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Effective Photoinduced Electron Transfer in Hetero-Deposited Redox Polymer LB Films Atsushi Aoki, Yumiko Abe, and Tokuji Miyashita* Institute for Chemical Reaction Science, Tohoku University Katahira 2-1-1, Aoba-ku, Sendai 980-8577 Japan Received October 28, 1998. In Final Form: December 2, 1998 The spatial arrangement of the ruthenium dipyridyl complex and ferrocene derivative in heterodeposited redox polymer Langmuir-Blodgett (LB) films is used for the direction control of the photocurrent flow resulting from the photoinduced electron-transfer reaction and the high quantum efficiency. Two kinds of heterodeposited structures are constructed by varying the deposition order of these redox polymer monolayers by the LB technique. The cyclic voltammograms of the heterodeposited redox polymer LB films show current rectifying and charge storage properties. On light irradiation, the anodic photocurrent is observed at the heterodeposited redox polymer LB films consisting of Ru copolymer LB film as an inner layer and Fc copolymer LB film as an outer layer on the ITO (Fc/Ru/ITO) electrodes, whereas a cathodic photocurrent is observed at the reverse layered structure (Ru/Fc/ITO) electrodes. The direction of photocurrent flow depends on the deposition order of the redox polymer LB films on the ITO electrode. A photocurrent quantum efficiency of 5.9% of the heterodeposited redox polymer LB film is achieved. The high photocurrent conversion efficiency is explained by the inhibition of the recombination of the photoinduced charge separation due to the presence of the heterodeposited LB film structures.
Electron-transfer processes in biosystems are controlled by regular molecular arrayed systems in performing a biofunctional activity.1 In particular, the photoinduced electron transfer process plays an important role in the photosynthesis of plants and bacteria in which solar energy is converted into chemical energy with high efficiency. Such highly efficient photoenergy conversion systems are supported by well-organized molecular assemblies in biomembranes. Various attempts to realize artificial photosynthesis have been carried out for application to photovoltaic devices and photoenergy conversion cells. However, the quantum efficiencies of most of these systems are very low compared with those of natural systems. To improve the quantum efficiency, the spatial arrangement of photosensitizers and donors (or acceptors) at a molecular level is a key factor in the inhibition of the reverse electrontransfer process. The Langmuir-Blodgett (LB) and self-assembled monolayer (SAM) techniques are attractive for fabricating ultrathin films with highly oriented molecular assemblies.2 Recently, the SAM technique has been employed for the construction of photoconversion molecular devices because a well-defined monolayer with self-organization property on substrates can be easily fabricated.3 On the other hand, the LB technique can make heterodeposited LB film structures by the heterodeposition of different kinds of monolayers. The heterodeposited LB films are of great interest for the application to artificial photosynthesis and molecular devices. Fujihira was the first to demonstrate that photocurrent flows at heterodeposited LB films consisting of acceptor, sensitizer, and donor monolayers on the electrodes and that the direction of the photocurrent flow was controlled by the deposition order of the mono(1) Albert, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.; Watson, J. D. Molecular Biology of the Cell; Garland Publishing: New York, 1989. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991; p 391. (3) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367.
layers.4 However, these heterodeposited LB films consisting of low-molecular weight compounds are generally unstable because of the aggregation and crystallization of functional groups in the LB films and flip-flop motion of amphiphiles. Recently, polymer LB films have received much attention due to their mechanical and thermal stability.5 Heterodeposited polyimide LB films consisting of acceptor, sensitizer, and donor units were constructed by Nishikata et al.6 The photocurrent quantum efficiency in those heterodeposited polymer LB films was low because of the disorder of the heterodeposited layer structure caused by thermal and chemical treatment of polyamic acid alkylamine salts for the formation of polyimides. Recently, we have found that a N-dodecylacrylamide (DDA) polymer has an excellent ability to form a stable monolayer on a water surface and an LB film.7 Furthermore, we have succeeded in the introduction of functional monomers to DDA polymers using a copolymerization method to functionalize the DDA polymer LB films. We previously prepared the redox amphiphilic polymers, poly(N-dodecylacrylamide-co-ferrocenylmethylacrylate) (Fc copolymer)8 and poly(N-dodecylacrylamide-co-(4-(acryloylmethyl)-4′-methyl-2,2′-bipyridine)bis(2,2′-bipyridine)ruthenium diperchlorate (ClO4))) (Ru copolymer)9 (Figure 1). Ru(bpy)32+ is well-known to be a redox-active photosensitizer with strong absorption in the visible light region and has been investigated with respect to its electrochemistry,10,11 electrochemiluminesence,12,13 photoinduced (4) (a) Fujihira, M. In Thin Films; Ulman, A., Ed.; Academic Press: San Diego, CA, 1995; Vol. 20, p 239. (b) Fujihira, M. Mol. Cryst. Liq. Cryst. 1990, 183, 59. (c) Fujihira, M.; Nishiyama, K.; Yamada, H. Thin Solid Films 1985, 132, 77. (d) Fujihira, M.; Sakomura, M.; Kamei, T. Thin Solid Films 1989, 180, 43. (e) Fujihira, M.; Sakomura, M. Thin Solid Films 1989, 179, 471. (f) Fujihira, M.; Yamada, H. Thin Solid Films 1988, 160, 125. (5) Miyashita, T. Prog. Polym. Sci. 1993, 18, 263. (6) Nishikata, Y.; Morikawa, A.; Kakimoto, M.; Imai, Y.; Hirata, Y.; Nishiyama, K.; Fujihira, M. J. Chem. Soc., Chem. Commun. 1989, 1772. (7) Mizuta, Y.; Matsuda, M.; Miyashita, T. Langmuir 1993, 9, 1158. (8) Aoki, A.; Miyashita, T. Macromolecules 1996, 29, 4662. (9) Aoki, A.; Miyashita, T. Chem. Lett. 1996, 563. (10) Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1985, 183, 1.
10.1021/la981527g CCC: $18.00 © 1999 American Chemical Society Published on Web 01/07/1999
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Aoki et al. Scheme 1
Figure 1. Chemical structures of amphiphilic redox polymers: (A) Ru copolymer; (B) Fc copolymer.
electron-transfer quenching,14,15 and photocurrent generation,16,17 in solutions, micelles, cast films, and LB films. The photoexcited ruthenium complex (Ru(bpy)32+*) can perform both oxidative and reductive electron-transfer reactions by means of acceptors and donors, resulting in the oxidized form, Ru(bpy)33+, and the reduced form, Ru(bpy)3+, respectively. Ru(bpy)33+ and Ru(bpy)3+ can be detected as a photocurrent at the electrode. The ferrocene derivative was chosen as a donor for Ru(bpy)32+ because of the rapid electron-transfer reaction. In this study, we investigate the influences of the film structure on photoelectrochemical properties using heterodeposited redox polymer LB films consisting of Fc and Ru copolymers on ITO electrodes. We found that the direction of photocurrent flow was controlled by the deposition order of redox polymer LB films on the electrodes and the photocurrent quantum efficiency was improved by the inhibition of the reverse electron-transfer reaction after the photoinduced electron-transfer reaction due to the presence of the heterodeposited structure. Experimental Section Materials. The Fc copolymer and Ru copolymer were prepared as described previously.8,9 Briefly, the Fc copolymer was prepared by free radical copolymerization of DDA with Fc in benzene at 60 °C with 2, 2′-azobis(isobutyronitrile). The Ru copolymer was prepared as follows; 4-hydroxymethyl-4′-methyl-2,2′-bipyridine, which was synthesized from 4,4′-dimethyl-2,2′-bipyridine according to Meyer’s method18 was reacted with acryloyl chloride in the presence of triethylamine in chloroform at room temperature to obtain 4-(acryloylmethyl)-4′-methyl-2,2′-bipyridine as a ligand monomer. The copolymer of DDA with the ligand monomer was copolymerized, as well as the Fc copolymer. The resulting copolymer was refluxed with cis-bis(2,2′-bipyridine)dichlororuthenium(II) in ethanol. The copolymer complexed with (11) Zhang, X.; Bard, A. J. Am. Chem. Soc. 1989, 111, 8098. (12) (a) Zhang, X.; Bard, A. J. Phys. Chem. 1988, 92, 5566. (b) Miller, C. J.; McCord, P.; Bard, A. Langmuir 1991, 7, 2781. (13) Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 10, p 1. (14) (a) Schmehl, R. H.; Whitesell, L. G.; Whitten, D. G. J. Am. Chem. Soc. 1981, 103, 3761. (b) Schmehl, R. H.; Whitten, D. G. J. Am. Chem. Soc. 1980, 102, 1938. (c) Sprintschnik, G.; Sprintschnik, H. W.; Kirsch, P. P.; Whitten, D. G. J. Am. Chem. Soc. 1977, 99, 4947. (15) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (16) (a) Jin, R.-J.; Kaneko, M. In Molecular Electronics and Molecular Electronic Devices; Sienicki, K., Ed.; CRC Press: Boca Raton, FL, 1993; Vol. 1, Chapter 6. (b) Kaneko, M.; Moriya, S.; Yamada, A. Electrochim. Acta 1984, 29, 115. (c) Oyama, N.; Yamaguchi, S.; Kaneko, M.; Yamada, A. J. Electroanal. Chem. 1982, 139, 215. (17) Downard, A. J.; Surridge, N. A.; Gould, S.; Meyer, T. J.; Deronzier, A.; Moutet, J.-C. J. Phys. Chem. 1990, 94, 6754. (18) Ciana, L. D.; Hamachi, I.; Meyer, T. J. J. Org. Chem. 1989, 54, 1731.
ruthenium ion was purified twice by precipitation in a large excess of diethyl ether from ethanol solution. The mole fractions of the redox species in the copolymers were determined from the UVvis absorption spectra to be 0.54 and 0.11 for Fc and Ru copolymers, respectively. Molar absorption coefficients of FcA homopolymer and Ru(bpy)32+ used as a standard compound for determination of copolymer composition are ) 110 M-1 cm-1 at 438 nm and 1.4 × 104 M-1 cm-1 at 454 nm, respectively. Bis(2-hydroxyethyl)-N,N′-4,4′-bipyridinium cation (V2+), which is an acceptor reagent, was prepared by reaction of 4,4′-bipyridine with 2-bromoethanol in ethanol for 12 h in reflux. All other chemicals were of reagent grade and used without further purification. General Methods. The measurement of surface pressure (π)area (A) isotherms and deposition of the monolayers were carried out with a computer-controlled Langmuir trough FSD-11 (USI) at 20 °C. Distilled and deionized water (Millipore Milli-Q) was used for the subphase. Chloroform was used as a solvent for spreading the monolayer on the water surface. An ITO electrode and a quartz substrate were employed as a substrate forming monolayers. The ITO electrode was cleaned by sonication in a chemical detergent solution, water, acetone, and chloroform, successively. The quartz substrate was cleaned by immersing in a methanol solution containing 5 wt % KOH, boiling in dilute HNO3 solution and then washing it with pure water. The hydrophobic quartz substrate was prepared by immersing the quartz substrate into CHCl3 solution containing a few percentage of dimethyldichlorosilane. The monolayers of the Fc and Ru copolymers were transferred onto these substrates by the vertical dipping method at a dipping speed of 10 mm min-1 under a surface pressure of 20 and 30 mN m-1, respectively, at 20 °C. All strucutres of the heterodeposited redox polymer LB films used in this study and their abbreviations are shown in Scheme 1. UV absorption spectra were measured with a Hitachi UV-vis absorption spectrometer. The X-ray diffraction (XRD) pattern was measured with an X-ray diffractometer (Rigakudenki RADC). The copolymer LB films with 21 layers were employed for XRD measurement. Emission spectra were obtained on a Hitachi 450 spectrofluorometer. The excitation wavelength for quenching experiments of Ru(bpy)32+ derivative was 450 nm. Electrochemical and Photoelectrochemical Measurements. Cyclic voltammetry and photocurrent measurement were performed using a potentiostat (HA-501, Hokuto) and a function generator (HB-104, Hokuto). Current-potential curves were recorded on an x-y recorder (WX1000, Graphtec). The photocurrent measurement system is illustrated in Figure 2. The photocurrent measurement was carried out at a constant applied potential on light irradiation. A 500 W xenon lamp equipped with IR cutoff filter (IRA-2S, Toshiba) and UV cutoff filter (VY43, Toshiba) was used as a light source. The light intensity at the irradiating substrate surface was measured with a thermopile, MIR-100C (Daiya Instrument). Photocurrent was measured on a y-t recorder. The electrochemical cell is equipped with a window
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Figure 3. Cyclic voltammograms for Fc/Ru/ITO electrodes in 1.0 M NaClO4 solution at 10 mV s-1 scan rate: solid line, the first scan; dotted line, the second scan.
Figure 2. Schematic representation of the photocurrent measurement system. for mounting an ITO electrode. The ITO electrode is mounted at the cell window using a silicon rubber O-ring (14 mmφ). An electrode area of 1.54 cm2 is exposed to the electrolyte solutions. A Pt wire is used as an auxiliary electrode and the potential is referenced to a saturated calomel electrode (SCE). 1.0 M NaClO4 solution is employed as an electrolyte solution. Solutions were initially purged with N2 for 30 min and then maintained under a flow of N2. A monochromatic light for the measurement of photocurrent action spectra was obtained from a xenon lamp filtered with metal interference filters MIF-W type (400-550 nm wavelength, 10 nm fwhm, Vacuum Optics Corporation of Japan).
Results and Discussion Monolayer and Electrochemical Properties of the Heterodeposited Redox Polymer LB Films. The π-A isotherms of Fc and Ru copolymers reported in the previous study9 indicate that stable condensed monolayers were formed on a water surface with a high collapse pressure. Since the limiting surface areas of both ferrocene and ruthenium dipyridyl complex moieties of these copolymers are almost zero, monolayer structures of Fc and Ru copolymers on the water surface are assumed to be alkyl side chains of amphiphilic DDA rising up from the water surface and ferrocene and ruthenium complex moieties placed in the water subphase. These Fc and Ru copolymer monolayers can be transferred onto quartz substrates and ITO electrodes as a Y-type film with a transfer ratio of unity. The cyclic voltammograms for the typical heterodeposited redox polymer LB film consisting of Ru copolymer LB film (two layers) as an inner layer and Fc copolymer LB film (three layers) as an outer layer on an ITO (Fc/Ru/ITO) electrode are shown in Figure 3. In the heterodeposited LB films, no oxidation current of ferrocene was observed around 0.34 V, which was observed in the Fc copolymer LB film on the ITO electrode. This means that ferrocene moiety of the outer layer in the heterodeposited LB films is not oxidized in the region of the potential oxidizing ferrocene species at the first scan because the inner Ru copolymer layer behaves as an insulator for the outer Fc copolymer layer. A current appeared at potential around 0.8 V, where Ru(bpy)32+ starts to be oxidized. At this potential, ferrocene moieties
in the outer layer are catalytically oxidized at the interface of Ru/Fc monolayers through the Ru(bpy)33+ moiety formed in the inner Ru copolymer layer. On the other hand, the current attributed to the reduction of ferrocenium moiety in the outer layer is not observed and the only current peak attributed to the reduction of Ru(bpy)33+ moiety in the inner layer is observed at the reverse potential scan. Because Ru(bpy)32+ moiety in the inner layer is thermodynamically incapable of mediating the reduction of the ferrocenium moiety in the outer layer. The outer Fc copolymer layer remains trapped in the ferrocenium state, which is regarded as a charge storage. Consequently, the voltammetric shape at the second scan is the same with that of the Ru monolayer. These results demonstrate that the unidirectional electron transfer across the interface between Ru and Fc copolymer monolayers is realized by the heterodeposited redox polymer LB films. The first heterodeposited redox polymer electrodes showing a rectifying property were fabricated using electropolymerization by Murray et al.19 We demonstrated that the heterodeposited redox polymers which are ultrathin films at a molecular level also showed a rectifying and charge storage properties. The surface concentrations of the redox species in the heterodeposited LB film were determined by integrating the anodic current in the cyclic voltammogram. The total surface concentration of ruthenium and ferrocene moieties and the surface concentration of ruthenium moiety were obtained to be 3.98 × 10-10 and 9.96 × 10-11 mol cm-2 at the first scan and the second scan, respectively. The surface concentration of ruthenium and ferrocene moieties per monolayer are 3.32 × 10-11 and 1.49 × 10-10 mol cm-2, respectively. These values are reasonably consistent with those of the Ru LB film (3.38 × 10-11 mol cm-2) and the Fc LB film (1.99 × 10-10 mol cm-2) obtained from the cyclic voltammograms of the monolayer LB films. Therefore, the heterodeposited LB film is fully electroactive at the scan rate of 10 mV s-1. We expect that high efficient photoinduced electron transfer would occur at the heterodeposited redox polymer LB films when light is irradiated to excite the Ru(bpy)32+ moiety in the heterodeposited redox polymer LB films. Photoinduced Electron-Transfer Process of the Heterodeposited Redox Polymer LB Films. In the preceding section, it was confirmed that the Ru and Fc copolymer monolayers and the heterodeposited LB films are working as a molecular electrochemical device. Moreover, Ru(bpy)32+ is well-known to be a redox-active photosensitizer with the strong absorption around visible light. The photoinduced electron transfer process is (19) Abruna, H. D.; Denisevich, P.; Umana, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1.
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Figure 4. Emission spectra for Ru copolymer monolayer and the heterodeposited redox polymer LB film consisting of Ru and Fc copolymers on the quartz substrates: (A) tail-to-tail deposition; (B) head-to-head deposition; solid line, Ru copolymer monolayer LB film; dotted line, the heterodeposited redox polymer LB film.
expected to be controlled by the heterodeposited LB films structure. The quenching of the photoexcited Ru copolymer monolayer by Fc copolymer monolayers was investigated by the emission spectra. Figure 4 shows the emission spectra for Ru copolymer monolayer and the heterodeposited redox polymer LB film consisting of Ru and Fc copolymer on the quartz substrates. The head-to-head LB film structure is constructed by the successive deposition of one Ru copolymer monolayer and one Fc copolymer monolayer on the hydrophobic quartz substrate (Scheme 1B). The tail-to-tail LB film structure is constructed by the successive deposition of one Ru copolymer monolayer and two Fc copolymer monolayers on the hydrophilic quartz substrate (Scheme 1B). Quenching efficiencies are determined to be 44.5% and 80.0% in the tail-to-tail and head-to-head structures, respectively. Energy transfer via a Fo¨rster resonance mechanism is unlikely in this system because the ferrocene derivative has no absorption transitions around the Ru(bpy)32+ emission wavelength (600 nm). As a consequence, the electron-transfer quenching from ferrocene in its ground state to the excited Ru(bpy)32+ would be performed in this system. The quenching efficiency in the head-to-head LB film structure is higher than that in the tail-to-tail LB film structure. Quenching is a result of the electron-transfer process from ferrocene to the photoexcited Ru(bpy)32+.20 Since the ferrocene and Ru(bpy)32+ moieties are placed in the hydrophilic part of the LB film, as indicated by the π-A isotherm and XRD measurements, the distance between the ferrocene and (20) Kondo, T.; Yamada, Y.; Hishiyama, K.; Suga, K.; Fujihira, M. Thin Solid Films 1989, 179, 463. (b) Fujihira, M.; Nishiyama, K.; Aoki, K. Thin Solid Films 1988, 160, 317.
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Ru(bpy)32+ in the head-to-head LB film structure is estimated to be 0.96 nm, shorter than the distance, 4.56 nm, in the tail-to-tail LB film structure.21 Consequently, the electron-transfer quenching becomes facile in the headto-head LB film structure. This film structure is used in the following photoelectrochemical experiments. Photocurrent Generation in the Heterodeposited Redox Polymer LB Films. The photocurrent responses for the heterodeposited redox polymer LB film electrodes were measured at a constant applied voltage, 0.0 V vs SCE, under nitrogen atmosphere with irradiation of light above 430 nm wavelength from a 500 W xenon lamp (Figure 5). Two kinds of heterodeposited LB film structures are constructed by varying the deposition order of Ru and Fc copolymer monolayers (Scheme 1C). One structure of the heterodeposited redox polymer LB films consists of Ru copolymer LB film (bilayer) as the inner layer and Fc copolymer LB film (three layers) as the outer layer on the ITO (Fc/Ru/ITO) electrode. The other is a reverse heterodeposited redox polymer LB film structure, which consists of Fc copolymer LB film (bilayer) as the inner layer and Ru copolymer LB film (three layers) as the outer layer on the ITO (Ru/Fc/ITO) electrode. The photocurrent response of Ru copolymer LB films (bilayer) on the ITO (Ru/ITO) electrode was also measured for comparison with those of the two heterodeposited redox polymer LB films. Spikelike transient current responses were observed for all LB film structures as soon as the light was turned on and off. The transient current may be attributed to capacitive current, based on the change in dielectric constant due to the production of the photoexcited Ru(bpy)32+ moiety from the ground state during light irradiation. A steady-state anodic photocurrent appeared at the Fc/Ru/ITO electrode during light irradiation (Figure 5B). On the other hand, a steady-state cathodic photocurrent was observed at the reverse, Ru/Fc/ITO electrode (Figure 5C). It is of interest that the direction of photocurrent flows was changed drastically by the deposition order of the Ru and Fc copolymer LB films. No steadystate photocurrent was observed at the Ru/ITO electrode (Figure 5A). We infer the unidirectional photocurrent flow mechanism of the heterodeposited redox polymer LB films to be as follows. After the photoinduced electrode transfer between the photoexcited Ru copolymer layer and the Fc copolymer layer takes place, the electrode reaction and the reverse electron transfer reaction due to charge recombination between the reduced ruthenium dipyridyl complex and the oxidized ferrocene occur competitively (Figure 6). An anodic photocurrent is produced by the electron-transfer reaction from the photoreduced Ru(bpy)3+ moiety in the inner layer to the ITO electrode at the Fc/Ru/ITO electrode (Figure 6A). Similarly, a cathodic photocurrent flows by electron transfer from the electrode to the ferrocenium moiety in the inner layer at the Ru/Fc/ITO electrode (Figure 6B). Hence, the direction of photocurrent flow is determined by whether the inner layer is Ru copolymer or Fc copolymer. Furthermore, the magnitude of the steady-state anodic photocurrent increases with the addition of a sacrificial donor, triethanolamine (TEOA), into the electrolyte solution at (21) The distances between the ruthenium and ferrocene centers were estimated as follows: The areas occupied by one ruthenium and one ferrocene in the LB films are 4.9 and 0.83 nm2 molecule-1, respectively. The molecular areas of ruthenium and ferrocene are reported to be 1.0 and 0.5 nm2. Therefore, as the ruthenium molecule has to be in contact with ferrocene in the face-to-face structure, the average distance is calculated to be 0.96 nm. On the other hand, the average distance is calculated to be 4.56 nm in the tail-to-tail structure because the thickness of the DDA monolayer is 1.8 nm.
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Figure 5. Photocurrent response for Ru/ITO (A), Fc/Ru/ITO (B), and Ru/Fc/ITO (C) in 1.0 M NaClO4 solution. Conditions: applied potential, 0.0 V vs SCE; light intensity, 80 mW cm-2.
Figure 6. Energy diagrams for the photoinduced electrontransfer reaction at the heterodeposited redox polymer LB films: (A) Fc/Ru/ITO; (B) Ru/Fc/ITO.
the Fc/Ru/ITO electrode. This means that the reverse electron transfer, between the Ru(bpy)3+ moiety in the inner layer and the ferrocenium moiety in the outer layer after photoinduced charge separation, is suppressed by the electron-transfer reaction between the ferrocenium moiety in the outer layer and TEOA in the solution. As a result of this inhibition, the anodic photocurrent becomes large. Similarly, in the case of the reverse heterodeposited structure (Ru/Fc/ITO), the magnitude of the steady-state cathodic photocurrent increases with the addition of the sacrificial acceptor, V2+, at the Ru/Fc/ITO electrode. The electron-transfer reaction between the Ru(bpy)3+ moiety in the outer layer and V2+ in the solution reduces the reverse electron-transfer process so that the cathodic photocurrent increases because the electrode reaction of ferrocenium moiety in the inner layer is facilitated.
Figure 7. Dependence of photocurrent on the electrode potential in each LB films: Fc/Ru/ITO (closed circles), Ru/ITO (closed triangles), and Ru/Fc/ITO (closed squares) in 1.0 M NaClO4 solution containing 0.733 M TEOA as a sacrificial donor. The dependence of dark current on the electrode potential in all LB films is shown by open triangles.
Figure 7 shows the dependence of the steady-state photocurrent on the applied potential in the heterodeposited redox polymer LB film (Fc/Ru/ITO) electrodes. The photocurrent dependence on the applied potential in the Ru copolymer LB film and the reverse heterodeposited structure LB film (Ru/Fc/ITO) electrode is also shown in
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Figure 8. Dependence of photocurrent on the light intensity at the heterodeposited redox polymer LB film on the ITO electrode: (A) Fc/Ru/ITO electrodes in 1.0 M NaClO4 solution containing 0.733 M TEOA as a sacrificial donor at the applied potential, 0.3 V vs SCE; (B) Ru/Fc/ITO electrodes in 1.0 M NaClO4 solution containing 40 mM V2+ derivative as a sacrificial acceptor at the applied potential, 0.0 V vs SCE.
Figure 7. The dark current is almost zero under the experimental potential condition. In all electrodes where Ru copolymer monolayers comprise the inner layer of the heterodeposited LB films, the magnitude of the anodic photocurrent increases with the anodic potential shift. The anodic photocurrent of the heterodeposited Fc/Ru/ ITO electrode is larger than that of the Ru copolymer LB film electrode. In addition, anodic photocurrent does not flow in the reverse heterodeposited Ru/Fc/ITO electrode. These results suggest that the heterodeposited structure is effective for inducing the overall reaction that produces the photocurrent, and that the deposition order is important in obtaining the photocurrent. The maximum photocurrent, 2.5 µA cm-2, is obtained in the Fc/Ru/ITO electrode at 0.3 V vs SCE, 80 mW cm-2, and 0.733 M TEOA. This value is twice that of the Ru copolymer LB film electrode. The dependence of the photocurrent on the applied potential at the Fc/Ru/ITO electrodes can be explained as follows. In the Ru copolymer LB film, photoinduced electron transfer from metal-to-ligand charge transfer (MLCT) excited-state Ru(bpy)32+* to the Fermi level potential of the ITO electrode occurs. After the photoinduced charge separation, the reverse electrontransfer reaction from the electron in the Fermi level potential of the ITO electrode to the Ru(bpy)33+ moiety in the inner layer occurs immediately at the cathodic potential, so no photocurrent flows. The reverse electron-
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Figure 9. Dependence of photocurrent on the concentration of sacrificial reagents at the Ru copolymer LB film and the heterodeposited redox polymer LB film on the ITO electrodes in 1.0 M NaClO4 solution at applied potential, 0.0 V vs SCE: (A) Fc/Ru/ITO, with TEOA used as the sacrificial donor; (B) Ru/Fc/ITO, with V2+ derivative used as the sacrificial acceptor. The other experimental condition is the same as in Figure 7.
transfer process is slow at the anodic potential because the Fermi level potential of the ITO electrode decreases with the shift of anodic potential and the electron-transfer reaction from TEOA to Ru(bpy)33+ becomes possible. Therefore, photocurrent flow depends on the applied potential. On the other hand, in the heterodeposited Fc/ Ru/ITO electrode, the photoinduced electron transfer from the ferrocene moiety in the outer layer to the photoexcited Ru(bpy)32+* moiety in the inner layer occurs, as discussed above. Both the electrode reaction and the reverse electron transfer reaction take place competitively depending on the electrode potential. The photoreduced Ru(bpy)3+ is oxidized on the ITO electrode when the electrode potential is anodic; i.e., the electrode potential is near the redox potential of the ferrocene moiety in the monolayer. The reverse electron transfer process mainly occurs when the electrode potential is cathodic. The anodic and cathodic photocurrents depend linearly upon the intensity of light irradiation on the Fc/Ru/ITO and Ru/Fc/ITO electrodes, respectively (Figure 8). These results indicate that the excitation of the ruthenium complex is the rate-determining process in obtaining photocurrent. The magnitudes of anodic and cathodic photocurrents increase with the concentrations of the electron donor, TEOA, and the electron acceptor, MV2+ derivative, in the electrolyte solution at the Fc/Ru/ITO and Ru/Fc/ITO electrodes, respectively (Figure 9). The maximum anodic photocurrent was observed to be above 0.5 M TEOA at the Fc/Ru/ITO electrode. The anodic photocurrent is dependent on the TEOA concentration below 0.5 M TEOA. This means that the magnitude of the
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468 nm using
φ)
i/e I(1 - 10-A)
(1)
Wλ hc
(2)
I)
Figure 10. Photocurrent action spectrum of Fc/Ru/ITO in 1.0 M NaClO4 solution. Conditions: applied potential, 0.3 V vs SCE; sacrificial donor, 0.733 M TEOA; Key: solid line, absorption spectrum of Ru copolymer LB film on the quartz.
anodic photocurrent is controlled by the rate of the electron-transfer reaction between the ferrocenium moiety in the outer layer and TEOA in the solution. Above 0.5 M TEOA, the photoinduced electron-transfer reaction from ferrocene to photoexcited Ru(bpy)32+ is the rate-determining step in the overall photocurrent process. The magnitude of the cathodic photocurrent also depends on the concentration of the V2+ derivative below 25 mM. However, no cathodic photocurrent flows with any V2+ concentration at the Ru copolymer LB film electrode because of the rapid reverse electron-transfer process after the photoinduced charge separation caused by V2+ reversibility. The photocurrent action spectrum of the heterodeposited redox polymer LB film is in good agreement with the absorption spectrum of the Ru copolymer LB film (Figure 10). These spectra indicate that the photocurrent was generated by MLCT excitation of the Ru(bpy)32+ moiety in the heterodeposited redox polymer LB film. The photocurrent quantum efficiency (φ) of the Fc/Ru/ITO electrode was calculated to be 5.9% at the wavelength of
where i is the photocurrent density, e is the elementary charge, I is number of photons per unit area and unit time, λ is the wavelength of light irradiation, A is absorbance of the Ru copolymer at λ nm, W is light power irradiated at λ nm, c is the light velocity, and h is the Planck constant. This value of the photocurrent quantum efficiency is higher than the 2.7% of the Ru copolymer LB film. Therefore, the efficiency of the overall reaction that produces a photocurrent is enhanced by the presence of heterodeposited redox polymer LB film structure because of inhibition of the reverse electron-transfer process at the interface between Ru and Fc monolayers at a molecular level. The electrochemical and photoelectrochemical properties of the heterodeposited redox polymer LB film on the ITO electrode were investigated through cyclic voltammetry and steady-state photocurrent measurement. The heterodeposited redox polymer LB film behaves as an electrochemical device, indicating current rectifying and charge storage properties. The direction of photocurrent flow is controlled by the deposition order of redox polymer LB films at a molecular level. A highly effective overall reaction that produces the photocurrent is achieved in the heterodeposited redox polymer LB films due to the inhibition of the reverse electron-transfer process. Acknowledgment. This work was partially supported by Grants-in-Aid for Scientific Research on the Priority Area of “Electrochemistry of Ordered Interfaces” (Nos. 09237209 and 10131209) from the Ministry of Education, Science, Sports, and Culture, Japan. LA981527G
