The influence of defects on the reactivity of oxides towards water is a field of a long lasting interest [1]. In dry environments, the MgO(100) cleavage surface is the most stable one, while other orientations can be stabilized by water adsorption. In any case, freshly cleaved MgO shows several defects such as steps, kinks, and vacancies. Although many investigations focused on water adsorption on defective oxide surfaces, very few combine experiments with numerical simulations. Theory indicates spontaneous water dissociation at steps, but the effect of water coverage was seldom investigated [2].
Here we study the interaction of water with point [3,4] and extended defects [5] on MgO(100), in comparison with more open faces, through first-principles simulations. A large variety of configurations is considered, in order to assess the effects due to water coverage and surface conformation on the adsorption enthalpy, the structural properties, and the vibrations of the hydroxyl groups. The surface Madelung potential, the formation of H bonds and the local interactions between adsorbates produce effects of different character and variable magnitude. We discuss the acid-base character of the OH groups, and compare their thermal stability against temperature or pressure induced desorption with available data. The computed stretching frequencies and mode intensities are discussed in relation to recent IR absorption spectra obtained on MgO smokes exposed to water [6]. Finally, the equilibrium shapes of MgO crystallites, either in dry or humid environments, are determined and compared to the morphology of MgO powders as observed by TEM [7].
In conclusion, we 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.
[1] M Henderson, Surf. Sci. Rep. 46 (2002) 1
[2] L Giordano, et al, PRL 81 (1998) 1271
[3] F Finocchi, J Goniakowski, PRB 64 (2001) 125426
[4] B Ealet et al, PRB 69 (2004) 054419
[5] D Costa et al, J. Chem. Phys. 125 (2006) 054702
[6] F Finocchi et al, submitted
[7] R Hacquart et al, Surf. Sci. 595 (2005) 172. |