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Immersion in the synthesis of nanoparticles with transmission electron microscopy in liquid

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Mastering the properties of nanomaterials requires controlling their size and shape. Developed in an empirical manner, wet-chemistry synthesis of anisotropic metal nanoparticles would greatly benefit from a better understanding of the nucleation and growth mechanisms. The main bottle neck arises from the difficulty of probing in liquid at the relevant time and spatial scales, the dynamic nature of atom-by-atom formation of nanoparticles. A new environmental transmission electron microscopy technique was recently implemented on the last generation microscope of the MPQ Lab (The Super TEM). This unseen in France method consist in imaging the dynamics of nano-objects in an encapsulated liquid solution within an electron-transparent microfabricated cell. The composition of the environment can be controlled with a micro-fluidic system which enables to mix different reaction solutions at the observation window. By combining the imaging capacities of the super TEM and this new type of sample-holder, we can image the atomic structure of NPs immersed in a solution with a resolution below 0.2 nanometre. This in situ technique was exploited for unravelling the growth mechanisms of gold nanoplates activated by the reduction of chloroauric acid in water. During this experiment, reducing agents were free radicals created by the electron beam via the radiolysis of water molecules. As the electron dose can be finely tuned in an electron microscope we can then modulate the speed of nanoparticle growth. By controlling the driving force of the nanoparticle synthesis, MPQ scientists have shown that the formation of 2D nanocrystals requires slow growth kinetics, below 3 atomic layer per second. Furthermore, 2D growth also depends on crystal defaults formed during the early stage of the nanoparticle growth. Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. This frequent process in the formation of metallic crystals induces symmetry breakings that favour anisotropic growth. As the adhesion of atoms is easier along the twin planes that separate two twinned crystal, the formation nanoplates requires those twin-planes to be parallel to each other. This study quantitatively explains the relevancy of the strategies used by colloidal chemists for fabricating anisotropic nanoparticles. Co-funded by the CNRS (Defi nano project) and the SEAM LabEx (Plas-Mag project) liquid-cell microscopy is a relevant technique to study dynamical processes at the interfaces between liquids and solids, opening many avenues in both materials and life sciences.

Figure :
Scanning electron microscope image of the two types of nanoparticles (nanoplatelets and three-dimensional) obtained by growth in liquid and followed in situ by transmission electron microscopy.

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Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy, D. Alloyeau, W. Dachraoui, Y. Javed, H. Belkahla, G. Wang, H. Lecoq, S. Ammar, O. Ersen, A. Wisnet, F. Gazeau, and C. Ricolleau, Nanoletters 15, 2574 (2015).