Field performances of nano-sized TiO2 coated limestone for a self-cleaning building surface in an urban environment

Over the last few years, photocatalytic titanium dioxide coatings have been explored in laboratory conditions to create building materials with self-cleaning and depolluting abilities. Assessing the performances of the photocatalytic surfaces under real conditions may provide basic knowledge to evaluate the potential of real applications of TiO2 coatings in buildings. In this study, the performance of photocatalytic limestone surfaces obtained through coating with waterdispersed TiO2 nanoparticles were investigated in an urban environment. Coated and uncoated samples were exposed to an urban site for one year. Before the exposure and periodically afterwards, optical microscopy observations, colour and contact angle measurements were performed on the sample surface. At the end of the exposure period, samples underwent a capillary water absorption test and self-cleaning efficiency was evaluated by a Rhodamine B photodegradation test. Ti and soluble fraction amounts on the sample surfaces were determined by X-ray Fluorescence (XRF) and ion chromatography, respectively. The overall results showed the TiO2 coating was better able to preserve the surface colour properties early after exposure. After eight months, this effect was lost and the self-cleaning efficiency was reduced to negligible final rates. No clear wettability results and no meaningful capillary behaviour were recorded. A decrease of the photocatalytic activity was due to both partial titania loss and deactivation phenomena. The role of soluble salt ions either adsorbed from the environment or produced by the photocatalytic abatement of pollutants was recognized in the deactivation of the photocatalysts and their accumulation deserves attention for possible stone damage risk implications.