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

Figure 1. Current pulse deposition of iron oxide film with repetition of cathodic pulse (current: Ic, time: tc = 1 s) and anodic pulse (current: Ia, time: ta = 1 s).

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].

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

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

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

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

Figure 1. Current pulse deposition of iron oxide film with repetition of cathodic pulse (current: Ic, time: tc = 1 s) and

) was prepared on a titanium substrate by

2. Electrochemical preparation of iron oxide film

2.1. Current pulse deposition of iron oxide film

An iron oxide film (geometric surface area of 1.0 cm2

conversion.

obtained [24–29].

150 Iron Ores and Iron Oxide Materials

mentioned as follows.

anodic pulse (current: Ia, time: ta = 1 s).

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).

$$\text{1}/\text{2O}\_2 + \text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{2OH}^- \tag{1}$$

$$\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}\_2 \tag{2}$$

$$\text{Fe(OH)}\_{2} \rightarrow \text{FeO} + \text{H}\_{2}\text{O} \tag{3}$$

$$\text{3FeO} + \text{H}\_2\text{O} \rightarrow \text{Fe}\_3\text{O}\_4 + 2\text{H}^+ + 2\text{e}^- \tag{4}$$

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.

Figure 3. XRD of the iron oxide film deposited on the titanium by current pulse method (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling for 100 s (the lower pattern), and heated at 600�C for 1 h in air (the upper pattern). ● titanium ○ hematite ☐ magnetite ◇ wustite ■ titanium dioxide (rutile).

$$\text{2FeO} + 1/2\text{O}\_2 \rightarrow \text{Fe}\_2\text{O}\_3\tag{5}$$

$$2/3\text{Fe}\_3\text{O}\_4 + 1/6\text{O}\_2 \to \text{Fe}\_2\text{O}\_3\tag{6}$$

Figure 4. SEM image of the iron oxide film by current pulse deposition (Ic = 7 mA, Ia = +1 mA, tc = ta = 1 s) for 100 s (a) and galvanostatic reduction (current: 7 mA) for 50 s (b) in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2

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Figure 5. 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 N2 bubbling for 100 s.

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

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)).

$$\text{Fe}^{2+} + 2\text{e}^- \rightarrow \text{Fe} \tag{7}$$

$$\text{Fe} + \text{H}\_2\text{O} \rightarrow \text{FeO} + 2\text{H}^+ + 2\text{e}^- \tag{8}$$

2FeO þ 1=2O2 ! Fe2O3 (5)

Fe2<sup>þ</sup> <sup>þ</sup> 2e� ! Fe (7)

Fe þ H2O ! FeO þ 2H<sup>þ</sup> þ 2e� (8)

2=3Fe3O4 þ 1=6O2 ! Fe2O3 (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

Figure 3. XRD of the iron oxide film deposited on the titanium by current pulse method (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling for 100 s (the lower pattern), and heated at 600�C for

1 h in air (the upper pattern). ● titanium ○ hematite ☐ magnetite ◇ wustite ■ titanium dioxide (rutile).

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

homogeneous deposition compared with the film by the current pulse method.

its thermal oxidation process (Eqs. (10)–(12)).

152 Iron Ores and Iron Oxide Materials

Figure 4. SEM image of the iron oxide film by current pulse deposition (Ic = 7 mA, Ia = +1 mA, tc = ta = 1 s) for 100 s (a) and galvanostatic reduction (current: 7 mA) for 50 s (b) in aqueous 10 mM FeCl2–0.15 M NaCl solution under O2 bubbling, and heated at 600C for 1 h in air.

Figure 5. 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 N2 bubbling for 100 s.

Eeq O2=OH� ð Þ¼ 0:401 � 0:0591 � log <sup>10</sup>aOH� (13)

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Eeqð Þ¼� Fe3O4=FeO 0:220 � 0:0591 � pH (14)

Eeq Fe<sup>2</sup>þ=Fe ¼ �0:<sup>473</sup> <sup>þ</sup> <sup>0</sup>:<sup>0296</sup> � log <sup>10</sup>aFe2<sup>þ</sup> (15)

Eeqð Þ¼� FeO=Fe 0:042 � 0:0591 � pH (16)

Eeqð Þ¼� Fe3O4=Fe 0:086 � 0:0591 � pH (17)

3Fe<sup>2</sup><sup>þ</sup> <sup>þ</sup> 4H2O ! Fe3O4 <sup>þ</sup> 8H<sup>þ</sup> <sup>þ</sup> 2e� (18)

2FeO þ H2O ! Fe2O3 þ 2H<sup>þ</sup> þ 2e� (20)

2Fe þ 3H2O ! Fe2O3 þ 6H<sup>þ</sup> þ 6e� (21)

2Fe3O4 þ H2O ! 3Fe2O3 þ 2H<sup>þ</sup> þ 2e� (22)

2Fe<sup>2</sup><sup>þ</sup> <sup>þ</sup> 3H2O ! Fe2O3 <sup>þ</sup> 6H<sup>þ</sup> <sup>þ</sup> 2e� (23)

Eeqð Þ¼� Fe2O3=FeO 0:083 � 0:0591 � pH (24)

Eeqð Þ¼� Fe2O3=Fe 0:055 � 0:0591 � pH (25)

EeqðFe2O3=Fe3O4Þ ¼ 0:192 � 0:0591 � pH (26)

Eeq Fe2O3=Fe2<sup>þ</sup> <sup>¼</sup> <sup>0</sup>:<sup>779</sup> � <sup>0</sup>:<sup>1773</sup> � <sup>p</sup><sup>H</sup> � <sup>0</sup>:<sup>0591</sup> � log <sup>10</sup>aFe2<sup>þ</sup> (27)

Eeq Fe3O4=Fe2<sup>þ</sup> <sup>¼</sup> <sup>1</sup>:<sup>072</sup> � <sup>0</sup>:<sup>2364</sup> � <sup>p</sup><sup>H</sup> � <sup>0</sup>:<sup>0886</sup> � log <sup>10</sup>aFe2<sup>þ</sup> (19)

The electrochemical formation of magnetite could be also considered in Eq. (18) and this

The corresponding equilibrium potential evaluated by using the standard chemical potential

These standard equilibrium potentials are slightly different from the values in Pourbaix Diagram [31] 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, Eeq(O2/OH�) is 0.969 (0.747), <sup>E</sup>eq (Fe3O4/FeO) -0.480 (�0.702), <sup>E</sup>eq (Fe2+/Fe) -0.532 (�0.754), <sup>E</sup>eq (FeO/Fe) -0.302 (�0.524), <sup>E</sup>eq (Fe3O4/Fe) -0.346 (�0.568), <sup>E</sup>eq (Fe3O4/Fe2+) 0.209 (�0.013), <sup>E</sup>eq (Fe2O3/FeO) -0.343 (�0.565), <sup>E</sup>eq (Fe2O3/Fe) -0.315 (�0.537), <sup>E</sup>eq (Fe2O3/Fe3O4) -0.068 (�0.290) and <sup>E</sup>eq (Fe2O3/Fe2+) 0.117 V vs. NHE (�0.105 V vs. Ag/AgCl). The electrode potential approached to the equilibrium potential of Fe3O4/FeO system with repetition of anodic current pulse as shown in

The electrochemical reactions concerning hematite are shown in Eqs. (20)–(23).

of hematite, �743.608 KJ/mol are represented in Eqs. (24)–(27).

Figures 3 and 5.

equilibrium potential is represented as Eq. (19).

Figure 6. XRD of the iron oxide film deposited on the titanium by current pulse method (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.15 M NaCl solution under N2 bubbling for 100 s (the lower pattern), and heated at 600�C for 1 h in air (the upper pattern). ○ hematite ☐ magnetite ◇ wustite △ iron ■ titanium dioxide (rutile).

Figure 7. SEM image of the iron oxide film by 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 N2 bubbling, and heated at 600�C for 1 h in air.

$$\rm{3Fe} + 4H\_2O \to \rm{Fe\_3O\_4} + 8H^+ + 8e^- \tag{9}$$

$$2\text{Fe} + 3/2\text{O}\_2 \rightarrow \text{Fe}\_2\text{O}\_3\tag{10}$$

$$\text{2FeO} + 1/2\text{O}\_2 \rightarrow \text{Fe}\_2\text{O}\_3\tag{11}$$

$$2/3\text{Fe}\_3\text{O}\_4 + 1/6\text{O}\_2 \to \text{Fe}\_2\text{O}\_3\tag{12}$$

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 [30] as <sup>μ</sup><sup>0</sup> (H2O), μ<sup>0</sup> (OH�), μ<sup>0</sup> (Fe2+), μ<sup>0</sup> (FeO) and μ<sup>0</sup> (Fe3O4), respectively. The equilibrium potentials (versus normal hydrogen electrode, NHE) of (1), (4), (7), (8) and (9) are represented in Eq. (13)–(17).

#### Photoelectrochemistry of Hematite http://dx.doi.org/10.5772/intechopen.73228 155

$$E\_{eq}(\text{O}\_2/\text{OH}^-) = 0.401 - 0.0591 \times \log\_{10} a\_{\text{OH}^-} \tag{13}$$

$$E\_{\rm eq}(\rm Fe\_3O\_4/FeO) = -0.220 - 0.0591 \times pH \tag{14}$$

$$E\_{\rm eq}(\rm Fe^{2+}/\rm Fe) = -0.473 + 0.0296 \times \log\_{10} a\_{\rm Fe^{2+}} \tag{15}$$

$$E\_{\rm eq}(\rm FeO/Fe) = -0.042 - 0.0591 \times pH \tag{16}$$

$$E\_{eq}(\text{Fe}\_3\text{O}\_4/\text{Fe}) = -0.086 - 0.0591 \times pH \tag{17}$$

The electrochemical formation of magnetite could be also considered in Eq. (18) and this equilibrium potential is represented as Eq. (19).

$$\rm{3Fe^{2+}} + 4H\_2O \rightarrow \rm{Fe\_3O\_4} + 8H^+ + 2e^- \tag{18}$$

$$E\_{\text{eq}}\text{(Fe}\_3\text{O}\_4/\text{Fe}^{2+}) = 1.072 - 0.2364 \times p\text{H} - 0.0886 \times \log\_{10} a\_{\text{Fe}^{2+}}\tag{19}$$

The electrochemical reactions concerning hematite are shown in Eqs. (20)–(23).

3Fe þ 4H2O ! Fe3O4 þ 8H<sup>þ</sup> þ 8e� (9)

2Fe þ 3=2O2 ! Fe2O3 (10)

2FeO þ 1=2O2 ! Fe2O3 (11)

(H2O), μ<sup>0</sup>

(OH�), μ<sup>0</sup>

(Fe2+), μ<sup>0</sup>

(FeO)

2=3Fe3O4 þ 1=6O2 ! Fe2O3 (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,

Figure 7. SEM image of the iron oxide film by 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 N2 bubbling, and heated at 600�C for 1 h in air.

Figure 6. XRD of the iron oxide film deposited on the titanium by current pulse method (Ic = �7 mA, Ia = +1 mA, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.15 M NaCl solution under N2 bubbling for 100 s (the lower pattern), and heated at 600�C for

1 h in air (the upper pattern). ○ hematite ☐ magnetite ◇ wustite △ iron ■ titanium dioxide (rutile).

(Fe3O4), respectively. The equilibrium potentials (versus normal hydrogen electrode,

�157.293, �91.2, �245.211 and �1015.359 KJ/mol [30] as <sup>μ</sup><sup>0</sup>

NHE) of (1), (4), (7), (8) and (9) are represented in Eq. (13)–(17).

and μ<sup>0</sup>

154 Iron Ores and Iron Oxide Materials

$$2\text{FeO} + \text{H}\_2\text{O} \to \text{Fe}\_2\text{O}\_3 + 2\text{H}^+ + 2\text{e}^- \tag{20}$$

$$\text{H}\cdot2\text{Fe} + 3\text{H}\_2\text{O} \rightarrow \text{Fe}\_2\text{O}\_3 + 6\text{H}^+ + 6\text{e}^- \tag{21}$$

$$\rm 2Fe\_3O\_4 + H\_2O \to 3Fe\_2O\_3 + 2H^+ + 2e^- \tag{22}$$

$$\text{2Fe}^{2+} + \text{3H}\_2\text{O} \rightarrow \text{Fe}\_2\text{O}\_3 + 6\text{H}^+ + 2\text{e}^- \tag{23}$$

The corresponding equilibrium potential evaluated by using the standard chemical potential of hematite, �743.608 KJ/mol are represented in Eqs. (24)–(27).

$$E\_{eq}(\text{Fe}\_2\text{O}\_3/\text{FeO}) = -0.083 - 0.0591 \times pH \tag{24}$$

$$E\_{eq}(\text{Fe}\_2\text{O}\_3/\text{Fe}) = -0.055 - 0.0591 \times p\text{H} \tag{25}$$

$$E\_{\rm eq}(\rm Fe\_2O\_3/Fe\_3O\_4) = 0.192 - 0.0591 \times \rm pH \tag{26}$$

$$E\_{eq} \text{(Fe}\_2\text{O}\_3/\text{Fe}^{2+}) = 0.779 - 0.1773 \times p\text{H} - 0.0591 \times \log\_{10} a\_{\text{Fe}^{2+}} \tag{27}$$

These standard equilibrium potentials are slightly different from the values in Pourbaix Diagram [31] 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, Eeq(O2/OH�) is 0.969 (0.747), <sup>E</sup>eq (Fe3O4/FeO) -0.480 (�0.702), <sup>E</sup>eq (Fe2+/Fe) -0.532 (�0.754), <sup>E</sup>eq (FeO/Fe) -0.302 (�0.524), <sup>E</sup>eq (Fe3O4/Fe) -0.346 (�0.568), <sup>E</sup>eq (Fe3O4/Fe2+) 0.209 (�0.013), <sup>E</sup>eq (Fe2O3/FeO) -0.343 (�0.565), <sup>E</sup>eq (Fe2O3/Fe) -0.315 (�0.537), <sup>E</sup>eq (Fe2O3/Fe3O4) -0.068 (�0.290) and <sup>E</sup>eq (Fe2O3/Fe2+) 0.117 V vs. NHE (�0.105 V vs. Ag/AgCl). The electrode potential approached to the equilibrium potential of Fe3O4/FeO system with repetition of anodic current pulse as shown in Figures 3 and 5.

#### 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 [28]. In this case, electrochemical reduction and oxidation occurs with repetition of a periodic change in the working electrode potential between cathodic potential (Ec) with the time of tc and anodic potential (Ea) with ta. The working electrode of titanium substrate and the counter electrode of graphite were connected to a potentio-galvanostat with a function generator. The aqueous solution of 10 mM FeCl2–0.1 M KCl (pH = 4.4) under O2 or N2 gas bubbling was used for the electrochemical deposition of iron oxide film.

Figure 9 shows the XRD of the film prepared by potential pulse deposition (E<sup>c</sup> = 1.0 V vs. Ag/ AgCl, tc = 1 s, E<sup>a</sup> = 0.2 V vs. Ag/AgCl, ta = 1 s) for 30 min under O2 bubbling to the solution, the upper representing the pattern of the film after heat treatment at the temperature of 500C for 1 h in air and the lower that before heat treatment. The magnetite peaks and the wustite peak were observed on the as-deposited film. Because the value of 0.2 V vs. Ag/AgCl of E<sup>a</sup> is more positive than the equilibrium potentials of Eeq (Fe2O3./FeO) of 0.565, Eeq (Fe2O3/Fe) of 0.537, <sup>E</sup>eq (Fe2O3/Fe3O4) of 0.290 and <sup>E</sup>eq (Fe2O3/Fe2+) of 0.105 V vs. Ag/AgCl, there is a possibility of anodic formation of hematite. But the hematite peaks did not appear on the as-deposited film. The hematite structure was confirmed after heat treatment. Figure 10 shows the XRD of the film by potential pulse deposition under N2 bubbling to the solution before and after heat treatment. The peaks of magnetite and wustite and hematite peaks were observed on the film before and after heat treatment, respectively. Figure 11 shows the SEM images of the hematite films prepared from the solution with O2 bubbling and with N2 bubbling. A different morphology of these hematite films was observed. The deposition state of iron oxide film was

Figure 8. Potential pulse deposition of iron oxide film with repetition of cathodic pulse (potential: Ec, time: tc) and anodic pulse (potential: Ea, time: ta).

dependent on current magnitude, potential value and pulse width. A photocurrent response of hematite film was strongly related to the deposition state of iron oxide by current and potential

Figure 10. XRD of the iron oxide film deposited on the titanium by potential pulse method (Ec = 1.0 V, Ea = 0.2 V vs. Ag/ AgCl, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling for 30 min (the lower pattern), and

Figure 9. XRD of the iron oxide film deposited on the titanium by potential pulse method (Ec = 1.0 V, Ea = 0.2 V vs. Ag/ AgCl, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.1 M KCl solution under O2 bubbling for 30 min (the lower pattern), and

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heated at 500C for 1 h in air (the upper pattern). ○ hematite ☐ magnetite ◇ wustite.

heated at 500C for 1 h in air (the upper pattern). ○ hematite ☐ magnetite ◇ wustite.

methods.

2.2. Potential pulse deposition of iron oxide film

156 Iron Ores and Iron Oxide Materials

electrochemical deposition of iron oxide film.

pulse (potential: Ea, time: ta).

Potential pulse method as shown in Figure 8 is also useful in preparation of iron oxide film [28]. In this case, electrochemical reduction and oxidation occurs with repetition of a periodic change in the working electrode potential between cathodic potential (Ec) with the time of tc and anodic potential (Ea) with ta. The working electrode of titanium substrate and the counter electrode of graphite were connected to a potentio-galvanostat with a function generator. The aqueous solution of 10 mM FeCl2–0.1 M KCl (pH = 4.4) under O2 or N2 gas bubbling was used for the

Figure 9 shows the XRD of the film prepared by potential pulse deposition (E<sup>c</sup> = 1.0 V vs. Ag/ AgCl, tc = 1 s, E<sup>a</sup> = 0.2 V vs. Ag/AgCl, ta = 1 s) for 30 min under O2 bubbling to the solution, the upper representing the pattern of the film after heat treatment at the temperature of 500C for 1 h in air and the lower that before heat treatment. The magnetite peaks and the wustite peak were observed on the as-deposited film. Because the value of 0.2 V vs. Ag/AgCl of E<sup>a</sup> is more positive than the equilibrium potentials of Eeq (Fe2O3./FeO) of 0.565, Eeq (Fe2O3/Fe) of 0.537, <sup>E</sup>eq (Fe2O3/Fe3O4) of 0.290 and <sup>E</sup>eq (Fe2O3/Fe2+) of 0.105 V vs. Ag/AgCl, there is a possibility of anodic formation of hematite. But the hematite peaks did not appear on the as-deposited film. The hematite structure was confirmed after heat treatment. Figure 10 shows the XRD of the film by potential pulse deposition under N2 bubbling to the solution before and after heat treatment. The peaks of magnetite and wustite and hematite peaks were observed on the film before and after heat treatment, respectively. Figure 11 shows the SEM images of the hematite films prepared from the solution with O2 bubbling and with N2 bubbling. A different morphology of these hematite films was observed. The deposition state of iron oxide film was

Figure 8. Potential pulse deposition of iron oxide film with repetition of cathodic pulse (potential: Ec, time: tc) and anodic

Figure 9. XRD of the iron oxide film deposited on the titanium by potential pulse method (Ec = 1.0 V, Ea = 0.2 V vs. Ag/ AgCl, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.1 M KCl solution under O2 bubbling for 30 min (the lower pattern), and heated at 500C for 1 h in air (the upper pattern). ○ hematite ☐ magnetite ◇ wustite.

Figure 10. XRD of the iron oxide film deposited on the titanium by potential pulse method (Ec = 1.0 V, Ea = 0.2 V vs. Ag/ AgCl, tc = ta = 1 s) in aqueous 10 mM FeCl2–0.1 M KCl solution under N2 bubbling for 30 min (the lower pattern), and heated at 500C for 1 h in air (the upper pattern). ○ hematite ☐ magnetite ◇ wustite.

dependent on current magnitude, potential value and pulse width. A photocurrent response of hematite film was strongly related to the deposition state of iron oxide by current and potential methods.

hematite electrode/electrolytic solution interface (C) was measured at a different electrode potential (E). At the semiconductor electrode/electrolyte interface, Motto-Schottky relation

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 elec-

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

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.

eNεε<sup>0</sup> <sup>E</sup> � Efb (28)

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

1 <sup>C</sup><sup>2</sup> <sup>¼</sup> <sup>2</sup>

can be observed as represented by Eq. (28).

trode.

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 for 1 h in air.
