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Abstract: STM, STS, LEED and XPS data for crystalline θ-Al2O3 and non-crystalline Al2O3 ultra-thin films grown on NiAl(001) at 1025K and exposed to water vapour at low pressure (1×10−7–1×10−5 mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O1s signal at 532.5–532.8eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al3+ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlO x OH y with x + y ∼2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H2O)=1×10−7 mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025K when the surface of the oxide film has been modified, but not when the bulk structure has been modified. [Copyright &y& Elsevier]

Abstract: The stabilization mechanism of the polar, copper terminated Cu2O(001) surface by means of complex surface reconstruction was studied theoretically with a combination of static and molecular dynamics calculations. The experimentally reported “3√2×1” surface structure was constructed and characterized for the first time. The combination of simulated annealing with molecular dynamics shows that Cu+–Cu+ dimers are formed in the first layer along the equivalent [011] and [01−1] directions at elevated temperature. There is a relaxation of the atoms that separates copper cations from nearest neighbor rows. Using the experimentally observed superstructure cell allows decoupling the symmetry equivalent dimers. The structural reconstructions were characterized by the electronic properties calculations. It is observed that the dimers are formed due to the d–d interaction of the copper atoms. Finally, the symmetry driven reconstructed structure was investigated by DFT STM. The simulated STM images show that copper atoms have higher density than oxygen atoms at the surface and produce the positive surface corrugation. [Copyright &y& Elsevier]

Abstract: The reactivity of the (0001)-Cr–Cr2O3 surface towards water was studied by means of periodic DFT+ U. Several water coverages were studied, from 1.2H2O/nm2 to 14.1H2O/nm2, corresponding to ¼, 1, 2 and 3 water/Cr at the (0001)-Cr2O3 surface, respectively. With increasing coverage, water gradually completes the coordination sphere of the surface Cr atoms from 3 (dry surface) to 4 (1.2 and 4.7H2O/nm2), 5 (9.4H2O/nm2) and 6 (14.1H2O/nm2). For all studied coverages, water replaces an O atom from the missing above plane. At coverages 1.2 and 4.7H2O/nm2, the Cr–Os (surface oxygen) acid–base character and bond directionality govern the water adsorption. The adsorption is molecular at the lowest coverage. At 4.7H2O/nm2, molecular and dissociative states are isoenergetic. The activation energy barrier between the two states being as low as 12kJ/mol, allowing protons exchanges between the OH groups, as evidenced by ab inito molecular dynamics at room temperature. At coverages of 9.4 and 14.1H2O/nm2, 1D- (respectively, 2D-) water networks are formed. The resulting surface terminations are –Cr(OH)2 and –Cr(OH)3– like, respectively. The increased stability of those terminations as compared to the previous ones are due to the stabilization of the adsorbed phase through a H-bond network and to the increase in the Cr coordination number, stabilizing the Cr (t 2 g ) orbitals in the valence band. An atomistic thermodynamic approach allows us to specify the temperature and water pressure domains of prevalence for each surface termination. It is found that the –Cr(OH)3-like, –Cr(OH)2 and anhydrous surfaces may be stabilized depending on (T, P) conditions. Calculated energies of adsorption and OH frequencies are in good agreement with published experimental data and support the full hydroxylation model, where the Cr achieves a 6-fold coordination, at saturation. [Copyright &y& Elsevier]

Abstract: The stabilization of the unstable, polar copper terminated Cu2O(111) surface by reconstruction and hydroxylation was studied theoretically with static and molecular dynamics calculations at ab initio density functional theory (DFT) level. Surface reconstruction was investigated using extensive finite temperature molecular dynamics (MD) combined with a simulated annealing technique. Both the global minimum energy structure obtained during annealing the system at higher temperature (300K) and the final ‘quenched’ structure which was obtained after cooling the system to 0K show the expected reconstruction of the adsorbate-free surface. The copper atoms in the first layer and oxygen atoms in the second and third layers are markedly displaced, and the atomic planes merge together to form a uniform mixed layer, thereby minimizing the polarity of the surface. Surface hydroxylation by adsorption of OH− or dissociated water was investigated using static optimization at 0K. The results show that adsorption is exothermic and that the reconstruction characterizing the annealed OH-free surface does not occur in the presence of adsorbed OH. A surface coverage of 50% results in the surface structure that is the closest to the unrelaxed bulk terminated surface. [Copyright &y& Elsevier]