Detailed analysis and formation mechanism of superhydrophobic antireflective coatings with adjustable refractive index from trimethylsilanized silica nanoparticles

This paper dealt with the fabrication of sol–gel-derived organically modified silica (ormosil) films on glasses from trimethylsilanized silica nanoparticles (SNPs) using hexamethylisilazane (HMDS) as the modification agent. The prepared sols were suitable for large-area processing with ease of coating and being directly applicable without requiring any pre- or post-treatment. By a dip-coating process, these HMDS-treated sols gave antireflective (AR) coatings exhibiting precisely tunable refractive indices ranging from 1.19 to 1.12, in a range that is not accessible to nonporous inorganic materials. Films produced a fair AR effect consisting of more than a 6 % increase in optical transmittance. The particle surface modification with HMDS significantly improved the hydrophobicity of the coated film. According to the dynamic light scattering (DLS) method, the HMDS modification at the surface of the pure spherical SNPs enlarged the SNP size from 12.4 to 22.3 nm. The relationship between the microstructure of silica sols and the properties of films was investigated. Besides this, the effects of various factors on the morphology of sols and films were studied to propose a mechanism for the formation of the adjustable refractive index thin films. Graphical Abstract