Titania-silica composite materials for self-cleaning applications on monumental stones

Abstract

Atmospheric pollution has an evidently adverse impact on the aesthetics of urban buildings and structures. Thus, the synthesis of photocatalysts capable of removing pollutants deposited on the surface of stone and other building materials is an interesting challenge to researchers. In the work undertaken for the Doctoral Thesis presented, mesoporous TiO2-SiO2 composites that have photocatalytic activity have been synthesized by mixing ethoxysilane oligomers and TiO2 nanoparticles in the presence of a non-ionic surfactant (noctylamine). The products synthesized have a clear practical application on buildings and other monumental heritage structures since they can be applied outdoors by means of a simple and low-cost process. The resulting nanomaterials give self-cleaning properties and create crack-free effective adhesive coatings for exterior stone surfaces. In addition, they improve the mechanical resistance of the stone. Another important advantage of these nanocomposites is that they substantially improve protection against salt crystallization degradation processes. By using of N2 physisorption, atomic force microscopy and electron tomography, together with 3D reconstructions, we have been able to conclude that the texture of the nanocomposites synthesized is a key parameter for controlling the photocatalytic activity. Specifically, we find that n-octylamine creates a mesoporous SiO2 structure within which TiO2 nanoparticles are embedded, and that TiO2 and SiO2 are present in separate domains in the bulk of the material. The mesoporous structure enhances the activity of the material by improving the access of light to photoactive sites. We optimize the effectiveness of these photocatalysts on stone by varying the loading and particle size of TiO2 in the starting sol. We find that the integration of around 4% w/v content of TiO2 nanoparticles into the SiO2 network significantly improves their effectiveness due to a higher availability of photoactive sites. For higher TiO2 loadings (10% w/v), photocatalytic activity decreases because the porous volume is drastically reduced and access to photoactive sites is more restricted. Regarding the effect of particle size, we observe that larger and sharper TiO2 nanoparticles enhance the photocatalytic activity.