Producing photoactive, transparent and hydrophobic SiO2-crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings

Nowadays, the enhancement of atmospheric pollution is dramatically increasing the presence of soiling on buildings in every city of the world. Thus, the development of photocatalysts as self-cleaning coatings is a promising challenge. The first object of this work was to develop a simple synthesis route for obtaining SiO2-crystalline TiO2 nanocomposites at ambient temperature. Thus, it meets the requirements to produce photoactive coatings on buildings and, additionally, it can be used in other applications requiring low temperature. The second objective was to gain insights into the structure of these materials and to establish the relationship between their structure and their performance as photocatalysts. The synthesis process involves mixing titanium and silicon alkoxides in the presence of oxalic acid. An organic silica oligomer is also added to reduce surface energy and consequently, to give hydrophobic properties to the material. These products were applied as a coating on marble specimens in order to investigate their effectiveness. The results obtained highlight that oxalic acid acts a chelating agent of the titanium precursor, giving rise to a homogeneous TiO2-SiO2 material. In addition and most importantly, oxalic acid produces photoactive anatase crystals at ambient temperature. From our investigation on the structure of these materials, we conclude that the formation of independent domains of TiO2 inside of SiO2 matrix, and the anatase crystal size are key factors for improving the photoactivity of the coatings. We also conclude that the oxalic acid concentration plays a crucial role in the formation of this optimum structure. (C) 2014 Elsevier B.V. All rights reserved.