Simple Physical Approach
to Reducing Frictional and
Adhesive Forces on a TiO<sub>2</sub> Surface via Creating Heterogeneous
Nanopores
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Abstract
A simple physical strategy to reduce the frictional and
adhesive
forces on TiO<sub>2</sub> films was proposed by constructing mesoporous
TiO<sub>2</sub> films with heterogeneously distributed nanopores on
the film surfaces. In comparison, TiO<sub>2</sub> 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 TiO<sub>2</sub>(B) phase exists
in the mesoporuos TiO<sub>2</sub> films but not in the densely packed
films. The existence of TiO<sub>2</sub>(B) plays a significant role
in creating and maintaining the nanopores in the mesoporous TiO<sub>2</sub> 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<sup>–3</sup> and 6.7 ×
10<sup>–2</sup> for the mesoporous and densely packed TiO<sub>2</sub> films, respectively. A model based on the atomic structures
of a thin film of water molecules adsorbed on TiO<sub>2</sub> surfaces
leading to hydrophobic effects was proposed to understand the lower
frictional and adhesive forces observed on the mesoporous TiO<sub>2</sub> films. This simple physical approach to reducing the frictional
and adhesive forces on TiO<sub>2</sub> films could have broad applications
to a variety of surface coatings