Polyphenols Extracted from Grape Pomace as Synthesis Directing Agents of Photoactive ZnO: A Morphology and Reactivity Study

ZnO can be easily obtained using different salts as precursors, and many examples are present in the literature describing the effect of several additives in the synthesis. In this paper, we study the effects of the addition of polyphenols present in the residues of the wine supply chain. The polyphenols are extracted from grape pomace and fractionated, exploiting a membrane-based process equipped with polysulfone ultrafiltration membranes (cut-off 1 kDa and 5 kDa) that can separate the plethora of molecules into larger than 5 kDa and smaller than 1 kDa. The extract and its fractions after the ultrafiltration process were used as additives for the thermal precipitation synthesis of ZnO from Zn acetate. The chemical and physical properties were studied with the aim of understanding the characteristics that influence the activity of the photocatalysts. To this purpose, a commercial system was used for comparison, and the photoactivity was analyzed with a caffeine solution upon irradiation, exploiting the UVA and VIS electromagnetic radiation for the activation of the catalytic materials. The kind of polyphenol fraction affects the surface behaviors of the nanoparticles. Morphology, presence of trapped hole/electron centers, and acidity/basicity of the surface sites of ZnO appear to be the most relevant features in the efficiency towards caffeine degradation.