Photoexcited Fe2O3 surfaces: Properties and chemisorption

Abstract

Transition–metal–oxide particles comprise a small but important fraction of atmospheric aerosols as they are among the few particles in the troposphere having band gaps less than the cutoff of solar radiation (about 4.3 eV), thus allowing photoexcited charge-transfer excitations. We have used single-crystal α-Fe2O3(0001) to study photoinduced charge-transfer processes and chemisorption of SO2, an atmospheric pollutant. Changes in electronic structure as a result of preparation method are presented which complement previous studies. Ultraviolet photoelectron spectroscopy (UPS) was used to study changes in the electronic structure of α-Fe2O3(0001) surfaces due to ultraviolet (UV) irradiation, and to differentiate them from thermal excitations. Intense UV irradiation of the surface by a Hg(Xe) arc lamp results in an increased density-of-states near EF similar to that produced by reduction of the surface; the increase is reversible when the irradiation is terminated. In addition, the upper edge of the valence band is observed to shift upon both UV irradiation and temperature change; however, the band edge shifts to higher binding energy upon UV irradiation, but to lower energy with increased temperature. UPS results show that photoexcited α-Fe2O3(0001) surfaces chemisorb much larger amounts of SO2 than does that surface in the dark; however, adsorbate molecular-orbital peaks were found at similar positions in both cases. X-ray photoelectron spectroscopy (XPS) showed that more SO2 chemisorbed on surfaces at 267 K than at 300 K, and that photoexcitation increased chemisorption at both temperatures, especially at low SO2 exposures. Based upon UPS and XPS results, the adsorbed species is identified as SO3 or SO4.