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 [27]. This hematite was prepared from thermal oxidation of iron at 600C 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 Efb value of hematite, the presence of a strong oxidizing agent as an electron acceptor may be necessary for occurrence of photoelectrochemical reaction on the hematite without application of electric power.

could be regarded approximately as 2.3 V vs. NHE in the solution (pH = 1) by referring to the band gap energy of 2.0 eV. The occurrence of photo-oxidation of Pb2+ to PbO2 deposition on hematite may be supported by this positive potential of valence band edge. For 0.1 M aqueous HNO3 solution containing 10 μM Pb(NO3)2, the removal rate of Pb2+ ions due to visible light irradiation was checked by atomic absorption analysis. In the case of using Ce4+ electron acceptor without applied voltage, 8.9% of the initial concentration of Pb2+ was removed from the solution after 6 h irradiation. By holding the potential of hematite at 1.50 V vs. Ag/AgCl, a marked increase in the removal rate was confirmed. The values of 27.0, 38.7 and 53.2% were

Figure 23. Dependence of photocurrent on irradiation time in the cell consisting of the hematite 0.1 M HNO3–10 μM Pb (NO3)2 and of the graphite cathode in 0.1 M H2SO4–10 mM Ce(SO4)2 solution (a) and also in the cell of the photoanode in the same solution and of the cathode in 0.1 M H2SO4–10 mM Fe2(SO4)3 solution (b). Hematite was prepared from thermal

Pb2<sup>þ</sup> <sup>þ</sup> 2H2O <sup>þ</sup> 2 h<sup>þ</sup> ! PbO2 <sup>þ</sup> 4H<sup>þ</sup> (30)

Pb2<sup>þ</sup> <sup>þ</sup> 2H2O <sup>þ</sup> 2Ce4<sup>þ</sup> ! PbO2 <sup>þ</sup> 4H<sup>þ</sup> <sup>þ</sup> 2Ce<sup>3</sup><sup>þ</sup> (32)

2Ce4<sup>þ</sup> <sup>þ</sup> 2e� ! 2Ce3<sup>þ</sup> (31)

obtained after 2, 4 and 6 h irradiation, respectively.

oxidation of iron at 600�C for 3 h in air.

170 Iron Ores and Iron Oxide Materials

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 Efb of the hematite in this solution by extrapolation of the linear line was 0.13 V vs. Ag/AgCl. On the polyaniline electrode in HClO4 solution, a linear relation due to the p-type semiconductor electrode/ electrolytic solution interface was observed. The state of undoped polyaniline could be regarded as the p-type semiconductor. The value of Efb of undoped polyaniline in this solution was 0.20 V vs. Ag/AgCl. On the polyaniline/hematite electrode in HClO4 solution, the two linear relations were observed. The linear portion at less positive potential than 0.10 V vs. Ag/AgCl was ascribed to the undoped polyaniline/electrolytic solution interface. Because the doped polyaniline has high electric conductivity to be regarded as metal, the linear portion at more positive potential than 0.12 V vs. Ag/AgCl reflects the

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

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

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

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

ion

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hematite/doped polyaniline interface.

HClO4 solution for 1, 2, 3and 4 h, respectively.

hematite to a new method for organic synthesis.

5. Conclusion

at 500

Figure 25. Mott-Schottky plots of the hematite electrode (a), the polyaniline-glassy carbon electrode (b) and the polyaniline-hematite electrode (c) in 0.1 M aqueous HClO4 solution. Hematite was prepared from thermal oxidation of iron at 600C for 3 h in air.

electrode in HClO4 solution, a linear relation due to the n-type semiconductor electrode/ electrolytic solution interface was observed. The value of Efb of the hematite in this solution by extrapolation of the linear line was 0.13 V vs. Ag/AgCl. On the polyaniline electrode in HClO4 solution, a linear relation due to the p-type semiconductor electrode/ electrolytic solution interface was observed. The state of undoped polyaniline could be regarded as the p-type semiconductor. The value of Efb of undoped polyaniline in this solution was 0.20 V vs. Ag/AgCl. On the polyaniline/hematite electrode in HClO4 solution, the two linear relations were observed. The linear portion at less positive potential than 0.10 V vs. Ag/AgCl was ascribed to the undoped polyaniline/electrolytic solution interface. Because the doped polyaniline has high electric conductivity to be regarded as metal, the linear portion at more positive potential than 0.12 V vs. Ag/AgCl reflects the hematite/doped polyaniline interface.

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.
