Effect of withdrawal speed on the microstructure, optical, and self-cleaning properties of TiO2 thin films

The transparent superhydrophilic TiO2 films were dip-coated on glass substrates by sol-gel method at different withdrawal speeds. The dependence of the morphological, transmittance, and self-cleaning properties of the films on withdrawal speed were investigated by scanning electron microscope (SEM), atomic force microscope (AFM), ultraviolet-visible (UV-Vis) spectrophotometer, photocatalytic degradation of a methyl blue, and water contact angle. The results indicated that the withdrawal speed had a great impact on the surface morphology, the transmittance, and self-cleaning properties of the TiO2 films. With the withdrawal speed increasing, the grains size, porosity, and RMS values of the films gradually increase, and the absorption edge shifts toward longer wavelength. These increase the radiation absorption and photocatalytic activity. The hydrophilic property enhances with the withdrawal speed increasing, due to the surface roughness and capillary force from pores between grains. All the prepared films are transparent in the visible spectrum (400-800 nm) with an average transmittance of more than 70%. Films with high transmittance (T similar to 90% at 500 nm) and good self-cleaning properties (WCA < 5 degrees) can be obtained at high withdrawal speed. [GRAPHICS] .