Elucidating the Role of Halides and Iron during Radiolysis-Driven Oxidative Etching of Gold Nanocrystals Using Liquid Cell Transmission Electron Microscopy and Pulse Radiolysis

Graphene liquid cell transmission electron microscopy (TEM) has enabled the observation of a variety of nanoscale transformations. Yet understanding the chemistry of the liquid cell solution and its impact on the observed transformations remains an important step toward translating insights from liquid cell TEM to benchtop chemistry. Gold nanocrystal etching can be used as a model system to probe the reactivity of the solution. FeCl₃ has been widely used to promote gold oxidation in bulk and liquid cell TEM studies, but the roles of the halide and iron species have not been fully elucidated. In this work, we observed the etching trajectories of gold nanocrystals in different iron halide solutions. We observed an increase in gold nanocrystal etch rate going from Cl^-- to Br^-- to I^--containing solutions. This is consistent with a mechanism in which the dominant role of halides is as complexation agents for oxidized gold species. Additionally, the mechanism through which FeCl₃ induces etching in liquid cell TEM remains unclear. Ground-state bleaching of the Fe(III) absorption band observed through pulse radiolysis indicates that iron may react with Cl₂·^- radicals to form an oxidized transient species under irradiation. Complete active space self-consistent field (CASSCF) calculations indicate that the FeCl₃ complex is oxidized to an Fe species with an OH radical ligand. Together our data indicate that an oxidized Fe species may be the active oxidant, while halides modulate the etch rate by tuning the reduction potential of gold nanocrystals.