The interaction of O2 with Cu/Ni(100) and Cu/NiO/Ni(100) surfaces studied by XPS

The interaction of O2 with Cu/Ni (100) and Cu/NiO/Ni (100) surfaces has been studied by X-ray photoelectron Spectroscopy (XPS) in an attempt to observe the variation of the chemical states of metal atoms at the topmost layer. By applying the 0.3 ML and 1.1 ML Cu/Ni (100) surfaces, the oxygen chemisorbed Cu was found to show chemical shifts in the binding energy of the Cu2p32 peak by 0.2 eV and in the kinetic energy of the Cu L3M4,5 Auger peak by −0.6 eV even at the Cu/O atomic ratio of 2 or less. The shift of the Cu LMM was much less than that for bulk Cu2O, −1.8 eV, indicating a less positive charge for the Cu atoms than for bulk Cu2O. At high O2 exposure the Cu atoms persisted to be in the low charge state contacting with metallic Ni underneath but were covered by NiO layers which are formed from Ni atoms segregated from the subsurface region to the topmost layer. The Cu atoms (<0.75 ML) deposited on 3 NiO layers previously formed on a Ni (100) surface seem to sit on the hollow sites surrounded by oxygen atoms. At coverage less than 0.4 ML, Cu atoms are mostly oxidized to Cu2O-like species showing shifts of 0.4 eV and −1.8 eV for the Cu2p32 peak and the Cu LMM peak, respectively. These Cu atoms were easily reduced to the oxygen chemisorbed state by heating at 423 K or oxidized to Cu2+ by exposure to O2. The latter species was verified by the Cu2p32 peak shifted by 1.5 eV and the appearance of the satellite peaks at higher binding energy side.