Improving adsorption and visible light photocatalytic performance of TiO2 via synergistic effect of nitrogen-doped graphene quantum dots and reduced mildly oxidized graphene oxide for cationic dye pollutants removal

Abstract

In this study, a new type of ternary composite for efficient removal of cationic dye pollutants, P25/nitrogen-doped graphene quantum dots/reduced mildly oxidized graphene oxide (P25/NGQDs/rmGO) was successfully prepared by a two-step hydrothermal process. P25/NGQDs/rmGO exhibited outstanding removal performance towards methylene blue (MB), crystal violet (CV) and Basic Red 46 (BR46) under visible light or solar light irradiation owing to the excellent adsorption-photocatalytic property resulting from the synergistic effect of NGQDs and rmGO. The negatively charged NGQDs significantly improved the adsorption capacity for positively charged dye based on the adsorption experiment results. The investigations of adsorption kinetics and isotherm for MB demonstrated that the adsorption process followed the pseudo-second-order kinetic model and Langmuir adsorption isotherm, respectively. Thermodynamics analysis indicated that the adsorption process was exothermic and spontaneous. Furthermore, NGQDs and rmGO could both extend photoresponse range and enhance charge separation and transportation based on UV–vis DRS, PL spectra, transient photocurrent and electrochemical impedance spectra results. The optimized P25/NGQDs/rmGO composite obtained the removal efficiencies of 96.6 %, 100 % and 82.4 % for MB, CV and BR46, respectively, after 30-min adsorption and 150-min photodegradation under visible light irradiation. In addition, solar light irradiation was more favorable for the removal of these cationic dye pollutants. The high adsorption and photocatalytic degradation performance of P25/NGQDs/rmGO make it an excellent candidate for removing cationic dye pollutants from the industrial effluents. This study also provided a new idea on the roles of NGQDs and rmGO in adsorption-photocatalysis systems and promoted the practical application of TiO2-based composites in environmental field.