Characterization of O−-Centers on Single Crystalline MgO(001)-Films

Abstract

Defects play an important role for understanding the properties of oxide surfaces. However, a detailed atomistic characterization of the properties of defects in particular of point defects is still a challenging task. On polycrystalline material it is the large variety of different species in terms of local environment, electronic properties as well as the metastable nature of most of these species, which complicates matters. EPR spectroscopy has proven to be a versatile tool to characterize electronic properties as well as local environment of paramagnetic point defects on oxide surfaces. In this study we elucidate the properties of O --centers on MgO surfaces under ultrahigh vacuum (UHV) conditions using a single-crystalline MgO(001) surface as a well-defined model system. The O --centers were produced by reaction of N 2 O with previously prepared F ? -centers, which were shown to be located at step edges of the MgO islands in a previous study. The experimental efforts are combined with ab-inito quantum chemistry calculations to gain a more detailed understanding of the electronic properties of the defects under consideration. The experimental and theoretical values of the g-tensor components are almost in quantitative agreement. In addition to the discussion of the properties of O --centers the paper will shed some light on the impact that doping of the surface in this case with Mo(V)-species present on the pristine surface has. In particular, we are able to provide evidence for the fact that there is redox chemistry between the O --centers and Mo-centers. The crosstalk between different redox active sites on the surface is an important phenomenon that is not limited to model systems as discussed here, but should also be taken into consideration if discussing the properties of high performance catalysts.

Single-Atom Catalysis: Insights from Model Systems

“… The underlying metal may contribute even more strongly if cations diffuse into the film and act as dopants. For example, MgO films grown on Mo(100) have been shown to contain Mo­(V) centers, which appear to be situated at the surface. , It seems likely that this would also influence the properties of adatoms on the MgO film, as Mo dopants in CaO(001) films, for example, have been shown to donate charge to adsorbed Au clusters, changing their shape …”

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

“…36,37 Electron paramagnetic resonance (EPR) shows that paramagnetic centers formed during the electron transfer process under molecular reaction conditions are preferably found at step edges and thicker MgO films. 38,39 However, the identification of non-paramagnetic centers at surface sites of oxide materials is less straightforward. 20 Density-functional theory (DFT) calculations have shown that surface chemical reactions can be controlled by defects located at the oxide-metal interface.…”

Location of Trapped Electron Centers in the Bulk of Epitaxial MgO(001) Films Grown on Mo(001) Usingin situW-band Electron Paramagnetic Resonance Spectroscopy

“…This result further corroborates the assignment of the observed species to trapped electron centers, as it shows that all three g-tensor components have to be quite similar. This excludes the presence of oxygenbased paramagnetic centers, which would have at least one g-tensor component shifted significantly to higher g values [22,[27][28][29][30][31]. Rotation of the magnetic field in the surface plane, as exemplified by a measurement with the magnetic field oriented along the [100] direction of the MgO lattice, results in the same resonance position.…”