In this paper, the photocatalytic activity of TiO2 NPs was improved twice: the first by engineering its morphology to narrow the band gap and to increase the surface area and the second by incorporating the modified TiO2 NPs with graphene quantum dots (GQDs) to reduce electron-hole () recombination rate. Three visible/solar light responsive photocatalysts including TiO2 nanoparticles (NPs) and two nanocomposites of TiO2 @ GQDs were prepared. The photocatalytic activity of the prepared photocatalysts, GQDs, and P25 against Rhodamine B (RhB) under visible light or sunlight radiation was investigated. The prepared TiO2 NPs possessed very small particle size of 7.62 nm, large surface area of 236 m2 g−1, and narrow band gap of 2.41 eV. TiO2 NPs could harvest the visible/solar light for degrading RhB. The photocatalytic activity of TiO2 @ GQDs nanocomposites prepared by hydrothermal and ultrasound methods under visible light radiation are much better than that of TiO2 NPs. Using PL spectra, it was found that GQDs in the nanocomposites can hinder (or delay) recombination and hence improve the photocatlytic performance of TiO2 NPs. The photocatalytic degradation of RhB over TiO2 NPs and TiO2 @ GQDs nanocomposites prepared by hydrothermal and ultrasound methods under visible light radiation followed the pseudo-first-order kinetics with the rate coefficients of 0.126, 0.170, and 0.149 min−1, respectively.

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