1448
J . Phys. Chem. 1985,89, 1448-1453
J Aggregate Sensitization of ZnO Electrodes As Studied by Internal Reflection Spectroscopy L. M. Natoli, M. A. Ryan, and M. T. Spitler*+ Department of Chemistry, Mount Holyoke College, South Hadley, Massachusetts 01075 (Received: October 29, 1984)
The photooxidation of J aggregates of cyanine dyes at ZnO single-crystal electrodes has been studied by using total internal reflection spectroscopy. A comparison of photocurrent action spectra was made with absorption spectra for three cyanine dyes. Quantum efficiencies for photocurrent production for these aggregates were found to be as high as 0.2, with surface-formed J aggregates being an order of magnitude more efficient than those adsorbed from solution.
In the studies of dye sensitization of semiconductor electrodes in literature a wide range of sensitizing species have been employed. These range from singly adsorbed molecule^^-^ to multilayers of dye deposited at thicknesses of thousands of angstroms.6-8 The models used to interpret the results of these experiments have ranged in a corresponding manner. For the molecular sensitizing agent, conventional theories of electron transfer a t semiconductor electrodes can be. used to describe the behavior of the ~ystem.Z~,~l’ For the thick-layer system the model of a p-n semiconductor junction has often been employed to explain charge separation at the dyesemiconductor interface.I2J3 Although it has been easy to define and categorize these two limiting cases, there are dye-semiconductor sensitizing systems which fall between these limits and are more difficult to examine and interpret. The most common example of this type involves the monomolecular dye system that has just begun to aggregate.lela Dimers, trimers, and extended aggregate systems can form to give H-band absorption and J-band absorption blue- and red-shifted with respect to absorption by the unassociated dye molecule. As is evident from the extensive work with photographic systems,’’ these aggregates have a surface photochemistry which can be significantly different from that of the monomer dye. Differences have also been observed in electrochemical systems where the photooxidation of H-band and J-band aggregation of cyanine dyes has been studied at semiconductor ~urfaces.’~*’~ This difference is reflected not only in the efficiency of current production for these aggregates but in the kinetics of this process as well. In an electrochemical cell where the electrolyte contains dye, this aggregation on the surface of the semiconductor is difficult to monitor and control. The investigation of these systems therefore requires the ability to measure the absorption spectrum of the adsorbed dye layer in situ, as the electrochemical reaction proceeds. In this way the action spectrum of the photocurrent produced when the surface is illuminated can be correlated with the amount of monomer and aggregate present on the surface at any one time. Given these considerations, spectroelectrochemical techniques have been developed so that an examination may be made in this work of the sensitization of ZnO singlecrystal electrodes by J-band aggregates of cyanine dyes. Two methods for the in situ spectral characterization of the adsorbed dye layer will be employed. One makes use of the total internal reflection of a laser beam within the electrode (laser ATR) to obtain 9,,the quantum efficiency of current production; the other is a form of internal reflection spectroscopy (IRS) which provides a 250-nm wide spectrum in the visible region within 5 ms.
Experimental Section The dyes used in these experiments are given in Figure 1. They are 9-ethyl-2,2’-diethylthiacarbocyanine (9E-Disc2(3)), 1,l’Presently on leave at the Solar Energy Research Institute, Golden, CO.
diethyl-2,2’-cyanine (DiQC2(l)), and 5,5’,6,6’-tetrachloro1,1’,3,3’-tetramethyl-2,2’-benzimidazolocarbocyanine(DiBACI(3)). DiBAC1(3) and 9E-DiSC2(3) were obtained from Accurate Chemical Co.; DiQC2(1) was purchased from Kodak. The purity of the dyes was examined by using TLC, which yielded no evident impurities. The ZnO single crystals were obtained from Professor R. Helbig of the University at Nuremburg-Erlangen in the form of long needles. The (1010) prismatic face was used as the electrode surface. The needles were cut, ground, and polished as prisms for use as internal reflection elements in the internal reflection experiments. These prisms were long enough to permit two reflections at the electrode surface with an angle of incidence of 4S0. The electrode surface was not polished or modified in any way from the “as-grown” condition and was cleaned after each experiment by rinsing in distilled water and hot methanol. The details of the laser ATR apparatus have been described in earlier r e p o r t ~ . ~InJ ~brief, the system provides a simultaneous measure of the number of photons absorbed by the adsorbed dye layer and the current produced by these photons. The quotient of these two gives the quantum efficiency of current production, @P*
The essential aspects of the IRS system are depicted in Figure 2. The electrode was placed in an arrangement that permitted both measurement of the photocurrent action spectrum and the absorption spectrum of the adsorbed dye through IRS. Photocurrent action spectra were taken with the standard photoelectrochemical setup. This involves a three-electrode electrochemical cell with a Pt counter electrode and an SCE reference electrode. The electrodes were biased at + O S V vs. SCE throughout these experiments. By use of a 150-W Xe lamp and a monochromator, the electrode was exposed to monochromatic light swept across the visible spectrum, and the resultant photocurrent was recorded as a function of the wavelength of the (1) Bressel, B.; Gerischer, H. Eer. Bunsenges. Phys. Chem. 1983,87, 398. (2) Kavassalis, C.; Spitler, M. T. J . Phys. Chem. 1983, 87, 3166. (3) Memming, R. Surf. Sci. 1980, 101, 331. (4) Nasielski, J.; Kirsch-De Mesmaeker, A,; Leempoel, P. Electrochim. Acta 1978, 23, 605. ( 5 ) Miyasaka, T.; Honda, K.Surf. Sci. 1980, 101, 541. (6) Breddels, P.; Blasse, G. Surf. Sci. 1981, 79, 209. (7) Tsubomura, H.; Matsumura, M.; Nomura, Y.; Amamiya, T. Nature (London) 1976, 261,402. (8) Matsumura, M.; Nomura, Y.; Tsubomura, H. Bull. Chem. SOC.Jpn. 1976,49, 1409. (9) Gerischer, H.; Willig, F. In “Topics in Current Chemistry”; Davison, A., Ed.; Springer: New York, 1976; Vol. 61, p 31. (10) Gerischer, H. Surf. Sci. 1980, 101, 518. ( 1 1) Sonntag, D. L.; Spitler, M. T. J. Phys. Chem., following paper in this issue. (12) Meier, H. Photochem. Photobiol. 1972, 16, 219. (13) Levi, B.; Lindsey, M. Photogr. Sci. Eng. 1972, 16, 389. (14) Spitler, M. T. In ‘Photoelectrochemistry: Fundamental Procases and Measurement Techniques”; Wallace, W., Nozik, A., Deb, S., Eds.; Electrochemical Society: Princeton, NJ, 1982; p 282. (15) Hada, H.; Yonezawa, Y.; Inaba, H. Ber. Bunsenges. Phys. Chem. 1981, 85, 425. (16) Tributsch, H. Eer. Bunsenges. Phys. Chem. 1969, 73, 5 8 2 . (17) Gilman, P. Photogr. Sci. Eng. 1974, 18, 418.
0022-3654185 12089-1448SO1SO10 0 1985 American Chemical Societv
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1449
J Aggregate Sensitization of ZnO Electrodes
I
Et
I
Et
1 , I '-Diethyl-2,2'-cyanine
DiXCJ2nt 1 )
550
450
650
9E-DiSCJ3) X = S, Y = H, R = 0 = Et
WAVELENGTH (NM)
DlBACJ3) Y = C1,O = CH,, R = H, X = N-CHI
Figure 1. Structure and nomenclature for the dyes used in this work. WHITE LIGHT
\I
FLOW CELL
Figure 3. Optimum conditions for J-aggregate participation in the photocurrent for 9E-DiSC2(3)at ZnO single crystals are seen to require high KCl concentration. As the KC1 concentration is increased from (a) 0.1 M to (b) 0.75 M to (c) 1.0 M the current from the aggregate steadily increases. The dye concentration was 5.5 X 10" M. The crystal was biased at + O S V vs. SCE.
WOTO DIODE ARRAY SPECTRWHOTOUETER
MONOCHROMATW LIGHT
Figure 2. Arrangement for electrochemicalcell which permits both the action spectrum and the absorption spectrum to be measured. White light internally reflected within the ZnO internal reflection element exits into a computer-controlled photodiode array spectrophotometer. At the same time the electrode surface was illuminated through the electrolyte by monochromatic light which was swept through the visible spectrum to induce photocurrent.
550
450
650
WAVELENGTH (NM)
incident light. In the arrangement of Figure 2 the ZnO is illuminated through the electrolyte solution of thickness 1.O mm. The action spectra were therefore corrected for absorption of light by dye in solution as well as for variations in the output of the lampmonochromator combination. The bandwidth of the monochromatic radiation was set at 2.5 nm to be comparable to the resolution of the IRS system. In the IRS measurements, white light from a 100-W tungsten lamp was internally reflected within the electrode to exit upon the entrance slit of a home-built photodiode array spectrophotometer. Light passing through this slit was diffracted off of a grating onto a reticon photodiode array with 128 elements. The optics of the system were adjusted so that a resolution of 2 nm was obtained over a spectral range of 250 nm. The data acquisition was controlled by an AIM 65 microcomputer which analyzed and normalized the data to produce an absorption spectrum. A spectrum could be obtained in 5 ms; it was also possible to signal-average over a longer time period. All experiments were done with perpendicularly polarized light; the spectra presented represent internal reflection spectra which have not been corrected. In the absorption range of interest, these spectra are qualitatively comparable to direct transmission spectra. For the IRS system, a flow cell was used so that the electrolyte could be changed easily. In a typical experiment, electrolyte solution was first introduced into the cell to obtain baselines for the absorption and action spectra and was then followed by the dye-containing electrolyte. The dye solution was kept flowing throughout the experiment. Results Conditions were first sought that would maximize the role of J aggregates in the photocurrent action spectrum. This was
p
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2
0
B
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,
, 450
,
,
,
,
,
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,
,
,
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Figure 4. (a) Action spectrum for 9E-DiSC2(3)after 2-min exposure to a 5.5 X 10" M solution in 1.0 M KC1 is compared with its absorption spectrum taken with the IRS arrangement. (b) After 25 min the J-aggregate contribution at 633 nm to the photocurrent has become pronounced, although it is not reflected to the same degree in the absorption
spectrum.
accomplished by varying the concentration of the dye in the electrolyte as well as that of the salt itself. In Figure 3 can be seen the end results of experimentation with 9E-DiSC2(3). With a constant dye concentration of 5.5 X lod M, J-band participation in the action spectrum increases as the KCl concentration is increased from 0.1 M to 0.75 M to 1.0 M. These spectra were
-
-
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-
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recorded at about 20 rnin after immersion of the electrode in the solution when the system had reached equilibrium. Similar experiments were conducted for DiQC2(1) with the result that at 2.9 X M dye and 1.0 M KCl, current from the J band dominated the action spectrum. In these experiments the dye concentration was sufficiently low that no absorption by J bands was evident in solution. For DiBAC1(3), however, J-band aggregates were present at concentrations as low as 1.5 X lo4 M in a 0.1 M KCI solution. Photocurrent from J bands was also present at all concentrations. Given these conditions for 9E-DiSCz(3) the action spectrum was measured as a function of time in conjunction with an absorption spectrum using IRS. After the dye had been flowing into the cell 2 rnin the action spectrum of Figure 4a was recorded, as was its absorption spectrum. Both exhibit the monomer peak a t 550 nm, but there is no J band present. After 25 min the spectra in Figure 4b were obtained. After this delay the J band has become the prominent feature of the action spectrum but is only a minor feature of the absorption spectrum in Figure 4b. Between the initial and final times in this and subsequent figures, the current and absorption change in a monotonic fashion until the equilibrium condition represented by the final time is attained. Similar behavior was observed for DiQC,( 1) as shown in Figure 4. At a concentration of 4 X M in a 1.0 M KCl solution, DiQC2(1) is present on the surface only as a monomer, and the action spectrum reflects this condition after a 1-min exposure to the dye. Twenty-two minutes later, however, the J-band peak dominates the action spectrum while appearing only as a slight shoulder on the absorption spectrum Figure 5b.
550
650
WAVELENGTH (NM)
WAVELENGTH (NM)
Figure 5. (a) Comparison of the action spectrum and absorption spectrum of DiQC2(1) at 4 X M in 1.0 M KCl after 1 min of exposure of the electrode to the dye solution. (b) After 22 min the absorption spectrum barely reflects the current at 583 nm by a J aggregate which dominate the action spectrum.
0.0 4 50
650
Action spectrum and absorption spectrum for DiBACI(3): (a) M dye and 0.1 M KCI where the J aggregate is after 1 min at 5 X present in solution; (b) after 30 min. Figure 6.
By use of the laser ATR system with 568-nm light from a dye laser, 0,for DiQC2(1) at the J-band peak was measured to be 0.06 f 0.01. For 9E-DiSQ.(3) a measurement at 632 nm yielded a value of 0.17 f 0.06 for the 0,of its J band. These compare with values of 0.011 f 0.001 for the monomer peak of 9EDiSC2(3) and 0.007 f 0.002 for the monomer of DiQCz(l). The conditions of the experiments were then changed so that J-band aggregates were present in the electrolyte before adsorption onto the semiconductor surface. As J-band aggregation was unavoidable with DiBAC2(3), this dye was not used until these experiments. As can be seen in Figure 6a, adsorption of the J-band aggregates occurs immediately, but the aggregate produces almost no current in the action spectrum. After 30 rnin the photocurrent reflects the presence of the aggregate, but with an efficiency which is much lower than the monomeric dye. The surface would appear to be thickly covered with the aggregate because the absorption at the J-band maximum is approaching the point where the relation between IRS absorption and concentration is no longer linear. For 9E-DiSC2(3) the aggregate does not adsorb as readily. The results shown in Figure 7 show that as the aggregate slowly forms on the surface, its contribution to the photocurrent increases, but as a rate no greater than that of the monomer. The aggregate formed in solution appears to be much less efficient than that formed at the surface in the experiments of Figure 4. Different behavior is observed for DiQCz(1) as can be seen in Figure 8. At a concentration of 1.7 X lo4 M in 0.2 M KCl the J aggregate adsorbs readily within 6 rnin and produces photocurrent with an efficiency comparable to the monomer dye. After 28-min exposure, photocurrent from the J-band absorption increases by about 50% relative to its absorption. the dashed portions
J Aggregate Sensitization of ZnO Electrodes
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1451 (a)
(0)
L SE-DISC2 (3)
0. 0
0. 0
550
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550
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WAVELENGTH (NM>
WA VELENGTH (NM)
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,
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,
,
,
,
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WAVELENGTH (NM) WAVELENGTH (NM)
Figure 7. Action spectrum and absorption spectrum of absorbed dye for M in 1.0 M KCI: (a) after 1 min in a situation 9E-DiSC2(5)at 1 X where the aggregate is present in solution; (b) after 8 min.
of the action spectrum indicate spectral regions where solution absorption of the light is too great to permit an accurate correction of the action spectrum.
Discussion It is evident that it is possible to study the photooxidation of aggregates which form by adsorption onto semiconductor electrodes. However, it is necessary to have a means of spectral characterization of the adsorbed dye layer. This is clearly demonstrated by the photocurrent action spectra of Figure 4-8, which are not a reliable reflection on the concentration of the adsorbed dye, be it monomer or aggregate. This is a consequence of the varying efficiencies of the different forms of the dye. Although the monomer and H aggregates have comparable @p, the J aggregates differ greatly, ranging from the more efficient in Figures 4 and 5 to the very inefficient of Figures 6, I, and 8. The spectral characteristics of the J-band absorption differ from those of the H band because of their aggregate structure.I* In Figure 9 are depicted two of the possible stacking arrangements of the molecules in H and J aggregates and illustrates how steric factors can influence the form preferred by the dye. As the monomer aggregates, the energy level of the excited state splits into two states of lower and higher energies. The structural characteristics of the J aggregate make the electronic transition to the lower energy state allowed, and a red shift of the absorption spectrum is observed. In a complementary fashion the H ag-
Figure 8. (a) Action spectrum and absorption spectrum of adsorbed dye for DiQC2(l) at 1.7 X loJ M in 0.2 M KC1 after 6 min exposure to a
solution containing aggregates; (b) After 28 min the current has climbed relative to the absorption. gregate structure results in a blue shift. The observed differences in aPfor the current-sensitizing J aggregates can be attributed to two kinds of aggregate that form in different locations. The aggregate in the electrolyte of the high-concentration experiments is visible to the eye and is clearly a three-dimensional cluster of dye aggregates. In Figures 6 and 8 this cluster is seen to adsorb rapidly onto the ZnO as can be confirmed by eye through examination of the crystal. The same spectral maxima are found for the solution aggregates and these adsorbed aggregates. Only a pottion of the cluster, however, can be in contact with the surface. The other type of aggregate is two dimensional and forms on the surface over a lO-2O-min period through the slow adsorption of the monomer building blocks. This is most clearly seen in Figures 4 and 5 . In the formation of this aggregate, these planar dyes initially adsorb flat on the surface but subsequently adsorb cooperatively in an edge-on manner to build a staggered brickwork structure similar to that depicted in Figure 9.19 The time required to reach equilibrium, usually 20 min, is roughly the same as that required for equilibration of non-aggregate-forming dyes with their adsorbed counterparts at ZnO and Ti02 single-crystal electrode surfaces.20v21 The two forms of the aggregate, surface and solution, have different aPwith the low-concentration experiments of Figure 4 and 5 , showing that the surface aggregate is the more efficient. In Figure 6 the solution aggregate of DiBAC1(3) adsorbs to the ~~~
(1 8) Stunner, D. M.; Heseltine, D. W.In "The Theory of the Photographic Process", 4th ed.; Macmillan: New York, 1977; p 194.
~
~~
(19) Hen, A. H. "The Theory of the Photographic Process", 4th ed.; Macmillan: New York, 1977; p 235. (20) Spitler, M.;Calvin, M. J . Chem. Phys. 1977, 66, 4294. (21) Spitler, M.; Calvin, M. J . Chem. Phys. 1977, 67, 5193.
1452 The Journal of Physical Chemistry, Vol. 89, No. 8, 1985
Natoli et al. J aggregate sensitization
Energy Scheme
H-BAND
_.
homo I
I
Et
Et
*--7
___
-/monomer
defect
Di SC.43)
J-BAND
exciton
dYLWNLM
{
-
defect
monomer
Et
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9E- Di SC,(3)
Figure 9. Two possible stacking arrangements of dyes on their side for H and J aggregates as seen from a vantage point which looks down at the crystal surface. For the case of 9E-DiSC2(3)the ethyl group in the meso position precludes the straight stack or H aggregate shown for DiSC,(3) and results in a staggered configuration characteristic of the J aggregate.
surface immediately but exhibits no immediate ability to sensitize. It is only after a 30-min delay that significant photocurrent is produced. For DiBAC1(3) and DiQC,(l) in Figures 6b and 8, there is a mixture of the two forms a t the surface giving rise to an apparent efficiency which is a weighted average of the two. These data can be understood with the consideration that the low 9,for monomers is due to a back reaction at the semiconductor surface following oxidation of the excited dye, D*.’*2,22*23 D*
-
e,;
+ D+
D+ + e s t
-
(1)
D
Here the electron is trapped at the surface, est, in close enough proximity to the oxidized dye to be able to return with high efficiency. Evidently the oxidation of the surface J aggregate proceeds in a manner which precludes this back reaction. We believe that an established model from the photographic literature” explains how the reaction of eq 2 is hindered. A schematic representation of this model is given in Figure 10. In this picture the exciton of the excited aggregate is broken up by a trapping of the hole by either an adjacent monomer or a surface defect. In the energetic scheme at the top of Figure 10 the monomer could function as a reducing agent which captures the hole; a defect could also perturb the ground-state energy of one of the aggregate components so that it functions as a hole trap. In either case, the electron soon becomes spatially separated from the hole, and transfer to the semiconductor occurs a t a distance from the hole large enough to hinder the recombination reaction of eq 2. It is also possible for electron transfer to occur from the exciton to the semiconductor before the hole is reduced.24 However, the work of Gi1manl7 shows that this does not affect the rate of eq 2. 9,for the surface aggregates is below 0.2 so that this mechanism is not totally efficient; the back reaction must still play a (22) Arden, W.; Fromherz, P. J . Electrochem. Soc. 1980, 127, 130. (23) Spitler, M., in preparation. (24) Penner, T. L.; Mobius, D. J . Am. Chem. Soc. 1982, 104, 7407.
Figure 10. In the lower portion is a pictorial representation of the hole-trapping mechanism where the exciton in the J aggregate is broken up through a trapping of the hole by either a monomer dye or an aggregate subunit perturbed by a neighboring defect. The consequent separation of electron and trapped hole precludes their recombination and results in a more efficient current-producing charge injection into the semiconductor. An energy level scheme is given for this mechanism at the top of the figure. The hole of the exciton can be localized at a more negative energy level provided by a defect-perturbed portion of the aggregate or a nearby monomer. This model is taken from ref 17.
major role in quenching the photocurrent producing reaction. The sensitized photocurrent from the solution clusters is small, and it can be shown that their 0,are no greater than those of their monomer forms. Most of the aggregate cluster adsorbed from solution is too distant from the surface to inject an electron; electron transfer through this disordered layer is obviously an inefficient process. Figure 6a shows that even those aggregates of the DiBAC’(3) cluster in contact with the surface do not sensitize photocurrent. In contrast, there is a simultaneous appearance of photocurrent and J-band absorption in Figure 8 for DiQC2(1). This difference in behavior can be explained with the use of the hole-trapping scheme. For DiBAC1(3) the oxidation potential of the monomer dye is more negative than the potential of the J aggregate hole stateeZ5 Therefore it is possible for the part of the aggregate cluster in contact with the surface to be photoreduced by adsorbed monomer and produce photocurrent, but this does not happen, indicated by the fact that its 9, is negligible. In contrast, the oxidation potential of the monomer of DiQC2(1) is 0.11 eV more positive than that of the aggregate?5 In this case hole trapping is not possible. Yet, J aggregate photocurrent is observed and one must conclude that it results from an accelerated formation of the surface aggregate owing to the high solution concentration of the dye. Of course, it is also possible for the adsorbed cluster to break up and re-form as the surface aggregate. However, the conclusion is the same. The aP for the DiQC2(1) solution cluster is less than that for its monomer. A similar acceleration in surface aggregation is seen for 9EDiSC,(3) in Figure 7a, but within 8 min the adsorption of the solution aggregate brings down the apparent 9, to that of the monomer. These comments and the data presented above indicate the degree of complexity involved in dealing with a sensitizing system of aggregates adsorbed on a semiconductor surface. The considerations go beyond those required to describe a simple molecular interaction with the substrate electrode and deal directly with complex photochemistry that can occur between the molecules of an adsorbed layer. (25) Gilman,
P. B. Photogr. Sci. Eng. 1968, 12, 230.
J . Phys. Chem. 1985,89, 1453-1457 Acknowledgment. For support of this work we are indebted to the National Science Foundation 69A Program for an instrumentation grant and to the Department of Energy, Office of Basic Energy Sciences. We are also grateful for the electronic work of A. Chace and the laboratory work of E. Fitzgerald. This work
1453
would not have been possible without the donation of the excellent samples of ZnO crystals by Prof. Helbig. Registry No. 9E-DiSC2(3),35077-88-4; DiQC,(l), 20766-49-8; DiBAC1(3),32690-13-4; ZnO, 1314-13-2;KCl, 7447-40-7.
Examination of the Energetic Threshold for Dye-Sensitized Photocurrent at SrTiOs Electrodes L. P. Sonntag and M. T. Spitler*+ Department of Chemistry, Mount Holyoke College, South Hadley, Massachusetts 01 075 (Received: October 29, 1984)
The energy threshold for electron transfer has been determined for the oxidation of an excited dye molecule over the conduction band of SrTi03singlecrystal electrodes. When the pH of the electrolyte is varied from 4 to 11, the conduction band energy of SrTi03 was shifted from below to above the energy of the electron in the excited state of two donor dyes, 2,2’-diethylthiacarbocyanine and 2,2’-diethyloxadicarbocyanine. The resulting threshold has been analyzed with the aid of several models for electron transfer at semiconductor electrodes.
The relationship between the rate of an electron-transfer reaction and the free energy has been well-defined for solution reactants. In most of these studies the transfer rate between an acceptor and a variety of electron donors was measured as a function of the energy change of the reaction;’-3 in other cases4 the reduction rate of a series of electron acceptors was determined by using a common electron donor. As the energy change of these reactions ranged from endothermic to exothermic the reaction rates increased exponentially until a diffusion-limited rate constant was observed. These characteristics have been analyzed in terms of current theories of electron transfer and have been used to suggest improvements and modifications in Similar correlations between heterogeneous rate constants and the reaction energies have been made in electrochemical systems. At the surface of organic molecular crystals the rate constants for hole injection into the electrode by oxidants follows theory very we11.8 At photoexcited semiconductor electrodes the oxidation of reductants as a function of their redox potentials yields relative rate constants which also rise exponentially to a diffusion-limited val~e.~J* At semiconductor electrodes it is also possible to determine the relation between the electron-transfer rates and free energy for just one electron donor or This can be done at the surface of oxide electrodes such as ZnO, SrTi03, and TiOz because of a pH-dependent double layer through which the energy levels of the solid can be shifted with respect to solution levels by 60 mV per unit change in pH. In this type of experiment, however, more refined information about electron transfer can be obtained than is possible in the experiments involving series of electron donors and acceptors. In particular the distribution function can be determined which describes the energy of the donor or acceptor species as a function of its particular solvation state. Such a distribution constitutes the instantaneous density of states for the electron donor or acceptor in solution. This point has been discussed in detail by Vanden Berghe et ai.’* in a study of the one-electron reduction of Fe(CN)63- at ZnO single-crystal electrodes in which the magnitude of the current density a t two p H values was used to calculate the rearrangement energy of the Fe(cN)63 ion. Further use of this feature of semiconductor electrochemistry has been made by Clark and SutinI3 in the estimation of the rearrangement ‘Presently on leave at the Solar Energy Research Institute, Golden, CO.
0022-3654/85/2089-1453$01.50/0
energies of several ruthenium complexes photooxidized from solution a t TiOa electrodes. However, there has been no comparable study of rearrangement energies and thresholds for electron transfer involving reactants adsorbed at the semiconductor surface at the interface of two differing dielectric media. In this work this method will be employed to determine the distribution function for the instantaneous electron energies of adsorted reductants and to assess the accuracy of the Gaussian function which has been proposed to describe it.I4J5 The photoelectrochemical system selected for this experimentation consists of a SrTi03singlecrystal electrode as the electron acceptor and two adsorbed cyanine dyes which serve as electron donors in their excited states. These dyes have been selected so that the average energy of the electron in the excited state equals that of the conduction band edge of SrTi03 a t about pH 4. Thus a threshold for electron transfer should be crossed as the electrolyte pH is increased because the energy of the SrTi03 conduction band moves above that of electron in the excited dye. The ability of these excited dyes to inject electrons into the conduction band of SrTi03 will be measured through a laser attenuated total reflection (ATR) technique which will provide QP, the quantum yield for current production at each pH. (1) Rehm, D.; Weller, A. Ber. Bunsenges. Phys. Chem. 1969, 73, 834. (2) Vogelmann, E.; Scbreiner, S.;Rauscher, W.; Kramer, H. Z . Phys. Chem. (Wiesbadenl 1976. 101. 321. (3) Bakrdini, R’.; Van& G:; Indelli, M.; Scandola, F.; Balzani, V. J. Am. Chem. SOC.1978, 100, 7219. (4) Nagle, J. K.; Dressick, M. J. Meyer, T. J. J. Am. Chem. Soc. 1979, 101,3993; ’ ( 5 ) Indelli, M.; Scandola, F. J. Am. Chem. SOC.1978, 100, 7733. (6) Efrina, S.; Bixon, M. Chem. Phys. 1976, 13, 447. (7) Van Duyne, R.; Fischer, S . Chem. Phys. 1974, 5, 183. (8) Willig, F. Adu. Electrochem. Electrochem. Eng. 1981, 12, 1. ( 9 ) Tamura, H.; Yoneyama, H.; Kobayashi, T. In “Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A., Ed.; American Chemical Society: Washington, DC, 1981; p 131. (IO) Inone, F.; Fujishima, A.; Honda, K. Bull. Chem. Soc. Jpn. 1979,52, 721 7.
(11) Schumacher, R.; Wilson, R. H. Harris, L. A. J . Electrochem. SOC. 1980, 127, 96. (12) Vanden Berghe, R. A,; Cardon, F.; Gomes, W. P. Surf. Sci. 1973, 39, 368. (13) Clark, W. D. K.; Sutin, N. J. Am. Chem. Soc. 1977, 99, 4676. (14) Gerischer, H.Adu. Electrochem. Electrochem. Eng. 1961, 1 , 139. (15) Gerischer, H. Photochem. Photobiol. 1972, 16, 243.
0 1985 American Chemical Society
