Preparation of silica coatings with continuously adjustable refractive indices and wettability properties via sol–gel method

Silica coatings with continuously adjustable refractive indices and wettability properties were prepared through a sol–gel base-catalyzed process. Adjustment of the molar ratio of water (H2O) to tetraethylorthosilicate (TEOS) was utilized to change the hydrolysis degree of the precursors, and hence change the morphology of the silica particles. With the increase in the H2O/TEOS molar ratio, the morphology of the silica particles changed from a linear net-work structure to a bead-like structure and then to a granular particle structure. A particle growth mechanism was proposed and verified by characterization. As the H2O/TEOS molar ratio increased from 0.3 to 21.0, the refractive indices of the silica coatings increased from 1.132 to 1.328. Meanwhile, a varied H2O/TEOS molar ratio also modulated the surface wettability of the silica coatings. The static water angle of the silica coatings decreased from 145° to 6° by increasing the H2O/TEOS molar ratio from 0.3 to 21.0. Different hydrophilic and hydrophobic coatings could be obtained by simply controlling the H2O/TEOS molar ratio. Silica coatings with different refractive indices and hydrophobic (or hydrophilic) properties were obtained at different H2O/TEOS molar ratios.