Preparation of hexagonal WO3 from hexagonal ammonium tungsten bronze for sensing NH3

Hexagonal tungsten oxide (h-WO3) was prepared by annealing hexagonal ammonium tungsten bronze, (NH4)0.07(NH3)0.04(H2O)0.09WO2.95. The structure, composition and morphology of h-WO3 were studied by XRD, XPS, Raman, 1H MAS (magic angle spinning) NMR, scanning electron microscopy (SEM), and BET-N2 specific surface area measurement, while its thermal stability was investigated by in situ XRD. The h-WO3 sample was built up by 50–100 nm particles, had an average specific surface area of 8.3 m2/g and was thermally stable up to 450 °C. Gas sensing tests showed that h-WO3 was sensitive to various levels (10–50 ppm) of NH3, with the shortest response and recovery times (1.3 and 3.8 min, respectively) to 50 ppm NH3. To this NH3 concentration, the sensor had significantly higher sensitivity than h-WO3 samples prepared by wet chemical methods.