We investigated the room-temperature chemisorption of oxygen on Cu(100) and Cu(111) using ambient-pressure X-ray photoelectron spectroscopy (APXPS). A shoulder-to-shoulder comparison between the oxygen-gas titration on the two surfaces reveals that Cu(100) is the more active for oxygen dissociative chemisorption when the surfaces are clean. The (2 root 2 x root 2)R45 degrees missing-row reconstruction appears in Cu(100)'s LEED image after about 10(4) Langmuir of oxygen exposure, whereas on Cu(111), no long-range ordering was observed throughout the whole experiment. An oxide layer consisting of cuprous and cupric oxide shows up on Cu(111) at an oxygen exposure that is significantly lower than for Cu(100). This observation suggests that the presence of (2 root 2 x root 2)R45 degrees missing-row reconstruction layer slows down Cu(100) oxidation. Literature has widely reported that surface morphology influences the copper oxidation process. This study provides an XPS demonstration that copper surface oxide formation in O-2 at room temperature depends on the surface crystallographic orientation.
