Visible light-driven photocatalytic performance of ZnO coupled with up-conversion phosphors for the removal of methylene blue

In this work it was investigated the photocatalytic activity of ZnO coupled with inorganic “up-conversion” phosphors for the treatment of aqueous solutions polluted with methylene blue (MB), in the presence of visible light irradiation. Specifically, the successfully prepared “up-conversion” phosphors nanoparticles emit, when excited with visible light, UV light with main emission at 365 nm. The photocatalytic tests were carried out in a cylindrical pyrex photoreactor equipped with a magnetic stirrer to ensure the continuous mixing of the physical mixtures of “up-conversion” phosphors and commercial ZnO nanoparticles in the solution and irradiated by visible light instead of UV light normally required to excite ZNO. Experimental results were analysed by evaluating the MB decolourization through spectrophotometric method and the total organic carbon (TOC) through high temperature combustion method. In particular, it was found that the photocatalytic performances obtained using ZnO nanoparticles coupled with “up-conversion” phosphors are strongly increased compared to the use of ZnO alone. These results confirmed the enhancement of photocatalytic performances induced by UV photons emitted by “up-conversion” phosphors nanoparticles, able to activate ZnO, demonstrating how their use in photocatalytic systems can be an alternative to the traditional UV lamps, thus preserving human health, environmental and, at the same time, allows to save the energy in process management.