Thickness-Dependent Hydroxylation of MgO(001) Thin Films Esther Carrasco, † Matthew A. Brown, † Martin Sterrer,* ,† Hans-Joachim Freund, † Karolina Kwapien, ‡ Marek Sierka,* ,‡ and Joachim Sauer ‡ Department of Chemical Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany, and Department of Chemistry, Humboldt-UniVersita ¨t zu Berlin, Unter den Linden 6, 10099 Berlin, Germany ReceiVed: June 9, 2010; ReVised Manuscript ReceiVed: September 15, 2010 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. 1. Introduction Metals that come in contact with the environment are commonly covered by oxide overlayers. According to the early work of Mott and Cabrera, activation of adsorbed oxygen by electrons represents a precursor for oxidation, which proceeds as long as tunneling of electrons through the growing oxide layer is possible. 1 Recently, oxide overlayers on metals have received renewed interest. On the one hand, submonolayer oxide islands on metal supports may be regarded as inverse catalysts, allowing the investigation of interactions at metal/oxide bound- aries. On the other hand, adsorption of suitable atoms or molecules on oxide layers of a few monolayers (ML) thickness was shown both theoretically and experimentally to be sub- stantially modified as compared to the surface of the corre- sponding bulk oxides. 2-8 The differences have been attributed to (i) lowering of the work function of the metal by the oxide overlayer, (ii) charge transfer, which can take place both into the adsorbate and into the substrate depending on the properties of the adsorbate, and (iii) polaronic distortion in the oxide layer to stabilize the resulting charged adsorbate. Thin layers of MgO supported by Ag(001) are well suited to study these effects due to the perfect epitaxial relationship between MgO(001) and Ag(001). Within this work, we report the MgO thickness dependence of water adsorption on the surface of MgO(001) thin films studied from UHV to mbar pressure. Water/oxide interaction is of paramount importance in fields as diverse as geochemistry, atmospheric chemistry, biology, catalysis, corrosion, materials science, and interstellar chemistry. These different aspects of water/oxide interaction have been summarized in detail in a number of review articles. 9-12 MgO(001), as the simplest oxide in terms of geometric and electronic structure, is among the oxides that have been most extensively studied with respect to water adsorption and dissociation, both experimentally and theoretically. There is now general consensus that H 2 O adsorbs molecularly at low coverage. 13,14 Spontaneous dissociation of water molecules is energetically not favored on the perfect MgO(001) surface and occurs only at defect sites, such as low-coordinated cation-anion pairs as present, for example, on steps and corners. 13,15-17 In the monolayer regime, however, an ordered overlayer with both molecular and dissociated water molecules in the unit cell is present. 18-20 Less clear is the mechanism of extensive hydroxylation and dissolution of the MgO(001) surface. Studies on MgO(001) single crystals revealed that a certain threshold pressure of water is necessary to transform the MgO surface from a state of low, defect-mediated hydroxylation, to a fully hydroxyl covered surface. 21 The interpretation of the threshold behavior was based on purely thermodynamic grounds. The fully hydroxylated surface may be regarded as the precursor for dissolution. There is ample theoretical and experimental evidence that the MgO(001) surface is not stable in equilibrium with water and transforms into MgO(111). 22,23 This transformation requires concerted mass transfer and is kinetically controlled. 24,25 AFM investigations provide evidence that the process of dissolution starts at defects such as steps, whereas the regular surface is rather unreactive. 26 Enhanced dissociation of water has been reported for sub- monolayer MgO islands deposited on Ag(001). 27,28 Even under typical ultrahigh vacuum conditions with low water background, the boundary ions between the MgO islands and the metallic substrate are easily hydroxylated. 29 This effect fades away with increasing MgO film thickness and has been attributed to the small size of the islands rather than to electronic effects caused by the metallic substrate. 30 Recently, the adsorption and dis- sociation of water on a 2 ML MgO film supported by Ag(001) has also been studied computationally. 31 It was found that dissociation of H 2 O is not favored on the regular surface of a supported thin film; however, it requires less energy than on bulk MgO(001). Although the thermodynamics of water adsorp- tion is not changed dramatically if MgO is supported, charge transfer between substrate and adsorbate seems to take place and changes the bonding character. 31 Herein, we present results of experiments performed for water adsorption and dissociation on Ag(001)-supported MgO(001) films * To whom correspondence should be addressed. E-mail: sterrer@ fhi-berlin.mpg.de (Sterrer), marek.sierka@chemie.hu-berlin.de (Sierka). † Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡ Humboldt-Universita ¨t zu Berlin. J. Phys. Chem. C 2010, 114, 18207–18214 18207 10.1021/jp105294e  2010 American Chemical Society Published on Web 10/01/2010