Self-cleaning stone Façades using TiO2 Microwave-Synthesised Coatings

Abstract

This study explores the development and characterization of self-cleaning coatings using titanium dioxide (TiO₂) nanoparticles for natural stone façades, particularly limestone. An energy-efficient, eco-friendly, fast (30 min), and low temperature (110 °C) microwave-assisted solvothermal method is reported for synthesising TiO₂ nanoparticles. These nanoparticles were integrated into coatings that were further applied to limestone substrates via spray-coating, maintaining the stone’s appearance while enhancing its self-cleaning properties. Characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), UV–VIS spectroscopy and Brunauer-Emmett-Teller (BET) surface area analysis were used to fully characterize the nanopowder. The anatase phase of TiO₂ nanoparticles and a band gap energy of about 3.24 eV were confirmed. SEM and STEM observations revealed that the nanopowder is formed by spherical particles with very fine nanocrystals highly agglomerated, however ensuing a high specific surface area of 199 m²/g. The self-cleaning properties of the coated limestone were assessed using static contact angle measurements. The results showed a significant enhancement in hydrophilicity, with the static contact angle of the coated limestone substrate reducing to nearly zero even without UV exposure, indicating complete wettability. The coating was also subjected to adhesion tests, confirming the presence of TiO₂ nanoparticles even after multiple cycles. The photocatalytic activity of the developed coating was evaluated using rhodamine B and methyl orange as model pollutants under solar radiation. The coating effectively degraded both model pollutants, and the photocatalytic cycling tests revealed a stable performance after multiple cycles. This research provides a promising approach for creating sustainable and low-maintenance building materials, contributing to preserving natural stone façades and reducing environmental impact in the construction industry.