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Critical role and modification of surface states in hematite films for enhancing oxygen evolution activity

Published online by Cambridge University Press:  11 January 2018

Myeongwhun Pyeon ,
Tero-Petri Ruoko ,
Jennifer Leduc ,
Yakup Gönüllü ,
Meenal Deo ,
Nikolai V. Tkachenko  and
Sanjay Mathur
Myeongwhun Pyeon
Affiliation:
Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany
Tero-Petri Ruoko
Affiliation:
Laboratory of Chemistry and Biotechnology, Tampere University of Technology, Tampere 33101, Finland
Jennifer Leduc
Affiliation:
Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany
Yakup Gönüllü
Affiliation:
Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany
Meenal Deo
Affiliation:
Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany
Nikolai V. Tkachenko
Affiliation:
Laboratory of Chemistry and Biotechnology, Tampere University of Technology, Tampere 33101, Finland
Sanjay Mathur*
Affiliation:
Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany
*
a)Address all correspondence to this author. e-mail: sanjay.mathur@uni-koeln.de
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Abstract

Hematite films deposited by plasma-enhanced chemical vapor deposition of iron pentacarbonyl [Fe(CO)5] in an oxygen plasma were modified by postdeposition (i) oxygen plasma treatment and (ii) short annealing treatments to reduce the defects and to modify the (sub)surface states and consequently the photoelectrochemical properties. The oxygen plasma treatment resulted in the increase of particle size and augmented surface roughening by densification of grains. Moreover, it induced saturated surface states with reactive oxygen species (O−, OH−), evident in the X-ray photoelectron spectroscopy (XPS). Under standard illumination (1.5 AM; 100 mW/cm2; 150 W xenon lamp), when compared to the pristine hematite coating (0.696 mA/cm2 at 1.23 V versus RHE and 0.74 Vonset) the oxygen plasma-treated films showed severe deterioration in photocurrent density of 0.035 mA/cm2 and an anodic shift in the onset potential (1.10 Vonset) due to oxygen rich surface. In a second set of experiments, the oxygen plasma-treated hematite films were briefly annealed (10 min at 750 °C) and the signals of Fe 2p and O 1s recovered to higher binding energies, indicating the formation of oxygen vacancies. In addition, a superior photocurrent density value of max. 1.306 mA/cm2 at 1.23 V versus RHE to that of the pristine hematite photoanode with 0.74 Vonset was obtained. Transient absorption spectroscopy further elucidated that the oxygen plasma-induced electron trap states acting as recombination centers that are unfavorable for photoelectrochemical activity. The alteration in Fe:O stoichiometry and thus photocurrent density are corroborated by determination of water oxidation rates in annealed (7.1 s−1) and oxygen plasma treated (2.5 s−1) samples.

Keywords

plasma-enhanced CVD (PECVD) (deposition)thin filmsurface chemistry
Type
Invited Feature Paper
Information
Journal of Materials Research , Volume 33 , Issue 4 , 28 February 2018 , pp. 455 - 466
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Copyright
Copyright © Materials Research Society 2018 

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Footnotes

Contributing Editor: Xiaobo Chen

This paper has been selected as an Invited Feature Paper.

References

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