Surface electronic structure and chemisorption on corundum transition-metal oxides: alpha -Fe2O3.

Abstract

Ultraviolet photoemission spectroscopy has been used to study the electronic structure of both nearly stoichiometric, well-ordered \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ surfaces and surfaces containing point defects. The interaction of ${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}$O, ${\mathrm{H}}_{2}$, and ${\mathrm{SO}}_{2}$ with both types of surfaces has also been investigated. The energy levels of the five 3d electrons on the ${\mathrm{Fe}}^{3+}$ ions overlap those of the filled O 2p orbitals, leading to a complex valence band about 10 eV wide. Oxygen-vacancy surface defects produced by ${\mathrm{Ar}}^{+}$-ion bombardment result in a conducting surface layer containing both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{0}$ ions. The well-ordered \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$(0001) surface is relatively inert with respect to all four molecules studied, with exposures greater than ${10}^{3}$ L (1 L\ensuremath{\equiv}${10}^{\mathrm{\ensuremath{-}}6}$ Torr sec) necessary before any chemisorption could be seen. For large exposures, the ${\mathrm{O}}_{2}$-surface interaction gives rise to a negative adsorbed species. ${\mathrm{H}}_{2}$O adsorbs dissociatively on both well-ordered and defect surfaces, resulting in adsorbed ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ ions. ${\mathrm{SO}}_{2}$ appears to bond primarily to surface oxygen ions, yielding a complex similar to ${\mathrm{SO}}_{4}$${\mathrm{}}^{2\mathrm{\ensuremath{-}}}$.