Adsorption of single Ag and Cu atoms on regular and defective MgO(0 0 1) substrates: an ab initio study

The DFT slab calculations were performed for Ag and Cu atoms adsorbed on both regular and defective MgO(0 0 1) substrates. Both metal atoms and surface O vacancies (F_s centers) were distributed uniformly with a concentration of one Ag, Cu or F_s per 2×2 surface supercell. Surface O²⁻ ions are energetically more preferable for metal-atom adsorption on a regular substrate as compared to Mg²⁺ ions. The nature of the interaction between Ag or Cu adatoms and a defectless MgO substrate is physisorption (despite the difference in the adsorption energies: 0.62 vs. 0.39 eV per Cu and Ag adatom, respectively). Above the F_s centers, metal atoms are bounded much stronger when compared with regular O²⁻ sites (2.4  vs. 2.1 eV per Cu and Ag adatoms, respectively). This is accompanied by a substantial charge transfer towards each adatom (Δq_Cu=0.41e and Δq_Ag=0.32e) as well as a formation of partly covalent Me-F_s bonds across the interface (Mulliken bond populations p_Cu-Fs=0.25e and p_Ag-Fs=0.33e).