Surface plasmon-induced photodegradation of methylene blue with single layer graphene quantum dots/Au nanospheres under visible-light irradiation

Abstract

Single-layer graphene quantum dots and gold (SLGQDs/Au) nanospheres composite was synthesized by hydrothermal and photochemical methods. Single-layer graphene quantum dot was synthesized by glucose precursor via hydrothermal method and SLGQDs/Au nanocomposite was prepared by adding HAuCl4 under UV irradiation. Characterization of sample was performed using XRD, Raman, FESEM, UV–vis spectroscopy and PL analysis. The quantum yield of SLGQDs/Au composite was calculated. The role of surface plasmon resonance (SPR) of gold nanoparticles in quantum yield reduction and the effect of quantum yield on photodegradation was discussed. Using the optimal sample, a photocatalyst test was performed on methylene blue, which near 90% photocatalytic degradation was achieved under visible light irradiation in 120 min. Compared to bare GQD the high photocatalytic efficiency of composite could be due to the presence of SPR level of Au nanoparticles between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of SLGQDs. Under light irradiation, SLGQDs and Au nanoparticles absorb visible light energy. By absorbing the photons, the electrons of SLGQDS could be excited to LUMO band and leave holes in HOMO band. Spontaneously the emitted photons excite the Fermi level electrons of Au nanoparticles to the SPR state. Created SPR level can suppress electron-holes recombination. Then the photo-exited electrons react with dissolved oxygen to generate ° O2− and ° OH radicals meanwhile, the holes could oxide OH− into OH°. Created radicals which are strong oxidants degrade the pollutants.