Hydrophilic/hydrophobic and optical properties of B2O3 doped TiO2 sol-gel thin films: Effect of B2O3 content, film thickness and surface roughness

Abstract

High transparent B2O3-doped TiO2 thin films were deposited on glass substrates using sol-gel method and spin coating. B2O3 content was varied from 0 to 20 wt %. Each film layer was calcined at 500 °C for 1 h. Film thickness was determined by profilometer and UV techniques. Monolayer thicknesses decrease from 55 nm to 47 nm when B2O3 content increases. For multilayer films, the thickness grows by steps of about 50 nm by layer and slightly decreases when boron content increases. The presence of B2O3 affected also roughness, light transmission and optical gap. As an example, for two layer films, Roughness decreases from 1.1 to 0.65 nm when B2O3 increases from 0 to 20%. Light transmission at λ = 500 nm and optical gap were enhanced from 93 to 97% and from 3.59 to 3.72 eV respectively. All these changes could be explained by a better melting of the thin layer at 500 °C, This was confirmed with XR diffraction and AFM analysis. The second part of this study concerned the variation of hydrophobic, hydrophilic properties. Water contact angle was measured in standard conditions. It was found for single layer samples, that θ increased from 16 to 61 ± 1° when B2O3 content varies from 0 to 20%. The study was performed also for samples having 2, 3 and 4 layers. Contact angle increased slowly with film thickness. The highest angle was equal to 75° (±1°); it was obtained for samples having four layers and doped with 20% B2O3. The dependence of θ with physical properties particularly with surface roughness and thickness was also examined.