2.95 at 470 and at 600 °C, respectively. Their structure, composition and morphology were analyzed by XRD, Raman, XPS, 1H-MAS NMR and SEM. In order to study the effect of crystal structure on the gas sensitivity of tungsten oxides, h- and m-WO 3 were tested as gas sensors to CH 4, CO, H 2, NO and H 2S (1000 and 10 ppm) at 200 °C. Monoclinic WO 3 responded to all gases, but its gas sensing signal was two magnitudes greater to 10 ppm H 2S than to other gases, and it also detected H 2S even at 25 °C. Hexagonal WO 3 responded only to 10 ppm H 2S. Its sensitivity was smaller compared to m-WO 3, however, the response time of h-WO 3 was significantly faster. The gas sensing tests showed that while m-WO 3 had relative selectivity to H 2S in the presence CH 4, CO, H 2, NO; h-WO 3 had absolute selectivity to H 2S in the presence these gases.