Observation of Cathodic Photocurrents at Nanocrystalline TiO2 Film

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J. Phys. Chem. B 2002, 106, 5878-5885

Observation of Cathodic Photocurrents at Nanocrystalline TiO2 Film Electrodes, Caused by Enhanced Oxygen Reduction in Alkaline Solutions Akira Tsujiko, Hiroki Itoh, Tetsuya Kisumi, Akira Shiga, Kei Murakoshi, and Yoshihiro Nakato* Department of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, and Research Center for Solar Energy Chemistry, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan ReceiVed: June 5, 2001; In Final Form: January 20, 2002

Nanocrystalline TiO2 (rutile and anatase) film electrodes usually show anodic photocurrents in aqueous electrolytes. Detailed photoelectrochemical studies have revealed that they also show cathodic photocurrents under particular conditions, e.g., in alkaline solutions containing dissolved oxygen under illumination from the TiO2-film side at short wavelengths such as 300 nm. The cathodic photocurrents appear in a potential region about 0.5∼0.9 V more positive than the flat-band potential (Ufb) of single-crystal n-TiO2 electrodes. It is shown that the appearance of the cathodic photocurrents can be attributed to efficient electron transfer from the conduction band of TiO2 particles to chemically adsorbed oxygen molecules, the density of which is largely increased in alkaline solutions through a charge-transfer interaction between surface anionic groups such as Ti-O- as an electron donor and oxygen molecules as an electron acceptor. The dependences of the cathodic and anodic photocurrents on the illumination wavelength, the illumination direction, the electrode potential, and the crystal form of TiO2 particles are discussed in relation with the photocatalytic activity of particulate TiO2 films.

Introduction

Experimental Section

Nanocrystalline TiO2 films, prepared on substrates, have been attracting much attention because of their interesting properties such as high photoelectrochemical or photocatalytic activity, high photoconductivity, high porosity, high surface areas, and high optical transparency. Much work has been done on such films in view of potential applications to energy conversion devices such as dye-sensitized solar cells,1-6 photocatalysts for decomposing pollutants and harmful bacteria,7-13 and electrochromic devices.14-16 The photoelectrochemical or photocatalytic activity of nanocrystalline TiO2 films depends on various factors such as the efficiency of light absorption, the rates of surface reactions and back reactions, those of surface carrier recombination, and the transport properties of charge carriers in the films. For obtaining high efficiencies or activity, the prevention of surface carrier recombination and back reactions is especially important, because no charge carrier separation due to band bending can be expected for insulating nanocrystalline TiO2 particles.17 In the course of studies on nanocrystalline TiO2 film electrodes,18,19 we have found that the electrodes in some cases show cathodic photocurrents, in addition to usually observed anodic photocurrents. The observation of cathodic photocurrents under particular conditions is briefly reported by Hodes et al.20 in relation with major anodic photocurrents for nanocrystalline CdS and CdSe films. The phenomenon is of much interest from the point of view of investigations of factors that govern the rates of anodic and cathodic processes on nanocrystalline particles. In the present paper, we report the behavior and characteristics of the cathodic photocurrents of the TiO2 films, together with a discussion on the generation mechanism.

Titanium dioxide (TiO2) powder, named JRC-TIO-3 and JRCTIO-4 by the Catalysis Society of Japan, was donated by the Society. The TIO-3 powder was prepared by a liquid-phase method using titanium disulfate as the starting material, and was composed of 100% rutile-type TiO2 particles. The TIO-4 powder was prepared by a gas-phase CVD method using titanium tetrachloride as the starting material, and was composed of about 75% anatase- and about 25% rutile-type TiO2. The TIO-4 powder was essentially the same as Degussa P25. Both TIO-3 and TIO-4 had the average diameter of about 30 nm. Nanocrystalline TiO2 film electrodes were prepared as follows.18 The TiO2 powder (2 g) was ground with 4 mL of water and 0.4 mL of acetylacetone. The mixture was added to 40 mL of 1.5% HNO3 under stirring. The colloidal TiO2 solution was coated on a transparent conductive oxide (F-doped SnO2) film deposited on a glass plate (Nippon Sheet Glass Co. Ltd., the sheet resistance about 20 Ω/square) with a spin coater at 2000 rpm, and heated at 150 °C. The transparent conductive oxide film was beforehand washed successively with boiling acetone, 30% HNO3, and pure water. The above coating procedure was repeated several times, and the TiO2 film was finally heated at 600 °C for 2 h in air. The thickness of the resulting TiO2 films was ca. 1.2 µm. Electrical contact with a copper wire was obtained at an edge of the SnO2 film with silver paste, and the whole part, except the 1.0 × 1.0 cm2 TiO2-film area, was covered with epoxy resin for insulation. Single-crystal rutile-type n-TiO2 was obtained by thermal reduction of commercial TiO2 wafers (Earth Chemical Co. Japan) in a hydrogen atmosphere.21 Single-crystal anatase-type n-TiO2 (∼1 Ω cm, natural product) was obtained from FOM (Instituut voor Atoom- en Molecuulfysica, Amsterdam, The

* Corresponding author. Fax. +81-6-6850-6236. E-mail: nakato@ chem.es.osaka-u.ac.jp.

10.1021/jp012144l CCC: $22.00 © 2002 American Chemical Society Published on Web 05/21/2002

Cathodic Photocurrents at TiO2 Film Electrodes

J. Phys. Chem. B, Vol. 106, No. 23, 2002 5879

Netherlands). Ohmic contact was obtained with an indiumgallium alloy at the rear surface. Current density (j) vs potential (U) curves were measured with a commercial potentiostat and a potential programmer, using a Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode. Photocurrents were measured using a Pyrex cell with a quartz window. A 300-W xenon lamp (Ushio UXL-300) was used as the light source, and monochromatic light was obtained with a monochromator (Jahrel-Ash JE-25E). The illumination was in most cases chopped with a light chopper (Scitec Instruments Ltd., 300HRG). Electrolyte solutions were prepared by use of pure water (Mill-Q water) and reagent grade chemicals without further purification. The temperature of the electrolyte solutions was kept at 25 °C ( 0.2 °C. Nitrogen or oxygen gas was bubbled through the electrolytes under magnetic stirring either to remove dissolved air (oxygen) or to dissolve oxygen. Photocurrent action spectra [or photocurrent quantum efficiency (ηPC) vs wavelength (λ) curves] were obtained by measuring the illumination intensity with a thermopile (Eppley). The light transmittance of nanocrystalline TiO2 film electrodes was measured with a Shimadzu 2500PC spectrophotometer, using an F-doped SnO2-coated glass plate as the reference sample. The diffuse reflectance of the electrodes was measured with the same spectrophotometer using an integral sphere attachment. The inspection of electrode surfaces was carried out with a high-resolution scanning electron microscope (SEM, Hitachi S-5000). X-ray diffraction patterns were obtained with a Philips X′Pert diffractometer using a Cu KR (λ ) 1.5417 Å) radiation. Results SEM inspection showed that the TiO2 films, prepared from JRC-TIO-3 and TIO-4 powder, were both particulate films similar to photographs reported in a previous paper.18 X-ray diffraction patterns confirmed the compositions of TIO-3 (100% rutile) and TIO-4 (about 75% anatase and about 25% rutile), as mentioned in the preceding section. Figure 1 shows the effect of dissolved oxygen on j-U curves for a nanocrystalline TiO2 (TIO-4, mainly anatase-type) film electrode in acidic (0.1 M HClO4, pH 1.1) and alkaline (0.1 M NaOH, pH 12.8) solutions. All curves (photo- and dark currents) were obtained under potential scans at a rate of 4 mV/s from positive to negative, with monochromatic illumination at 300 nm being chopped at 0.2 Hz (further details will be explained later in Figure 2). The flat-band potential (Ufb) of single-crystal anatase n-TiO2 reported22 is included for reference. We can see that nanocrystalline TiO2 film electrodes usually show anodic photocurrents, especially in a region of positive potentials, independent of the solution pH and the presence or absence of dissolved O2. The anodic photocurrent starts at around Ufb in acidic N2- and O2-bubbled solutions (Figure 1, parts A and B). On the other hand, in O2-bubbled 0.1 M NaOH (Figure 1D), the dark cathodic current increases much, and moreover, a cathodic photocurrent appears at potentials more negative than about -0.20 V vs Ag/AgCl, which is about 0.90 V more positive than the Ufb. Figure 2A shows an actually measured j-U curve under chopped illumination, with U scanned from positive to negative, under the same conditions as Figure 1D. Figure 2, parts B and C, are time courses of j under chopped illumination at constant potentials of 0.2 V and -0.4 V, respectively. The anodic photocurrent in general reached a steady-state value just after the start of illumination, whereas the cathodic photocurrent

Figure 1. Effect of dissolved oxygen on j-U curves for a particulate TiO2 (TIO-4, anatase 75% + rutile 25%) electrode. Electrolyte: (A) N2-bubbled and (B) O2-bubbled 0.1 M HClO4 (pH 1.1), and (C) N2bubbled and (D) O2-bubbled 0.1 M NaOH (pH 12.8). The electrode was illuminated at 300 nm from the TiO2-film side.

initially decayed and then reached a steady-state value in around 2.5 s. For long illumination times of, say, 10 s, much longer than in Figure 2C, the cathodic photocurrent was kept constant without any decay after around 2.5 s till 10 s. The steady-state values are plotted in Figure 1 as the photocurrent. The dark cathodic current increased much in O2-bubbled 0.1 M NaOH, as mentioned above (Figure 1D). This made the measurements of cathodic photocurrents at more negative potentials than about -0.7 V very difficult. To clarify the origin of the increased dark cathodic currents, Figure 3A shows the whole behavior of the currents for a TiO2 (TIO-4, mainly anatase) film electrode, compared with that for a TiO2-free SnO2 (substrate) electrode. Figure 3B is a simple expansion of Figure 3A in the j-scale. All the plotted currents represent steady-state values at constant potentials, measured 30 min after steps to the potentials. The currents for the TiO2 film electrodes were of the same order of magnitude as those for the TiO2-free SnO2 electrodes, but changed from electrode to electrode, and therefore, in Figure 3 the saturated values in -0.9 to -1.0 V are normalized to those for the SnO2 electrode. It is to be noted that the j-U curves for the increased dark cathodic currents show quite the same onset and shape between the TiO2 and the SnO2 electrodes, indicating that the dark currents come from the SnO2/solution interface. It will be evident that the increased dark cathodic currents can be attributed to O2 reduction because they appear prominently in O2-bubbled alkaline solutions. They also show

5880 J. Phys. Chem. B, Vol. 106, No. 23, 2002

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Figure 3. (A) Dark cathodic currents for a TiO2 (TIO-4, mainly anatase) film electrode, compared with that for a TiO2-free SnO2 (substrate) electrode, and (B) a simple expansion of (A) in the j-scale. The electrode and the electrolyte: (b) TiO2 and (2) SnO2 both in O2bubbled 0.1 M NaOH, and (O) TiO2 and (4) SnO2 both in N2-bubbled 0.1 M NaOH.

Figure 2. (A) An actually measured j-U curve under chopped illumination, with U scanned from positive to negative, for a particulate TiO2 (mainly anatase) in O2-bubbled 0.1 M NaOH. (B) and (C) are time courses of j under chopped illumination at U ) 0.2 and -0.4 V, respectively.

saturations in a region of -0.9 to -1.0 V (Figure 3A) attributable to the diffusion limitation for O2, in harmony with the above argument. The equilibrium redox potential for the O2 reduction in alkaline solution (O2 + 2H2O + 4e- f 4OH-) is 0.26 V vs Ag/AgCl at pH 13.23 The onset of the increased dark cathodic currents at around -0.50 V (Figure 3) implies the presence of an overvoltage for the O2 reduction of about 0.76 V, which will be reasonable for the SnO2 (metallic conductor) electrode. The fairly large dark cathodic current in N2-bubbled 0.1 M NaOH (Figures 1C and 3B) is most probably due to a small amount of dissolved O2 that could not be removed by N2 bubbling for 2 h. Further long N2 bubbling did not diminish the current. Figure 4 shows j-U curves for a nanocrystalline TiO2 (TIO3, 100% rutile) film electrode, obtained in a similar way to the case of mainly anatase-type TiO2 (Figure 1). The Ufb of singlecrystal rutile n-TiO224,25 is included in the figure. Essentially the same results as in Figure 1 are obtained, including the appearance of cathodic photocurrents in O2-dissolved 0.1 M NaOH, though the saturated anodic photocurrents in acidic solutions (Figure 4, parts A and B) are much lower than the corresponding values in Figure 1. Higher photocurrent quantum efficiencies for anatase-type TiO2 than those for rutile-type under illumination at short wavelengths (λ) around 300 nm were reported in a previous paper19 for the oxidation of water and

ethanol (see also Figures 6 and 7 and Figure 9 of the present paper), and explained tentatively by taking account of high mobilities of “hot” electrons generated by short-λ illumination. Figure 5 shows the effect of dissolved oxygen on dark j-U curves for single-crystal, rutile-type and anatase-type n-TiO2 electrodes. Contrary to the case of acidic solution (0.1 M HClO4), the j-U curves in O2-bubbled alkaline solutions clearly show dark cathodic currents attributable to the O2 reduction. It is to be noted also that the currents strongly depend on the solution pH. The onset potential of the dark cathodic currents, especially that for rutile-type n-TiO2, is far more negative than that for the (metallic) SnO2 electrode shown in Figures 1D or 3. This is because the onset potential for single-crystal n-TiO2 is determined by the Ufb owing to the presence of upward band bending at potentials more positive than Ufb. The onset potential for anatase n-TiO2 is shifted to the positive compared with that for rutile n-TiO2 (Figure 5). This might be attributed to the presence of high-density defects in natural anatase crystals used in the present work, as expected from the observation of negligibly low photocurrents. The cathodic currents near Ufb for single-crystal rutile n-TiO2, attributable to the reduction of intermediates of oxygen photoevolution reaction, were investigated in detail by Salvador et al.26,27 Figure 6 shows the effect of dissolved oxygen on photocurrent action spectra for a nanocrystalline TiO2 (100% rutile, TIO-3) film electrode. The ordinate represents the photocurrent quantum yield, ηPC(λ), at a wavelength of λ, which is defined by the equation,

ηPC (λ) ) 1240 × j(λ)/λ[I0(λ) - Itrans(λ) - Iref(λ)] (1) where j(λ) is the photocurrent density under illumination at λ, I0(λ) the incident light intensity at λ, Itrans(λ) the transmitted light intensity at λ, and Iref(λ) the reflected light intensity at λ. The Itrans(λ) and Iref(λ) are calculated from I0(λ) using the



Figure 5. Effect of dissolved oxygen on dark j-U curves for (A) rutile and (B) anatase single-crystal (001)-cut n-TiO2 electrodes in acidic (right) and alkaline (left) solutions.

Figure 4. Effect of dissolved oxygen on j-U curves for a particulate TiO2 (TIO-3, rutile 100%) electrode. Electrolyte: (A) N2-bubbled and (B) O2-bubbled 0.1 M HClO4 (pH 1.1), and (C) N2-bubbled and (D) O2-bubbled 0.1 M NaOH (pH 12.8). The illumination was carried out at 300 nm from the TiO2-film side.

absorption and the diffuse reflectance spectra of the TiO2 film, respectively. Equation 1 gives correct ηPC values when the photocurrent density, the light intensity, and the wavelength are given in units of mA cm-2, W cm-2, and nm, respectively. The light reflection at the cell window is neglected. The anodic current is taken to be positive in the sign, and thus negative ηPC indicates that a cathodic photocurrent is observed. Figure 6A shows that ηPC in acidic solution (0.1 M HClO4) is positive all over the wavelengths measured, independent of the presence or absence of O2 in the solution, and increases with decreasing λ in a long-λ region. Also, the ηPC in N2-bubbled 0.1 M NaOH (Figure 6B) is positive throughout the wavelengths measured. On the contrary, the ηPC in O2-bubbled 0.1 M NaOH (Figure 6B) is negative at wavelengths shorter than about 370 nm. The decrease in ηPC in N2-bubbled 0.1 M NaOH in short wavelengths is most probably due to the effect of a small amount of O2 not removed by the N2 bubbling. Figure 7 shows the photocurrent action spectra (ηPC-λ curves) for a nanocrystalline TiO2 (TIO-4, mainly anatase-type) film electrode. Essentially the same results as rutile-type (Figure 6) are obtained, except that the ηPC values in Figure 7 are in general much higher than those in Figure 6. It is to be noted also that small cathodic photocurrents (negative ηPC) are observed in a region from 390 to 400 nm, too, in O2-bubbled 0.1 M NaOH (Figure 7B).

Figure 6. Photocurrent action spectra for a particulate TiO2 (TIO-3, rutile 100%) electrode with and without dissolved oxygen, obtained (A) at -0.15 V vs Ag/AgCl in 0.1 M HClO4 and (B) at -0.55 V vs Ag/AgCl in 0.1 M NaOH. The illumination was performed from the TiO2-film side.

Figure 8 shows the potential dependence of the ηPC-λ curves in O2-bubbled 0.1 M NaOH for (A) rutile-type (TIO-3) and (B) mainly anatase-type (TIO-4) nanocrystalline TiO2 electrodes. At a positive potential of 0.5 V, ηPC is positive for both the electrodes throughout the wavelengths measured. As the potential becomes more negative, ηPC becomes negative at short wavelengths, and the wavelength at which ηPC changes the sign from positive to negative shifts to longer values. It is to be noted again that for anatase-type TiO2, negative ηPC is observed in a region near 400 nm in an intermediate potential range, as


Tsujiko et al.

Figure 7. Photocurrent action spectra for a particulate TiO2 (TIO-4, anatase 75% + rutile 25%) electrode with and without dissolved oxygen, obtained (A) at -0.00 V vs Ag/AgCl in 0.1 M HClO4 and (B) at -0.40 V vs Ag/AgCl in 0.1 M NaOH. The illumination was performed from the TiO2-film side. Figure 9. Illumination-direction dependence of the ηPC-λ curves in O2-dissolved 0.1 M NaOH, (A) for TIO-3 (rutile 100%) at -0.55 V vs Ag/AgCl and (B) for TIO-4 (anatase 75% + rutile 25%) at -0.40 V. The illumination direction is shown in the figure.

(Figures 1, 2, 4, 6-8), for illumination from the SnO2-substrate side (marked by solid triangles), positive ηPC (anodic photocurrent) is observed at short wavelengths for both the electrodes. It may be noted also that for illumination from the SnO2substrate side, negative ηPC is observed in a long-wavelength region near 400 nm, especially for a mainly anatase-type TiO2 electrode. Discussion

Figure 8. Potential dependence of the ηPC-λ curves in O2-bubbled 0.1 M NaOH for (A) TIO-3 (rutile 100%) and (B) TIO-4 (anatase 75% + rutile 25%) film electrodes. The electrode potentials indicated in the figure are measured vs Ag/AgCl. The illumination was performed from the TiO2-film side.

mentioned above. In a very negative potential of -0.60 or -0.65 V, ηPC is negative all over the wavelengths measured. Figure 9 shows the effect of the illumination direction on the ηPC-λ curves in O2-bubbled 0.1 M NaOH for (A) rutiletype and (B) mainly anatase-type nanocrystalline TiO2 electrodes. Contrary to the case of illumination from the TiO2-film side (marked by solid circles), which was explained thus far

It has been reported in the literature18,19,28-32 that nanocrystalline TiO2 film electrodes usually show anodic photocurrents in aqueous electrolytes, as really observed in Figures 1 and 4 in the present work. However, the present work has revealed that the nanocrystalline TiO2 electrodes also show cathodic photocurrents under particular conditions, e.g., in alkaline solutions containing dissolved oxygen under illumination from the TiO2-film side at short wavelengths such as 300 nm. The cathodic photocurrents start to flow at a potential 0.6-0.9 V more positive than the flat-band potential (Ufb) of single-crystal n-TiO2 electrodes. Let us first consider the increased dark cathodic currents in O2-bubbled 0.1 M NaOH (Figures 1D and 4D) because they are much larger in magnitude than the cathodic photocurrent. We mentioned in the preceding section that they were attributed to the O2 reduction at the SnO2(substrate)/solution interface. This conclusion is reasonable theoretically because, at potentials more positive than the Ufb of n-TiO2, the Fermi level of the SnO2 electrode (as a metallic conductor) is below the bottom of the conduction band of TiO2 particles and thus electrons in the SnO2 electrode cannot enter the TiO2 film. This implies that almost no dark cathodic current can flow at the TiO2/solution interface in these potentials, or in other words, only photocurrents can flow at the TiO2/solution interface in these potentials. The generation of cathodic photocurrents can be explained by a model shown in Figure 10. A similar model was reported


Figure 10. Schematic illustrations of expected mechanisms of generation of (A) anodic and (B) cathodic photocurrents for nanocrystalline TiO2 film electrodes in an aqueous electrolyte.

by Hodes et al. for nanocrystalline CdS and CdSe films.20 The illumination generates electron-hole pairs in TiO2 particles. The anodic photocurrent flows in the case where the holes react efficiently at the TiO2 surface with the electrolyte (water) and the electrons migrate to the SnO2-substrate through the TiO2 film, as shown in Figure 10A. On the other hand, the cathodic photocurrent flows in the case where the electrons react efficiently with an oxidative reagent in the electrolyte, such as dissolved oxygen, and the holes migrate to the SnO2 substrate. Actually, both the processes will occur simultaneously in nanocrystalline TiO2 film electrodes, because no band bending is present in nanosized TiO2 particles, contrary to the case of single-crystal n-TiO2 electrodes in which the band bending and thus the Ufb position solely determine the surface processes. Thus, the observed photocurrent for the nanocrystalline TiO2 film electrode is given by a difference in the rates of the above processes of the opposite directions. The aforementioned results that anodic photocurrents are usually observed imply that the reaction of holes with water is in most cases faster than the capture of electrons by an electron acceptor such as dissolved oxygen. On the other hand, the observation of cathodic photocurrents in O2-dissolved alkaline solutions indicates that the rate of the electron capture becomes much higher and exceeds the rate of the hole reaction in this solution. It is to be noted here that single-crystal n-TiO2 electrodes show clear cathodic dark currents attributable to the O2 reduction in alkaline solutions (Figure 5). The aforementioned increase in the dark cathodic current at the SnO2/solution interface in O2-bubbled 0.1 M NaOH also indicates that the O2 reduction at the metal-oxide surface becomes efficient in alkaline solutions. The surface of TiO2 in alkaline solutions should be covered with anionic groups such as Ti-O-, formed by deprotonation of surface Ti-OH, because the point of zero charge of TiO2 is about 5.5.33,34 We can thus assume that chemical adsorption of oxygen molecules occurs on the TiO2 surface in alkaline solutions through a charge transfer (CT) interaction between surface Ti-O- group as an electron donor and O2 molecules as an electron acceptor, as shown in Figure 11A. This will lead to an increase in the density of chemically


Figure 11. Expected structures of adsorbed O2 molecule in (A) alkaline and (B) acidic electrolytes.

adsorbed O2 molecules as well as the rate of electron transfer from the conduction band of TiO2 to the adsorbed O2 molecules, in contrast to the case of only weak physical adsorption of O2 in acidic and neutral solutions (Figure 11B). This argument is supported by the fact that the cathodic current attributable to the O2 reduction increases with increasing solution pH (Figure 5), because the density of surface anionic groups is expected to increase with the solution pH. Thus, the generation of cathodic photocurrents in O2-dissolved alkaline solutions can be attributed to the effective capture of photogenerated electrons in TiO2 particles by chemisorbed O2 present in a high density in the solutions. The essential mechanism for the generation of cathodic photocurrents can be explained as discussed above,35 but they show rather complicated behavior such as the potential dependence (Figures 1, 4, and 8), the illumination-wavelength dependence (Figures 6 and 7), the illumination-direction dependence (Figure 9), and the crystal-form dependence, the elucidation of which will be important in clarifying factors that govern the rates of anodic and cathodic processes in nanocrystalline TiO2 film electrodes. Although the details are not fully understood yet, some possible explanations on main features may be given as follows. Let us first consider the behavior of the photocurrent for the rutile-type TiO2 film electrode. As mentioned before, anodic and cathodic processes, such as shown in Figure 10, parts A and B, will occur simultaneously in the TiO2 film electrode, the observed photocurrent thus being given by a difference in the rates of such processes. Figure 12 schematically shows the most important process that will determine the sign of the photocurrent, under several experimental conditions, together with the photogeneration rate for electron-hole pairs as a function of the distance (x) from the SnO2/TiO2 interface, which is in proportion to the intensity of penetrating light. The penetration depth of illuminated light at 350 nm is estimated36 to be of the order of several hundred nanometers, considerably smaller than the thickness of the TiO2 film of about 1.2 µm in the present work. The penetration depth decreases with decreasing λ, as the absorption coefficient of TiO2 increases with


Figure 12. Schematic illustrations of the most important process that will determine the sign of the photocurrent, under several experimental conditions, together with the photogeneration rate for electron-hole pairs as a function of the distance (x) from the SnO2/TiO2 interface in a nanocrystalline TiO2 film electrode.

decreasing λ.19 Accordingly, for illumination from the TiO2film side at short λ, high-density electron-hole pairs are generated only in the TiO2 film near the TiO2/solution interface, as shown in Figure 12A. The observation of the anodic photocurrent in acidic solutions under this condition (Figure 6A) can thus be explained by assuming that the reaction of holes with water is faster than the capture of electrons by dissolved O2 owing to its weak physical adsorption, as mentioned before, and as illustrated in Figure 12A. In an O2-bubbled alkaline solution (0.1 M NaOH), on the other hand, the cathodic photocurrent is observed under illumination at short λ, whereas the anodic photocurrent is observed at long λ (Figure 6B). The former result can be explained by assuming that the electron capture by the chemisorbed O2 in alkaline solution surpasses the hole oxidation of

Tsujiko et al. water, as shown in Figure 12B. Now, how can the latter result be explained? For illumination at long λ, electron-hole pairs are generated throughout the TiO2 film, including the part near the TiO2/substrate (SnO2) interface, as shown in Figure 12C. In such a case, we can assume that high-mobility electrons, generated in the TiO2 film near the TiO2/substrate (SnO2) interface, can enter effectively into the substrate, resulting in an anodic photocurrent (Figure 12C). If this type anodic photocurrent exceeds the cathodic photocurrent generated in the TiO2 film near the TiO2/solution interface by the mechanism of Figure 12B, then an anodic photocurrent is observed. The explanation is supported by the increase in the anodic photocurrent for illumination from the SnO2 side at short λ (Figure 9A), which can be explained easily by taking account of a high generation rate for electron-hole pairs near the TiO2/substrate (SnO2) interface, as shown in Figure 12D. The mainly anatase-type TiO2 film electrodes show behavior similar to that of the rutile-type discussed above, except that the former electrodes show the additional cathodic photocurrent under illumination in a long-λ region near 400 nm (Figure 9B). The additional cathodic photocurrent, which is peculiar to the mainly anatase-type TiO2 film, may tentatively be explained by taking account of the mixing of rutile- and anatase-type TiO2 particles in the films. For illumination at long λ, the light will be absorbed more effectively by rutile-type particles with the smaller band-gap (Eg ) 3.0 eV) than anatase-type (Eg ) 3.2 eV). Moreover, photogenerated electrons in the mixed TiO2 film will have a smaller mobility than in the pure rutile- or anatasetype TiO2 film owing to uneven conduction bands in the TiO2 film. This implies that the mechanism of Figure 12C, leading to the generation of an anodic photocurrent, becomes less important in the mixed TiO2 film, especially under illumination at long λ, resulting in the generation of a cathodic photocurrent. Finally let us consider the potential dependence of the cathodic photocurrents. Both the rutile- and mainly anatasetype electrodes show cathodic photocurrents only in a potential region about 0.5∼0.9 V more positive than the Ufb of singlecrystal n-TiO2 electrodes, or in other words, they show anodic photocurrents in highly positive potentials even in O2-dissolved 0.1 M NaOH (Figures 1 and 4). For nanocrystalline TiO2 film electrodes, no band bending can be assumed in TiO2 particles, and thus such a potential dependence will be ascribed to the properties of electrical junction at the TiO2-particles/SnO2 contact. The observation of the cathodic photocurrent in potentials about 0.5∼0.9 V more positive than the Ufb implies that valence-band holes in TiO2 particles can enter into the SnO2 substrate in such potentials (see Figure 10B). On the other hand, the disappearance of the cathodic photocurrent and the appearance of the anodic photocurrent in highly positive potentials indicate that the hole transfer from TiO2 particles to the SnO2 substrate is inhibited in the highly positive potentials. We reported in a previous paper18 that the electrical contacts between the TiO2 particles and SnO2 substrate were composed of a mixture of the Bardeen-type junction and the extended Schottky junction. At the extended Schottky junction, at which no band bending is present at the TiO2-particle/SnO2 interface,18 the valence-band holes in TiO2 particles can enter into the SnO2 substrate. It is to be noted here that no band bending at the TiO2-particle/SnO2 interface for the extended Schottky junction is maintained by an adsorption equilibrium of H+ or OH- ions between the interface and the electrolyte.18 Thus, the full coverage, or inversely complete exhaustion of adsorbed H+ and OH- at the TiO2-particle/SnO2 interface in highly positive potentials may lead to the formation of band bending (or an

Cathodic Photocurrents at TiO2 Film Electrodes energy barrier for hole transfer) even at the extended Schottky junction, resulting in the disappearance in the cathodic photocurrent. In conclusion, the present work has revealed that nanocrystalline TiO2 (rutile and anatase) film electrodes show not only anodic but also cathodic photocurrents, depending on experimental conditions. Which photocurrent is observed depends on the light penetration depth (or the profile of the photogeneration rate for electron-hole pairs) as well as the relative rates of various electronic and chemical processes in the TiO2 film. The analyses of the behavior are useful to clarify the factors that govern the photoelectrochemical or photocatalytic activity of nanocrystalline TiO2 films. Acknowledgment. This work was partly supported by Grantin-Aid for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (No. 09237105) from the Ministry of Education, Science, Sports and Culture, Japan. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature(London) 1991, 335, 737. (2) Hagfeldt, A.; Gra¨tzel, M. Sol. Energy Mater. Sol. Cells 1994, 32, 243. (3) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (4) Gru¨nwald, R.; Tributsch, H. J. Phys. Chem. B 1997, 101, 2564. (5) Zaban, A.; Meier, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 7985. (6) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (7) Cai, R.; Hashimoto, K.; Itoh, K.; Kubota, Y.; Fujishima, A. Bull. Chem. Soc. Jpn. 1991, 64, 1268. (8) Maruska, H. P.; Ghosh, A. K. Sol. Energy 1978, 20, 443. (9) PhotocatalysissFundamentals and Applications; Serpone, N., Pellizzetti, E., Eds.; Wiley: New York, 1989. (10) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (11) Semiconductor Nanoclusters; Kamat, P. V., Meisel, D., Eds.; Studies in Surface Science and Catalysis 103; Elsevier: Amsterdam, 1996. (12) Takeda, N.; Ohtani, M.; Torimoto, T.; Kuwabata, S.; Yoneyama, H. J. Phys. Chem. B 1997, 101, 2644. (13) Matsumoto, Y.; Shimizu, T.; Sato, E. Electrochim. Acta 1982, 27, 419.

J. Phys. Chem. B, Vol. 106, No. 23, 2002 5885 (14) Granqvist, C. G. Handbook of Inorganic Electrochromic Materials; Elsevier: Amsterdam, 1995; Chapter 15. (15) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608. (16) So¨dergren, S.; Siegbahn, H.; Rensmo, H.; Lindstro¨m, H.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. 1997, 101, 3087. (17) So¨dergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 98, 5552. (18) Shiga, A.; Tsujiko, A.; Ide, T.; Yae, S.; Nakato, Y. J. Phys. Chem. B 1998, 102, 6049. (19) Shiga, A.; Tsujiko, A.; Yae, S.; Nakato, Y. Bull. Chem. Soc. Jpn. 1998, 71, 2119. (20) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (21) Tsujiko, A.; Kisumi, T.; Magari, Y.; Murakoshi, K.; Nakato, Y. J. Phys. Chem. B 2000, 104, 4873. (22) Kavan, L.; Gra¨tzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716. (23) Chemical Handbook Basic Part II; The Chemical Society of Japan, Ed.; Maruzen: Tokyo, 1993; p II-467. (24) Finklea, H. O. Semiconductor Electrodes; Finklea, H. O., Ed.; Studies in Physical and Theoretical Chemistry 55; Elsevier: Amsterdam, 1988; pp 43-145. (25) Gandia, J.; Pujadas, M.; Salvador, P. J. Electroanal. Chem. 1988, 244, 69. (26) Salvador, P. J. Phys. Chem. 1985, 89, 3863. (27) Pujadas, M.; Salvador, P. J. Electrochem. Soc. 1989, 136, 716. (28) Wahl, A.; Ulmann, M.; Carroy, A.; Jermann, B.; Dolata, M.; Kedzierzawski, P.; Chatelain, C.; Monnier, A.; Augustynski, J. J. Electroanal. Chem. 1995, 396, 41. (29) Rensmo, H.; Lindsto¨m, H.; So¨dergren, S.; Willstedt, A.-K.; Hagfeldt, A.; Lindquist, S.-E. J. Electrochem. Soc. 1996, 143, 3173. (30) Zhou, M.; de Tacconi, N. R.; Rajeshwar, K. J. Electroanal. Chem. 1997, 421, 111. (31) Boschloo, G. K.; Goossens, A.; Schoonman, J. J. Electroanal. Chem. 1997, 428, 25. (32) de Jongh, P. E.; Vanmaekelbergh, D. J. Phys. Chem. B 1997, 101, 2716. (33) Yates, D. E.; James, R. O.; Hearly, T. W. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1. (34) Salvador, P. Sol. Energy Mater. 1982, 6, 241. (35) As to the origin of the cathodic photocurrent, it is difficult to assume another mechanism than the hole migration shown in Figure 10B, because the cathodic photocurrent is really steady, lasting for 10 s or more, and shows the unique dependences on the potential, the illumination wavelength, the illumination direction, and the crystal form, as discussed later in the text. (36) Eagles, D. M. J. Phys. Chem. Solids 1964, 25, 1243.

Observation of Cathodic Photocurrents at Nanocrystalline TiO2 Film

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