Photosensitization of Nanocrystalline ZnO Films by Bis(2,2'-bipyridine

An incident photon to current conversion efficiency (IPCE) of 14% has been obtained for unbiased .... The Journal of Physical Chemistry C 0 (proofing)...
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Langmuir 1997, 13, 2398-2403

Photosensitization of Nanocrystalline ZnO Films by Bis(2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)ruthenium(II) Idriss Bedja,† Prashant V. Kamat,‡ Xiao Hua,§ A. G. Lappin,§ and Surat Hotchandani*,† Groupe de Recherche en E Ä nergie et Information Biomole´ culaire, Universite´ du Que´ bec a` Trois-Rivie` res, C.P. 500, Trois-Rivie` res, Que´ bec, Canada G9A 5H7, Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received November 18, 1996. In Final Form: February 11, 1997X Spectral sensitization of nanocrystalline ZnO films has been carried out with ruthenium complex, bis(2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium(II) or (Ru(II)). An incident photon to current conversion efficiency (IPCE) of 14% has been obtained for unbiased ZnO/Ru(II) photoelectrochemical cells. This low IPCE has been attributed to the poor light harvesting efficiency (LHE) and charge collection (ηc) efficiency. The weak interaction between Ru(II) and ZnO surface which results in poor uptake of Ru(II) on ZnO is responsible for a lower LHE while various charge recombination processes are the cause of poor ηc. To better understand the process of photosensitization of ZnO by Ru(II), the dependence of IPCE and fluorescence on applied bias, and other photoelectrochemical measurements have also been carried out.

Introduction Spectral sensitization, also referred to as dye sensitization or simply photsensitization, of semiconductors by adsorbed dyes is of a great importance in photography,1-3 in electrophotography,4 and in the development of efficient organic solar cells.5-11 In this, the charges are generated in semiconductors via visible excitation of the dye molecules adsorbed on them. It is thus a process which makes the semiconductors sensitive to the light of wavelengths longer than their intrinsic absorption and permits the direct conversion of solar energy into electricity. It is now well accepted that the dominant mechanism of the dye sensitization is the charge injection from the excited dye into the semiconductor.12-16 Thus, for an n-type semiconductor, the dye sensitization is the result of electron transfer from excited dye to the conduction band of the semiconductor. The semiconductors that have * To whom correspondence should be addressed. † Groupe de Recherche en E Ä nergie et Information Biomole´culaire, Universite´ du Que´bec a` Trois-Rivie`res. ‡ Radiation Laboratory, University of Notre Dame. § Department of Chemistry, University of Notre Dame. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) West, W.; Gilman, P. B. Jr. The theory of the photographic processes, 4th ed.; James, T. H., Ed.; Macmillan: New York, 1977; Chapter 10. (2) James, T. H. Photogr. Sci. Eng. 1974, 18, 100. (3) (a) Tani, T. Photogr. Sci. Eng. 1982, 26, 2213. (b) Tani, T. J. Imaging Sci. 1987, 31, 263. (c) Tani, T. J. Imaging Sci. 1990, 34, 143. (4) Inoue, E. Current problems in electrophotography; Borg, W. E., Hauffe, K., Eds.; De Gruyter: Berlin, 1972; p 146. (5) Clark, W. D. K.; Sutin, N. J. Am. Chem. Soc. 1977, 99, 4676. (6) Jaeger, C. D.; Fan, F. R. F.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 2592. (7) Usui, Y.; Misawa, H.; Sakuragi, H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1987, 60, 1573. (8) Mallouk, M. A.;Webber, S. E.; White, J. M. J. Phys. Chem. 1988, 92, 1872. (9) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (10) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (11) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98, 4133. (12) Neslon, R. C. J. Mol. Spectrosc. 1967, 23, 213. (13) Meunter, A. A. J. Phys. Chem. 1976, 80, 2178. (14) Tani, T. J. Appl. Phys. 1987, 62, 2456. (15) Sakata, T.; Hashimoto, K.; Hiramoto, M. J. Phys. Chem. 1990, 94, 3040. (16) Gerisher, H. Photochem. Photobiol. 1972, 16, 243.

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been widely employed are the large bandgap ones, for example, TiO2 (Eg ) 3.2 eV), ZnO (Eg ) 3.0 eV), SnO2 (Eg ) 3.8 eV), SrTiO3 (Eg ) 3.2 eV), etc. This is primarily because they are quite stable and because they predominantly absorb in the UV. As a result, there is a negligible overlap between the absorption of semiconductor and the luminescence of dye (in the visible), thus minimizing the chances of energy transfer from excited dye to the semiconductor. A lot of work on the dye sensitization of TiO2,17-20 ZnO,21-25 and SnO2,26-29 with various dyes, for example, rhodamine, rose bengal, methylene blue, cresyl violet, ruthenium complexes, etc., has been reported in the past from various labs around the world. In these studies, the semiconductors employed had been largely in the single or polycrystalline form. With the advent of nanocrystalline film electrodes obtained from nanometer-sized colloidal semiconductor particles, there has been a great deal of interest in spectral sensitization of these nanocrystalline films.10,11,30-35 These films are easily prepared and are readily surface modified with dyes. Further, they are (17) Gulino, D. A.; Drickman, H. G. J. Phys. Chem. 1984, 88, 1173. (18) Spitler, M.; Calvin, M. J. Chem. Phys. 1977, 66, 4294. (19) Hadda, H.; Yonezawa, Y.; Inaba, H. Ber. Bunsenges. Phys. Chem. 1981, 85, 425. (20) Matsumura, M.; Mitsuda, K.; Yoshizawa, N.; Tsubomura, H. Bull. Chem. Soc. Jpn. 1981, 54, 692. (21) Spitler, M.; Lu¨bke, M.; Gerisher, H. Ber. Bunsenges. Phys. Chem. 1979, 83, 663. (22) Yamase, T.; Gerisher, H.; Lu¨bke, M. Ber. Bunsenges. Phys. Chem. 1979, 83, 658. (23) (a) Spitler, M. J. Phys. Chem. 1986, 66, 90, 2156. (b) Spitler, M.; Calvin, M. J. Chem. Phys. 1977, 67, 5193. (24) Beneking, C.; Heiland, G. J. Lumin. 1989, 43, 9. (25) Heiland, G.; Bauer, W.; Neuhaus, M. Photochem. Photobiol. 1972, 16, 315. (26) Haraguchi, A.; Yonezawa, Y.; Hanawa, R. Photochem. Photobiol. 1990, 52, 307. (27) Shimura, M.; Shakushiro, K.; Shimura, Y. J. Appl. Electrochem. 1986, 16, 683. (28) Sato, H.; Kawasaki, M.; Kasatani, K.; Higuchi, Y.; Azuma, T.; Nishiyama, Y. J. Phys. Chem. 1988, 92, 754. (29) Krishnan, M.; Zhang, X.; Bard, A. J. J. Am. Chem. Soc. 1984, 106, 7371. (30) (a) O’Regan, B. ; Gra¨tzel, M. Nature 1991, 353, 737. (b) Redmond, G.; Fitzmaurice, D.; Gra¨tzel, M. Chem. Mater. 1994, 6, 686. (c) O’Regan, B.; Moser, J.; Anderson; M.; Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720. (d) Kay, A.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. 1994, 98, 952. (31) (a) Meyer, G. J.; Searson, P. C. Interface 1993, 23. (b) Heimer, T. A.; Bignozzi, C. A.; Meyer, G. J. J. Phys. Chem. 1993, 97, 11987.

© 1997 American Chemical Society

Photosensitization of Nanocrystalline ZnO Films

highly porous and possess large surface to volume ratios. As a result, the light harvesting ability of the dye, adsorbed on nanocrystalline films, is tremendously increased leading to an improved efficiency of the solar cell. A power conversion efficiency of as high as 11-15% in diffuse daylight has been reported for a solar cell based on a ruthenium complex adsorbed on a nanocrystalline TiO2 electrode.10 We, on our part, have also been involved in the fabrication, photoelectrochemical characterization, and spectral sensitization of various nanocrystalline films,11,36 e.g., SnO2, WO3, ZnO, etc. Further, to improve the efficiency of nanocrystalline photoelectrochemical cells, the coupled semiconductor systems, such as, ZnO-CdS, SnO2-CdSe, and TiO2-CdSe were also fabricated.36c,d,f,g An improved charge separation in coupled systems, for example, ZnOCdS was definitely observed relative to ZnO alone. The spectral sensitization of ZnO-CdS with chlorophyll a (Chl a) was also found to be better than that for ZnO alone.36c In our recent studies, we reported the spectral sensitization of nanocrystalline SnO2 film by a ruthenium complex, bis(2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)ruthenium(II), i.e., [Ru(bpy)2(dcbpy)2+],11 and Chl b.36a The IPCEs (incident photon to current conversion efficiencies) of ∼10 and 27% were, respectively, obtained for Chl b and Ru(bpy)2(dcbpy)2+. Beside these studies, experiments pertaining to kinetic and mechanistic aspects of photosensitization were also performed. In our continued interest in the spectral sensitization of nanocrystalline film electrodes, we have coated Ru(bpy)2(dcbpy)2+ on nanocrystalline ZnO and report various photoelectrochemical measurements with regard to the generation of sensitized photocurrent in the ZnO/Ru(II) system. A further objective behind such a study was that while much work has appeared on the sensitization of single-crystalline and polycrystalline ZnO, very little work has been reported on the sensitization of nanocrystalline ZnO films.30b,36c Experimental Section Materials. Optically transparent electrodes (OTE) were cut from an indium tin oxide coated glass plate (1.3 mm thick, 20 Ω per square) obtained from Donelley Corp., Holland, MI. Zinc acetate (Aldrich) and LiOH (Fluka) were of highest available purity and were used without further purification. Ru(bpy)2(dcbpy)2+ was prepared according to the method reported in the literature.37 All other chemicals were analytical reagents of highest available purity. Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer. Preparation of ZnO Particulate Films. The concentration of ZnO colloids was 0.05 M and was prepared by the method described by Spanhel and Anderson.38 The diameter of these colloidal particles was in the range of 20-50 Å. A small aliquot (32) Dabestani, R.; Bard, A. J.; Campon, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1987, 92, 1872. (33) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (34) Lindstrom, H.; Rensmo, H.; Sodergren, S.; Solbret, A.; Lindquist, S.-E. J. Phys. Chem. 1996, 100, 3084. (35) O’Regan, B.; Schwartz, D. T. Chem. Mater. 1995, 7, 1349. (36) (a) Bedja, I.; Hotchandani, S.; Carpentier, R.; Fessenden, R. W.; Kamat, P. V. J. Appl. Phys. 1994, 75, 5444. (b) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 11064. (c) Hotchandani, S.; Kamat, P. V. Chem. Phys. Lett. 1992, 191, 320. (d) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (e) Bedja, I.; Hotchandani, S.; Fessenden, R. W.; Kamat, P. V. Langmuir 1994, 10, 17. (f) Nasr, C.; Hotchandani, S.; Kamat, P. V. J. Electroanal. Chem. in press. (g) Liu, D.; Kamat, P. V. J. Phys. Chem. 1993, 97, 10769. (h) Vinodgopal, K.; Hua, X.; Dahlgreen, R. L.; Lapin, A. G.; Patterson, L. K.; Kamat, P. V. J. Phys. Chem. 1995, 99, 10883.; (i) Kamat, P. V.; Bedja, I.; Hotchandani, S.; Patterson, L. K. J. Phys. Chem. 1996, 100, 4900. (j) Fessenden, R. W.; Kamat, P. V.; J. Phys. Chem. 1995, 99, 12902. (37) (a) Elliott, M. C.; Hershanhart, E. J. J. Am. Chem. Soc. 1982, 104, 7519. (b) Giordano, P. J.; Bock, C. R.; Wrighton, M. S.; Interrante, L. V.; Williams, R. F. X. J. Am. Chem. Soc. 1977, 99, 3187. (38) Spanhel, L.; Anderson, M. J. J. Am. Chem. Soc. 1991, 113, 2826.

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Figure 1. Photocurrent action (‚‚‚) and absorption (;) spectra of an OTE/ZnO/Ru(II) electrode. IPCE at various wavelengths was determined using eq 1; electrolyte was 0.05 M I2 and 0.5 M LiI in acetonitrile. For absorption spectrum, OTE/ZnO electrode was used as a reference. (0.1-0.8 mL) of ZnO solution was applied to the conducting surface 0.8 × 4 cm2 of OTE. The films (0.5-3 µm) were dried in air and then sintered at 673 K for 1 h. The sintered semiconductor films adhered strongly to the glass surface and were stable in neutral and alkaline solutions. This electrode will be referred to as OTE/ZnO or simply ZnO electrode. Modification of ZnO Films with Ru(bpy)2(dcbpy)2+. The method for the modification of OTE/ZnO electrode with Ru(bpy)2(dcbpy)2+ was similar to the one employed for the modification of TiO2 and SnO2 particulate films with the derivatives of a ruthenium complex.10,11 Warm OTE/ZnO plates (∼353 K) were dipped in 10-3 M ethanolic solution of Ru(II) complex for a period of 8-10 h. The electrodes were then washed with acetonitrile and used in photoelectrochemical measurements. The yelloworange coloration of the film confirmed the adsorption of the dye. These ruthenium-modified ZnO electrodes will be referred to as OTE/ZnO/Ru(II) or ZnO/Ru(II) in the following discussion. Spectroelectrochemical and Photoelectrochemical Measurements. The 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) and counter (Pt gauze) electrodes. The description of the cell can be found elsewhere.36b The design of the cell is such that we can insert it into the sample compartment of the absorption or emission spectrophotometer (Perkin-Elmer and SLM 8000, respectively) and carry out the measurements under the influence of an applied bias. For further details regarding these measurements see ref 11. Time-Resolved Fluorescence and Microwave Absorption Measurements. Luminescence measurements were made by exciting Ru(bpy)2(dcbpy)2+ adsorbed on ZnO nanocrystalline film with 532 nm pulses of light from a Quanta-Ray DCR1 YAG laser system.39 The laser output was suitably attenuated to less than ∼2 mJ/pulse and defocused to minimize multiphoton processes. Microwave absorption measurements were made using an apparatus described previously40 with modifications to improve time response. Excitation of the sample at 532 nm was achieved by the same YAG laser.

Results and Discussion In order to examine the feasibility of sensitization of ZnO nanocrystalline film with Ru(bpy)2(dcbpy)2+, hereafter referred to as Ru(II), various photoelectrochemical parameters have been measured and are described below. 1. Photocurrent Action Spectrum. The photocurrent action spectrum of Ru(II) adsorbed on ZnO is shown in Figure 1. The incident photon to current conversion efficiency (IPCE), defined as the number of electrons collected per incident photon, was evaluated from the short (39) Federici, J.; Helman, W. P.; Hug, G. L.; Kane, C.; Patterson, L. K. Comput. Chem. 1985, 9, 171. (40) Fessenden, R. W.; Carton, P. M.; Shimamori, H. Scaiano, J. C. J. Phys. Chem. 1982, 86, 3803.

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Figure 2. Band energy diagram of ZnO and the redox potentials (E°) of ground and excited states of Ru(II). kr, knr, kinj, and kb represent the rate constants for radiative, nonradiative, charge injection, and back electron transfer processes, respectively.

circuit photocurrent measurements at different wavelengths and using the following expression11

IPCE(%) )

Isc(A/cm2) × 1240 λ(nm) Iinc(W/cm2)

× 100

(1)

where Isc is the short-circuit photocurrent and Iinc is the light power incident on the OTE/ZnO/Ru(II) electrode. The resemblance between absorption and action spectra (Figure 1) suggests that the photosensitization mechanism is operative in the generation of photocurrent. Upon excitation with visible light, the excited ruthenium complex, Ru(II)*, injects electrons into ZnO particles which are then collected at the back OTE contact to generate anodic photocurrent. The oxidized sensitizer, Ru(III), can either capture back the injected electrons resulting in wasteful back electron transfer or be reduced by the electron donor present in the electrolyte, for example, the iodide, to regenerate the original sensitizer for reuse. The whole sequence for the generation of sensitized photocurrent in ZnO by Ru(II) can be described as follows, and is depicted in Figure 2 kr

Ru(II) + hν

Ru(II)*

Ru(II) + hνf (2)

knr

Ru(II) Ru(II)* + ZnO Ru(III) + ZnO(e) 2Ru(III) + 3I–

kinj

Ru(III) + ZnO(e) kb

Ru(II) + ZnO 2Ru(II) + I3–

(3) (4) (5)

where kr, knr, kinj, and kb are, respectively, the rate constants for radiative, nonradiative, electron injection, and back electron transfer processes, and ZnO(e) represents ZnO with injected electron. A maximum IPCE of 14% is obtained under back face, i.e., substrate side, illumination, which is slightly greater than that obtained with front face illumination. This is due to the fact that under back face illumination the charges injected in ZnO are close to the OTE surface and, as a result, are collected in larger number. This IPCE of 14% with ZnO nanocrystalline film is much superior to IPCE of 1-2% reported for several dyes deposited on ZnO crystals.41 However, it is somewhat smaller than that (22%) observed by us when the same ruthenium complex (41) (a) Natoli, L. M.; Ryan, M. A.; Spitler, M. T. J. Phys. Chem. 1985, 8, 1448. (b) Kavassalis, C.; Spitler, M. T. J. Phys. Chem. 1983, 87, 3166. (c) Spitler, M. T. J. Phys. Chem. 1986, 90, 2156.

was adsorbed on SnO2 nanocrystalline electrode.11 This is most probably due to the larger driving force present in the SnO2/Ru(II) system compared to that for ZnO/Ru(II), since the conduction band edge, ECB, of SnO2 (∼-0.2 V vs Ag/AgCl) lies at more positive energy relative to that for ZnO (∼-0.69 V vs Ag/AgCl). Further, this IPCE is comparable to that for RuL2(NCS)2 complex adsorbed on ZnO30b but is significantly lower than the IPCE of 98% for RuL2(NCS)2 adsorbed on TiO210 (ECB ∼-0.69 V vs Ag/ AgCl). This is rather surprising as the driving forces for both ZnO/Ru(II) and TiO2/RuL2(NCS)2 are almost similar. The reasons for this difference are varied and may be linked to the factors that determine IPCE, namely, the light harvesting efficiency (LHE) of the dye, the charge injection yield (Φinj) from the excited dye to the semiconductor, and the charge collection efficiency, ηc, of the system. The IPCE in terms of these parameters is written as follows10

IPCE (%) ) LHE(%) Φinj ηc

(6)

In the present case of ruthenium complex on ZnO, LHE is about 45%, which may partly explain the lower IPCE. A similar LHE was also observed for RuL2(NCS)2 adsorbed on ZnO nanocrystalline electrode,30b although the same molecule adsorbed on TiO2 nanocrystalline film exhibited surprisingly a very high LHE (98%).10 The weaker interaction between dye and ZnO, in terms of the poor degree of chelation of Ru(bpy)2(dcbpy)2+ through carboxylate groups of the bipyridyl ligands to surface Zn2+ atoms, is possibly responsible for the lower uptake of the dye. With LHE of 45%, the product of Φinj and ηc is 0.3. Let us now evaluate the individual contribution of Φinj and ηc to IPCE. 2. Charge injection yield. Since charge injection from Ru(II)* into the conduction band of ZnO occurs in competition with the radiative and nonradiative modes of deactivation of the excited state, one can express the charge injection yield as

φinj )

kinj kinj + kr + knr

(7)

It is thus clear from above that for an efficient charge injection, kinj should be much higher than the sum of kr and knr. We have employed the time-resolved luminescence and microwave conductivity techniques to determine kinj. (a) Time-Resolved Fluorescence and Microwave Conductivity Measurements. It has been observed by us and by several other researchers that there occurs a pronounced quenching in fluorescence of various dyes when adsorbed on semiconductors,30,36i,h,42-45 for example, TiO2, SnO2, etc., compared to when they are deposited on a nonreactive insulator surface, such as, Al2O3 or SiO2. Further, a shortening in fluorescence lifetimes was also noticed. These observations have been explained in terms of electron transfer from the excited dye to the semiconductor. (42) Itoh, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J. Am. Chem. Soc. 1984, 106, 1620. (43) (a) Takemura, H.; Saji, T.; Fujihara, M.; Aoyagui, S.; Hashimoto, K.; Sakata, T. Chem. Phys. Lett. 1985, 122, 496. (b) Hashimoto, K.; Hiramoto, M.; Sakata, T.; Muraki, H.; Takemura, H.; Fujihara, M. J. Phys. Chem. 1987, 91, 6198. (c) Hashimoto, K.; Hiramoto, M.; Lever, A. B. P.; Sakata, T. J. Phys. Chem. 1988, 92, 1016. (d) Hashimoto, K.; Hiramoto, M.; Sakata, T. J. Phys. Chem. 1988, 92, 4272. (e) Hashimoto, K.; Hiramoto, M.; Sakata, T. Chem. Phys. Lett. 1988, 148, 215. (f) Sakata, T.; Hashimoto, K.; Hiramoto, M. J. Phys. Chem. 1990, 94, 3040. (g) Kajiwara, T.; Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1982, 86, 4516. (44) (a) Eichberger, R.; Willig, F. Chem. Phys. 1990, 141, 159. (b) Willig, F.; Eichberger, R.; Sundaresan,N. S.; Parkinson, B. A. J. Am. Chem. Soc. 1990, 112, 2702. (45) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 3822.

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Figure 3. Luminescence decay monitored at 640 nm following 532 nm laser pulse excitation of the OTE/ZnO/Ru(II) electrode. The solid line shows the biexponential kinetic fit of the emission decay using the expression, F(t) ) a1 exp(-t/τ1) + a2 exp(-t/τ2). The inset shows the growth of the microwave absorption signal.

As reported earlier, the luminescence of Ru(II) adsorbed on a neutral insulator surface such as silica shows a simple exponential decay with a lifetime of ∼0.26 µs.11 However, when adsorbed on ZnO nanocrystalline film, it exhibits a faster and multiexponential decay (Figure 3). This multiexponential behavior is most probably due to the existence of a range of adsorption and injection sites on ZnO. The decay rate has been satisfactorily fitted to biexponential decay kinetics with lifetimes of 9.6 and 83 ns. (The amplitude of the faster component was 0.78 of the total.) One can note that these lifetimes are significantly shorter than the 0.26 µs obtained on insulator SiO2. The shortening of fluorescence lifetimes in ZnO/Ru(II) has been attributed to the electron transfer from Ru(II)* to ZnO. By substituting the respective values of lifetimes on ZnO (τs) and on silica (τin), we have obtained the values of 108 and 8 × 106 s-1 for fast and slow components of kinj, respectively, from the following expression11

kinj )

1 1 τs τin

(8)

Similar results have also been obtained by us when this ruthenium complex was adsorbed on SnO2 nanocrystalline electrode.11 As suggested by Hashimoto et al.,43c the fast component of kinj can be ascribed to Ru(II)* in direct contact with the ZnO surface while the slower one with longer lifetime may be assigned to Ru(II)* located farther away from the surface. Such electron transfer from Ru(II)* to ZnO is also supported by the microwave conductivity measurements where the absorption signal arises as a result of mobile charge carriers produced in the semiconductor.36j Since the growth of microwave absorption represents the charge injection process, it should match the decay of fluorescence. This, indeed, is the case as seen from the inset of Figure 3 where the growth in microwave signal following excitation of the ZnO/Ru(II) electrode with a 532 nm laser pulse is shown. The analysis of the signal gives 1.1 × 108 s-1 as the value of kinj, which is in excellent agreement with that obtained from fluorescence decay experiments. The above observations of electron transfer from Ru(II)* to ZnO are consistent with electron transfer theory which states that the probability of electron transfer from a donor to an acceptor depends on the free energy change or the driving force between the donor and acceptor.46,47 For a dye adsorbed on a semiconductor the driving force (46) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (b) Marcus, R. A. J. Chem. Phys. 1956, 24, 966. (47) Closs, G. L.; Miller, J. R. Science 1988, 240, 440.

for photoinduced electron transfer from the dye to the semiconductor is the energy gap, ∆E, defined as, ∆E ) -q(Eox* - EFB),43c,e,f where Eox* is the oxidation potential of the excited dye, EFB is the flat band potential of the semiconductor, and q is the electronic charge. Thus, in view of energetic diagram shown in Figure 2, the more negative value of Eox*, i.e., EoRu(II)*/Ru(III) (∼-0.72 V vs Ag/ AgCl) compared to that of EFB of ZnO (∼-0.69 V vs Ag/ AgCl) will permit an activationless electron transfer from excited ruthenium to ZnO conduction band which is composed of a continuum of energy states. In the case of insulator SiO2 (ECB ) -4.7 eV vs NHE),43g its conduction band lies at much higher energy than Eox* of Ru(II) and, as a result, is not able to accept an electron from the dye. The values for kinj obtained for ZnO/Ru(II) in the present study are comparable to those cited by other researchers for several ruthenium complexes adsorbed on various semiconductor surfaces.36h,43,45,48 However, they are much smaller than those for various organic dyes such as cresyl violet, rhodamine, and squaraines where charge injection has been shown to occur within 20 ps.44b,49,50 Fast electron transfer has also been observed with RuL2(NCS)2 on TiO210 and for Ru(H2O)22- on TiO2 at very low dye coverage.51 The lower values of kinj in the present case are most probably due to the poor degree of delocalization of the π* state of Ru(II)* into the conduction band of ZnO which results in weak electronic coupling between the occupied states of excited dye and the conduction band manifold of ZnO. Such a weak electronic coupling has also been considered by others to explain the lower values of kinj in ruthenium complexes adsorbed on various semiconductors.30b,43 With these rates for electron transfer (∼108 and 107 s-1) for Ru(II) on ZnO and equating kr + knr to 1/τin, the corresponding values of Φinj evaluated from eq 7 are unity and 0.70, or an average of 0.85. This value is much higher than the Φinj of 0.17 reported for RuL2(NCS)2 also adsorbed on ZnO.30b We, however, feel that their value is underestimated as it is not determined experimentally but has rather been indirectly evaluated from eq 6 assuming a collection efficiency of 100%. In the present case, Φinj has been determined experimentally, and it is rather the collection efficiency that is much lower than 100% as described below. 3. Charge Collection Efficiency. With Φinj of 0.85 and LHE ∼45%, ηc from eq 6 is evaluated to be 0.31 or 31%. This means that 70% of the injected charges are lost on their way to the back contact. A major source for this loss is the reverse or back electron transfer, kb, from the conduction band to the oxidized dye. It should be mentioned that because of the nanometer size of ZnO particles (∼50 Å) used in the preparation of the electrodes, the space charge layer is absent at the semiconductor/electrolyte interface. This is in contrast with the conventional photoelectrochemical (PE) cells where a significant junction potential exists and aids in charge separation. In nanocrystalline electrode based PE cells, it is, however, the differing rates of electron or hole transfer to the electrolyte that control the charge separation.36g,52,53 As a result, the charge trapping and (48) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellino, F. S.; Meyer, G. J. Inorg. Chem. 1994, 33, 5741. (49) Kamat, P. V.; Das, S.; Thomas, G. K.; George, M. V. Chem. Phys. Lett. 1991, 178, 75. (50) Crackel, R. L.; Struve, W. S. Chem. Phys. Lett. 1985, 120, 473. (51) Shwarzburg, K.; Willig, F. Appl. Phys. Lett. 1991, 58, 2520. (52) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (53) (a)Hagfeldt, A.; Bjosksten, U.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1992, 27, 293. (b) Hagfeldt, A.; Lindquist, S.-E.; Gra¨tzel, M. Sol. Energy Mater. Sol. Cells 1994, 32, 245.

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Figure 4. Dependence of IPCE and fluorescence intensity on applied potential.

recombination at grain boundaries, surface defects, etc., or, in other words, at various surface states, can become quite significant. The surface states may, in fact, directly compete with the conduction band for the injected electrons from the excited dye. The trapped charges in these surface states can recombine with the oxidized dye before even being transferred to the conduction band. This recombination route, i.e., the electrons in surface states recombining with the oxidized dye, will thus be in addition to the one between conduction band electrons and the oxidized dye. Another recombination route that may be at play is the capture of injected electrons into ZnO by the electrolye, the triodide in the present case, i.e.

I3- + 2ZnO(e) f 3I- + 2ZnO

Figure 5. Variation of short-circuit photocurrent (Isc) of OTE/ ZnO/Ru(II) electrode measured at 470 nm as a function of incident light intensity, Iinc (electrolyte, 0.05 M I2 and 0.5 M LiI in acetonitrile). The inset shows the log-log plot of Isc vs Iinc.

(9)

Such a possibility has been discussed by Nazeerudin et al. with RuL2(NCS)2 adsorbed on TiO2 and was rationalized in terms of surface states mediating the charge transfer process.10 Thus all recombination pathways described above are possibly, to a differing degree, responsible for the lower collection efficiency in Ru(II) adsorbed on ZnO nanocrystalline film. 4. Dependence of IPCE on Applied Bias. To further examine the photosensitization of ZnO by Ru(II), IPCE was recorded as a function of applied bias and the results are presented in Figure 4. As seen, the IPCE rises from zero at cathodic potential of ∼-0.75 V to a maximum of 24% at an anodic bias of 0.1 V. This is due to the fact that the external bias directly controls the Fermi level of the semiconductor and hence modifies the energy gap. Under unbiased conditions the average IPCE is ∼12%. With application of a negative bias, the Fermi level of ZnO is shifted to negative energies and, at a potential of -0.75 V, it becomes higher in energy or more negative than Eox of Ru(II)*, which is at -0.72 V, thus resulting in an inefficient electron transfer from Ru(II)* to ZnO and leading to zero IPCE. With positive bias, the Fermi level is shifted to more positive energies. This increases ∆E and leads to an efficient electron transfer resulting in a higher Φinj and a higher IPCE. It should, however, be mentioned that the increase in IPCE with positive bias may also be due to the increase in charge collection efficiency as the photoinjected electrons will be more efficiently withdrawn and transported to the back contact. In the present case, since the injection yield of unbiased cells is already high (∼0.85), the observed increase in IPCE with positive bias may, in large part, be due to the increase in charge collection efficiency. Similar dependence of IPCE on applied bias has also been observed by others.30b,c The dependence of IPCE on applied bias is in parallel with the results of fluorescence vs applied bias, also shown in Figure 4. Since Ru(II)* deactivates via radiative,

Figure 6. Dependence of open-circuit photovoltage (Voc) of the OTE/ZnO/Ru(II) electrode measured at 470 nm on incident light intensity, Iinc (electrolyte, 0.05 M I2 and 0.5 M LiI in acetonitrile). The inset shows the semilog plot of Isc vs Voc.

nonradiative, and electron injection processes (Figure 2), and since nonradiative decay is less likely to be influenced by the applied bias, any increase in one (radiative or electron injection) would cause a decrease in the other. This is exactly what is being observed in Figure 4. When the IPCE is increasing, the exact opposite (decrease) is happening to the fluorescence intensity, and vice versa. This is, once again, related to the energy gap. At negative potentials, the Fermi level is pushed upward to more negative energies, which decreases the driving force for electron injection and leads to a higher probability of radiative transition to the ground state. With anodic bias, the opposite situation prevails; i.e., the fluorescence decreases and IPCE increases. 5. Other Photoelectrochemical Characteristics. To further characterize the ZnO/Ru(II) system, the following measurements were also performed. (a) Dependence of Short-Circuit Photocurrent (Isc) and Open-Circuit Photovoltage (Voc) on Incident Light Intensity. Isc and Voc were measured at various incident light intensities (Iinc) and are shown in Figures 5 and 6, respectively. The photocurrent generation in ZnO/Ru(II) with incident light intensity follows the relation Isc ) RIincγ, where γ is the light exponent.54 The logarithmic plot of Isc vs Iinc (inset of Figure 5) yields the value of γ ) 0.98 (∼1), which suggests that the photogeneration of charge carriers is a monophotonic process. The variation of Voc with Iinc is initially large but becomes smaller at higher light intensities. The inset of Figure 6

Photosensitization of Nanocrystalline ZnO Films

Langmuir, Vol. 13, No. 8, 1997 2403

Figure 7. Current-voltage characterstics of an OTE/ZnO/ Ru(II) electrode (a) in dark, and (b) under illumination: excitation wavelength ) 475 nm. Scan rate is 5 mV/s and electrolyte is 0.05 M I2 and 0.5 M LiI in acetonitrile.

shows that Voc varies logarithmically with Isc (and hence also with Iinc). This behavior is similar to the photoelectrochemical cells employing single crystal or polycrystalline electrodes operating on Schottky barrier principle where the space charge layer is present at the semiconductor/electrolyte interface and is responsible for charge separation. Voc in such cells is related to Isc by the following expression55

Voc )

(

)

Isc nkT +1 ln q I0

(10)

where k, T, and q are, respectively, the Boltzmann constant, absolute temperature, and electronic charge, and n and I0 are the diode quality or the ideality factor and reverse saturation current, respectively. As mentioned earlier, the charge separation in nanocrystalline films does not depend upon the built-in electric field; it is, however, the interfacial charge transfer kinetics that control the photoinduced charge separation in these films. Recently, Sodergren et al. have derived an expression for photocurrent vs applied bias in microporous semiconductor based photoelectrochemical cell.56 The expression is similar to that for a Schottky barrier solar cell shown in eq 10. The values of n and I0 obtained from the plot of ln(Isc) vs Voc, shown in the inset of Figure 6, are 6.0 and 0.24 µA/cm2, respectively. The relatively high value of the reverse saturation current indicates that the charge collection at the back OTE contact is not efficient and may point to the loss of injected charge carriers via recombination route involving the electrolyte (reaction 9). The high value of n suggests a leaky nature of the ZnO/Ru(II) electrode. Its current-voltage characterstics are shown in Figure 7. (b) Power Conversion Efficiency. The power conversion efficiency (η) of the cell employing the ZnO/Ru(II) electrode was determined from the following expression54

η(%) )

IscVocff × 100 Iinc

(11)

where ff is the fill factor and other terms have already been defined. For a typical cell operation, Voc and Isc, respectively, were 400 mV and 40 µA/cm2 at the excitation wavelength of 470 nm with an Iinc of 1.2 mW/cm2. With a ff of 0.40 the value of η determined from expression 11 is 0.6%. Since only about 50% of the incident light is (54) Segui, J.; Hotchandani, S.; Baddou, D.; Leblanc, R. M. J. Phys. Chem. 1992, 95, 8807. (55) Fan, F.-R.; Faulkner, L. J. Chem. Phys. 1978, 69, 3341. (56) So¨dergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 98, 5552.

Figure 8. IPCE and Voc measured at 470 nm as a function of ZnO film thickness (electrolyte, 0.05 M I2 and 0.5 M LiI in acetonitrile).

absorbed by the ruthenium complex, the power conversion efficiency is expected to be 1.2%. This value is comparable to that obtained with Ru(II) adsorbed on SnO211 and that obtained for RuL2(NCS)2 adsorbed on ZnO.30b (c) Effect of ZnO Film Thickness. One of the ways to improve the performance of a ZnO/Ru(II) based photoelectrochemical cell is to optimize the thickness of the ZnO particulate film. We have studied the dependence of Voc and IPCE at different thicknesses of the ZnO film modified with Ru(II). These results are shown in Figure 8. The thickness of the film in these experiments was increased by increasing the amount of ZnO colloids deposited on the OTE plate. Voc grew rapidly between 0 and 0.5 µm and tended to saturate at greater thicknesses. The IPCE, however, increases linearly with ZnO thickness up to 3 µm. Further increase in the thickness was not possible as the films started to crack. Conclusion The photosensitization study of ZnO modified with Ru(bpy)2(dcbpy)2+ shows that a maximum IPCE (incident photon to current conversion efficiency) of 14% can be obtained for unbiased cells. This IPCE is comparable to that obtained for RuL2(NCS)2 also adsorbed on ZnO but is significantly lower than the 98% IPCE of RuL2(NCS)2 adsorbed on TiO2. The lower IPCE is attributed to poor light harvesting efficiency (LHE) and charge collection efficiency (ηc) of the ZnO/Ru(II) system. The weaker interaction of Ru(II) with the ZnO surface is the cause of lower LHE, while various charge recombination processes lead to a poor ηc. The results of IPCE and fluorescence as a function of applied bias show that the charge injection and collection can be controlled by external bias. Power conversion efficiency of 0.6%, although comparable to that for RuL2(NCS)2 on ZnO, is still low. The optimization of various experimental parameters with respect to the choice of the electrolyte, the annealing temperature and time of the film, etc., is necessary to improve the performance of ZnO/Ru(II) based photoelectrochemical cells. Acknowledgment. We thank C. Nasr for helpful discussions. We also thank Professor R. W. Fressenden for microwave conductivity measurements and helpful discussions. The work described herein was supported by the Natural Sciences and Engineering Research Council of Canada (S.H.) and by the Office of Basic Energy Sciences of the U.S. Department of Energy (P.V.K.). This is contribution No. 3986 from the Notre Dame Radiation Laboratory. LA9620115