Noncovalent copper oxide framework on Cu(111) with open honeycomb structure

Understanding the oxidation process of copper is essential for controlling its surface properties and optimizing its performance in catalysis, electronics, and corrosion protection. Despite extensive studies on various covalent copper oxide films, the oxidation mechanisms and structural evolution of copper surfaces under low oxygen partial pressures remain insufficiently understood. In this work, we directly visualized the atomic structure of a noncovalent copper oxide film formed on Cu(111) under low oxygen content using qPlus-based noncontact atomic force microscopy. Combined with density functional theory calculations and atomic force microscopy simulations, we reveal that the copper oxide network consists of Cu₃O₂ building blocks, which self-assemble into an ordered open honeycomb (OHC) framework through noncovalent interactions. Force curve analysis confirms the presence of both upper-layer and lower-layer O atoms within Cu₃O₂ building blocks. The OHC framework exhibits properties similar to those of the covalent copper oxide film on Cu(111), including comparable electronic structures and identical structural phase transitions upon alkali metal (K) doping. This study not only provides atomic-level insights into copper oxidation under low oxygen environments but also enriches the phase diagram of surface oxides on Cu(111).