Nanocrystalline TiO2 coatings by sol-gel: photocatalytic activity on Pietra di Noto biocalcarenite

Self-cleaning photocatalytic coatings based on TiO2 nanoparticles obtained by sol-gel techniques at two different pH (1.3-10.6), have been investigated on carbonatic stone. The selected material is the yellowish lithofacies of Palazzolo Formation calcarenite, commonly named Pietra di Noto, widely used in the Baroque monuments of the Noto Valley (Italy, Sicily). SEM-EDS, XRD and Raman investigations were carried out to characterize the TiO2 nanoparticles (mainly nanocrystalline anatase) obtained by the acid and basic nanosols and the corresponding coatings on pietra di Noto. To evaluate the effects of the treatments at two different pH values, changes of the stone color appearance, water absorption by capillarity and the behavior for salt crystallization were measured. The photocatalytic activity of the coatings was evaluated under UV irradiation, by monitoring methyl orange and methylene blue dye degradation as a function of time. The results highlight the compatibility of both treatments with respect to the properties of the calcarenite stone, showing no chromatic changes, no alteration of physical properties, an improvement of the resistance to salts crystallization and the good photocatalytic activity on both dyes. The experimental data suggest that the basic coating has to be preferred for carbonatic stones for its harmlessness and better performances in term of self-cleaning action and protection against water and salts.