Preparation and characterization of titania thin films from aqueous solutions

Dip- or spin-coating and characterization of titania (TiO2) thin films from various aqueous solutions have been studied. The aqueous titanium solutions mainly used in this study were halogen- and chelate-free solutions with the concentrations up to 1.4 M derived from titanium isopropoxide (TIP) with tetramethylammonium hydroxide (TMAOH) or some alkylamines, while aqueous and alcoholic solutions containing titanium atoms stabilized chelating ligands were examined for comparison. The TiO2 films prepared from the TIP-TMAOH solution were already crystallized at 350degreesC to anatase form and those formed at 600degreesC had high transparency and refractive indices of 2.40. No carbon residue in the film prepared at 400degreesC was detected by XPS. The pure anatase form was sustained up to 850degreesC. Interestingly, it was found that the (004) preferentially oriented anatase films were obtained from TIP-lactic acid (LA) system until 700degreesC. The solutions containing citric acid (CA) or alkanolamines yielded anatase and rutile form fired at the temperatures equal to or higher than 600degreesC. Carbon residue was detected in the film fired at 400degreesC. The film thickness monotonically decreased from the upper to the bottom ends of the substrate. However, it was found that the thickness uniformity was drastically improved by an addition of sucrose to the aqueous solutions. The effects of the solution composition and polyhydroxy compounds on the crystal modifications of formed films and the film uniformity are discussed.