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Layered Nano-TiO2 Based Treatments for the Maintenance of Natural Stones in Historical Architecture.

Layered treatments of natural stones based on dispersions of experimental nano-TiO2 and commercial TEOS showing photocatalytic and self-cleaning properties were set up and tested. To enhance nano-TiO2 efficacy, a surface pre-treatment with tetraethyl orthosilicate was proposed to avoid the penetration of NPs into the crystalline porous substrates and to improve their adhesion to the stone. Two treatment applications (wet-on-wet and wet-on-dry) were compared, showing different results. A strong interaction Si-O-Ti was the key factor for the successful treatment, leaving the band gap and relevant properties of nano-TiO2 unaltered. The layered treatments were tested on a porous calcarenite (Noto stone) and a very compact marble (Carrara marble). The combined SiO2 -nano-TiO2 treatments can find application in suitable cases where a surface consolidation is needed, ensuring a depolluting and self-cleaning durable activity.