Gold is the first metal used by humans, and it is deeply involved in human life. This was possible because gold was found to be almost pure in nature and easy to process without heating or melting. As one of the most chemically stable metals, gold adopts only one crystal structure.

The crystal structure is closely related to the intrinsic properties of materials. When the crystal structure is changed, physical and chemical properties, including electrical and optical properties, are changed. Therefore, it is important to control the polymorphism to achieve reliable material or device properties.

The crystal structure of gold is face-centered cubic (fcc) with 3-fold rotational symmetry by nature. For the first time, Prof. SHIN, Ms. LEE, and Dr. BAE (Department of Energy Science at Sungkyunkwan University) have published a stable hexagonal non-close packed structure (ncp-2H) of gold in Advanced Materials (IF 19.791) as a cover story. They report not only excelent stability of ncp-2H gold, but also different physical properties different from those of fcc.

Schematic illustration of the experimental procedures for the confined growth of gold nanowires

They fabricated the gold nanowires in a confined system of TiO2 nanotubes via the photoelectrochemical reduction of gold ions. During ultraviolet illumination, excited photoelectrons in the conduction band of TiO2 reduced the Au ions to metallic Au. At the early stage of the nucleation process, various polymorphs are able to be formed; however, they are eventually transformed to fcc, the most thermodynamically stable form in nature. They suggest that isolated ncp-2H gold seeds at the nucleation stage survive and grow by a diffusion-limited process under the nanoscale confinement provided by TiO2 nanotubes. They supported the possibility that nanoscale confinement influenced polymorph formation through control experiments.

This study is particularly noteworthy that it suggests that different theories from that of classical thermodynamics are required for nanoscale confined systems.

Published article:

S. Lee, C. Bae, J. Lee, S. Lee, S. H. Oh, J. Kim, G.-S. Park, H. S. Jung and H. Shin, "Fabrication of a Stable New Polymorph Gold Nanowire with Sixfold Rotational Symmetry," Advanced Materials, 30, 1706261 (2018). (Cover Illustration)