CO2 electro-reduction is a promising method for sustainable production of carbon fuels as well as valuable chemicals. Herein, we investigated a direct electro-catalytic CO2 conversion to CO by nanostructured gold catalysts from the small nanoparticles to aggregated clusters to layered film. The selectivity of CO formation was found to increase as the gold amount increased and it reached a saturation point of 78% (-0.59 V vs. RHE) at the morphological transition from aggregated cluster to layered film. Furthermore, the similar to 4 nm gold nanoparticle exhibited a remarkable CO formation mass activity value at 166.1 A/g (-0.59 V vs. RHE). (C) 2015 Elsevier B.V. All rights reserved.