Desenvolvimento de nanomateriais de TiO2/SiO2 pelo método sol-gel para aplicações nas áreas de energia e meio ambiente

Abstract

The objective of this work was to obtain xTiO2/(100-x)SiO2 thin films and particles (with x = 0, 20, 40, 60, 80 and 100%) by the sol-gel process for applications in energy and environment areas. The method of precursor solutions mixing, in the sol-gel process, allowed a systematic variation of the composition (TiO2/SiO2 ratio) and the evaluation of this change in the optical, structural, textural and surface properties of the obtained materials. In addition, the repetition of the drying and deposition steps allowed a good control of the films thickness in the dipcoating process. 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 (n), extinction coefficient (k), and band gap (Eg) 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 superhydrophilicity that makes them promise in application as selfcleaning surfaces. In the soiling test, TiO2 films showed an optimal performance with transmittance losses lower than 8% over the entire test interval, while TiO2/SiO2 films presented better performance in the rainy season. TiO2/SiO2 particles presented a micromesoporous structure, demonstrated by the DFT pore size distribution. The decrease in intensity and the broadening of the peaks in the xTiO2/(100-x)SiO2 diffractograms with the increasing of %SiO2 demonstrated a better dispersion of smaller TiO2 anatase particles in the silica matrix. Transmission electron microscopy images confirmed that TiO2 anatase particles are homogeneously distributed within the amorphous and porous SiO2 matrix. The textural properties of the xTiO2/(100-x)SiO2 particles were evaluated by comparing the BET, DR, Langmuir and DFT theories. The TiO2/SiO2 particles showed a superior adsorption capacity and photocatalytic activity comparing to TiO2 against methylene blue and quinoline contaminants.