Large-scale computer simulations of metal/oxide interfaces with defects

Ab initio slab simulations have been performed for silver adhesion to the perfect and defective MgO(001) surfaces. For 1/4 Ag monolayer (ML) coverage of perfect substrate, we observe small silver adhesion energies over both O ^2- and Mg²⁺ ions on a regular MgO(001) substrate (0.23 and 0.22 eV per Ag atom, respectively), with negligible interfacial charge transfer towards metal atoms. For larger Ag coverages (beginning with 1/2 ML), silver adsorption over regular O^2- ions is much more favorable. We demonstrate that point surface defects on a magnesia surface increase markedly the metal adhesion energy and cause a redistribution of the electron density across the interface. The results for electron (F_s° = O vacancy with two trapped electrons) and hole (V_s° = Mg vacancy with two holes trapped by nearest O^2- ions) centers in the Ag atom adhesion at different surface coverages are analyzed. For Ag atoms positioned over the point surface defects, the substrate binding energies increase by more than an order of magnitude (to 7.6 and 12.7 eV, respectively) compared to a regular interface and are associated with marked charge transfer (∼1 e towards a Ag atom over a F_s center and -1.5 e towards the nearest O^2- ions from a Ag atom over a V_s center). A comparison of these results with silver adhesion on α-Al₂O₃(0001) surfaces shows two similarities in the nature of the adhesion for Ag adsorption: (i) on the perfect MgO(001) and Al-terminated (stoichiometric) corundum (0001) substrates we observe physisorption with a weak atomic polarization, whereas (ii) over a V_s center on the defective MgO and for O-terminated corundum, strong interfacial ionic bonding takes place.