From Bulk-To-Nano: Advances Towards Optimization of Electrochemical Studies for Liquid-Phase Transmission Electron Microscopy

Liquid-phase transmission electron microscopy (LPTEM) has advanced considerably, evolving from rudimentary static cells [1] to advanced commercial systems capable of performing a variety of functions from temperature control to electrochemistry [2]. A core limitation of any TEM study is the assumption that the limited spatial area and sampling size inherent to TEM specimen preparation accurately represents the bulk state of the material [3]. This limitation is felt even more strongly for in-situ TEM studies, where the researcher must not only contend with determining if the sample itself is representative of its bulk state, but if that sample’s environment and its operando behavior remain consistent with its bulk processes and mechanisms.<br/><br/>Here we describe recent advancements in hardware, MEMs chip designs, and software with the end goal of improving the accuracy of replicating bulk scale results in-situ. First, integration of an external, standard reference electrode (SRE) via a “metal bridge” enables integration of conventional Ag/AgCl reference electrodes. Employing an SRE eliminates the shifting potentials introduced by on-chip pseudo-reference electrodes, such as platinum, without introducing a significant ohmic drop [4], and enables more accurate comparisons between bulk and in-situ results.<br/><br/>Second, it is well understood that the narrow gap between chips necessary to maintain thin enough liquid layers for good resolution in LPTEM experiments, introduces a myriad of potential artifacts due to confinement, including incomplete mixing, slow exchange of liquids and ion depletion [5]. Recently, we developed a new E-chip configuration designed to balance both thin liquid layer required for optimal resolution and a deeper flow channel to improve liquid flow characteristics. This design utilizes a 10 micron channel etched into the silicon substrate of the E-chip. The viewing region, containing the amorphous silicon nitride membrane, is isolated within the center of the E-chip on an island type structure, such that the deep flow channel surrounds it like a moat and does not interfere with the gap between the top and bottom membranes. This significantly reduces the distance over which the liquid must diffuse to reach the narrow gap between the viewing windows from &gt;1 mm to a few tens of microns, significantly improving liquid exchange within the critical region [6].<br/><br/>Finally, comprehensive analysis of the electron dose during in-situ studies is necessary to disentangle beam-induced changes and behavior from a samples’ inherent chemical or electrochemical behavior. We utilize a state-of-the-art machine vision software, AXON Dose, to calibrate and accurately track electron dose exposure throughout an experiment to create a record of both the electron flux, and the samples’ cumulative dose exposure, on a pixel-by-pixel basis [7]. Taken in concert, these new features bring us closer to achieving the goal of accurately replicating, measuring, and observing bulk electrochemical processes at the nanoscale.<br/><br/>References:<br/><br/>1. de Jonge, N.; Ross, F. M. <i>Nature Nanotech</i> <b>2011</b>, <i>6</i> (11), 695–704.<br/><br/>2. Yoshida, K. et al., <i>Microscopy</i> <b>2023</b>, dfad044.<br/><br/>3. D. B. Williams and C. B. Carter, “Transmission Electron Microscopy: A Textbook for Materials Science,” 2nd Edition, Springer, New York, 2009, pp. 3-22<br/><br/>4. Choudhary, S. et al., <i>J. Electrochem. Soc.</i> <b>2022</b>, <i>169</i> (111505).<br/><br/>5. Merkens, S. et al., <i>Ultramicroscopy</i> <b>2023</b>, <i>245</i>, 113654.<br/><br/>6. Merkens, S. et al., Towards sub-second Solution Exchange Dynamics in Liquid-Phase TEM Flow Reactors, 01 August 2023, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-3208774/v1]<br/><br/>7. Dukes, M. D. et al., <i>JoVE</i> <b>2023</b>, No. 196, 65446