Spinel nanoparticles on stick-like Freudenbergite nanocomposites as effective smart-removal photocatalysts for the degradation of organic pollutants under visible light

Abstract A series of mixed nanocomposite materials was synthetized, containing a Ferrite phase type Zn1-xNixFe2O4 and a Freudenbergite phase type Na2Fe2Ti6O16, where x = 0; 0.2; 0.4; 0.6; 0.8; 1. The choice for this combination is based on the good adsorption properties of Freudenbergite for dye molecules, and the small bandgap energy of Ferrite spinel, allowing activation of the catalysts under visible light irradiation. A two steps synthesis protocol was used to obtain the smart-removal nanocomposites. Firstly, the spinel structure was obtained via the co-precipitation route followed by the addition of the Ti-source and formation of the Freudenbergite system. The role of cations on the formation mechanism and an interesting interchange of cations between spinel and Freudenbergite structures was clarified by a TEM-study. Part of the Ti4+ penetrated the spinel structure and, at the same time, part of the Fe3+ formed the Freudenbergite system. The photocatalytic activity was studied under visible light, reaching for the best catalysts a 67% and 40% mineralization degree for methylene blue and rhodamine 6G respectively, after 6 h of irradiation. In the same conditions, the well-known commercial P25 (Degussa) managed to mineralize only 12% and 3% of methylene blue and rhodamine 6G, respectively. Due to the remarkable magnetic properties of Ferrites, a convenient recovery and reuse of the catalysts is possible after the photocatalytic tests. Based on the excellent catalytic performance of the nanocomposites under visible light and their ease of separation out of the solution after the catalytic reaction, the newly developed composite catalysts are considered very effective for wastewater treatment.