Stability and Controlled Composition of Hexagonal WO₃

Abstract

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.