Antifogging and Self-Cleaning
Fogging on eyeglasses, car windows, and bathroom mirrors is caused by condensation of water, which forms small droplets on the glass surface. The wetting of a solid surface with water where air is the surrounding medium is dependent on the contact angle. In general, 0° contact angle means completely wet, whereas 180° means completely non-wet. Cleaning by soap is based on low contact angle by reducing the water surface tension. The contact angle of water on glass and other inorganic materials is in the range of 20-30° but is not less than 10° for any surfaces under any conditions. The contact angle can be decreased without detergents but by putting a thin layer on the material surface. A hydrophobic surface is the main method to prevent this fogging by repelling water molecules and then removing water drops by blowing or shaking. Materials such as ceramic tiles, glass, or plastics can develop super-hydrophilic materials by surface coating with the photocatalyst Ti02.
The antifogging, antibacterial, and self-cleaning functions of Ti02-coated materials are obtained without using any chemicals, but with only UV sunlight and rainwater. The water contact angle of the illuminated surface with UV light of anatase Ti02 is less than 1°. The super-hydrophilic effect of Ti02 occurs when the surface is exposed to light and the water contact angle approaches zero after a certain time of moderate illumination [22].
The application of Ti02 photocatalysis on surface materials has enabled the degradation of a range of organic and inorganic compounds that are aggressive toward material properties and the environment. Ti02 photocatalysis has been applied in different cementation materials, and the applications varied from selfcleaning to air cleaning especially for indoor. Ti02 coating on ceramic tiles is used for its antibacterial properties to increase the lifecycle of cement-based materials and to substantially reduce the concentration of air pollutants in hospital rooms, care facilities, kitchens, baths, and schools [23]. A particularly interesting aspect is clear synergy between Ti02 and cement composites, which makes this material an ideal substrate for environmental photocatalysis. Photocatalytic building materials used outdoors, e.g., PVC fabric, aluminum wall, glass, and exterior tiles, are coated with Ti02, where they could be easily washed by water, raining, rainwater, and rainfall. This is because water soaks between the highly hydrophilic Ti02 surface and the materials.
Rutile has been reported to have a higher surface enthalpy and higher surface free energy than anatase due to different crystal structures and associated exposed planes [24]. Therefore, it is expected that anatase wetting by water is less than that of rutile since higher surface free energies commonly contribute to hydrophilicity [25]. Rutile (100) and (110) faces exhibit a higher hydrophilicity than the (001) face, which are favorable for increasing the number of OH groups. The (100) and (110) faces have twofold oxygens, which are higher in position and energetically more reactive than their surrounding atoms and are called "bridging site oxygen." On the other hand, the rutile (001) has threefold oxygens, which are lower in position and energetically more inactive [26].
Photocatalysts for Water Treatment and Air Purification
In the 1970s, Frank and Bard discovered that illuminated Ti02 and other metal oxides can be served as a photocatalyst for photocatalytic decomposition of pollutants in water [27-29]. A wide range of organic and inorganic as well as toxic compounds, including cyanide, sulfite, phenol, and aniline, were decomposed (oxidized) by the illuminated polycrystalline Ti02 [27, 28, 30]. These reports demonstrated that water and air purification can be achieved by photo catalysis reaction, which contributes to the decomposition of pollutants for a cleaner and greener environment. Regarding air purification, another critical environmental issue is the substantial climate change primarily caused by the increased concentration of greenhouse gases. In particular, the excess emission of C02 is attributed to the large amount of human activities because of rapid industrial revolution and excessive combustion of fossil fuels. In the 1990s, Anpo et al. designed an efficient Ti02 photocatalyst via metal-ion implantation within Ti02 to address the decomposition of pollutants in water, which has been mentioned above, as well as for the photoreduction of C02 [31-33]. The illuminated Ti02 again serves as a photocatalyst to transform C02 into other nontoxic or reusable compounds such as H2, H20, and other hydrocarbons (methane, ethane, methanol, acetic acid, formic acid) [34-36]. Consecutively, some of the derived compounds can be utilized in another green solar fuel, which in turn reduces the use of fossil fuels [37, 38].
The three-dimensional (3D) spaces such as water, air, and soil are much more difficult than that of the two-dimensional (2D) surfaces of photocatalytic building materials. In general, the total amount of reactants in 3D spaces is higher than on the 2D surface, indicating that much more light energy is necessary for the purification of 3D space [39]. Moreover, the photocatalytic reactions are surface reactions, and thus the reactants must be captured by the photocatalyst surface. Fortunately, the construction of practical purification systems for wastewater from agriculture and polluted soil has been successful with volatile organic compounds (VOCs), which were based on Ti02 photocatalysts and use only solar light. Nanosized Ti02 photocatalyst powders were dispersed on substrates with extremely large surface areas, spreading them on the ground widely to collect sunlight [39].
TiO2 Photobioreactor
In the food and environmental industries, anatase photocatalysts as a photobioreactor have attracted great attention for sterilizing selected food-borne pathogenic bacteria such as Listeria monocytogenes, Vibrio parahaemolyticus, and Salmonella choleraesuis subsp [40]. Various anatase concentrations and UV illumination time have been used to study the photocatalytic reaction. On all bacterial suspensions, the bactericidal effect of UV/anatase was much higher than without anatase. As the anatase concentration increased to 1.0 mg/ml, bactericidal effect increased, but it was rapidly abbreviated at higher than 1.25 mg/ml anatase concentration to all selected bacteria. The time of anatase/UV illumination affected the viability of bacteria with different death rates drastically. After 3 h of UV illumination, S. choleraesuis subsp and V. parahaemolyticus were completely killed, whereas about 87% death ratio was for the killed Listeria monocytogenes [40].
