Nanometric TiO2-based materials for environmental remediation and self-cleaning

In recent years , the increased environmental awareness of public opinion and legislators has prompted the research activity in the field of environmental protection and remediation. Photocatalytic oxidation of pollutants by TiO2 is one of the most promising technologies in this field, especially for the removal of low concentration pollutants in slightly contaminated atmospheres.[1] Moreover, nano-TiO2 based materials with modified wettability properties have been successfully applied to the self-cleaning of surfaces. [2] Combining photocatalytic activity and wettability modulation would allow to achieve efficient self-cleaning materials, for applications such as the protection of the outdoor cultural heritage. Here, the two complementary approaches are investigated in detail. The characteristics of the nano-oxide are modulated by combining a template synthesis and Bi doping. The resulting oxides are deposited in thin films and tested for their ability to promote the degradation of methylene blue (MB) stains. Photocatalytic results show a marked enhancement of the photocatalytic activity. In fact, the template synthesis increases the oxide surface area oxides, while the Bi doping leads to an effective reduction of the recombination rate of photogenerated charge. Nano-TiO2 is then functionalized with several siloxanes molecules to tailor its surface wettability. The effect of the siloxane structure and amount on the interaction with the TiO2 surface is investigated. Depending on the type of siloxane employed, films showing superhydrophobic behaviour, i.e. water contact angles higher than 150° and low contact angle hysteresis, can be obtained. Their Cassie-Baxter-type wetting behaviour derives from the combined effect of low surface energy siloxanes and TiO2-derived film roughness. The composite films are tested for their self-cleaning ability using an aqueous solution of MB. The dye interaction with the film surface is evaluated by ATR-FTIR spectroscopy. All materials are thoroughly characterized for their structural (XRD, CP/MAS NMR), morphological (BET, HR-TEM), and surface properties (dynamic contact angle measurements).