Despite the pivotal role played by the reduction of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the solution phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by reduction through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the reduction kinetics involved. Our quantitative analyses of the reduction kinetics of PdCl42- and PdBr42- by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl42- was reduced in the solution phase while PdBr42- was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr42- by ascorbic acid could be switched from surface to solution by raising the reaction temperature.
Keywords: Nanocrystal synthesis; kinetic model; precursor reduction; seed-mediated growth.