An environmentally friendly titania–silica core–shell nanoparticles coating for protection of tiled facade of cultural-historical buildings

Abstract

The purpose of the current research was to synthesize and characterize a hydrophobic ultraviolet (UV) absorbent coating for protection of tiled facade of cultural-historical buildings. Considering the environmental and conservational issues for such applications, using toxic materials is limited. A silicone-based nanocomposite coating was prepared in which organically modified silicates (ormosil) and titania–silica core–shell nanoparticles were used as matrix and reinforcing phase, respectively. Nanocomposite coatings were prepared by sol–gel method. Hydroxy-terminated polydimethylsiloxane (PDMS-OH) and tetraethoxysilane (TEOS) were used to form the ormosil matrix. Titania nanoparticles in anatase crystalline phase were used. To control their photocatalytic activity, silica–titania core–shell nanoparticles were prepared. Nanocomposite coatings were applied on microscope slides and historic tiles by dip-coating and simple brushing. The coatings were characterized by Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), water contact angle measurements and photocatalytic activity measurements. The results revealed that a transparent hydrophobic coating of prepared ormosils could obtain by adding a maximum amount of 20 wt% of PDMS-OH to the initial sols without addition of highly toxic solvents and catalysts. The FT-IR spectra of ormosil gels exhibited Si–O–Si bond corresponding to silica gels. It also showed that PDMS formed covalent bonds to silica networks. The XRD patterns showed that titania maintained its anatase crystalline structure after core–shell treatment. TEM images of core–shell treated nanoparticles illustrated the formation of continuous silica shell with less than 4 nm thickness around titania cores. Water contact angle measurements on tiles showed an increase in contact angle from 30° to 97° after coating. The results of UV–Vis spectrophotometry confirmed that the continuous silica layer acted as a barrier between photocatalytic titania and Methylene blue. Overall, the best composition for obtaining an environmentally friendly, transparent, hydrophobic, and UV absorbing nanocomposite coating could contain 20 wt% of PDMS-OH and about 0.025 wt% of silica–titania core–shell nanoparticles.