Evaluation of Silver-Modified ZnAl-Layered Double Hydroxide Coatings for Photocatalytic Self-Cleaning of Mortar Substrates

Abstract The preservation of stone and mortar elements in cultural heritage structures is increasingly challenged by environmental degradation, pollution, and biological fouling. Photocatalytic coatings represent a promising approach for developing self-cleaning and pollutant-degrading surfaces. In this study, a silver-modified ZnAl-layered double hydroxide (Ag@ZnAl-LDH) was synthesized via coprecipitation and applied as a coating on mortar substrates. The structural and compositional characteristics of the prepared materials were investigated using X-ray diffraction (XRD) and wavelength-dispersive X-ray fluorescence (WDXRF). The XRD pattern confirmed the formation of a ZnAl-LDH phase (JCPDS 48-1022) with carbonate as the main interlayer anion, along with additional reflections attributed to ZnO (JCPDS 99-0111) and silver-containing species. The photocatalytic activity was evaluated through the degradation of methylene blue under natural illumination (700 W/m²), monitored via CIELAB colorimetric parameters. The results demonstrated that Ag@ZnAl-LDH coatings promote the photodegradation of organic dyes, indicating potential for developing functional, self-cleaning coatings for the protection of heritage materials.