The impact of Carbon Quantum Dots (CQDs) on the photocatalytic activity of TiO2 under UV and visible light

In this study, the impact of carbon quantum dots (CQDs) on the photocatalytic performance of TiO2 in aqueous medium was investigated under UV and visible light. CQDs were synthesized through a solvothermal procedure, and the effect of synthesis conditions on their optical properties were optimized. The CQDs/TiO2 nanocomposites were prepared via a hydrothermal method using CQDs with the extreme band-gaps (i.e. CQDs450 and CQDs550). The physicochemical properties of CQDs and CQDs/TiO2 samples were studied by various characterization techniques. According to the High-Resolution Tunneling Electron microscopy (HRTEM) micrographs, the synthesized spherical CQDs had diameters smaller than 5 nm. The Diffuse Reflectance Spectroscopy (DRS) analysis revealed that incorporation of CQDs into nanocomposite samples causes a remarkable shift in absorption edge to the visible region, in comparison with pure TiO2. According to the experimental data, the nanocomposite sample containing CQDs with the higher absorption edge, i.e. CQDs550/TiO2, showed a superior performance in photocatalytic removal of methylene blue (MB) under visible light. In this regard, the removal percent of MB on CQDs550/TiO2 and CQDs450/TiO2 nanocomposites, with 4%wt CQDs, under visible light (λ > 420 nm) were 92.1 % and 53.4 % after 120 min of irradiation at pH = 11 and 150 mgcat./L catalyst dosage, which are respectively 2.4 and 1.4 times higher than that on pure TiO2. However, under UV irradiation the observed photocatalytic performances were in the reverse order. Finally, two different reaction mechanisms have been proposed to explain the obtained results on CQDs/TiO2 samples, based on possible electron transfer between TiO2 and CQDs nanoparticles under UV and Visible lights.