Zinc oxide exhibits a number of extraordinary properties and is one of the most technologically important metal oxides [1]. Presently, there is great interest in the doping of ZnO by foreign atoms due to its effects on changing the electrical, optical and catalytic properties of ZnO. In this contribution, the interaction of hydrogen and Copper atoms with ZnO(000-1) was studied by high resolution electron energy loss spectroscopy (HREELS) which recently has been successfully applied to perfect and defective metal oxide surfaces [2].

 

Exposing the fully hydroxylated ZnO(000-1) surface to atomic hydrogen leads to a significant broadening of the quasi-elastic peak in HREELS, which corresponds to the existence of free charge carriers at ZnO surface region [3]. These free charge carriers result from the thermal excitation of electrons at the donor level into the conduction band. The shallow donor states are created via diffusion of H atoms into the bulk. The analysis of the temperature dependence yields a shallow donor ionization energy of 25 ± 5 meV [4].

 

Cu deposition on O-ZnO leads to the formation of well-defined islands with the Cu(111) facets. For the small clusters the partial oxidation of Cu0 into Cu+ was clearly identified by the characteristic C-O stretching frequencies. Upon heating the Cu atoms undergo, instead of desorption, diffusion into the bulk. The doping of ZnO by Cu leads also to the formation of shallow donor states, in which the electrons can be thermally excited into the conduction band and, as a result, giving rise to the plasmon-induced broadening of the quasielastic peak in HREELS. From the observed temperature dependence, the donor level ionization energy was determined. This unexpected doping effect of ZnO by Cu has important consequences for its chemical activity, as confirmed by the detailed studies on CO2 adsorption.

 

[1] Ch. Wöll, Prog. Surf. Sci. 82 (2007) 55.

[2] Y. Wang, Z. Phys. Chem. 222 (2008) 927.

[3] Y. Wang, B. Meyer, X. Yin, M. Kunat, D. Langenberg, F. Traeger, A. Birkner, Ch. Wöll, Phys. Rev. Lett. 95 (2005) 266104.

[4] H. Qiu, B. Meyer, Y. Wang, Ch. Wöll, Phys. Rev. Lett. 101 (2008) 236401.