Surface defects and their impact on the electronic structure of Mo-doped CaO films: an STM and DFT study

Abstract

The functionality of doped oxides sensitively depends on the spatial distribution of the impurity ions and their interplay with compensating defects in the lattice. In our combined scanning tunneling microscopy (STM) and density functional theory (DFT) study, we analyze defects occurring in Mo-doped CaO(001) films at the atomic scale. By means of topographic imaging, we identify common point and line defect in the doped oxide, in particular Mo donors and compensating Ca vacancies. The influence of charged defects on the oxide electronic structure is analyzed by STM conductance spectroscopy. The experimentally observed defect features are connected to typical point defects in the CaO lattice by means of DFT calculations. Apart from the identification of individual defects, our study reveals a pronounced inhomogeneity of the oxide electronic structure, reflecting the uneven spatial distribution of dopants in the lattice. Our results provide the basis for a better understanding of adsorption and reaction patterns on doped oxides, as widely used in heterogeneous catalysis.

Reactions of ethane with CO2 over supported Au

“…Finally, we may deal with the possible role of the defect structure of the oxidic supports. As mentioned in the introduction, in the explanation of the effectiveness of the TiO 2 as a support an increased attention is paid to the role of its oxygen vacancy [23][24][25]. As the carrier effect of TiO 2 (containing O vacancy) and that of ZnO (with excess Zn the interstitial position) is comparable, we may conclude that in this reaction the electronic property of the supporting oxides is more important than the nature of their defect structure.…”

“…After the first use of TiO 2 as a support [20,21] a dramatic increase in its application occurred [22]. Recently a greater attention is paid to the role of O vacancy in the catalytic and support effect of TiO 2 [23][24][25]. As the n-type character of TiO 2 is due to the O vacancy [26], it is not easy to differentiate between the role of O vacancy and the electric property.…”

Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

“…If we can place an electron source within the bulk of an oxide support to mimic the corresponding energetics, a similar situation should be encountered, as anticipated in Section 2.2.2. 120,159,206,[243][244][245] Fig . 23 shows STM images of a 60 ML thick CaO film in a (100) geometry without a Mo dopant (Fig.…”

“…The state of the dopant may be probed using XPS, whereby the depth distribution of the dopants may be analyzed using angle dependent XPS or photon energy dependent spectra via synchrotron radiation studies via an effective variation of the electron escape depth. 120,[159][160][161] Another way to bring the dopant into the film is successive evaporation of a film of a given thickness, followed by the evaporation of the dopant material and a second controlled film deposition step to bury the dopant. This allows for a controlled deposition of the dopant at a given depth below the oxide surface.…”

A DFT Study of the Reactivity of Anatase TiO2 and Tetragonal ZrO2 Stepped Surfaces Compared to the Regular (101) Terraces

“…[1] These are either low-coordinated cations or anions at the surface of the oxide which exhibit enhanced activity due to their modified acidicbasic properties, or point defects such as cation or anions vacancies [2] or impurity atoms. [3] For instance, the reactivity of an alkaline-earth oxide like MgO is completely different on the flat (100) terraces,w hich are almost completely unreactive, and on the four-, or three-coordinated sites at steps, kinks and corners where most of the chemistry of this oxide takes place. Oxide catalysts are usually employed in the form of nanoparticles where the surface-to-bulk ratio is large.…”