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).

$$\frac{1}{C^2} = \frac{2}{eN\varepsilon\varepsilon^0} \left( E - E\_{\text{fb}} \right) \tag{28}$$

where e is the quantity of charge on an electron, N the carrier density, ε is the dielectric constant of electrode material, ε<sup>0</sup> is the permittivity of free space and Efb is the flat-band potential corresponding to the potential indicating no band bending of semiconductor electrode.

Figure 12a, b shows the plots of 1/C<sup>2</sup> against E in 0.1 M aqueous Na2SO4 solution (pH = 5.7) for the hematite electrodes prepared from current pulse deposition (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) under O2 bubbling and N2 bubbling for 100 s, respectively. The capacitance measurement was carried out with the frequency of 1 kHz. The values of flat-band potential (Efb) and carrier density (N) of these hematite electrodes were �0.57 V vs. Ag/AgCl (�0.35 V vs. NHE) and 1.35 � 1018 cm�<sup>3</sup> (a), �0.33 V vs. Ag/AgCl (�0.11 V vs. NHE) and 3.53 � 1018 cm�<sup>3</sup> (b) from the intercept of the linear portion extrapolated to the potential axis and its slope by using ε<sup>0</sup> of 120. The Mott-Schottky relation was also confirmed on the hematite electrode

3. Photoelectrochemical characterization of hematite

are mentioned as follows.

for 1 h in air.

158 Iron Ores and Iron Oxide Materials

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

Figure 11. SEM image of the iron oxide film by potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under O2 bubbling (a) and N2 bubbling (b), and heated at 500C

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

3.1. Mott-Schottky relation of hematite electrode/electrolytic solution interface

Figure 12. Plots of 1/C<sup>2</sup> against E in 0.1 M aqueous Na2SO4 solution (pH = 5.7) for the hematite electrodes prepared from current pulse deposition (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) under O2 bubbling (a) and N2 bubbling (b) for 100 s.

prepared from potential pulse deposition (E<sup>c</sup> = 1.0 V vs. Ag/AgCl, E<sup>a</sup> = +0.2 V vs. Ag/AgCl, tc = ta = 1 s) under N2 bubbling for 30 min and heat treatment at 500C for 1 h in air. In this case, the values of Efb of 0.00 V vs. Ag/AgCl (+0.22 V vs. NHE) and <sup>N</sup> of 4.52 1018 cm<sup>3</sup> were obtained in 1.0 M aqueous Na2SO4 solution (pH = 5.9). According to the other researchers, the hematite prepared by thermal oxidation of iron showed Efb of 0.95 V vs. SCE (0.68 V vs. NHE) and <sup>N</sup> of about 3 <sup>10</sup><sup>18</sup> cm<sup>3</sup> in 1.0 M aqueous NaOH solution [1], and that prepared by a spray-pyrolytic method Efb of 0.74 V vs. SCE (0.47 V vs. NHE) and <sup>N</sup> of 2.2 <sup>10</sup><sup>20</sup> cm<sup>3</sup> in 1.0 M NaOH solution [9]. Supposing a pH dependence of Efb was 59 mV/pH, the value of Ffb for above-mentioned hematite in the solution (pH = 5.7) would be 0.19 and 0.02 V vs. NHE. These mean that the values of Efb and N for hematite depend on preparation methods. Because the value of Efb for n-type semiconductor electrode is approximately equal to the potential for the bottom of conduction band, the hematite from current pulse deposition under O2 bubbling to the solution may have more negative potential of the conduction band.

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

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

The photocurrent response of the iron oxide depending on heat treatment temperature (100–600C) in air is shown in Figure 15. In this case, the iron oxide film was prepared from potential pulse deposition (E<sup>c</sup> = 1.0 V, E<sup>a</sup> = +0.2 V, tc = ta = 1 s) under N2 bubbling for 30 min. The iron oxide treated at different temperatures was irradiated with the visible

AgCl in 0.1 M aqueous Na2SO4 solution. The XRD of the film with the corresponding heat treatment temperatures is also shown in Figure 16. No photocurrent was detected on the

Figure 14. Photocurrent response of the hematite prepared from current pulse deposition (Ic = 7 mA, Ia = +1 mA, tc = ta = 1 s) for 100 s in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling (a) and N2 bubbling (b), and

) was supplied

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) at the potential of 1.0 V vs. Ag/

irradiation of the visible light (wavelength: 490 nm, light intensity: 3.8 mW/cm<sup>2</sup>

light (wavelength: 490 nm, light intensity: 3.8 mW/cm<sup>2</sup>

prepared under O2 bubbling.

heated at 600C for 1 h in air.

#### 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 flatband 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 13. Interface of n-type semiconductor electrode/electrolytic solution VB: Valence band, CB: Conduction band, e: Photo-generated electron, h<sup>+</sup> : Photo-generated hole.

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/cm<sup>2</sup> ) 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.

prepared from potential pulse deposition (E<sup>c</sup> = 1.0 V vs. Ag/AgCl, E<sup>a</sup> = +0.2 V vs. Ag/AgCl, tc = ta = 1 s) under N2 bubbling for 30 min and heat treatment at 500C for 1 h in air. In this case, the values of Efb of 0.00 V vs. Ag/AgCl (+0.22 V vs. NHE) and <sup>N</sup> of 4.52 1018 cm<sup>3</sup> were obtained in 1.0 M aqueous Na2SO4 solution (pH = 5.9). According to the other researchers, the hematite prepared by thermal oxidation of iron showed Efb of 0.95 V vs. SCE (0.68 V vs. NHE) and <sup>N</sup> of about 3 <sup>10</sup><sup>18</sup> cm<sup>3</sup> in 1.0 M aqueous NaOH solution [1], and that prepared by a spray-pyrolytic method Efb of 0.74 V vs. SCE (0.47 V vs. NHE) and <sup>N</sup> of 2.2 <sup>10</sup><sup>20</sup> cm<sup>3</sup> in 1.0 M NaOH solution [9]. Supposing a pH dependence of Efb was 59 mV/pH, the value of Ffb for above-mentioned hematite in the solution (pH = 5.7) would be 0.19 and 0.02 V vs. NHE. These mean that the values of Efb and N for hematite depend on preparation methods. Because the value of Efb for n-type semiconductor electrode is approximately equal to the potential for the bottom of conduction band, the hematite from current pulse deposition under O2 bubbling to the solution may have more negative potential of the conduction

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 flatband 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

Figure 13. Interface of n-type semiconductor electrode/electrolytic solution VB: Valence band, CB: Conduction band, e:

: Photo-generated hole.

3.2. Photocurrent response of hematite to visible light irradiation

photoanodic current may flow on the electrode.

Photo-generated electron, h<sup>+</sup>

band.

160 Iron Ores and Iron Oxide Materials

The photocurrent response of the iron oxide depending on heat treatment temperature (100–600C) in air is shown in Figure 15. In this case, the iron oxide film was prepared from potential pulse deposition (E<sup>c</sup> = 1.0 V, E<sup>a</sup> = +0.2 V, tc = ta = 1 s) under N2 bubbling for 30 min. The iron oxide treated at different temperatures was irradiated with the visible light (wavelength: 490 nm, light intensity: 3.8 mW/cm<sup>2</sup> ) at the potential of 1.0 V vs. Ag/ AgCl in 0.1 M aqueous Na2SO4 solution. The XRD of the film with the corresponding heat treatment temperatures is also shown in Figure 16. No photocurrent was detected on the

Figure 14. Photocurrent response of the hematite prepared from current pulse deposition (Ic = 7 mA, Ia = +1 mA, tc = ta = 1 s) for 100 s in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling (a) and N2 bubbling (b), and heated at 600C for 1 h in air.

From this, both maghemite and hematite have a photocurrent response to visible light, but the response of hematite is much higher. Figure 17 shows the photocurrent response of the iron oxide film treated at the temperature of 400–500C. The XRD of the corresponding iron oxide films is shown in Figure 18. All the films showed a photoanodic current response. A marked increase in photocurrent was observed on the film treated at 450C.

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Figure 17. Dependence of photocurrent response of the iron oxide upon heat treatment temperature at 400–500C in air. Iron oxide was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling. The heat treatment temperatures of as-deposited film were

Figure 18. XRD of the iron oxide treated at different temperatures of 400–500C in air. The heat treatment temperatures

400C (a), 410C (b), 420C (c), 430C (d), 450C (e) and 500C (f).

were 400C (a), 410C (b), 420C (c), 430C (d) and 500C (e).

Figure 15. Dependence of photocurrent response of the iron oxide upon heat treatment temperature at 100–600C in air iron oxide was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling. The as-deposited film (a) was heated at different temperatures of 100C (b), 200C (c), 300C (d), 400C (e), 500C (f) and 600C (g).

Figure 16. XRD of the iron oxide with same heat treatment as that in Figure 15. ○ hematite ☐ magnetite ◇ wustite ▽ maghemite.

as-deposited film. This suggests that both magnetite and wustite may have no ability as a photoelectrode. The iron oxide film heat treated at 200C or higher temperature showed a photoanodic current response and a remarked photocurrent was observed on the film heat treated at 500C. It has been reported that the DTA peak of transition from magnetite to maghemite (γ-Fe2O3) and that of transition from maghemite to hematite appeared at about 150 and 480C, respectively, on the thermal analysis for magnetite particles [32]. From this, both maghemite and hematite have a photocurrent response to visible light, but the response of hematite is much higher. Figure 17 shows the photocurrent response of the iron oxide film treated at the temperature of 400–500C. The XRD of the corresponding iron oxide films is shown in Figure 18. All the films showed a photoanodic current response. A marked increase in photocurrent was observed on the film treated at 450C.

Figure 17. Dependence of photocurrent response of the iron oxide upon heat treatment temperature at 400–500C in air. Iron oxide was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling. The heat treatment temperatures of as-deposited film were 400C (a), 410C (b), 420C (c), 430C (d), 450C (e) and 500C (f).

Figure 18. XRD of the iron oxide treated at different temperatures of 400–500C in air. The heat treatment temperatures were 400C (a), 410C (b), 420C (c), 430C (d) and 500C (e).

as-deposited film. This suggests that both magnetite and wustite may have no ability as a photoelectrode. The iron oxide film heat treated at 200C or higher temperature showed a photoanodic current response and a remarked photocurrent was observed on the film heat treated at 500C. It has been reported that the DTA peak of transition from magnetite to maghemite (γ-Fe2O3) and that of transition from maghemite to hematite appeared at about 150 and 480C, respectively, on the thermal analysis for magnetite particles [32].

Figure 16. XRD of the iron oxide with same heat treatment as that in Figure 15. ○ hematite ☐ magnetite ◇ wustite ▽

Figure 15. Dependence of photocurrent response of the iron oxide upon heat treatment temperature at 100–600C in air iron oxide was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling. The as-deposited film (a) was heated at different temper-

atures of 100C (b), 200C (c), 300C (d), 400C (e), 500C (f) and 600C (g).

162 Iron Ores and Iron Oxide Materials

maghemite.

The intensity of maghemite XRD peaks on the films decreased with heat treatment temperature from 400 to 420C and hematite peaks appeared on the film treated at 430C as shown in Figure 18.

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 500C. 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/cm<sup>2</sup> ) to the hematite, the onset potential of photoanodic current was about 0.0 V vs. Ag/AgCl, almost equal to the value of Ffb. In the irradiation of visible light (wavelength: 490 nm, intensity: 4.8 mW/cm<sup>2</sup> ), the onset potential was more positive than Ffb. This means a necessity of fair band bending for the separation of photogenerated electron-hole pair in the absorption of visible light.

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 500C. The photocurrent quantum efficiency (ηelec) represents the ratio of the number of electrons for photocurrent to photon number of incident light. The value of ηelec in the presence of hydroxyl acid was much higher than that in aqueous Na2SO4 solution containing no hydroxyl acids. The values of ηelec at wavelength in the range of 400–500 nm were 22–7, 17–5 and 14–4% in the presence of citric acid, tartaric acid and malic acid, respectively. The lower value of 5–2% in aqueous Na2SO4 solution may reflect a slow transfer of photo-generated holes in the valence band of hematite to water molecules. The highest photocurrent response obtained in the presence of citric acid was probably due to a rapid hole transfer to citric acid molecules. According to the reports [33, 34] on zinc oxide and

rutile photoelectrodes in aqueous formic acid solution, a distinct increase in photocurrent could be observed because formic acid might act as the hole scavenger of these photoelectrodes. But, a clear increase in photocurrent was not observed on the hematite photoelectrode in formic

Figure 20. Relationship between photocurrent quantum efficiency and wavelength of irradiation light on the hematite at 1.0 V vs. Ag/AgCl in 0.1 M aqueous Na2SO4 solution containing 1 mM hydroxyl acid. Hematite was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.15 M NaCl solution under N2 bubbling, and heated at 500C for 1 h in air. Each of citric acid (a), tartaric acid (b) and malic acid

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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+

4.1. Photo-oxidation of citric acid on hematite in aqueous solution under visible light

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 [29]. This hematite was

4. Photoreaction of chemical species on hematite photoelectrode

ion and aniline on the hematite photoelectrode.

acid solution.

(c) was added to aqueous Na2SO4 solution (d).

irradiation

Figure 19. Relationship between electrode potential and photocurrent on the hematite in 0.1 M aqueous Na2SO4 solution under irradiation of visible light and UV light. Hematite was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling, and heated at 500C for 1 h in air. Three relation curves of a, b and c are corresponding to no irradiation, visible light and UV light irradiation, respectively.

The intensity of maghemite XRD peaks on the films decreased with heat treatment temperature from 400 to 420C and hematite peaks appeared on the film treated at 430C as

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 500C. 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

anodic current was about 0.0 V vs. Ag/AgCl, almost equal to the value of Ffb. In the irradiation

positive than Ffb. This means a necessity of fair band bending for the separation of photo-

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 500C. The photocurrent quantum efficiency (ηelec) represents the ratio of the number of electrons for photocurrent to photon number of incident light. The value of ηelec in the presence of hydroxyl acid was much higher than that in aqueous Na2SO4 solution containing no hydroxyl acids. The values of ηelec at wavelength in the range of 400–500 nm were 22–7, 17–5 and 14–4% in the presence of citric acid, tartaric acid and malic acid, respectively. The lower value of 5–2% in aqueous Na2SO4 solution may reflect a slow transfer of photo-generated holes in the valence band of hematite to water molecules. The highest photocurrent response obtained in the presence of citric acid was probably due to a rapid hole transfer to citric acid molecules. According to the reports [33, 34] on zinc oxide and

Figure 19. Relationship between electrode potential and photocurrent on the hematite in 0.1 M aqueous Na2SO4 solution under irradiation of visible light and UV light. Hematite was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling, and heated at 500C for 1 h in air. Three relation curves of a, b and c are corresponding to no irradiation, visible light and UV light

) to the hematite, the onset potential of photo-

), the onset potential was more

shown in Figure 18.

164 Iron Ores and Iron Oxide Materials

irradiation, respectively.

(wavelength: 365 nm, intensity: 5.5 mW/cm<sup>2</sup>

of visible light (wavelength: 490 nm, intensity: 4.8 mW/cm<sup>2</sup>

generated electron-hole pair in the absorption of visible light.

Figure 20. Relationship between photocurrent quantum efficiency and wavelength of irradiation light on the hematite at 1.0 V vs. Ag/AgCl in 0.1 M aqueous Na2SO4 solution containing 1 mM hydroxyl acid. Hematite was prepared from potential pulse deposition (Ec = 1.0 V, Ea = 0.2 V vs. Ag/AgCl, tc = ta = 1 s) for 30 min in aqueous 10 mM FeCl2–0.15 M NaCl solution under N2 bubbling, and heated at 500C for 1 h in air. Each of citric acid (a), tartaric acid (b) and malic acid (c) was added to aqueous Na2SO4 solution (d).

rutile photoelectrodes in aqueous formic acid solution, a distinct increase in photocurrent could be observed because formic acid might act as the hole scavenger of these photoelectrodes. But, a clear increase in photocurrent was not observed on the hematite photoelectrode in formic acid solution.
