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In the present work, we demonstrated the upcycling technique of effective wastewater treatment via photocatalytic hydrogen production by using the nanocomposites of manganese oxide-decorated activated carbon (MnO2-AC). The nanocomposites were sonochemically synthesized in pure water by utilizing MnO2 nanoparticles and AC nanoflakes that had been prepared through green routes using the extracts of Brassica oleracea and Azadirachta indica, respectively. MnO2-AC nanocomposites were confirmed to exist in the form of nanopebbles with a high specific surface area of ~109 m2/g. When using the MnO2-AC nanocomposites as a photocatalyst for the wastewater treatment, they exhibited highly efficient hydrogen production activity. Namely, the high hydrogen production rate (395 mL/h) was achieved when splitting the synthetic sulphide effluent (S2− = 0.2 M) via the photocatalytic reaction by using MnO2-AC. The results stand for the excellent energy-conversion capability of the MnO2-AC nanocomposites, particularly, for photocatalytic splitting of hydrogen from sulphide wastewater.

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Capping ligands are vital in stabilizing various nanostructures and semiconductor quantum dots in which unusual optical properties, especially white light emission, have been realized. Oleic acid (OA) is a widely used capping ligand. Here, we report blue emission from OA in its free molecular form and further demonstrate this by anchoring OA over the surfaces of Al2O3, ZnAl2O4(ZA), ZnAl2O4:Eu3+ (ZA:Eu3+), and Y2O3:Eu3+. White light emission was observed from OA-modified ZA:Eu3+ nanophosphor due to mixing of broad blue emission of OA and red emission of Eu3+ through energy transfer from OA to Eu3+. A detailed study revealed the characteristic binding modes of OA and their dependence on Eu3+ concentration, structural inversion in ZA, and the optical properties and surface states in the pristine and OA-modified ZA:Eu3+. First principles density functional theory calculations were employed to provide an insight into the HOMO-LUMO levels of OA molecule and, electronic structure of pristine and OA-modified ZA surface. The binding of OA with the ZA:xEu3+ surface changes from bridging bidentate to chelating bidentate with increasing Eu3+ concentration in the lattice. The surface binding nature of the carboxylate group with the optimized surface of ZA and the creation of mid-gap states were deduced theoretically by using butanoic acid instead of OA. The blue emission from OA and its mixing with Eu3+ emission was further confirmed experimentally by anchoring it over Y2O3:Eu3+ red phosphor. These results show the multifunctional roles of OA as capping ligand, blue emitter and sensitizer in tuning the emission colour of red phosphors into white.