Control of Nucleation in Solution Growth of Anatase TiO₂ on Glass Substrate

Low-temperature preparation of ceramic thin films in aqueous solution is an important new field in the synthesis of functional materials. This work examines control of nucleation in the growth of anatase titania (TiO₂) on a common glass substrate at 55−80 °C using an inorganic precursor, TiF₄. It is found that TiO₂ on fused silica is first formed as single-crystalline nuclei during an initial heterogeneous nucleation. These TiO₂ crystallites elongate along the [001] direction with their [110] axis perpendicular to the glass substrate; the elongated nuclei later grew into a round flowerlike morphology owing to a significant increase in homogeneous nucleation rate during the continued growth. The flowerlike polycrystalline TiO₂ islands, either as discrete crystallites or as lined crystallite arrays, subsequently develop into a planar crystallite monolayer. In general, low-pH condition suppresses heterogeneous nucleation and growth; this allows a self-alignment of thin threadlike crystallites. In contrast, higher pH favors the heterogeneous nucleation and subsequent planar growth but results in polydispersivity of crystallites. All of these observations can be related to the hydrolysis of TiF₄ and formation of Si−O−Ti and Ti−O−Ti linkages. In particular, localized HF concentration enrichment resulting from the TiF₄ hydrolysis is responsible for the sequential lined crystallite formation and planar-branching, as well as creation of unsaturated surface bonds of fused glass for heterogeneous nucleation. High nucleation densities have been achieved by mechanical and chemical treatments, and nanocrystalline TiO₂ thin films have been prepared using etched substrates.