Controllable synthesis of Au nanocrystals with systematic shape evolution from an octahedron to a truncated ditetragonal prism and rhombic dodecahedron

Abstract

Controllable preparation of nanocrystals (NCs) is a prerequisite to reveal their shape-dependent physicochemical properties. Herein, by using a one-pot synthesis method, ethylene glycol (EG) as a solvent and a reducing agent, poly(diallyldimethylammonium chloride) (PDDA) as a ligand, and AgNO₃ as an additive, we successfully prepared octahedral, rhombic dodecahedral (RD), tetrahexahedral (THH) and truncated ditetragonal prism (TDP) Au NCs. And by simply tuning the reaction temperature and thus the reducing power of EG, we not only realized the shape transformation of Au NCs among low-index facet crystals ({111} to {110} facet) and low-index facet crystals to high-index facet crystals ({111} to {210} facet), but also achieved the shape evolution among high-index facet crystals ({210} to {310} facet). At a relatively high reaction temperature of 195 °C, (b.p. 198 °C) stepwise evolution of Au NCs from a {111}-bounded octahedron to a {210}-bounded THH and then to a {310}-bounded TDP was achieved. Furthermore, by simply reducing the reaction temperature to 160 °C, the {111}-bounded octahedral Au NCs would transform into RD Au NCs enclosed with the {110} facet. And the morphology evolution of Au crystals caused by different experimental parameters was systemically investigated. We found that the evolution of the crystal morphology is the result of the synergistic effect of the UPD effect of Ag⁰ and the oxidative etching of O₂/Cl⁻. In addition, high-index facet THH Au NCs with controllable sizes of 65–120 nm were prepared by adjusting the amount of the PDDA solution added to the growth solution. This work not only provides a new method to synthesize Au NCs with tunable shape and size, but is also helpful in understanding the crystal growth habit.