Photoelectrochemical Process on Semiconductor Electrodes

Abstract

For the purpose of conversion of solar energy to chemical energy and water purification, we have investigated the photoelectrochemical process on semiconductor electrodes. As the electrodes, we focused on TiO₂, ZnO, α-Fe₂O₃, and Cu₂O. The TiO₂ electrode was prepared from the anodic oxidation of Ti. The sintered ZnO and the electrochemically deposited film were used as ZnO electrodes. The α-Fe₂O₃ and Cu₂O electrodes were prepared from the deposited film by pulse electrolysis. The flat-band potential and photocurrent were checked for these semiconductor electrodes in the solution. The HPLC analysis was carried out for intermediates generated due to the photoelectrochemical reaction of organic materials. The performance of the apparatus for water purification was tested.

1. Introduction

The earth can be regarded as a closed system from the point of view of thermodynamics. On the earth, solar energy comes in and heat energy goes out into the universe. Keeping energy balance is necessary to sustain life on Earth. The performance of conversion of solar energy to chemical energy is important to suppress carbon dioxide emissions. As the semiconductors have the property to generate holes and electron under irradiation of the light with a shorter wavelength than that corresponding to the band gap energy, they may be suitable for an effective utilization of solar energy. Thus, photoelectrochemical processes such as the photodecomposition of water on the titanium dioxide photoelectrode (Honda-Fujishima effect) have been attracting attention. There are a lot of reports of photoelectrodes and photocatalysis concerning water decomposition and environmental purification [1, 2, 3, 4, 5, 6, 7, 8, 9]. We have been investigating photoelectrochemical processes on semiconductor electrodes [10, 11, 12, 13, 14, 15, 16, 17]. In the case of oxidation and reduction of chemical species in the solution, using a semiconductor electrode is preferable because reaction control may be possible by an applied voltage. For the semiconductor electrodes, it is necessary to prepare a large surface area. We carried out mainly the preparation of semiconductor films by electrochemical methods. They have the merits of simplicity, low cost, good reproducibility, and homogeneity for the formation of semiconductor films. We focused on TiO₂, ZnO, α-Fe₂O_3, and Cu₂O electrodes to investigate the preparation, characterization, and reactivity of organic materials in aqueous solution.

2. Processes of photocatalysis and photoelectrode

In order to cause photoreaction of chemical species by using functional semiconductors under light irradiation, there are two processes of photocatalysis and photoelectrode as shown in Figure 1(A and B). In the photocatalysis system, the holes in the valence band and the electrons in the conduction band photogenerated due to light irradiation have the possibility to oxidize and reduce chemical species, respectively. In this case, the photooxidation and photoreduction may occur simultaneously on the surface of the photocatalysis particle. On a photoelectrode system consisting of a semiconductor electrode and a counter electrode, the oxidation and reduction of chemical species occur separately at each electrode. In the case of using an n-type semiconductor electrode, photoanodic oxidation proceeds on the surface of the photoelectrode. A p-type semiconductor photoelectrode causes a photocathodic reaction on its surface.

Figure 1.

Photocatalysis (A) and photoelectrode (B).

In the semiconductor electrode/electrolyte interface, the space charge layer may be formed from the surface to the inside of the semiconductor electrode. The band bending of the space charge layer can be controlled by the potential of the semiconductor electrode. An effective separation of hole-electron pairs photogenerated due to light absorption could occur in the presence of band bending. The flat-band potential and the photocurrent are important factors as the feature of the semiconductor electrode/electrolyte interface. The analysis of the chemical product generated on the photoelectrode may provide valuable information concerning the photooxidation and reduction process. The semiconductor photoelectrode system may be suitable for chemical synthesis and water purification.

3. Preparation and characteristics of semiconductor materials

The preparation and characteristics of TiO₂, ZnO, α-Fe₂O₃, and Cu₂O electrodes are mentioned below.

3.1 TiO₂ electrode

Titanium dioxide was prepared from the anodic oxidation of the titanium plate and its heat treatment in air. This method is useful in the aspects of simplicity, reproducibility, and low cost. The properties of titanium dioxide film may depend on the kinds of electrolytic solution for the oxidation of titanium plate. We used mainly nitric acid [12, 13] because the titanium dioxide from this solution showed a higher photocurrent response than those from sulfuric acid, chloric acid, and phosphoric acid.

Figure 2a shows the FE-SEM image of the surface of the sample prepared from anodic oxidation of titanium plate (size: 50$\times$50$\times$0.2 mm) with a constant current of 300 mA for 60 min in aqueous 0.1 M HNO₃ solution. The porous surface with about 100 nanometer diameter was confirmed. Figure 2b shows the cross-section SEM of this sample. The boundary between titanium oxide and titanium substrate could not be distinguished. Figure 2c shows the relationship between the Ti:O ratio and the depth for the sample heat-treated at 500$°\mathrm{C}$for 60 min. The thickness of the titanium oxide film was 1.5 μm. From the depth corresponding to the Ti: O ratio of 33: 67, the thickness of titanium dioxide was 0.2 μm. The XRD illustrated the dependence of the structure of the anodically oxidized sample on heat treatment temperature in air as shown in Figure 3. The peaks of titanium dioxide were not recognized on the sample without heat treatment. The anatase peaks of TiO₂ were observed on the samples heated at 400, 500, and 650$°\mathrm{C}$The rutile peaks appeared clearly on that heated at 650°C, and the intensity of the rutile peaks increased instead of the much weaker anatase peaks on that heat treated at 800$°\mathrm{C}$. Although there was no appearance of titanium dioxide peaks on the sample without heat- treatment, it might be amorphous because energy-dispersive X-ray spectroscopy (EDX) analysis showed the Ti: O ratio of 36.5: 63.5 for it. The anatase peaks were not recognized on the sample prepared from the titanium heat-treated only. Anodic oxidation of titanium may be necessary to obtain the anatase structure of titanium dioxide.

Figure 2.

FE-SEM image of the surface (a), cross-section SEM (b), and relationship between the Ti:O ratio and the depth (c) for the sample heat-treated at 500$°\mathrm{C}$ for 60 min after anodic oxidation of titanium in aqueous 0.1 M nitric acid.

Figure 3.

XRD patterns of the samples (a) without heat treatment, and heat-treated at (b) 400$°\mathrm{C}$ for 60 min, (c) 500$°\mathrm{C}$ for 60 min, (d) 650$°\mathrm{C}$ for 60 min, and (e) 800$°\mathrm{C}$ for 10 min after anodic oxidation treatment in aqueous 0.1 M nitric acid A: Anatase, R: Rutile and T: Titanium.

In order to characterize semiconductor electrodes, evaluation of the flat-band potential (E_fb) of semiconductor electrodes in electrolytic solution is important. The value of E_fb can be determined from the relationship between capacitance in the semiconductor electrode/electrolyte interface (C) and electrode potential (E) as shown in Eq. (1).

$$\frac{1}{C^2}=\frac{2}{\mathit{eN\epsilon }{\epsilon }^o}\left(E-E_{\mathit{fb}}\right)$$E1

e is the quantity of charge on an electron, N is the carrier density, ε dielectric constant, ε^o the permittivity of free space, and E_fb is the flat-band potential corresponding to the potential causing non-band bending inside the semiconductor electrode. In the linear plots of 1/C² versus E (Mott-Schottky plots), the values of E_fb and N can be obtained by the intercept to the potential axis and the slope of the linear portion, respectively. The sign of slope is positive in the n-type semiconductor electrode, and that is negative in the p-type semiconductor electrode. The value of E_fb in the n-type semiconductor electrode may be close to the potential for the lower edge of the conduction band. Thus, the n-type semiconductor electrode with a more negative value of E_fb may have a higher reduction ability of photo-generated electrons. The potential for the upper edge of the valence band of the n-type semiconductor electrode could be regarded as the value of F_fb plus band gap energy. This potential implies the oxidation ability of a photo-generated hole. Figure 4 shows the Mott-Schottky plot of TiO₂ electrode in an aqueous Na₂SO₄ solution. In this case, TiO₂ is prepared from anodic oxidation of Ti in aqueous HNO₃ solution and heat-treated at 500$°\mathrm{C}$. The value of E_fb of −0.53 V vs. Ag/AgCl and that of N of 3.5 $\times$ 10¹⁷ cm⁻³ were estimated from the extrapolation of the potential axis and slope of the linear potion, respectively. From the Mott-Schottky plots of the TiO₂ electrodes prepared from different heat-treatment temperatures of 400, 650, and 800$°\mathrm{C}$ the values of E_fb were − 0.54 V, −0.53 V, and − 0.54 V vs. Ag/AgCl, and the values of N were 1.9$\times$10¹⁸, 4.7$\times$10¹⁶, and 7.2$\times$10¹⁵ cm⁻³, respectively.

Figure 4.

Mott-Schottky plot for the TiO₂ (prepared from heat treatment at 500$°\mathrm{C}$for 60 min after anodic oxidation treatment in aqueous 0.1 M nitric acid) electrode in aqueous 1 M Na₂SO₄ solution.

With regard to the anodic oxidation of titanium, it was reported that nano-porous titanium dioxide could be formed in an electrolytic solution in the presence of an F⁻ ion [18]. We carried out the anodic oxidation of Ti in aqueous 0.1 M (COOH)₂ solution containing 0.2 Wt % NH₄F. Figure 5 shows the FE-SEM of the sample from the anodic oxidation of Ti plate in this solution with an applied voltage of 10 V (a), 15 V (b), and 20 V (c) for 60 min and then its heat treatment at 500$°\mathrm{C}$ for 60 min. The nano-porous structure was observed. The size of the nano-pore was dependent on an applied voltage. The porous surface of about 40, 70, and 100 nm pores was obtained by the anodic oxidation with an applied voltage of 10, 15, and 20 V, respectively, but the destruction of the porous structure was recognized in the case of higher voltage than 20 V. Figure 6 shows the Mott-Schottky plot of the sample prepared from the anodic oxidation of Ti with 10 V in the same solution and its heat-treatment at 500$°\mathrm{C}$ in air. The value of E_fb was −0.38 V vs. Ag/AgCl and the value of N was 2.37 $\times$ 10¹⁹ cm⁻³. From the Mott-Schottky plots for the nano-porous TiO₂ electrode prepared from the applied voltage of 15 V and 20 V, the values of E_fb were − 0.01 V and 0.06 V vs. Ag/AgCl, and the values of N were 1.89$\times$10¹⁹ and 1.67$\times$10¹⁹ cm⁻³, respectively. In comparison with the TiO₂ electrode prepared from anodic oxidation of titanium in nitric acid, these nano-porous electrodes showed a more positive E_fb value.

Figure 5.

FE-SEM images of the surface of the samples heat-treated at 500$°\mathrm{C}$ for 60 min after anodic oxidation of titanium with the voltage of 10 V (a), 15 V (b), and 20 V (c) in aqueous 0.1 M (COOH)₂ solution containing 0.2 Wt % NH₄F.

Figure 6.

Mott-Schottky plot for the TiO₂ (prepared from heat treatment at 500$°\mathrm{C}$for 60 min after anodic oxidation treatment of titanium with the voltage of 10 V in aqueous 0.1 M (COOH)₂ solution containing 0.2 Wt % NH₄F) electrode in aqueous 1 M Na₂SO₄ solution.

3.2 ZnO electrode

It is previously known that photoanodic reaction proceeds on the surface of ZnO photoelectrode [19]. The simple preparation of the ZnO electrode we carried out is the sintering of ZnO powders [16]. In this case, the powders were pressed at a pressure of 2 MPa and heated at different temperatures for 60 min in air. The sample was connected to a lead wire through a Ga-In ally and covered with epoxy resin to protect leakage current. Figure 7a shows the SEM images of the samples heated at temperatures of 1000$°\mathrm{C}$ (A), 1100$°\mathrm{C}$ (B), and 1200$°\mathrm{C}$ (C). The aggregation and combination of particles proceeded with temperature and the grain with a larger size than 5 μm was observed on the sample heated at 1200$°\mathrm{C}$. Figure 7b shows the relationship between electrode potential and photocurrent on the sintered ZnO electrode (geometric surface area: 1.54 cm²) with heat-treatment temperatures of 1000$°\mathrm{C}$ (A), 1100$°\mathrm{C}$ (B), and 1200$°\mathrm{C}$ (C) under irradiation of UV light (wavelength: 360 nm and intensity: 30 mW/cm²). The photocurrent on the 1200$°\mathrm{C}$-treated electrode was much higher than that on the 1100$°\mathrm{C}$- and 1000$°\mathrm{C}$-treated electrodes, and it had a tendency to be saturated at more positive potential than 2.0 V vs. Ag/AgCl. This means that photo-generated holes and electron pairs could be separated smoothly on the 1200$°\mathrm{C}$-treated electrode. The dependence of magnitude of photocurrent on electrode potential suggests that much more band bending may be necessary for the separation of photo-generated hole and electron pairs on the 1000$°\mathrm{C}$- and 1100$°\mathrm{C}$-treated electrodes.

Figure 7.

SEM images of the surface (a) and relationship between photocurrent and electrode potential (b) for the ZnO electrode sintered at different temperatures of 1000$°\mathrm{C}$ (A), 1100$°\mathrm{C}$ (B) and 1200$°\mathrm{C}$ (C).

The photocurrent quantum efficiency $\varphi$is represented as the following equation.

$$\varphi =\frac{N_e}{N_{\mathit{h\nu }}}\times 100=\frac{\frac{I_{\mathit{ph}}}{e}}{\frac{\mathit{WS}}{h\frac{c}{\lambda }}}\times 100$$E2

N_e is the number of electrons as photocurrent, N_hv the number of photons of the incident light, I_ph the photocurrent, e the quantity of charge on an electron, W the intensity of light, S the surface area of the electrode, h the Planck constant, c the speed of light, and λ the wavelength of light. The values of photocurrent at 2.0 V vs. Ag/AgCl were 1.4, 3.1, and 10.8 mA for 1000$°\mathrm{C}$-, 1100$°\mathrm{C}$-, and 1200$°\mathrm{C}$-treated electrodes, respectively. From Eq. (2), the values of ϕ were 10.4, 23.2, and 80.6% for each electrode. We had previously investigated the quantum efficiency by using the photo-thermal method (η_pt) for semiconductor photoelectrodes [20, 21, 22]. The value of 80.6% was close to the value of η_pt of 85% for the sintered ZnO electrode. Figure 8 shows the Mott-Schottky plot of 1200$°\mathrm{C}$-treated ZnO electrode in an aqueous 0.1 M Na₂SO₄ solution. The value of E_fb was –o.24 V vs. Ag/AgCl and the value of N was 2.2$\times$10¹⁸ cm⁻³. The 1100$°\mathrm{C}$-treated electrode provided the values of −043 V vs. Ag/AgCl and 2.9$\times$10¹⁷ cm⁻³ as E_fb and N, respectively. A remarked difference of N between 1200$°\mathrm{C}$- and 1100$°\mathrm{C}$-treated electrodes was recognized. The 1000$°\mathrm{C}$-treated electrode did not show a linear relationship between 1/C² and E clearly.

Figure 8.

Mott-Schottky plot for the 1200$°\mathrm{C}$-sintered ZnO electrode in aqueous 0.1 M Na₂SO₄ solution.

For the performance of water purification and photoelectrochemical synthesis on semiconductor electrodes, it is necessary to prepare the semiconductor films with a large surface area. Therefore, we carried out the electrochemical deposition of ZnO film on a titanium substrate by the following procedure. The electrode potential of a titanium working electrode (size: 50$\times$ 50 mm²) was held at −0.9 V vs. Ag/AgCl for 3 hours in aqueous 0.1 M Zn(NO₃)₂ solution with O₂ gas bubbling. The temperature of the solution was kept at 62${}^{°}\mathrm{C}$. This electrolysis caused the deposition of the ZnO film on the surface of the titanium electrode in accordance with the following equations [23].

$$\mathrm{N}{{\mathrm{O}}_3}^{-}+{\mathrm{H}}_2\mathrm{O}+2e^{-}\to \mathrm{N}{{\mathrm{O}}_2}^{-}+2\mathrm{O}{\mathrm{H}}^{-}$$E3

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_2$$E4

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\to \mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}$$E5

Figure 9a shows the FE-SEM image of the film deposited on the titanium substrate. From the cross-section analysis, the thickness of the film was about 20 μm (Figure 9b). Figure 9c shows the XRD of the film before (below illustration, A) and after (upper, B) heat treatment at 500$°\mathrm{C}$ for 60 min in air. The diffraction peaks due to ZnO were recognized on the as-deposited film, but the intensity of the peaks increased markedly after heat treatment. Figure 10 shows the relationship between electrode potential and photocurrent on the ZnO electrode (geometric surface area: 25 cm²) prepared from the electrochemical deposition and heat treatment under irradiation of UV light (wavelength: 365 nm and intensity: 2.35 mW/cm²). The tendency to saturation of photocurrent was observed at more positive potential than 2.0 V vs. AG/AgCl. The photocurrent quantum efficiency at 2.0 V vs. Ag/AgCl for this electrode was estimated at 82.7% from Eq. (2) with I_ph of 14.3 mA, W of 2.35 mW/cm², S of 25 cm², and λ of 365 nm. The value of ϕ for the electro-deposited ZnO was high as well as that of ϕ for the 1200$°\mathrm{C}$-sintered ZnO. This reflects a rapid separation of photo-generated holes and electron pairs on the ZnO electrode.

Figure 9.

FE-SEM image of the surface (a), cross-section SEM (b), and XRD patterns (c) for the film deposited electrochemically at the constant potential of$-0.9$V vs. Ag/AgCl for 3 hours in aqueous 0.1 M Zn(NO₃)₂ solution with the temperature of 62$°\mathrm{C}$. The below (A) XRD pattern is for the film before heat treatment, and the upper (B) for the film after heat treatment at 500$°\mathrm{C}$ for 60 min in air.

Figure 10.

Relationship between electrode potential and photocurrent on the ZnO electrode prepared from electrochemical deposition in aqueous Zn(NO₃)₂ solution and heat treatment at 500$°\mathrm{C}$.

3.3 α-Fe₂O₃ and Cu₂O

As α-Fe₂O₃ (hematite) and Cu₂O are responding to visible light, they are expected as candidate semiconductors to play a part in artificial photosynthesis. Hematite electrode is prepared from thermal oxidation of iron easily [15]. We carried out the deposition of iron oxide film by current and potential pulse electrolysis on the titanium electrode in an aqueous KCl solution containing FeCl₂. The hematite electrode was obtained by heat treatment of the deposited iron oxide film at 500$°\mathrm{C}$ in air. The details of the preparation, characterization, and phtoelectrochemical response of hematite are summarized in the book [10].

For the preparation of Cu₂O, we used the electrochemical deposition of a film on the titanium electrode in an aqueous solution (pH = 10) of 10 mM CuSO₄, 0.2 M sodium lactate, and 20 mM NaOH. The electrolysis in this case was done for 60 min by the potential pulse method represented by the positive potential (E_p), the negative potential (E_n), and pulse width (W) as shown in Figure 11. Figure 12a shows the FE-SEM of the film deposited in the case of E_p = 0.0 V, E_n = −0.4 V vs. Ag/AgCl and W = 0.5 sec. The granular deposits with a size of about 80 nm were observed. From the cross-section analysis (Figure 12b), the thickness of the film was about 0.8 μm. Figure 12c shows the XRD of this film. The peaks of Cu₂O were confirmed in addition to Ti peaks. With regard to the electrochemical deposition of Cu₂O film in the basic solution containing lactate ion (Lac⁻), the following process could be considered. The cu²⁺ ion forms the complex of Cu²⁺ (Lac⁻)₂. This complex is reduced electrochemically to form Cu₂O through an intermediate of CuOH as shown below.

Figure 11.

Potential pulse electrolysis with negative potential (E_n), positive potential (E_p), and pulse width (W).

Figure 12.

FE-SEM image of the surface (a), cross-section SEM (b) and XRD pattern (c) for the film deposited from potential pulse electrolysis (E_n: $\mathit{-}\mathit{0.4}$ V vs. Ag/AgCl, E_p: $\mathit{0.0}$ V vs. Ag/AgCl and W: 0.5 sec) for 60 min in aqueous basic solution of 10 mM CuSO₄, 0.2 M sodium lactate and 20 mM NaOH.

$${\mathrm{Cu}}^{2+}{\left({\mathrm{Lac}}^{-}\right)}_2+e^{-}+{\mathrm{OH}}^{-}\to \text{CuOH}+2{\mathrm{Lac}}^{-}$$E6

$$2\text{CuOH}\to {\mathrm{Cu}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$E7

Figure 13a shows the Mott-Schottky plot of the Cu₂O electrode in aqueous 1 M Na₂SO₄ solution. The negative slope of the linear portion indicated the p-type semiconductor electrode. The values of E_fb and N were 0.22 V vs. Ag/AgCl and 5.73$\times$10¹⁸ cm⁻³, respectively. Figure 13b shows the photocurrent response of the Cu₂O electrode to the irradiation of visible light (wavelength: 490 nm and intensity: 4.8 mW/cm²). The cathodic photocurrent of the feature of the p-type semiconductor electrode was clearly observed.

Figure 13.

Mott-Schottky plot for the Cu₂O electrode in aqueous 1 M Na₂SO₄ solution (a) and photocurrent response of the Cu₂O electrode to the irradiation of visible light (wavelength: 490 nm, intensity: 4.8 mW/cm²) in aqueous 0.1 M Na₂SO₄ solution (b).

4. Photoelectrochemical reaction on semiconductor electrodes

For the progress of organic synthesis and water purification based on photoelectrochemical reactions, it is necessary to make clear the reactivity of photo-generated holes and electrons on semiconductor electrodes to chemical species. We carried out an HPLC analysis of the products generated from a starting organic material on the photoelectrode by using a cell shown in Figure 14. In this cell, the solution of photoelectrode and that of the counter electrode were separated by a cation-exchange membrane (Selemion CMVm AGC Engineering). We investigated the photoanodic oxidation process of Veratryl Alcohol (VA, Figure 15) on the semiconductor electrode in an aqueous solution from the aspects of intermediates detected by applying the HPLC method to the photoelectrode solution.

Figure 14.

Cell assembly for photoelectrochemical reaction of organic material in aqueous solution consisting of photoelectrode and counter electrode sides separated by a cation-exchange membrane.

Figure 15.

Chromatograms before and after irradiation (wavelength: 365 nm, intensity: 3.0 mW/cm²) for 6 hours to the TiO₂ electrode in aqueous 10 mM NaCl solution containing initial concentration of 50 μm Veratryl alcohol. A) Veratryl alcohol B) Veratraldehyde C) Veratric acid D) Vanillil alcohol E) Vanillin F) Vanillic acid G) Protocatechualdehyde H) Protocatechuic acid I) Benzentriol.

In the case of using the TiO₂ electrode (geometric surface area: 25 cm²) prepared from anodic oxidation of Ti in HNO₃ solution and then heat-treatment at 500$°\mathrm{C}$, the intermediates due to oxidation of VA were clearly detected [12]. In this case, the electrolytic solution (volume: 420 ml) of two electrodes was aqueous 10 mM NaCl solution containing 50 μM VA. The solution of the Pt counter electrode (geometric surface area: 25 cm²) was bubbled with oxygen gas. The light source was a black light (wavelength: 365 nm, intensity: 3.0 mW/cm²). Figure 15 shows the chromatograms for the solution of the TiO₂ electrode before and after irradiation of UV light. The potential of the TiO₂ electrode was held at 2.0 V vs. Ag/AgCl. Before irradiation, only the peak A due to VA was observed at a retention time of 6.9 min. After irradiation for 6 hours, the new peaks of Veratraldehyde (B), Veratric Acid (C), Vanillil Alcohol (D), Vanillin (E), Vanillic Acid (F), Protocatechualdehyde (G), Protocatechuic Acid (H), and Benzentriol (I) appeared. The structure of these intermediates is shown in Figure 16. Protocatechuic Acid and Benzentriol are highly oxidized intermediates. Figure 17 shows the relationship between the concentration of intermediates and irradiation time, and between the relative concentration of VA and irradiation time on the TiO₂ photoelectrode. In addition to the main intermediates of Vanillil Alcohol, Veratraldehyde, and Vanillin, the formation of Benzentriol and Protocatechuic Acid is shown clearly. The HPLC analysis for organic acid revealed the formation of ketomalonic acid. This suggests the occurrence of aromatic ring cleavage on the TiO₂ photoelectrode. High response to VA with the formation of ketomalonic acid was also recognized on the TiO₂ prepared from the anodic oxidation of Ti in (COOH)₂ –NH₄F solution.

Figure 16.

Intermediates from Veratryl alcohol.

Figure 17.

Relationship between concentration of intermediate and irradiation time on the TiO₂ electrode Veratryl alcohol (

) Veratraldehyde (

) Veratric acid (

) Vanillil alcohol (

) vanillin (

) Vanillic acid (

) Protocatechualdehyde (

) Protocatechuic acid (

) Benzentriol (

).

In the photoanodic oxidation of 50 μM VA on the ZnO electrode in aqueous NaCl solution under UV irradiation (wavelength: 365 nm, intensity: 2.6 mW/cm²), the main intermediates were Veratraldehyde and Vanillil Alcohol. The concentration of Vanillin was much lower than that of Veratraldehyde and Vanillil Alcohol. The formation of Veratric Acid, Isovanillin, Protocatechuic acid, and Benzentriol was recognized on the ZnO photoelectrode. Except for alkaline solution, the photoanodic dissolution may occur in an aqueous solution under UV irradiation as below.

$$\mathrm{ZnO}+2{\mathrm{h}}^{+}\to {\mathrm{Zn}}^{2+}+\frac{1}{2}{\mathrm{O}}_2$$E8

Because of this side reaction, a difference of intermediate between ZnO and TiO₂ might appear.

On the α-Fe₂O₃ electrode in an aqueous NaCl solution, the visible light irradiation (wavelength: 490 nm, intensity: 4.8 mW/cm²) caused the photoanodic oxidation of VA to generate Veratraldehyde, Vanillil Alcohol, and Vanillin. The HPLC peaks of Protocatechuic Acid and Benzentriol were not observed. After the photoanodic oxidation of VA with the initial concentration of 100 μM for 9 hours, 24.7 μM of VA decreased, and 17.5 μM of Veratraldehyde, 5.0 μM of Vanillil Alcohol, and 1.1 μM of Vanillin were formed. The ratio of the total concentration of these intermediates to the concentration of oxidized VA was 95.5%. This implies a possibility of selective oxidation of organic materials by α-Fe₂O₃ photoelectrode.

The E_fb of the p-type semiconductor electrode is close to the upper edge of the valence band. As the E_fb of the Cu₂O electrode we prepared is 0.22 V vs. Ag/AgCl, namely 0.42 V vs. NHE in an aqueous Na₂SO₄ solution, the lower edge of the conduction band can be regarded as −1.68 V vs. NHE assuming that the band gap of 2.1 eV. Thus, the photo-generated electrons could reduce the chemical species with more positive redox potential than −1.68 V vs. NHE. Figure 18 shows the chromatograms of aqueous 10 mM Na₂SO₄ solution containing an initial concentration of 100 μM p-benzoquinone on the Cu₂O photoelectrode for different irradiation times. In this case, the potential of the Cu₂O electrode (geometric surface area: 1 cm²) was held at −0.2 V vs.Ag/AgCl. The wavelength of irradiation light was 550 nm and the intensity was 4.8 mW/cm². The solution was bubbled with N₂ gas. As shown in this figure clearly, the concentration of p-benzoquinone decreased and that of hydroquinone increased with irradiation time. The photocathdic reduction of p-benzoquinone to hydroquinone was confirmed. The current efficiency of this process was 95.8%. The occurrence of photocathodic reduction of 1,4-Naphthoquinone to 1,4-Dihydroxynaphthalene on the Cu₂O photoelectrode was also confirmed by the HPLC analysis. In this case, the current efficiency was 83.5%.

Figure 18.

Change of chromatogram with irradiation time on the Cu₂O electrode.

Lastly, we would like to mention the apparatus for water purification [13] as shown in Figure 19a. The photoelectrode is 30 sheets of the TiO₂ electrode (5$\times$5 cm²) prepared from anodic oxidation of Ti in nitric acid solution and heat treatment at 500$°\mathrm{C}$. The counter electrode is a carbon cloth. The light source is four black lights (wavelength: 365 nm, intensity: 3.0 mW/cm²). Figure 19b shows the test of performance of this apparatus for the waste liquid (volume: 20 L). After 4 days with an applied voltage of 3.0 V, the color of the liquid disappeared and The COD value of 120 decreased to a value of 20. Further application such as water purification in the field of food and medicine is expected.

Figure 19.

The apparatus for water purification consisting of 30 sheets of titanium dioxide electrodes, carbon cloth counter electrode, four black lights, a power supply, and a stirring implement (a) and its performance for 20 L wastewater after 4 days of treatment (b).

5. Conclusion

We have been investigating the preparation, characteristics, and photoelectrochemical response to chemical species for semiconductor electrodes. The semiconductor films of TiO₂, ZnO, α-Fe₂O₃, and Cu₂O were prepared by electrochemical methods. From the Mott-Schottky plot for semiconductor electrodes in aqueous Na₂SO₄ solution, the values of flat-band potential and carrier density of each electrode were estimated. In the case of the Cu₂O electrode, a negative slope of the linear plot indicating the p-type semiconductor electrode was recognized. The Cu₂O electrode showed a cathodic photocurrent response under visible light irradiation. With regard to the photocurrent quantum efficiency of the ZnO electrode, both the 1200${}^{°}\mathrm{C}$-sintered electrode and the electrochemically deposited film electrode showed a high efficiency of over 80%. On the TiO₂, ZnO, and α-Fe₂O₃ electrodes, the photoanodic reaction of Veratryl Alcohol was analyzed by the HPLC method. On the TiO₂ and ZnO electrodes, the highly oxidized intermediates of Protocatechuic Acid and Benzentriol were formed, but these intermediates were not detected on the α-Fe₂O₃ electrode. There was also a difference in the appearance of intermediates between TiO₂ and ZnO electrodes. These reflect a difference in the reactivity of photo-generated holes to chemical species on the TiO₂, ZnO, and α-Fe₂O₃ electrodes. The phocathodic reduction of p-benzoquinone to hydroquinone and of 1,4-Naphthoquinone to 1,4-Dihydroxynaphthalene was confirmed on the Cu₂O electrode. The photoelectrochemical system by using α-Fe₂O₃ and Cu₂O is expected to apply to artificial photosynthesis.

Acknowledgments

I am grateful to Dr. D. Kodama, Mr. M. Itono, Mr. H. Mitsuyoshi, Mr. T. Kitagawa, Mr. A. Konemura, Mr. T. Nakamura, Mr. Y. Kanada, Mr. K. Miura, and Dr. Y. Kohno for experimental contribution in Photoelectrochemical Process on Semiconductor Electrodes.