The influence of defects on the reactivity of oxides towards water is a main concern in many fields, such as geo-chemistry, catalysis and chemical physics [1]. Although many experimental and theoretical investigations focused on water adsorption on defective oxide surfaces, very few ones combined measurements with numerical simulations. As a prototypical case, I will focus on magnesium oxide, which is highly hygroscopic, and discuss water dissociation at surface point defects [2, 3] and steps [4], through DFT-based simulations at low water coverage. Many configurations have been considered, in order to assess how water coverage and surface conformation affect adsorption enthalpy, structural properties and vibrations of hydroxy groups. We found that the surface Madelung potential, formation of hydrogen bonds and local electrostatic interactions between co-adsorbed hydroxy groups or water ad-molecules play an important role. We analyze the acid-base character of the OH groups and discuss the computed frequencies and mode intensities in relation to infrared (IR) absorption spectra that have been obtained on MgO smokes exposed to water [5].
At higher water coverage, we have determined equilibrium shapes of MgO crystallites, either in dry or humid environments [6] and compared them to the morphology of MgO powders as observed by transmission electron microscopy (TEM) [7]. The role of steps on the transformation from cubic to octahedral crystallites has also been investigated, as well as the effect of water on thermodynamic stability of ledges [8]. Moreover, I will discuss the outcome of a recent work where the role of the surface electric field on the structure of water layers at the surface has been analyzed in detail [9].
In conclusion, I will show that the issue of water adsorption on real, defective MgO surfaces can be resolved and the spectroscopic signatures of OH groups adsorbed on specific sites on MgO faces interpreted, by combining accurate IR measurements and TEM with systematic ab initio simulations. Computed equilibrium (Wullf) shape of MgO crystallites in humid conditions (PH2O = 1 atm) as a function of temperature.
References :
- [1] M. A. Henderson, “The interaction of water with solid surfaces: fundamental aspects revisited”, Surf. Sci. Reports 46, 1 (2002).
- [2] F. Finocchi and J. Goniakowski, “Interaction of a water molecule with the oxygen vacancy on MgO(100) surfaces”, Phys. Rev. B 64, 125426 (2001).
- [3] B. Ealet, J. Goniakowski, and F. Finocchi, “Water dissociation on a defective MgO(100) surface: Role of divacancies ”, Phys. Rev. B 69, 054419 (2004).
- [4] D. Costa, C. Chizallet, B. Ealet, J. Goniakowski, and F. Finocchi, “Water on extended and point defects at MgO surfaces ”, J. Chem. Phys. 125, 054702 (2006).
- [5] F. Finocchi, R. Hacquart, C. Naud, and J. Jupille, “Hydroxyl-defect complexes on hydrated MgO smokes”, J. Phys. Chem. C 34, 13226 (2008).
- [6] F. Finocchi and J. Goniakowski, “The effects of exchange and correlation on the computed equilibrium shapes of wet MgO crystallites”, Surf. Sci. 601, 4144 (2007).
- [7] P. Geysermans, F. Finocchi, J. Goniakowski and J. Jupille, ”Combination of (100), (110) and (111) facets in MgO crystals shapes from dry to wet environments”, Chem. Phys. Phys. Chem. 11, 2228 (2009).
- [8] F. Finocchi, P. Geysermans and A. Bourgeois, ”The effect of hydroxylation on the step stability and step-step interaction: a first-principles study of vicinal MgO surfaces”, Chem. Phys. Phys. Chem. 14, 13692 (2012).
- [9] S. Laporte, F. Finocchi, L. Paulatto et al, ”Strong electric fields at a prototypical oxide/water interface probed by ab initio molecular dynamics: MgO(001)”, Chem. Phys. Phys. Chem. 17, 20382 (2015).
Institut des Nano-Sciences de Paris (INSP), Paris 6