In this work, we report a systematic study of the relationship between photocatalytic properties of hydrogen evolution and structures and morphologies of g-C3N4 prepared by different precursors (urea, melamine and dicyandiamide). The photocatalytic performances of H-2 production are affected by the method and degree of polymerization, the degree of protonation, and the morphology of g-C3N4 prepared by different precursors. Furthermore, a novel metal-free N-GQDs/g-C3N4 catalyst was designed and synthesized, which shows much better photocatalytic activity for H-2 evolution from water splitting than that of g-C3N4 due to the unique and multiple roles of N-GQDs. A mechanism is put forth to explain the roles of N-GQDs and the detailed enhancement of photocatalytic performance of the N-GQDs/g-C3N4. (C) 2016 Elsevier B.V. All rights reserved.