Sol-gel synthesis of TiO2-SiO2 hybrid films with tunable refractive index for broadband antireflective coatings covering the visible range

A series of TiO2-SiO2 composite sols have been synthesized using titanium (IV) isopropoxide and tetraethyl orthosilicate as precursors under acidic conditions. The stability of the binary sols is improved by a prehydrolysis step of tetraethyl orthosilicate. Assisted by heat treatment, the refractive index of the TiO2-SiO2 hybrid films obtained can be tuned in a wide range. On the basis of film optical constants derived from fitting of transmittance spectrum, two types of antireflection coatings with quarter-half and quarter-half-quarter multilayer structures are designed, and ordered mesoporous SiO2 film and dense SiO2 film are proposed to be used as the top layer, respectively. According to theoretical design requirements, the two antireflection coatings are successfully constructed by selecting the TiO2-SiO2 films with appropriate refractive index as the other layers. The average transmittance of the triple-layer coating at 400–800 nm is 98.74%, and that of the double-layer coating even reaches 99%. Meanwhile, the two types of multilayer coatings show good mechanical properties, which benefit from the tough skeleton of each layer. X-ray reflectivity measurements were also performed on the multilayer structures and the obtained thickness of each layer is consistent with that in the theoretical design. The results show that precise control of film thickness and refractive index is achievable using sol-gel techniques. Owing to excellent control on sol composition, the practical sol-gel route has high potential for the production of antireflective coatings.