Chemical Processes in Photogalvanic Cells (7) N. A. P. Kane-Maguire, J. E. Pfifer, and C. G. Toney, Inorg. Cbem., 15, 593 (1976). (8) N. A. P. Kane-Maguire, D. E. Richardson, and C. G. Toney, J. Am. Cbem. Soc., 98, 3996 (1976). (9) D. Sandrini, M. T. Gandolfi, L. Moggi, and V. Balzani, J . Am. Cbem. Soc., 100, 1463 (1978). (10) M. Maestri, F. Bolletta, L. Moggi, V. Balzani, M. S. Henry, and M. 2 . Hoffman, J . Am. Cbem. Soc., 100, 2694 (1978). (11) A. R. Gutierrez and A. W. Adamson, J. Pbys. Cbem., 82, 902 (1978). (12) R. T. Walters and A. W. Adamson, Acta Cbem. Scand., Sect. A , 33, 53 (1979). (13) S. C. Pyke and M. W. Windsor, J. Am. Cbem. Soc., 100,6518 (1978). See also A. D. Kirk, P. E. Hoggard, G. B. Porter, M. G. Rockley, and M. W. Windsor, Cbem. Pbys, Lett., 37, 199 (1976). (14) A. W. Adamson, R. T. Walters, R. Fukuda, and A. R. Gutierrez, J . Am. Cbem. Soc., 100, 5241 (1978). (15) D. Sandrini, M. T. Gandolfi, A. Juris, and V. Balzani, J . Am. Cbem. Soc., 99, 4523 (1977). (16) C. L. Rollinson and J. C. Bailar, Jr., Inorg. Syn., 2, 198 (1946). (17) C. S. Garner and D. A. House, TransitionMetalchem., 8, 69 (1970). (18) (a) G. Schlesssinger, "Inorganic Laboratory Preparation", Academic Press, New York, 1962. (b) A. Benrathy and H. Steinrath, Z . Anorg. Allg. Cbem., 194, 351 (1930).
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(19) D. W. Hoppenjans and J. B. Hunt, Inorg. Cbem., 8 , 505 (1969). (20) See J. W. Hunt and J. K. Thomas, Radiat. Res., 32, 149 (1967). We used the RCA 4840 PMT instead of their RCA 1P28 because of the better red response (S-20 vs. S-5). (21) M. W. Dowley, K. B. Eisenthal, and W. L. Peticolas, phys. Rev. Left., 18, 531 (1967). (22) W. Geis and H. L. Schlafer, Z . Anorg. Allg. Cbem., 271, 115 (1953). (23) Two isomers may be present, M. Cimolino and R. G. Linck, private communication. (24) E. Jtrgensen and J. B. Bjerrum, Acta Cbem. Sand., 13, 2075 (1959). (25) F. Basolo and R. G. Pearson, "Mechanisms of Inorganic Reactions", 2nd ed, Wiley, New York, 1958, p 180. (26) R. G. Wilkins, "The Study of Kinetics and Mechanism of Reactions of Transition Metal Complexes", Allyn and Bacon, Boston, 1974, p 318. (27) A. W. Adamson, Adv. Cbem. Ser., No. 150, 128 (1976). (28) The conclusion that the bisc path was dominant' was proposed as the preferred interpretation of quenching results; the results, however, are not inconsistent with k as the dominant reaction of D,'. (29) Literature values for Cr(en)iSt are 1.33 ps at -20 OC6 and 1.85 ps at 20 OC.'* (30) See ref 7, 13, and 11. Note also ref 8 and 31. (31) C. Conti and L. S. Forster, J. Am. Cbem. Soc., 99, 613 (1977).
Chemical Processes and Electric Power in Photogalvanic Cells Containing Reversible or Irreversible Reducing Agents Hiroshl Tsubomura,* Yasuhlro Shlmoura, and Shigeakl Fujiwara Depatfment of Cbemistty, Faculty of Engineering Science, Osaka lJnivers/ty, Toyonaka, Osaka, Japan 560 (Received March 6, 1979)
The photovoltages and photocurrents in various photogalvanic cells, containing a dye (thionine, riboflavin, or proflavin) and a reducing agent (Fez+,hydroquinone, EDTA, or triethanolamine), were examined. The results indicate that those systems having EDTA or triethaiolamine as a reducing agent give much higher photocurrents than those having FeZt or hydroquinone. It has been concluded that such a difference by the reducing agent is due to irreversible reactions of EDTA or triethanolamine taking place after their photooxidation. The energetics in these photogalvanic systems was discussed qualitatively in relation to their feasibility as energy transformers. The photochemical effect of methyl viologen on excited proflavin was also studied. The enhancement of photocurrents by methyl viologen in the proflavin-EDTA photogalvanic systems has been explained from experimental results.
Introduction The photogalvanic effect arising from photochemical redox reactions of species in solutions has been attracting many authors. Most of the previous work has been done on thionineferrous ion (Fez+)systerns.l4 We studied some features of .these systems and presented a theory for the currents and voltages in the photogalvanic cells.7 The photocurrents in these cells are rather low owing to (1)the recombination of photochemically produced active species in solutions, and (2) the two-way electrode processes of the active species. Various trials to improve the efficiencies of photogalvanic cells have been made,8-14but as yet no drastic progress to improve the efficiency has been made. We have also examined the influence of semiconductor electrodes on the efficiency of photogalvanic cells, but have not obtained results leading to much improvement of the efficiencies.15 Kamiya and Okawara reported some years ago that especially high photovoltages and currents were obtained from photogalvanic cells with dyes and aliphatic amines as reducing agents.16-19 Other authors reported similar results with similar reducing agents.2q20 Kamiya and Okawara also reported an interesting result that the photoeffect in the proflavin-triethanolamine system is markedly enhanced by the addition of methyl viologen 0022-365417912083-2 103$01.0010
(MV2+).19In the present paper, we will describe our results including some new aspects of the effect of aliphatic reducing agents and methyl viologen.
Experimental Section Figure 1 shows the apparatus for the measurement of photocurrents and photovoltages. Platinum plates were used as light and dark electrodes. The electrode potentials with respect to a saturated calomel electrode (SCE) and the currents between two platinum electrodes were measured by use of a Shimadzu 601 electrometer. The electrodes were polished with alumina abrasives and washed with dilute nitric acid and with deionized water. Nitrogen gas was bubbled into the solution for 20 min before each measurement and kept bubbling duringcthe measurements. A 500-W xenon lamp was used as a light source, with light of wavelength shorter than about 360 nm being cut off by a glass filter (Toshiba UV-39). Commercial chemicals of special reagent grade were used without further purification. Unless otherwise specified, aqueous solutions containing 1.0 X mol dm3 (M) dye, 1.0 X M reducing agent, and 2.0 X 10- M potassium sulfate as a supporting electrolyte were used. For the analysis of EDTA, the pH of the sample solution was adjusted to 10 by adding a buffer solution, then, in
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H. Tsubomura, Y. Shimoura and Thionine-TEA
Thionine-Fez'
ID, MA light on off
0
0
20
In, PA
40
60 min
hu "p2 mVlighton
I &
L
thionine
riboflavin
proflavin
rhodamine B rose bengal
Fez+ hydro uinone EDTA1 TEAC TEA t MVd hydroquinone EDTA TEA TEA t MV hydroquinone hydroquinone t MV EDTA EDTA + MV TEA TEA t MV MV EDTA EDTA + MV EDTA EDTA t MV
Vp, mV
off &
I
LP 9
uA/cmZ 3.3 2.8 12 18 18 0.83 58 35 37 0.43
0.41 0.79 25 0.25 6.5 0.0 38 0.092 0.092 0.025
0.017
0
20
40
60 min
TABLE I: Photocurrents (ip) and Photovoltagcs ( Vp) in Various Photogalvanic Cellsa redox agent
rnin
L
Flgure 1. Apparatus for the measurement of photocurrents and photovottages in a photogaivanic cell: 1, R electrode; 2, magnetic stirrer; 3, nitrogen gas inlet; 4, glass wool partition; 5, glass filter; 6, quartz window.
dye
S.Fujiwara
0
10
20 min
Figure 2. The change of photocurrent (i,) and photovoltage ( Vp) with irradiation.
VP mV 9
140 20 190 250 210 20 720 500 430 1.9 2.4
140 350
60 320 5.0
28 10 19 20
a The concentrations are M for the dye, except thionine ( 5 X M), M for the reducing agent, M for methyl viologen. The pH is ca. 6. The polarity of the irradiated electrode is negative in all sysEthylenediarninetetraacetic acid. Triethanoltems. amine. Methyl viologen.
the presence of eriochrome black T as an indicator, the EDTA in the solution was titrated with a magnesium(I1) standard solution.
Results Table I shows the photocurrents and photovoltages for various combinations of dyes and reducing agents. As shown in this table, especially high photocurrents and photovoltages were obtained in the systems with EDTA or TEA as reducing agents against thionine and riboflavin, compared with the case for the Fez+ or hydroquinone as a reducing agent. For proflavin, rhodamine B, and rose bengal, much smaller photocurcents and photovoltages were observed even with EDTA or TEA as reducing agents. The photoresponses for proflavin-EDTA or -TEA systems were enhanced drastically by the addition of methyl viologen (MV2+). It is noteworthy, however, that these enhanced values were the same order of magnitude as those of thionine or riboflavin with EDTA or TEA as reducing agents and with no MV2+. Such an enhancement by MV2' could not be seen for thionine and riboflavin, nor for proflavin with hydroquinone. To summarize, the effect of MV2+ arises only for the proflavin-aliphatic amine
systems. In these systems, the solution turned dark green, the color of MV+-, after a few minutes of irradiation. Figure 2 shows the responses of the photocurrents and photovoltages in some typical systems. In the thionineFez+system, both the photocurrent and photovoltage arise and disappear instantaneously with the on-off of the light. On the other hand, in the thionine-TEA system, they arise and decay slowly with the on-off, taking ca. 20 min to become constant. The similar slow and high response was observed in the thionine-EDTA, riboflavin-EDTA, and riboflavin-TEA systems, that is to say, in all cases of aliphatic reducing agents. In order to check the chemical stability of the photooxidation product of EDTA, one of the electrodes of a photogalvanic system comprising 1.0 X M riboflavin, 2.01 X M EDTA, and 0.10 M potassium nitrate was continuously illuminated for 7 h at the short circuit condition, and the EDTA left in the solution was analyzed by means of chelatometric titration. It turned out that the concentration of EDTA decreased from 2.01 to 1.48 mM. This corresponds to a decrease of 4.5 X mol, about 20% of the originally present EDTA, while the total electrical quantity passed was 1.3 C. If one tentatively assumes that a molecule of EDTA is lost by anodic consumption of an electron, the result leads to a quantum efficiency for the reaction of EDTA+ of 0.29. (As some of the reaction products of EDTA' might still hold the chelating ability, the above number for the decrease of EDTA is regarded to be the lower limit for the irreversible chemical change.) In the proflavin-EDTA system, the EDTA consumption under the same irradiation level was found to become much higher in the presence of methyl viologen than without it. For the purpose of clarifying the effect of methyl viologen, the photochemistry of methyl viologen with proflavin was studied. The fluorescence of proflavin in aqueous solutions was found to be quenched by methyl viologen. The fluorescence intensity followed the Stern-Volmer equation, with Kq7 = 51 dm3 mol-l. It was also found that, when an ethanol solution of 1 X M M methyl viologen frozen at 77 K proflavin and 1 X was irradiated with a 250-W mercury lamp through a glass filter for about 15 min, a green color developed, indicating the formation of reduced methyl viologen, MV+.. The difference spectrum of the sample before and after irradiation (Figure 3) agreed well with the spectrum of MV'. reported by Kosower et aL2l The reduction of methyl viologen was not observed by similar photolysis in a low
The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
Chemical Processes in Photogalvanic Cells 1
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2105
El irreversible recucipg agent
A I Reversible reducing agent
-\
D*+ EDTA
D* + Fez+
',,
Product + ( H C I
400
500
Flgure 5. Schematic energy diagrams for the photochemical processes in photogalvanic cells.
600
Wavelength ( n m )
Figure 3. (a) A difference spectrum between an ethanol solution of proflavin and methyl viologen at 77 K and that irradiated. (b) An absorption spectrum of reduced methyl viologen (MV'.).
t r
w
-0,5 -l'O
0
wl
I
Prof lavi n
Edamine
0 Rose bengal Riboflavin 0
Thion ine
cz Methyl viologen
Hyd roqui Don e I
._
cm-2 and 100 mV, respectively, and their response is quick. In the second case, the photocurrents and photovoltages are about one order higher, and their response is slow. From general chemical evidence, as well as from part of our experimental results, it is thought that the reducing agents (generally donoted as R) of the above first case are reversible: R P R+ f eIn other words, the oxidized form R+is sufficiently stable so that they return to the reduced form reversibly. The reducing agents in the second case, that is, EDTA, TEA, allyl thiourea, etc., are irreversible, in other words, the oxidized form undergoes further reaction quickly and does not return to the reduced form: R e- R+ e- R'' Though the details of the ensuing reactions of the amines are not understood, it seems highly probable that electron elimination from the nitrogen lone pair occurs as the first step of the photooxidation, then a proton e l h ination takes place as follows:
-
+1.0
I
- - - - _ _ - l i m i t of
I
I
measurement I
Figure 4. The on-set potentials for the cathodic 0 and the anodic E current at pH 7.8.
temperature ethanol matrix, when the concentration of M. methyl viologen was less than 1 X The fluorescence of riboflavin was quenched by methyl viologen with a quenching constant K4r about one fifth that for proflavin. The formation of MV+. by a similar irradiation of MV2+with riboflavin in a low temperature matrix of ethanol was also weaker than for the case of proflavin. The fluorescence of riboflavin was quenched very slightly (-4%) by the addition of M EDTA. That of thionine was practically unchanged by EDTA. In order to see the relative positions of the redox potentials of dyes and reducing agents used in the present work, the current-potential curves in the electrolyte solutions containing these substances were measured with platinum or glassy carbon electrodes under potentiostatic condition in borate buffer solutions (pH 7.8), and the on-set potentials for the oxidation or reduction currents were obtained as shown in Figure 4. The reduction potential of proflavin could not be observed because it was too much negative. In this case, therefore, a half-wave potential obtained from polarography was employed. The oxidation potentials for thionine and riboflavin could not be obtained because of the oxidation of water. Hence, the oxidation potential for these dyes are more positive than +1.1 eV.
Discussion As Table I and Figure 2 indicate, there is a clear distinction in the photogalvanic behavior between (1)the case where Fez+ or hydroquinone is used as a reducing agent and (2) the case where an aliphatic amino compound is used as a reducing agent. In the first case, the photocurrents and photovoltages are small, of the order of pA
+
-
+
followed by complicated reactions of the radical thus formed: bond cleavage, double bond formation, condensation, etc. As pointed out earlier in this paper, the weakness of the photocurrents in the thionine-Fez+ system is mainly due to the decline of photochemically produced active species by their recombination. This is fatal in any systems having reversible reducing agents. In the case of irreversible reducing agents, the oxidized form is thought to undergo rapid irreversible chemical change and turn into electrochemically inactive materials, that is to say, materials having a highly negative on-set potentials for cathodic reaction, so that only the reduced dye remains in the solution as an active agent. With continued illumination, the concentration of the reduced dye will be increased in the vicinity of the electrode and the photovoltage and current will be increased gradually with time. Such a scheme explains all the experimental results very nicely. Though riboflavin is known to be photochemically degraded, it is reported to be fairly stable in the presence of suitable reducing agents such as EDTAz2 Figure 5 shows a schematic energy diagram for photogalvanic cells containing a dye (D) and a reducing agent, where Fez+ represent the reversible, and EDTA the irreversible reducing agent. In the former, the photon energy (hv)absorbed by the system is partially converted to the chemical energy of D- -l- Fe3+,and a small fraction of this chemical energy is converted into electrical energy by the electrode reaction. The efficiency of this last conversion is rather low, owing to the back reaction dissipating most of the free energy into heat. h the case of the dye-EDTA system, it changes into D- EDTA+ by
+
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Vol. 83, No. 16, 1979
TABLE 11: E n e r g y D i f f e r e n c e b e t w e e n t h e Ground State, G, a n d t h e F i r s t E x c i t e d Singlet State, S, , of t h e D y e s -4E(S,,GI, 4 E( SI ,G I, dye
eV
eV
dye I
thionine riboflavin proflavin
2.0 2.6 2.6
rose bengal rhodamine B
2.2 2.2
photoexcitation. The EDTA' ion causes further reactions losing free energy and forms stable products which are electrodically inactive. D- ions then accumulate around the illuminated electrode giving high photocurrent and photovoltage. This type of photogalvanic cells produce electrical power with high efficiency but the reducing agent is consumed by the irreversible chemical change. If the free energy of the products of the reaction is higher than that of EDTA, then in the cell some light energy is transformed and stored as a chemical energy. If, on the other hand, the free energy of the products is lower than that of the original reducing agent, then the system is regarded to be a light driven chemical battery. In either case, one has to supply a reducing agent so as to keep the cell working. Albery and Archer also pointed out from theoretical reasoning that photogalvanic cells work efficiently only if one of the redox couples in solution is irrever~ible.~~ Effect of Methyl Viologen. From Table I, it can be seen that the photocurrent and photovoltage for a combination of proflavin and an irreversible reducing agent are very low compared to those of thionine and riboflavin, but are enhanced by addition of methyl viologen, while the photocurrents and voltages for thionine and riboflavin are not changed by the addition of methyl viologen. These results, together with the high negative value for the reduction potential of proflavin, seem to indicate that this dye in the excited state cannot oxidize TEA or EDTA by itself. Similarly, rhodamine B and rose bengal, having high negative redox potentials, cause a very small photogalvanic effect with EDTA, suggesting that these dyes cannot oxidize EDTA even in excited electronic states. As has been described in the Results section, excited proflavin by itself can probably reduce methyl viologen in its singlet excited state. Therefore, it is most probable that in a solution containing proflavin, EDTA (or TEA), and methyl viologen, the excited proflavin first reduces methyl viologen to form the rather stable MV'. ion and the proflavin cation. Then, this proflavin cation and EDTA (or TEA) react together forming EDTA+ and proflavin, the former causing rapid and irreversible changes into stable products. As a result, MV'. is accumulated around the illuminated electrode, making it negatively charged. For the case of riboflavin and thionine, the excited dyes are such strong oxidants that they can oxidize EDTA or TEA efficiently. These results are reasonably related with the energy level diagram of Figure 4. The potentials for cathodic reactions are thought to be shifted positively by the energy difference between the excited and ground states, and
H. Tsubomura, Y.
Shimoura and
S.Fujiwara
those for anodic reactions are thought to be shifted negatively by the same amount. As proflavin has the highest negative potentials for anodic and cathodic reactions, it is regarded to be the most reductive and the least oxidative in the excited state among dyes studied, if the energy difference between excited and ground states is not much different among these dyes. (See Table 11. In some cases the electron transfer may proceed via the triplet (TI)states, but the energies for TI and the lowest singlet (SI)states are mostly parallel.) Thionine and riboflavin have fairly low levels for the cathodic reaction and therefore their strong oxidative nature in the excited state can be understood. Rose bengal and rhodamine B may be regarded to be in the intermediate situation. Conclusion From our analysis of the results obtained, the strong photogalvanic effect observed for the combinations between some dyes and aliphatic amino compounds is related to the chemical free energy change of the photooxidized latter compounds. Constant supply of these reducing agents is necessary to keep the photocclrrents in such systems flowing. It might be worthwhile to note that photoregenerative solar cells such as those proposed by Eisenberg and Silverman2 years ago seem to work only in cases where irreversible reducing agents are used. In a cell systems in which the solution is circulated, the loss due to back reactions arising for the case of reversible reducing agent is much more serious than simple-type photogalvanic cells. In fact the materials employed by them and others in their cells are all composed of irreversible redox systems. References a n d Notes E. Rabinowitch, J . Chem. Pbys., 8, 551, 560 (1940). M. Eisenberg and H. P. Silverman, Electrocbim. Acta, 5, 1 (1961). W. D. K. Clark and J. A. Eckert, Sol. Energy, 17, 147 (1975). K. Shigehara and E. Tsuchida, J. Phys. Cbern., 81, 1883 (1977). P. D. Wildes and N. N. Lichtin, J . Pbys. Chem., 82, 981 (1978). P. D. WiMes, N. N. Lichtin, M. 2. Hoffman, L. Andrews, and kl. Linschitz, Photochem. Photobiol., 25, 21 (1977). T. Sakata, Y. Suda, J. Tanaka, and H. Tsubomura, J . Phys. Chem., 81, 537 (1977). D. E. Hall, W. D. K. Clark, J. A. Eckert, N. N. Lichtin, and P. D. Wildss, Bull. Am. Ceram. Sac., 56, 408 (1977). D. E. Hall, J. A. Eckert, N. N. Lichtin, and P. D. Wikles, J. Eh?ctrmbem. Soc., 123, 1705 (1976). D. E. Hall, J . Electrochem. Soc., 124, 804 (1977). P. D. Wildes, D. R. Hobart, and N. N. Lichtin, Sol. Energy, 19, 567 (1977). K. Shigehara, M. Nishimuaa, and E. Tsuchida, Bull. Chem. SOC.Jpn., 50, 3397 (1977). K. Shigehara, M. Nishimura, and E. Tsuchida, Electrochim. Acta, 23, 855 (197%). H. T. Tien and J. M. Mountz. J. Electrochem. Soc., 125, 885 (1978). Y. Suda, Y. Shimoura, T. Sakata, and H. Tsubomura, J. mys. Chem., 82, 268 (1978). N. Kamiya and M. Okawara, J. Electrochem. Soc. Jpn., 37, 81 (1969). N. Kamiya and M. Okawara, Denki Kagaku, 36, 506 (1968). N. Kamiya and M. Okawara, Denki Kagaku, 88, 273 (1970). N. Kamiya and M. Okawara, Kogyo Kagaku Zasshi, 72, 96 (1969). M. Kaneko and A. Yamada, J . Pbys. Chem., 81, 1213 (1977). E.M. Kosower and J. L. Cotter, J. Am. Chem. Soc., 86,5524 (1964). W. M. Moore, J. T. Spence, F. A. Raymond, and S. D. Colson, J . Am. Chem. Soc., 85, 3367 (1963). W. J. Albery and M. D. Archer, J. Electrochem. Soc., 124, 688 (1977).
