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

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Abstract
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 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.