Preparation of superhydrophilic silicate coating by sol-gel for double-wavelength broadband antireflective coatings

In the process of realizing double-wavelength broadband antireflection (AR) coatings, it is very important to control the refractive index in a wide range, but at present, many methods of adjusting the refractive index are too complex or expensive. Here, a simple and inexpensive sol-gel route is proposed to prepare the superhydrophilic silica (SiO2) thin films having ultra-low refractive indices by the SiO2 particles surface corrosion with hydrofluoric acid (HF) in sol; this treatment effectively reduces the refractive index of the coating from 1.22 to 1.11. The formation mechanism of the coatings with ultra-low refractive index was discussed in detail. The corrosion of HF on the particles surface significantly improves the hydrophilicity of the coating, and reduces the water contact angle of the coating from 27 degrees to 2 degrees. The bottom layer is prepared by mixing the acid-base complex silica sol. When the refractive indices of the bottom and top layers were set as 1.29 and 1.13, the double-layer double-wavelength AR coating with excellent optical properties has the transmittance of 99.95% and 100.00% at 532 and 1064 nm, respectively. Broadband antireflective superhydrophilic antifogging coatings with high transmistttance are successfully constructed based on double-layer system by a dip-coating method. Particle-chains porous silica nanoparticles as building blocks increase the roughness of coating surface, and reduce the refractive index of coating.