The kinetics of solution-phase Ag nanowire growth mediated by PVP: A theoretical perspective

A broad collection of Ag nanostructures can be synthesized in solution with the aid of polyvinylpyrrolidone (PVP). Among these, the growth of fivefold-twinned nanowires is particularly confounding, as it requires a growth rate 10²–10³ times faster along the nanowire axis than laterally. These structures grow from Mark’s decahedron seeds, which possess re-entrant “notches” at each twin boundary on the {100} side facets. These notches serve as “super-highways” to direct a high atom diffusion flux to the {111} ends, and thus elongate the nanowire. We calculate relevant adatom diffusion energy barriers for a bare Ag nanowire, incorporate these into finite Markov chain calculations to obtain the net growth rates of the nanowire length and diameter, and find that nanowires cannot form from bare Ag because atom accumulates on the nanowire sides, leading a lateral growth. However, by accounting for the solution environment and sampling the free energies around the edges and sides, we find that the region near the edge on the {111} end can become more energetically favored due to the semi-flexibility of PVP, and thus an atom’s transition towards the end becomes faster than the reverse path, leading to high-aspect-ratio Ag nanowires. Our results can explain several aspects of experiments.