  Transition metal doping is known to increase the photosensitivity to visible
light for photocatalytically active ZnO. We report on the electronic structure
of nano-crystalline Fe:ZnO, which has recently been shown to be an efficient
photocatalyst. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the
surface region of the nano-crystalline material. Electronic states
corresponding to low-spin Fe 2+ are observed and attributed to crystal field
modification at the surface. These states can be important for the
photocatalytic sensitivity to visible light due to their deep location 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 a crucial factor limiting the photocatalytic
functionality of ZnO, where the most efficient doping concentration have been
shown to be 2-4% for Fe doping. Using resonant photoemission spectroscopy we
determine the location of Fe 3d states with sensitivity to the charge states of
the Fe ion even for multi-valent and multi-coordinated Fe.
