Hydrophobic, Photoactive Titania-Alumina Nanocrystallites and Coatings by an Aqueous Sol-Gel Process

Hydrophobic coatings of titania-alumina nanocrystallites showing enhanced photocatalytic activity under sunlight were synthesized by an aqueous sol-gel route from titanyl sulfate. The influence of the addition of alumina in the range 5-30 mol-% on the phase transition and grain growth of nanocrystalline titania was investigated. A titania-alumina composition containing 5 mol-% Al2O3 (TA-5), annealed at 700 degrees C, showed the highest photocatalytic activity among all investigated compositions. While pure titania in the present study started to transform to rutile at 700 degrees C, all the titania-alumina compositions retained 100% anatase phase up to 800 degrees C. The specific surface area was measured as 73 m(2)g(-1) for the titania containing 5 mol-% alumina annealed at 700 degrees C, whereas pure titania had 38 m(2)g(-1) under identical conditions. By photon correlation spectroscopy, the particle size of pure titania sol was measured as 29 nm, while that of titania sol containing 5 mol-% alumina was 43 nm. Nanocoatings applied over glazed ceramic surfaces indicated its hydrophobic character though the composition did not contain any hydrophobic material, which produces a high water contact angle. The uniformly roughened surface, preferably with nanostructured morphology that lowers the surface energy, is proposed to be the reason for the observed hydrophobicity.