Organic-inorganic hybrid: the role played by surface energy and structure of the hydrophobizing molecules

The functionalization of nano-TiO2 by siloxanes gives rise to robust covalent bonds by mild reactions upon hydrolysis of –OR’ moieties producing a very promising class of materials, in which siloxanes may also serve as powerful linkers for different functionalities. These linkages bear technological relevance to widely different fields, from the promotion of self-cleaning processes, to the prevention of clotting in artificial blood vessels, the protection of outdoor cultural heritage or in enhancing the electron transfer in photovoltaic devices. In this work the features of hydrophobic layers deposited onto rough TiO2 films composed by home-made nanometric particles are investigated. Three different kinds of siloxanes were chosen, as functionalizing agents, both commercial and laboratory made showing different end-groups. For the three molecules the surface energy, together with their polar and disperse components, is calculated on the grounds of contact angle measurements of different solvents on thick siloxane coatings deposited onto smooth glass slides. The morphology of the hybrid TiO2-siloxane films is analyzed by AFM and EDX and their wettability by water is evaluated by receding and advancing contact angle determinations. The structure of the surface layer and the modes of attachment of Si atoms are investigated by combining data of contact angle hysteresis, surface mapping by EDX and CP/MAS NMR analyses performed on samples with increasing siloxane content. The chemical specificity introduced in the TiO2 hybrids by the different siloxane end-groups is discussed on the grounds of the composite interactions with different solvents. Finally self cleaning tests by using Alizarine Red dye are performed on the siloxane-clad TiO2 layers to evaluate the actual hydrophobicity of the hybrid materials.