Solving the Long-Standing Cu(111) "8" Surface Oxide Puzzle

Due to the interesting properties of copper oxides, in particular for catalytic applications, ultrathin oxide layers formed by Cu oxidation have attracted sustained interest. Experimental observations of several surface phases, including the so-called "5-7", "8", "29" and "44" structures, have sparked ongoing debate. However, their precise atomic configurations remain incompletely resolved. Among these, the "8" phase is particularly challenging, as existing structural models rely on limited experimental evidence. Here, we combine surface X-ray diffraction, direct fitting of experimental data, and density functional theory (DFT) calculations, to determine both the composition and atomic structure of this phase. The resolved structure reveals a dense, interwoven network of four-membered Cu-O rings, comprising two distinct Cu cation species with markedly different structural and electronic characteristics. DFT results show that a substantial electron transfer from the Cu(111) substrate stabilizes its relatively oxygen-rich composition, Cu5O4. This "8" phase is energetically more stable than the most recent Cu8O4 model [Kim, H. J. et al. ACS Nano 2024, 18, 4559-4569] across the full range of oxygen chemical potential and is structurally and electronically distinct from the "29" and "44" surface oxides. Its bonding motifs and charge distribution resemble those of Pd5O4 and Ag5O4, formed on oxidized (111) surfaces of palladium and silver, but its formation requires less oxygen-rich conditions, reflecting the stronger affinity of Cu for oxygen.