Persistent superhydrophilicity of sol-gel derived nanoporous silica thin films

In this investigation, sol-gel synthesized nanoporous silica thin films, annealed at different temperatures, with long time superhydrophilic property have been studied. Two kinds of sol-gel silica thin films were fabricated by dip-coating of glass substrates in two different solutions; with low and high water. The transparent coated films were dried at 100 degrees C and then annealed in a temperature range of 200-500 degrees C. The average water contact angle of the silica films prepared with low water content and annealed at 300 degrees C measured about 5 degrees for a long time (6 months) without any UV irradiation. Instead, adding water into the sol resulted in silica films with an average water contact angle greater than 60 degrees. Atomic force microscopic analysis revealed that the silica films prepared with low water had a rough surface (similar to 30 nm), while the films prepared with high water had a smoother surface (similar to 2 nm). Using x-ray photoelectron spectroscopy, we have shown that with a decrease in the surface water on the film, its hydrophilicity increases logarithmically.