The dynamic redox process of surface oxide layers on metal surfaces is of great significance for understanding the active phase in catalytic reactions. We studied the formation of surface oxide layers on Cu(111) and Cu(110) in O₂, as well as the subsequent reduction by CO using in situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). By monitoring and comparing the oxidation process of Cu(111) and Cu(110) surfaces, we found a crystal-plane-dependent reaction mechanism, which also applies to the reduction of surface oxide layers on Cu surfaces. We found XPS Cu spectra cannot be used to identify the various surface oxide layer on Cu surfaces, suggesting their presence in catalytic reactions might have been overlooked. The combination of STM and XPS studies are thus advantageous in identifying surface oxide structures and pinpointing the active phases in the redox process, which paves the way for engineering the catalyst and reaction environment for optimized catalytic performances.

Two-dimensional (2D) cuprous oxide (Cu₂O) nanostructures (NSs) of monolayer thickness were synthesized on Au(111) and characterized using atomic-resolution scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. The surface and edge structures of 2D Cu₂O were resolved at the atomic level and found to exhibit a graphene-like lattice structure. Cu₂O NSs grew preferentially at the face centered cubic (fcc) domains of Au(111). Depending on the annealing temperature, the shapes and structures of Cu₂O NSs were found to vary from elongated islands with a defective hexagonal lattice (mostly topological 5–7 defects) to triangular NSs with an almost-perfect hexagonal lattice. The edge structures of Cu₂O NSs also varied with the annealing temperature, from predominantly the arm-chair 56 structure at 400 K to almost exclusively the zig-zag structure at 600 K. DFT calculations suggested that the herringbone ridges of Au(111) confined the growth and structure of Cu₂O NSs on Au(111). As such, the arm-chair edges of Cu₂O NSs, which are less stable than the zig-zag edges, could be exposed preferentially at 400 K. Cu₂O NSs developed into the thermodynamically-favored triangular form and exposed zig-zag edges at 600 K, when the Au(111) substrate became mobile. The confined growth of 2D cuprous oxide on Au(111) demonstrated the importance of metal-oxide interactions in tuning the structures of supported 2D oxide NSs.