Metal Monolayers on Command: Underpotential Deposition at Nanocrystal Surfaces: A Quantitative Operando Electrochemical Transmission Electron Microscopy Study

Underpotential deposition (UPD) enables the generation of metal monolayers whose electronic properties can be significantly different from the bulk. While studied extensively for bulk electrodes, UPD kinetics for individual nanoparticles can deviate from their bulk counterparts due to the distinct differences in the distribution of local electric fields. UPD’s nature remains largely unexplored due to the lack of nanoscale time-resolved operando/in situ techniques at interfaces. This study presents an investigation on a model system, the UPD of monolayer Cu at single-crystal Au nanocube surfaces, based on a correlative analysis of electrochemical results and high-resolution chemical mapping. Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) directly visualizes the potential-dependent and spatially resolved Cu electrodeposition kinetics, classified as planar, island-shaped, or dendritic growth. Operando EC-STEM reveals the well-defined diffusion-controlled processes during Cu electrodeposition in sub-μm thick liquid with both quantitative electrochemistry and quantitative imaging analysis. 4D-STEM shows that the Cu electrodeposition is guided by the crystallographic orientation of the Au substrate. This methodology of operando EC-STEM for quantitative electrochemistry can serve as a generalized platform to advance fundamental understanding of (electro)­chemical reaction dynamics.