Thickness-Dependent Hydroxylation of MgO(001) Thin Films

Hydroxylation of MgO surfaces has been studied from UHV to mbar pressure for MgO(001) films of different thickness grown on Ag(001) by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and density functional theory calculations. In agreement with earlier studies on MgO(001) single crystals, a threshold water pressure on the order of 10(-4) mbar is found for extensive hydroxylation of thick, bulklike MgO films. Decreasing the MgO film thickness shifts the threshold pressure to lower values, being 10(-6) mbar in the limit of 2 monolayer MgO(001)/Ag(001). This result is explained on the basis of the precursor state of periclase MgO(001) dissolution involving hydrolysis of Mg-O surface bonds. The enhanced structural flexibility (polaronic distortion) of the ultrathin MgO film facilitates surface hydroxylation by lowering the barrier for hydrolysis.