DECOMPOSITION OF NO2 ON METAL-SURFACES - OXIDATION OF AG, ZN, AND CU FILMS

The dissociation probability Of O2 on Cu(111), Ag(111), and Zn(001) su rfaces is quite low (< 10(-2)). The oxidation process using molecular oxygen is not practical for work under ultrahigh vacuum conditions. We have found that NO2 is a very good oxygen source for the oxidation of Ag, Zn, and Cu surfaces. At elevated temperatures (300-500 K), the di ssociation probability of NO2 on thick Ag, Zn, and Cu films supported on Ru(001) is close to one. The decomposition of NO2 produces a large amount of adsorbed oxygen and gaseous NO and N2. The thermal stability and electronic properties of the O/Ag, ZnO(x), and CuO(x) films were examined using temperature desorption spectroscopy, x-ray photoelectro n spectroscopy (XPS), and x-ray Auger electron spectroscopy. Large amo unts of oxygen can be adsorbed and dissolved in Ag films without oxida tion of die noble metal. The zinc oxide films displayed the typical O 1s XPS and Zn L3M4,5M4,5 Auger spectra of polycrystalline ZnO. In addi tion, the high-resolution electron energy-loss spectroscopy spectra of these films exhibited the characteristic phonon losses of bulk ZnO. I n the case of Cu films the NO2-oxidation leads to the formation of a m ixture of CuO and CU2O. The CuO species is relatively unstable and can be reduced to CU2O by annealing at 700 K (2 CuO-->Cu2O+1/2O2) or by d irect reaction with metallic Cu at 300 K (CuO+Cu-->Cu2O).