title: Photoinduced reduction of surface states in Fe:ZnO.
abstract: We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been found to be an efficient photocatalyst. Using resonant photoemission spectroscopy, we determine the binding energy of Fe 3d states corresponding to different valencies and coordination of the Fe atoms. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material due to the formation of oxygen vacancies. Electronic states corresponding to low-spin Fe(2+) are observed and attributed to crystal field modification at the surface. These states are potentially important for the photocatalytic sensitivity to visible light due to their location deep in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is limiting the photocatalytic functionality of ZnO, where the most efficient Fe doping concentration has been shown to be 1%-4%.
authors.affiliation: Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: R
authors.initials: R
authors.lastname: Knut
authors.affiliation: Department of Chemistry Ångström, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden.
authors.forename: U
authors.initials: U
authors.lastname: Lagerqvist
authors.affiliation: Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Palmgren
authors.affiliation: MAX-Laboratory, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Pal
authors.affiliation: Department of Engineering Sciences, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Svedlindh
authors.affiliation: Department of Chemistry Ångström, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden.
authors.forename: A
authors.initials: A
authors.lastname: Pohl
authors.affiliation: Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: O
authors.initials: O
authors.lastname: Karis