1129
Langmuir 1991, 7, 1129-1137
Transient Near-Infrared Spectroscopy of Visible Light Sensitized Oxidation of I- at Colloidal Ti02 Donald J. Fitzmaurice and Heinz Frei’ Chemical Biodynamics Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received March 20, 1990. I n Final Form: November 26, 1990 Visible light induced photooxidation of iodide at dye-sensitized Ti02 colloid has been monitored in real time by near-infrared absorption spectroscopy. Sensitizers used were phenylfluorone (PF) and tris(2,2’biphenyl-4,4’-dicarboxylate)ruthenium(II)dichloride (RuL3). In the case of ethanolic and aqueous PF/ Ti02 sol, a transient was observed in the 700-800 nm region whose spectral profile and.extinctioncoefficient allowed assignment to 12- (2n 22,+). The decay of the transient followed a second-order rate law with a rate constant consistent wita disproportionation of 12- radical to 13- and I-. At low iodide concentration, the signal rise could be resolved and was found to be linear in [I-] with k = (1.2 f 0.2) X 108 L mol-’ s-l (ethanol). The most probable explanation for the observed rise is reaction of surface-adsorbed I atoms with I- from the bulk of the solution to give 12-. Adsorbed I atoms would be formed within the duration of the nanosecond laser pulse by reaction of oxidized sensitizer with surface-adsorbedI-. The efficiency of I- photooxidation at the PF/Ti02 colloid was found to be much lower than the corresponding photoelectrochemical efficiency at a PF/Ti02 polycrystalline electrode. We attribute it to recapture of photoinjected conduction band electrons by adsorbed I atoms. An unexpected deviation from a monotonous increase of the 12- yield as a function of I- concentration in the range of lo-‘ to 0.2 M is interpreted in terms of a wide spread of the rates of electron recapture by adsorbed I atoms. In the case of aqueous RuL3 coated Ti02 colloid, a transient observed in the 700-800-nm range has an extinction coefficient 6 times larger than that of 12-. The signal decayed according to a single exponential law with a rate constant of k = 3 X 103 s-1 that was independent of iodide concentration. The absorption is assigned to a charge transfer transition of an initially formed Ru1*L3I2-ion pair. It dissociates subsequently to 12- and sensitizer. The rise of the transient, and the observed low oxidation yields compared to those reported for corresponding photoelectrochemical cell experiments are interpreted similarly to the PF/Ti02 case.
-
I. Introduction Halide to halogen oxidation by visible light photosensitization of stable oxide semiconductor materials has recently attracted considerable attention because of its potential use for chemical storage of solar photon energy and for light to electrical energy conversion in photodriven regenerative electrochemical Particularly promising is photooxidation of halide by dye sensitization of Ti02 (band gap 3.2 eV). Using RuL3 (tris(2,2’-bipyridyl4,4’-dicarboxylate)ruthenium(II)dichloride) adsorbed on high surface area polycrystalline Ti02 films, Gratzel and co-workers achieved very high incident photon to current conversion efficiencies of 73% and 56% at the blue absorption maximum of the dye for oxidation of I- and Br-, respectively.’ These photoelectrochemical studies were supported by laser flash kinetic experiments with colloidal TiO2, which allowed determination of electron injection and back electron transfer rate constants for RuL3 adsorbed a t this semiconductor material.4 Similarly high efficiencies for iodide oxidation by sensitization of Ti02 in the visible region were obtained with ci~-RuLz(H20)2~(550 nm)2 and phenylfluorone (475nm).5 Knowledge of elementary steps of halide photooxidation at dye-sensitized TiO2, including their energetics and rates are crucial for the design of systems with intrinsically low driving forces as encountered when employing long wavelength visible and near-infrared sensitizers. Extension of the wavelength response of systems that accomplish (1) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gritzel, M. J. Am. Chem. SOC.1988,110,1216, and references therein. (2) Liska, P.;Vlachopoulos, N.; Nazeeruddin,M. K.; Compte, P.; Gritzel, M. J. Am. Chem. SOC.1988,110,3686. (3) Kiwi, J.; Gritzel, M. J. Chem. SOC.,Faraday Trans.2 1982,78,931. (4) Desilvestro, J.; Gritzel, M.; Kavan, L.; Moser, J. J. Am. Chem. SOC. 1985,107, 2988. (5) Frei, H.; Fitzmaurice, D. J.; Gritzel, M. Langmuir 1990, 6, 198.
photooxidation of halides into this spectral range critical for energy conversion is the main goal of our work. Mechanistic studies of halide to molecular halogen oxidation at Ti02 colloids have previously been conducted by Hengleid and Gratze17in the case of direct band gap excitation at 350 nm. The main finding of these laser flash kinetic studies was that for C1-, Br-, and I-, the oneelectron oxidation product X2- is formed. Results were interpreted in terms of exclusiveformation of these species by reaction of valence band holes with surface-adsorbed halide ions. No detailed studies on the reaction path of halide oxidation at dye loaded Ti02 have been reported to date. Taking advantage of our sensitive near-infrared transient absorption spectrometer based on a continuous wave (CW) probe laser,8p9 we have sought insight into elementary reaction steps of I- oxidation at sensitized Ti02 colloid by attempting to monitor in real time the build-up and decay of 12-, the expected product following one-electron oxidation of iodide. 12- has a characteristic absorption in the 700-800 nm region (211g 2Zu+,4750 nm) = 2800 L mol-’ cm-l)10 which can be used to monitor radical concentration if the visible and UV spectral range is not suitable for optical measurements due to strong sensitizer absorption. Basis for the choice of the two sensitizers used, phenylfluorone and RuL3, was that interfacial electron transfer kinetics and nature of interaction with the Ti02 surface had previously been established for both.4~~ Furthermore, these sensitizers do not exhibit optical absorption in the 650-800 nm range, which allowed us to monitor 12transient absorption with high sensitivity. A detailed
-
(6) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 241. (7) Moser, J.; Gritzel, M. Helu. Chim. Acta 1982, 65, 1436. (8) Chou, P. T.; Frei, H. J. Chem. Phys. 1987,87, 3843. (9) Braathen, G.; Chou, P. T.;Frei, H. J. Phys. Chem. 1988,92,6610. (10) Devonshire, R.; Weiss, J. J. J . Phys. Chem. 1968, 72, 3816.
0743-7463/91/2407-ll29$02.50/0 0 1991 American Chemical Society
1130 Langmuir, Vol. 7, No. 6, 1991 investigation of iodide oxidation with these two sensitizers adsorbed on Ti02 colloid by time-resolved spectroscopy is reported here.
11. Experimental Section For time-resolved absorption measurements a tuned CW dye laser probe system (Coherent Model 599-01with laser dyes LDS 751, LDS 698, and DCM (Exciton),pumped by an Ar ion laser Coherent Model Innova 90-5) was used in combination with a pulsed Nd:YAG photolysis laser (Quanta Ray Model DCR-2A). This system has been described in detail in an earlier report.* Briefly, air or Ar saturated colloidal solutions in a 1-cm quartz cell were irradiated with 532- or 355-nm pulses of 54s duration, the laser beam covering the whole width of the cell. Transient absorption was measured perpendicular to the photolysis laser beam. The dual beam absorption system was equipped with a differential photodiode detector consisting of two silicon photodiodes (United Detector Technology Model PIN-1OD) which were protected against 532-nm scattered light by optical filters (Schott Glass Model OG570). A dc voltage proportional to the difference in intensity of the light incident on the two photodiodes was sent to an EG&G amplifier Model Ortec 574. The amplified signal was digitized by a 100-MHz LeCroy transient digitizer, Model TR8818A. Static UV and visible spectra were obtained with a HewlettPackard 8452 diode array spectrometer or a Shimadzu UV-2100 spectrometer. Fluorescence spectra were obtained by using a Spex Fluorolog 212L system. The sensitizer RuLa (Tris(2,2'-bipyridyL4,4'-dicarboxylate)ruthenium(I1)dichloride) and colloidalTi02solutionswere kindly provided by Professor M. Griitzel. Aqueous Ti02 colloid was prepared in his laboratory according to a published procedure.' The stock solution contained 5.1 g/L Ti02 and was kept at pH = 2.0 (HC104)to avoid particle growth or agglomeration. The mean diameterof the colloidal particleswas5.8 nm. Experiments were typically performed with solutions of Ti02 concentration of 1 g/L, which corresponds to a semiconductor particle concentration of 6.3 X 10-7 M.6.11 For dilution, Millipore filtered distilled water was used throughout. The pH was maintained at 2.0 by adding HClOd for all photosensitization experiments unless indicated otherwise. The concentrationof the ethanoliccolloid, prepared as described in an earlier report: was 8.2 g/L. To prevent particle growth, the colloidal solution was acidified by addition of 2.5 X 10-8 M HC104. Average particle diameter was 3.5 nm. For photosensitizationexperiments this stock solution was diluted with ethanol (Rossville USP Gold Shield) to 1 g/L TiO2. Polyvinyl alcohol (PVA Moviol lO-98, Hoechst, Germany) was used as protective agent at 0.25 wt % . For preparation of RuL3-coated Ti02 colloid, the sensitizer was dissolved in basic aqueous solution (pH = 8.0) and its concentrationdetermined by visible spectroscopy (thesolubility of RuL3 in acidic solution is too low for direct absorption of the colloid). The RuL3 solution was added to the Ti02 sol, after which the pH was lowered to 2.0 and the solution allowed to stand overnight. The sensitizerphenylfluorone (PF,Kodak) was purified by recrystallization from water before use. Ethanolic and aqueous PF coated Ti02colloids were prepared as previously described.6 Potassium iodide (Fisher Scientific)was added to the colloid in solution form. Stocksolutionswere adjustedto givethe desired KI concentration in the resulting sol upon addition of a volume equal to 1%of the volume of colloid. 111. Results We first report transient absorption signals observed upon excitation of RuL3 and PF sensitizer adsorbed at aqueous and ethanolic colloidal Ti02 (section 1). These experiments furnished blanks necessary to analyze transient absorptions observed upon photosensitized oxidation of iodide in these colloidal solutions, presented in section 2. Then, attempts to generate transients upon oxidation of I- by Ru"'L3 in colloid-free solution are described (11) Griitzel, M.; Moser, J. J. Am. Chem. SOC.1983, 105, 6547.
Fitzmaurice and Frei (section 3). In section 4, we will discuss how injected conduction band electrons influence the primary product yield of photosensitized I- oxidation at Ti02 colloid. 1. Near-Infrared Spectroscopy of Sensitized Electron Injection into Colloidal TiO2. UV-vis spectra of aqueous and ethanolic Ti02 solutions showed the wellknown band gap absorption around 340 nm, with a tail extending into the blue spectral range.lZ An aqueous colloidal solution (6.3 X lo-' M) in the presence of 1.0 X 10" M Ru"L3 had an additional visible absorption with a peak at 465 nm (e = 21 000 L mol-l cm-l).l Since the spectra of dissolved and TiOz-adsorbed Ru11L3 are very similar, fluorescence spectroscopy was used to monitor sensitizer ad~orption.~ Compared with the aqueous 1.0 X lo4 M RuUL3solution (pH = 5.21, a colloidal solution (pH = 2.0) with the same sensitizer concentration showed a 5 times smaller Ru11L3 fluorescence at 650 nm. This indicates quenching by surface adsorption of the sensitizer, predominantly through electron injection into the conduction bands4 The residual luminescence is most probably not due to dissolved Rur1L3because at pH < 2.5 RuL3 becomes zwitterionic, rendering the sensitizer insoluble4 (the solubility of RuIIL3 a t 25 "C and pH = 3 is 3 X lo4 M).13 Indeed, when we further acidified the colloidal solution to pH = 0.5 and allowed it to stand overnight, a further fluorescence intensity loss of merely 30% ' was observed. Fluorescence quenching behavior was independent of whether the stabilizing agent Moviol (PVA) was added to the colloid,ornot. Most probably, the residual fluorescence stems from RuIIL3 species that are adsorbed on the colloidalsurface in a configuration that is inefficient for electron injection; hence they remain luminescent. Quantitative condensation of Ru"L3 onto the colloidal particles was also indicated by the observation that centrifugation of the sensitizer-loaded colloid resulted in a red precipitate and a supernatant with negligible RumL3 optical absorption. Visible absorption and fluorescence spectra of PF and those of the sensitizer in ethanolic colloidal Ti02 solution have been reported previously (ref 5, Figures 1 and 2). Briefly, the visible spectrum of a M ethanolic solution of PF shows a single broad band with an absorption maximum a t 462 nm (e = 1.5 X lo4 L mol-' cm-l). The M sensitizer at the same concentration in 6.3 X colloidal Ti02 (pH = 2.0) yielded an intensely red colored solution with an absorption maximum at 544 nm (e = 9.1 X lo4 L mol-' cm-l). Concurrently, the intense 610-nm fluorescence of PF in acidic, colloid-free solution was completely quenched in the presence of Ti02 sol. Similar observations were made with aqueous PF colloids. This spectral behavior has been interpreted in terms of quantitative chelation of the sensitizer PF on the surface of the semiconductor.6 Before we studied RuLa/TiOz and PF/Ti02 sensitized oxidation of I-, a series of pulsed-excitation transientabsorption experiments with sensitizer-loaded colloid in the absence of halide was performed. While, as expected, no absorption signal was observed upon excitation of bare Ti02 colloid with 532-nm quanta, excitation a t 355 nm (5.0 mJ cm-2 pulse-') gave a broad transient absorption in the red spectral range of several microseconds duration, which is characteristic for trapped conduction band electrons generated by direct bandgap irradiati~n.'~Part (12) Duonghong, D.; Borgarello, E.; GrHtzel, M. J. Am. Chem. SOC. 1981,103,4685.
(13) Furlong, D.N.;Wells, D.; Sasse, W. H. F. J.Phys. Chem. 1986,
90,1107.
(14) Rothenberger, G.;Moser, J.; Griitzel, M.; Serpone, N.; Sharma, D.
K.J. Am. Chem. SOC.1985, 107, 8054.
Langmuir, Vol. 7, No. 6, 1991 1131
Oxidation of I- at Colloidal Ti00
1.ox10-2
2x10 -3 m
z
e:
P 9
5 8
2
1~1.0-3
9 TiOp / 35513111
0 1
0
0.0 500
600
700
1
I
4
6
800
of the spectrum of the transient is shown in Figure 1, curve a. Excitation at 532 nm of aqueous colloid with adsorbed Ru11L3 resulted in a transient exhibiting two decays of similar amplitude but widely different duration of a few microseconds and several milliseconds,respectively. The spectrum is shown in Figure 1, trace b. It is a superposition of two absorption signals,namely oxidized sensitizer RuXnL3l6and trapped conduction band electron. At 650 nm the extinction coefficients of RuUIL3 (c = 500 L mol-' cm-l for Ru(bpy)3)leand trapped electron (c = 800 L mol-' cm-l)17 are similar; hence the two species contribute probably about equally to spectrum b. While the fast decay agrees with the rate constant k b = 4 X lo6 s-l for electron recapture of Ru"L3 reported by Griitzel and coworkers: our observation of an additional long-livedsignal indicates that a substantial fraction of the RumL3species is ineffective in electron recombination. This spread in electron recapture rates points to a spread in the way the sensitizer is adsorbed (interacts with the surface). The lack of observation of an RumL3concentrationdependence of the long-lived component (which would be manifested in a shortening of its decay with increasing laser pulse energy) makes it unlikely that the long-lived absorption originates from RuI"L3 that has desorbed from the surface.le Excitation of RuI1L3 loaded colloid at the shorter, 355-nm wavelength gave essentially the same spectrum of Ru111L3 (Figure IC)but, in addition, an absorption at wavelengths shorter than 630 nm not present in the 532 nm irradiation experiment. At this wavelength 10% of the absorbed laser photons excited directly the adsorbed RuL3, the remainder producing electron-hole pairs in the particles. This additional absorption may be due to reduced sensitizer Ru1L3 produced by a redox process in which valence band holes from Ti02 bandgap excitation oxidized the protective PVA to give macroradicals, which in turn would reduce Ru11L3.17*1g120 Ru'L3 has (15) Kalyanaeundaram, K. Coord. Chem. Reu. 1982,46,159. (16) McCaffery, A. J.; Maeon, S. F.; Norman, B.J. J . Chem. SOC.A 1969, 1482. (17) K6lle, U.; Moeer, J.; GrBtzel, M. Inorg. Chem. 1985,24, 2253. (18) M o w , J.; Gritzel, M. J. Am. Chem. SOC.1984,106, 6557. (19) Duonghong,D.;Ramsden, J.; Gritml, M. J. Am. Chem. SOC.1982, 104,2977.
8
Time (psec)
Wavelength (nm)
Figure 1. Transient absorptionspectra of bare and RuLloaded aqueous Ti02 colloid taken 10 ps after excitation pulse: (a) bare Ti02 colloid (6.3 x lo" M, pH = 2) excited at 355 nm (5.0 mJ cm-gpulee-l);(b) RuLa loaded Ti02 colloid excited at 532 nm (5.0 mJ cm-l pulse-'); (c) same solution as in part b excited at 355 nm.
I
I
I
1
2
2x106
n "
1
0.000
0.002
0.004
0.006
0.008
0.010
0.012
concenbation(md I '1)
Figure 2. Rise of transient observed upon 532-nm excitation of ethanolic PF/Ti02 colloid. (a) Absorbance at 756 nm in solution containing 1,5, and 10 X 10-9 M potassium iodide. Each trace was obtained by subtracting the signal before adding iodide from that after KI addition. (b) Peeudo-first-order rate constant of the rise of the absorption as a function of iodide concentration. an absorption maximum at 510 nm.15 Alternatively, the absorption may be due to trapped holes that do not have sufficientlypositive potential to oxidize adsorbed PVA.21 Transient conduction band electron absorption (600800 nm) was also observed upon 532-nm excitation of PF derivatives Ti02 colloid in ethanol as reported in earlier work.s 2. Near-InfraredSpectroscopy of Photosensitized
I- Oxidation at Colloidal Ti02 (A) Phenylfluorone/ TiO2. When PF-loaded ethanolicTi02colloid was excited at 532 nm in the presence of potassium iodide, a transient absorption was observed in the near-infrared region. At sufficiently low I- concentration the rise of the signal could be resolved, as shown in Figure 2a. Its spectrum exhibits a weak, broad absorption in the 700-800 nm range which is characteristic for I2-.lo Constants hobof the exponential rise of the transient were found to be linear in Iconcentration, Figure 2b. The slope of kobvs [I-] was (1.2 (20) Bahnemann, D.; Henglein, A.; Lilie,J.; Spanhe1,L.J. Phys. Chem. 1984,88,709. (21) Henglein, A. Top. Curr. Chem. 1988,143,113.
Fitzmaurice and Frei
1132 Langmuir, Vol. 7, No. 6,1991 2x10-2
1.o
0.5
0.0
1.5
2.0
Time (msec)
i
A 750nm
/
[KI](mol I -')
Figure 4. Initial absorbance of the transient at 750 nm as function of I- concentrationwhen exciting PF loaded ethanolic Ti02 colloid at 532 nm (50 mJ cm-2 pulse-').
5x10 6
3x10 6
1x106
0.0
1.o
0.5
1.5
2.0
Time (msec)
Figure 3. (a) Decay of the transient observed upon 532-nm excitation of ethanolic PF Ti02 colloid containing 1 X 10-* M iodide. (b)Plot of (A/r12- 700 nm)l)-' as function of time. f 0.2) X 108 L mol-' s-l; the intercept was (3.9 f 0.3) X l o 6 s-'. As shown in Figure 3, the decay of the transient is second order with a rate constant of (3 f 1) X 109 L mol-' s-l if c = 2200 L mol-' cm-1, the extinction coefficient of 12- at 700 nm, is used."J This is consistent with the rate constant of 7 X log L mol-' s-l reported for the disproportionation process22
+ 1;
-
1,- + I(1) and further points, as expected, to 12- as the observed transient. Two observations indicated that the yield of transient 12- was far below that expected from the high efficiency of I- oxidation measured previously in a regenerative photoelectrochemical cell a t an anodically polarized, P F loaded Ti02 e l e c t r ~ d e An . ~ estimate of the extinction coefficient of the observed transient at 700 nm based on the initial concentration of oxidized sensitizer PF+ (calculated from irradiation pulse energy, transmittance at 532 nm, and known electron injection efficiency9 and I- oxidation efficiency in the photoelectrochemical experiment gave a value about 10times lower than that of 12-when comparing data for [I-] = 0.1 M. Furthermore, the 12- buildup as function of iodide concentration, shown in Figure 4, exhibits a plateau around [I-] = 0.01 M, but then a further steep increase at higher I- concentration. This unexpected behavior indicates that upon photoinduced I- oxidation I,
(22) Crossweiner, L. I.; Matheson, M. S. J. Phys. Chem. 1957,61,1089. These authors determined the rate constant at I- concentrations orders of magnitude lower than that used here. It waa noted that the constant decreases with increasing iodide concentration.
at sensitized Ti02 colloid some quenching process limits the reaction efficiency. However,since the buildup of the final oxidation product 13-was found to be proportional to the extent of exposure to 532-nm radiation (i.e. number of laser flashes) over several hundred laser pulses (air saturated solution, 22 mJ cm-2 pulse-'), the concentration of the transient per photolysis pulse, and hence its extinction coefficient,could be accurately established by static UV measurement of the 13-product concentration. A 6.3 X lo-' M colloidal solution containing 2.3 X 10-6 M PF and 5 X 10"' M KI was irradiated with 100green laser pulses. The difference of UV-vis spectra taken before and after irradiation was found to be identical with an authentic spectrum of the final oxidation product The absorbance growth at the peak wavelength of 360 nm was 0.038, corresponding to an 13-buildup of 1.46 X 10-8M per laser pulse (€(I3-,360 nm) = 26 OOO L mol-' cm-l). Considering that the total volume of the solution exceeded that of the laser irradiation volume by a factor of 3.2, and taking into account the stoichiometry of process 1,we calculate that 9.3 X 10-8M transient is produced per pulse. The absorbance of the transient measured a t 750 nm is 2.5 X lo4, which yields an extinction coefficient of 2700 L mol-l cm-'. This is close to the value of c = 2800 for 12- at that wavelength'O and, hence, supports the identification of the transient species as 12-. The same experiments were conducted with argonsaturated colloidal solution. While the transient 12- signal was exactly the same as the one observed in air (or 0 2 ) saturated solution, the 13-product concentration was only 20% of what was calculated from the transient 12- absorbance and the stoichiometry of process 1. Apparently, in the absence of oxygen, Is- is partly consumed by subsequent chemical reaction. Ti,me-resolvedand static experiments described above were also performed on aqueous PF colloid. The spectral properties of the aqueous sol differ markedly from those of ethanolic colloid. These differences and their structural origin have been previously discussed in detail.5 The similarity of the near-infrared spectrum of the transient observed upon 532-nm excitation of the aqueous PF-colloid with that of 12-, and the fact that the absorption signal decays by a second-order law with k = (3 f 1) X lo9 L mol-' s-l (based on t(Iz-, 750 nm) = 2800 L mol-' cm-I), indicates that the absorbing species is 12-. The rise of the transient agreed within error limits with that measured (23) Awtrey, A. D.; Connick, R. E. J.Am. Chem. SOC.1951, 73,1842.
Langmuir, Vol. 7, No. 6, 1991 1133
Oxidation of Z- ut Colloidal Ti02
5p e c
2 2x10-3
w
25 p e c
9 1x10-3
0.5
0.0
-5,63
1.o Time (msec)
1.5
2.0
Slope: (3kl)XlO3 sec -1
-6 63 I
-7,63
%L 2x10
-
[KI] = 4X10-4 M
~~~
1x10-
-9,63 1.o
0.5
0.0
I.5
2.0
0 0
Time (msec) A700nm
5X10-4
[KI] (mol I
i
Figure 7. Initial absorbance of the transient at 700 nm as function of I- concentration when exciting RuLa/TiOa aqueous colloid at 532 nm (24 mJ cm-2pulse-1). Inserts show decay at [I-] = 4 x lo4 M and 0.1 M.
3x10-3
0
5
10
15
Time(pec) Figure 5. Transient absorption at 700 nm of RuL3(1X 104M) loaded aqueousTi02 colloid containing4 x lo-' M KI (excitation at 532 nm, 23.8 m3 cm-2pulse -I): (a) signal in absence of iodide has been subtracted; (b) logarithmic plot; (c) rise at [I-] = 1, 2, 3, and 4 X 10-4 M. in the case of ethanol PF/Ti02 colloid, and the 12absorption as a function of [I-] reached a plateau around 0.01 M like in ethanolic solution. Accumulation of 13over multiple laser pulses was confirmed by static UV spectroscopy. In contrast to ethanolic PF colloid, buildup of [I3-] in air-saturated aqueous solution corresponded to only 7% of what was expected from the yield of 12- per laser pulse, and no 13-at all was observed in argon-saturated solution. (B)RuLs/TiOz. Upon addition of potassium iodide to a RunLs loaded aqueous Ti02 colloid, 532-nm excitation resulted in a broad near-infrared transient absorption with a maximum around 750 nm. Figure 5a displays the transient signal at 700 nm of a 4 X 10-4 M KI solution along with a logarithmic plot of the absorbance decay,
Figure 5b. Part c of Figure 5 shows, on an expanded time scale, the I-concentration dependence of the rise. Spectra in the 730-800 nm range taken 5 and 25 ps after the green excitation pulse are presented in Figure 6. The similarity between the two spectra indicates that the near-infrared absorption originates from a single species. As in the case of the P F colloid, initial estimates of the concentration of the transient based on the extinction coefficient of 12- showed that the concentration of the transient was far below that expected from photochemical oxidation of I- at a RuL3 coated polycrystalline Ti02 electrode.' For example, at an iodide concentration of 0.01 M, from the known photochemical efficiency' we would predict an 12- yield of at least 0.15. However, with a measured transient absorbance at 700 nm of 2.5 X 10-3, Figure 7, we calculate a yield of only 2.5% based on a Ru"'L3 concentration of 4.5 X M just after the laser excitation pulse (if 12- were the absorber, with 42-, 700 nm) = 2200 L mol-' cm-9. We conclude that I- oxidation efficienciesmeasured in photoelectrochemical experiments cannot be used to establish the extinction coefficient of the one electron oxidation product observed in colloidal solution. In order to determine the extinction coefficient of the transient, an air-saturated 6.3 X 10-7 M colloidal solution M Ru"L3 and 1.0 X loa M I- was containing 1.0 X irradiated with 20green laser flashes (16.2 mJ cm-2pulse-'). The difference of UV-vis spectra taken before and after irradiation is that of the final oxidation product 13- with an absorbance growth of 0.025 at 360 nm. At the pulse
1134 Langmuir, Vol. 7, No. 6, 1991
Fitzmaurice and Frei 3x10-3 2x10-3 1X10-3 a
0
0
4
10
20
30
Time (pec)
3x10-3 0
0
I
I
0
energy and number of laser flashes used, the absorbance growth was found to be linear in the extent of exposure. With t(I3-, 360 nm) = 26 000 L mol-' cm-I, the 13concentration was 9.6 X lob7M; hence 4.8 X M was produced per 532-nm laser pulse. If we assume that Isoriginates from disproportionation of 12- (process 1)as in the case of P F sensitized oxidation, and if we take into account the factor of 3.2 difference between solution volume and irradiation volume, we calculate 3.1 X M 12- generated per laser pulse. The absorbance of the transient measured at 700 nm is 4.1 X For a species M, that would yield an of concentration 3.1 X extinction coefficient 6 = 13 200 L mol-l cm-l, which nm) = 2200) exceeds the extinction coefficient of 12- (~(700 and by a factor of 6. This suggests that the observed transient one-electron oxidation product is a precursor to 12- rather than the species it~elf.2~ Analysis of the decay kinetics of the transient observed upon 532 nm photolysis of aqueous Rur1L3/Ti02/I- also suggests that the initial photoproduct is not free 12-. The decay of the transient, Figure 5a, is close to single exponential with a rate of (3 f 1) X 103 s-l, independent of I- concentration. The rate was determined from experiments in the [I-] range between 1.0 and 4.0 X lo4 M, at a pulse energy of 24 mJ cm-2. No significant change of the decay rate was observed at higher iodide concentration as can be seen from the inserts in Figure 7. This behavior contrasts with the second-order decay observed in the case of the PF-sensitized photooxidation, with a rate consistent with 12- disproportionation. Observed l / e times of the rise of the transient increase with increasing iodide concentration. However, the data scatter considerably more than in the case of PF/TiOz, as can be seen from Figure 8. Bimolecular rate constants derived from Figure 8 range from 2 X l o 9 to 3 X log L mol-' s-l. The initial amplitude of the transient signal was constant at all iodide concentratons above 5 X lo4 M (Figure 7). Measurements were conducted up to 0.1 M, the concentration beyond which coagulation of the colloid occurred. It is important to note that for both decay and rise the transient exhibited the same kinetic behavior independent of whether or not the Ti02 colloid was protected by PVA. In terms of 13- buildup, the aqueous RuL3 case resembles that of aqueous PF, but differs considerably from ethanolic PF colloid. The yield of 1s- in static experiments (24) Explanation by 12- formed by secondary photolysis of accumulated 13- at appropriate concentrations.
Is- could be ruled out on the basis of blank experiments with
I
20 Time (psec)
10
I
30
Figure 9. Quenching of MV+ absorption upon photosensitized I- oxidation: (a) Transient absorption at 622 nm following 532nm excitation (2 mJ cm-z pulse-') of ethanolic PF/Ti02 colloid (4.3 X M, pH = 2); (b) solution a after adding 3 X 10-8 M MVz+;(c) solution b after adding 1 X 10-3M KI. All solutions were degassed by bubbling Ar for 3 min prior to irradiation. dropped significantly below that expected on the basis of the first few tens of laser pulses if exposure exceeded 100 pulses (24 mJ cm-2pulse-'). No 13-was detected in argonsaturated RuL3 colloid even upon prolonged 532-nm irradiation, despite the fact that the amplitude and temporal behavior of the transient signal were identical in argon-, air-, and oxygen-saturated solution. 3. Ru(III)L3 + I- Reaction in Colloid-FreeSolution. In an attempt to obtain an authentic spectrum of the transient observed upon RuL3 photosensitized oxidation of I-at colloidal TiOz, reaction of oxidized sensitizer RumL3 with I-was studied in aqueous solution. In order tomonitor the reaction in real time, Ru111L3 was prepared by a nanosecond laser pulse by exciting RuX1L3(1.0 X 10" M) at 532 nm and using Is- as electron acceptor. The concentration of the latter was 2.8 X 10-5 M in a 0.5 M Isolution. Both decay (single exponential, (3 i 1) X lo3 s-l) and near-infrared spectrum of the observed transient were found to agree within uncertainties with the transient observed upon photosensitized oxidation of I- in the case of colloidal RuLa/TiOa. 4. Evidence for Quenching of Transient I Atoms by Conduction Band Electrons. A possible reason for much lower yields of I--sensitized photooxidation at colloids compared to photoelectrochemical experiments5 is a competition between reaction of surface adsorbed I atoms (produced by reaction of oxidized sensitizer with surface adsorbed I-) with iodide to form 12-, on the one hand, and capture of a conduction band electron to regenerate I-, on the other hand. In order to test this possibility, a series of experiments was performed in which MV2+ was used as a probe for quenching of conduction band electrons by I atoms. Figure 9 demonstrates the effect of addition of MV2+ to an ethanolic Ti02 colloid (4.3 X lo-' M) coated with PF sensitizer (2.3 X M). When exciting with a 532-nm laser pulse in the absence of MV2+ and I-, the transient absorption due to conduction band electrons is observed at 622 nm (Figure 9, trace a). In the presence of 3 X M methylviologen, an additional absorption appears at 620 nm (curve b) that is assigned to MV+ produced by fast electron transfer of conduction band electrons to the acceptor.5As
Langmuir, Vol. 7,No. 6, 1991 1135
Oxidation of I- at Colloidal Ti02
Figure 9c shows, subsequent addition of I- at 1 X M quenches the MV+ absorption. This is consistent with our interpretation that I atoms produced on the Ti02 surface by sensitized photooxidation of I- compete successfully with MV2+for conduction band electrons. The same quenching of MV+ absorption was observed upon addition of I-in a similar experiment where direct bandgap excitation of bare Ti02 colloid at 355 nm was employed. However, the quenching effect of added 1- was limited to low iodide concentration, as no quenching of MV+ absorption occurred when [I-] was raised above 0.01 M.
IV. Discussion We will discuss, in turn, I- oxidation at the surface of a colloidal Ti02 particle sensitized by adsorbed phenylfluorone and RuL3. (A) Phenylfluorone/TiO2. We can conceive of three possible interpretations of the kinetics of the 12- rise and the iodide concentration dependence of the 12- yield. The first, our preferred interpretation, is reaction of oxidized sensitizer PF+with surface-adsorbed iodide Iads to give adsorbed I atom, which then reacts with I- from the bulk of the solution to form 12-
+ I,&
PF+ + I-,ds-PF I,,
+ I-
-
(2)
k
I,-
(3)
The first process is expected to occur within the 5-ns duration of the laser excitation pulse, while the reaction of Ia&with bulk I- would proceed a t a rate around the diffusion limited value. Hence the observed rise of 12would originate from reaction 3. This mechanism (EleyRideal mechanismP is consistent with the observed linear dependence of the rate constant of the rise on [I-], Figure 2. The fact that the measured bimolecular rate constant k = 1.2 X lo8 L mol-' s-1 is over an order of magnitude below the diffusion limited value of 6 X lo9 L mol-' s-' may reflect a lower reactivity of I- with I chemisorbed on a Ti02 surface as compared to free I atoms. The close to diffusion controlled value of the 12- decay suggests that the disproportionation process (1)occurs predominantly in solution rather than on the surface of the colloid. In the framework of this mechanism,one could interpret the plateau in the 12- yield vs [I-] plot around [I-] = 0.01 M, Figure 4, in terms of saturation coverage of the colloid surface by iodide. However, the further steep increase of the 12- yield at [I-] > 0.01 M suggests that there is an additional reservoir of I atoms (or oxidized sensitizer) that reacts with I- to give 12- only at these higher iodide concentrations, but not at [I-] < 0.01 M. On the basis of the observed quenching of MV+ growth upon photosensitization of electron injection at ethanolic PF/Ti02 in the presence of I- (at [I-] < 0.01 M), we propose that this additional pool of adsorbed I atoms consists of those which compete for injected conduction band electrons
kb*
I,,
+ e-cb
I-
(4)
and therefore react with I- to give 12- only at high Iconcentration. Accordingly, the monotonous increase of the 12- yield a t iodide concentrations above 0.01 M could reflect increased competition of reaction (3) with electron capture (4). This is consistent with our observation that I- quenches formation of reduced MV+ at low I- concentration, while MV+growthappears again when [I-] is raised (25) Atkins, P.W. Physical Chemistry,3rd ed.; Freeman: New York, 1988,p 782.
above 0.01 M. Hence we propose that there are two pools of transient I atoms formed on the Ti02 surface. One consists of I atoms that do not efficiently recombine with injected conduction band electrons (kb' on the order of lo5 5-l or lower) and gives rise to the 12-signal observed at [I-] < 0.01 M. The other pool comprises those I atoms that combine efficiently with conduction band electrons and therefore can only be detected at concentrations sufficiently high for reaction 3 to compete with process 4 ([I-] > 0.01 M). One can make an order of magnitude estimate of the rate constant kb' of reaction 4 for this second pool of I atoms on the basis of the data plotted in Figure 4. At an I- concentration of 0.05 M, for example, we find that at most 15% of the surface I atoms react with bulk I(assumingthat at [I-] = 0.15 M all of them do, which may be an overestimate). Hence the competition between reactions 3 and 4 would give k[I-]/(k[I-] + kb') = 0.15. Therefore, kbl would have a value around 4 X lo7s-'. This is 2 order of magnitude faster than the rate of capture of conduction band electrons by PF+, kb = 2.8 X lo5s-'.~ We think that a much higher efficiency for back electron transfer to I on the Ti02 surface compared to oxidized phenylfluorone is not unreasonable because the highly reactive I atoms may coordinate to surface titanium more tightly than phenylfluorone and, hence, may constitute more effective conduction band electron traps, despite the fact that PF forms a chelate with surface Ti.5 (It is important to note that process 4 would be suppressed in the correspondingreaction in a photoelectrochemicalcell because of the band bending introduced by the applied electrode potentialJ5 The origin of the two pools of transient I atoms with widely different electron recapture rates may lie in different ways in which transient I atoms are adsorbed on the surface of the colloid (heterogeneity). This would be analogous to the wide spread of back electron transfer rates already found in the case of adsorbed oxidized sensitizer (section 111.1). The small 12- yield in the plateau region indicates that the reservoir of I atoms which constitutes inefficient trapping sites for conduction band electrons is relatively small. Assumption of two pools of I atoms is most likely an oversimplification as there may exist a continuous range of kb' rates. The second possible mechanism differs from the one just described in the interpretation of 12-growth a t [I-] < 0.01 M, while the same explanation in terms of competition between reactions 3 and 4 is maintained for the rise of 12yield a t iodide concentrations above 0.01 M. Assuming that a fraction of adsorbed sensitizer molecules P F have no surface adsorbed I- neighbor, it is conceivable that they react after photoinjection of an electron with I- from the bulk of the solution according to k'
PF++I--PF+I
(5)
kd
1+1--1,-
-
PF+ + e-cb
(6)
kb
PF
(7)
All the other PF+species would not contribute to 12-growth at [I-] < 0.01 M as they react quantitatively according to quenching processes 2 and 4. k' would be the bimolecular rate constant for oxidation of I- by PF+, kd the diffusioncontrolled rate constant in ethanol of 6 X loBL mol-' sec-l, and k b the rate constant for back transfer of the conduction band electron of 2.8 X 105 s-l reported earlier.5 For this simple model, the integrated law for the concentration dependenceof 12- is (assuming steady-state approximation
Fitzmaurice and Frei
1136 Langmuir, Vol. 7, No. 6, 1991 for I atom concentration)
I / P 2 we calculate driving forces of 0.53 and 0.16 V for Ru"IL3 + I- to yield 12- or I, respectively. According to the estimated extinction coefficient and the decay kinetics of the observed transient, the initial oxidation product is not free Iz. The fact that the transient decays as a single exponential with a rate constant that is independent of the I- concentration over a wide range suggests that the species decays by unimolecular dissociation. Products are most probably 12- and RuL3, as this would readily account for formation of 13-by disproportionation reaction 1. Thus, we propose that the observed transient is an ion pair, RuIIL~Iz-,formed by
The slope of the plot of the observed rate constant vs [I-], Figure2b,givesiz'= (1.2f0.1) X lOBLmol-'s-'forreaction 5. The intercept is (3.9 f 0.3) X lo6 s-', in excellent agreement with the rate constant of process 7 obtained in other worksVbAlso, the plateau of the I2-yield around [I-] = 0.01 M agrees with this model. However, the agreement could be fortuitous. A third possible interpretation assumes that capture of RUII~L, + 21RU~IL,I,(8) conduction band electrons by surface adsorbed I atoms, produced by reaction 2, is negligible. In the low iodide Ru"L3I; Ru"L3 + 1,concentration range, reaction 3 would explain the rise of the signal and, on reaching I-surface saturation coverage, Ion pair (or outer-sphere coordination ~ o m p l e xfor)~~~~~ the plateau of the 12- buildup around [I-] = 0.01 M. At mation is expected on the basis of electrostatic attraction higher iodide concentrations, exchange of adsorbed I atoms between the inner sphere complex R u L ~ and ~ + I z a s these with bulk I- through electron transfer could result in two species emerge from reaction 8. Dissociation of the additional 12- yield if the exchange were fast on the time weakly bound ion pair would regenerate the sensitizer. scale of the 5-ns laser pulse duration. However,we consider We do not consider interaction of 12- with the inner an efficient exchange between surface I and bulk I- by coordination sphere as likely (substitution of a Ru-bpy electron transfer as unlikely at these modest iodide bond, or expansion of the inner coordination sphere) since concentrations.28 Furthermore, this exchange model would it is expected to result in a loss of sensitizer through be inconsistent with the MV2+quenching results. chemical reaction at the metal center, contrary to observation. Common to all possibilities discussed is surface reaction We can conceive of two possible explanations regarding 2. There are precedents for a dominant role of surfacethe origin of the near-infrared absorption of the proposed adsorbed species over those in the bulk in redox processes at semiconductor colloids and powders in genera11By27*28 RuL&-ion pair, namely transition to a bipyridyl-if charge and halide oxidation at Ti02 in p a r t i c ~ l a r .For ~ ~ ~ ~transfer ~ ~ ~ ~state ~ or enhancement of the 211g+ 2&+ transition of 12- by interaction with the RuL3 complex. The latter example, Herrmann and Pichat found that halide phocase is not very probable since the 12- 211r 28,+ band is tooxidation a t Ti02 powder suspensions in water by direct a spin and orbital allowed electric dipole t r a n s i t i ~ n . ~ ~ bandgap excitation is limited to surface adsorbed ions, as Hence there is no obvious way in which interaction with indicated by Langmuir type behavior of product buildup RuL3 could enhance the absorption cross section. as function of halide c~ncentration.~g Gratzel' and HenOn the other hand, if we take the ionization potential glein6 concluded that the same holds in the case of 350-nm of bipyridyl of 8.54 eV36 and the electron affinity of 2.4 eV photooxidation of halides in aqueous colloidal Ti02 as for we find that a 750-nm charge transfer absorption indicated by Langmuir adsorption isotherms for buildup would correspond to a separation of the charge centers by of one-electron oxidation products X2-. In addition, the 3.2 A in a simple Coulombic model. This is a reasonable rise of X2- occurred within the duration of the excitation separation for an ion pair. Further support for assignment laser pulse, from which the authors concluded that only of the observed band to a charge transfer transition is the surface-adsorbed halide is oxidized. Note, however, that fact that the absorption is unstructured and observation halide concentrations employed in this earlier work may of a 20-nm blue shift of its maximum from aqueous to have been too high to permit observation of the rise of Xzethanolic solution. The excited state is expected to be according to reaction 3. better stabilized in water than in the less polar ethanol. (B) RuL3/TiOz. The energetics of electron injection We conclude that the bipyridyl-12- charge transfer abfrom excited RuL3 into conduction band of Ti02 colloid sorption of a RuL312- ion pair is the most probable origin and of the reaction of oxidizedsensitizer with I-can readily of the transient observed upon excitation of RuLs/TiOz be obtained from known redox potentials. The standard in iodide solution. Such a second sphere ion pair charge reduction potential of the excited sensitizer is estimated transfer absorptions involving transition-metal complexes a t -0.6 V (NHE) based on Eo of Rur11L3/Rur1L3of +1.56 are not unprecedented, and various examples have been V30 and an excited-state energy of 2.12 V.15 This gives reviewed by Balzani et a1.3' Examples include C P sufficient driving force for injection of an electron into the (bpy)2Clz+I-ion pairs in aqueous solution, which exhibit Ti02 colloid since the conduction band edge at pH = 2.0 a second sphere charge transfer absorption around 475 is around -0.2 V.12 Rate constants for electron injection nm.38 Charge transfer interaction between bipyridyl ligand from excited Ru(II)L3, and back electron transfer to and 12- would add to the stability of the ion pair beyond oxidized sensitizer have been reported by Gratzel and ~ + the 12- radical. coworkers as 3.2 X lo7sec-l and 4 X lO6sec-l, re~pectively.~ mere electrostatic interaction of R u L ~and With redox potentials of 1.03 V and 1.40 V for Iz-/I-3' and
-
-
-
(26) Nord, G.; Pederson, B.; Farver, 0. Znorg. Chem. 1978, 17, 2233. (27) Nosaka, Y.; Fox, M. A. J. Phya. Chem. 1988,92, 1893. (28) Griitzel, M., Ed. Energy Resources through Photochemistry and Catalysis; Academic Press, Inc.: New York, 1983. (29) Hermann, J. M.; Pichat, P. J. Chem. Soc., Faraday Tram 1 1980, 7 .43-, 1128 - - --. (30) Deeilvestro, J.; Duonghong, D.; Kleijn, M.; Gratzel, M. Chimia 1985, 39, 102.
(31) Henglein, A. Radiat. Phys. Chem. 1981, 15, 151.
(32) Berdnikov, V. M.; Bazhin, N. M. Russ. J . Phys. Chem. 1970,44, 395. (33) Beck, M. T. Coord. Chem. Rev. 1968,3,91. (34) Person, W. B. J. Chem. Phys. 1963,38, 109. (35) Herzberg,G. Spectra ofDiatomicMolecules;Van Nostrand New York, 1950. (36) Kato, K.; Sasaki, K.; Aida, K. Spectrochim. Acta, Part A 1982, %A, 1011. (37) Balzani,V.; Sabbatini,N.; Scandola, F. Chem. Rev. 1986,86,319. See also ref 26. (38)Baker, W. A.; Phillips, M. G. Inorg. Chem. 1965,4,915.
Oxidation of Z- at Colloidal Ti02 Our finding that the buildup of transient RuL312- as a function of iodide concentration, Figure 7, reaches an asymptotic limit around [I-] = 0.01 M that lies well below the yield expected from photoelectrochemical efficiencies' indicates that only a small fraction of Ru(III)L3 formed on the surface reacts to produce RuL&-. As in the case of PF/Ti02 we interpret the low yields by conduction band electron capture of adsorbed I atoms, process 4. The fact that the RuL& yield stays constant in the range 0.001 M < [I-] < 0.1 M implies that kb' for the pool of I atoms with fast electron recapture rates is l l O g s-l, which would suggest substantially more efficient reduction of Iads by conduction band electrons than in the ethanolic PF/Ti02 case. We propose the same two alternatives for the interpretation of the rise of the transient, Figure 512,as discussed above for photooxidation of I- a t ethanolic PF/Ti02 colloid. The first is explanation of the rise in terms of the rate-limiting reaction 3, which would be preceded by the much faster reaction of Ru"IL3 with surface-adsorbed I-. Observed reaction would again originate only from the pool of adsorbed I atoms that have kb' constants S105s-l. Alternatively, the observed rise could originate from RurT1L3species that are not reductively quenched by Iab and hence would live sufficiently long to react with I- from the bulk of the solution (analogous to reactions 5-7). However, we consider this interpretation as more speculative than the former. Assuming that the observed rise in both the RuLa/TiOz and PF/Ti02 case is dictated by the rate of the reaction of I& with bulk I-, it is interesting to compare the corresponding bimolecular rate constants. The rate constant in the aqueous RuLs/TiOz case of (2-3) X 109 L mol-l s-l is more than an order of magnitude larger than for ethanolic PF/Ti02 colloid. This may reflect greater accessibility of adsorbed I atoms in the RuL3/Ti02 sol (at least those I& which have low electron capture rates). The rise constant for RuLa/TiOz shows considerable scattering (Figure 8), which may originate from I- concentration gradients in the vicinity of the positively charged colloidal surface a t pH = This adds further uncertainty to the determination of the rate constant of reaction 3 by using bulk iodide concentrations. As indicated by the dramatic loss of Is- yield upon removal of oxygen from aqueous RuLa/Ti02 and ethanolic PF/Ti02 colloidal solutions, 13-acts as an acceptor of conduction band electrons if 0 2 is not available. In fact, the much lower yield of 13- per 12- found for air-saturated aqueous colloid, as compared to the ethanolic case, suggests that in the former case accumulated It- competes for conduction band electrons even in the presence of 0 2 . This indicates a higher stability of trapped conduction band
Langmuir, Vol. 7,No. 6, 1991 1137
electrons in the case of aqueous Ti02 colloid. Most trapped electrons may have sufficient driving force to reduce 13(EO= +0.54 V), but not 02 (EO= -0.33 V (NHE)).39 Our finding that only the yield of the final oxidation product 13-,but not that of transient 12- (or RuL3 12% depended on the concentration of dissolved 0 2 points to conduction band electron scavenging as the only role of oxygen under the conditions used here.
V. Conclusions In the first time-resolved spectroscopic investigation of the photooxidation of a halide at dye-sensitized TiOz, we have confirmed that in the case of iodide the reaction path involves 12- in aqueous and ethanolic solution. Aside from the distinct sensitizer and solvent dependence of the observed near-infrared signal associated with the formation of this radical, we have found a substantial spread in the rates of redox processes induced at the colloidal surface. This is clearly manifested in the dependence of the 12yield on iodide concentration. The details of this heterogeneity in terms of rates were only uncovered by extending the spectroscopic investigation to low iodide concentrations. According to our preferred interpretation, there is a wide spread in the efficiency of recapture of the photoinjected conduction band electrons by surface adsorbed I atoms, with a substantial fraction of adsorbed I atoms acting as efficient conduction band electron traps which are responsible for much lower oxidation yields in colloidal solution compared to the corresponding yields in photochemical cell experiments. This would imply that iodide photooxidation efficiencies in sensitized colloids depend critically on the specific environment in which the primary oxidation product (I atom) is formed. Clearly, further time-resolved experiments with Ti02 colloids in different pH regimes (to vary the Ti02 surface charge) and with other types of solvents (in particular nonprotic solvents like acetonitrile) are necessary in order to establish more firmly the factors that control halide oxidation paths and their efficiencies at these colloid surfaces.
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S.Department of Energy under Contract No. DE-AC03-76-SF00098. The authors thank Professor M. Gratzel for the generous supply of colloid solutions and RuL3 sensitizer. Registry No. PF,975-17-7; RuL3,97333-46-5; I-, 20461-54-5; TiOn,13463-67-7; 11, 12190-71-5; I, 14362-44-8. (39) Bard,A. J.; Pareon,R.;Jordan,J. StandardPotentiakin Aqueou Solution; Marcel Dekker, Inc.: New York, 1985; p 65.
