Aerosol assisted chemical vapour deposition of MoO3 and MoO2 thin films on glass from molybdenum polyoxometallate precursors; thermophoresis and gas phase nanoparticle formation

Aerosol assisted chemical vapour deposition (AACVD) of molybdenum polyoxometallates dissolved in acetonitrile or water yielded adhesive thin films of molybdenum oxides on glass. At substrate temperatures of 300 - 350 degrees C single phase MoO3 was obtained, from 350 - 500 degrees C mixed phases of MoO3 - MoO2 were formed and at 500 - 550 degrees C single phase MoO2 was observed. The morphology of the as-deposited molybdenum oxide films was found to be dependent upon a number of factors including the nature of the precursor used, the deposition temperature and position of the film within the reactor. Needles, spheres, agglomerates and platelets formed depending on the conditions. The films with a needle-like microstructure displayed enhanced hydrophobicity to water droplets (125 degrees contact angle). X-Ray diffraction showed that the MoO3 films had typical cell constants of a = 3.96, b = 13.85, c = 3.69 angstrom and the MoO2 films had typical cell constants of a = 5.62, b = 4.84, c = 5.56 angstrom, beta = 119.32 degrees. The MoO2 films were readily converted to MoO3 by annealing in air for 30 minutes at 600 degrees C. The MoO3 films functioned as gas sensors showing a linear change in electrical resistance upon exposure to trace amounts of ethanol vapour in air.