9468
J. Phys. Chem. B 2000, 104, 9468-9474
Long-Lived Photoinduced Charge Separation in Ru(Bpy)32+/Viologen System at Nafion Membrane-Solution Interface Xiu-Yu Yi, Li-Zhu Wu, and Chen-Ho Tung* Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China ReceiVed: April 3, 2000; In Final Form: July 20, 2000
The photoinduced electron transfer from the excited state of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)32+) incorporated into Nafion membranes to propylviologen sulfonate (PVS°) in the surrounding solution has been examined both by photochemical and photoelectrochemical measurements. N,N′-Tetramethylene-2,2′bipyridinium (DQ2+) entrapped in the Nafion membranes is used as an electron relay. Luminescence quenching studies indicate that the quenching reaction of Ru(bpy)32+ with DQ2+ is both of dynamic and static nature. Ru(bpy)33+ generated from the luminescence quenching remains in the Nafion matrix, while DQ+• migrates to the Nafion-water interface by an electron hopping mechanism, which transfers an electron to PVS° to produce PVS-•. The negatively charged PVS-• is repelled into the bulk solution by the anionic Nafion surface. The isolation of the photoinduced oxidized species Ru(bpy)33+ in Nafion from the ultimate reduced species PVS-• in solution prevents them from undergoing back electron transfer, and a long-lived (up to a few hours) charge separation state is achieved. The low quantum yield for the charge separation was demonstrated to be mainly originated from the back electron transfer in the initial Ru(bpy)33+/DQ+• pair. An electrode was fabricated by coating Ru(bpy)32+-DQ2+-incorporated Nafion film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in PVS° solution to be ca. 350 mV when the light intensity was ca. 60 mW cm-2. This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in cathodic photocurrent, and there is no net chemical change associated with the functioning of the cell which converts light to electricity.
Introduction The approach to photochemical conversion of solar energy generally involves electron transfer from an excited molecule to a ground-state molecule to form an ion pair.1
D* + A f D+ + A-
(1)
D+ + A- f D + A
(2)
To make the energy conversion efficient, the excited molecule D* must be readily quenched by the acceptor A (eq 1), and the energy-wasting back electron transfer (eq 2) must be effectively minimized. Since the ion pair stores a considerable amount of energy, the back electron transfer is thermodynamically allowed, and is usually diffusion-controlled in homogeneous solutions. In attempt to impede the back electron transfer, many efforts have been performed.2-10 Gust and Moore and others2 used donor-acceptor combinations held in well-defined geometries by spacer groups to prevent back electron transfer, and achieved long-lived charge separation. Willner and co-workers3 studied the photosensitized electron transfer between tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)32+) and propylviologen sulfonate (PVS°), and found that in the presence of colloidal silica particles the back electron transfer was slowed, since the PVS-• radical anion was electrostatically repelled by the negatively * Author to whom correspondence should be addressed.
charged surface groups on silica. Slama-Schwok et al.4 obtained charge separation by using a mobile electron relay to transfer an electron from excited-state pyrene to viologen, both held rigidly in a sol-gel glass. Gra¨tzel and co-workers5 studied the photoinduced electron transfer in molecular assemblies composed of an electron donor and a sensitizer, ruthenium(II) bisterpyridine complex, adsorbed to the TiO2 semiconductor surface. Upon excitation, one electron is promoted from the metal center to the conduction band of TiO2, then the donor transfers an electron to the oxidized ruthenium complex. The half-lifetime of the charge separation state can be as long as 300 µs. Yoon and co-workers6 demonstrated that excitation of charge-transfer complexes between pyridinium acceptors and arene donors in zeolites generates radical ion pairs that are orders of magnitude longer-lived than in solution, suggesting the substantial potential of the zeolite cage for controlling back electron transfer. Krueger et al.7 observed long-lived charge separation in a covalently held Ru(bpy)32+-viologen complex by transferring the electron from the covalently bound viologen to another viologen in zeolite L cavities. Dutta and Borja8 trapped the donor Ru(bpy)32+ in the supercages of zeolite Y and used PVS° as the acceptor in the surrounding solution. Electron transfer from the excited Ru(bpy)32+ to PVS° was mediated by N,N′-tetramethylene-2,2′-bipyridinium ions (DQ2+) loaded in the zeolite. Isolation of the donor within the zeolite from the acceptor in the solution outside prevented them from undergoing back electron transfer. Recently, Sykora and Kincaid9
10.1021/jp001284c CCC: $19.00 © 2000 American Chemical Society Published on Web 09/20/2000
Charge Separation in Ru(Bpy)32+/Viologen System
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9469 Results and Discussion
Figure 1. Schematic representation of the two-phase cluster network model for Nafion membrane.
arranged the donor (Ru(mmb)32+, where mmb is 5-monomethyl2,2′-bipyridine), the acceptor (DQ2+) and the sensitizer (Ru(bpy)2-bpz2+, where bpz is 2,2′-bipyrazine) in adjacent cages of Y-zeolite, and suspended the zeolite in PVS° solution. Since the back electron transfer in the initial ion pair (Ru(mmb)33+/ DQ+•) is prevented, they achieved a high level of net chargeseparation efficiency. More recently, Johnston and Ramamurthy and co-workers10 studied the photoinduced electron transfer reactions between excited singlet cyanoaromatic sensitizers and arylalkenes within zeolites and obtained long-lived radical cations, indicating that the confined space of the zeolite interior provides an ideal environment for carrying out photosensitized electron transfer reactions, and in particular for overcoming the limitation of the energy-wasting back electron transfer. However, in all the above studies on zeolite systems, only the surface of the zeolite particles could access to the irradiation. In the present study, we use transparent Nafion membranes as the reaction medium to conduct the electron transfer between Ru(bpy)32+ and PVS°. With the isolation of the donor within Nafion membranes from the acceptor in the surrounding solution and by using DQ2+ as the electron relay, we observed long-lived charge separation. Nafion represents a novel and unique family of polymers which consist of a perfluorinated backbone and short pendant chains terminated by sulfonic groups. When swollen in water, the structure of Nafion resembles that of reversed micelles (Figure 1).11 The hydrated SO3- headgroups are clustered together in a water-containing pocket of ca. 40 Å in diameter which are interconnected by short channels (ca. 10 Å in diameter) within the perfluorocarbon matrix. It has been established12-13 that water-swollen Nafion can incorporate high concentrations of aromatic hydrocarbons and organic dyes. Because of the negatively charged surface of the watercontaining pocket, the positively charged donor Ru(bpy)32+ and charge carrier DQ2+ might be easily incorporated into Nafion clusters, while neutral PVS° should stay in the solution outside. Thus, we prepared the samples by the double loading technique whereby first one guest (either the donor or the charge carrier) and then a second one were introduced into Nafion membranes, and then suspended the sample in PVS° aqueous solution. Electron transfer from the excited Ru(bpy)32+ to PVS° is mediated by DQ2+. The spatial separation between the donor and the ultimate acceptor inhibits the back electron transfer, and an exceedingly long-lived (up to a few hours) chargeseparated pair was achieved.
Incorporation of the Donor and the Electron Relay into Nafion Membranes. The Nafion membranes used in the present study were in the sodium form (Nafion-Na+). Ru(bpy)32+ is a positively charged compound. Due to electrostatic and hydrophobic interactions, it is easily adsorbed into Nafion by immersing the polymer in a well-stirred aqueous solution of Ru(bpy)32+Cl2-. Similarly, the charge carrier DQ2+ can also be easily entrapped into Nafion. The samples incorporating both the donor and the charge carrier were prepared by the double loading technique. First, we prepared Ru(bpy)32+-Nafion samples, then immersed them in an aqueous solution of DQ2+Br2- to give Ru(bpy)32+-DQ2+-Nafion samples. The loading of Ru(bpy)32+ used in the present work was generally ca. 3.5 µmol/g Nafion and that of DQ2+ was ca. 250 µmol/g Nafion. At this loading level of DQ2+, we found that no detectable Ru(bpy)32+ having been incorporated into Nafion was displaced by DQ2+. Using the parameters reported in the literature,11c,14 we could calculate the average occupancy numbers (the number of guest molecules contained in each water-cluster of Nafion) of the samples to be ca. 0.2 for Ru(bpy)32+ and ca. 15 for DQ2+. Considering the hydrophobicity and the positive charge of these compounds, it is likely that the molecules of Ru(bpy)32+ and DQ2+ are located in the fluorocarbon/water interface of the clusters of the membranes. This allows electrostatic interactions with the sulfonate headgroups as well as interactions with the hydrophobic domain of Nafion. In contrast, PVS° is difficult to be adsorbed into Nafion membranes. We immersed Nafion membranes in a 0.01 M PVS° aqueous solution for 2 h and found no perceptible uptake of PVS° from the solution into the Nafion. The inhibition of PVS° from adsorption to the membrane is obviously due to the lack of electrostatic interactions between the neutral molecules of PVS° and the sulfonate headgroups of Nafion. Thus, we could isolate Ru(bpy)32+ molecules in Nafion from the acceptor PVS° in outside solution. Long-Lived Charge-Separation in the Ru(bpy)32+-DQ2+Nafion and PVS0 System. The absorption spectrum of Nafionentrapped Ru(bpy)32+ is almost identical with that in aqueous solution.15 The two MLCT d f π* transition bands are centered at ca. 240 and 447 nm, respectively, and the band of LC π f π* transition occurs at 285 nm. Both DQ2+ and PVS° do not absorb light at wavelengths above 300 nm. Thus, irradiation of the samples with λ > 330 nm light would selectively excite Ru(bpy)32+. Photolysis of Ru(bpy)32+-Nafion samples immersed in a 0.01 M aqueous solution of PVS° results in the production of the viologen radical in solution. Figure 2 shows the growth of the radical (measured at the 395 nm band, molar absorptivity 37000 dm3 mol-1 cm-1)8c for irradiation of a 2.5 × 0.8 cm2 sample (the loading of Ru(bpy)32+, was ca. 3.5 µmol/g Nafion, 0.2 molecules per supercage of Nafion) in 4 mL of 0.01 M PVS° aqueous solution with a 450 W Hanovia lamp as a function of photolysis time. As mentioned in the above section, no perceptible amount of PVS° was uptaken from the solution into the Nafion membranes, due to the lack of the driving force for ion-exchange into the Nafion. Thus, the photoinduced charge transfer from *Ru(bpy)32+ to PVS° must be occurring at the Nafion-solution interface. The diameter of the water cluster11 in Nafion is ca. 40 Å, and the number of headgroups in a single cluster15b was calculated to be ca. 70 for Nafion with EW ) 1100. Thus, the charge density expressed as negative charges per nm2 is ca. 1.4. This value is much greater than that of the colloidal SiO2 surface3a where the charge density is 0.1-0.25 OH- ions per nm2, and comparable with that of alumino-
9470 J. Phys. Chem. B, Vol. 104, No. 40, 2000
Figure 2. Growth of PVS-• radical as a function of photolysis time. 2.5 × 0.8 cm2 samples immersed in 4 mL of 0.01 M PVS° solution; [Ru(bpy)32+] ) 3.5 µmol/g Nafion; light wavelength λ > 330 nm; (9) Ru(bpy)32+-Nafion; (O) Ru(bpy)32+-MV2+-Nafion,[MV2+] ) 250 µmol/g Nafion; (2) Ru(bpy)32+-DQ2+-Nafion, [DQ2+] ) 250 µmol/g Nafion.
Figure 3. Absorption spectra of PVS-• radical as a function of photolysis time (every 20 min) for the Ru(bpy)32+-DQ2+-Nafion samples in 0.01 M PVS° solution; irradiation conditions were same as Figure 2.
silicates16 where the charge density can readily exceed 2 negative charges per nm2. Therefore, the generated negatively charged viologen radical PVS-• would be repelled by the anionic Nafion surface, thereby promoting charge separation. The enhancement of charge separation due to electrostatic repulsion has been observed with other charged interfaces such as micelles,17 polyelectrolytes,18 vesicles,19 semiconductors,20 and SiO2 colloid.3 Although the long-lived charge separation observed above is interesting, the yield of the photoinduced redox species is very low, since only the entrapped Ru(bpy)32+ with direct access to the outside PVS° in solution participates in the photochemical process. To enhance the yield of the photoinduced redox species, we incorporated DQ2+ into the Ru(bpy)32+-Nafion samples as described in the above section. The loading of DQ2+ in the samples was quite high, ca. 15 molecules per cluster of the Nafion. Thus, all of the Ru(bpy)32+ molecules were surrounded by DQ2+. Photolysis of the Ru(bpy)32+-DQ2+-Nafion samples in a 0.01 M PVS° solution under conditions identical to that of Ru(bpy)32+-Nafion samples readily produced the PVS-• radicals. Figure 3 shows the absorption spectra of the generated PVS-• radical in solution at various photolysis time. In the measurements of these spectra, we pulled the Nafion film off the solution. Thus, there is no contribution from DQ+. in the PVS-•
Tung et al.
Figure 4. Schematic representation of electron transfer from Nafion to solution. Reduction potentials of the components were adapted from ref 17.
absorption spectra shown in Figure 3. The spectra clearly document the continuous growth of solution-phase PVS-•. Figure 2 compares this growth with that of the Ru(bpy)32+Nafion system under our experimental arrangement. The rate of radical formation is 4 times greater than that of Ru(bpy)32+Nafion. For samples thoroughly deoxygenated and kept in an anaerobic atmosphere the charge-separation state may survive up to a few hours. After 1 day the PVS-• in solution disappeared, and the Ru(bpy)32+ in Nafion recovered without any loss as detected by UV absorption. This suggests that persistence of PVS-• is indeed due to the charge-separation via the Nafionsolution interface rather than a loss of Ru(bpy)32+ induced by some adventitious sacrificial reagent. We also found that if the Nafion film was pulled off the solution after irradiation, the charge separation could retain for several weeks. The proposed mechanism for the photoinduced electron transfer in the above system involves the electron transfer from the excited state of Ru(bpy)32+ to DQ2+ in the cluster of Nafion, the propagation of the generated charge on DQ+. from the cluster to the Nafion/water interface, and the electron transfer from DQ+. to PVS° at the interface. Figure 4 shows the schematic representation of the photoinduced electron transfer from the Nafion membrane to solution. The reduction potentials of the components are also given in this figure.21 Obviously, the electron transfer processes are thermodynamically favorable. The different steps in the above proposed charge separation mechanism can be examined separately. First, we examined the luminescence quenching of Ru(bpy)32+ by DQ2+ in Nafion clusters. It has been established15b that the emission spectrum of Ru(bpy)32+ in Nafion is ca. 12 nm blue shift and 10 nm narrowing of the width at half-height as compared to that in aqueous solution. These spectral shifts in Nafion stem from the interaction of Ru(bpy)32+ with the flurocarbon chain rather than with the sulfonate headgroups. For the samples of Ru(bpy)32+DQ2+-Nafion immersed in plain water the emission intensities of Ru(bpy)32+ at 610 nm (excitation at 480 nm) as a function of increasing DQ2+ concentration are shown in Figure 5. Here I0 and I represent the intensities of the emission in the absence and presence of the quencher, respectively. In the calculation of the DQ2+ concentration, we assumed that 36% of the total volume of the swollen Nafion membrane is available to Ru(bpy)32+ and DQ2+ (the volume of the clusters and channels occupied by water).15b The emission lifetimes of this ruthe-
Charge Separation in Ru(Bpy)32+/Viologen System
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9471
Figure 5. Stern-Volmer plots for quenching of *Ru(bpy)32+ by DQ2+ in Nafion membranes: (2) I0/I; (3) τ0/τ; [DQ2+] was calculated by assuming 36% of the total volume of the swollen Nafion is available.
nium(II) complex were also measured by a single photon counting apparatus. The emission decay profiles could be well fit by a monoexponential model. Using a biexponential model did not improve the fit very much. The emission lifetime of Ru(bpy)32+ incorporated into water-swollen Nafion membrane (ca. 940 ns) in the absence of DQ2+ is longer than that in aqueous solution and in consistent with the literature.22 The plot of τ0/τ (where τ0 and τ are the lifetimes in the absence and presence of DQ2+, respectively) vs [DQ2+] is also shown in Figure 5. Both curves follow the Stern-Volmer dependence:
I0/I ) 1+ kqτ0 [DQ2+]
(3)
τ0/τ ) 1+ kq′τ0 [DQ2+]
(4)
The slopes of the I0/I and τ0/τ plots are 5.95 and 1.33 M-1, respectively. By assuming τ0 is 940 ns, kq and kq′ were calculated to be 6.33 × 106 and 1.41 × 106 M-1 s-1, respectively. Evidently, the quenching is primarily static in nature. This observation resembles the luminescence quenching of Ru(bpy)32+ by MV2+ in Nafion clusters reported by Meisel and Lee.15b This suggests that the mobilities both of Ru(bpy)32+ and DQ2+ in Nafion are remarkably restricted due to association with the fluorocarbon/water interface. The second step in the overall process of the charge separation is the charge transportation from the Nafion bulk to the Nafionwater interface. In general, the charge propagation occurs via physical diffusion and/or charge hopping between the adjacent redox species. In the present study, the electron relay DQ2+ is incorporated in the Nafion cluster via the electrostatic interactions between the positive charge and the anionic headgroups of the Nafion. Therefore, physical diffusion should be suppressed. On the other hand, the concentration of DQ2+ used is quite high, which could favor charge hopping. Thus, we proposed that the charge transportation in our case might mainly take place by charge hopping along the intra-membranous adjacent DQ2+ molecules. The charge propagation in Nafion membranes by this mechanism has been well established,23 and the diffusion coefficient has been evaluated by spectrocyclic voltammetry or potential-step chronoamperospectrometry. The importance of the last step in the above proposed mechanism of the charge separation was also examined with a system in which the DQ2+ in the Ru(bpy)32+-DQ2+-Nafion samples was replaced by methylviologen (MV2+). Since the reduction potential of MV2+ (E° ) -0.44 V) is comparable to
that of PVS°, the driving force for interfacial electron exchange might be absent. The rate of the PVS-• radical generation for the samples of Ru(bpy)32+-MV2+-Nafion immersed in PVS° solution is shown in Figure 2, and is similar to that of Ru(bpy)32+-Nafion sample. This suggests that the interfacial electron transfer from DQ+• to PVS° is essential for high yields of photoinduced charge separation. The quantum yield of the charge separation for photolysis of the Ru(bpy)32+-DQ2+-Nafion samples in 0.01 M PVS° solution was measured to be 7.3 × 10-5 based on per photon absorbed by Ru(bpy)32+• The low quantum yield obviously originates from the combined inefficiencies of two steps in the overall process. First, the charge transfer from DQ+• to PVS° at the Nafion-solution interface might be inefficient. Second, the initial generated ion pair Ru(bpy)33+/DQ+ in Nafion clusters may undergo the back electron transfer, which plagues the photoinduced charge separation scheme (Figure 4). To estimate the importance of the two factors, we included triethylamine (TEA) as a sacrificial electron donor in the Ru(bpy)32+-DQ2+Nafion system.24 The Nafion cluster was saturated with TEA. Thus, the initial photoinduced ion pairs Ru(bpy)33+/DQ+• are surrounded by TEA molecules. Efficient reaction between Ru(bpy)33+ and TEA would inhibit the charge recombination and results in “permanent” DQ+•.
Ru(bpy)33+ + TEA f Ru(bpy)32+ + TEA+
(5)
TEA+ f decomposition products
(6)
The rate of PVS-• formation was found to be ca. 40 times greater than that for the samples absent of TEA. Thus, the quantum yield for PVS-• generation in this case was estimated to be ca. 2.9 × 10-3 by assuming the quantum yield in the absence of TEA was 7.3 × 10-5 as mentioned above. This value is comparable with that observed for the system of Ru(bpy)32+, PVS°, and TEA in homogeneous solution3a (φ ) 5 × 10-3). Indeed, back electron transfer in the initial Ru(bpy)33+/DQ+• pair is the main obstacle for high efficient charge separation. Photoelectrochemical Behavior. The evidence for the longlived charge separation based on the photochemical study is further strengthened by the photoelectrochemical measurements. We have used the system described in the above section to constitute a photoresponsive device. A measured volume of Nafion solution was coated onto an ITO electrode. After the solvent was evaporated from the surface of the film, the Nafioncoated electrode was loaded with Ru(bpy)32+ and/or DQ2+ by the method as described above. The photovoltage measurements were conducted with a saturated calomel reference electrode in PVS° aqueous solution. Irradiation was performed with visible light (λ > 400 nm, light intensity 60 mW cm-2) from the ITO electrode side. In a control experiment, we irradiated a Ru(bpy)32+-DQ2+-Nafion-ITO electrode in plain water, and could not observe any photovoltage. In this case the initial electron transfer from the excited Ru(bpy)32+ to DQ2+ should definitely occur. Since the generated ion pair Ru(bpy)33+/DQ+• rapidly undergoes charge recombination, no charge separation state is produced. However, in the presence of PVS° in solution, a high photovoltage was detected. Curve b in Figure 6 gives the photovoltage changes induced by switching on and off the irradiation for the system of Ru(bpy)32+-Nafion-ITO electrode in 0.01 M PVS° solution. Evidently, irradiation of the Ru(II) complex results in the photovoltage, and the rise and decay of the photovoltage can take place repeatedly by switching the irradiation on and off. This photovoltage originates from the electron transfer from the excited Ru(bpy)32+ to PVS° at the
9472 J. Phys. Chem. B, Vol. 104, No. 40, 2000
Tung et al.
Figure 6. Photovoltage changes induced by switching on and off the irradiation. The photovoltage measurements were conducted with a saturated calomel reference electrode in 0.01 M PVS° solution. (a) Ru(bpy)32+-DQ2+-Nafion-ITO electrode; (b) Ru(bpy)32+-Nafion-ITO electrode.
Nafion-water interface and the subsequent charge-separation, resulting in positive charge in the ITO electrode and the negative charge in solution. Inspection of Curve b in Figure 6 reveals that by switching on the irradiation the photovoltage is immediately produced and increases rapidly to a saturated value (ca. 80 mV), while the photovoltage decay is relatively slow when the irradiation is switched off. The difference in the rise and decay rates of the photovoltage is probably due to the fact that the generated negatively charged species PVS-• is repelled by the anionic Nafion surface, thereby retarding the charge recombination, while in the forward electron-transfer such a repulsion between PVS° and the surface is absent. Because only the Ru(bpy)32+ with direct access to the outside PVS° in solution may undergo electron transfer, the yield of the photoinduced redox species is low, thus the voltage generated by irradiation is small. However, when DQ2+ was introduced into the membranes as an electron relay, a significant enhancement of the photoresponsibility was observed. For example, irradiation of Ru(bpy)32+-DQ2+-Nafion-ITO electrode in 0.01 M PVS° solution under conditions identical to that for Ru(bpy)32+Nafion-ITO electrode led to a photovoltage of ca. 4.5 times greater than the later. Although this measurement was not optimized, the photovoltage could reach 350 mV. Curve a in Figure 6 shows the photovoltage change induced by switching on and off the irradiation. Obviously, DQ2+ molecules as charge carriers transport electrons from the site of the excited Ru(bpy)32+ to the Nafion-water interface and donate the electrons to PVS° in solution. The isolation of the oxidized species Ru(bpy)33+ in Nafion from the ultimate reduced species PVS-• in solution prevents them from back electron transfer. Since all the Ru(bpy)32+ incorporated into the Nafion may undergo electron transfer, the photoinduced voltage is higher with respect to the Ru(bpy)32+-Nafion-ITO electrode. The photocurrent experiments also demonstrated the enhancement of the charge separation via Nafion-solution interface and the role of the electron carrier in relaying the electron from photoexcited Ru(bpy)32+ to the viologen electron acceptor PVS°. These measurements employed a platinum counter electrode. The working electrode, either a Ru(bpy)32+-DQ2+-Nafion-ITO or a Ru(bpy)32+-Nafion-ITO, and the counter electrode were placed in a 0.01 M PVS° aqueous solution (containing 0.1 M KCl) to construct a photogalvanic cell. Irradiation was performed with visible light (λ > 400 nm) through the ITO working electrode. As in the case of photovoltage measurements, if the PVS° aqueous solution was replaced by plain water, irradiation
Figure 7. Photocurrent changes induced by switching on and off the irradiation. The photogalvanic cell was constructed with a platinum counter electrode in 0.01 M PVS° solution. (a) Ru(bpy)32+-DQ2+Nafion-ITO as the working electrode; (b) Ru(bpy)32+-Nafion-ITO as the working electrode.
could not produce any photocurrents. However, in the presence of PVS° in the interelectrode space cathodic photocurrent with respect to the Ru(II) complex-incorporated ITO working electrode was detected during irradiation. Figure 7 shows the current changes induced by switching on and off the irradiation. Evidently, for the cell consisting of the Ru(bpy)32+-DQ2+Nafion-ITO electrodes the excited Ru(bpy)32+ undergoes electron transfer with DQ2+ in the Nafion membrane, leading to the formation of the ion pair Ru(bpy)33+/DQ+•. The generated DQ+• is transported to the Nafion-water interface by means of electron hopping, and where donates an electron to PVS° resulting in the regeneration of DQ2+ and formation of PVS-•. Subsequently, the generated PVS-• diffuses through the bulk solution toward the counter electrode, where it is oxidized electrochemically to PVS°. On the other hand, the oxidized species Ru(bpy)33+ is reduced back to Ru(bpy)32+ by the conduction band electron of the ITO electrode. Thus, there is no net chemical change associated with the functioning of the cell that converts light into electricity. We found that the operational time of such a photogalvanic cell might be very long. For example, we once continuously irradiated the cell for 20 h and no change in the cathodic photocurrent was detected. The combination of photochemical and electrochemical reactions described above has been well established at semiconductor electrodes coated with a polymer membrane containing photoreaction components.25,26 As indicated in Figure 7, the photoinduced currents are relatively smaller for the cell consisting of Ru(bpy)32+-Nafion-ITO electrode compared with that for the Ru(bpy)32+-DQ2+-Nafion-ITO electrode under identical irradiation conditions. This is attributed again to the fact that for the former cell only the Ru(bpy)32+ adsorbed at the Nafion-water interface may undergo photoinduced electron transfer to the PVS° in solution. Conclusions In summary, we have achieved an extremely long lifetime (up to a few hours) of photoinduced charge-separated pair, although in low yield at present, by isolation of Ru(bpy)32+ within the Nafion membranes from PVS° in solution outside. DQ2+ as an electron relay completely surrounds the Ru(bpy)32+
Charge Separation in Ru(Bpy)32+/Viologen System
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9473
molecules, and efficiently quenches the luminescence of Ru(bpy)32+ in Nafion with the constant of 6.33 × 106 M-1 s-1 both by dynamic and static quenching mechanisms. The generated charge on DQ+• is transported to the Nafion-water interface via an electron hopping mechanism, and then is transferred to PVS° in solution. The ultimate photoinduced reduced species PVS-• and the oxidized species Ru(bpy)33+ are physically separated, preventing the charge recombination. The low quantum yield for the photoinduced charge separation state mainly originates from the back electron transfer in the initial generated ion pair Ru(bpy)33+/DQ+• in Nafion clusters as evidenced by TEA interception experiments. A photoelectrochemical cell constructed on the basis of the above system displays the function of conversion of light to electricity. Irradiation of a Nafion (which has incorporated Ru(bpy)32+ and DQ2+)-coated ITO electrode in a PVS° aqueous solution by visible light with intensity of ca. 60 mW cm-2 results in a ca. 350 mV photovoltage and 2.23 µA photocurrent, although the measurement conditions are not optimized. Furthermore, this photoelectrochemical cell operates in a regenerative mode: PVS° is reduced to PVS-• at the irradiated Nafion-ITO electrode, and the later is oxidized back to the original state at the dark counter electrode; Ru(bpy)32+ is oxidized to Ru(bpy)33+ via photoinduced electron transfer, and the generated Ru(bpy)33+ is reduced back to its original state by the conduction band electron of the ITO electrode.
The samples prepared above were immersed in a 0.01 M PVS° aqueous solution in a Pyrex cell. Prior to irradiation, the samples were deoxygenated with argon for at least 30 min. A 450-W medium-pressure mercury lamp was used as the light source, and a filter was used to cut off the light with λ < 330 nm. After irradiation, the Nafion membranes were pulled off the solution, and the photoproduct PVS-• in the solution was determined by UV spectrum. Photoelectrochemical Measurements. An ITO-coated electrode was coated with a Nafion membrane by casting 30 µL of a 5% Nafion solution and dried under air. The membrane thickness was estimated to be 1.3 µm from the coated Nafion amount by assuming the membrane density as 1.98 g/cm3 for dry Nafion film.15b The Nafion-coated electrode was loaded with Ru(bpy)32+ and/or DQ2+ by the method described above. The measurements of photovoltage were conducted with a saturated calomel reference electrode in 0.01 M PVS° aqueous solution, while those of photocurrent with a platinum counter electrode in water containing 0.01 M PVS° and 0.1 M KCl. Irradiation of the cell with visible light (λ > 400 nm) was performed by a 450-W medium-pressure Hanovia mercury lamp through cutoff filters from the ITO electrode side. The light intensity was measured to be 60 mW cm-2. The voltage and current changes induced by switching on and off of the light were recorded. The current direction was expressed based on the ITO electrode as the working electrode.
Experiment Section
Acknowledgment. We thank the Nation Science Foundation of China and the Bureau for Basic Research, Chinese Academy of Sciences for financial support.
Materials and Instruments. Tris(2,2′-bipyridine)ruthenium(II) chloride (Ru(bpy)32+) was purchased from Aldrich and was recrystallized twice from water. Propyl viologen sulfonate (PVS°) and N,N′-tetramethylene-2,2′-bipyridinium bromide (DQ2+) were synthesized according to the literatures.27,28 Nafion membrane 117 in acid form (Nafion-H+) with an equivalent weight of 1100 and thickness of 0.0178 cm was a product of Du Pont. Prior to use, the membrane was cleaned by boiling in concentrated nitric acid for 4 h, and then thoroughly washed with distilled water and finally immersed in water for 24 h. The membrane in sodium form (Nafion-Na+) was obtained by immersing the pretreated Nafion-H+ membrane in 1 N NaOH aqueous solution. Excess base was then removed by stirring the samples in water. Doubly distilled water was used throughout this work. 5% Nafion (EW ) 1100) solution in lower alphatic alcohol was purchased from Aldrich. UV absorption spectra were recorded on a Shimadzu-1601 spectrometer. Steady-state luminescence spectra were run on a Hitachi F-4500 spectrofluorimeter. Luminescence decay measurements were performed with a Horiba NAES-1100 single photon counting nanosecond fluorescence spectrometer. Photoelectrochemical measurements were carried out by Digital Multimeter Thurlby 1503. Photolysis of the Samples. For preparation of the photolysis samples, the weighed Nafion membranes were immersed in an aqueous solution of Ru(bpy)32+, and the solution was continuously stirred. To reach equilibration, the samples were kept in the solution for at least 24 h. The amount of uptake of the substrate by the membranes was determined by comparison of the UV absorptions of the solutions before and after the membrane immersion. Generally, the experimental error is less than 2%. To prepare the samples incorporating both Ru(bpy)32+ and DQ2+, the membranes having adsorbed Ru(bpy)32+ prepared above were soaked in an aqueous solution of DQ2+. The solution was continuously stirred for 1 day. The amount of uptake of DQ2+ by the membranes was measured by the UV absorbance as described for the case of Ru(bpy)32+.
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