Water chemisorption on a sputter deposited uranium dioxide film — Effect of defects

• The DRS technique “sees” surface hydrogen and the surface geometry of OH groups.
• Dissociation + adsorption routes of water molecules
• Sputter-defected vs. stoichiometric-annealed UO2 surfaces
• A modified water chemisorptions model is proposed.

The characteristics of water vapor chemisorptions on stoichiometric and on sputtered reduced UO2 thin film surfaces, obtained by the reactive sputter deposition technique, were studied by utilizing direct recoil spectrometry and X-ray photoelectron spectroscopy over a temperature range of 300–500 K. It seems that dissociative chemisorption takes place on both types of surfaces but surface coverage and adsorption rates are different. For the stoichiometric surface, DRS results indicate that water partially dissociate on this surface up to 350 K, probably on sparse defect sites. On the other hand, the kinetics of water dissociation fragments (H, O, OH) on the defected surface displayed a complex behavior, caused by the healing effect of surface defects, being temperature dependent. A modified water chemisorption model is proposed, assuming dense clusters of hydroxyls with a mix of perpendicular and tilted OH bonds formed on the UO2 − x sputtered surface. Perpendicular isolated hydroxyls are assumed to be predominant on UO2 stoichiometric surface.