Direct Visualization and Quantitative Insights into the Formation and Phase Evolution of Cu Nanoparticles via In Situ Liquid Phase 4D‐STEM

Abstract Copper (Cu)‐based nanomaterials are one of the most efficient heterogeneous electrocatalysts for the CO₂ reduction reaction. However, their selectivity and stability are strongly determined by their morphology, crystal structure, composition, grain size, grain boundary density, etc. Hence, gaining quantitative insights into their dynamic evolution under synthesis and/or working conditions is critical for developing optimal Cu‐based electrocatalysts and unveiling their structure‐property relationship. In this work, the possibility of addressing these issues is demonstrated by integrating in situ liquid phase transmission electron microscopy (LP‐TEM) with 4D scanning transmission electron microscopy (4D‐STEM). Here, the dynamic morphology and phase evolution of Cu nanoparticles during electrodeposition and electrooxidation processes are revealed in liquid. Virtual imaging and selected area electron diffraction provide novel insights into the evolution of defective nanocrystalline Cu nanoparticles during electrodeposition. It is shown that virtual off‐axis dark field imaging can be used to map the distribution of Cu₂O and Cu within partially oxidized Cu nanoparticles, opening new opportunities for quantitatively probing electrocatalysts under operando conditions.