Aerosol-assisted chemical vapour deposition of WO3 thin films using polyoxometallate precursors and their gas sensing properties

Aerosol-assisted chemical vapour deposition (AACVD) of polytungstates in acetonitrile or water yielded thin films of tungsten oxide on glass. AACVD reactions of [NH4]6[W12O39] and [NH4]10H2[W2O7]6 and of [nBu4N]4[W10O32] and [nBu4N]2[W6O19] at substrate temperatures exceeding 500 °C resulted in the formation of yellow films comprised of randomly-orientated crystalline monoclinic WO3. In contrast, the films deposited from [nBu4N]3[WO4] were blue and showed preferred orientation along the <010> direction, with the direction of crystallites becoming increasingly randomised with increasing deposition temperature. Annealing these films in air for 30 minutes at 550 °C yielded yellow films in which the crystallites were randomly orientated. The WO3 films functioned as gas sensors showing a linear change in electrical resistance upon exposure to trace amounts of ethanol and nitrogen dioxide vapour in air, with responses comparable to that of screen-printed sensors and a faster speed of response. Furthermore, the CVD sensors gave a maximum response to nitrogen dioxide at a significantly lower temperature (250 °C) than the screen-printed sensor.