Effect of pore morphology and surface roughness on wettability of porous titania films

Surface hydrophobicity/hydrophilicity of titania (TiO2) films, spin-coated on silicon wafers, were tuned by introducing surface mesopores with various morphologies using a triblock copolymer F38 as the template agent of different weight ratios via a sol-gel method. It is found that both the porosity (2.92 similar to 33.03%) and the surface roughness (0.22 similar to 0.43 nm for arithmetic mean roughness and 0.28 similar to 0.58 nm for root mean square roughness) of the films increase monotonically as increasing F38 ratio from 5 to 25 wt%, accompanied by distinct changes of pore morphology from isolated mesopores with pore sizes of 5 similar to 7 nm to longer worm-like pores (30 similar to 100 nm in length). The apparent static contact angle (*) of the films with isolated mesopores is enhanced from ca. 90.6 degrees to 100.1 degrees as indicated by an increase of the roughness factor with incresing F38 from 5 to 15 wt%, which is in qualitative agreement with the Wenzel's equation. Interestingly, the films with interconnected worm-like pores show obvious hydrophilicity (* = 80.7 degrees) with further increasing F38 ratio higher than 20 wt%. The reversed surface wettability show that not only surface roughness but also pore morphology could significantly affect the wettability of the mesoporous TiO2 films.