J. Phys. Chem. 1993, 97, 9040-9044
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Electrochemically Assisted Photocatalysis. Ti02 Particulate Film Electrodes for Photocatalytic Degradation of 4-Chlorophenol K. Vinodgopa1,'Vt Swat Hotchandani,bf and Prashant V. Kamatfs Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408, Centre de Recherche en Photobiophysique, Universith du Quhbec h Trois RiviPres, Trois RiviPres, Quhbec, Canada G9A 5H7, and Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: May 17. 1993'
A commercially available Ti02 powder (Degussa P25) has been used to prepare thin particulate films on conducting glass plates. These semiconductor particulate films are photoelectrochemically active with properties similar to an n-type semiconductor. The recombination between the photogenerated charge carriers can be suppressed by applying an external anodic bias. Electron scavengers, such as oxygen, affect the photocurrent generation by competing for the photogenerated electrons. These particulate films provide a convenient method for accelerating the photocatalytic reaction by applying an external bias. For example, the rate of photocatalytic degradation of 4-chlorophenol greatly increases when the Ti02 particulate film electrode is maintained at an external anodic bias (0.6 V vs SCE) during the UV photolysis.
Introduction Thin semiconductorfilms generated from colloidal suspensions exhibit interesting electrochemical and photoelectrochemical properties.1-1s These particulate films possess highly porous structure and can be easily surface-modified with sensitizers, redox couples, and short-bandgap semiconductor particles. Colloidal suspensions of Zn0,l-S Ti02,'1-14and W036 have been employed to develop photoelectrochemically active thin semiconductor films. The most striking feature of such a particulate film is the ability to retain the photophysical and photochemical properties of individual semiconductor particles and thus carry out the photocatalytic reactions with similar selectivity and efficiency as in the semiconductor particle suspensions. For the first time we have employed a commercially available anatase Ti02 (Degussa P-25) to develop photoelectrochemically active films on optically transparent electrodes. There is considerable interest in this commercial product because of its photocatalytic property of degrading organic contaminants from air and water.16 For example, complete mineralization of halogenated phenols can be achieved with excitation of the suspended anatase Ti02 (Degussa P-25) particles. The decontamination process in slurry reactors requiresremoval of the catalyst by filtration. Any process that could avoid the filtration step (for example, immobilizing the photocatalyst particles on a solid substrate) would be of great practical benefit.l 7 In a suspension, semiconductor particles behave as shortcircuited microelectrodes under bandgap excitation and thus promote oxidation and reduction on the same particle. One of the disadvantages of such a system is the high degree of recombination between photogenerated charge carriers. The charge separation is facilitated by the space charge layer region at the semiconductor electrolyte interface. However, in a semiconductor particulate film the differing rates of electron or hole injection into the solution control the charge separation. If one can drive away the photogenerated electrons from the semiconductor particle (for example, with the application of an electric field), it should be possible to improve the efficiency of oxidation (i.e., reaction with holes) at the semiconductorelectrolyte interface.
* To whom correspondence should be addressed. Universitd du Qudbec a Trois Rivibres.
+ Indiana University.
1 University of Notre Dame. *Abstract published in Advance ACS Abstracts, August 15, 1993.
0022-3654/93/2097-9040$04.00/0
Although few initial electrochemical studies have been carried out with Ti02 slurries,1*-20it has not been possible to influence a photocatalytic reaction in a semiconductor particulate system with an externally applied bias potential. The semiconductor particulate films provide a convenient way of manipulating the photocatalytic reaction by electrochemical methods. The photoinduced charge separation in a semiconductor particulate film is illustrated in Scheme I. The photoelectrochemical characterization of the Ti02 particulate film electrodes and their application in the electrochemically assisted degradation of 4-chlorophenol are presented here. Experimental Section Materials. Optically transparent electrodes (OTE) were cut from a conducting glass plate obtained from Donelley Corp., Holland, MI. Ti02 powder (Product name P-25, particle size 30 nm, surface area 50 m2/g) was a gift sample from Degussa Corp. The major fraction of this sample consists of anatase form of Ti02. All other chemicals were analytical reagents of highest available purity. Preparation of Ti02 Particulate Films. A suspension of Ti02 was prepared by suspending a known amount of the Degussa P-25 powder in water (1.6 g/L) and sonicating for 10 min. A 0.5-mL sample of the Ti02 slurry was applied to a conducting surface of 0.8 X 4 cm2 of OTE and was dired in air at elevated temperatures on a warm plate. The TiO2-coated glass plate was then sintered at 673 K for 1 h. The semiconductor thin film sintered at 673 K adhered strongly to the glass surface and was stable in the pH range 1-13. From gravimetric analysis the thickness of Ti02 film was estimated to be 1Wm. Such optically transparent conducting glass electrodes covered with thin Ti02 particulate film will be referred to as OTE/Ti02 electrodes. Absorption spectra were recorded with a Perkin Elmer 3840 diode array spectrophotometer. Electrochemical and Photoelectrochemical Measurements. These measurements were carried out with a standard threecompartment cell consisting of a Pt wire gauze counter electrode and a saturated calomel electrode (SCE) as reference. A Princeton Applied Research (PAR) Model 173 potentiostat and Model 175 universal programmer or BAS 100 electrochemical analyzer were used in electrochemicaland spectroelectrochemical measurements. Photocurrent measurements were carried out with a Kiethley Model 617 programmable electrometer. A collimated light beam from a 250-W xenon lamp or a 1000-W halogen lamp
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0 1993 American Chemical Society
Electrochemically Assisted Photocatalysis of 4-Chlorophenol
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9041
anodic photocurrent which increases with increasing anodic bias. Thegeneration of anodic photocurrent is a characteristic behavior of an n-type semiconductor.21 In the conventionalphotoelectrochemicalcells employingsingle crystal or polycrystalline materials, the charge separation is facilitated by the space charge layer at the electrode/electrolyte interface. The potential gradient of the space charge layer region promotes the flow of electrons and holes in the oppositedirections. However, the mechanism of photocurrent generation at a semiconductorparticulate film is quitedifferent. These individual particles in the film are too small (Dp 30 nm) to form a space charge layer at the electrolyte interface. Even if some band bending may occur as a result of heavy doping, the charge recombination is still a dominant process in such small particles. Charge separation in these particles can only be achieved from the differing rates of electron and hole transfer at the solution interface. For example, Hodes and co-workers15haveshown that preferential hole injection into the electrolyte makes thin CdSe films an n-type semiconductor. It is evident from the i-V characteristics in Figure 1 that photogenerated holes quickly migrate to the particle/solution interface while electrons move toward OTE, thus giving the film an n-type semiconducting behavior. The application of an anodic bias to an OTE/Ti02 electrode further provides a potential gradient within the film to drive away the photogenerated holes and electrons in different directions efficiently. This energy gradient within the particulate film is very similar to the band bending observed in conventional PE cells. Thus, one can minimize charge recombination with the application of an anodic bias to the Ti02 film. Application of this simple logic of achieving better charge separation in particles can be important for improving the efficiency of photocatalytic degradation of organic contaminants. The electrode reactions responsible for the generation of photoanodic current can be summarized as follows:
-
t
+0.8
I I
I I
+0.4
v
0
I I
-0.4
-0.8
vs. SCE Figure 1. i-Ycharacteristicsof OTE/Ti02 (RE:SCE, scan rate 5 mV/s) in (a) Nz-saturated and (b) 02-saturated aqueous solution of 0.05 M NaOH. The traces were recorded in dark and under illumination with UV light.
SCHEME I: Photoinduced Charge Separation in a Semiconductor Particulate Film
at TiO, photoanode:
TiO,
+ hv
-
TiO,(h--e)
-
TiO,(h)
+ OH-s,,, or (OH-)
TiO,(h)
+ OH--
(1)
OH'
or
at dark Pt cathode: Electrolyte
was used for excitation of the electrode. A Bausch and Lomb high-intensity grating monochromator or a 300-nm cutoff filter was introduced into the path of the excitationbeam for the selective excitation of the T i 0 2film. All measurements were carried out at room temperature (-296 K). Results and Discussion Photoelectrochemical Propertiesof OTE/Ti02 Electrode. As indicated in Scheme I, thin semiconductor particulate films prepared from particulate suspensions consist of small particles which are in close contact with each other and are capable of exhibiting photoelectrochemical properties similar to polycrystalline semiconductor films. The i-V characteristics of OTE/ Ti02 electrode in N2- and 02-saturated solutions are shown in Figure 1. In the dark, only cathodic current is seen as a result of accumulation of electronsin these particles. The optical effects arising as a result of such accumulation in Ti02 and WO3 particulate films have been studied earlier.6JO Under UV illumination (A > 300 nm), the OTE/Ti02 electrode exhibits an
e
'/.,02+ '/,H20
+ 0,
-
0;
(2b) (3)
Although 0 2 evolution (reaction 2b) is a major process on the rutile we did not observe any 0 2 formation on the Ti02 particulate film in the present experiments. The Ti02 particles in this film are predominantly anatase and are likely to have high overvoltage for the 0 2 generation. Thus, reaction 2a dominates the anodic process. Surface hydroxyl groups are likely to be the primary hole scavengers leading to the generation of hydroxyl radicals. Another interesting behavior of the i-V characteristics is the observation of a zero current in the potential range of -0.05 to -0.75 V. This potential, which is similar to a flatband potential of an n-type semiconductor, indicates that all the photogenerated chargecarriers are lost in the recombinationprocess. Comparison of i-V profiles in Figure la,b indicates that the presence of 0 2 shifts the zero-current potential slightly to the positive in the dark. However, this shift is significant under UV illumination. For example, under illumination the zero-current potential shifts from -0.75 V in deaerated solution to -0.05 V in oxygenated solution. Oxygen is a good electron scavenger (D(02/02-) = -0.33 V vs NHE) and can react readily with the photogenerated electrons from a Ti02 particle (ECB= -0.75 V vs NHE at pH
Vinodgopal et al.
9042 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 ILL.
r-
ILL. OFF
ON
N2
a
Y
a 600
cr, 0
0
c
2
a
CE
IE
E
c
I
Potentiostat
4001 I
! w
0 0
IO;
0
Pt
0
200 0
f
0
b
0
-1
I
1
I.
4
6
a
1
2
0
Time, min
0
0
ILL. OFF
ON
0
0
Figure 2. Open-circuit photovoltage response of OTE/Ti02 ( R E SCE) to UV illumination. The electrolyte (0.05 M NaOH) was saturated with (a) N2 and (b) 0 2 . ILL.
0
0
D
4CP
+ OH' * Products \
I
glass frit
hv
Figure 4. Schematic diagram of the electrochemical cell employed to carry out photocatalytic degradation of 4-CP.
is significantly lower with a maximum photopotential of only 60 mV. Oxygen adsorbed on the Ti02 surface is a good scavenger for the photogenerated electrons (reaction 9,and as a result of Ti02(e,) + 0, --+ TiO,
--
t I
I I
I
I
I
I
0
I
2
3
4
Time, min Figure 3. Photocurrent of OTE/Ti02 following UV illumination at an applied potential of 0.02 V vs SCE. The electrolyte (0.05 M NaOH) was saturated with (a) N2, (b) air, and (c) 0 2 .
12). The experiment described in Figure 1b indicates that the electrons trapped near the semiconductor surface are quickly transferred to the adsorbed 0 2 , thus yielding a higher cathodic current. This cathodic process seems to be the reason for the shift in the zero-current-potential. The i-V profiles shown in Figure 1 confirm the important role of 0 2 adsorbed on the Ti02 particles by directly participating in the cathodic process. Effect of Oxygen on the Photovoltage and Photocurrent Generation. In the electrochemical study of Ti02 slurries, Gerischer and Heller have highlighted the rate of 0 2 reduction as the limiting step in the oxidation of organic molecules at the irradiated Ti02 particles.20 In order to assess the effect of oxygen on the performance of the OTE/Ti02 electrode, photovoltagetime and photocurrent-time profiles were recorded (Figures 2 and 3). In N2-saturated solutions a quick rise in the open-circuit potential is seen as the electrons are accumulated at the particle surface. Immediately after the photoinduced charge separation (reaction l), the electrons are quickly trapped at the surface (reaction 4) TiO,(e)
-
TiO,(e,)
(4) where Ti02(et) represents trapped electrons presumably at the Ti4+sites. Such an accumulation of electrons in the particulate -film collectively gives rise to an open-circuit photopotential of -880 mV. The photopotential attains a steady state in less than a minute. Upon stopping the illumination, the potential decays slowly as the stored electrons leak out of the film to react with the trace oxygen. In 02-saturated solutions,the observed potential
+ 0;
(5)
this no significant accumulation of electrons occurs in the Ti02 particulate film. This electron scavenging effect is also reflected in the quick decay of the photopotential upon stopping the illumination. The photocurrent response of the OTE/Ti02 electrodeis shown in Figure 3. A prompt generation of photocurrent is seen when the OTE/Ti02 electrode is illuminated with the UV light. The photocurrent was maximum in the absence of 0 2 . The observed effect of 0 2 in decreasing the photocurrent is similar to the one reported by Gerischer and Heller in a Ti02 slurry system.20cIt is important to note that in the case of a single-crystal semiconductor electrode the oxygen dissolved in an electrolyte plays a very minor role. The formation of a space charge layer at the electrode/electrolyte interface in these systems provides the necessary potential gradient to drive away the electrons from the electrolyte interface. However, in thin particulate films it is difficult to achieve an ideal space charge layer at the electrode/ electrolyte interface.13J5 Because of this, the electrons easily leak out and react with the 0 2 adsorbed on the semiconductor particle. Thus, the efficiency of electron delivery to the counterelectrode via an external circuit is diminished in the presence of an electron scavenger, such as 0 2 . Photocatalysis with OTE/Ti02 Electrodes. Considerable interest has been shown in recent years in utilizing semiconductor particulate systems for carrying out photocatalytic reactions. In particular, anatase Ti02 is found to be very effectivein degrading organic contaminants from air and ~ater.23.2~ One such system that has been extensively studied in this context is the mineralization of 4-chlorophenol (4-CP) in an UV-irradiated Ti02 slurry system. In order to assessthe ability of OTE/Ti02 electrode to promote the photocatalytic process, we monitored the degradation of 4-CP in a photoelectrochemical operation. The schematic diagram of the photoelectrochemical cell used to carry out the degradation of 4-CP is shown in Figure 4. By separating the working and counter electrodecompartments with a fine glass frit, it was possible to control the oxidation and reduction processes in separate compartments. N2 was bubbled continuously in the working electrode compartment so that the oxidation of 4-CP is initiated only by the photogenerated holes at the semiconductor/electrolyteinterface. In Ti02 slurry systems it has been shown that oxygen is essential for facilitating
Electrochemically Assisted Photocatalysis of 4-Chlorophenol I.Or
The Journal of Physical Chemistry, Vol. 97,No. 35, I993 9043
I
Wavelength (nm) Figure 5. Absorption spectrum of 0.8 mM 4-CP recorded at different
time intervals following the photolysis of the OTE/Ti02 electrode: (a) 0, (b) 25, (c) 60, and (d) 155 min. The OTE/Ti02 electrode was maintained at an anodic bias of +0.6 V vs SCE, and the solution (pH 9 ) was continuously bubbled with a slow stream of nitrogen. photocatalytic degradation of 4-CP as it scavengesthe conduction band electrom20 Upon scavenging the photogenerated electrons, the survivability of photogenerated holes increases and thus the efficiency of the oxidation process improves. However, in the experiments described here, the dissolved oxygen is not a limiting factor since the photogenerated electrons are driven away to the counter electrode via an external circuit. The absorption spectra of a 4-CP solution recorded following the excitation of an OTE/Ti02 electrode are shown in Figure 5. An anodic bias of +O.6 Vvs SCE was applied to facilitate efficient charge separation in the Ti02 film. The absorption peaks corresponding to the 4-CP disappear completely following the photolysis, indicating thereby complete disappearance of 4-CP. The observation of increased absorbance in the 235-280-nm region a t short irradiation times is attributed to the formation of reaction intermediates such as hydroquinone. These observations are similar to those observed during the photolysis of Ti02 powder or suspension containing 4-CP.24,25 It should be noted that 4-CP alone does not undergo oxidation at an anodic potential of +0.6 V vs SCE. When the photolysis is carried out with a bare OTE electrode replacing OTE/Ti02, negligibly small changes in the 4-CP concentration are seen. Preliminary HPLC analysis of the sample drawn at the end of the photocatalytic degradation has indicated mineralization of 4-CP. This confirms the photocatalytic role of the Ti02 particulate film is very similar to that observed in slurry systems. The irradiation of the OTE/Ti02 electrode results in the generation of electron-hole pairs. The photoelectrochemical studies described above suggest that, in an Nz-saturated solution, the electrons are quickly driven to the counter electrode through external circuit while the holes react with OH-,,f or OH- at the electrolyteinterface to generate OH' radicals (reaction 2a). These OH' radicals then oxidize 4-CP. The mechanistic details of oxidation of 4-CP by OH'radicals aredescribed in earlier studies (see, for example, ref 24-26). The results described here indicate that Ti02 particulate film electrode is photocatalytically active, and its ability to degrade 4-CP is similar to that of suspended Ti02 particles in slurries. The immobilized catalyst surface employed in the present study also has the advantage of utilizing an anodic bias for improved charge separation. Electrochemically Assisted Photocatalysis Using OTE/TiOl Electrode. The i-V profiles in Figure 1 show that the charge separation in the Ti02 particulate film can easily be controlled by an externally applied bias. It would be interesting to see how such an applied potential influences the rate of 4-CP degradation on the Ti02 particulate film. The dependence of rate of photocatalytic degradation of 4-CP * on the applied potential is shown in Figure 6. The degradation
-
__
t 40
80
120
Irradiation Time (min)
Figure 6. Dependenceof 4-CPdegradation rate on the externally applied bias. The OTE/Ti02 electrode was maintained at constant potential of (a) -0.6, (b) 0.0, and (c) +0.6 V vs SCE during the photolysis, and the solution (pH 9) in the working electrode compartment was continuously bubbled with a slow stream of nitrogen.
of 4-CP was monitored from the decrease in the absorbance a t 225 nm a t various irradiation times. It is evident from these measurements that the photocatalytic degradation occurs a t a faster rate when the applied potential is maintained at +0.6 V while little degradation occurs when the potential is maintained a r o u n d 4 6 V. Since the charge separation in theTi02 particulate film is maximum when an anodic bias is applied to the OTE/ Ti02 electrode, one observes a higher efficiency for photocatalytic degradation. At potentials close to the flatband potential all the electron-hole pairs are lost in the recombination process, and hence it is not possible to carry out the oxidation of 4-CP. In a slurry system, the irradiated particles behave as short-circuited microelectrodes, and thus the charge recombination competes with the interfacial charge-transfer process. This situation closely resembles the experimental condition in which the OTE/Ti02 is maintained a t 0.0 V (curve b in Figure 6). The rate of disappearance of 4-CP at 0.0 V applied bias was similar to that obtained in the absence of any bias but in the presence of 02. Thus, with the aid of an applied electrochemicalbias it is possible to control the photocatalytic oxidation of organic contaminants such as 4-CP. Also, such electrochemicallyassisted photocatalysis overcomes the limitation of electron scavenging process which one encounters in the slurry system. HPLC analysis of the photocatalytically degraded samples has indicated mineralization of 4-CPover OTE/Ti02 surface. Further studies to compare the products and reaction mechanisms of 4-CP oxidation by electrochemical and photocatalytic methods are currently in progress. Conclusion Thin particulate films which exhibit photoelectrochemical properties similar to those of an n-type semiconductor have been successfully prepared on conducting glass plates with a commercially available anatase Ti02 powder. These thin semiconductor films provide a convenient way to enhance the efficiency of the photocatalytic degradation process. Such a beneficial aspect of electrochemicallyassisted photocatalysis can find its application in photocatalytic reactors with immobilized semiconductor particles. Acknowledgment. We thank Prof. Kimberly Gray and Mr. Ulick Stafford for helpful discussions. We also thank Degussa
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Corp. for supplying us the gift sample of Ti02 powder. K.V. acknowledges the support of Indiana University Northwest through a Summer Faculty Fellowshipand a Grant-in-Aid. P.V.K acknowledges the support of the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL3556 from the Notre Dame Radiation Laboratory. References and Notes (1) Spanhel, L.; Anderson, M. J . Am. Chem. SOC.1990, 112, 2278. (2) Spanhel, L.; Anderson, M. A., J. Am. Chem. SOC.1991,113,2826. (3) Hotchandani, S.,Kamat, P. V. Chem. Phys. Let?. 1992. 191, 320. (4) Hotchandani, S.;Kamat, P. V.J. Electrochem. Soc. 1992,139,1630. (5) Hotchandani, S.;Kamat, P. V. J . Phys. Chem. 1992, 96, 6834. (6) Hotchandani, S.;Bedja, I.; Kamat, P. V. Lungmuir, in press. (7) (a) Anderson, M. A,; Xu, Q.;Gieselmann, M. J. J . Membr. Sci. 1988, 39, 243. (b) Xu,Q.;Anderson, M. A. J. Mater. Res. 1991, 6, 1073. (c) O’Regan, B.; Moser, J.; Anderson, M.; Grltzel, M. J. Phys. Chem. 1990, 94. 8720. (8) ORegan, B.; Moser, J.; Gritzel, M.; Fitzmaurice, D. Chem. Phys. Leu. 1991, 183, 89. (9) ORegan, B.; Moser, J.; Grltzel, M.; Fitzmaurice, D. J . Phys. Chem. 1991,95, 10525. (10) Rothenberser, G.;Fitzmaurice, D.; Gritzel, M. J.Phys. Chem. 1992, 96, 5983. (1 1) ORegan, B.; Grltzel, M. Nature 1991, 353, 737. (12) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (13) Liu, D.; Kamat, P. V. J . Electroanal. Chem. 1993, 347, 451.
Vinodgopal et al. (14) Ennaoui, A.; Fighter, S.;Tributsch, H.;Giersig,M.; Vogel, R.; Weller, H. J. Elecrrochem. Soc. 1992, 139, 2514. (15) Hodes, G.; Howell, I. D. J.; Peter, L. M. J . Electrochem. Soc. 1992, 139, 3136. (16) (a) Ollis, D.; Pellizzetti, E.; Serpone, N. Bnuiron. Sci. Technol. 1991, 25, 1522. (b) Ollis, D.; El-Akabi, H. Ti02 Photocatalytic Purification and Treatment of Water and Air; Elsevier: New York, 1993. (17) (a) Sabate, J.; Anderson, M. A.; Kikkawa, H.; Edwards, M.; Hill, C. G. J . Catal. 1991,127, 167. (b) Matthews, R. W. J. Phys. Chem. 1987, 91, 3328. (c) AI-Ekabi, H.; Safarzadeh-Amiri, A.; Sifton, W.; Story, J. Int. J. Environ. Pollut. 1991, 1, 125. (18) (a) Dunn, W. W.; Aikawa, Y.;Bard, A. J. J . Elecrrochem. Soc. 1981, 128,222. (b) Ward, M. D.; Bard, A. J. J . Phys. Chem. 1986,32, 8599. (c) Ward, M. D.; White, J. R.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 27. (19) Peterson, M. W.; Turner, J. A.; Nozik, A. J. J. Phys. Chem. 1991, 95, 221. (20) (a) Gerischer, H.; Heller, A. J . Phys. Chem. 1991, 95, 5261. (b) Gerischer,H.; Heller,A.J. Electrochem.Soc. 1992,139,113. (c) Wang, C.; Heller, A.; Gerischer, H. J. Am. Chem. Soc. 1992, 114, 5230. (21) Memming, R. Top. Curr. Chem. 1988,143, 81. (22) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (23) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A,; Draper, R. B. Lungmuir 1989, 5, 250. (24) (a)Turchi,C.S.;Ollis,D. F.J.Catal. 1990,113,178. (b)Augugliaro,
P.; Palmisano, L.; Sclafani, A,; Minero, C.; Pelizzetti, E. Toxicol. Environ. Chem. 1988, 16, 89. (25) Stafford, U.; Gray, K.; Kamat, P. V.; Varma, A. Chem. Phys. Lett. 1993, 205, 55. (26) Jaeger, C. D.; Bard, A. J. J . Phys. Chem. 1979,83, 3146.
