Using Graphene Liquid Cell Transmission Electron Microscopy to Study <em>in Situ</em> Nanocrystal Etching

Abstract

Graphene liquid cell electron microscopy provides the ability to observe nanoscale chemical transformations and dynamics as the reactions are occurring in liquid environments. This manuscript describes the process for making graphene liquid cells through the example of graphene liquid cell transmission electron microscopy (TEM) experiments of gold nanocrystal etching. The protocol for making graphene liquid cells involves coating gold, holey-carbon TEM grids with chemical vapor deposition graphene and then using those graphene-coated grids to encapsulate liquid between two graphene surfaces. These pockets of liquid, with the nanomaterial of interest, are imaged in the electron microscope to see the dynamics of the nanoscale process, in this case the oxidative etching of gold nanorods. By controlling the electron beam dose rate, which modulates the etching species in the liquid cell, the underlying mechanisms of how atoms are removed from nanocrystals to form different facets and shapes can be better understood. Graphene liquid cell TEM has the advantages of high spatial resolution, compatibility with traditional TEM holders, and low start-up costs for research groups. Current limitations include delicate sample preparation, lack of flow capability, and reliance on electron beam-generated radiolysis products to induce reactions. With further development and control, graphene liquid cell may become a ubiquitous technique in nanomaterials and biology, and is already being used to study mechanisms governing growth, etching, and self-assembly processes of nanomaterials in liquid on the single particle level.

Gold Nanocrystal Etching as a Means of Probing the Dynamic Chemical Environment in Graphene Liquid Cell Electron Microscopy

“…In these liquid cell transmission electron microscopy (TEM) experiments, premade gold nanocrystals are added to an aqueous solution of FeCl 3 and Tris-HCl, and the solution is encapsulated between graphene sheets for imaging in the TEM. 53 The concentrations of FeCl 3 are low enough that the gold nanocrystals do not begin to be etched until electron beam irradiation reaches a sufficiently high dose rate. Through a combination of the preloaded FeCl 3 and beam-induced radiolysis species, the nanocrystals undergo oxidative etching and the nanocrystals' etching trajectories can be observed ( Figure 1A).…”

“…Graphene liquid cells were fabricated following a reported procedure. 53 Graphene (3−5 layers, ACS materials) was transferred onto holey amorphous carbon, gold quantifoil TEM grids (SPI Supplies, 300 mesh, R1.2/1.3), and these grids were used to encapsulate a solution of 104 mM FeCl 3 (Sigma-Aldrich) in 22 mM HCl (Fisher Chemical), 100 mM tris-(hydroxymethyl)aminomethane hydrochloride, Tris-HCl, (Fisher Biotech), and the nanocrystals of interest. The volumes of FeCl 3 / HCl and Tris-HCl were modulated to yield the desired final FeCl 3 concentration.…”

Preparation of Graphene-Supported Microwell Liquid Cells for <em>In Situ</em> Transmission Electron Microscopy

“…In the first approach, the liquid is encapsulated in a cavity between two Si 3 N 4 membranes produced via Si process technology 17 , whereas in the second, small liquid pockets are formed between two graphene or graphene oxide sheets 10,18 . The handling of both silicon-based liquid cells (SiLCs) and graphene-based liquid cells (GLCs) has been demonstrated 19,20,21 . Although both approaches have undergone significant improvements 22,23,24,25 , they still lack in the combination of the respective advantages.…”

“…Furthermore, the prevalence of particle dissolution proves that a wet-chemical reaction is present which would not be possible without a successful liquid enclosing. Other typical indications for enclosed liquids are beam-induced bubble formation19 or particle motion. The presence of Au particles in graphene-featured cells alone does not conclusively indicate a liquid environment, since the particles could also stem from the graphene-induced reduction of HAuCl 4 A quantification of the oxygen peaks of the enclosed liquid via electron energy loss spectroscopy (EELS) can also be performed to verify a liquid environment41.…”

Imaging Graphene-Encapsulated Microtubules at Room Temperature with Electron Microscopy

“…We obtained the highest number of intact microtubules with the graphene coated sample supports. It was recently reported that the number of stable liquid pockets obtained in a graphene liquid cell strongly depends of the solvent conditions [7]. Figure 1a shows the schematic representation of graphene-encapsulated microtubules for electron microscopy.…”