Abstract
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.
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