**4. Antireflective coatings**

High transmittance characteristics of antireflective coatings have great significance in high-performance devices along with the property of transmittance such as flat panel display, solar panels, lenses, telescope, etc. TiO2 causes a high refractive index and refraction when it is applied to the transparent glass substrate. A very low concentration of TiO2 in the self-cleaning coatings reduces the photocatalytic activity but at the same time minimized the surface refraction. Therefore for the practical application of self-cleaning coating there should be between balance antifogging and anti-reflective properties of the self-cleaning coating. Preparation of TiO2 composite with SiO2 adopted to have self-cleaning anti-reflective properties with a high refractive index. A hierarchical macro-mesoporous SiO2 thin film with very high porosity showed a significant broadband anti-reflection with an average reflectance of 3.45%

#### *Nano TiO2-Based Smart Superhydrophilic Self-Cleaning Surfaces DOI: http://dx.doi.org/10.5772/intechopen.108157*

in the wavelength range from 350 nm to 1200 nm [21] The macroporoporous template is superior to the conventional flat SiO2/TiO2 composite film.

The sandpaper abrasion test results show that the porous composite film was superhydrophilic with a water contact angle of 2.4° and this contact angle was retained even after 50 cycles of the abrasion test. Raspberry-like core-shell nanoparticles of SiO2/TiO2 were synthesized by a sol–gel single pot method. These synthesized SiO2/TiO2 core-shell nanoparticles were then fabricated onto glass by layer-by-layer self-assembly dip coating technique. The prepared coating exhibited superhydrophilicity both in the presence and absence of UV irradiation [22]. The SiO2 particle with the size in the range of sub-micron provides a porous structure and anti-reflective property to the self-cleaning coating. However, the larger surface area of nanoparticles and higher surface roughness in the coating are the main factors in imparting its superhydrophilic property. An example of a high antireflective self-cleaning coating is Ag/TiO2/Si forest-like hierarchical nano/ microstructures like a moth-eye. This type of coating shows solar-weighted reflectance values of 3.5% and 3.3%, respectively, over the wavelength range of 300 − 1000 nm [23]. The estimated contact angle over the surface was <5o which exhibits outstanding antifogging and self-cleaning properties by the use of plasmonic silver nanoparticles [24]. The self-cleaning coating also shows photocatalytic dye degradation and self-cleaning under UV–vis light irradiation. Along with these combinations, several researchers have developed double-layered and multilayered TiO2-SiO2 films with reduced reflectance [25, 26]. Three-dimensional TiO2 nanostructures such as nanopores, nanorods, and hierarchical structures with larger surface area have been reported with reduced surface reflectance losses but with enhanced photocatalytic activity [27, 28]. TiO2 can also be used as a protective layer for thermochromic self-cleaning coating in Vanadium (IV) oxide (VO2). Vanadium oxide VO2 is a unique material practically used for thermochromic applications due to its large optical and electronic behavior and because first-order phase transitions from monoclinic to tetragonal geometry [29, 30]. A multifunctional transparent VO2/SiO2/TiO2 thin film showed considerable improvement in visible light transmittance which is accompanied by thermochromic and self-cleaning properties. Therefore, these coatings can be applied for energy-efficient, intelligent window applications [31]. TiO2/VO2 bilayer composite coating is also capable of supporting the TiO2 for increasing air purification by absorbing Infrared rays.

### **4.1 Coatings for building materials**

Self-cleaning coatings are the probable substitute for high energy consumption. TiO2-coated self-cleaning surfaces has been used in construction material such as in cement, tiles, limestone, glass can behold their artistic appearance without contamination or getting dirt throughout their lifetime [32–38]. A number of esthetic images and buildings reported preserving with the help of TiO2 as a photocatalytic material [39–42]. Sedimentation methods and spray coating methods are usually used for the building materials. According to the report by n-Tech Research in 2015 the self-cleaning market is expected to reach a market of US \$3.3 billion by 2020. Pilkington has been marked as the first commercially available coating over the glass. Additionally, these self-cleaning coatings should withstanding varying climatic changes to the outdoor environments. During the development of self-cleaning coatings, a number of weather variables like light, temperature, humidity, etc. should be considered [39–42]. Humidity plays a very important role in stain removal, by influencing the hydration state of TiO2 this will increase the cumulative hydroxyl ion concentration that will lead to sustainable self-cleaning materials for buildings [43–50].
