Aerosol-Assisted CVD of Titanium Dioxide Thin Films from Methanolic Solutions of Titanium Tetraisopropoxide; Substrate and Aerosol-Selective Deposition of Rutile or Anatase

Titanium dioxide (TiO2) thin films are synthesized using aerosol-assisted (AA) CVD of titanium (IV) isopropoxide (TTIP) in methanol. Deposition is carried out on glass, steel, and titanium substrates at 400-550 degrees C. The films produce morphologies that are radically different to those from typical aerosol-assisted processes, and from the use of TTIP in low or atmospheric pressure (AP) CVD. The films show some substrate-dependent morphology and properties. In particular at 550 degrees C the films on steel show needle-and rod-like particles. X-ray diffraction (XRD) and Raman spectroscopy of the TiO2 films show that on steel or titanium substrates only the rutile form can be obtained, whereas on glass either anatase, anatase/rutile mixtures, or rutile can be obtained, depending on substrate temperature. The TiO2 films formed at 550 degrees C on all substrates are hydrophobic to water droplets, with contact angles in the range 101-110 degrees. These films become hydrophilic on heating to above 100 degrees C in air, or superhydrophillic when irradiated under 254nm radiation generating water-contact angles less than 5 degrees. Surprisingly, use of TTIP under APCVD on steel substrates without an aerosol form exclusively the anatase form of TiO2 at 400-550 degrees C, whereas use of a methanolic aerosol delivery system for the TTIP forms rutile. Hence use of the methanol aerosol has a controlling influence on the deposition chemistry. The TiO2 thin films are shown to be active photocatalysts using a dye-ink test, and are also shown to be able to photo-split water in a sacrificial system to evolve oxygen.