Green and fast synthesis of amino-functionalized graphene quantum dots with deep blue photoluminescence

Graphene quantum dots (GQDs) were prepared using a top-down approach with a green microwave-assisted hydrothermal synthesis from ultrathin graphite, previously ultrasound delaminated. Results obtained by transmission electron microscopy and atomic force microscopy indicate that the so-fabricated GQDs are plates with 6 nm of average diameter, mostly single- or bi-layered. Photoluminescence characterization shows that the strongest emission occurs at 410-415 nm wavelength when the samples are excited at 310-320 nm wavelength. In addition to these down-conversion features, GQDs also exhibit up-conversion photoluminescence when excited in the range 560-800 nm wavelength, with broad emission peaks at 410-450 nm wavelength. Analysis of X-ray photoelectron spectroscopy measurements indicates a higher proportion of C-C sp(2) than sp(3) bonds, with the sp(3) ones mainly located at the GQD surfaces. Also evidences of C-O and C-N bonds at the GQD surface have been observed. The combination of these results with Raman and ultraviolet-visible absorption experiments allows envisaging the GQDs to be composed of amino-functionalized sp(2) islands with a high degree of surface oxidation. This would explain the photoluminescent properties observed in the samples under study. The combined up-and down-conversion photoluminescence processes would made these GQDs a powerful energy-transfer component in GQDs-TiO2 nanocomposite systems, which could be used in photocatalyst devices with superior performance compared to simple TiO2 systems.