Nanostructured titania–silica sol–gel multilayer coatings with spectrally selective light transmission and water-repellent functionality

Spectrally selective multilayer titania–silica coatings with water-repellent functionality were developed on glass substrates by combining thin-layer optical modelling with sol–gel processing. A two-layer optical model was used to define suitable ranges of layer thickness and effective refractive index, which served as a basis for the experimental development and optimization of three- and four-layer coatings. The optimized glass/TiO 2 /porous SiO 2 /hybrid SiO 2 and glass/TiO 2 /compact SiO 2 /porous SiO 2 /hybrid SiO 2 systems increased the average transmittance in the 400–700 nm range by 2.3 ± 0.4% and 2.3 ± 0.2%, respectively, while reducing the average near-infrared transmittance by 9.4 ± 0.6% and 7.8 ± 0.2%. Maximum transmittance values of 99.17 ± 0.11% at 498 ± 4 nm and 99.62 ± 0.07% at 554 ± 5 nm were achieved for the respective systems. The hybrid silica top layer enhanced visible-light transmission and provided water-repellent functionality, as demonstrated by low contact angle hysteresis and efficient droplet removal under a water jet. The coatings also showed sub-nanometer surface roughness and good optical stability under dry-heat exposure and water immersion, whereas abrasion caused visible surface scratches and a moderate increase in the root mean square error values. The results demonstrate that the combination of optical modelling and sol–gel processing provides an effective route to multilayer titania–silica coatings combining tailored spectral transmission with water-repellent surface functionality, offering a promising approach for functional photovoltaic cover-glass surfaces.