Character of Cu and O in partially oxidized "29" CuxO/Cu(111) from Auger Photoelectron Coincidence Spectroscopy.

The controversial formal oxidation state of partially oxidized copper has been determined by the enhanced surface and chemical selectivity of Cu-L3VV, Cu-M3VV, and O-KVV Auger photoelectron coincidence spectroscopy (APECS) and first-principles calculations. We compare metallic Cu(111) of formal oxidation state (0), Cu2O cuprous Cu(I) oxide, and CuO cupric Cu(II) oxide to partially oxidized "29" CuxO/Cu(111). The oxidation state of Cu within the top three layers of "29" CuxO/Cu(111) is found as Cu(0.3) close to the formal Cu(0) oxidation state of metallic Cu(111). A distribution of oxygen chemical species is observed in agreement with the proposed three types of oxygen bonding. This clarifies essential steps for copper-based materials and chemistry by the enhanced selectivity of APECS made applicable to materials.