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 [29]. This hematite was

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

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

CH2ð Þ COOH C OH ð Þð Þ COOH CH2ð Þ! COOH CH2ð Þ COOH COCH2ð Þ COOH

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. Nh<sup>ν</sup> is the photon number of incident light to the surface of hematite electrode, Qelec is the amount of electric quantity for the photocurrent flowing during irradiation, Nelec is the number of electrons from Qelec, ηelec is the photocurrent quantum efficiency represented as the term of (Nelec/Nhν) �100, ΔC is the change in concentration of citric acid during photo-oxidation from HPLC data, Nmole is the number of citric acid molecules oxidized, ηmole is the quantum efficiency represented as the term of (Nmole/Nhν) � 100 and Ieffi is the current efficiency in the photo-oxidation process of citric acid to acetonedicarboxylic acid. The value of Ieffi was evaluated from the term of (2ηmole/ηelec) �100 by consideration of Eq. (29). The value of 100.0% was obtained as Ieffi in the process under visible light irradiation. All the photocurrent was derived from the photo-oxidation of citric acid to acetonedicarboxylic acid. This means that the holes photo-generated by visible light could

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

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.

Nh<sup>ν</sup> Qelec (C) Nelec ηelec (%) ΔC (μmol) Nmole ηmole (%) Ieff (%) 3.04 � 1020 0.273 1.71 � <sup>10</sup><sup>18</sup> 0.56 1.42 8.65 � <sup>10</sup><sup>17</sup> 0.28 100

Table 1. 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:

after irradiation of UV light (wavelength: 360 nm, intensity: 4.2 mW/cm<sup>2</sup>

<sup>þ</sup>CO2 <sup>þ</sup> 2H<sup>þ</sup> <sup>þ</sup> 2e� (29)

) for 9 h to the surface of hematite electrode,

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). A distinct decrease

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

oxidize only the citric acid molecules.

Eq. (29).

3.8 mW/cm2

) for 9 h.

Figure 21. A chromatograms of the citric acid solution before and after irradiation of visible light (wavelength: 490 nm, intensity: 3.8 mW/cm2 ) to the hematite electrode at 1.0 V vs. Ag/AgCl for 9 hours, b: Relationship among concentration of citric acid, concentration of acetonedicarboxylic acid and irradiation time. Hematite was 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, and heated at 600C for 1 h in air.

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 600C 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/cm<sup>2</sup> ) 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).

$$\begin{aligned} \text{CH}\_2(\text{COOH})\text{C}(\text{OH})(\text{COOH})\text{CH}\_2(\text{COOH}) &\rightarrow \text{CH}\_2(\text{COOH})\text{COCH}\_2(\text{COOH})\\ + \text{CO}\_2 + 2\text{H}^+ + 2\text{e}^- \end{aligned} \tag{29}$$

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. Nh<sup>ν</sup> is the photon number of incident light to the surface of hematite electrode, Qelec is the amount of electric quantity for the photocurrent flowing during irradiation, Nelec is the number of electrons from Qelec, ηelec is the photocurrent quantum efficiency represented as the term of (Nelec/Nhν) �100, ΔC is the change in concentration of citric acid during photo-oxidation from HPLC data, Nmole is the number of citric acid molecules oxidized, ηmole is the quantum efficiency represented as the term of (Nmole/Nhν) � 100 and Ieffi is the current efficiency in the photo-oxidation process of citric acid to acetonedicarboxylic acid. The value of Ieffi was evaluated from the term of (2ηmole/ηelec) �100 by consideration of Eq. (29). The value of 100.0% was obtained as Ieffi in the process under visible light irradiation. All the photocurrent was derived from the photo-oxidation of citric acid to acetonedicarboxylic acid. This means that the holes photo-generated by visible light could oxidize only the citric acid molecules.

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


Table 1. 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.

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 600C 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

Figure 21. A chromatograms of the citric acid solution before and after irradiation of visible light (wavelength: 490 nm,

citric acid, concentration of acetonedicarboxylic acid and irradiation time. Hematite was 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

) to the hematite electrode at 1.0 V vs. Ag/AgCl for 9 hours, b: Relationship among concentration of

intensity: 3.8 mW/cm2

166 Iron Ores and Iron Oxide Materials

bubbling, and heated at 600C for 1 h in air.

hematite electrode under UV light irradiation for 9 h. The higher value of ηelec and ηmole means that the UV light could facilitate a transfer of hole of the hematite to the citric acid molecule. This may be ascribed to the photo-generation of a pair of hole and excited electron in the vicinity of the surface of the hematite irradiated with UV light. The Ieffi value of 62% suggests the occurrence of the photo-oxidation of not only citric acid but also water. This reflects the competitive process of hole transfer to the molecules of chemical species and water in aqueous solution. The Ieffi value of 100% in the visible light irradiation could be interpreted in terms of the photo-generation of a pair of hole and excited electron at the inside of space charge layer due to a deeper penetration of visible light. In this case, the hole moving from the inside to the surface might prefer citric acid molecule to water molecule probably from the aspect of oxidation rate. The hematite also showed a distinct photo-oxidation response to other hydroxyl acids such as tartaric acid, malic acid and glycolic acid. These results imply possibility of photo-oxidation synthesis of organic materials by using hematite under visible light

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4.2. Photo-oxidation of Pb2+ ion to PbO2 on hematite in acid solution under visible light

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 600C 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 photodeposition 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<sup>+</sup>

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,

, for reaction (32), the positive value 48.2 kJ/mol could be evaluated by using the standard equilibrium potential of Pb4+/Pb2+, 1.69 V and that of Ce4+/Ce3+, 1.44 V vs. NHE. This means that the reaction (32) is not spontaneous reaction and the light energy causes it to proceed. The hematite prepared from thermal oxidation of iron showed the value of Efb of 0.31 V vs. NHE in the 0.1 M aqueous HNO3 solution. Therefore, the position of valence band edge of hematite

) and

irradiation.

irradiation

ΔG<sup>0</sup>

Figure 22. A chromatograms of the citric acid solution before and after irradiation of UV light (wavelength: 360 nm, intensity: 4.2 mW/cm2 ) to the hematite electrode at 1.0 V vs. Ag/AgCl for 9 hours, b: Relationship among concentration of citric acid, concentration of acetonedicarboxylic acid and irradiation time. Hematite was prepared by the same method as that in Figure 21.

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


Table 2. 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 UV light (wavelength: 360 nm, intensity: 4.2 mW/cm2 ) for 9 h.

hematite electrode under UV light irradiation for 9 h. The higher value of ηelec and ηmole means that the UV light could facilitate a transfer of hole of the hematite to the citric acid molecule. This may be ascribed to the photo-generation of a pair of hole and excited electron in the vicinity of the surface of the hematite irradiated with UV light. The Ieffi value of 62% suggests the occurrence of the photo-oxidation of not only citric acid but also water. This reflects the competitive process of hole transfer to the molecules of chemical species and water in aqueous solution. The Ieffi value of 100% in the visible light irradiation could be interpreted in terms of the photo-generation of a pair of hole and excited electron at the inside of space charge layer due to a deeper penetration of visible light. In this case, the hole moving from the inside to the surface might prefer citric acid molecule to water molecule probably from the aspect of oxidation rate. The hematite also showed a distinct photo-oxidation response to other hydroxyl acids such as tartaric acid, malic acid and glycolic acid. These results imply possibility of photo-oxidation synthesis of organic materials by using hematite under visible light irradiation.
