Abstract
Nonreducible oxides are characterized by large band gaps and are therefore unable to exchange electrons or to form bonds with surface species, explaining their chemical inertness. The insertion of aliovalent dopants alters this situation, as new electronic states become available in the gap that may be involved in charge-transfer processes. Consequently, the adsorption and reactivity pattern of doped oxides changes with respect to their nondoped counterparts. This Account describes scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS) experiments that demonstrate the impact of dopants on the physical and chemical properties of well-defined crystalline oxide films. For this purpose, MgO and CaO as archetypical rocksalt oxides have been loaded either with high-valence (Mo, Cr) or low-valence dopants (Li). While the former generate filled states in the oxide band gap and serve as electron donors, the latter produce valence-band holes and give rise to an acceptor response. The dopant-related electronic states and their polarization effect on the surrounding host material are explored with XPS and STM spectroscopy on nonlocal and local scales. Moreover, charge-compensating defects were found to develop in the oxide lattice, such as Ca and O vacancies in Mo- and Li-doped CaO films, respectively. These native defects are able to trap the excess charges of the impurities and therefore diminish the desired doping effect. If noncompensated dopants reside in the host lattice, electron exchange with surface species is observed. Mo ions in CaO, for example, were found to donate electrons to surface Au atoms. The anionic Au strongly binds to the CaO surface and nucleates in the form of monolayer islands, in contrast to the 3D growth prevailing on pristine oxides. Charge transfer is also revealed for surface O2 that traps one Mo electron by forming a superoxo-species. The activated oxygen is characterized by a reinforced binding to the surface, an elongated O-O bond length, and a reduced barrier for dissociation, and represents an important intermediate for oxidation reactions. The charge-transfer processes described here are quenched if Li is inserted into the oxide lattice, neutralizing the effect of the extra electrons. The specific behavior of doped oxides has been explored on a mechanistic level, i.e. on thin-film model systems at ultrahigh vacuum and low temperature. We believe, however, that our results are transferrable to realistic conditions and doping might thus develop into a powerful method to improve the performance of nonreducible oxides in surface-catalyzed reactions.
Highlights
Oxide materials play a fundamental role in surface-catalyzed chemical reactions
In order to verify the existence of dopants in the oxide lattice and to probe their electronic impact, we have investigated the state-density of doped oxide films via X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) conductance spectroscopy (STS)
This Account aimed at exploring the nature of doped oxides and at correlating charge-transfer processes with the distinct adsorption behavior of the material
Summary
Oxide materials play a fundamental role in surface-catalyzed chemical reactions. They fulfill two major purposes in heterogeneous catalysis. They serve as support material for the chemically active species, for example, an ensemble of metal particles On the other, they may participate in the chemical reaction themselves, e.g., by providing charges for redox processes or balancing the oxygen supply. Dopants residing directly in the surface may interact with adsorbates, further increasing the chemical versatility of the material In this Account, we review our experimental expertise to prepare doped oxides and discuss the impact of the impurities on structural and electronic properties of the host lattice. We take a fundamental viewpoint to discuss the physics and chemistry of doped oxides, as we use scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to characterize our samples Both methods are capable of providing mechanistic insights into the role of dopants in the oxide lattice
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