Ag-TiO2/PDMS nanocomposite protective coatings: Synthesis, characterization, and use as a self-cleaning and antimicrobial agent

The protection of our artifacts becomes a necessity due to several decay phenomena induced by pollution, microorganism colonization, and water. This research study has been focused on elaborating photo-activated nanocomposites to protect outdoors heritage buildings. Hence, novel Ag-TiO2/PDMS hydrophobic nanocomposites (with low doping rates) thin films with enhanced photo-response activity have been developed to be used as self-cleaning durable protective coatings in the field of monument conservation. The protective coatings elaborated in this work are intended to have multifunctional properties (water repellent features, self-cleaning activity, antimicrobial agent). To reach our objective, silver-doped titanium dioxide nanoparticles (Ag-TiO2 NPs) at different doping amounts (0.1, 1, 3, and 5 mol%) were firstly synthesized by the sol-gel method. As-prepared NPs were characterized and their performance was evaluated comparatively to find out the appropriate low doping rates of the dopant by testing photo-degradation as well as photo-killing activities. The second part of this research is focused on elaborating Ag-TiO2/PDMS hydrophobic nanocomposites (selected low doping rates) as self-cleaning protective coatings. For this purpose, Ag-doped TiO2 NPs were dispersed in a polymeric binder (polydimethylsiloxane, PDMS) and applied to the very porous stone substrate, i.e., Lecce stone (LS). The suitability of the resulting nanocomposites (optimal Ag doped TiO2 NPs/ binder ratio; optimal doping amount) on the stone substrate was evaluated by performing different experimental analyses: chromatic measurements, contact angle measurements, water capillary absorption test, permeability to water vapour, pencil and Vickers hardness tests, optical microscope analyses, and scanning electron microscopic analyses combined with energy-dispersive X-ray spectroscopy (SEM-EDS), microbiological experiments, and self-cleaning test. Moreover, the durability of the nanocomposite coatings was also assessed after exposing to different ageing cycles. Results suggested that the samples doped with low concentrations (≤1 mol%) of silver (Ag-doped TiO2) together with PDMS can be considered as efficient and durable protective coatings for the highly porous bio-calcarenite stone. Such doping amount is considered very low compared to the other research works that have been performed in the same field.