SURFACE ELECTRONIC STRUCTURE AND CHEMISORPTION ON CORUNDUM TRANSITION-METAL OXIDES.

摘要

The surface electronic structures of all of the conducting corundum oxides of the fourth-period transition-metals have been studied using an ultra-high vacuum (UHV), multiple-technique surface analysis system. This system, which permits comprehensive determination of surface properties, includes ultra-violet and x-ray photoemission, low energy electron diffraction (LEED), Auger electron spectroscopy, and electron energy-loss spectroscopy. The complementary nature of the information obtained from these techniques plays an essential role in understanding fundamental properties of this class of oxides.; Measurements of the properties of perfect oxide surfaces are extremely important in order to provide model systems for developing theories on the structure of complex materials. Nearly perfect (047) surfaces of Ti(,2)O(,3) and V(,2)O(,3) have been prepared by cleaving, while the (001) surfaces of (alpha)Fe(,2)O(,3) have been prepared by sputter-etching followed by heating in ultrahigh vacuum. The electronic and geometric structures of these oxide surfaces have been characterized. Although the cleaved (047) surface valence-band structures are similar to those of the bulk, the surface-cation core levels have been found to be shifted, due to the decreased surface ligand coordination.; The active sites in such technologically important processes as catalysis, photoelectrolysis, corrosion, etc. are often thought to be surface defect sites. In order to understand such sites, the nature of surface defects, produced in a controlled manner by inert-ion bombardment, has been investigated. On these oxide surfaces, such defects are associated with oxygen vacancies and result in surface states located on and above the O 2p valence band. The density of occupied states near the Fermi level also increases due to the population of d-electron levels of reduced, metallic surface species.; The effects of adsorbates of catalytic interest have been investigated on both perfect and defect oxide surfaces. On all of the surfaces studied, adsorbed O(,2) has a profound effect on the surface band structure, depopulating the d-electron bands, bending the O 2p bands up, and increasing the work function. Exposure to H(,2)O results in adsorbed OH radicals on all surfaces except the cleaved Ti(,2)O(,3) (047), where only molecularly adsorbed H(,2)O was observed. On (alpha)Fe(,2)O(,3) (001), exposure to H(,2) produces surface OH radicals.