Developing efficient nonprecious electrocatalysts to accelerate the hydrogen evolution reaction ( HER) is of importance for the hydrogen energy technology. Herein, we report the in situ growth of single-crystalline gamma-Cu2S nanoplates on copper foam ( CF) in a hydrothermal system, with the assistance of a small amount of cobalt( II) acetate. The presence of cobalt( II) acetate in the synthesis system has been proven to have multiple roles: ( i) inhibiting the formation of copper( I) oxide ( Cu2O); ( ii) directing the fromation of the crystal phase of gamma-Cu2S; and ( iii) controlling the morphology of the as-formed gamma-Cu2S. Furthermore, we show that the resulting gamma-Cu2S/CF material can serve as an efficient integrated 3D electrode toward HER at neutral pH. The gamma-Cu2S/CF delivers a current density of 10 mA/cm(2) at a small overpotential of 190 mV, gives 100% Faradaic yield during HER, and maintains its electrocatalytic activity for > 10 hours. To the best of our knowledge, this is the first time that a copper( I) sulfide-based material is demonstrated to electrocatalyze the HER efficiently. Identifying copper( I) sulfide as the active phase for HER and constructing advantageous 3D gamma-Cu2S nanostructure via an ion-induced method might open a door for the further investigation of Cu-based hydrogen-evolution electrocatalysts. (C) 2016 Elsevier Ltd. All rights reserved.