Critical role and modification of surface states in hematite films for enhancing oxygen evolution activity
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ro-Petri Ruoko Laboratory of Chemistry and Biotechnology, Tampere University of Technology, Tampere 33101, Finland
Jennifer Leduc, Yakup Gönüllü, and Meenal Deo Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany
Nikolai V. Tkachenko Laboratory of Chemistry and Biotechnology, Tampere University of Technology, Tampere 33101, Finland
Sanjay Mathura) Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany (Received 14 September 2017; accepted 29 November 2017)
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 s1) and oxygen plasma treated (2.5 s1) samples.
I. INTRODUCTION
In the context of alternative energy concepts, an economically viable hydrogen cycle carries the potential for becoming the fuel of the future.1,2 The conventional hydrogen production relies on the reforming of hydrocarbons obtained from petrochemicals, which makes this technology less environmental friendly due to the significant carbon footprint. Since Fujishima and Honda3 had demonstrated water photolysis of TiO2 under UV illumination in 1972, massive attention to the splitting of water
Contributing Editor: Xiaobo Chen a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2017.465
into hydrogen and oxygen in a photoelectrochemical (PEC) way has been drawn as a promising alternative ro
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