Upconversion and Downconversion Fluorescent Graphene Quantum Dots: Ultrasonic Preparation and Photocatalysis

A facile ultrasonic route for the fabrication of graphene quantum dots (GQDs) with upconverted emission is presented. The as-prepared GQDs exhibit an excitation-independent downconversion and upconversion photoluminescent (PL) behavior, and the complex photocatalysts (rutile TiO2/GQD and anatase TiO2/GQD systems) were designed to harness the visible spectrum of sunlight. It is interesting that the photocatalytic rate of the rutile TiO2/GQD complex system is ca. 9 times larger than that of the anatase TiO2/GQD complex under visible light (λ > 420 nm) irradiation in the degradation of methylene blue.