Co-based bimetallic nanocrystals with three-dimensional(3D)branches are synthesized by heterogeneous nucleation of Co atoms onto the prenucleated seeds such Pd or Cu through a facile wet-chemical route.The peripheral branches(rod-like)of Co-Pd(Figure 1)and Co-Cu(Figure 2)nanocrystals outspread along(001)direction and are enclosed by(101)facets.Here,the higher electronegativity of Pd caused Pd2+ions to be first reduced by propylene glycol(served as the solvent and reductant)to form neutral Pd atoms.Positive Co2+ions attracted the electron cloud of the Pd atoms and the Pd atoms were polarized towards Co2+center.The continuous supply of electrons from propylene glycol to Pd(Pd as an acceptor,reducing agent as a donator)made electron density shift from Pd to Co until the formation of Co-Pd nucleation,and subsequently Co-Pd nucleation grew up towards Co-Pd nanocrystal.Cu plays similar role on the anisotropic growth of Co.By switching the prenucleated metals to form robust Co-Pd or Co-Cu bimetallic nanocatalysts,the selectivity of CO hydrogenation could be purposely adjusted towards heavy paraffins,light olefins or oxygenates.The Anderson-Schulz-Flory chain-lengthening probabilities for products are up to 0.9 over Co-Pd nanocrystals,showing that long-chain hydrocarbons can be formed with high selectivity using the targeted design of Co-Pd nanocrystal catalysts.These Co-based bimetallic nanocrystals with 3D structure exhibit superior catalytic activities to the corresponding Co-based nanoparticles for synthesis gas conversion(Figure 3a-b).CoPd0.001 nanocrystals can achieve more heavy hydrocarbons(Cn,n>12)than CoPd(0.1 wt%Pd)nanoparticles(Figure 3c).However,CoCu0.1 nanocrystals and CoCu(10 wt%Cu)nanoparticles were found to be similar catalytic selectivity for syngas conversion(Figure 3d).The different phenomena suggest that the catalytic selectivity of Co-based bimetallic catalysts is ascribed ont only to the structure effect but also to the synergistic action of metals.