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 TiO(2)/GQD and anatase TiO(2)/GQD systems) were designed to harness the visible spectrum of sunlight. It is interesting that the photocatalytic rate of the rutile TiO(2)/GQD complex system is ca. 9 times larger than that of the anatase TiO(2)/GQD complex under visible light (λ > 420 nm) irradiation in the degradation of methylene blue.