Mechanistic Transformation of CuI Nanoparticles Into Oxidation‐Resistant 2D Copper Nanoplates

Abstract Unconventional phase transformations reveal new crystallization mechanisms, yet direct observation of such pathways during nanoscale solution‐phase synthesis remains challenging. This study uncovers an atypical growth process in which thermodynamically stable CuI nanoparticles (NPs) transform into high‐energy 2D Cu plates. Using a combination of in situ transmission electron microscopy, ex situ structural analysis, and density functional theory calculations shows that the formation of structural defects induced by hexadecylamine and chloride ions facilitates the transformation by promoting surface iodine vacancies. The resulting Cu{111} nanoplates, with ultrathin thicknesses (≈4 nm) and exceptionally high aspect ratios (≈450), display enhanced oxidation resistance and long‐term stability under ambient conditions. This resistance is attributed to the close‐packed {111} facets, which suppress chemical oxidation even after extended exposure to air over 100 days. These findings provide new insights into non‐classical crystallization pathways in metal nanomaterials and suggest a versatile approach for preparing oxidation‐resistant, structurally defined Cu nanostructures.