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 X 10(-3) and 6.7 X 10(-2) for the mesoporous and densely packed TiO2 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.