Metal nanowires are essentially sticks of metal with diameters usually in the tens of nanometers and lengths in the tens of micrometers, giving them aspect ratios around 100-1000. Despite the fact that an increasing number of applications rely on nanowires grown in solution-phase syntheses, the mechanism by which nanowires grow to have a high aspect ratio has largely remained a mystery. This presentation will show how measurements with single-crystal electrodes can be used to reveal the facet-selective chemistry that drives anisotropic growth of metal nanowires.  Solution-phase synthesis generally involves heating a solution containing a metal salt, a reducing agent, and a shape-directing organic additive (usually referred to as a capping agent) that is necessary for anisotropic growth. Pentagonally-twinned nanowires of Cu, Ag, and Au have (100) facets on their sides and (111) facets on their ends. Researchers have hypothesized that organic capping agents direct anisotropic growth by selectively inhibiting atomic addition to the (100) facets. This presentation will show this hypothesis is wrong for two different syntheses of Cu nanowires.  For a Cu nanowire synthesis in which ethylenediamine serves as the capping agent, ethylenediamine acts as a facet-selective promoter of Cu nanowire growth by keeping the Cu(111) surface relatively free of surface oxidation in the highly basic growth solution (&gt;12 M NaOH). This resulted in a ~6 minute window during which hydrazine reduced Cu ions onto Cu(111) while reducing surface oxides on Cu(100). The ~6-minute period of facet-selective Cu deposition closely matched the time scale over which Cu nanowires switch from longitudinal to lateral growth in the actual nanowire growth solution. In another synthesis in which hexadecylamine is a capping agent, electrochemical measurements on Cu(111) and Cu(100) surfaces show that, in contrast to previous hypotheses, hexadecylamine does not selectively passivate Cu(100); it passivates both facets equally. However, the introduction of Cl<sup>-</sup> in a narrow range of concentrations selectively disrupts the alkylamine monolayer on Cu(111), causing Cu to preferentially deposit onto (111) facets on the ends of the nanowires.<br/>

Single-crystal electrochemistry reveals why nanowires grow

256th National Meeting (2018)

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