Super-hydrophilic and photocatalytic properties of Ag-TiO2 thin films prepared by sol-gel technique

A comparative study of photocatalytic degradation of methyl orange (liquid-solid interface) and methyl stearate (solid-solid interface) was performed with three different kinds of transparent pure and Ag-doped TiO2 films, prepared on glass by sol-gel dip-coating technique. The optical, morphological and hydrophilic properties of the thin films were characterized by UV-Vis spectrophotometer, atomic force microscope (AFM) and water contact angle technique, respectively; whereas the crystalline form, the specific surface area and the chemical state/composition were determined by X-ray diffraction, BET nitrogen sorption analysis and X-ray photoelectron spectroscopy (XPS), respectively. Ag doping increases the thin films surface roughness and inhibits the agglomerates structure formation of nano-size TiO2 particles during heat treatment. The XPS results showed that the silver is present in the ionic state and that its concentration is much higher at the surface of the film than in the inner layers, suggesting Ag ions migration from the interior to surface of the film during film preparation and thermal treatment. The influence of air contamination and UV irradiation on water contact angle was systematically examined in order to understand the hydrophobic/hydrophilic properties and their effect on the photocatalytic activity for methyl stearate photodegradation. It has been observed that small dopant concentration enhanced photocatalytic activity of TiO2 film remarkably towards methyl orange aqueous solution, whereas the same thin film showed slightly lower activity than pure TiO2 towards methyl stearate layer. The photodegradation results of methyl stearate and methyl orange indicate that the activity of pure TiO2 thin films is favored at the solid-solid interface, while Ag doped TiO2 is more active at the liquid-solid interface.