Simple physical approach to reducing frictional and adhesive forces on a TiO₂ surface via creating heterogeneous nanopores

A simple physical strategy to reduce the frictional and adhesive forces on TiO₂ films was proposed by constructing mesoporous TiO₂ films with heterogeneously distributed nanopores on the film surfaces. In comparison, TiO₂ films with densely packed nanoparticles were also prepared. The crystal structure and morphology of the films were characterized with Raman spectroscopy, field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). It was found that the TiO₂(B) phase exists in the mesoporuos TiO₂ films but not in the densely packed films. The existence of TiO₂(B) plays a significant role in creating and maintaining the nanopores in the mesoporous TiO₂ films. The frictional and adhesive forces were measured on both films using AFM. The mesoporous films exhibit two typical adhesion forces of around 3 and 12 nN in the force distribution profile whereas the densely packed films show only one around 12 nN. The frictional coefficients were 2.6 × 10^-3 and 6.7 × 10^-2 for the mesoporous and densely packed TiO ₂ films, respectively. A model based on the atomic structures of a thin film of water molecules adsorbed on TiO₂ surfaces leading to hydrophobic effects was proposed to understand the lower frictional and adhesive forces observed on the mesoporous TiO₂ films. This simple physical approach to reducing the frictional and adhesive forces on TiO₂ films could have broad applications to a variety of surface coatings.