Hydroxyl groups on oxide surfaces: NiO(100), NiO(111) and Cr₂O₃(111)

Hydroxyl groups at the surfaces of NiO(100), NiO(111), and Cr₂O₃(111) have been studied using different surface sensitive spectroscopies. The OH groups are readily formed by the interaction of the oxide surfaces with the residual gas atmosphere or by dosing of water. They can be removed by annealing at temperatures T ≥ 600 K (NiO) or T ≥ 540 K (Cr₂O₃). OH does not bond to regular NiO(100) sites so that for a cleaved NiO(100) single crystal surface no OH adsorption could be observed. For the more defect containing NiO(100)/Ni(100) film the existence of OH could be verified by isotope exchange with OD. As indicated by TDS (thermal desorption spectroscopy) of an NO adsorbate, OH groups fully block the (111) oriented surface of NiO for NO adsorption which indicates that OH groups bond to regular NiO(111) surface sites. For Cr₂O₃(111) thermal decomposition of water at defect sites and photochemical dissociation is observed. The latter path seems to involve water molecules in the second layer and leads most likely to an occupation of regular surface sites.