Multi-functional TiO₂-based nanocomposite coating with durable superhydrophobicity and enhanced photocatalytic and antimicrobial properties for the sustainable maintenance of building stones

Superhydrophobic surfaces have demonstrated great potential in fundamental research and functional applications, yet their durability and antimicrobial property shall be improved. To mitigate the degradation of building stones, which severely undermines the safety, economic and aesthetic value of contemporary and historical constructions, a superhydrophobic coating endowed with photocatalytic and antimicrobial properties is presented. The nanocomposite (Si-PFE-TiO₂) was obtained by covalently connecting TiO₂ nanoparticles with a fluorinated siloxane via Ti-O-Si linkages. To apply, superhydrophobic surfaces can be effortlessly prepared by brushing/deposing the alcoholic suspension of Si-PFE-TiO₂ on substrates. Upon protective performance and physiochemical compatibility evaluation, with a very low amount applied (8.5 g/m²), Si-PFE-TiO₂ coating exhibited much higher water inhibition efficiency, residual vapor diffusivity while less surface chromatic changes, compared with three reference coatings (TiO₂-fluoroelastomer formulations). Owing to the Ti-O-Si linkages which improve photon efficiency and stimulate the oxidation process, Si-PFE-TiO₂ coating showed enhanced photoactivity and antibacterial action. The degradation efficacy of colorants on Si-PFE-TiO₂ coating was>1.6 times higher than reference coatings. Considering the amount of TiO₂ incorporated in Si-PFE-TiO₂ (∼0.92 g/m²), its photoactivity outperforms other recently reported TiO₂-containing coatings. Moreover, compared with anatase nano-TiO₂, Si-PFE-TiO₂ coating showed higher antimicrobial effects against Gram-positive and Gram-negative bacteria, being 1.94, 1.32 times higher for E. Coli and 1.93, 1.59 times higher for S. aureus, with the concentration 0.5 g/L and 1.2 g/L, respectively. Lastly, Si-PFE-TiO₂ coating is also chemically and mechanically robust, enabling application in harsh conditions. This study sheds light on designing multi-functional superhydrophobic surfaces for in-situ and outdoor applications.