Polyol Synthesis of Uniform Silver Nanowires:  A Plausible Growth Mechanism and the Supporting Evidence

We have recently demonstrated an approach based on the polyol process for the large-scale synthesis of silver nanowires with uniform diameters (see Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833. Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736). Although the capability and feasibility of this method have been successfully illustrated with the production of silver nanowires 30−60 nm in diameter and 1−50 μm in length, the growth mechanism of this process is yet to be elucidated. Here we report some progress on this matter: First, electron microscopy studies on microtomed samples indicated that the cross sections of such silver nanowires had a pentagonal shape, together with a 5-fold twinned crystal structure. Second, the side surfaces (bounded by {100} facets) and the ends (bounded by {111} facets) of each nanowire were shown to have significant difference in reactivity toward dithoil molecules, with the side surfaces being completely passivated by poly(vinyl pyrrolidone) (PVP) and the ends being partially passivated (or essentially uncovered) by PVP. This result implied that the PVP macromolecules interacted more strongly with the {100} planes than with the {111} planes of silver. On the basis of these new results, we proposed that each silver nanowire evolved from a multiply twinned nanoparticle (MTP) of silver with the assistance of PVP at the initial stage of the Ostwald ripening process. The anisotropic growth was maintained by selectively covering the {100} facets with PVP while leaving the {111} facets largely uncovered by PVP and thus highly reactive.