Electron trapping in misfit dislocations of MgO thin films

Benia, Hadj M.; Myrach, Philipp; Gonchar, Anastasia; Risse, Thomas; Nilius, Niklas; Freund, Hans‐Joachim

doi:10.1103/physrevb.81.241415

Cited by 62 publications

(91 citation statements)

References 37 publications

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“…Moreover, the surface density of those vacancies was found to be higher in MgO than in CaO films at similar doping levels, reflecting the higher V M formation energy in the latter case. Additional electron traps are known to be present along the dislocation lines of the MgO and CaO films [38], which render a more quantitative discussion at this point difficult. By dosing the dopants at particular stages of film growth we are able to control their position within the film, and with respect to the oxide surface.…”

Section: Resultsmentioning

confidence: 99%

Model Studies on Heterogeneous Catalysts at the Atomic Scale

2014

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Thin single crystalline oxide films comprise perfect supports to grow nanoparticles of metals and other catalytically relevant materials. The model systems thus created can be thoroughly investigated with respect to structure and/or chemical activity applying both, the techniques of surface science under ultrahigh vacuum conditions as well as the traditional techniques applied in catalysis to study chemical kinetics under ambient conditions. We discuss here in particular the oxidation of methanol to formaldehyde on ceria supported vanadia nanoclusters as well as the effect of transition-metal dopants in single crystalline oxides, such as CaO, on the activation of molecular oxygen

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Section: Resultsmentioning

confidence: 99%

Model Studies on Heterogeneous Catalysts at the Atomic Scale

2014

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“…2(a)]. We note that the temperature effect is most pronounced for an intermediate film thickness (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) and fades away for thicker films. Conversely, the band onset can be tuned by varying the film thickness and keeping the annealing temperature constant at 1000 K. This trend is 035418- 2 FIG.…”

Section: (E)mentioning

confidence: 89%

“…The limits in the oxygen activation are rather set by the total number of Mo ions in the oxide lattice, their mean separation from the surface, and their charge state. Note that the latter is largely controlled by the presence of parasitic electron traps in the film, such as the metal-oxide interface and electron-accepting point or line defects [24,25]. As electron transfer into the O 2 molecules was found to be efficient, the structural quality of our CaO layers is apparently high enough to maintain the donor character of the oxide [8].…”

Section: Interplay Between Electronic Structure and Adsorption Behmentioning

confidence: 93%

Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

et al. 2015

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The electronic structure of CaO films of 10-60 monolayer thickness grown on Mo(001) has been investigated with synchrotron-mediated x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Upon annealing or reducing the thickness of the film, a rigid shift of the CaO bands to lower energy is revealed. This evolution is explained with a temperature-induced diffusion of Mo ions from the metal substrate to the oxide and their accumulation in the interface region of the film. The Mo substitutes divalent Ca species in the rocksalt lattice and is able to release electrons to the system. The subsequent changes in the Mo oxidation state have been followed with high-resolution XPS measurements. While near-interface Mo transfers extra electrons back to the substrate, generating an interface dipole that gives rise to the observed band shift, near-surface species are able to exchange electrons with adsorbates bound to the oxide surface. For example, exposure of O 2 results in the formation of superoxo species on the oxide surface, as revealed from STM measurements. Mo interdiffusion is therefore responsible for the pronounced donor character of the initially inert oxide, and largely modifies its adsorption and reactivity behavior.

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“…[9] Defects will definitely fulfill a key function with respect to the activation of either methane or oxygen by changing the electronic structure of the wide band gap magnesium oxide particularly with regard to facilitate the transfer of electrons between the solid surface and the adsorbed molecules, which undergo a redox reaction. [10][11][12][13][14][15] The present work addresses relations between the nature and abundance of morphological surface defects such as steps and corners on pure magnesium oxide and its reactivity in the oxidative coupling of methane.…”

Section: Li-doped Mgo Was Discovered By Lunsford Et Al As An Active mentioning

confidence: 99%

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

Schwach

Hamilton

Thum

et al. 2015

Journal of Catalysis

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The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano-structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol–gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments. The initial activity of MgO in the OCM reaction is clearly structure-sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface-mediated coupling mechanism involving direct electron transfer between methane and oxygen. The two molecules are weakly adsorbed at structural defects (steps) on the surface of MgO. The proposed mechanism is consistent with high methane conversion, a correlation between methane and oxygen consumption rates, and high C₂H₄ selectivity after short times on stream. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas-phase chemistry becomes more important, which includes the formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C₂H₆. The mechanistic concepts proposed in this work (Part I) will be substantiated in Part II by spectroscopic characterization of the catalysts (Schwach et al. [1])

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Electron trapping in misfit dislocations of MgO thin films

Cited by 62 publications

References 37 publications

Model Studies on Heterogeneous Catalysts at the Atomic Scale

Model Studies on Heterogeneous Catalysts at the Atomic Scale

Evolution of the electronic structure of CaO thin films following Mo interdiffusion at high temperature

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

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