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Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Hu Yelin, Boudoire Florent, Hermann-Geppert Iris, Bogdanoff Peter, Tsekouras George, Mun Bongjin Simon, Fortunato Giuseppino, Graetzel Michael, Braun Artur,
Project Fundamental Aspects of Photocatalysis and Photoelectrochemistry / Basic Research Instrumentation for Functional Characterization
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Original article (peer-reviewed)

Journal JOURNAL OF PHYSICAL CHEMISTRY C
Volume (Issue) 120(6)
Page(s) 3250 - 3258
Title of proceedings JOURNAL OF PHYSICAL CHEMISTRY C
DOI 10.1021/acs.jpcc.5b08013

Abstract

The origin, the nature, and the electronic structure of surface defects causing surface states on metal oxides and their role in solar water splitting have been under scrutiny for several decades. In the present study, the surface of hematite films is treated with an oxygen plasma and then subject to a detailed investigation with electroanalytical methods and element orbital specific X-ray spectroscopy. We observe a systemic variation of photoelectrochemical properties with oxygen treatment time. Fe 2p and O is core level X-ray photoelectron spectra and resonant valence band photoemission at the Fe 3p edge reveal the filling of prevalent oxygen vacancies with concomitant oxidation of Fe2+ to Fe3+ upon the oxygen treatment. The dc bias dependent impedance spectra confirm how a prevalent capacitive surface state, which evolves parallel with the photocurrent onset potential, becomes diminished upon oxygen treatment. Surface states of iron induce higher reactivity toward water oxidation than oxygen surface states. correlation between oxygen vacancy filling, concentration of surface states, and photocurrent density in the course of treatment confirms that the surface defects are of a capacitive nature and that the onset of water splitting can be considered as a result of dielectric breakdown in an interfacial hydroxide layer between photoanode and water.
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