Electronic Structure and Chemisorption Properties of Chromium and Nickel Oxides.

Resonant photoemission measurements have been performed on single-crystal Cr_2O _3 (1012) surfaces for the first time. A strong resonant enhancement of the highest-lying valence-band states was observed across the Cr 3p to 3d excitation threshold. In light of the similarity of the resonance in Cr_2O _3 with that of the Mott-Hubbard oxides Ti_2O_3 and V_2O_3, and differences from those of the charge-transfer oxides MnO, Fe_2O_3, CoO, NiO and CuO, it is suggested that Cr_2 O_3 is a Mott-Hubbard insulator. However, configuration-interaction cluster calculations were applied to study the nature of the satellite in the Cr 2p core -level XPS spectrum. It is argued that the satellite can be understood as a charge-transfer satellite, and Cr _2O_3 is found to be actually situated at the boundary between the Mott-Hubbard and the charge-transfer regimes. The values of the charge -transfer energy, Delta, the Coulomb correlation energy, U, and the ligand 2p-cation 3d hybridization energy, T, found from fitting the Cr 2p core-level XPS spectrum were used to analyze the valence-band UPS spectrum. The comparison between the experimental spectrum and the theoretical prediction is good. Interestingly, defective Cr_2O _3 (1012) surfaces are found to preferentially lose Cr upon Ar^+-ion bombardment, while most of the 3d-transition-metal oxides that have been studied tend to lose O rather than their metal cations. The surface electronic structure is modified by chromium-vacancy surface defects. Surface Cr vacancy defects only cause an insignificant change in SO_2 adsorption characteristics on Cr_2O_3 (1012). The SO_2 adsorption on both nearly perfect and defective Cr_2O _3 (1012) surfaces at room temperature is associative and very weak, yielding a small S 2p peak after a large SO_2 exposure. However, it is found that the reactivity of SO_2 is somewhat weaker on the defective Cr _2O_3 (1012) surface, as compared to that on the cleaved surface. This shows that the role of Cr vacancies in SO_2 adsorption is very different from that of oxygen vacancies on the other 3d-transition-metal oxides, since surface oxygen vacancies are, in general, known to enhance the reactivity toward SO_2. Unlike Cr_2O_3 , oxygen-vacancy defects on reduced NiO (100) play an important role in SO_2 adsorption. SO _2 reacts only weakly with the stoichiometric NiO (100) surface at room temperature, yielding associatively adsorbed SO_2 molecules. In contrast, the interaction of SO_2 with the reduced NiO (100) surface is much stronger. At room temperature, the SO_2 adsorption on reduced NiO (100) is both associative and dissociative. It was found that the dissociation is due to a thermally activated mechanism that involves adsorption at oxygen-vacancy sites only. A simple approach has been developed that includes explicitly the secondary-electron cascade process in the determination of inelastic backgrounds in low-energy electron spectra such as UPS. The new approach fits the spectra much better than does the iterative integral background, which is generally used in higher-energy spectroscopies such as XPS. While the new approach gives a good description of the inelastic background in UPS spectra from transition -metal oxides for low photon energies, discrepancies are found between the computed and actual backgrounds for UPS spectra taken at intermediate photon energies. The reason for this may have to do with experimental parameters. (Abstract shortened by UMI.).