A systematic study of multifunctional xTiO2/(100 − x)SiO2 thin films prepared by sol–gel process

It is known that TiO2/SiO2 nanocomposite thin films have several properties improved when compared with the TiO2 or SiO2 pure thin films. In this work, a systematic study of xTiO2/(100 − x)SiO2 films (x = 0, 20, 40, 60, 80, and 100%) with different numbers of layers (1–5), i.e., different thicknesses, prepared by sol–gel process using dip-coating deposition was made. The films were thermally treated and characterized by UV–Vis spectroscopy and ellipsometry to evaluate the optical properties and thicknesses. Transmittance of the films were discussed by means of the interference fringes and refractive index modulation as a TiO2/SiO2 ratio function. The values of refractive index, extinction coefficient, and band gap obtained from ellipsometric analyses were related to the changes in UV–Vis spectra. The construction of the xTiO2/(100 − x)SiO2 structural models for a precise ellipsometric analyses were based on a comprehension of theoretical models. The glass substrate and SiO2 film were modeled by Cauchy; TiO2 film by Tauc–Lorentz and TiO2/SiO2 by Bruggeman theories. A graded refractive index function was necessary to fit the thicker films with low mean square error (MSE). Moreover, the refractive index values permitted the calculation of total porosity of the films, using Lorentz–Lorentz equation. Atomic force microcopy images showed clearly the effect of SiO2 addition on TiO2, reducing the particle size and root mean square roughness (from 1.4 to 0.3 nm). The lowest roughness of TiO2/SiO2 films was correlated with their enhanced hardness. All TiO2/SiO2 films showed a persistent super hydrophilicity that makes them suitable for applications in environmental and energy fields.