Natural superhydrophilicity of sol–gel derived SiO2–TiO2 composite films

Sol–gel SiO2–TiO2 mixed films have been deposited from a polymeric SiO2 solution and either a polymeric TiO2 mother solution (MS) or a derived TiO2 crystalline suspension (CS). The chemical and structural compositions of MS and CS mixed films heat-treated at 110 or 500 °C have been investigated by Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Water contact angle measurements show that TiO2 rich MS films exhibit photo-induced superhydrophilicity, but cannot maintain a zero contact angle in the absence of UV light. In contrast, CS mixed films exhibit a natural and persistent superhydrophilicity for a large range of compositions. Superhydrophilic properties are analyzed in terms of enhanced acidity at the SiO2–TiO2 interfaces and discussed with respect to the chemical and structural composition of MS and CS films.