High quantum yield graphene quantum dots decorated TiO2 nanotubes for enhancing photocatalytic activity

Graphene quantum dots (GQDs) with high quantum yield (about 23.6% at an excitation wavelength of 320   nm) and GQDs/TiO 2 nanotubes (GQDs/TiO 2 NTs) composites were achieved by a simple hydrothermal method at low temperature. Photoluminescence characterization showed that the GQDs exhibited the down-conversion PL features at excitation from 300 to 420   nm and up-conversion photoluminescence in the range of 600–800   nm. The photocatalytic activity of prepared GQDs/TiO 2 NTs composites on the degradation of methyl orange (MO) was significantly enhanced compared with that of pure TiO 2 nanotubes (TiO 2 NTs). For the composites coupling with 1.5%, 2.5% and 3.5% GQDs, the degradation of MO after 20   min irradiation under UV–vis light irradiation (λ   =   380–780   nm) were 80.52%, 94.64% and 51.91%, respectively, which are much higher than that of pure TiO 2 NTs (35.41%). It was inferred from the results of characterization that the improved photocatalytic activity of the GQDs/TiO 2 NTs composites was attributed to the synergetic effect of up-conversion properties of the GQDs, enhanced visible light absorption and efficient separation of photogenerated electron-holes of the GQDs/TiO 2 composite.