PHOTOEXCITED FE2O3 SURFACES - PROPERTIES AND CHEMISORPTION

Transition-metal-oxide particles comprise a small but important fracti on 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 alpha-Fe2O3(0001) to study photoinduced c harge-transfer processes and chemisorption of SO2, an atmospheric poll utant. Changes in electronic structure as a result of preparation meth od are presented which complement previous studies. Ultraviolet photoe lectron spectroscopy (UPS) was used to study changes in the electronic structure of alpha-Fe2O3(0001) surfaces due to ultraviolet (UV) irrad iation, and to differentiate them from thermal excitations. Intense UV irradiation of the surface by a Hg(Xe) are lamp results in an increas ed density-of-states near E-F similar to that produced by reduction of the surface; the increase is reversible when the irradiation is termi nated. In addition, the upper edge of the valence band is observed to shift upon both UV irradiation and temperature change; however, the ba nd edge shifts to higher binding energy upon UV irradiation, but to lo wer energy with increased temperature. UPS results show that photoexci ted alpha-Fe2O3(0001) surfaces chemisorb much larger amounts of SO2 th an does that surface in the dark; however, adsorbate molecular-orbital peaks were found at similar positions in both cases. X-ray photoelect ron 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. (C ) 1998 American Vacuum Society. [S0734-2101(98)03203-5].