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

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2013-08-02Advisor
Mosquera Díaz, María Jesús; Arlindo Jorge, Sá de BegonlhiDepartment
Química FísicaAbstract
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.
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