Silica Functional Surfaces With Special Wettability Preparation And Research

Transparent materials have important applications, including mirrors, glasses, goggles, laser safety eye protective lenses, face masks, windows for vehicles, and solar cells. However, fogging usually occurs when their surface temperature is lower than that of their air surrounding, which has caused serious influences in industry and our daily life, and antifogging coatings have been attracting much attention of scientists and investors.To date, several approaches have been developed to prepare antifogging coatings. The first involves the use of photochemically active materials such as TiO2 or ZnO that become superhydrophilic after exposure to UV, or after suitable chemical modifications and exposure to visible radiation. The second involves spraying an appropriate surfactant or polymer solution to lower the surface energy. The third case is the use of rough surfaces to fabricate superhydrophilic (water droplet contact angle <5°within 0.5 s or less )or superhydrophobic (water droplet contact angle >150°and low contact angle hysteresis ) surfaces. The theory and development of anti-fogging and antireflective materials have been reviewed in chapter 1. In a word, antifogging coatings with good mechanical durability, transmittance, and stability have been attracting much attention.In this work, we have designed and fabricated several functional coatings based on silica with low reflectivity, good hydrophilicity and good mechanical durability. The functional coatings include antifogging coatings, superhydrophilic and antireflective coatings, and superhydrophobic coatings with high contact angle and low sliding angle. The main works are listed as follows:1. Raspberry-like silica nanospheres were prepared by electrostatic self-assembly of polyelectrolytes and monodisperse silica nanoparticles of two different sizes, and their coatings were fabricated via layer-by-layer assembly with polyelectrolytes and following calcination. The morphology of the raspberry-like silica nanospheres and their coatings were observed by scanning and transmission electron microscopies. The surface properties of these coatings were investigated by measuring their water contact angles, and the results showed that such hierarchically structured coatings had unique superhydrophilic and antifogging properties. Finally, the formation mechanism and the property–structure relationship were discussed in details.2. Raspberry-like organic/inorganic composite spheres are prepared by stepwise electrostatic assembly of polyelectrolytes and silica nanoparticles onto monodisperse polystyrene spheres. Hierarchically structured porous films of silica hollow spheres are fabricated from these composite spheres by layer-by-layer assembly with polyelectrolytes followed by calcination. The morphologies of the raspberry-like organic/inorganic composite spheres and the derived hierarchically structured porous films are observed by scanning and transmission electron microscopies. The surface properties of these films are investigated by measuring their water contact angles, water-spreading speeds, and antifogging properties. The results show that such hierarchically structured porous films of silica hollow spheres have unique superhydrophilic and antifogging properties. Finally, the formation mechanism of these nanostructures and property–structure relationships are discussed in detail on the basis of experimental observations.3. Superhydrophilic and antireflective coatings were fabricated from silica nanoparticles of 30 nm (S-30), 150 nm (S-150) and polyelectrolytes of poly(diallyldimethylammonium) chloride (PDDA) and sodium poly(4- styrenesulfonate) (PSS) via layer-by-layer assembly and post-calcination. The time for a droplet to spread flat decreases to as short as 0.28 s by applying a coating of (PDDA/S-150)3/ (PDDA/S-30)2 while the maximum transmittance reaches as high as 98.5% by applying a coating of (PDDA/S-30)8. Factors that affect the superhydrophilic and antireflective properties of coatings, such as the number of deposition cycles, size of nanoparticles and surface roughness, were investigated in details by observing their surface morphologies and by measuring their water contact angles, water spreading time and transmittances. Systematic investigation gave an optimal structure of calcinated (PDDA/S-30)8/(PDDA/S-150)2/(PDDA/S-30)2 for the superhydrophilic and antireflective coating. Its maximum transmittance and the time for a droplet to spread flat reached 97.1% and <0.5 s, respectively, indicating that both antireflective and superhydrophilic properties were achieved by a single coating. Finally, the roles of S-30 and S-150 nanoparticles in enhancing the superhydrophilic and antireflective properties were discussed.4. Hierarchically structured coatings were fabricated on glass substrates by one-step hydrothermal method. The surfaces of the coatings are rough, and are composed of flower-like particles assembled by nanoflakes or urchin-like particles constructed by nanowires. These rough surfaces exhibit superhydrophilicity, their water contact angles reaching 0o in less than 40 ms. After surface modification by 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane, the wetting properties of these coatings switch from superhydrophilicity to superhydrophobicity, with water contact angles as high as 160o and slide angle as low as 1o. The formation mechanism of the hierarchically structured coatings is discussed in details on the basis of experimental results.