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(9) L. W. Reeves and A. S. Tracey, J. Am. Chem. Soc.,97,5729 (1975). (10) F. Fujiwara and L. W. Reeves, J. Am. Chem. Soc., 98, 6790 (1976). (11) (a) S. Marcelja, Nature (London), 241, 451 (1973); (b) Biochim. bbphys. Acta, 367, 165 (1974); (c) J. Chem. Phys., 60, 3599 (1974). (12) (a) J. Seelig and W. Niederberger, Biochemistry, 13, 1565 (1974); (b) J. Am. Chem. Soc., 96, 2069 (1974). (13) F. Y. Fujiwara, L. W. Reeves, A. S. Tracey, and L. A. Wilson, J. Am. Chem. Soc., 96, 5249 (1974). (14) B. J. Forrest and L. W. Reeves, Chem. Phys. Lipids, 24, 183 (1979). (15) M. Acimls and L. W. Reeves, submitted for publication in Can. J . Chem . (16) L. W. Reeves, A. S. Tracey, and Y. Lee, Can. J. Chem., in press.
(17) L. Hecker, L. W. Reeves, and A. S. Tracey, Mol. Cryst. Liq. Cryst., 53, 77 (1979). (18) P. 0. de Gennes, "The Physics of Liquid Crystals", Clarendon Press, Oxford, 1974. (19) F. Y. Fujiwara and L. W. Reeves, Can. J. Chem., 56, 2178 (1978). (20) L. W. Reeves, J. Sauches de Cara, M. Suzuki, and A. S. Tracey, Mol. Phys., 25, 1481 (1973). (21) L. W. Reeves, A. S. Tracey, and M. M. Tracey, J. Am. Chem. Soc., 95, 3799 (1973). (22) 8. J. Forrest and L. W. Reeves, Mol. Cryst. Liq. Cryst., in press. (23) D. M. Chen, F. Y. Fujiwara, and L. W. Reeves, Can. J. Chem., 55, 2396 (1977).
Measured Dependence of Photovoltage on Irradiance in the Gold-Rhodamine B Photoelectrochemical Cell T. I. Quickenden" and G. K. Yim' Department of Physical and Inorganic Chemistty, University of Western Australia, Nediands, W.A., 6009, Australia (Received May 23, 1979; Revised Manuscript Received December IO. 1979)
A linear dependence of open circuit photovoltage on irradiance has been observed for the gold-rhodamine B photoelectrochemical cell in the investigated irradiance range of (0-1.2) X lo" einsteins m-2 s-l. Care was taken to ensure that thermogalvanic effects did not contribute significantly to the measured photovoltages. The observations conformed with the predictions of a previously published comprehensive photoelectrochemical model which predicts linearity under the low photovoltage and low irradiance conditions of the present investigation.
Introduction Despite a growing i n t e r e ~ t l -in~ the possible use of photoelectrochemical effects for the conversion and storage of solar energy, photoelectrochemical studies are still relatively undeveloped. This is partly due to the complexities introduced into the t h e ~ r y of ~ -photoelectro~ chemical effects by the fusion of the diverse fields of photochemistry and electrochemistry and partly due to the shortage of reliable data on the electrodic and photolytic properties of the fluorescent dyes commonly used in photoelectrochemical cells. In particular, it is surprising that so little experimental data are available on the fundamental dependence of photovoltage on irradiance in cells containing fluorescent dyes. We have previously derived6t7theoretical relationships between open circuit photovoltage, V and irradiance, E, for several situations and, recently, have developed a comprehensive model for cells containing inert electrodes and a fluorescent dye. This model predicts a relationship of the form (1)
where the constant K is a function of the dark concentrations of the electrolyte species and of the rate constants for excitation, fluorescence emission, intersystem crossing, *To whom reprint requests should be directed. On leave from the University of Western Australia until February 1980, as a Visiting Scientist in the Department of Physical Chemistry,at the University of Cambridge, Cambridge, CB2 lEP, U.K. Nire Tan. Recently appointed to a Postdoctoral Research Assistanship in the Department of Physical Chemistry, University of Cambridge. The present work was carried out at the University of
Western Australia.
0022-3654/80/2084-0670$01 .OO/O
TABLE I : Classification of Photoelectrochemical Cells type I inert metal electrodes immersed in a nonelectronically excited liquid electrolyte type I1 inert metal electrodes coated with a solid fluorescent dye and immersed in a
nonelectronically excited liquid electrolyte type I11 inert metal electrodes immersed in a liquid electrolyte which is electronically excited by the incident light type IV semiconductor electrodes immersed in a nonelectronically excited liquid electrolyte type V semiconductor electrodes coated with a solid fluorescent dye and immersed in a nonelectronically excited liquid electrolyte type VI semiconductor electrodes immersed in a liquid electrolyte which is electronically excited by the incident light
photochemical reaction, the various types of quenching, diffusion, and electrodic and nonelectrodic electron transfer. R is the gas constant, T is the absolute temperature, and F is the Faraday. At low photovoltages, eq 1 simplifies8 to the linear relationship RT V,, = -KE (2) F It is the purpose of this present paper, to test the above relationships for the cell Aulrhodamine B, FeCl,, FeCl,, HC1, HzOIAu where one of the two identical, semitransparent, gold electrodes is illuminated and the other gold electrode is kept in the dark. At the same time, it is desired to obtain reliable V,, vs. E dependences for this cell, unaffected by electrode heating and photochemical decomposition which may have unfortunately complicated the few previous experimental determinations in this area. 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No.
PhotoelectrochemicalCell
Before surveying previous experimental work, it is useful to adopt the categorization of photoelectrochemical cells in Table I. ‘The distinction between type I1 and type I11 cells is particularly relevant to the present study and it should be noted that, although a fluorescent dye will be involved in both cases, the type I1 cell contains the dye as a solid layer on the photoelectrode, whereas in the type I11 cell, the fluorescent dye is dissolved in the electrolyte solution. In the latter type of cell, diffusion of photolysis products to the photoelectrode is believed6v7to be an important step in the voltage generating process. The gold-rhodamine B cell used in this study would normally be classified as a type I11 cell, as the fluorescent dye is dissolved in a liquid electrolyte. Although the primary typlt? I11 character of this cell is not contested, recent workgJOindicates that, over a period of time, some type I1 characteristics start to develop. The only photovoltage-irradiance measurements previously reported for type I11 cells,are those by Levin and Whitell and by Miller.12 The former workers studied an illuminated platinum electrode immersed in a solution containing blenzoin dissolved in ethyl alcohol. Unfortunately, Levin and White did not present any absolute irradiance calibration for their source, nor did they present actual photovoltages, but only published uncalibrated deflections which were proportional to photovoltage. It is therefore very difficult to relate their work, in any quantitative manner, to the results of other investigators. Their results for monochromatic light were not conclusive, since only three points were taken over a fairly wide irradiance range and no error bars were given. Their results for polychrornatic light were somewhat more extensive, but it is unclear ]how seriously work with very broad bandpass polychromatic illumination should be regarded. Miller12 has reported the measured photovoltage-irradiance dependence for platinum electrodes immersed in an Fe3+/ Fez+solutioii containing thionine dye. This work is not subject to the same criticisms as can be made of Levin and White’sll investigation, but the interpretation of Miller’s work at highler irradiances is probably very complex due to electrode heating and consequent thermogalvanic voltages. Miller has not considered the complications due to electrode heating and it is noted that the dependence which he obtained was complex and did not even become linear when replotted as photovoltage vs. the logarithm of irradiance. Although there is apparently no further photovoltageirradiance data available for type I11 cells, several sets of data are avaiilable for type I1 cells, but this work has only peripheral relevance to the present study. The most pertinent of these studies is that by Nga13 who in 1935 studied a plaltinum electrode coated with solid rhodamine B and found a photovoltage-irradiance dependence which conforms reasonably well to the logarithmic predictions of eq 1. The other notable set of data is by Anderson et al.14for a platinum electrode coated with solid victoria blue B dye. An extensive linear dependence was observed up to an irradiance of 9.4 W m-2, but a complex fall from linearity was observed at high irradiances and a plot of the logarithm of the irradiance against photovoltage fails to linearize the dependence. Experimental Section The electriical arrangements and the procedures used in the measurement of open circuit photovoltages were identical with those described previ~usly.~ The optical system used for the irradiance dependence measurements was also the siame as that described previously,4except that a supplementary arrangement for the interposition of a
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Figure 1. Photovoltage action spectrum for the gold-rhodamlne B cell (uncorrectedfor the spectral distribution of the xenon light source and monochromator). The dotted line representsthe absorption spectrum of the electrolyte solution. The error bars represent 95% confkdence intervals.
series of calibrated neutral density filters was included, to allow for the variation of irradiance by known amounts. In order to choose an appropriate wavelength for irradiance dependence measurements, the photovoltage action spectrum of the cell was determined (Figure 1). This was carried out by using a high-pressure xenon lamp (Bausch and Lomb 33-86-20-01) and a Bausch and Lomb high intensity monochromator fitted with a UV-visible gr,ating (33-86-79) and operated at a bandpass of 5.0 nm. From the action spectrum, the wavelength of 573 f 24 nm was chosen for the irradiance study, being reasonably near the maximum of the monomer photovoltage peak but well away from the smaller dimer peak. The absolute irradiance of the light used in the irradiance dependence study einsteins rrr2s-l was determined to be (1.2 f 0.15) X at the outer surface of the cell, in the absence of any neutral density filter. Calibration was by comparison with a standard source, previously4 calibrated by visible actinometry. The composition of the aqueous electrolyte used in the cell was as follows: rhodamine B (twice re~rystallizedl~ “spot test” reagent), 1.17 X mol dm-3; HC1 (A.R.), 0.050 mol dm-3; FeC13(A.R.) 0.156 mol dm-3; FeC12(A.R.), 1.3 X mol dm-3. The water used was prepared by deionization, followed by a single distillation. Experiments were carried out on three cells of a previously described4 design, with each having different thicknesses for the semitransparent gold electrodes which were formed by vacuum deposition. Each cell was :filled with a freshly prepared electrolyte solution and thermostatted at 298 f 0.02 K to obtain a steady dark voltage which was usually close to zero if the dark and light electrodes were well matched for thickness. In order to measure the photovoltage in the presence of a slowly drifting dark voltage, the procedure illustrated in Figure 2 was adopted. The dark voltage was recorded for a period of ca. 25 min and the cell was then illuminated until a steady total voltage was achieved. This usually took ca. 25 min. The illumination was then terminated and the decaying dark voltage recorded until a steady (i.e., constant or smoothly drifting) dark voltage was again recorded. The photovoltage was then obtained by substraction, as slhown in Figure 2. The initial and final dark voltages did not usually differ greatly. In order to ascertain the magnitudes of any drifts in photovoltage which might occur during irradiance dependence measurements, the effect of prolonged illumination on open circuit photovoltage was determined (Figure 3)
672
The Journal of Physical Chemistry, Vol. 84, No.
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Quickenden and Yim
> E
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Flgure 2. Schematic diagram showing the method of measuring the photovoltage, V,, in the presence of a slowly drifting dark voltage, ABDE. Illumination is turned on at B and turned off at C.
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Flgure 4. The measured variation of open circuit photovoltage with incident irradlance for three different gold-rhodamine 6 cells with electrode thicknesses of 25.0, 28.,, and 31 .5 nm, in decreasing order of slopes. The error bars represent 95% confidence Intervals.
z 0 L 0;
40
80 TIME
120
160
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Figure 3. The effect of prolonged illumination on open circuit photovoltage for two different gold-rhodamine B cells. The error bars represent 95 % confidence intervals.
for two different cells having the same electrode thicknesses. Both cells showed a progressive increase in photovoltage over a period of 200 h, and the photovoltage still appeared to be rising slowly at the cessation of measurements. The maximum slope in Figure 3 represents a rate of change of photovoltage of ca. 1.5% h-l, and this provides an indication of the largest error likely to arise from this cause. The rise in photovoltage was accompanied by the formation of a rhodamine-coloreddeposit on the electrode surface. The deposit was removable by prolonged soaking and rinsing of the contaminated surface in water over a period of several days. After such cleaning, the electrode reverted to its original, lower photovoltage.
Results The photovoltage vs. irradiance plots for three different cells with different electrode thicknesses and containing freshly prepared rhodamine B solutions are shown in Figure 4. The error bars are 95% confidence intervals and they represent random errors determined by repeated illumination of a cell containing the same electrolyte. To check for slow changes in the cell response, three sets of voltage-irradiance measurements were carried out on each cell. In two such runs (symbols A and 0) the cell was illuminated by successively decreasing irradiances, while in the other run (symbol 0)the irradiances were successively increased. The results in Figure 4 show good linear relationships between open circuit photovoltage and irradiance, with correlation coefficients in excess of 0.997 in all cases. The straight lines also pass acceptably close to the origin as would be expected. In order to compare the characteristics of these three separate cells which have different electrode thicknesses, all data were accumulated into Figure 5 in the form of a single graph of photovoltage vs. the irradiance transmitted to the gold-electrolyte interface. It is seen that once the transmittances of the electrodes are compensated for in
IRRADIANCE/
Einsteins m-2 s - ' ~
Flgure 5. Accumulated open circuit photovoltage-irradiance data for the three cells in Flgure 4, plotted as a function of the irradiance transmitted to the metal-electrolyte interface. The error bars represent 95 % confidence intervals.
this way, the data from all three cells show remarkably good conformity.
Discussion The highest irradiance used in Figure 4 was 1.2 X einsteins m-2 s-l. As thermogalvanic voltages may complicate photoelectrochemical measurements at high irradiances, care was taken to ascertain whether heating of the electrode or the closely adjacent solution accounted for a significant portion of the measured photovoltage. To this end, a clean cell containing the usual electrolyte minus the fluorescent dye was illuminated with an irradiance of 1.2 X einsteins m-2 s-l. A black polythene shield was interposed between the two electrodes, to prevent light reaching the dark electrode, which was no longer shielded by the strongly absorbing fluorescent dye. Under such circumstances, the only modes of voltage generation available were direct photoemission (as in type I cells in Table I) or thermogalvanic voltage generation due to heating of the illuminated electrode and/or the adjacent electrolyte solution. Direct photoemission would be unlikely at the wavelengths used in the present study. Under the above conditions, a small positive thermal voltage was observed on illumination, but it did not exceed 1.4% of the photovoltage which would normally have been generated at this irradiance, and, furthermore, it decayed
Gold-Rhodamilne B PhotoelectrochemicalCell
Flgure 6. Outliine of the Quickenden-Yim comprehensive model' for photovoltage generation in photoelectrochemical cells containing inet3 metal electrodes and fluorescent electrolytes, when the photolyzed species, R, yields an electron-acceptingphotoproduct. In the present case, R represents rhodamine B and the subscripts b and s represent species in the bulk of the solution and at the electrode surface, respectively. Aslerisks represent electronically excited species. Electrons above and below arrows designate electrodlc electron transfers. The various side rajactions have been detailed elsewhere' and have been omitted from the present diagram for the sake of clarity.
over a periold of ca. 10 min. It only recurred if subsequent reillumination was deferred for some time. As this effect would not cause significant errors in the present photovoltage measurements, it was ignored. However, caution would appear to be necessary in the interpretation of measuremeints made at substantially higher irradiances than those used in the present investigation. Complications of the above type are likely to have affected the irradiance dependences reported by Miller12for the iron-thiionine cells as his data extend to irradiances ca. 575 times larger than the maximum irradiance used here. Neither eq 1 nor eq 2 are able to account for the complex irradiance dependence reported by Miller, although it is possible that his measurements would also have exhibited linearity had they been carried out in the much lower irradiance range used in the present study. As pointed out in the Introduction, the only irradiance dependence work on a type I11 cell is that by Levin and White,ll but their work does not give sufficient information to enable quantitative comparisons to be made. However, it is noted that the photovoltages which they measured in the platinurn-benzoin cell do show some signs of linearity with irradiance, at low irradiances. Comparisons with Theory. Providing thermogalvanic complicatioins do not intervene, the photoelectrochemical model of Quickenden and Yim8 predicts the relationship between open circuit photovoltage and irradiance in type I11 cells. Figure 6 shows the essential features of the Quickenden-Yim model for the case where photolysis produces an electron-accepting species, R+., from the rhodamine B, which is designated as R. The subscripts b and s designate species in the bulk of the solution and at the electrode surface, respectively. The superscripts 1 and 3 designate singlet and triplet excited species, respectively, and electrodic electron transfers are designated by the placeiment of electrons above and below the relevant arrows. The various side reactions and deactivating steps have all been omitted from Figure 6 , so that the salient points of the mechanism directly relevant to the present paper will riot be obscured. The mechanism has been described and discussed in detail in a previous publication? In Figure 6 , rhodamine B, either in the bulk of the solution, b, or at the electrode surface, s, is excited by the absorption of light and proceeds via singlet and triplet excited states to form a semioxidized radical ion, R+.,by
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reaction with the excess Fe3+present in the solution. The evidence for the involvement of semireduced and semioxidized radical ions generally, in photoelectrochemical systems containing fluorescent dyes, has been discussed previously.8 In the case of rhodamine B, the semioxidized species, R"., has been identified by Korobov et a1.16 in photolyzed solutions containing the dye and these workers have also observed 3R*,which they implicate as a precursor of the R+.. The semireduced radical ion, R-.,can also be observed16in photolyzed rhodamine B solutions, but following the work of Kruger and Memming" on other dyes, this species would not be expected to be important in the oxidizing medium provided by the excess Fe3+ in the present study. For the above reasons, the mechanism (Figure 6 ) involving photolysis of the dye to form an electron-accepting species has been selected from the two options presented in our previous publication.8 In this mechanism, the sign of the photovoltage can be determined by two factors. In the event that the dye and ferric couples are both highly reversible at the electrode and providing that diffusion from the bulk of the solution is of primary importance, we feel that the relative magnitudes of the diffusion coefficients of R+. and Fez+ will determine the sign of the photovoltage. The positive photopotentials observed in the present study are not explainable on these grounds, in view of the higher diffusivity of the small Fez+ion. The second and much more likely explanation is that provided by Albery and Archer6who point out that the sign of the photopotential will commonly be determined by the relative reversibilities of the dye and inorganic ion couples. In the mechanism outlined in Figure 6 , the positive photopotential implies6that the dye couple is more revemible than the couple involving ferrous and ferric ions.l8 The predicted dependence of open circuit photovoltage on irradiance in such a case is given by8 eq 1 and 2. 'There is no doubt, that the data in Figures 4 and 5 fit the linear prediction of eq 2 very well indeed. Linearity is expectede if V ,
