Fabrication of a Stable New Polymorph Gold Nanowire with Sixfold Rotational Symmetry

Abstract

Gold is known as the most noblest metal with only face-centered cubic (fcc) structure in ambient conditions. Here, stable hexagonal non-close-packed (ncp) gold nanowires (NWs), having a diameter of about 50 nm and aspect ratios of well over 400, are reported. Au NWs are grown in the confined system of nanotubular TiO₂ arrays via photoelectrochemical reduction of HAuCl₄ precursors. Some of the resulting Au NWs are proved to have sixfold rotational symmetry, observed by transmission electron microscopy tilting experiments. This new polymorph is identified as a hexagonal ncp-structure with lattice parameters of a = 2.884 Å and c = 7.150 Å, showing quite a large interplanar spacing (c/a ≈ 2.48). That is, Au atoms are close-packed along the ab plane, but each plane is not closely stacked along the c axis like in graphite. The structure is usually expected to be unstable, but the present ncp-2H gold is stable under ambient conditions and intense electron beam irradiation, and shows thermal stability up to 400 °C. Moreover, the resulting physical properties as a result of the corresponding change in electronic structures are investigated by comparing the optical properties of fcc and ncp-2H Au NWs.

Stable new polymorph gold nanowires are fabricated by a diffusion-limited process under nanoscale confinement. The crystal structure is carefully identified as a hexagonal non-close-packed structure (ncp-2H) by specimen titling in transmission electron microscopy. Ncp-2H gold nanowires show considerable stability and different physical properties from the thermodynamically stable form of gold.

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