R. Paolini, M. Sleiman, M.-P. Pedeferri, M.V. Diamanti
Politecnico di Milano,
Keywords: TiO2 nanoparticles, self-cleaning, near-infrared reflectance, aging
Summary:A manifold positive contribution to climate change mitigation and in the improvement of air quality of built environments can be provided by the use of titanium dioxide (TiO2), also – but not exclusively – as coating for building envelope materials in order to spread its beneficial effect on the largest surface area possible. The benefits in applying TiO2 coatings –containing in particular the anatase phase – rely, first of all, on the UV-activated photocatalytic degradation of pollutants mediated by it, which can mitigate pollution arising from industrial sources, heating and transportation. Moreover, changes in wettability upon UV irradiation lead to a superhydrophilic state which, coupled with photocatalysis, results in the so called self-cleaning effect, allowing materials to retain a cleaner and more reflective surface over time. Reducing the impact of aging on the optical-radiative performance of built environment surfaces has a strong influence on the thermal comfort and energy consumption of buildings. Here we present a novel effect observed on long-term durability investigations of commercial siloxane paints containing anatase nanoparticles: an increase in reflectance in the near infrared (NIR) wavelength range upon material aging. This was correlated with the interaction between the TiO2-containing surface and the environment, in particular the potential products of its photocatalytic activity towards pollutants. In fact, laboratory tests showed that the effect was reproduced by immersing the materials in diluted HNO3 solutions, which are representative of hydrolyzed NOx degradation products. Investigations were extended to pure commercial TiO2 nanoparticles, which allowed to correlate the increase in NIR reflectance to alterations in the TiO2 structure and composition, namely, a decrease in crystallinity, surface protonation, and adsorption of nitrate ions. These findings open the door for HNO3 treatment of TiO2 as a method to boost the self-cleaning efficiency of superhydrophilic surfaces, as well as to improve the optical-radiative properties of TiO2 coatings and admixtures in construction materials for a better energy performance of the buildings.