Fluoroalkylsilane Modification of Sol–Gel SiO2-TiO2 Coatings

Sol–gel chemistry of SiO_{2(1 – x)}TiO_2(x) involving the partial pre-hydrolysis of tetraethoxysilane (TEOS) and subsequent reaction with titanium isopropoxide has been used to produce materials of high area and variable absorption edge in the UV-visible spectrum. As x increases so the total surface area of these phases unexpectedly passed through a maximum, but the UV-vis cut off moved to greater wavelength. On the other hand, X-ray photoelectron spectroscopy (XPS) measured O_1s binding energies and temperatures of maximum rates of reduction (T_max) that were intermediate between those of pure SiO₂ and TiO₂. Both suggested that SiO_{2(1 – x)}TiO_2(x) samples were an intimate mixture of Si^{4 +} and Ti^{4 +} in the sol–gel matrix. Perfluoroalkylsilane (PFAS) was strongly adsorbed on this sol–gel coating making it strongly hydrophobic. This mode of modification was better than PFAS incorporation at the start of the sol–gel synthesis. Such sol–gel chemistry might in the future be optimised for water-repelling surfaces.