Thermal Stability and Optimal Photoinduced Hydrophilicity of Mesoporous TiO₂ Thin Films

Abstract

Using the triblock copolymer Pluronic F127 (EO 106 PO 70 EO 106 ) as the templating agent and Ti(OBu n ) 4 as the titanium source, mesostructured TiO 2 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 TiO 2 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 TiO 2 thin films. It was shown that the synthesized mesoporous TiO 2 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 °C, the mesoporous framework was stable and the pore size was from 7.3 to 7.8 nm. Above 600 °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 TiO 2 thin films showed excellent hydrophilicity without light illumination; for example, the film obtained after being treated at 600 °C had a contact angle of about 22.5°, whereas the sample treated at 500 °C (the particle and pore sizes were 10.2 and 7.5 nm, respectively) showed the optimal photoinduced hydrophilicity, with the contact angle of about 9.5°.