Formation of anisotropic nanocrystals from isotropic single-crystal precursors requires an essential symmetry breaking event. Single-crystal gold nanorods have become the model system for investigating the synthesis of anisotropic nanoparticles, and their growth mechanism continues to be the subject of intense investigation. Despite this, very little is known about the symmetry breaking event that precedes shape anisotropy. In particular, there remains limited understanding of how an isotropic seed particle becomes asymmetric and of the growth parameters that trigger and drive this process. Here, we present direct atomic-scale observations of the nanocrystal structure at the embryonic stages of gold nanorod growth. The onset of asymmetry of the nascent crystals is observed to occur only for single-crystal particles that have reached diameters of 4-6 nm and only in the presence of silver ions. In this size range, small, asymmetric truncating surfaces with an open atomic structure become apparent. Furthermore, {111} twin planes are observed in some immature nanorods within 1-3 monolayers of the surface. These results provide direct observation of the structural changes that break the symmetry of isotropic nascent nanocrystals and ultimately enable the growth of asymmetric nanocrystals.

The phenomenon of symmetry breaking-in which the order of symmetry of a system is reduced despite manifest higher-order symmetry in the underlying fundamental laws-is pervasive throughout science and nature, playing a critical role in fields ranging from particle physics and quantum theory to cosmology and general relativity. For the growth of crystals, symmetry breaking is the crucial step required to generate a macroscopic shape that has fewer symmetry elements than the unit cell and/or seed crystal from which it grew. Advances in colloid synthesis have enabled a wide variety of nanocrystal morphologies to be achieved, albeit empirically. Of the various nanoparticle morphologies synthesized, gold nanorods have perhaps been the most intensely studied, thanks largely to their unique morphology-dependent optical properties and exciting application potential. However, despite intense research efforts, an understanding of the mechanism by which a single crystal breaks symmetry and grows anisotropically has remained elusive, with many reports presenting seemingly conflicting data and theories. A fundamental understanding of the symmetry breaking process is needed to provide a rational framework upon which future synthetic approaches can be built. Inspired by recent experimental results and drawing upon the wider literature, we present a mechanism for gold nanorod growth from the moments prior to symmetry breaking to the final product. In particular, we describe the steps by which a cuboctahedral seed particle breaks symmetry and undergoes anisotropic growth to form a nanorod. With an emphasis on the evolving crystal structure, we highlight the key geometrical and chemical drivers behind the symmetry breaking process and factors that govern the formation and growth of nanorods, including control over the crystal width, length, and surface faceting. We propose that symmetry breaking is induced by an initial formation of a new surface structure that is stabilized by the deposition of silver, thus preserving this facet in the embryonic nanorod. These new surfaces initially form stochastically as truncations that remove high-energy edge atoms at the intersection of existing {111} facets and represent the beginnings of a {011}-type surface. Crucially, the finely tuned [HAuCl]:[AgNO] ratio and reduction potential of the system mean that silver deposition can occur on the more atomically open surface but not on the pre-existing lower-index facets. The stabilized surfaces develop into side facets of the nascent nanorod, while the largely unpassivated {111} facets are the predominant site of Au atom deposition. Growth in the width direction is tightly controlled by a self-sustaining cycle of galvanic replacement and silver deposition. It is the [HAuCl]:[AgNO] ratio that directly determines the particle size at which the more open atomic surfaces can be stabilized by silver and the rate of growth in the width direction following symmetry breaking, thus explaining the known aspect ratio control with Ag ion concentration. We desc...

Single crystal gold nanorods remain one of the most important and intensively studied anisotropic nanocrystals. The aspect ratio of the nanorods is controlled during the colloidal synthesis using silver nitrate; however, the mechanisms for the underlying control are not well understood. Here, we investigate the growth of gold nanocrystals at the stage where they break symmetry and begin anisotropic growth into nanorods. Using high resolution electron microscopy, we determine directly the size and atomic structure of the nanocrystals at the symmetry breaking point. We find that silver nitrate controls the size of the crystal at which symmetry breaking occurs. The seed crystal undergoes a symmetry breaking event at a critical diameter between 4 and 6 nm that depends upon the [HAuCl4]:[AgNO3] ratio. The smallest diameter for symmetry breaking, ∼4 nm, is observed at the lowest [HAuCl4]:[AgNO3] ratio (i.e., the highest AgNO3 concentration) corresponding to the minimum size at which a “truncation” can form, a precursor to a {110} facet. The diameter of the nanocrystal at the symmetry breaking point becomes the width of the nascent nanorod, and this in turn determines the final nanorod width. Surprisingly, the [HAuCl4]:[AgNO3] ratio has little effect on the final nanorod length. Our observations explain why the nanorod aspect ratio is constrained within a limited range. This provides a rational framework for controlling width and aspect ratio in the growth of single crystal gold nanorods.

A new mechanism for reactivity of multiply twinned gold nanoparticles resulting from their inherently strained structure provides a further explanation of the surprising catalytic activity of small gold nanoparticles. Atomic defect structural studies of surface strains and quantitative analysis of atomic column displacements in the decahedral structure observed by aberration corrected transmission electron microscopy reveal an average expansion of surface nearest neighbor distances of 5.6%, with many strained by more than 10%. Density functional theory calculations of the resulting modified gold d-band states predict significantly enhanced activity for carbon monoxide oxidation. The new insights have important implications for the applications of nanoparticles in chemical process technology, including for heterogeneous catalysis.

Metal nanocrystals can be grown in a variety of shapes through the modification of surface facet energies via surfactants. However, the surface facets are only a few atoms wide, making it extremely challenging to measure their geometries and energies. Here, we locate and count atoms in Au nanorods at successive time intervals using quantitative scanning transmission electron microscopy. This enables us to determine the atomic-level geometry and the relative stability of the facets and to expound their relationship to the overall three-dimensional nanocrystal shape and size. We reveal coexisting high- and low-index facets with comparable stability and dimensions and find the geometry of the nanorods is remarkably stable, despite significant atom movements. This information provides unique insights into the mechanisms that govern growth kinetics and nanocrystal morphology.