Traditional Venetian marmorino: Effect of zinc-based oxides on self-bleaching properties

Abstract

• Zinc-promoted materials exhibits increased self-bleaching activity. • Improved surface area does not improve the self-bleaching. • Surface insulating calcium hydroxide layer limits charge-transfer processes. • Multiple-heterojunctions contributed to a remarkable photoactivity enhancement. The Venetian marmorino is a traditional material used for wall coating in Venice city, composed of hydrated lime and marble powders, and is part of the cultural heritage of the city. Several surface damage processes can negatively affect its aesthetic properties, such as the formation of stain deposits of different origin (i.e. pollution, bio-deterioration, vandalism) which eventually change the colour of this material. An appealing way to avoid these deposits rely on inducing self-bleaching features by introducing a photocatalyts on this material. In the present work, TiO 2 -based and ZnO-based photocatalysts were introduced within the marmorino , then evaluating how the addition of these materials influenced the compositional, structural and morphological properties though X-ray diffraction (XRD), N 2 physisorption and electron microscopy (SEM). ZnO-modified samples, which exhibited comparable morphological and structural properties, were further analyzed to get information concerning the surface compostion and the opto-electronic properties, through attenuated total reflectance (ATR) spectroscopy and photoluminescence (PL), respectively, in turn related to the presence of defects and heterojunctions. The self-bleaching activity was evaluated by irradiation in mild condition (room temperature) in air using a dye as model stain compound. Finally, the morphological, surface and opto-electronic properties of the photocatalyst-modified marmorino were correlated to the activity results, aiming to understand how they can influence the material performances, and thus how to design an active self-bleaching Venetian marmorino .