The Influence of Halides on the Solution-Phase Growth of Cu Nanowires and Microplates: A Multi-Scale Theoretical Study

In experimental studies, chloride promotes the growth of Cu nanowires, with {100} sides and {111} ends, in the presence of hexadecylamine (HDA) capping agent, but iodide promotes the growth of microplates with {100} sides and large, basal {111} facets. We used ab initio thermodynamics based on quantum density functional theory (DFT) to understand the role of chloride and iodide in these syntheses. Our studies reveal that HDA forms self-assembled monolayers (SAMs) on both surfaces at sufficiently low halide solution-phase chemical potentials. As the chloride chemical potential is increased, we find a range where HDA is displaced from {111} facets but remains strongly bound to {100} facets. These features suggest Cu ions are easily deposited on the ends of growing nanowires, but that deposition is suppressed on the nanowire sides and are consistent with the experimentally observed growth of Cu nanowires. Additionally, adsorbed chlorine reverses the facet preference of Cu atom binding and surface diffusion from the bare Cu surface, such that Cu binding is stronger, and diffusion is slower on the {111} nanowire ends than on the {100} sides. Calculations using the theory of absorbing Markov chains indicate that a combination of facet-selective surface diffusion and deposition leads to the growth of nanowires with aspect ratios over 1000.

Unlike chloride, iodide displaces HDA from both Cu(100) and Cu(111) when it is present at relatively low solution-phase concentrations. Additionally, iodide enhances the binding and surface-diffusion tendencies of bare Cu(100) and Cu(111), such that Cu atoms are basically immobile on the Cu(100) nanoplate sides and they are highly mobile on the basal {111} facets. Calculations using the theory of absorbing Markov chains indicate that facet-selective surface diffusion leads to the growth of microplates with dimensions in the experimental range.

Overall, the profound effect of halide adsorption in promoting/suppressing molecular binding on metal surfaces extends beyond Cu surfaces and I will discuss examples from our very recent work, in which similar scenarios occur on Pd, Au, and Au surfaces.

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