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Abstract Shape control has been a major theme of nanocrystal research in terms of synthesis, property tailoring, and optimization of performance in a variety of applications. Among the possible shapes, bipyramids are unique owing to their symmetry, planar defects, and exposed facets. In this article, we focus on the colloidal synthesis of noble‐metal nanocrystals featuring a triangular bipyramidal shape, together with highlights of their properties and applications. We start with a brief discussion of the general classification and requirements for the nucleation and growth of bipyramidal nanocrystals, followed by specific aspects regarding the synthetic methods with a focus on the roles of reduction, etching, and capping, as well as controls of facet, size, aspect ratio, and corner truncation. In the end, we illustrate how these aspects affect the properties of bipyramidal nanocrystals for plasmonic and catalytic applications, together with future perspectives. 

Abstract A relatively unexplored aspect of noble‐metal nanomaterials is polymorphism, or their ability to crystallize in different crystal phases. Here, a method is reported for the facile synthesis of Ru@Pd core–shell nanocrystals featuring polymorphism, with the core made of hexagonally close‐packed (hcp)‐Ru while the Pd shell takes either anhcpor face‐centered cubic (fcc)phase. The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking thehcpphase whereas the thicker ones revert tofcc. The injection rate provides an experimental knob for controlling the phase, with one‐shot and drop‐wise injection of the Pd precursor corresponding tofcc‐Pd andhcp‐Pd shells, respectively. When these nanocrystals are tested as catalysts toward formic acid oxidation, the Ru@Pd_hcpnanocrystals outperform Ru@Pd_fccin terms of both specific activity and peak potential. Density functional theory calculations are also performed to elucidate the origin of this performance enhancement.