TiO2 nanocoatings for architectural heritage: Self-cleaning treatments on historical stone surfaces

The development and application of nano-engineered surface treatments on stones could become a useful tool for the realization of smart systems to better preserve and maintain architectural surfaces. Titanium dioxide nanoparticles can be used to realize transparent self-cleaning coatings applicable directly on preexisting surfaces, limiting cleaning actions and conservation processes, thus reducing their costs. The aim of this investigation is to evaluate the potential use of TiO2 on stone surfaces, especially in the field of architectural heritage. An aqueous colloidal dispersion based on titanium dioxide, obtained by sol-gel and hydrothermal processes, was applied by spray coating on travertine, a limestone largely used in buildings, both historical and modern. The maintenance in the original appearance of treated substrates was evaluated monitoring both colour and gloss changes produced by the treatments. Physical changes induced to stone by titanium dioxide were studied by wettability analyses. The efficiency of TiO2 photocatalysis was assessed by depolluting and soiling removal tests under ultraviolet light. The effects of deposited amount of titania on treated surfaces were also evaluated. Obtained results seem to allow the use of selected TiO2 treatments on the selected substrate, travertine, without altering in an evident and harmful way the original properties of limestone. Photoinduced effects (hydrophilicity, degradation of pollutants and decolourization of soiling) are very evident, and the combination of these properties may lead to an actual self-cleaning effect.