Cost-effective nanoporous SiO2–TiO2 coatings on glass substrates with antireflective and self-cleaning properties

Simultaneous antireflective and self-cleaning properties have been realized by depositing double-layered SiO2–TiO2 coatings onto glass substrates using a combined sol–gel dip-coating process in which the two oxide layers were deposited in succession. The first layer is composed of a hybrid methyl-functionalized nanoporous SiO2 material that provides on average a 6% anti-reflection gain. The degree of antireflectivity can be controlled by adjusting the thickness and refractive index by selecting suitable solvents and pore-forming agents in the coating mixture. The second layer is an ultrathin TiO2 nanoporous layer deposited on top of antireflective layer. The high hardness of the TiO2 top layer acts as a protection barrier toward mechanical damage and ensures the surface is self-cleaning. An average anti-reflectivity of 3.4% achieved over a spectrum range of 400–800nm for the optimal SiO2–TiO2 bilayers is obtained. It is found that a homogeneous pore size distribution in the SiO2 layer introduced by using isopropanol as the solvent can compensate for the TiO2 band edge absorption based on the optical interference that occurs between the two layers. These systems are easy to produce on a large scale at low cost and exhibit high mechanical and chemical durability. These materials are thus suitable for use as bifunctional antireflective and self-cleaning coatings for use as large substrates for solar cells.