Structural and Mechanical Evolution of Mesoporous Films With Thermal Treatment: The Case of Brij 58 Templated Titania

Abstract

Mesoporous titania thin films (MTTFs) with well ordered cubic array of mesopores were synthesized on glass and silicon substrates using Brij 58 as a template. The effect of the thermal treatment and the substrate on the structural parameters (thickness, porosity, pore order, and crystallinity) and the mechanical properties of MTTFs were determined by electron microscopy, X-ray diffraction, Raman spectroscopy, 2D-small angle X-ray scattering, ellipsometric porosimetry, and nanoindentation. Clear differences in the mesostructural order evolution and crystallization behavior were observed as a function of the substrate and the thermal treatment. In particular, the anatase crystallization process occurs at lower temperatures for samples prepared on silicon when compared with samples prepared on glass, due to the balance between nanocrystals formation, mass diffusion, and Na+ migration from the substrate. As a consequence of such phenomena, the MTTFs mechanical properties are also dependent on the substrate. For samples prepared on glass the 325–350 °C range is the optimal annealing temperature to maximize the mechanical properties (E value of 45 GPa), while higher temperatures can be used for the Si supported oxides, to reach E values of 60 GPa. The obtained anatase crystal dimensions (below 4–5 nm) are restricted by the wall thickness, indicating the chosen thermal treatment prevents the mesoporous structure from collapsing even when the oxide presents thin walls and small pores, preserving high porosity and high porous ordering. As a consequence, the presented Brij 58 templated MTTFs exhibits smaller crystalline domains than analogous materials with thicker walls. Such properties could be exploited for applications in photocatalysis and titania-based solar cells.