O − radical ions on MgO as a tool to unravel structure and location of ionic vacancies at the surface of oxides: a coupled experimental and theoretical investigation

Abstract

The present paper analyzes the results of experiments consisting in the bleaching of color centers (F s +(H) or F s +(D)) at the surface of polycrystalline MgO after exposure to N 2O, and in their regeneration following admission of hydrogen in the dark (no UV irradiation). The reactions have been followed by EPR, using systematically computer simulation for the interpretation of the experimental results. After hydrogen exposure and irradiation, the spectra give evidence of the presence of trapped electrons close to a surface hydroxyl group, while after N 2O reaction with F centers the dominant species are a family of O − radical ions; in the latter case the spectra present well resolved features which permit the accurate determination of the g factors of the different radical species. Different sites are characterized by different g values of O − ions which are trapped there after reaction with N 2O. Up to three cycles (bleaching – regeneration of F centers in the dark) can be performed prior to final surface saturation. The alternation of hydrogen and deuterium admission has clarified that the F centers produced in the dark reaction are not the same as formed initially by UV irradiation. The evidence collected suggests that the MgO sample presents a family of “hot sites” where heterolytic dissociation of molecular hydrogen can take place, and a more abundant family of “cold sites”, where the former reaction is not possible, but atomic hydrogen can be ionized to generate an F center. An attempt to associate specific structural models with the various sites has been performed by means of quantum mechanical simulation of simple surface defects. The energy of several reactions (H 2 heterolysis, N 2O decomposition, H ionization, H 2 reaction with surface O −) has been calculated, and the g factor of O − surface ions in different environments has been estimated. This study seems to exclude that the isolated vacancy at the (0 0 1) face may act either as a hot or a cold site.