Durable Self-Cleaning Coatings for Architectural Surfaces by Incorporation of TiO2 Nano-Particles into Hydroxyapatite Films

To prevent soiling of marble exposed outdoors, the use of TiO2 nano-particles has been proposed in the literature by two main routes, both raising durability issues: (i) direct application to marble surface, with the risk of particle leaching by rainfall; (ii) particle incorporation into inorganic or organic coatings, with the risk of organic coating degradation catalyzed by TiO2 photoactivity. Here, we investigated the combination of nano-TiO2 and hydroxyapatite (HAP), previously developed for marble protection against dissolution in rain and mechanical consolidation. HAP-TiO2 combination was investigated by two routes: (i) sequential application of HAP followed by nano-TiO2 (H+T); (ii) simultaneous application by introducing nano-TiO2 into the phosphate solution used to form HAP (HT). The self-cleaning ability was evaluated before and after prolonged exposure to simulated rain. H+T and HT coatings exhibited much better resistance to nano-TiO2 leaching by rain, compared to TiO2 alone. In H+T samples, TiO2 nano-particles adhere better to HAP (having flower-like morphology and high specific surface area) than to marble. In HT samples, thanks to chemical bonds between nano-TiO2 and HAP, the particles are firmly incorporated in the HAP coating, which protects them from leaching by rain, without diminishing their photoactivity and without being degraded by them.