Bio-Inspired Titanium Dioxide Materials with Special Wettability and Their Applications

Titanium dioxide (TiO2) is one of the most widely used nanomaterials in our daily life. In 1972, Fujishima and Honda reported the photo electrolysis of water into H2 and O2 utilizing an electrochemical cell in which the TiO2 single-crystal electrode is connected with a Pt electrode. This is analogus with the natural photosynthesis that produces oxygen through oxidizing water and reducing carbon dioxide using sunlight, where solar energy is converted into chemical energy. Since that time, photocatalysis has received considerable attention owing to its important applications in the conversion of light energy into useful chemical energy. In 1997, Fujishima et al. first reported the photogeneration of a superamphiphilic (both superhydrophilic and superoleophilic, where the contact angle of water and oil on a surface is almost 0°, respectively) TiO2 surface under UV light irradiation, showing self-cleaning and antifogging characteristics. This breakthrough work expanded the research field of TiO2 materials and marked the beginning of a new era in TiO2-based self-cleaning materials. Since then, an important effort has been focused on the understanding of the fundamental mechanism of this novel function and on the development of selfcleaning materials for a wide range of applications in energy, environmental, and industrial fields, resulting in the generation of new markets. Although photocatalysis and photoinduced superhydrophilicity can take place simultaneously on the same TiO2 surface, they are intrinsically different processes. In recent years, environmental pollution and damage on a global scale have emerged as a serious issue. The viable environmental cleanup has attracted a great deal of attention to achieve important breakthroughs in the design of advanced materials and in the development of new technology. Now, a variety of TiO2-based materials have been commercialized arising from their unique photoinduced properties. Furthermore, these commercial products demonstrate their importance in the environmental cleanup.