Visualizing Metal Nanoparticle Electrochemical Dissolution Atom by Atom

Gaining insight into the early stages of the electrochemical dissolution of metal nanoparticles (NPs) provides crucial insights into mechanisms that control this important process. Being able to do this under conditions where atom loss from individual NPs can be quantified is especially challenging. Here, we use identical-location, annular dark field, scanning transmission electron microscopy (IL-ADF-STEM) to provide "time-stamped" snapshots of the dissolution of gold NPs on electron-transparent carbon electrodes. Dissolution is carried out in aqueous chloride (mM) solutions, at anodic potentials, over millisecond timescales. IL-ADF-STEM analysis of the integrated image intensities is employed to estimate the number of atoms within each NP, allowing atom loss (and gain) to be tracked for the same NP, over time, on a particle-by-particle basis. 3D reconstruction of NPs enables changes in 3D morphology to be visualized. Hemispherical-shaped gold NPs ≤4 nm in diameter are interrogated, with the smallest showing the largest atom loss. NPs are revealed to flatten during dissolution, as opposed to a gradual reduction in diameter, and the number of isolated gold atoms on the surface increases. Considerable interactions between NPs also occur, including the formation of single atom bridges and coalescence events. The vertical growth of NPs is also observed.