Nanostructured TiO2 for stone coating: assessing compatibility with basic stone's properties and photocatalytic effectiveness

Many building materials have been functionalized to achieve photocatalytic properties, namely selfcleaning and depolluting abilities, through the application of photocatalytic TiO2 nanoparticles to those materials. These nanoparticles are able to preserve building fac¸ades by blocking the deposition of airborne particulates in polluted urban environments, and they are able to purify the air, thus benefiting the environment. In this study, the application of nanostructured TiO2 as a photoactive coating on two types of natural stone was investigated. A TiO2 sol obtained by sol-gel synthesis followed by hydrothermal processing was applied via spray deposition onto a compact limestone and a highly porous calcarenite. The effects of this coating on some basic properties of the stone, such as its color and water absorption, and the photocatalytic effectiveness of the coated surface were then studied. Scanning electron microscopy and energy-dispersive X-ray spectroscopy showed that the coating presented a uniform morphology on both types of stone, with the TiO2 nanoparticles penetrating\1 lm into the stone. The coating was found to be compatible with the properties of the investigated types of stone. Colorimetry indicated that the change in the color of the stone due to the coating was negligible. Measurements of the static contact angle and the results of the capillary water absorption test showed that photoinduced superhydrophilicity did not increase the amount of the water absorbed by the coated stone. A photodegradation test of rhodamine B demonstrated the self-cleaning ability of the coating on both types of stone. Conversely, the photocatalytic effectiveness of the coating-- as measured by a nitrogen oxide abatement test--was found to be higher for the porous calcarenite than for the compact limestone, and to depend on the porosity and roughness of the substrate.