Exploring the Formation of Symmetric Gold Nanostars by Liquid-Cell Transmission Electron Microscopy

Abstract

The shape-dependent properties of gold nanostars (NSs) have motivated massive research efforts in the field of colloidal chemistry to gain a better control over the morphology of these promising nanostructures. Nevertheless, this challenge requires a better understanding of the atomic-scale processes leading to the formation of stellated nanoparticles. We hereby report an unprecedented in situ study focused on the seed-mediated synthesis of symmetric gold NSs performed by radiolysis in methanol. We take advantage of the spatial and temporal resolutions of liquid-cell transmission electron microscopy to unravel the key effects of the growth speed, seed-crystal morphology, and dimethylamine functionalization on the formation mechanisms, shape, and stability of NSs enclosed by high-index facets. Surprisingly, the stellation processes transforming icosahedral nanoparticles into NSs with 20 sharp arms entails a continuous restructuring of NS facets driven by surface diffusion, which provide a fresh look at faceting mechanisms.

Selective shortening of gold nanorods: when surface functionalization dictates the reactivity of nanostructures

“…Liquid STEM imaging was performed using a liquid-cell TEM holder commercialized by Protochips Inc. A colloidal suspension of citrate-coated gold NRs dispersed in an acidic saline solution containing 1 M of NaCl and 10 −2 M of HCl was encapsulated in the liquid-cell by using the conventional loading process. 22,23 The Au spacers of the liquid cell were 150 nm thick and the experiments were performed in the corners of the observation window, where the liquid thickness is minimum. The acidic saline solution was continuously flowing into the liquid-cell during the experiment with a flow rate of 5 µL min −1 .…”

“…In that regards, liquid cell transmission electron microscopy (LCTEM) has become a method of choice to observe the oxidative etching of individual nanostructures such as metallic nanoparticles [9][10][11][12][13][14][15][16][17] and nanoalloys [18][19][20][21] and it has provided a unique observation window on the intermediate nanostructures formed during these dynamic processes. Like the nucleation and growth of nanoparticles, 22,23 the dissolution mechanisms are simultaneously driven by kinetic and thermodynamic effects that depend on the reaction speed and the structural stability of nanomaterials, respectively. Therefore, LCTEM allowed highlighting the effects of oxidation potential, nanoparticle faceting and crystal defects on selective-etching processes.…”

In Situ Tracking of Colloidally Stable and Ordered Assemblies of Gold Nanorods

“…In situ electron microscopy (EM) observation of nanoparticle dispersions under dynamic conditions has recently allowed for real-time visualization of chemical and physical events at the single nanoparticle level. Some remarkable examples include the observation of how spherical 13 and anisotropic nanoparticles grow, 14 nanoparticle surface oxidation, 15 or dehydrogenation. 16 More recently, in situ EM has also been used to investigate self-assembly processes.…”

Shape-controlled synthesis and in situ characterisation of anisotropic Au nanomaterials using liquid cell transmission electron microscopy

“…[16][17][18][19][20][21][22][23] In particular, this technique can complement spectroscopy and X-ray based methods to track nanoparticle (NP) growth trajectories through direct visualisation of crystal formation in liquid environments, with a unique combination of temporal and spatial resolutions. 6,16,18,20,22,[24][25][26][27][28][29][30][31][32][33] On the other hand, the irradiating electrons can change the solution chemistry in the submicron layer of liquid encapsulated between two electron transparent membranes (e.g., silicon nitride or graphene), by creating transient radical products, including ℎ − (hydrated electrons), H • , OH • , H2O2 and H3O + . 20,[34][35][36] Among these reactive species, ℎ − are known to reduce metal ions and initiate crystal growth in solution or on the silicon nitride (SixNy) membranes of the liquid cell.…”