Oxygen Vacancies on the MgO Surface

For the experimentahst, F+ and F centers are most interesting because they represent spectroscopically active species. Both exhibit electronic transitions in the visible range of the optical spectrum, and the F+ center is, due to its unpaired electron, a paramagnetic entity that can be investigated by electron paramagnetic resonance (EPR). Optical absorption and EPR have been extensively used to characterize F centers in the bulk of MgO single crystals. With the advent of MgO thin films, this insulating oxide became also accessible to classic surface science spectroscopic tools that made experimental characterization of surface F centers possible. 

Until now, there is no experimental evidence for a rich abundance of F centers (at least within the experimental detection limit) on freshly prepared MgO thin films. Methods that have been described in the literature to produce F centers on MgO thin films include high-temperature annealing and subsequent quenching, electron bombardment, or Ar+ bombardment. Among these methods, electron bombardment has been most widely used for the generation and subsequent characterization of F centers on MgO. 

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Figure 15.32 (a) Electron density plots for the three charge states of oxygen vacancies on the MgO surface, (b) Dependence of ionization potentials of electrons trapped in oxygen vacancies located in different positions on the MgO surface from Sushko, 2000 [143],... 

The topic of defect sites at oxide surfaces therefore becomes crucial in order to fully understand the metal-oxide bonding. This subject has been addressed theoretically only recently. In this review we have shown how defect sites at both MgO and Si02 surfaces play a fundamental role in both stabilization and nucleation, but also that they modify the cluster electronic properties. In particular, some defect centers that act as electron traps like the oxygen vacancies at the MgO surface are extremely efficient in increasing the electron density on the deposited metal atoms or clusters, thus augmenting their chemical activity toward other adsorbed molecules. Understanding the metal-oxide interface and the properties of deposited metal clusters also needs a deeper knowledge of nature, concentration and mechanisms of formation, and conversion of the defect sites of the oxide surface. 

Fig. 2.4. EEL spectra of an MgO thin film after electron bombardment to create oxygen vacancies on the surface. Reproduced from [137], Copyright 2002 Elsevier...

Fig. 2.13. Optimal geometry of M(CO)2 complexes adsorbed on the MgO surface, (left) Pd(CO)2 complex formed at an oxygen vacancy (F center) (right) Rh(CO)2 complex formed at a step...

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

Oxygen vacancies on MgO(lOO) have been described by numerous authors [69,88-97]. The formation energy E f of an isolated vacancy in bulk MgO is large of the order of 10 eV. It decreases monotonically when the vacancy is located closer and closer to the surface [91], or in surface sites of... 

Fig. 13 Characteristics of the saddle point configuration in the diffusion path of an oxygen vacancy on MgO(lOO) (top view), (a) atomic configuration (b) iso-density surface for the vacancy gap state, plotted at about 15% of the maximum state density. The magnesiums and oxygens are shown as large and small circles, respectively (from Ref. 69).

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