Self-cleaning materials active in the visible range based on porphyrin-sensitised titanium dioxide

Abstract

In the last decades, nanostructured semiconductors played a central role in the material science scene because of their numerous applications spanning from renewable energy to organic/hybrid electronics up to photocatalysis. Titanium dioxide is one of the most used semiconductors because of its low cost, chemical stability, sustainability and versatility. Indeed, it is widely employed as photo-active or charge- transporting material in electronic devices, as photocatalyst in water de-pollution treatments etc.. Remarkably, one of the most interesting application of titanium dioxide consists of the protection and conservation of cultural heritages. Actually, TiO2 photocatalytic properties are exploited for self- cleaning materials able to passivate artifact surfaces and degrade organic pollution, thus preventing blackening while reducing maintenance costs. Unluckily, the photoactivity of TiO2 is limited to the UV range limiting its performance under the natural sunlight exposition or artificial illumination. In order to enhance the performance in both outdoor and indoor conditions, it is crucial to extend its light absorption range up to the visible region. To this goal, in this work we prepared novel TiO2-based materials functionalized in-bulk with meso-tetra(carboxyphenyl)-porphyrin (TCPP) and its metal derivatives (Me-TCPP), which served as visible-light sensitizers. The particular wet sol-gel route employed for the synthesis makes the porphyrin dye bonded inside the TiO2 structure and not easily DYE P HO T O DE GR A DAT I O N adsorbed to the surface. The material structure, composition and electronic properties were investigated by XRD, Raman and photoelectron spectroscopies (XPS and UPS) while photocatalytic properties under both UV and visible light were studied by following the degradation kinetic of carminic acid by UV-Vis spectrophotometry. The visible-light sensitised materials showed an enhanced photocatalytic activity with respect to commercial titanium dioxide in different illumination conditions, indicating that the inclusion of dyes in-bulk is a valuable strategy to obtain efficient visible-light active TiO2 photocatalysts.