Sol-gel preparation of TiO2 ceramic coating films from aqueous solutions of titanium sulfate (IV) containing polyvinylpyrrolidone

TiO2 ceramic coating films were deposited on silica glass substrates by the sol- gel method using aqueous solutions of Ti(SO4)2 containing polyvinylpyrrolidone (PVP) as the coating solutions. Unless PVP was added to the solution, the wettability of the substrate was poor, leading to failure in gel film formation. When PVP was added to the solution, on the other hand, homogeneous gel films could be deposited on the substrate, which could be converted to crack- free, transparent TiO2 ceramic thin films about 0.1 μm in thickness. The TiO2 films thus obtained had refractive index, porosity, and surface roughness Ra (the arithmetic average deviation of the assessed profile) of 2.68, 2.1%, and 0.94 nm, respectively. For comparison, a TiO2 film was also prepared from an alcoholic solution of Ti(OC3H7i)4, showing refractive index, porosity, and Ra of 2.65, 3.0%, and 0.98 nm, respectively. These values indicate that the TiO2 films prepared from the Ti(SO4)2 aqueous solutions containing PVP are as smooth and dense as those prepared from conventional alkoxide solutions. The solutions were found to be stable in viscosity for more than five months at room temperature in sealed containers. Thermal analysis, infrared absorption spectra measurement, and x- ray diffraction analysis indicated that the gel films are converted to ceramic films on firing via (i) vaporization of H2O; (ii) decomposition of H2SO4 and/or vaporization of H2SO4 · H2O; (iii) decomposition of SO42− coordinating Ti atoms and oxidation and decomposition of PVP, the latter of which leads to formation of residual carbons; (iv) oxidation of the carbonaceous residues; and (v) crystallization of the titania phase. During the gel- to- ceramic film conversion, the thickness decreased by 90%.