Efficient Separation of Electron–Hole Pairs in Graphene Quantum Dots by TiO₂ Heterojunctions for Dye Degradation

Abstract

Water-soluble, single-crystalline, and amine-functionalized graphene quantum dots (GQDs) with absorption edge at ∼490 nm were synthesized by a molecular fusion method, and stably deposited onto anatase TiO 2 nanoparticles under hydrothermal conditions. The effective incorporation of the GQDs extends the light absorption of the TiO 2 nanoparticles from UV to a wide visible region. Moreover, amine-functionalized GQD–TiO 2 heterojunctions can absorb more O 2 than pure TiO 2, which can generate more ·O 2 species for MO degradation. Accordingly, the heterojunctions exhibit much higher photocatalytic performance for degrading methyl orange (MO) under visible-light irradiation than TiO 2 alone. At optimum GQD content (1.0 wt %), an apparent MO decomposition rate constant is 15 times higher than that of TiO 2 alone, and photocurrent intensity in response to visible-light excitation increases by 9 times. Compared with conventional sensitization by toxic, photounstable quantum dots such as CdSe QDs, the sensitization by environmentally friendly GQDs shows higher visible-light photocatalytic activity and higher cycling stability. Monodispersed QD-based heterojunctions can effectively inhibit the fast recombination of electron–hole pairs of GQDs with a large exciton binding energy. The photogenerated electron transfer, energy-band-matching mechanism of GQD/TiO 2, and possible MO decomposition pathways under visible-light irradiation are proposed.