Control of Symmetry Breaking Size and Aspect Ratio in Gold Nanorods: Underlying Role of Silver Nitrate

Abstract

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.

Disconnecting Symmetry Breaking from Seeded Growth for the Reproducible Synthesis of High Quality Gold Nanorods

“…For small nanocrystals (4-6 nm) this could lead to truncating surfaces appearing only at certain intersections between {111} facets and the rupture of the nanocrystal symmetry. [13][14][15] Then, the presence of silver ions could render preferential stabilization of newly formed {110} facets 10 over the {001} and {111} ones, via an underpotential deposition mechanism and the growth of the nascent AuNRs develops orthogonally on the new facets (along the ⟨100⟩ direction) 14,31 In addition to silver ions, CTAB has also been proposed to play a key role in the stabilization of emerging facets. 32,33 As such, CTAB remains the most employed surfactant for the synthesis of AuNRs.…”

“…12 Moreover, seeded growth has proven to be the most efficient method toward gaining insight into the growth mechanisms of anisotropic nanocrystals. [13][14][15] The concept was first introduced by Murphy et al for the synthesis of gold nanorods (AuNRs), 7 which is probably the most alluring case of anisotropic noble metals synthesis. Due to the high-symmetry facecentered cubic lattice of gold (as well as of other noble metals), its growth into low-symmetry nanocrystals is highly unfavored.…”

“…In this regard, the aspect ratio is determined by the [HAuCl ]/[AgNO ] ratio, symmetry breaking occurring at the smaller seed sizes for high silver concentration, thereby producing AuNRs of higher aspect ratio. [13][14][15] Finally, solution pH, temperature, and concentration of ascorbic acid (weak reducing agent) also influence the outcome of AuNR growth. Therefore, symmetry breaking occurs only within a narrow range of reactant concentrations, seed sizes, and experimental conditions.…”

Surfactant‐Assisted Symmetry Breaking in Colloidal Gold Nanocrystal Growth

“…5-7 nm, point at which the emergence of truncations is more favorable at the {111} intersections without reducing nanocrystal symmetry. [41,45,46] The final single crystal gold nanorod products are typically enclosed by {100} and {111} facets at the tips, while higher index planes such as {110} or {250} can be found at the sides ( Figure 3D). [47][48][49][50][51] The aspect ratio of gold nanorod colloids, their quality (dispersion in size and aspect ratio), and the shape yield of the synthesis (hindered by spherical impurities) are primary determined by the efficiency of the symmetry-breaking process.…”

Photochemical Formation of Tunable Gold Nanostructures Using Versatile Water‐Soluble Thiolate Au(I) Precursor

“…Gold plating on AgNPs was shown to alleviate stability problems, yet for many applications, pure AuNPs are preferred. Several approaches to AuNP preparation have been developed . One of the most successful shape‐selected preparations of AuNPs is the rod synthesis .…”