Revealing the symmetry breaking, growth and surface properties of gold nanorods

Abstract

The unique properties of nanoparticle structures are directly determined by particle composition, size, shape and surface faceting; each offering a tuneable parameter with which one can tailor properties to a given application. Advances in wet‐chemistry techniques now enable the synthesis of nanoparticles with shape anisotropy and novel surface faceting that exhibit exciting optical and catalytic properties. Here, we develop and apply quantitative scanning transmission electron microscopy methods to observe key structural changes that enable anisotropic growth; characterise the nanorod faceting in three dimensions; and probe the relative surface “energies” of both high and low index facets. Gold nanorods are an archetypal system with which to study anisotropic nanoparticle growth. Yet despite intense research it remains unclear how or why a single crystal seed particle, with a cubic lattice, grows preferentially in two of six nominally symmetry‐equivalent directions. Observations at various stages of gold nanorod growth reveal the onset of asymmetry occurs only in single crystal seed particles that have reached diameters between 4 and 6 nm. In this size range only, small, asymmetric truncating surfaces with an open atomic structure become apparent, and in the presence of Ag + ions are stabilised, becoming side facets in the embryonic nanorod structure [1,2]. These results provide the first direct observation of the structural changes that break the symmetry of the seed particle and provide key insights into the mechanism of anisotropic growth. The various facets exhibited by the nanoparticle [3] and their relative surface energies are a crucial driver of shape control whilst also directly determining how the resulting particle will interact with its environment. We apply a quantitative STEM technique [4] to count the number of atoms in each atomic column as identified in STEM images of a single crystal gold nanorod orientated in two different zone axes. Using this method we are able to determine the morphology and facet crystallography of the nanorod, finding it is comprised of both high {0 1 1+√2} and low {110}, {100} index side‐facets, each of comparable size and shape. Furthermore, by applying this method at successive time intervals and comparing the images it is possible to quantify atomic movement on the surface and therefore determine the relative stability of different crystallographic facets and the overall stability of the nanoparticle shape [5]. These results provide important information on the effect of surfactants on the relative surface energies of high and low index facets, and shed new light on the growth kinetics of Au nanorods.