Thermal Stability and Optimal Photoinduced Hydrophilicity of Mesoporous TiO2 Thin Films

Using the triblock copolymer Pluronic F127 (EO106PO70EO106) as the templating agent and Ti(OBun)(4) as the titanium source, mesostructured TiO2 thin films were constructed through the sol-gel and evaporation-induced self-assembly method. The effect of the calcination temperature on the mesostructure and on the hydrophilicity of the obtained mesoporous TiO2 thin films was investigated. Small-angle and wide-angle X-ray diffractions, transmission electron microscopy, N-2 adsorption-desorption, and contact angle measurements were used to characterize the as-synthesized TiO2 thin films. It was shown that the synthesized mesoporous TiO2 materials exhibited an excellent thermal stability and possessed pores with a diameter larger than 7 nm and a narrow pore-size distribution, and thick inorganic walls composed of nanocrystalline anatase. The calcination temperature affected the stability of the mesoporous structures. In the range of 450-600 degrees C, the mesoporous framework was stable and the pore size was from 7.3 to 7.8 nm. Above 600 degrees C, however, the structure collapsed partially. On the basis of the film structure, a four-coordinate channel mode was proposed and the collapse criterion for the mesoporous structure was established through thermodynamic calculation. The synthesized mesoporous TiO2 thin films showed excellent hydrophilicity without light illumination; for example, the film obtained after being treated at 600 degrees C had a contact angle of about 22.5 degrees, whereas the sample treated at 500 degrees C (the pat-tide and pore sizes were 10.2 and 7.5 nm, respectively) showed the optimal photoinduced hydrophilicity, with the contact angle of about 9.5 degrees.