This paper discusses the formation of nanosized hexagonal tungsten oxide (h-WO 3 ) during the annealing of hexagonal ammonium tungsten bronze (HATB), (NH 4 ) 0.33-x WO 3-y . This process was investigated by TG/DTA-MS, XRD, SEM, Raman, XPS, and 1 H-MAS NMR analyses. Through adjusting the temperature and atmosphere of annealing HATB, the composition (W oxidation state, residual NH 4 + and NH 3 content) of h-WO 3 could be controlled. The effect of composition on the conductivity and gas sensitivity of h-WO 3 was studied. New structural information was obtained about both HATB and h-WO 3 . It was found that NH 4+ and NH 3 could be situated at three different positions in HATB. Residual NH 4 + and NH 3 in the hexagonal channels seemed to be vital for stabilizing h-WO 3 : when they were completely released, the hexagonal framework collapsed. We propose that the structure of h-WO 3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels.

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Hexagonal (h-) WO3 was prepared through heating hexagonal ammonium tungsten bronze (HATB), (NH4)0.07(NH3)0.04(H2O)0.09WO2.95. By adjusting the heating temperature and atmosphere of HATB, we could control the oxidation state of tungsten atoms and the residual NH3/NH4 + content in h-WO3. The as-produced h-WO3 nanoparticles with different composition were tested as gas sensors and the effect of composition on gas sensing properties was studied. Our results showed that oxidized h-WO3 had the best sensitivity to H2S.