Decahedra and Icosahedra Everywhere: The Anomalous Crystallization of Au and Other Metals at the Nanoscale

Throughout history, objects with fivefold symmetry have been a popular topic of interest for artists, philosophers, and scientists. This may be because fivefold symmetry is very conspicuous in nature. In the case of crystals, fivefold rotational symmetry is mathematically forbidden, and macroscopic crystals exhibiting fivefold symmetry have never been shown to exist. Nevertheless, in the nanoworld, nanoparticles are often found with decahedral and icosahedral shapes that have an overall pseudo‐fivefold symmetry. These structures are observed at many length scales, from 1 nm up to 1 µm. In this review, several reasons for the stability of fivefold nanoparticles are discussed. These include the formation of twin boundaries, surface reconstruction faceting, and other factors. Fivefold symmetry is common in macroscopic nature; however, it is not seen in metallic crystals at the macroscopic scale. But, when looking at the nanoscale, metallic nanoparticles and nanoalloys exhibit decahedral and icosahedral pseudo‐fivefold symmetry. This is a review of gold and other metals alloyed in nanoparticles and the prevalence of fivefold symmetry.