Honeycomb 3D graphene-like Co9S8 nanocrystallines were successfully synthesized using cobalt and Na2S center dot 9H(2)O as starting materials without a template via a simple hydrothermal route. The composition, structure, morphology, and catalytic activity of the as-prepared Co9S8 were characterized by investigation of XRD, SEM, EDS, XPS, GC -MS and HPLC. It was found that an appropriate concentration of inexpensive NaHCO3 play a critical role for the formation of honeycomb Co9S8 nanocrystallines. On the other hand, Co9S8 acted as a catalyst in the hydrothermal reaction, producing formic acid reduced from NaHCO3 as high as 3680 mg/L. It indicates that the present study not only proposes a simple way of honeycomb Co9S8 synthesis, but also provides a potential approach to CO2 reduction. (C) 2015 Elsevier B.V. All rights reserved.