A Rationally Designed Route to the One-Pot Synthesis of Right Bipyramidal Nanocrystals of Copper

Lined with planar defects, nanoscale right bipyramids (RBPs) are of great interest for a variety of applications owing to the presence of a single type of {100} facets and two oppositely positioned right-angle corners as well as a higher surface-to-volume ratio than that of a cube. Here we report a rationally designed route to the facile synthesis of Cu RBPs with edge lengths controlled in the range 30-70 nm. The one-pot protocol involves the sequential reduction of Pd(II) and Cu(II) precursors, mixed at a molar ratio of 1:200, by glucose in the presence of hexadecylamine, a capping agent with strong binding to the Cu(100) surface. Because of the difference in standard reduction potential between the Pd(II)/Pd and Cu(II)/Cu pairs, the Pd(II) precursor is reduced first for the generation of Pd seeds lined with parallel planar defects, followed by the slow deposition of Cu atoms for the formation of RBPs. By varying the growth time and/or the amount of Pd(II) relative to Cu(II), Cu RBPs with tunable edge lengths can be obtained with purity over 80%. Because Pd(II) is used at a trace amount relative to Cu(II), the Pd has essentially no impact on the elemental composition and optical properties of the RBPs. For Cu RBPs with edge lengths greater than 56 nm, we observed two localized surface plasmon resonance peaks at 586 and 621 nm, and their positions are both red-shifted as the edge length is increased. The availability of Cu RBPs with uniform and controlled sizes opens the door to a range of applications related to catalysis, photocatalysis, and plasmonics, among others.