Charging phenomena on doped wide-gap oxides: identifying the key parameters by DFT calculations.

The use of transition metals (TM) as dopants is a versatile way to change the properties of oxide materials. One example is the possibility to induce charge transfer from the TM to species adsorbed on the oxide surface. In the case of wide-gap alkaline-earth oxides, such as MgO and CaO, the charge transfer is determined by the relative position of the energy levels of the impurity and the adsorbate and resides in the capability of the TM to be stabilized in various oxidation states [1]. The structural and electronic properties of Cr- and Mo-doped MgO and CaO are investigated by Density Functional Theory (DFT) calculations, while the problem of band gap description and energy levels alignment in DFT is addressed by comparing the results obtained with standard GGA, GGA+U and hybrid exchange-correlation functionals. The ability of the impurity ions to transfer electrons to the adsorbed species is considered for the case of electronegative Au adatoms. The study of the thermodynamic stability of the TM ions shows that Mo impurities are stable in the oxide lattice as Mo3+ or Mo+4 but can assume also higher oxidation states and are therefore able to transfer electrons to adsorbed gold. Conversely, Cr impurities are stabilized as Cr3+ and the high cost of further oxidation results in the absence of charge transfer to gold on both Cr-doped MgO and Cr-doped CaO [2,3]. [1] Shao, X.; Prada, S.; Giordano, L.; Pacchioni, G.; Nilius, N.; Freund, H.-J. Angew. Chem. Int. Ed. 2011, 50, 11525-11527. [2] Stavale, F.; Shao, X.; Nilius,N.; Freund, H.-J.; Prada, S.; Giordano, L.; Pacchioni, G. J. Am. Chem. Soc. 2012, 134, 11380-11383. [3] Prada, S; Giordano, L.; Pacchioni, G. J. Phys. Chem. C 2013, 117, 9943-9951.