Iron Resonant Photoemission Spectroscopy on Anodized Hematite Points to Electron Hole Doping during Anodization

Authors

  • Dr. Artur Braun,

    Corresponding author
    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    • Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
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  • Qianli Chen,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    2. Swiss Federal Institute of Technology, ETH Zürich, Physics Department, 8057 Zürich (Switzerland)
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  • Dorota Flak,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    2. Faculty of Material Science and Ceramics, AGH University of Science and Technology, 30-059 Krakow (Poland)
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  • Dr. Giuseppino Fortunato,

    1. Laboratory for Advanced Fibers, Empa. Swiss Federal Laboratories for Materials Science and Technology, 9014 St. Gallen (Switzerland)
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  • Dr. Krisztina Gajda-Schrantz,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    2. Research Group of Environmental Chemistry, Department of Inorganic and Analytical Chemistry, University of Szeged, 6720 Szeged (Hungary)
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  • Prof. Dr. Michael Grätzel,

    1. Laboratory for Photonics and Interfaces, Ecole Polytechnique Federal de Lausanne, 1015 Lausanne (Switzerland)
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  • Prof. Dr. Thomas Graule,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
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  • Dr. Jinghua Guo,

    1. Advanced Light Source, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)
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  • Dr. Tzu-Wen Huang,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
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  • Dr. Zhi Liu,

    1. Advanced Light Source, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)
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  • Anastasiya V. Popelo,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    2. Laboratory for Photonics and Interfaces, Ecole Polytechnique Federal de Lausanne, 1015 Lausanne (Switzerland)
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  • Dr. Kevin Sivula,

    1. Laboratory for Photonics and Interfaces, Ecole Polytechnique Federal de Lausanne, 1015 Lausanne (Switzerland)
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  • Dr. Hiroki Wadati,

    1. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-0032 (Japan)
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  • Pradeep P. Wyss,

    1. Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    2. Fachhochschule Nordwestschweiz, Institut für Chemie und Bioanalytik, 4132 Muttenz (Switzerland)
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  • Liang Zhang,

    1. Advanced Light Source, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)
    2. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230029 (China)
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  • Prof. Dr. Junfa Zhu

    1. Advanced Light Source, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)
    2. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230029 (China)
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Abstract

Anodization of α-Fe2O3 (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that an additional current wave occurs in the cyclic voltammogram. The energy position of this current wave is closely below the potential of the anodization treatment. Continued cycling or exchanging of the electrolyte causes depletion of this new feature. The O 1s and Fe 2p core-level X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra of such conditioned hematite exhibit a chemical shift towards higher binding energies, in line with the general perception that anodization generates oxide species with dielectric properties. The valence band XPS and particularly the iron resonant valence band photoemission spectra, however, are shifted towards the opposite direction, that is, towards the Fermi energy, suggesting that hole doping on hematite has taken place during anodization. Quantitative analysis of the Fe 2p resonant valence band photoemission spectra shows that the spectra obtained at the Fe 2p absorption threshold are shifted by virtually the same energy as the anodization potential towards the Fermi energy. The tentative interpretation of this observation is that anodization forms a surface film on the hematite that is specific to the anodization potential.

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