Langmuir 1995,11, 1777-1783
1777
Electrochemical and Photoelectrochemical Properties of Monoaza-15-crownEther Linked Cyanine Dyes: Photosensitization of Nanocrystalline SnOz Films Chouhaid Nasr,? Surat Hotchandani,? and Prashant V. &mat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
Suresh Das,* K. George Thomas, and M. V. George*!$ Photochemistry Research Unit, Regional Research Laboratory (CSIR), Trivandrum 695 019, India Received October 27, 1994. I n Final Form: January 17, 1995@ Photophysical, electrochemical, and photosensitization properties of newly synthesized crown ether derivatives of cyanine dyes have been investigated. These dyes exhibit strong absorption in the visible (450-550 nm) range with a solvent-dependentfluorescencequantum yield ranging from 0.006 to 0.11 and adsorb strongly on SnOz nanocrystallites. The singlet excited state is readily quenched when adsorbed on SnOz nanocrystallites as a result of a charge injection process. The photosensitization properties of these cyanine dyes in extending the photoresponse of SnOz semiconductor have been investigated by probing the dependence of photocurrent and photovoltage on the wavelength and intensity of excitation. A maximum photon to photocurrent conversion efficiency of -1% has been observed.
Introduction The synthesis and study of the photophysical properties of fluorophores covalently linked to ionophoric units such as crown ethers are active areas of research, because of their potential applications in the selective and quantitative estimation of trace quantities of metal ions.l-12 Several reports exist on the photoinduced changes in the metal ion binding capacity of crown ether linked chromoionophores; however, most of these systems involve structural changes in the chromophore unit brought about by photoisomerization. A photoinduced intramolecular charge transfer process in a crown ether linked merocyanine dye has recently been reported by Martin et ale3 In this study they also observed ejection of metal ions complexed to the crown ether moiety. These results are potentially interesting, since a wide range of molecules possessing intramolecular charge transfer transitions can t Permanent address: Centre de Recherche e n Photobiophysique, Universit6 d u Qu6bec 21 Trois RiviBrs, Trois RiviBrs, Quebbc, G9A 5H7,Canada. Also at Notre Dame Radiation Laboratory Notre Dame, I N
46556. Abstract published in Advance A C S Abstracts, April 1, 1995. (1)Lehn, J.-M. Angew. Chem. Int. Ed. Engl. 1990,29,1304. (2)Sutherland, I. 0.in Crown Compounds Towards Future Application; Cooper, S . R. Ed.; VCH: New York, 1992,pp 235-260. (3)Martin, M. M.; Plaza, P.; Hung, N. D.; Meyer, Y. H.; Bourson, J.; Valeur, B. Chem. Phys. Lett. 1993,202,425. (4)Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993,65,1705. (5)Letard, J. F.; Lapouyade, R.; Rettig, W. Pure Appl. Chem. 1993, 65,1705. (6)de Silva, A. P.; Gunaratne, H. Q. N.; McRoy, C. P. Nature 1993, 364,42. (7)Cazaux, L.; Faher, M.; Lopez, A.; Picard, A.; Times, P. J. Photochem. Photobiol. A: Chem. 1994,1771,217. ( 8 ) Das, S.;Thomas, K. G.; Thomas, K. J.; Kamat, P. V.; George, M. V. J. Phys. Chem. 1994,98,9291. (9)Thomas, K. J.;Thomas, K. G.; Manojkumar, T. K.; Das, S.;George, M. V. Proc. Indian Acad. Sci. (Chem. Sci.) 1994 (in press). (10)Barzykin, A. V.;Fox, M. A,; Ushakov, E. N.; Stanislavsky, 0.B.; Gromov, S. P.; Fedorova, 0. A.; Alfimov, J . V. J. Am. Chem. SOC.1992, 114,6381. (11)Jonker, S. A,; Van Dijk, S. I.; Goubitz, K.;Reiss, C, A.; Schuddeboom, W.; Verhoeven, J. W. Mol. Cryst. Liq. Cryst. 1990,183, 273. (12)Bourson, J.;Valeur, B. J . Phys. Chem. 1989,93,3871. @
be utilized for the design of molecules for photocontrolled release of metal ions. Organic dyes have been extensively used in sensitizing large bandgap semiconductors (see, for example, refs 1316). Such systems have important applications in imaging science and solar energy conversion. Previous studies have indicated the ability of cyanine dye modified semiconductor electrodes to generate photocurrent under visible light excitation (see, for example, refs 17-22). With (13)(a) Memming, R. Ber. Bunsenges. Phys. Chem. 1987,91,353. (b) Memming, R. Chem. Soc., Faraday Disc. 1974,85,261.(c) Memming, R. Prog. Surf Sci. 1984,17,7.(d) Memming, R. In Topics in Surface Chemistry, Kay, E., Bagus, P. S., Ed.; Plenum Press: New York, 1978; p 1. (e) Memming, R. In Electroanalytical Chemistry; Bard, A., Ed.; Marcel Dekker: New York and Basel, 1979;Vol. 11,p 1. (0 Gerischer, (g)Bressel, H.; Tributsch, H. Ber. Bunsenges, Phys. Chem. 1968,72,437. B.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1983, 87, 963. (h) Willig, F. Top. Curr. Chem. 1976,61,31. Gerischer, H.; (14)(a) Arden, W.; Fromherz, P. Ber. Bunsenges. Phys. Chem. 1978, 82, 868.(b) Arden, W.; Fromherz, P. J. Electrochem. SOC.1980,127, 370.(c) Fromherz, P. ; Arden, W. Ber. Bunsenges. Phys. Chem. 1980, 84 1045.(d) Fromherz, P.; Arden, W. J. Am. Chem. SOC.1980,102, 6211. (15)(a) Kirsch-De Mesmaeker, A,; Leempoel P.; Nasielski, J . Sol. Energy 1980, 25, 117. (b) Nasielski, J.; Kirsch-De Mesmaeker A.; (c)Nasielski, J.; KirschLeempoel, P. Electrochemica Acta 1978,23,605. DeMesmaeker A.; Leempoel, P. Nouu. J. Chem. 1978,2,497.(d) KirschDe Mesmaeker A.; Kanicki, J.;Leempoel, P.; Nasielski, J. Bull. Chem. SOC.Belg. 1978,87,849. (e)Biesmans,G.;van der Auweraer, M.; Cathry, C.; De Schryver, F. C.; Yonezawa, Y.; Sato, T.Chem. Phys. 1992,60, 97.(0 Biesmans, G.; van der Auweraer, M.; Cathry, C.; Meerschaut, D.; De Schryver, F. C.; Storck W.; Willig, F. J.Phys. Chem. 1991,953771. (16)(a)Kamat, P. V. Prog. React. Kinet. 1994,19,277.(b)Kamat, P.V. Chem. Reu. 1993,93,267.(c) Das, S.;Thomas. K. G.; Kamat, P. V.; George, M. V. Proc. Indian Acad. Sci. (Chem. Sci.) 1993,105,513. (d) Hotchandani, S.;Kamat, P. V. Chem. Phys. Lett. 1992,191,320.(e) Kamat, P. V.; Chauvet, J. P. Radiat. Phys. Chem. 1991,37, 705.(0 Kamat, P. V.; Das, S.; Thomas, K. G.; George, M. V. Chem. Phys. Lett. 1991,178,75.(g) Gopidas, K.R.; Kamat, P. V. J. Phys. Chem. 1989, 93,6428.(h) Kamat, P.V. J. Phys. Chem. 1989,93,859.(i)Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986,90,1389.(i) Kamat, P. V.; Fox, M. A. Chem. Phys. Lett. 1983,102,379. (k) Kamat, P. V.; Fox, M. A. J. Electrochem. SOC.1984,131,1032. (17)(a) Spitler, M.; Parkinson, B. A. Langmuir 1986,2,549. (b) (c) Natoli, Sonntag, L. P.; Spitler, M. T. J. Phys. Chem. 1986,89,1453. L. M.; Ryan, M. A.; Spitler, M. T. J. Phys. Chem. 1986,89,1448.(d) Spitler, M. T. J. Chem. Educ. 1983,60, 330. (18)Heimer, T. A,; Bignozzi, C. A.; Meyer, G. J . J . Phys. Chem. 1993, 97,11987. (19)Lenhard, J.R.; Hein, B. R.; Mueter, A. A. J. Phys. Chem. 1993, 97,8269.
0743-746319512411-1777$09.00/0 0 1995 American Chemical Society
Nasr et al.
1778 Langmuir, Vol. 11, No. 5, 1995 Chart I
Wavelength (nm)
Figure 1. Absorption spectra of dyes in acetonitrile: (a) 1 (13 pm),(b) 2 (8pm), and (c) 3 (13 pm).
3
the recent developments in the area of nanocrystalline semiconductor films (see, for example, refs 23-30), we have now focused our attention on employing organic dyes to extend the photoresponse of large-bandgap semiconductors. Organic dye molecules such as squaraines,3O oxazines,3l and chlorophyll derivative^^^,^^ have already been employed to sensitize nanocrystalline semiconductors such as TiO2, ZnO, and SnO2. Nanocrystalline SnO2 films prepared from colloidal suspension exhibit a highly porous morphology and can easily be surface-modified with sensitizing dye m o l e ~ u l e s .We ~ ~ have therefore chosen nanocrystalline SnO2 (bandgap 3.6 eV) a s the semiconductor system in the present study to investigate the photosensitization properties of two crown ether derivatives of cyanine dyes, 1 and 2 (Chart 1). Their photoelectrochemical performance is also compared with the model compound 3.
Experimental Section Materials. Crown ether bearing cyanine dyes were synthesized, adopting procedures similar to those of aldol condensation^.^,^^ The purity of all these compounds was established on the basis of analytical and spectral data. Optically transparent electrodes (OTE) were cut from a n indium tin oxide coated glass plate (1.3 mm thick, 20 R/square) obtained from Donnelley Corp., Holland, MJ. A SnOn colloidal suspension (18%,particle size 30-50 A) was obtained from Alfa Chemicals and used without (20) (a) Tani, T.; Suzumoto, T.; Kemnitz, K.;Yoshihara, K. J. Phys. (b) Tani, T.;Suzumoto, T.; Ohzeki, K. J. Phys. Chem. 1992,96,2778. Chem. 1990,94,1298. (21) Hayashi, Y.; Ogawa, S.; Sanada, M.; Hirohashi, R. J . Imaging Sei. 1989,33,124. (22) Iwasaki, T.; Oda, S.; Kamada, H.; Honda, K. J. Phys. Chem. 1980,84, 1060. (23) Hodes, G.; Howell, I. D. J.;Peter, L. M. J. Electrochem. SOC. 1992,139,3136. (24) Sakohara, S.; Tickanen, L. D.; Anderson, M. A. J. Phys. Chem. 1992,96,11086. (25) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994,98,3183. (26) ORegan, B.; Gratzel, M. Nature 1991,353,737. (27)Hagfeldt, A.; Bjorksten, U.;Lindquist, S.-E.Sol. Energy Mater., Sol. Cells 1992,27,293. (28)Bjorksten, U.; Moser, J.;Graetzel, M. Chem. Mater. 1994,6, 858. (29) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98,4133. (30)Hotchandani, S.;Das, S.; Thomas, K. G.;George, M. V.; Kamat, P.V. Res. Chem. Intermed. 1994,20, 927. (31) Liu, D.; Kamat, P. V. J. Electrochem. SOC. 1995,142, 835. (32) Bedja, I.; Hotchandani, S.; Carpentier, R.; Fessenden, R. W.; Kamat, P. V. J. Appl. Phys. 1994,75,5444. (33) Kay, A.; Gratzel M. J. Phys. Chem. 1993,97,6272. (34)Dix, J. P.; Vogtle, F. Chem. Ber. 1980,113,457.
further purification. Colloidal silica (#2326,14.5%, 50 A, pH 9.0) was a gift sample from NALCO. All other chemicals were analytical reagents and used as supplied. Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer. Quantum yields of fluorescence were measured by the relative method using optically dilute solutions. Rhodamine 6B (CDf = 0.9) in ethanol was used as a standard. Preparation of SnOz Particulate Films. The synthetic procedure for casting transparent thin films of SnO2 on a n optically transparent electrode (OTE) was reported earlier.29 A small aliquot (usually 0.1 mL) of the diluted SnO2 colloidal suspension (2%)was applied to a conducting surface of 0.8 x 3 cm2 of OTE and was dried in air on a warm plate. The SnO2 colloid-coated glass plates were then annealed in air a t 673 K for 1 h. The thin film semiconductor electrode is referred to a s OTE/Sn02. The typical thickness of the SnO2 film was 0.5-1 ,um. These nanocrystalline films are highly porous and can adsorb cationic dyes very efficientlyfrom aqueous solutions. OTE/ SnO2 electrodes were then modified with cyanine dyes by dipping them in dye solution (acetonitrile) for a period of 8-10 h and carefully washing with acetonitrile. The orange coloration of the film confirmed adsorption of the dye on the SnO2 surface. Spectroelectrochemical and Photoelectrochemical Measurements. These measurements were carried out in a thin layer cell consisting of a 2 or 5 mm path length quartz cuvette with two side arms attached for inserting reference (Ag/AgCl or SCE) and counter (Pt gauze) electrodes. The description of the cell can be found elsewhere.35 A Princeton Applied Research (PAR) model 173 potentiostat and model 175 universal programmer and BAS model 100 electrochemical analyzer were used in electrochemical and spectroelectrochemical measurements. Photocurrent measurements were carried out with a Keithley model 617 programmable electrometer. Collimated light beam from a 250 W Xenon lamp was used to excite the electrode surface. A Bausch and Lomb highintensity grating monochromator was introduced into the path of the excitation beam for selecting the excitation wavelength.
Results and Discussion Absorption and Emission Properties. The absorption spectra of 1-3 are shown in Figure 1. The broad absorption of these dyes in the visible (400-600nm) region makes them suitable as sensitizers to extend the response of large-bandgap semiconductors such as SnOz. The absorption properties of 1 and 2 are summarized in Table 1. The red shift in the absorption band of 2 arises because (35) Bedja, I.; Hotchandani, S.; Kamat, P.V. J. Phys. Chem. 1993, 97,11064.
Photosensitization of Nanocrystalline SnOz Films
Langmuir, Vol. 11, No. 5, 1995 1779
".I
I
A
I
Q)
9 -0.1 /
t1.2
t1.0
+0.4
1.1 v
-0.2
300
400
500
0.4
E (V)
600
0.8
1.2
vs. Agi AgCl 8
700
Wavelength (nm) 0.1
1 t1.2
B I
I
I
1
ti .o
8
0.0
e ca
e g -0.1 9
E (Volt) Figure 2. Cyclic voltammograms of cyanine dyes (-1 mM in acetonitrilecontaining0.1M tetrabutylammoniumperchlorate, reference electrode, Ag/AgCl; scan rate, 100 mV/s): (a) 1, (b) 2, and (c) 3.
-0.2
300
Table 1. Photophysical Properties of 1 and 2 in Different Solvents 1
solvents
n-butanol methanol acetonitrile water
A$&,,, 490 477 474 455
~k%,nm 606 608 615 604
2 Qf
ig&,nm ~ z ! , n m Of
0.114
533
0.026
528
0.014
521 500
0.006
597 593 598 590
0.047 0.060
0.008 0,009
the benzothiazolium moiety (present in 2) is a better ~t electron acceptor (Brooker acid361 than the pyridinium moiety (present in 1). Both 1 and 2 show a hypsochromic shift with increasing solvent polarity, thus indicating a large dipole moment superposed on a completely delocalized positive charge. The fluorescence emission yield of these dyes varies from 0.006 to 0.114. The fluorescence quantum yield is quite low in polar solvents. It has been shown recently that oxazine dyes adsorbed on SnOz nanocrystallites form H-aggregates as they exhibit a blue shift in the absorption maximum.31 We also checked the possibility of aggregate formation in the present case since cyanine dyes are known to form aggregates on surface^.^' The absorption characteristics of the dyes, 1,2,or 3,adsorbed on SnOz nanocrystalline films were very similar to the solution spectra. This suggests that the dyes on SnOz nanocrystallites exist in the monomeric form. (The absorption characteristics of adsorbed dyes are compared with the photoconversion efficiencies in a later section.) Electrochemical Measurements. The dyes 1-3 were found to undergo two successive oxidations in the potential range of 600-1100 mV vs Ag/AgCl. The cyclic voltammograms of these three dyes are shown in Figure 2. Except for 2, these dyes underwent irreversible oxidation. This shows that the presence of the benzothiazolium moiety in 2 makes its oxidation reversible. The oxidation potentials determined from square wave voltammetry are summarized in Table 2. The second oxidation was quite prominent and the chemical changes (36) (a)Brooker, L. G. S.; Keys, G . H.; Heseltine, J. Am. Chem. Soc. 1951,73,5350. (b) Brooker, L. G. S.; Craig, A. C.; Heseltine, D. W.; Jenkins, P. W.; Lincoln, L. L. J. Am. Chem. SOC.1966,87,2443. (37) Horng, M.-L.; Quitevis, E. L. J . Phys. Chem. 1993,93,6198.
400
500
600
700
I IO
Wavelength (nm)
Figure 3. Changes in the absorption spectra of (A) 1 and (B) 2 in a thin layer cell during the electrochemical scan using conducting glass (OTE)as the working electrode,platinum wire as the counter electrode, and Ag/AgCl as reference electrode. The concentration of the dye was -1 mM. The electrolytewas 0.1 M tetrabutylammonium perchlorate in acetonitrile. The scan rate was 5 mV/s. The insets show the changesin the ground state absorption band of the dye.
associated with this oxidation were probed from the changes in the absorption spectra. The spectroelectrochemical measurements of the dye solution were carried out in a thin layer cell (2 mm) with OTE plate as a working electrode. The thickness of the solution within the cell was -0.5 mm. The chemical changes that occur during the oxidative scan were continuously monitored by simultaneous recording of the absorption changes of the dye solution. Some representative spectra are shown in Figure 3A,B. For compound 1, no significant change in the differenceabsorption spectrum was seen up to a n applied potential of +0.5 V (vs Ag/ AgC1). When the potential was increased to +0.8 V, an absorption band appeared at 360 nm, indicating the formation of a n oxidized form of the dye, presumably the cation radical. Further increase in the potential makes this absorption peak decrease, with simultaneous bleaching of the 470 nm band. The inset shows the absorption changes recorded at different potentials. Complete bleaching of the dye was observed a t potentials greater than 1.1 V. These changes remained permanent and couldn't be reversed by reversing the electrochemical scan. This observation is consistent with the irreversible oxidation peak observed in the cyclic voltammogram of 1 in Figure 2.
Compound 2 exhibited similar absorption changes with increasing potential. The difference absorption spectra recorded in the potential range of0.82-1.3 V(vs Ag/AgCl) are shown in Figure 3B. With increasing applied potential, an increase in the absorption a t -380 nm and a simultaneous bleaching in the 525 nm band was observed. The presence of a n isosbestic point suggested the conversion of 2 to its cation radical. Unlike the case of 1,the oxidation of 2 can be reversed; the original dye spectrum could be
1780 Langmuir, Vol. 11, No. 5, 1995
Nasr et al.
Table 2. Electrochemical and Photoelectrochemical Properties of Dye-Modified SnOz Thin Films oxidation potentials
photoelectrochemical parameters
dye
lEoox,"mV
2Eoox,a mV
1 2
720 750 -720
890 1050 820
3
nm
Voc,mV
i,e,pNcm2
ff'
120 90 135
5.67 1.4 1.33
0.24 0.27 0.3
460 520 470
71:
IPCE, %
%
1.15 0.35 0.25
0.012 0.0035 0.0039
and EZoxwere determined from square wave voltammetry. Excitation wavelength with Iincin the range of 0.95-1.38 mW/cm2. Fill factor, ff = P m a / ( V o c i d where P,, is the maximum power output of the cell. Net power conversion efficiency, 7 (%) = (Pma/IinC) a Elox
x 100.
E
.-0 .-g
0.2
Time ( 5 )
E
W
0.0'
'
500
.
'
550
"
600
"
650
"
700
Wavelength (nm)
Figure 4. Emission spectra of 1 adsorbed on nanocrystalline thin films of (a) SiOz, and (b) SnOz. The absorbance of both 0.2) at the excitation these samples was matched (A wavelength of 460 nm. The spectra were recorded in a front face geometry.
-
restored by reversing the scan. The absorption changes a t 520 nm during the forward and reverse scans are shown in the inset of Figure 3B. These results show that the dye gets completely bleached as it is oxidized to its cation radical and reversal of the scan results in the recovery of the 520 nm absorption band. Only a small fraction of the dye failed to recover. It is to be noted that these experiments were performed under slow scan (5 mV/s) and it is likely that a small fraction ofthe dye is undergoing some permanent changes a t high anodic potentials. The hysterisis observed in the inset of Figure 3B is attributed to the limitations arising from mass transfer and/or kinetics of heterogeneous electron transfer. Thus, spectroelectrochemical experiments described here provide a convenient way to characterize the cation radicals of 1 and 2. Excited State Quenching by SnOz Nanocrystallites. The dye molecule, 1, adsorbs strongly on nanocrystalline oxide films prepared from colloidalsuspension. The Sn02 and Si02 films coated on glass plates were immersed separately in a n acetone solution of 1. In both these samples the absorbance a t 470 nm of 1 was matched to about 0.2 by controlling the time of adsorption. The emission spectra of these dye-modified films were then recorded in a front face excitation geometry. The emission spectra of 1 adsorbed on Si02 and SnOz are shown in Figure 4. Both these samples exhibit emission maximum around 610 nm. Although the absorbance of 1 coated on SnOz and Si02 a t the excitation wavelength is similar, the emission yield of 1 on SnOz is significantly lower (-10% that of Si02 sample). This is indicative of the fact that a n efficient quenching of the excited state is occurring on the SnOz surface. As seen earlier with other sensitizing dyed6such excited state quenching is associated with the charge transfer to the semiconductor. The formation and decay of excited state processes on these metal oxide (MO) surfaces are summarized in reactions (1)-(3).
1
+ hv
[1]*
-
-
1
[11*
(1)
+ hvf
(2)
bu
Iu I \I 40
OO
80
120
Time (s)
Figure 5. Generationof (A)photocurrent and (B)photovoltage at an OTE/Sn02 electrode modified with 1. Excitation wavelength, 460 nm; electrolyte, 0.5 M LiI and 0.04 M 12 in acetonitrile.
[11*
+ MO
-
[ll"
+ MO(e)
(3)
where MO(e) represents electrons trapped within the metal oxide particle. While reactions (1)and (2)dominate on the Si02 surface, reactions (1)and (3) are the major processes that occur on the S n 0 2 surface. Photoelectrochemical Effect with Dye-Modified Nanocrystalline Films. If indeed the quenching observed in Figure 4 was due to a charge injection process, it should be possible to utilize the injected charge in generating electricity. A photoelectrochemical cell was constructed utilizing dye-modified Sn02 film coated on conducting glass plate (referred to as OTE/SnOz/dye) as a photoelectrode. The geometry of the cell was similar to the one employed in spectroelectrochemical measurements. Illumination of this electrode in a photoelectrochemical cell with visible light resulted in the generation of anodic photocurrent. Upon excitation with visible light the excited sensitizer molecules inject electrons into the SnOz particles (reaction 3). These electrons are then collected a t the OTE surface to generate an anodic photocurrent. The redox couple 13-A- present in the electrolyte quickly regenerates the sensitizer (reaction 4).
2[11*
+ 31- - 2[lf1 + I,-
(4)
The relevance of similar redox systems in sensitizer regeneration as well as photocurrent stability has been addressed in earlier studies.38 The photoelectrochemical response of this electrode was recorded with several on-off cycles of illumination. Some representative traces are shown in Figure 5. The generation of both open-circuit photovoltage and short-circuit photocurrent was prompt and steady during the course of illumination. Photoelectrochemical properties of various dye-modified SnOz films are summarized in Table 2.
Photosensitization of Nanocrystalline SnOz Films
Langmuir, Vol. 11, No. 5, 1995 1781
photosensitization. (The absence of additional bands in both the absorption and IPCE spectra ruled out existence of dye aggregates on the SnOz film.) The photocurrent response in the UV-region arises from the direct excitation of SnOz particles. This is evident from spectrum c in Figure 6, which was recorded with an OTE/SnO2 electrode without any surface modification. However, the observed response in the visible is entirely due to the surface modification with cyanine dyes. Of the three dye systems investigated, the maximum IPCE obtained was about 1%for SnOz film modified with 1 (Table 2). Since the maximum absorbance of these cyanine dye Wavelength (nm) modified semiconductor film is around 0.25, only a fraction Figure 6. (a)Action spectrum and (b) absorption spectrum of of the light is absorbed. If we account for the unabsorbed OTE/Sn02 electrode modified with 1. The spectrum c shows light, the photon-to-photocurrent efficiencyis expected to photocurrent response of a bare OTE/Sn02 electrode recorded be higher (-factor of 2) than 1%.These IPCE values are before dye modification (electrolyte,0.5 M LiI and 0.04 M 12 in acetonitrile).The photocurrentsmeasured at differentexcitation comparable to the ones reported for other sensitization wavelengths were converted t o IPCE values using eq 5. experiments with organic dyes. However, the IPCE values for all three dyes are 0.4 I 1 0.25 significantly smaller than the values obtained for the Ti02 J and SnOz particulate films with ruthenium complex 0.3 sensitizer^.^^^^^ One reason is the low quantum yield (QS 0) 0 I0.1, Table 1)of the singlet excited state of cyanine dyes E 5 0.2 (1-3). Therefore a major fraction of the excitation energy - 0.20 m is lost in the nonradiative deactivation of the excited dye. u) Afast back-electron transfer between the injected electron 2 0.1 and oxidized sensitizer would also result in lower IPCE values. By designing dye molecules with higher excited singlet yield and choosing suitable redox couple to compete 0.0' ' " " 0.15 450 500 550 600 with back-electron transfer, it should be possible to Wavelength (nm) improve the IPCE of these dye-modified semiconductor films. Figure 7. (a)Action spectrum and (b) absorption spectrum of OTE/Sn02electrodemodified with 2 (electrolyte,0.5 M LiI and The power characteristics ofthis cell were evaluated by 0.04 M 12 in acetonitrile). monitoring the photovoltage and photocurrents a t various load resistances. The fill factor for all the three dye 0.3 7 0.25 1 systems was similar (0.24-0.3). The net power conversion efficiency was also low for the photoelectrochemical cell 0.20 p) employing dye-modified SnOz nanocrystalline electrodes 0 (Table 2). The greatest power conversion efficiency, r , of E a 0.012% was observed for the dye system 1. Further 0.15 $ optimizations of the operating conditions are necessary fn n to improve the performance of photoelectrochemical cell 0.10 4 employing such organic dyes. Dependence of Photovoltage and Photocurrent on the Incident Light Intensity. In order to further 0.00.05 400 500 600 assess the photoelectrochemical behavior of dye-modified Wavelength (nm) SnOz films, the short-circuit photocurrent (is,) and opencircuit photovoltage (V0J were measured a t different Figure 8. (a)Action spectrum and (b) absorption spectrum of OTE/Sn02 electrode modified with 3 (electrolyte,0.5 M LiI and incident light intensities (Iinc). The dependence of is, and 0.04 M 12 in acetonitrile). V,, on the intensity of incident light is shown in Figures 9 and 10, respectively. In a photoelectrochemical cell the The photocurrent action spectra of dye-modified SnOz observed short-circuit photocurrent can be related to Iin, electrodes are shown in Figures 6-8. The values of by expression 6. photon-to-photocurrent conversion efficiency (IPCE)were evaluated from the short circuit photocurrent measure(6) ments a t different excitation wavelengths and by using expression 5, where y is the number of photons required to generate a n electron in the external circuit. The linearity of the IPCE(%) = (1240/A)(isJIinc)x 100 (5) logarithmic plot of is, vs 4,, with a slope of 0.9 (Figure 9) confirms the validity of expression 6. Moreover, the slope where is, is the short-circuit photocurrent (A/cm2),Ii,, is of this plot which is close to unity indicates that the the incident light intensity (W/cm2),and2 is the excitation primary charge injection process responsible for the wavelength (nm). The dye-modified electrodes show photocurrent generation is monophotonic. excellent photoresponse in the visible region. The close The dependence of V,, on the intensity of incident light match between the IPCE spectra and the absorption shows a sharp increase initially but attains saturation at spectra of the dyes in Figures 6-8 shows that the higher light intensities. The maximum attainable phophotosensitization mechanism (reaction 3) is operative in extending the photocurrent response of the nanocrystalline (38)J. Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; film into the visible. These spectra also confirm that only Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J.Am. Chem. SOC. 1993,115, 6382. the monomeric form of the dye is responsible for the h
.
g
H
'
I
'
1782 Langmuir, Vol. 11, No. 5, 1995
Nasr et al.
Scheme 1. Mechanism of SensitizedPhotocurrent Generation in a Nanocrystalline Semiconductor Filma
."I
10
-8.0
4.0
-6.0
I" 'lnc
0
2
6
4
8
1
0
1
2
I jnc (mW/cm2)
Figure 9. Dependence of short-circuit photocurrent on the incidentlight intensity. The OTE/Sn02electrodemodified with 1 was excited at 460 nm in a thin layer cell containing 0.5 M LiI and 0.04 M I2 in acetonitrile. The inset shows the linear dependence of In is, versus In Ijnc with a slope of 0.9. ---I
I
9 E
Y
I,,
(mW/cm*)
Figure 10. Dependence of open-circuit photovoltage on the incidentlight intensity. The OTE/Sn02 electrodemodified with 1 was excited at 460 nm in a thin layer cell containing 0.5 M LiI and 0.04 M 12 in acetonitrile. The inset shows the plot of In is, versus V,. tovoltage in such a cell is determined by the difference between the conduction band of the semiconductor and the redox potential of the Is-/I- couple. Under the present experimental conditions we obtain a maximum opencircuit voltage of 180 mV. For a photoelectrochemicalcell operating on a Schottkybarrier principle, V, can be related to is, by expression (7V9
(7) where k, T,and q are, respectively, the Boltzmann constant, absolute temperature, and electric charge, and n and io are the diode quality factor and the reverse saturation current, respectively. The linear dependence of V, on In i , (and hence also on lint) shown in the inset of Figure 10 suggests the validity of expression 7 in evaluating the photoelectrochemicalperformance of a dyemodified SnO2 film. The diode quality factor, n, and reverse saturation current, io, determined from the In i, vs V, plot were 2.65 and 0.7pA/cm2,respectively. Similar behavior of the photovoltage dependence has also been observed for organic and inorganic semiconductor thin films (see, for example, refs 39-42). (39)Memming, R. Top. Cum. Chem. 1988,143,81. (40) Fan, F.-R.; Faulkner, L. J. J. Chem. Phys. 1978,69,3341. (41) Segui, S.;Hotchandani, S.;Baddou, D.; Leblanc, R. M. J. Phys. Chem. 1991,95,8807. (42) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992,96,6834.
a The schematicdiagram shows the varyingdegree of electron accumulationwithin the particlesfollowingthe charge injection Such an inhomogeneous electron from an excited sensitizer(S*). distribution alters the energetics of the quasi-Fermilevel (E'f) resulting in the formation of a potential gradient along the width of the film. .
Mechanism of Charge Transport Within the Nanocrystalline Semiconductor Film. Although space charge layer formation at the semiconductor/electrolyte interface controls the charge separation in single crystal semiconductor systems, such a configuration is not fepible in semiconductor nanocrystallites of diameter -50 A. The diameter of individual nanocrystallites is considered to be too small to permit formation of a depletion layer. It has been proposed that interfacial charge transfer kinetics controls the photoinduced charge separation at a nanocrystalline semiconductor thin film.23 Thus, immediately following the charge injection from the excited dye into the semiconductor nanocrystallites it is essential to reduce dye cation radicals with a redox couple (reaction 4). Quick regeneration of the dye molecule suppresses the backelectron transfer and thus leads to the accumulation of electrons within the semiconductor nanocrystallites. (The concentration of the 13-4- couple near the semiconductor surface is sufficiently high to compete with the backelectron transfer.) The degree of electron accumulation within the semiconductor particles alters the energetics of the quasi-Fermi level and creates a potential gradient within the thin film (Scheme 1). Formation of such a potential gradient provides the necessary driving force for the electron transport to the collecting surface of OTE. Although it is difficult to establish the exact nature of this overall potential gradient, the validity of expression 7 in the present experiments indicates it to be qualitatively similar to that of the Schottky barrier observed in a single crystal semiconductor system. Since this potential gradient is not a n ideal type of Schottky barrier, significant loss of electrons is encountered during the transit because of recombination at the grain boundaries. This is also evident from the high value of reverse saturation current (0.7,uA/cm2),which suggests the poor rectification property of the nanocrystalline film in transporting electrons to the conducting OTE surface. As shown earlier with Ti021 CdSe s y ~ t e m s , 4it~is*possible ~ to rectify the flow of charge carriers by coupling two different semiconductor systems of different conduction and valence band energy levels. Currently efforts are underway to use this approach for improving the efficiency of photosensitization. Conclusions Crown ether derivatives of cyanine dyes are capable of sensitizing nanocrystalline SnO2 films. Excited state quenching and anodic photocurrent generation confirm (43) Liu, D.; Kamat, P. V. J. Electroanal. Chem. 1993,347, 451. (44) Liu, D.; Kamat, P. V. J. Phys. Chem. 1993,97, 10769.
Langmuir, Vol. 11, No. 5, 1995 1783
Photosensitization of Nanocrystalline Sn02 Films the charge injection from excited dye molecules into the semiconductor nanocrystallites. The lower yield of excited dye singlet and poor rectification properties of the nanocrystalline semiconductor film seem to limit the photonto-photocurrent efficiency to about 1%. It has been proposed that a potential gradient similar to that of a Schottky barrier acts as a driving force for transporting photoinjected electrons to the collecting surface.
Acknowledgment. We would like to thank our colleagues K. J . Thomas, Abraham Joy, T. K Manojkumar,
and Idriss Bedja for their help in synthesizing dyes and SnOz films. The work described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy (P.V.K. and M.V.G.), National Sciences and Engineering Research Council of Canada (C.N. and S.H.), and the Council of Scientific and Industrial Research, Government of India (S.D., K.G.T., and M.V.G). This is contribution No. 3770 from the Notre Dame Radiation Laboratory and RRLT-PRU-57 from RRL, Trivandrum. LA940849R
