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

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 TiO2 nanoparticles under hydrothermal conditions. The effective incorporation of the GQDs extends the light absorption of the TiO2 nanoparticles from UV to a wide visible region. Moreover, amine-functionalized GQD–TiO2 heterojunctions can absorb more O2 than pure TiO2, which can generate more ·O2 species for MO degradation. Accordingly, the heterojunctions exhibit much higher photocatalytic performance for degrading methyl orange (MO) under visible-light irradiation than TiO2 alone. At optimum GQD content (1.0 wt %), an apparent MO decomposition rate constant is 15 times higher than that of TiO2 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/TiO2, and possible MO decomposition pathways under visible-light irradiation are proposed.