Langmuir 1999, 15, 3731-3737
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Suppression of Surface Recombination on TiO2 Anatase Photocatalysts in Aqueous Solutions Containing Alcohol O. A. Semenikhin,† V. E. Kazarinov,† L. Jiang,‡ K. Hashimoto,‡,§ and A. Fujishima*,| A. N. Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninskii prosp. 31, 117071 Moscow, Russia, Chinese Academy of Sciences, Institute of Chemistry, Zhong Guan Chun, Beijing, 100080, China, Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received October 14, 1998. In Final Form: March 10, 1999 Intensity-modulated photocurrent spectroscopy (IMPS) was applied to studying the mechanism of photoelectrochemical decomposition of a model organic compound, ethanol, on a TiO2 anatase photoelectrode in aqueous solution. The frequency spectra of the intensity-modulated photocurrents observed on the TiO2 photoelectrode at low band bending were drastically changed in the presence of alcohol. Without the alcohol, the frequency dependence of the modulated photocurrents were similar to those reported previously for TiO2 electrodes and were typical for the case of surface recombination occurring on surface states, while upon addition of alcohol the photocurrent frequency dependence was no longer observed. This fact suggests that alcohol suppresses the recombination processes at the surface of the TiO2 photoelectrode. Possible mechanisms for this phenomenon are discussed. The results demonstrate that IMPS is a powerful tool for studying the mechanism of photocatalytic decomposition of organic pollutants on semiconductor electrodes.
1. Introduction The photocatalytic degradation of organic compounds on TiO2 photoelectrodes has emerged as a rather promising process for purification of water containing organic pollutants.1 The mechanism of this process and the problem of enhancing the catalytic efficiency of various forms of TiO2 photoelectrodes have been extensively studied in the past decade.2-10 However, despite the considerable success that has been achieved in this area, the scientific basis for the selection of new and more efficient forms of the catalysts has yet to be adequately developed. The mechanism of the photocatalytic activity of various types of TiO2 photocatalysts in the degradation of organic compounds is still disputed. * To whom correspondence should be addressed. Tel: +81-33812-2111 ext. 7245. Fax: +81-3-3812-6227. E-mail:
[email protected]. † Russian Academy of Sciences. ‡ Chinese Academy of Sciences. § Research Center for Advanced Science and Technology, The University of Tokyo. | Department of Applied Chemistry, School of Engineering, The University of Tokyo. (1) Ollis, D. E.; Pelizzetti, E.; Serpone, N. In Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; WileyInterscience: New York, 1989; pp 603-638. (2) Wahl, A.; Ulmann, M.; Carroy, A.; Jermann, B.; Dolata, M.; Kedzierzawski, P.; Chatelain, C.; Monnier, A.; Augustynski, J. J. Electroanal. Chem. 1995, 396, 41. (3) Fujishima, A.; Nagahara, L. A.; Yoshiki, H.; Ajito, K.; Hashimoto, K. Electrochim. Acta 1994, 39, 1229. (4) Ikeda K.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 437, 241. (5) Malati, M. A. Environ. Technol. 1995, 16, 1093. (6) Nogueira, R. F. P.; Jardim, W. F. Sol. Energy 1996, 56, 471. (7) Serpone, N.; Maruthamuthu, P.; Pichat, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol., A-Chem. 1995, 85, 247. (8) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 69. (9) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1996, 415, 183. (10) Trillas, M.; Peral, J.; Domenech, X. J. Chem. Technol. Biotechnol. 1996, 67, 237.
One example of a point that still requires clarification is the nature of the effect of alcohols and some other organic compounds on the photocatalytic efficiency of various types of TiO2 catalysts. We were interested, in particular, in the mechanism of the photodegradation of organic compounds at low band bending, which is close to actual operating conditions of most practical photoelectrodes in the water treatment process when no external bias is applied. As was demonstrated earlier for TiO2 anatase photoelectrodes,2,11 addition of alcohols as well as formic and oxalic acids to the solution results in a considerable increase in the photocurrent magnitude (measured under controlled potential, e.g., as a photocurrent-potential curve), particularly at low band bending, as well as in a considerable shift of the photocurrent onset potential toward more negative values. Similar effects were also noted by Fermin et al.12 for a single-crystal rutile TiO2 electrode in the presence of formic acid as well as by Schoenmakers et al.13 for a single-crystal ZnO electrode in the presence of methanol. In principle, there are several possible reasons for such behavior. First, it should be noted that the increase in the photocatalytic activity of semiconductor photoelectrodes in the presence of organic compounds cannot be related to mere competition between the processes of the photooxidation of the organic additive and water photoelectrolysis. It is well-known (see, e.g., refs 2 and 11) that alcohols and other organic compounds are able to efficiently compete for photoexcited holes with water adsorbed at the surface of TiO2 photoelectrodes. However, this fact alone cannot account for the observed increase in the photocurrent magnitude because, if the amount of (11) Augustynski, J. Struct. Bonding 1988, 69, 1. (12) Fermin, D. J.; Ponomarev, E. A.; Peter, L. M. In Proceedings of Symposium on Photoelectrochemistry: Electrochemical Society Proceedings; Electrochemical Society, Inc.: Pennington, N. J., 1997; Vol. 97-20; p 62. (13) Schoenmakers, G. H.; Vanmaekelbergh, D.; Kelly, J. J. J. Chem. Soc., Faraday Trans. 1997, 93, 1127.
10.1021/la981437b CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999
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photoexcited holes that are transferred to species in the solution remains constant, the nature of such species cannot affect the photocurrent magnitude, provided that no special cases such as photocurrent multiplication (see below) are involved. Next, the increase in the photocatalytic activity cannot be related to the fact that the shift of the photocurrent onset potential favors the occurrence of the associated reduction reaction. This mechanism is important in treating the photocatalytic activity of photoelectrodes operating in open-circuit conditions, for instance, particulate TiO2 photocatalysts, but it cannot explain the above observations made under potentiostatic control because in this case the photocurrent values are compared at a fixed electrode potential. The remaining factors to be considered are photocurrent doubling, which is known to occur in photooxidation of alcohols and some other organic species on semiconductor electrodes,2,11,14 as well as competition between the processes of photooxidation of the organic additive and surface recombination. With the photocurrent doubling mechanism, the photoprocess involves photogeneration of intermediates that undergo further oxidation, with injection of electrons into the conduction band of the semiconductor electrode. This effect is particularly pronounced at high band bending, where it accounts for the increase in the magnitude of saturation photocurrents observed with organic compounds in the solution. However, the probability of such injection decreases at low band bending (see, e.g., ref 2) and, as will be shown below, while it is likely that this process does occur at high anodic potentials, we observed no photocurrent doubling on anatase TiO2 photoelectrodes at low band bending. The factor that is likely to be especially important at low band bending is surface electron-hole recombination. Surface recombination can strongly affect the efficiency of the overall photoprocess near the flatband potential. In particular, surface recombination can produce the effects, such as the shift of the photocurrent onset potential, noted above. These as well as other indications2,11,15 suggest that organic additives can in principle affect the rate of surface recombination processes on semiconductor electrodes. However, until recently, no direct evidence of this effect has been obtained. An efficient means for detailed examination of the mechanism of photoelectrochemical decomposition of organic compounds on semiconductor electrodes and, in particular, for direct demonstration of the effect of organic additives on the surface recombination rate and/or occurrence of the photocurrent doubling mechanism is to use intensity-modulated photocurrent spectroscopy (IMPS). The IMPS technique, which is also referred to as optoelectrochemical impedance, has proven to be a powerful tool for studying the kinetics of photoelectrochemical processes at semiconductor electrodes14,16-22 including TiO2 electrodes,23,24 but, until recently, there has been no (14) Herrasti, P.; Peter, L. M. J. Electroanal. Chem. 1991, 305, 241. (15) Gerischer, H. J. Electroanal. Chem. 1983, 150, 553. (16) Li, J.; Peat, R.; Peter, L. M. J. Electroanal. Chem. 1986, 200, 333. (17) Li, J.; Peter, L. M. J. Electroanal. Chem. 1986, 199, 1. (18) Li, J.; Peter, L. M. J. Electroanal. Chem. 1985, 193, 27. (19) Peat, R.; Peter, L. M. J. Electroanal. Chem. 1986, 209, 307. (20) Peter, L. M. Chem. Rev. 1990, 90, 753. (21) Rotenberg, Z. A.; Semenikhin, O. A. J. Electroanal. Chem. 1991, 316, 165. (22) Kelly, J. J.; Minks, B. P.; Verhaegh, N. A. M.; Stumper, J.; Peter, L. M. Electrochim. Acta 1992, 37, 909. (23) Semenikhin, O. A.; Rotenberg, Z. A.; Teplitskaya, G. L. Elektrokhimiya 1991, 27, 209. (24) Goossens, A. Surf. Sci. 1996, 365, 662.
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attempts to apply this technique to the studies of photodecomposition of organic compounds on such electrodes. Only lately two papers12,13 were published that presented the results of IMPS measurements on single-crystal TiO2 (rutile) and ZnO electrodes in the presence of organic compounds. However, both of these papers were aimed mainly at studying the photocurrent doubling reactions that occurred on such electrodes, and therefore most of the IMPS data presented there were taken at high band bending, where no surface recombination could be observed. The effect of addition of formic acid on the surface recombination rate on a single-crystal rutile electrode as inferred from IMPS data was noted in ref 12; however, no detailed analysis of this effect was performed. In this communication, we demonstrate, using IMPS, that the addition of a model organic compound, ethanol, results in virtual suppression of surface recombination on anatase TiO2 photoelectrodes. This phenomenon is likely to play the key role in enhancing their catalytic activity. The mechanisms of the photoelectrochemical processes occurring both with and without ethanol in the solution as inferred from the IMPS data are discussed. For instance, the IMPS data provide no evidence for the occurrence of photocurrent doubling at low band bending. With the IMPS technique, the electrode is illuminated in potentiostatic conditions with intensity-modulated light. The modulation is performed using an electrooptical modulator or by mechanically interrupting the light flux using a light chopper. The in-phase (real) and quadrature (imaginary) components of the resulting ac photocurrent are measured as a function of the light modulation frequency using the lock-in technique. From the form and parameters of the frequency dependence of modulated photocurrents, one can derive important information on the kinetics of surface processes that occur at semiconductor electrodes. Further details regarding the IMPS technique and the principles of the frequency spectra analysis can be found elsewhere.20,21,23 Here we note only that the IMPS technique allows easy determination of the rate of the surface recombination process17,18,20,21,23 as well as detection of the photocurrent multiplication mechanism.14,16,19,20,22 When the surface recombination is the dominant process, the amplitude of the ac photocurrent is dependent on the light modulation frequency and increases from zero to some limiting value g, which is called the generation current and is determined chiefly by the magnitude of the band bending. Alternatively, when the dominant process is the charge transfer to the species in the solution, as is the case at high values of band bending, no frequency dependence (photocurrent relaxation) is observed. The photocurrent is equal to the generation current in the whole frequency range. When the two processes are competing, the photocurrent relaxation does occur, but the low-frequency limit of the photocurrent is no longer zero and corresponds to the steady-state photocurrent that is observed when the electrode is illuminated with nonmodulated light. The above reasoning can be illustrated by simple calculations. In this section we limit ourselves solely to the simplest case of the photoelectrochemical process that involves photocurrent relaxation on a single type of surface state occurring along with the charge transfer to the solution. For simplicity, the latter is taken to occur only directly from the valence band, and the only surface-state process to be considered is surface recombination. No follow-up chemical reactions are considered. The bulk recombination is taken to occur much more rapidly than the surface recombination and is taken into account via potential dependence of the generation current g, which
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corresponds to the flux of photoexcited carriers that reach the surface of the semiconductor electrode. In most cases the generation current g can be considered frequencyindependent, at least, within the frequency range usually employed in the IMPS measurements (up to several kHz). The balance for photoexcited holes on the surface of a semiconductor electrode under modulated illumination for the above mechanism can be expressed as23
iωp ) γg - kp
(1)
Here g is the generation current, p is the complex amplitude of the surface concentration of photoexcited holes, γ is the fraction of photoexcited holes captured by the surface states, k is the pseudo-first-order rate constant of surface recombination, ω is the angular light modulation frequency, and i is the imaginary unit. The intensitymodulated photocurrent jphoto at a frequency ω can be found as the difference between the generation current (the flux of photoexcited holes that reach the surface of the electrode) and the recombination flux expressed here via the pseudo-first-order rate constant of surface recombination:
jphoto ) g - kp ) g{1 - γ/(1 + iω/k)}
(2)
The corresponding equations for the real and imaginary components of the modulated photocurrent take the form:23
{ ( {(
)} )}
Re(jphoto) ) g 1 - γ/ 1 +
ω2 k2
(3)
Im(jphoto) ) gγω/ k 1 +
ω2 k2
(4)
The photocurrent frequency dependence is usually presented either in the form of so-called Nyquist plots (dependence of the real and imaginary components of the photocurrent on the logarithm of the modulation frequency) or as Cole-Cole plots (imaginary part vs the real part of the photocurrent, the modulation frequency being the implicit parameter). Both types of plots for frequency spectra calculated by eq 1 and 2 are presented in Figure 1. From the calculated Nyquist plot (Figure 1a), one can see that both the real component Re(jphoto) and amplitude A of the photocurrent,
(5)
Figure 1. (a) Frequency dependence of (1) normalized amplitude, (2) real, and (3) imaginary components of the ac photocurrent calculated by eq 3-5 and presented as a Nyquist plot. (b) Corresponding Cole-Cole plot. The value of γ was taken to be 0.2.
increase with the modulation frequency, reaching at high frequencies a limiting value equal to generation current g. The imaginary component Im(jphoto) passes through its maximum value at ω* ) k. When presenting the results in the complex plane as a Cole-Cole plot, the corresponding diagram takes the form of semicircle (Figure 1b). The low-frequency and the high-frequency intercepts correspond to the steady-state current (1 - γ)g and the generation current g, respectively. It should be noted here that, as follows from eq 1-5, the presence of the photocurrent relaxation in the form of a semicircle in quadrant I of the complex plane and, in particular, the increase in the photocurrent amplitude with the light modulation frequency, is characteristic of the occurrence of surface recombination. When surface recombination is suppressed (k ) 0 or γ ) 0), such photocurrent relaxation is absent. The photocurrent amplitude is independent of the modulation frequency, while the corresponding Cole-Cole plots converge to a
single point on the real axis, which represents the generation current g. (This statement may be invalid if the mechanism of the photoprocess differs from that treated in this section and involves additional follow-up reactions such as photocurrent multiplication (see below), or if no correction of the experimental data for the “cell effect” was performed.) As follows from eqs 3 and 4, the experimental photocurrent frequency spectra allow one to find the values of the kinetic parameters of the photoprocess such as the recombination rate constant k, the fraction of photoexcited holes captured by the surface states γ, and the generation current g. Furthermore, from the low-frequency limit of the photocurrent Re(jphoto)ω)0, one can estimate the value of steady-state photocurrent that would be observed under nonmodulated light. As was already noted, the above equations were derived for the simplest case of surface recombination occurring on a single type of surface states along with charge transfer
A ) {Re(jphoto)2 + Im(jphoto)2}1/2
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to the solution. This mechanism is the one that is usually observed with semiconductor electrodes at low band bending. However, in some cases this mechanism can be complicated by follow-up electrochemical reactions,14,16,19,22 diffusion of reacting species,25 or photocurrent relaxation occurring at several types of surface states.21 Also, we should note additional high-frequency photocurrent relaxation that is due to uncompensated IR drop between the electrode and the tip of the Luggin capillary and is known as “cell effect”.20 Additional photocurrent relaxation can be also observed if the photoprocess involves additional steps that result in photocurrent doubling (multiplication). The latter case is of special importance for photoelectrochemical reactions involving organic species. With photocurrent doubling, the Cole-Cole plots of the ac photocurrent take a very special form and involve a semicircle in quadrant IV of the complex plane (i.e., the photocurrent lags behind the light intensity).14,16,19,20,22 This fact allows easy detection of the occurrence of the photocurrent doubling mechanism. If no such relaxation is observed, the photocurrent doubling is absent. It should be noted, however, that a similar effect could be produced by the relaxation due to the “cell effect”,20 except that the lowfrequency limits of the photocurrent will differ. However, the relaxation due to photocurrent doubling will persist even after correcting for the uncompensated IR drop in the solution, which allows one to distinguish these two cases. To this end, analogue compensation of the IR drop can be employed. We believe that this technique has some advantages as compared with the procedure used by Fermin et al. 12 because in our approach there is no need of making voluntary assumptions on the nature of the electrode impedance.
phase calibration for each modulation frequency was performed at a potential of +0.2 V, where no photocurrent relaxation was observed. The setup and procedure for the modulated photocurrent measurements are further detailed elsewhere.23 The data acquisition and processing were performed with a PC-type computer with specially designed software.
2. Experimental Section
3. Results and Discussion
The TiO2 photocatalyst samples used were in the form of ITO glass sheets with the deposited thin anatase TiO2 layer prepared by using a spray-pyrolysis technique. Further details on the preparation procedure and photocatalyst characterization can be found elsewhere.26 Electrochemical and photoelectrochemical measurements were performed in a glass cell with separated anode and cathode compartments. The electrolyte was a 0.2 M solution of Na2SO4 (reagent-grade) in doubly distilled water. In some experiments, 2 vol % of ethanol was added to the solution to compare the surface and photoelectrochemical behavior of the TiO2 catalysts during the photodegradation of organic compounds and in background electrolyte. Before and during the measurements, the solution was deaerated by purging with argon gas. The potentials were measured and are presented vs an aqueous Ag/AgCl/KCl(sat) reference electrode. Polarization of the electrodes was performed using a model PI-50-1.1 potentiostat with a model PR-8 universal programmer (Russia). For the photoelectrochemical measurements, the electrode was illuminated with a 365-nm light, which was obtained from a 250-W medium-pressure mercury lamp by means of a notch filter. The light intensity was modulated using a mechanical light chopper, which allowed the modulation frequency to be varied over a frequency range from 5 to 850 Hz. The amplitude and phase of the resulting modulated photocurrent were measured with a PAR 5204 lock-in analyzer. The photocurrent frequency spectra were measured under potentiostatic conditions with analogue compensation of the IR drop enabled. The degree of the IR compensation was controlled with an oscilloscope. As was verified in special experiments with both dummy cells and actual semiconductor electrodes, such analogue compensation allows one to completely eliminate undesirable additional photocurrent relaxation due to the cell effects. Zero-
Figure 2 presents the potential dependence of the photocurrent obtained in solutions with and without alcohol under nonmodulated light. In both cases the form of the photocurrent-potential dependence is typical of that observed for n-type semiconductor electrodes: after initial growth, the photocurrent monotonically increases with the electrode potential, tending to some limiting value. As can be seen from the figure, our data are consistent with the results of previous studies:2,11 the photocurrent values observed with alcohol are larger than those in the background solution in the whole potential range. Furthermore, with alcohol the photocurrent onset potential is shifted to more negative values by ca. 200 mV. The increase in the saturation photocurrent observed with organic species at high band bending must be related to the occurrence of photocurrent doubling2,11 because at these potentials the rate of the surface recombination is negligible and all of the photoexcited holes that reach the surface of the semiconductor electrode are transferred to species in the solution. Therefore, to produce the increase in the saturation photocurrent in the presence of organic species, the photoprocess must involve additional steps with injection of electrons into the conduction band of the semiconductor electrode. This is known as photocurrent multiplication.13,14,16,19,22 However, the situation may be different at low band bending, where the surface recombination cannot be neglected. Furthermore, the probability of electron injection decreases with a decrease in band bending.2,11 Therefore, the nature of the increase in the photocurrent values in the presence of organic species at low band bending may differ from that observed at high band bending.
(25) Semenikhin, O. A.; Rotenberg, Z. A. Elektrokhimiya 1992, 28, 1199. (26) Yanagi, H.; Ohoka, Y.; Hishiki, T.; Ajito, K.; Fujishima, A. Appl. Surf. Sci. 1997, 113-114, 426.
Figure 2. Potential dependence of the photocurrent obtained for an anatase TiO2 photoelectrode (1) under nonmodulated 365-nm light in 0.2 M Na2SO4 (dashed line) and (2) in the same solution upon addition of 2 vol % of ethanol (solid line). The potential scan rate was 50 mV/s.
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Figure 4. Frequency dependence of the photocurrent amplitude for the potentials of (1, 2) -0.1 V and (3, 4) 0.3 V measured in 0.2 M Na2SO4 (curves 2, 3) and in the same solution on addition of 2 vol % of ethanol (curves 1, 4).
Figure 3. (a) 3D plot of the real and imaginary components of the modulated photocurrent vs the electrode potential. The photocurrent frequency spectra were measured under potentiostatic conditions in 0.2 M Na2SO4 (0) and in the same solution upon addition of 2 vol % of ethanol (b). (b) Corresponding 2D complex plane plots for electrode potentials of -0.3 V (open symbols) and -0.1 V (filled symbols) taken in the same solution (b) with and (0) without ethanol.
To examine this point in more detail, we measured a series of photocurrent frequency spectra at different electrode potentials near the photocurrent onset potential in solutions both with and without alcohol. The results are presented in Figure 3a as a 3D plot and, for selected electrode potentials, in Figure 3b in the form of conventional complex plane plots (Cole-Cole plots). In the 3D plot, the x-axis is the electrode potential, while the y- and z-axes are the real and imaginary components of the ac photocurrent, the modulation frequency being a parameter. This form of data presentation allows one to follow the evolution of the photocurrent frequency spectra with the potential. From the figures one can see that the photocurrent complex plane (Cole-Cole) plots in the background solution are similar to those predicted by the model calculations (cf. Figure 1b) and represent partially depressed semicircles in quadrant I of the complex plane. As was shown above, such behavior is indicative of the occurrence of surface recombination. The same conclusion follows from Figure 4, which presents the frequency dependence of the photocurrent amplitude for the poten-
tials of -0.1 V and -0.3 V. One can see that, in the absence of the alcohol, the photocurrent amplitude monotonically increases with the modulation frequency, as must be the case for surface recombination occurring at the surface states. Using the approach outlined above, we can determine the values of the kinetic parameters of the photoprocess, including the recombination rate constant k and the fraction of photoexcited holes that escape recombination and give rise to photooxidation processes in the solution (1 - γ). Corresponding potential dependence for the case when no alcohol is added to the solution is presented in Figure 5. One can see that the closer the potential to its flat band value, the higher the recombination rate and the lower the fraction of photoexcited holes that are involved in the charge transfer to the solution, or, in other words, the lower the rate of photooxidation of species in the solution. This behavior is typical for semiconductor electrodes; in particular, similar results were obtained earlier for TiO2 rutile electrodes in sulfuric acid solutions.23 The situation, however, is drastically changed when alcohol is added to the solution. In this case the complex plane plots converge to a single point on the real axis (Figure 3), while the photocurrent frequency dependence is no longer observed (Figure 4). Even at the most negative potentials, the photocurrent is independent of the modulation frequency. These facts suggest that addition of alcohol totally suppresses the recombination process on the surface of TiO2 photocatalyst, and, even at low band bending, all of the photoexcited holes that reach the surface of the semiconductor electrode can now participate in the oxidation of species in the solution, thus giving rise to an increase in efficiency of the overall photoprocess. This is also illustrated in Figure 6, which presents the potential dependence of the high frequency and low-frequency intercepts of the photocurrent complex plane plots. As was noted above, the low-frequency photocurrent limit corresponds to the photocurrent that would be observed
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Figure 5. Potential dependence of (1) the recombination rate constant k (0) and (2) the fraction of photoexcited holes transferred to the solution 1-γ (4) derived from the photocurrent frequency dependence obtained without the alcohol.
Figure 6. Potential dependence of (1) frequency-independent photocurrent observed in 0.2 M Na2SO4 with 2 vol % ethanol as well as (2) high-frequency and (3) low-frequency intercepts of the photocurrent complex plane plots obtained in the same solution without alcohol.
with nonmodulated illumination, which corresponds to typical operating conditions of the photocatalysts. As follows from the figure, these values are higher with alcohol (curve 1) than for the background solution (curve 3) in the whole potential range, which is in good agreement with the data of Figure 2 as well as literature data.2,11
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This observation is very important also from the viewpoint of validating the conclusions derived from analysis of the photocurrent frequency dependence. Because the experimental frequency range is limited by the capabilities of the modulation technique employed (in our case the frequency range was 5-850 Hz), it could be argued that addition of alcohol resulted in such a change in the surface recombination rate that the corresponding photocurrent relaxation is simply no longer observed (but would be observed if we extended the experimental frequency range). However, if this were the case, for instance, if the recombination rate increased on addition of alcohol to such extent that it became much higher than it had been without the alcohol, this would have given rise to a considerable increase of the recombination losses. Therefore, both the photocurrent values observed under nonmodulated light and the low-frequency photocurrent limit would have been much lower on addition of alcohol than in background solution. As can be seen from Figures 2 and 6, the situation is opposite, which supports our conclusion that addition of alcohol suppresses surface recombination on TiO2 photoelectrodes. Again, it should be noted here that the photocurrent spectra provide no evidence of the occurrence of photocurrent doubling. If this were the case, the photocurrent Cole-Cole plots would have involved a semicircle in quadrant IV of the complex plane, whereas the photocurrent frequency dependence would have demonstrated a decrease in the photocurrent amplitude at high frequencies.19,20 However, no evidence of such behavior is observed in Figures 3 and 4. Therefore, the IMPS data indicate that addition of alcohol to the solution gives rise to a considerable increase in the photocurrent efficiency that is due to suppression of the recombination process on the surface of the TiO2 photocatalyst. The nature of such a phenomenon could involve two possible situations. First, alcohol can reduce the surface recombination rate but preserve the possibility of hole capture by surface states (k ) 0; γ * 0). In this case, the only process occurring at the surface states would be their charging-discharging under modulated illumination. Second, addition of alcohol can result in suppression of the entire process of charge transfer to the surface states (γ ) 0). These two cases cannot be distinguished by mere analysis of the photocurrent frequency spectra; however, we can draw some conclusions by examining the potential dependence of the generation current g, which is equal to the high-frequency photocurrent limit in the case of surface recombination or to the frequencyindependent photocurrent observed without surface recombination. Such dependences are given in Figure 6. If the process of charge transfer from/to the surface states and, therefore, the potential distribution across the semiconductor/solution interface were unaffected by the addition of alcohol and the only effect of alcohol were the reduction in the surface recombination rate (case I above), the values of the generation current g, which can be considered to be determined only by the magnitude of band bending (via the competition between the processes of separation and bulk recombination of photogenerated electron-hole pairs) and independent of the rate of surface processes, would have been the same both with and without alcohol in the solution. From Figure 6 one can see that this is not the case. Furthermore, whereas g is strongly dependent on the electrode potential in the alcoholcontaining solution, this dependence is much less pronounced for the background solution. This could be considered as an indication of strong Fermi level pinning,
Suppression of Surface Recombination
which is observed in the background solution and virtually vanishes upon addition of alcohol. The nature of the Fermi level pinning is the potential redistribution between the space-charge layer in the semiconductor bulk and the Helmholtz layer in the solution. If the position of the Fermi level becomes close to the energy level of a surface state, further variations in the electrode potential will result in only minor changes in the potential drop across the space-charge layer and therefore in the generation current. This situation corresponds to Figure 6, curve 2. However, if no states are present at the surface of a semiconductor, or if such states are blocked so that the charge transfer from/to them is suppressed, no Fermi level pinning can be observed. The dependence of the generation current on the electrode potential will then become more pronounced (cf. Figure 6, curve 1). From the above discussion it follows that in our case addition of alcohol results not merely in a reduction of the rate of surface recombination but in virtual suppression of the entire process of charge transfer to the surface states. The most probable explanation is that alcohol adsorbs at the TiO2 surface and blocks surface active sites that act as recombination centers. A similar situation was described by Schoenmakers et al.13 for a single-crystal ZnO electrode. Using the electrochemical impedance technique, they observed that addition of alcohol resulted in changes in the potential dependence of a parallel capacitance associated with the charge transfer to/from the surface states. Without the alcohol, the parallel capacitance passed through its maximal value at an electrode potential when the Fermi level was close to the energy of that surface state. However, when alcohol was added to the solution,
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this capacitance peak associated with the charge transfer to the surface states completely vanished. The recombination centers on the surface of TiO2 electrode are usually associated with surface Tis-Ogroups, which also act as effective mediators of water oxidation.2,11 Therefore, these results present direct evidence that alcohol is able to effectively compete for photoexcited holes with not only water photooxidation but also with surface recombination processes. This mechanism is likely to play a key role in the superior catalytic activity of anatase TiO2 photocatalysts in the photodegradation of organic pollutants. 4. Concluding Remarks Comparison of the frequency spectra of modulated photocurrents obtained with and without alcohol in the solution shows that alcohol suppresses surface recombination over a wide potential range near the flat band potential. Such behavior could be responsible for the high catalytic activity of anatase in the photodecomposition of organic pollutants. The results demonstrate the potential of IMPS for the study of the mechanism for the photocatalytic degradation of organic compounds on semiconductor photoelectrodes. Acknowledgment. The authors thank Dr. D. A. Tryk for carefully reading the manuscript and making valuable comments. The work was partially supported by the Russian Foundation for Basic Research, Grant no. 96-15-97320. LA981437B
