Cryogenic Atom Probe Tomograph to Resolve Confined Electrochemical Interfaces

Atom Probe Tomography (APT) is an analysis technique capable of providing a 3-dimensional, element-sensitive map of needle-shaped samples (where the tip radius is about 50 nm) with sub-nanometre resolution [1]. APT analysis is carried out at temperatures in the range of 25-75 K to limit the thermal motion of surface atoms which improves the spatial and mass resolution. As a result, this enables the analysis of frozen liquids, and by extension solid-liquid interfaces, by so-called “cryo-APT”. While this was historically challenging, recent advancements in UHV-cryogenic transfer and sample preparation methods now make it possible for the atomic scale analysis of interfacial species in electrochemical systems [2-4]. In this study, we will attempt to explore the effect of nanoscale geometrical confinement on the distribution of interfacial ions for the first time and to resolve the electric double layer, using cryo-APT. We will describe the workflow for cryo-APT sample preparation to study confined solid-liquid interfaces, using nanoporous gold as a template material with a frozen sodium iodide solution inside its pores. APT reconstructions will allow 3-dimensional mapping of aqueous interfacial species, helping to develop our understanding of the structure of the electric double layer under nanoscale geometrical confinement. The attached Figure shows an SEM image of the later stages of cryo-FIB milling of an APT tip of frozen deionised water and 50 mM sodium iodide and a segment of the related APT atom map reconstruction. References [1] Erwin W. Müller, John A. Panitz, and S. Brooks McLane. “The Atom-Probe Field Ion Mi- croscope”. In: Review of Scientific Instruments 39.1 (1968), pp. 83–86. issn: 0034-6748. [2] A. A. El-Zoka et al. “Enabling near-atomic–scale analysis of frozen water”. In: Science Advances 6.49 (Dec. 4, 2020). Publisher: American Association for the Advancement of Science, eabd6324. [3] T. M. Schwarz et al. “Field evaporation and atom probe tomography of pure water tips”. In: Scientific Reports 10.1 (Nov. 20, 2020). Number: 1 Publisher: Nature Publishing Group, p. 20271. issn: 2045-2322. [4] Stephenson LT, Szczepaniak A, Mouton I, Rusitzka KAK, Breen AJ, Tezins U, et al. (2018) The Laplace Project: An integrated suite for preparing and transferring atom probe samples under cryogenic and UHV conditions. PLoS ONE 13(12): e0209211. Figure 1