Phase-controlled preparation of TiO2 films and micro(nano)spheres by low-temperature chemical bath deposition

TiO2 films and micro(nano)spheres have been simultaneously prepared by means of low-temperature chemical bath deposition (CBD). The films and precipitates are found to be constructed of the same building blocks regardless of the bath conditions. Thin nanosheets of ca. 20 nm in thickness serve as the building blocks of rutile TiO2 grown in an acidic bath. They aggregate to form spheres to reduce the surface energy of the precipitates, while in the films they grow into discrete crystallites on the seeded substrate. Fine control over the crystalline phase of TiO2 is achieved by replacing some water with ethanol or the addition of NH4F. A gradual transition from rutile to anatase is observed as the ethanol and NH4F contents are increased. Moreover, the crystallite size of TiO2 is significantly reduced and there exists monodisperse nanoparticles and aggregate microspheres in the products modified with ethanol and NH4F, respectively. This size-dependent phase transition is consistent with previous theoretical and thermodynamic studies. The facile tune of the TiO2 phase by low-temperature CBD method may be useful to improve the performance of TiO2 for its various applications.