We synthesized single-phase CoS2 on a large scale by adding graphene oxide of sufficient quantity via the hydrothermal method using cobalt acetate and thioacetamide as precursors; this produced the hybrid of CoS2 with reduced graphene oxide which exhibited high electrocatalytic activity in the hydrogen evolution reaction.Electronic supplementary materialThe online version of this article (doi:10.1186/s40580-016-0066-x) contains supplementary material, which is available to authorized users.