Self cleaning antireflective coatings constructed by novel raspberry-like hollow SiO2@TiO2 nanocomposites

Normally, ideal antireflective coating such as the type of coatings applied on solar photovoltaic and photo thermal components, should meet specific attributes like high transmittance and high self-cleaning capability. These coatings serve the main purpose of promoting optical conversion efficiency. Titanium oxide has recently been discovered to possess super-hydrophobic properties and photocatalytic self-cleaning attributes. However, it has a demerit in that it has a high refractive index that results in deterioration of light transmittance, which inhibits its applicability in preparing coatings of self-cleaning attributes. A way around this is to prepare nanocomposites with low-refractive-material such as silica. In such endeavors, the silica core become completely encapsulated by the titanium oxide shell, an attribute which has adverse effect on the optical property of the resultant coating. In addition, the preparation of a highly homogeneous raspberry-like structure of the silica encapsulated titania (SiO₂@TiO₂) nano spheres is still challenging.

Recently, a team of researchers led by professor Ruoyu Chen at Changzhou University and Dr. Lin Sun at Yancheng Institute of Technology developed a uniform raspberry-like hollow SiO₂@TiO₂ composite nano spheres with different internal cavity volumes as well as tunable refractive-index and self-cleaning coatings via a colloidal assembly method. Their work is currently published in the research journal, Thin Solid Films.

The research method employed commenced with the utilization of hydrogen peroxide as a hydroxyl modifier to repair the surface silicon-oxygen bonds of hollow silica and to promote the density and activity of surface hydroxyl groups. Next, the effects of silanol content on density and stabilization of grafted 3-aminopropyltriethoxysilane molecules were studied. They then dispersed the titanium oxide nanoparticles on the surface of the amine-functionalized hSiO₂ via hydrolysis of tetrabutylorthotitanate. Lastly, antireflective coatings were fabricated on glass substrates via a sol-gel dipcoating method.

The authors observed that under the optimal conditions, the average transmittance and refractive index of the coatings derived from uniform raspberry-like hollow silica encapsulated titania (urb-hSiO₂@TiO₂) were 99.04% and 1.218 in the range 400–800 nm, respectively, showing excellent antireflection performance. Moreover, TiO2 nanoparticles were seen to endow the composite coatings with high photocatalytic activity and photo-induced super-hydrophilic after irradiation by UV light for 2.5h, thereby improving the durability of the composite coating in outdoor applications.

In summary, the study presented the successful development of urb-hSiO2@TiO2 from small-grain-size TiO₂ nanoparticles well distributed on the surface of a hSiO₂ core. A series of characterizations, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, thermogravimetry, and derivative thermogravity, were carried out to analyze the composition, structure and morphology of the composite. Altogether, it was seen that the developed unique structure with refractive index of 1.218 and transmittance over 99% exhibited excellent self-cleaning capability when served as antireflective coatings.