Patterned hydrothermal synthesis of TiO2 rods-array and its application to oil repellent surface in water

Titanium dioxides (TiO2) nano-rods array were fabricated on a fluorine-doped tin oxide (FTO) substrate leaving an array of star-like photoresist patterns array with regular spacing. The patterns were fabricated with a lithography, and the photoresist masked the hydrothermal process, in which chemical reactions proceeds in a sealed container at elevated temperature and pressure to produce regular structure of specific materials. Synthesized TiO2 rods became single crystal rods and aligned perpendicular to the substrate because its lattice constant is similar with that of the substrate. Typical height was 3.4 μm after 4 hours synthesis. Using the superhydrophilicity of TiO2 after irradiation with ultraviolet light, the hydrophobic star-like areas were surrounded by hydrophilic TiO2 area. On the tapered area of each star, the spread oil is ideally gathered at the center of the star due to the imbalance of interfacial tensions in water, which helps the oil droplet repellency. The functionality was evaluated with two experiments. One is the floating test in which the substrate was dipped into water with an oil droplet. The best case showed the droplet float in water. Another is the sliding angles measurement of oil droplet in water. It was found that both of the samples with/without pattern showed small sliding angle as approximately 10 °. The patterned sample showed longer functional life than that without pattern after one week. It was also found that the water trapped between the TiO2 rods assists in repelling the oil droplet.