Paper SS-ThM5
Are MgO Thin Films More Reactive Than Bulk MgO Surfaces?

Thursday, November 3, 2011, 9:20 am, Room 107

The ubiquity of the interface between water and oxide materials has prompted a tremendous activity to determine the adsorption mechanisms at the microscopic scale on crystalline surfaces of simple oxides. Among these, magnesium oxide MgO offers the advantage of having various morphologies of good crystalline quality, involving cleaved crystal surfaces, high surface area samples and supported films, all of these dominated by the low index (100) orientation. On bulk MgO, the fivefold coordinated atoms of the basal (100) surface do not dissociate isolated H₂O molecules. Conversely, H₂O is easily dissociated at low-coordinated sites such as steps and kinks. A puzzling case is the adsorption of H₂O on metal-supported MgO films in the submonolayer range of which coverage by OH groups has been estimated to 60 to 70% of a monolayer [1,2]. Similar OH coverages were obtained by aging freshly prepared films in the residual atmosphere of the vacuum chamber [1], although the observation was contradicted by the finding of a more modest effect that cast doubt on OH being the cause of aging [2]. The extraordinary uptake of OH groups was attributed to the peculiarities of the electronic properties of the thin supported MgO films [3]. However, density functional theory hardly supports this view. Little changes in both the electronic structure and the capacity to dissociate H₂O are predicted for monolayer-thick MgO(100) islands that, at variance with experiment, are only expected to dissociate H₂O molecules along their borders [4].

In an attempt to solve the discrepancies between experiments and between experiment and simulation, MgO films of different stoichiometry were grown on Ag(100) by reactive deposition of Mg in an O₂ partial pressure, prior to being exposed to H₂O vapor and/or aged in vacuum [5]. Films were observed by scanning tunneling microscopy (STM) and water uptake was analyzed by x-ray photoemission spectroscopy (XPS). The stoichiometry and, consequently, the chemical activity towards hydroxylation of the MgO(100) films, was shown to strongly depend on the O₂ pressure during the film growth. Oxygen-deficient films undergo dramatic oxygen uptake either by exposure to H₂O or by aging in vacuum. Conversely, on stoichiometric MgO islands, XPS analysis and STM images are consistent with the prediction that H₂O only dissociates at the island edges.

[1] S. Altieri et al., Phys. Rev. B. 76 (2007) 205413.

[2] L. Savio et al., J. Phys. Chem. B 108 (2004) 7771.

[3]S. Altieri et al., Thin Solid Films 400 (2001) 9.

[4] A. M. Ferrari et al., Phys. Chem. Chem. Phys. 9 (2007) 2350.