It have been recognized that the coupling of graphene quantum dots (GQDs) with semiconductor photocatalysts endow the resulting nanocomposites with enhanced photocatalytic performances, however, the essential roles of GQDs have not been clearly revealed yet. Herein, we report that a high efficiency of the photocatalytic H-2 evolution was achieved using strongly coupled nanohybrids of CdS with GQDs (CdSIGQDs) as visible-light-driven photocatalysts. CdSIGQDs nanohybrids were synthesized by a facile hydrothermal method in which the crystallization of CdS precursor and coupling of GQDs could be accomplished in one-step. GQDs are firmly decorated on the surface of CdS nanoparticles, forming "dot on -particle" heterodimer structures. GQDs have no significant influence on the crystallite structure of CdS but render the nanohybrids with strong light absorption at the wavelength beyond the band edge of CdS. Under visible light irradiation (>420 nm), CdSIGQDs nanohybrids reach the highest H-2 production rate of 95.4 mol h-1, about 2.7 times higher than that of pure CdS nanoparticles, at GQDs content of 1.0 wt%, and the apparent quantum efficiency (AQE) was determined to be 4.2% at 420 nm. Incident light wavelength dependent experiments reveal that the light absorption of CdS dominated the performance of nanohybrids, and the excess light absorption coming from GQDs hardly contributes to the observed higher activity. Photocurrent response, steady-state and time-resolved PL, and EIS measurements suggest that the high activity of CdSIGQDs is attributed predominantly to the graphene-like nature of GQDs, which can act as an efficient electron acceptor to induce an efficient charge separation. This work clearly reveals that GQDs mainly played a role of electron acceptor instead of a photosensitizer in enhancing the photocatalytic H-2 evolution performances of CdSIGQDs nanohybrids, which offers a new insight to understand the essential roles of GQDs in semiconductor/GQDs nanohybrids for efficient solar energy conversion applications. (C) 2017 Elsevier B.V. All rights reserved.