Adsorption of Water on Cu2O and Al2O3 Thin Films

The initial stages of water condensation, approximately 6 molecular layers, on two oxide surfaces, Cu 2 O and Al 2 O 3, have been investigated by using ambient pressure X-ray photoelectron spectroscopy at relative humidity values (RH) from 0 to >90%. Water adsorbs first dissociatively on oxygen vacancies producing adsorbed hydroxyl groups in a stoichiometric reaction: O lattic + vacancies + H 2 O = 2OH. The reaction is completed at ∼1% RH and is followed by adsorption of molecular water. The thickness of the water film grows with increasing RH. The first monolayer is completed at ∼15% RH on both oxides and is followed by a second layer at 35−40% RH. At 90% RH, about 6 layers of H 2 O film have been formed on Al 2 O 3 .