1. Introduction

Hematite (α-Fe2O3), one of iron oxides, has merits of abundance, harmlessness and stability. Hematite is expected to be utilized as a photo-functional material for the purpose of conversion of visible light energy to chemical and electric energy because it is an n-type semiconductor with band gap energy of about 2.0 eV. There are several reports concerning photoelectrochemical characteristics [1–7], photo-oxidation of water [8–15] and photocatalytic water purification [16–21] by using hematite. It is known that oxygen evolution due to photo-oxidation of water could occur on the hematite irradiated with visible light. This may be an interesting and important process from the viewpoint of artificial photosynthesis. Hematite is also one of the candidates for photocatalyst acting under visible light irradiation. Titanium dioxide with band gap energy of about 3.0 eV shows strong photocatalytic performance for environmental purification such as air purification, anti-soiling, self-cleaning, deodorizing, water purification and anti-bacterial [22–23], but it has disadvantage for utilization of visible light energy. In order to use hematite for photocatalysis and photosynthesis effectively, it is necessary to make clear its

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

photoresponse to chemical species. Knowledge about hematite electrode/electrolytic solution interface is important to understand a reactivity of photo-generated hole in the valence band of hematite to chemical species during irradiation. For the use of hematite as a photofunctional material, preparation of hematite film may be useful from the aspect of its repetitive performance. We have prepared the hematite film by electrochemical deposition of iron oxide and its heat treatment, and studied photo-oxidation of organic and inorganic materials on the hematite photoelectrode. Investigation of photo-oxidation of organic materials on hematite may lead to a new development of organic materials synthesis based on visible light energy conversion.

the counter electrode of iron plate were connected to a potentio-galvanostat with a function generator. The aqueous solution of 10 mM FeCl2–0.15 M NaCl (pH = 4.4) under oxygen gas bubbling was used for the electrochemical deposition of iron oxide film. The temperature of

Figure 2 shows the potential of titanium working electrode during the electrolysis by repetition of cathodic pulse (Ic = �7 mA, tc = 1 s) and anodic pulse (Ia = +1 mA, ta = 1 s) in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling for 100 s. The potential changed periodically with the cathodic and anodic current pulses. The potential depending on anodic current pulse approached to the value of �0.68 V vs. Ag/AgCl gradually. In the case of galvanostatic deposition with the current of �7 mA for 50 s in the same solution as above, the

Figure 3 shows the XRD of the film by current pulse deposition (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) for 100 s, the upper representing the pattern of the film after heat treatment at the temperature of 600�C for 1 h in air and the lower pattern corresponding to the as-deposited film before heat treatment. The diffraction peaks of Fe3O4 (magnetite) and FeO (wustite) and the peaks of α-Fe2O3 appeared on the film before and after heat treatment, respectively. On the as-deposited film by galvanostatic reduction (current: �7 mA) for 50 s in the presence of O2, the diffraction peaks of Fe(OH)2, FeO and Fe were confirmed, but the peak of Fe3O4 was not observed. From a consideration of the XRD result, the reaction for the formation of iron oxide film by current pulse deposition in the solution with O2 gas bubbling could be shown as Eqs. (1)–(4). The reaction in the heat treatment of film in air could be represented as Eqs. (5)

Figure 2. Change of electrode potential of working electrode by repetition of cathodic pulse (Ic = �7 mA, tc = 1 s) and anodic pulse (Ia = +1 mA, ta = 1 s) in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling for 100 s.

1=2O2 þ H2O þ 2e� ! 2OH� (1)

Photoelectrochemistry of Hematite

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http://dx.doi.org/10.5772/intechopen.73228

Fe<sup>2</sup><sup>þ</sup> <sup>þ</sup> 2OH� ! Fe OH ð Þ<sup>2</sup> (2)

Fe OH ð Þ<sup>2</sup> ! FeO þ H2O (3)

3FeO þ H2O ! Fe3O4 þ 2H<sup>þ</sup> þ 2e� (4)

this solution was kept constant at 25�C by circulation of thermo-stated water [29, 26].

potential of working electrode became almost constant value of �1.60 V vs. Ag/AgCl.

and (6).

In this chapter, I would like to describe photoelectrochemistry of hematite in terms of electrochemical preparation of iron oxide film, photoelectrochemical characterization of hematite and photo-oxidation reaction of chemical species on hematite mainly based on the results we have obtained [24–29].
