Visible light-driven photocatalysts have been attracting attention for harvesting solar energy. The ransition-metal oxides with empty or filled 3d states have actively been studied upon tuning of their band-gaps by chemical doping. In contrast, those with partially filled 3d states have been poorly understood because of complex d-d transition.
In this study, we have examined a model solid-solution composed of α-Fe2O3, a Photocatalyst available from abundant minerals, and isostructral α-Cr2O3, whose band-gaps are 2.1 and 3.0 eV, respectively.
The α-(CrxFe1-x)2O3 thin films were prepared by using pulsed-laser
deposition on c-sapphire substrates. The films were epitaxially grown along the c-axis orientation and lattice parameters were a good greement with values of reference powder samples in the entire composition range. In addition, film surfaces were confirmed to be optically flat by inspections with atomic force microscopy. Moreover,
new absorption band appeared above 1.7 eV in the intermediate composition films (0.2 < x < 0.9) [1]. To clarify the origin of the band-gap narrowing, electronic structures of the solid-solution films
were characterized by X-ray photoelectron spectroscopy and X-ray
absorption spectroscopy using synchrotron radiation. It is found that
the top of valence band consists of Cr 3d and O 2p states, whereas, the bottom of conduction band consists of Fe 3d state, exhibiting a type-II
band alignment. Thus, the lowest transition level is attributed to
interatomic charge transfers from occupied Cr t2g and O 2p states to
empty Fe t2g* state. Our results indicate that the use of high-quality
epitaxial films will be a suitable approach to explore visible
light-driven photocatalyst based on complex transition-metal oxides.
1. H. Mashiko, T. Oshima, and A. Ohtomo: Appl. Phys. Lett. 99 (2011) 241904.
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