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)(0.09)WO2.95. The structure, composition and morphology of h-WO3 were studied by XRD, XPS, Raman, H-1 MAS (magic angle spinning) NMR, scanning electron microscopy (SEM), and BET-N-2 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 m(2)/g and was thermally stable up to 450 degrees 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. (C) 2008 Elsevier Ltd. All rights reserved.