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

Abstract

Surface hydrophobicity/hydrophilicity of titania (TiO _2 ) 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 ∼ 33.03%) and the surface roughness (0.22 ∼ 0.43 nm for arithmetic mean roughness and 0.28 ∼ 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 ∼ 7 nm to longer worm-like pores (30 ∼ 100 nm in length). The apparent static contact angle ( θ *) of the films with isolated mesopores is enhanced from ca. 90.6° to 100.1° 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 ( θ * = 80.7°) 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 TiO _2 films.

Keywords

• titania film
• pore morphology
• surface structure
• wettability