Band Bending in Mg-Colored and O₂-Activated Ultrathin MgO(001) Films

Abstract

Ultrathin MgO films grown on Ag(001) have been investigated using X-ray and ultraviolet photoemission spectroscopies for oxide films successively exposed to Mg and O 2 flux. Studying work functions and layer-resolved Auger shifts allows us to keep track of band profiles from the oxide surface to the interface and reveal the charge-transfer mechanisms underlying the controlled creation of Mg-induced surface color centers and the catalytic enhancement of O 2 activation. Our results demonstrate that one can intimately probe the catalytic properties of metal-supported ultrathin oxide films by studying the electronic band alignment at interfaces.

CO adsorption on MgO thin-films: formation and interaction of surface charged defects

“…32 Furthermore, the thin-film growth proceeds by successively exposing the growing film to different background atmospheres and this can affect the formation energies of charged vacancies at MgO thin-film surfaces. 17 In addition, as the Ag support affects charging at the thin-film surface, the stability of V centers can also depend on the presence of a MgO/Ag interface. In order to investigate these effects, we computed the formation energies of the neutral and charged Mg and O vacancies, 3MgO(001) and 3MgO(001)/4Ag(001) surfaces in the Mg-rich and O-rich thermodynamic limits.…”

“…SI For the slab calculations, the support structure is composed by 4 ML Ag and 3 ML MgO, where 1 ML is defined as one-half of the Ag (1 ML = 2.08 Å) or MgO (1 ML = 2.12 Å) lattice constants (Table SI 1, ESI †). We chose 3 ML MgO because it corresponds to the well-established experimental thickness of MgO thin films in the MgO/Ag system, [14][15][16][17] hence it is appropriate for evaluating the effects of charged vacancies in this system. 32 The 3MgO/4Ag interface displays a non-polar (001) structure, which corresponds to the most stable surface in a cubic rock-salt structure such as MgO and it displays the preferred on-top arrangement between the O anions and Ag atoms.…”

“…In the case of vacancy formation, previous studies focused only on the stability of charged F centers. [13][14][15][16][17] However, other types of vacancies can be present at the MgO(001) surface, such as Mg vacancies and divacancies, formed by removing a MgO unit with trapped electrons. 32 Furthermore, the thin-film growth proceeds by successively exposing the growing film to different background atmospheres and this can affect the formation energies of charged vacancies at MgO thin-film surfaces.…”

“…3 Combined effects of structural relaxation, electrostatic compression (Pauli's push-back effect) and charge transfer are fundamental properties of band bending effects in an oxide thin-film, with the Fermi level pinning and band-offset shifts in the interface. [11][12][13][14][15][16][17] Consequently, the surface charge density distribution and the dielectric material modify the surface dipole and the work-function of the metal supported oxide material.…”

“…40 Unlike the case of unsupported MgO surfaces, 19,27,[41][42][43][44][45][46] an MgO/Ag thin-film favors the formation of surface charged defects thanks to the charge transfer processes between MgO and the Ag support. 17,19,27 Accordingly, the experimentally measured shift of the work function may be substantially affected by the position, charge state and density of defects in the MgO thin-film, thereby leading to strong local potential shifts associated with electron acceptor and donor centers. 13,32 From the simulation of defects and trapped electrons at the MgO surface, 20,37,[47][48][49][50] the knowledge of the electronic structure of charged intrinsic vacancies at thin-films may gather further insights on interfaces and 2D materials with catalytic potential.…”