Initial oxidation of Zn(0001) surfaces: H2 adsorption and XPS studies

Abstract

H2 adsorption capacity measurements have been combined with XPS intensity measurements to study the kinetics of O2 uptake on Zn(0001) surfaces. On clean Zn(0001) the temperature programmed desorption spectrum of H2 shows half-order desorption kinetics and an apparent desorption energy of 11 kcal/mole. As the surface is oxidized by progressive dosing with O2, two features are noted: The H2 desorption spectrum characteristic of clean Zn(0001) is attenuated in a nonlinear fashion and a new H2 desorption state with an activation energy of 18 kcal/mole is observed for surfaces with less than saturation oxygen coverage. XPS results indicate that H2 adsorption is not completely blocked until an oxygen coverage corresponding to approximately 3 monolayers is achieved. Both H2 uptake and XPS intensity measurements also indicate that the oxygen sticking coefficient increases with oxygen coverage up to about one half of the O2 saturation coverage. These results indicate that the initial oxidation of Zn surfaces involves the formation and growth of multilayer oxide islands, where oxygen incorporation at the perimeter of the islands is the rate limiting step.