Double layer SiO2–TiO2 sol–gel thin films on glass for antireflection, antifogging, and UV recoverable self-cleaning

Double layer thin films, mechanically stable and adhering to glass, were produced through the sol–gel process, using tetraethyl orthosilicate and titanium butoxide as precursors. The refractive index of the titania and silica–titania composite layers were typically 2.1 and 1.7, and their physical thicknesses were approximately 65 nm and 81 nm, respectively, as determined by ellipsometry. These optical constants allowed attainment of quarter-wave optical thicknesses at the center of the visible spectrum (550 nm) as designed, with an increase of 3.4% in transmittance. The nanometric surface roughness, measured by optical profilometry, was effective to decrease light scattering and water contact angles to below 10°. As novelty in dip-coated sol–gel films, superhydrophilicity for self-cleaning, antifogging, and antireflection in the mid-visible spectrum were simultaneously attained with durability of 9 weeks in the dark. Further application of UV light allowed regeneration of contact angles for self-cleaning.