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

Abstract

Ultraviolet photoemission spectroscopy has been used to study the electronic structure of both nearly stoichiometric, well-ordered α-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ 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 α-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$(0001) surface is relatively inert with respect to all four molecules studied, with exposures greater than ${10}^3$ L (1 L≡${10}^{\mathrm{-}6}$ 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 ${\mathrm{OH}}^{\mathrm{-}}$ ions. ${\mathrm{SO}}_2$ appears to bond primarily to surface oxygen ions, yielding a complex similar to ${\mathrm{SO}}_4$ ${\mathrm{}}^{2\mathrm{-}}$.