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Ni Substitutional Defects in Bulk and at the (001) Surface of MgO from First-Principles Calculations

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journal contribution
posted on 04.11.2016, 00:00 by Aliaksei Mazheika, Sergey V. Levchenko
Electronic and structural properties of nickel substitutional defects in the bulk and at (001) terraces, steps, and corners of magnesium oxide, as well as adsorption of CO, CO2, CH4, and H2 on them, are studied using hybrid density-functional theory, coupled-cluster model with single, double, and perturbative triple substitutions [CCSD­(T)], and perturbative GW approximation. The amount of exact exchange (α) in the HSE­(α) hybrid functional is validated against the higher-level methods. We find that with α = 0.3, formation energies of NiMg defects and adsorption energies of CO, CO2, and H2 calculated with HSE are close to ones obtained with CCSD­(T), whereas for ionization of NiMg α = 0.44–0.5 is needed to reproduce G0W0@HSE­(α) ionization energy. The dependence of the adsorption energies on α is found to be weaker than the dependence of formation and ionization energies: changing α from 0.25 to 0.44 results in variation of adsorption energies on the order of up to 0.2 eV. HSE calculations with the optimal α revealed that NiMg is most stable at corner sites on MgO(001), followed by the subsurface, bulk, and step sites, which are very similar in energy (within 0.03 eV), and then the terrace sites (0.45 eV less stable than at corner sites). In the bulk, NiMg defects serve as traps for both holes and electrons. The presence of these defects at the MgO surface is found to have a weak effect on energies of CH4 physisorption and CO2 chemisorption, whereas it favors chemisorption of CO and dissociation of H2 and methane.

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