Understanding Crystallization of Anatase into Binary SiO2/TiO2 Sol-Gel Optical Thin Films: An in Situ Thermal Ellipsometry Analysis

SiO2/TiO2 xerogel ultrathin protective coatings, with tunable refractive index between 1.45 and 2.20, have been prepared by dip-coating sol-gel solutions of TiCl4 and tetraethyl orthosilicate with various molar compositions. In situ thermal ellipsometry analysis is used to follow the modifications of optical properties of these binary oxide systems upon thermal treatment up to 820 degrees C. Involved phenomena, such as dehydration, condensation, viscous sintering, crystallization, and diffusive sintering have been clearly identified and studied with respect to the Ti/Si atomic proportion and the treatment conditions. Due to the influence of the crystal size on the semiconducting properties of titania nanoparticles, its crystallization kinetic has been specifically investigated. We show that the crystallization of TiO2 is globally taking place between 300 and 600 degrees C depending on the silica to titania ratio and on the heating rate. Films containing more than 40% of silica remain amorphous at 820 degrees C. Poor silica contents tend to be associated to lower crystallization temperature (T-c) and larger crystal sizes. The size of the smallest detected anatase nanoparticles was close to 4 nm, while those of the largest were limited by the film thickness around 35 nm. Interestingly, we observed that, while faster heating rates induce high T-c for pure TiO2, the inverse tendency is observed for binary oxide films, which is explained by the diminution of the atomic diffusion due to the progressive condensation of the silica network. Electronic microscopy investigations revealed that smoother surface topologies and more homogeneously structured films are obtained in the presence of silica.