Stability and controlled composition of hexagonal WO3

This paper discusses the formation of nanosized hexagonal tungsten oxide (h-WO3) during the annealing of hexagonal ammonium tungsten bronze (HATB), (NH4)(0.33-x)WO3-y. This process was investigated by TG/DTA-MS, XRD, SEM, Raman, XPS, and H-1-MAS NMR analyses. Through adjusting the temperature and atmosphere of annealing HATB, the composition (W oxidation state, residual NH4+ and NH3 content) of h-WO3 could be controlled. The effect of composition on the conductivity and gas sensitivity of h-WO3 was studied. New structural information was obtained about both HATB and h-WO3. It was found that NH4+ and NH3 could be situated at three different positions in HATB. Residual NH4+ and NH3 in the hexagonal channels seemed to be vital for stabilizing h-WO3: when they were completely released, the hexagonal framework collapsed. We propose that the structure of h-WO3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels.