Values in relation to the photoanodic oxidation on the hematite electrode in 0.1 M aqueous Na2SO4 solution containing initial concentration of 200 μM citric acid under irradiation of visible light (wavelength: 490 nm, intensity: 3.8 mW/cm2) for 9 h.
It was possible to prepare a hematite film by electrochemical deposition of iron oxide in aqueous solution and its heat treatment at 500°C or higher temperature in air. The deposition process of iron oxide film from current and potential pulse methods was mentioned in relation to the equilibrium potential for iron oxide. The hematite in aqueous solution showed a clear photoanodic current due to visible light irradiation. The photo-oxidation response of hematite electrode to organic and inorganic materials in aqueous solution was summarized through the examples of citric acid, Pb2+ and aniline.
- iron oxide
- electrochemical deposition
- visible light
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, 2, 3, 4, 5, 6, 7], photo-oxidation of water [8, 9, 10, 11, 12, 13, 14, 15] and photocatalytic water purification [16, 17, 18, 19, 20, 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 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 photo-functional 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.
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, 25, 26, 27, 28, 29].
2. Electrochemical preparation of iron oxide film
On photocatalysis and photo-conversion by using hematite, preparation of hematite film is useful from the point of view of repetitive performance. Iron oxide film is prepared by spray pyrolysis, electrochemical deposition, sputtering method and so on. Electrochemical method may provide easy and reproducible preparation of iron oxide film. Here, based on the results we have obtained, the current and potential pulse deposition process of iron oxide film is mentioned as follows.
2.1. Current pulse deposition of iron oxide film
An iron oxide film (geometric surface area of 1.0 cm2) was prepared on a titanium substrate by current pulse deposition with repetition of cathodic pulse (current (Ic); time (tc): 1 s) and anodic pulse (current (Ia); time (ta): 1 s) as shown in Figure 1. The surface of titanium substrate was polished with alumina powder, immersed in aqueous HCl solution, washed with pure water and cleaned ultrasonically before electrolysis. The working electrode of titanium substrate and 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 this solution was kept constant at 25°C by circulation of thermo-stated water [29, 26].
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 potential of working electrode became almost constant value of −1.60 V vs. Ag/AgCl.
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) and (6).
Figure 4a shows the SEM image of the film deposited on the titanium 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, and heated at 600°C for 1 h in air. The film with the thickness of about 1.0 μm had the network morphology. As shown in Figure 4b, the film prepared by galvanostatic reduction with the current of −7 mA for 50 s in the same solution as above, and heated in the same condition showed the similar morphology, but less homogeneous deposition compared with the film by the current pulse method.
The current pulse deposition of iron oxide film in the solution under N2 gas bubbling was compared with that in the solution under O2 gas bubbling. Figure 5 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 N2 bubbling for 100 s. The appearance of iron, wustite and magnetite XRD peaks was confirmed in the as-deposited film due to the repetition of current pulse in this solution as shown in Figure 6. The film after heat treatment at 600°C for 1 h in air had the hematite structure. The SEM image of this hematite film is shown in Figure 7. The deposition of particles was observed in this film. The hematite film preparation under N2 bubbling could be represented as the process of current pulse deposition of iron oxide film (Eqs. (7)–(9), (4)) and its thermal oxidation process (Eqs. (10)–(12)).
For the electrochemical reactions of (1), (4), (7), (8) and (9), the corresponding equilibrium potentials can be evaluated by using the values of standard chemical potential of −237.178, −157.293, −91.2, −245.211 and −1015.359 KJ/mol  as
These standard equilibrium potentials are slightly different from the values in Pourbaix Diagram  due to the used standard chemical potential of component. With regard to the equilibrium potential in the case of pH = 4.4 and Fe2+ concentration of 10 mM,
2.2. Potential pulse deposition of iron oxide film
Potential pulse method as shown in Figure 8 is also useful in preparation of iron oxide film . In this case, electrochemical reduction and oxidation occurs with repetition of a periodic change in the working electrode potential between cathodic potential (
Figure 9 shows the XRD of the film prepared by potential pulse deposition (
3. Photoelectrochemical characterization of hematite
In order to characterize the hematite film in the aspect of a functional material for photocatalytic water purification and artificial photosynthesis, understanding of hematite electrode/electrolytic solution interface is important. Here, the Mott-Schottky relation and photocurrent response are mentioned as follows.
3.1. Mott-Schottky relation of hematite electrode/electrolytic solution interface
The measurement of capacitance for hematite electrode/electrolytic solution interface is useful in understanding of properties of hematite as an n-type semiconductor. The hematite film connected to a lead wire was used as a hematite working electrode. The capacitance of hematite electrode/electrolytic solution interface (C) was measured at a different electrode potential (E). At the semiconductor electrode/electrolyte interface, Motto-Schottky relation can be observed as represented by Eq. (28).
Figure 12a, b shows the plots of 1/
3.2. Photocurrent response of hematite to visible light irradiation
In the n-type semiconductor electrode/electrolytic solution interface, the Schottky barrier due to the band bending is formed at more positive potential of semiconductor electrode than flat-band potential as shown in Figure 13. In this case, no currents may flow on the electrode because of the existence of the Schottky barrier in the dark. Under irradiation of the light with higher energy than band gap, the transfer of photo-generated electrons to the bulk and that of holes to the surface of n-type semiconductor could proceed due to the band bending and thus photoanodic current may flow on the electrode.
Figure 14a, b shows the photocurrent response of the hematite prepared current pulse deposition under O2 and N2 bubbling to the solution, respectively. In this case, repetitive on–off irradiation of the visible light (wavelength: 490 nm, light intensity: 3.8 mW/cm2) was supplied to the surface of the hematite electrode at 1.0 V vs. Ag/AgCl in 0.1 M aqueous Na2SO4 solution. A clear photoanodic current was observed in both the hematite electrodes, but the hematite from N2 bubbling showed a higher photocurrent. The hematite prepared from potential pulse deposition under N2 bubbling also had a higher photocurrent response compared with that prepared under O2 bubbling.
The photocurrent response of the iron oxide depending on heat treatment temperature (100–600°C) in air is shown in Figure 15. In this case, the iron oxide film was prepared from potential pulse deposition (
Figure 19 shows the relationship between electrode potential and photocurrent on the hematite in 0.1 M aqueous Na2SO4 solution during irradiation. This hematite was prepared from the potential pulse deposition under N2 bubbling and heat treatment at 500°C. In the dark, anodic current did not flow up to the potential of 1.2 V vs. Ag/AgCl. In the irradiation of UV light (wavelength: 365 nm, intensity: 5.5 mW/cm2) to the hematite, the onset potential of photoanodic current was about 0.0 V vs. Ag/AgCl, almost equal to the value of
Figure 20 shows the relationship between photocurrent quantum efficiency and wavelength of irradiation light on the hematite at the potential of 1.0 V vs. Ag/AgCl in 0.1 M aqueous Na2SO4 solution containing 1 mM hydroxyl acid. The hematite was prepared from the potential pulse deposition under N2 bubbling and heat treatment at 500°C. The photocurrent quantum efficiency (
4. Photoreaction of chemical species on hematite photoelectrode
A photoelectrochemical system consisting of a semiconductor working electrode and a counter electrode may be suitable for performance of water purification and artificial photosynthesis because an effective separation of photo-generated hole and electron pair under irradiation could proceed due to the existence of space charge layer at the semiconductor electrode/electrolytic solution interface. In the case of using n-type semiconductor, photoanodic oxidation and cathodic reduction occur at a working and a counter electrodes, respectively. Photodecomposition of water by using titanium dioxide electrode, Honda-Fujishima effect, is well known as a typical photoelectrochemical process. In order to understand photo-oxidation response of hematite to chemical species, we checked oxidation behavior of citric acid, Pb2+ ion and aniline on the hematite photoelectrode.
4.1. Photo-oxidation of citric acid on hematite in aqueous solution under visible light irradiation
The HPLC analysis of organic acids in the solution was carried out to reveal the reaction process of citric acid on hematite photoelectrode in aqueous solution . This hematite was prepared from the current pulse deposition (Ic = −7 mA, Ia = +1 mA, tc = ta = 1 s) under O2 bubbling for 100 s and heat treatment at 600°C for 1 h in air. Figure 21a shows the chromatogram of the aqueous solution of 0.1 M Na2SO4 and citric acid (initial concentration: 200 μM) at the hematite electrode potential of 1.0 V vs. Ag/AgCl. Before irradiation, the only peak due to citric acid was observed at the retention time of 16.5 min. This chromatogram was not changed after immersion of the hematite electrode for 9 h in the dark. After irradiation of the visible light (wavelength: 490 nm, intensity: 3.8 mW/cm2) for 9 h to the surface of hematite electrode, the intensity of the citric acid peak was decreased and a new peak due to acetonedicarboxylic acid appeared at the time of 19.0 min. Figure 21b shows the relationship between concentration of citric acid and irradiation time as well as between concentration of acetonedicarboxylic acid and irradiation time. The concentration of citric acid decreased and that of acetonedicarboxylic acid increased with irradiation time. From this result, the photo-oxidation of citric acid to acetonedicarboxylic acid proceeded on the hematite photoelectrode according to Eq. (29).
Table 1 summarizes the significant values of photocurrent quantum efficiency, quantum efficiency and current efficiency derived from the data of photocurrent measurement and HPLC analysis concerning the photo-oxidation of citric acid on the hematite photoelectrode in aqueous solution under visible light irradiation for 9 h.
|3.04 × 1020||0.273||1.71 × 1018||0.56||1.42||8.65 × 1017||0.28||100|
The HPLC analysis of citric acid under UV light irradiation was carried out to make clear a difference between visible light and UV light affecting the hematite electrode. Figure 22a shows the chromatogram of the aqueous solution of 0.1 M Na2SO4 and citric acid (initial concentration: 200 μM) at the hematite electrode potential of 1.0 V vs. Ag/AgCl before and after irradiation of UV light (wavelength: 360 nm, intensity: 4.2 mW/cm2). A distinct decrease in the intensity of citric acid peak and also a distinct increase in that of acetonedicarboxylic acid peak were observed after UV irradiation to the surface of the hematite electrode for 9 h. Figure 22b shows the relationship between concentration of citric acid and irradiation time as well as between concentration of acetonedicarboxylic acid and irradiation time. The UV light irradiation accelerated the photo-oxidation of citric acid to acetonedicarboxylic acid. Table 2 summarizes the values concerning photocurrent quantum efficiency, quantum efficiency and current efficiency for the photo-oxidation of citric acid to acetonedicarboxylic acid on the hematite electrode under UV light irradiation for 9 h. The higher value of
|2.47 × 1020||1.91||1.19 × 1019||4.82||6.14||3.70 × 1018||1.50||62|
4.2. Photo-oxidation of Pb2+ ion to PbO2 on hematite in acid solution under visible light irradiation
The photo-oxidation treatment of Pb2+ ions in aqueous solution was examined by using hematite for the purpose of elimination of them from the solution [25, 24]. The hematite in this case was prepared from thermal oxidation of iron plate at 600°C for 3 h in air. The cell consisting of the hematite working electrode in 0.1 M HNO3–10 μM Pb(NO3)2 and of the graphite counter electrode in 0.1 M H2SO4–10 mM Ce(SO4)2 aqueous solution was used as a photocell performing without applied voltage. The flow of photocurrent was observed by irradiation of visible light to the hematite electrode. Figure 23 shows the dependence of photocurrent on irradiation time in this cell and also in the cell with the graphite electrode solution of 0.1 M H2SO4–10 mM Fe2(SO4)3 aqueous solution. In this case, the hematite and graphite electrodes acted as a photoanode and a cathode, respectively. It is clear that the presence of Ce4+ in the cathode solution was effective for the performance of the cell based on the hematite photoanode. Since the standard equilibrium potential of Ce4+/Ce3+ system of 1.44 V vs. NHE is more positive than that of Fe3+/Fe2+ system of 0.771 V vs. NHE, Ce4+ may act as a stronger electron acceptor. Figure 24 shows the SEM image of the surface of the hematite electrode before and after irradiation for 6 h in 0.1 M HNO3–10 μM Pb(NO3)2 solution. The photo-deposition of many particles was observed on the surface of hematite. The XRD peak due to PbO2 was confirmed on the hematite after irradiation. This suggests the occurrence of the following photoelectrochemical reactions in Eqs. (30)–(32). The photo-generated hole (h+) and electron (e−) pair was separated to oxidize Pb2+ to PbO2 by hole at the hematite photoanode and reduce Ce4+ to Ce3+ by electron at the graphite cathode according to Eqs. (30) and (31). The total reaction is represented as Eq. (32). With regard to the standard Gibbs free energy change, Δ
4.3. Photo-polymerization of aniline on hematite and characteristics of polyaniline/hematite electrode under visible light irradiation
The photo-polymerization of aniline was carried out by the photoelectrochemical cell consisting of the separated parts of hematite photoanode in 0.1 M HClO4–0.1 M aniline and the graphite cathode in 0.1 M H2SO4–10 mM Ce(SO4)2 aqueous solutions under visible light irradiation . This hematite was prepared from thermal oxidation of iron at 600°C for 3 h in air. These electrolytic solutions were connected by a KCl salt bridge. The deposition of many particles was observed on the surface of the hematite electrode after irradiation. The photoanodic polymerization of aniline due to the photo-generated hole could proceed on the hematite electrode without applied voltage by using Ce4+ ions as an electron acceptor. Because the potential for the bottom of conduction band is positive as understood from the positive
The polyaniline/hematite electrode has a unique property. Figure 25 (a–c) shows the Mott-Schottky plots of the hematite electrode, the polyaniline electrode prepared from anodic deposition of polyaniline film on the glassy carbon and the polyaniline/hematite electrode in 0.1 M aqueous HClO4 solution, respectively, at the frequency of 1 kHz. On the hematite electrode in HClO4 solution, a linear relation due to the n-type semiconductor electrode/electrolytic solution interface was observed. The value of
The photocurrent on the hematite under visible light irradiation decayed immediately with time at less positive potential than 0.70 V vs. Ag/AgCl in aqueous HClO4 solution. This means that the band bending of hematite is not enough in this potential range because of slow transfer of photo-generated hole to water molecule. The polyaniline/hematite electrode showed a stable photocurrent response to visible light at less positive potential than 0.70 V vs. Ag/AgCl in aqueous HClO4 solution. On the polyaniline/hematite electrode, the rapid transfer of photo-generated hole to the polyaniline and simultaneous occurrence of ClO4− ion doping may proceed. The polyaniline/hematite electrode showed a distinct increase in photocurrent in the presence of glycolic acid. The linear relationship between photocurrent and concentration of glycolic acid (1–10 mM) was recognized under visible light irradiation. On the hematite electrode, the linear dependence of photocurrent on concentration was not observed. This implies a possibility of application of the polyaniline/hematite electrode to an amperometric sensor for glycolic acid. Hematite has the demerit that iron dissolution may proceed in acid solution. The amount of iron dissolution after immersion of hematite in 0.1 M aqueous HClO4 solution for 1, 2, 3 and 4 h was 4.89, 11.77, 15.18 and 17.55 ppm, respectively, by atomic absorption analysis. The polyaniline/hematite showed the suppression of iron dissolution. The high stability of the polyaniline/hematite in acid solution was supported by the amount of iron dissolution of 0.01, 0.02, 0.03 and 0.04 ppm after immersion in 0.1 M aqueous HClO4 solution for 1, 2, 3and 4 h, respectively.
As a preparation method of hematite film, the process for electrochemical deposition of iron oxide and its heat treatment in air was mentioned in relation to the equilibrium potential of iron oxide in aqueous solution. The current and potential pulse electrolysis may be useful in deposition of homogeneous iron oxide film. The hematite from the heat treatment of iron oxide at 500°C or higher temperature in air showed a clear photocurrent response and brought the photo-oxidation of chemical species such as citric acid, Pb2+ ion and aniline under visible light irradiation. On the hematite electrode in aqueous solution containing organic materials under visible light irradiation, photo-oxidation processing of organic materials with suppression of water photo-oxidation may be possible. This will lead to application of photo functionality of hematite to a new method for organic synthesis.
I am grateful to Dr. Y. Morinaga, Dr. D. Kodama, Mrs. Y. Itoh, Mr. H. Yoshida, Mr. H. Hamada, Mr. K. Ota and Mr. Y. Kanada for experimental contribution in photoelectrochemistry of hematite.