Three-dimensionally (3D) structured Cu nanoparticles (NPs)-embedded graphene hydrogels were synthesized from inexpensive graphite and low-cost copper acetate by a novel self-assembly and in-situ slow-release photoreduction method. The cross-linked microporous 3D reduced graphene oxide framework ensures full contact of the photocatalyst with water and promotes electron transfer. The EDA-reduced Cu-rGH hydrogel with an 11.3% Cu NPs mass ratio exhibits the best hydrogen evolution rate (16.92 mmol.g(-1) .h(-1)). This rate is almost 17 times faster than pure Cu NPs. EDA played an important role in both forming a hydrogel as the reducing agent and slowly releasing Cu NPs as a ligand in the process of in-situ photoreduction, allowing small sized and uniformly distributed Cu NPs. Therefore, the number of reaction sites in the composite increases and the recombination of photoinduced electron-hole pairs in Cu NPs decreases. Under irradiation, the SPR-excited (surface plasmon resonance) hot electrons from Cu NPs quickly transfer through the rGH channel. 3D rGH was found to be a promising substrate for boosting hydrogen production by Cu NPs. (C) 2020 Elsevier Inc. All rights reserved.