Origins of electronic band gap reduction in Cr/N codoped TiO2.
Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d3 levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA.
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Parks Cheney
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA.
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Vilmercati
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA.
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Martin
Dipartimento di Matematica e Fisica and Interdisciplinary Laboratories for Advanced Materials Physics, Università Cattolica del Sacro Cuore di Brescia, Via Musei 41, Brescia 25121, Italy.
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Chiodi
Dipartimento di Matematica e Fisica and Interdisciplinary Laboratories for Advanced Materials Physics, Università Cattolica del Sacro Cuore di Brescia, Via Musei 41, Brescia 25121, Italy.
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Gavioli
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
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Regmi
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
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Eres
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA.
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Callcott
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
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Weitering
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA.
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Mannella