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

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 O2 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 O2 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. ■ INTRODUCTION Metal−supported ultrathin oxide films have been widely studied both experimentally and theoretically in the field of heterogeneous catalysis due to their pivotal role in controlling charging mechanisms, adsorption properties, and catalytic activation of metal adatoms and molecules. The favored electron tunneling through the oxide film brought by the ultrathin limit opens new catalytic pathways for charge-transfer mechanisms, as demonstrated by the charging of Au atoms and the spontaneous activation of carbon monoxide and molecular oxygen upon adsorption on ultrathin MgO(001) films. The catalytic properties of metal−supported ultrathin oxide films are also strongly impacted by the presence of defects. For example, depositing metal adatoms on oxides is known to boost the reactivity via electron transfer to adsorbed molecules. It has been further shown that metal clusters deposited on defectrich MgO films were catalytically more active than the stoichiometric oxide. Theoretical calculations have provided strong support that this was related to neutral and singly charged oxygen vacancies at the surface of MgO, also known as the Fs and Fs + surface color centers. Therefore, ultrathin, colored oxide films can, in principle, allow us to reach optimal catalytic properties. However, the design of such advanced catalytic devices is not only facing the complexity of ultrathin limit and controlled defect generation but also requires a detailed knowledge of electronic structures at interfaces that can host a variety of entangled charge transfer mechanisms. Recent electron paramagnetic resonance (EPR) experiments have reported the ability of producing Fs + color centers on 20 ML-thick MgO(001) single-crystalline films by the deposition of small amounts of Mg at low temperatures. These centers have been attributed to unpaired electrons trapped at morphological defects of the MgO surface in contrast with color centers prepared by electron bombardment that originate in electron trapping within oxygen vacancies. We report spectroscopic evidence of the controlled creation of Mg-induced color centers on ultrathin MgO films grown on Ag(001). We further show that they serve as host for subsequent O2 activation by favoring superoxide O2 − anion formation. The associated charge-transfer mechanisms are identified by using X-ray and ultraviolet photoemission spectroscopy (XPS-UPS) for monitoring, layer-by-layer, the impact of Mg and O2 treatments on the electronic band profiles, that is, on band bending. ■ EXPERIMENTAL METHODS All experiments were performed in a multichamber ultrahigh vacuum (UHV) system with base pressures below 2 × 10−10 mbar. A molecular beam epitaxy chamber equipped with reflection high-energy electron diffraction (RHEED) is interconnected with XPS-UPS analysis chamber via a transfer chamber. The (001)-oriented Ag single crystal was cleaned by several cycles of Ar ion bombardment and annealing at 670− 720 K for 30 min. The cycles were repeated until a good RHEED pattern and a reproducible work function resulted. The final stable value of the work function of Ag(001) was φm = 4.38 ± 0.05 eV. The 3 monolayers (ML) thick MgO films (one monolayer is defined as one-half MgO lattice parameter, i.e., 1 ML = 2.105 Å) were grown on the prepared Ag(001) surface by evaporation of Mg in O2 background atmosphere 1 ht tp :// do c. re ro .c h Published in "The Journal of Physical Chemistry C 121(8): 4363–4367, 2017" which should be cited to refer to this work.