Optoelectronic Photoinduced Charge Transfer System with μ3PhN

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Optoelectronic Photoinduced Charge Transfer System with µ3PhN-Ru3 Cluster Functionalized Single-Walled Carbon Nanotubes Wonjoo Lee,† Gumae Koo,† Satishchandra B. Ogale,*,‡ and Sung-Hwan Han*,† †

Department of Chemistry, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-ku, Seoul, Korea 133-791, and Physical and Materials Chemistry DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India ReceiVed: February 24, 2009; ReVised Manuscript ReceiVed: June 10, 2009

Optoelectronic photoinduced charges have been attracting great attention lately in view of their applicability to diverse optoelectronic device systems. Here, we report the design and implementation of a charge transfer system based on triruthenium (Ru) cluster compounds on viologen modified single-walled carbon nanotubes (V-SWNTs) by an in situ preparation method. We examine the photoinduced (intensity dependent and temporal) changes in the transport properties of the Ru-cluster functionalized V-SWNT coating and identify the possible charge transfer mechanism. Furthermore, we demonstrate the applicability of such Ru-cluster functionalized V-SWNT films to photoelectrochemical cells. Introduction Nanomaterials offer a set of electronic states that can be engineered to harness optical and electronic properties, and the use of organic/organometallic compounds as electronic materials offers the advantages of low cost and implementation flexibility.1-3 Especially, carbon nanotubes (CNTs) have emerged as an attractive choice for conducting composite materials on account of their extraordinary physical, electrical, and chemical properties.4-7 Composites incorporating CNTs show percolation dominated conductivity with a much lower volume threshold (volume fraction ≈ 10-5), as compared to those with nanoparticles, as is indeed expected based on the large aspect ratio of the former.4,5 Two-thirds of CNTs are generally p-type semiconductors with holes as the charge carriers. For the active layer of optoelectronic devices, incorporation of single-walled carbon nanotubes (SWNTs) enhances the charge separation and facilitateschargetransport,therebyimprovingthedeviceperformance.6-8 Among numerous applications, charge transfer systems based on SWNTs are at the focus of scientific attention in recent times, in view of their envisioned applicability to several key areas such as energy conversion, storage, photocatalysis, and sensors, etc.6-8 The CNT-organometallic compound hybrid composite system benefits from the quasi-one-dimensional (1-D) electron transport along the nanotubes, which allows the use of faster electron transfer properties of carbon nanotubes and the higher photoelectrochemical properties of organometallic compounds to excite electrons and transportation to CNTs.9,10 To prepare nanoscale devices of CNT-based composite systems, appropriate modification of CNTs needs to be implemented.4-8 As an innovative approach, the modification of CNTs with organometallic compounds has been investigated, providing rich diversity in designing photoinduced charge transfer systems.4,6 Organometallic compounds have many unique properties in the presence of d-orbitals, whose energy levels can be easily * Corresponding authors. S.-H. Han and S. B. Ogale. Telephone: +8222220-0934. Fax: +822-2299-0762. E-mail: [email protected] (S.-H. Han) or [email protected] (S. B. Ogale). † Hanyang University. ‡ National Chemical Laboratory.

controlled by changing the metal center and/or coordinating ligands. Upon irradiation, electrons in the highest-occupiedmolecular-orbital (HOMO) can excite to the lowest-unoccupiedmolecular-orbital (LUMO). Through metal-to-ligand-chargetransfer (MLCT), the excited electrons move to the ligand side and further to electron acceptors (i.e., CNTs). The organometallic cluster is a classification which contains metal-metal bonds with more than one metal in a compound.11,14 If the number of metals increases, the total number of d electrons increases. Then, the electronic and optical properties will be very much improved compared to those of single-metal organometallic compounds. Interestingly, the organometallic clusters are not much investigated as an optoelectronic element to form a charge transfer system, and their applications to optoelectronic devices on CNTs are totally unexplored. Although researchers have demonstrated that organometallic compound-CNT systems lead to high charge transfer efficiency,4-8 the preparation of organometallic compounds and cluster compounds on CNTs is difficult to fabricate. Therefore, it should be necessary that a facile methodology be developed to directly form organometallic cluster compounds on CNTs. In this work, we employ the click chemistry approach (in situ preparation) to station triruthenium (Ru) cluster compounds on SWNTs without purification steps, where viologen molecules are used as a linker between SWNTs and cluster compounds. In other words, the organometallic Ru-cluster compounds [Ru3(Br)(CO)11]- (denoted as Ru-1) and [Ru3(µ2-Br)(CO)10]- (denoted as Ru-2), and [Ru3(µ3-NPh)(Br)(CO)9]- (denoted as Ru-3) are directly anchored on viologen-modified SWNTs (denoted hereafter as V-SWNTs), using click chemistry, and are examined for the optoelectronic charge transfer device application. Experimental Section Materials. Ru3(CO)12 was purchased from Strem Chemicals, Inc., and nitrosobenzene (PhNO) was obtained from Aldrich and was used as received. SWNTs (90% purity) were obtained from Iljin Nanotech (Korea), and these were prepared by the arc discharge method. The synthesis of di(3-aminopropyl)viologen was as follows. A mixture (1:2 mol ratios) of 4,4′-

10.1021/jp901683g CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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bipyridine and 3-bromopropylamine in acetonitrile solution was stirred for 24 h at 80 °C under a nitrogen atmosphere. As the reaction proceeded, the reaction mixture changed from a white to a more viscous yellow. The resulting solution was poured into excess diethyl ether solution to precipitate the molecules. The precipitated molecules were filtered and dried in air. ITO (10 Ω · cm) glass was purchased from Samsung Corning Co. The ITO substrates were washed with acetone, ethanol, and deionized (18.2 MΩ · cm) water in an ultrasonication bath for 15 min with a final wash in isopropanol. Preparation of Viologen Modified Single-Walled Carbon Nanotubes (V-SWNTs). The V-SWNTs were prepared as reported.9 The SWNTs were oxidized in a concentrated acid mixture of H2SO4/HNO3 ) 3:1 by volume under ultrasonication for 20-24 h at 50-60 °C, to produce shortened SWNTs with terminal carboxylic acid groups. The resulting solution was filtered through a poly(tetrafluoroethylene) membrane with a 100 nm pore size. The filtrate was washed with water by decantation to remove any remaining acid, followed by drying in an oven at 100-110 °C. Then, the V-SWNTs were prepared by dispersing the SWNTs in 2 mM DAPV/0.1 M phosphate buffer (pH 7.0) at room temperature for 1 h. The amide bond between SWNTs and DAPV was formed by a 1-ethyl-3-(3dimethylaminopropyl)carbodiimide (EDC) coupling reaction. Finally, the V-SWNT powder was filtered through a poly(tetrafluoroethylene) membrane with a 100 nm pore size. The concentration of viologen in V-SWNTs is 3.72 × 10-4 mol/g. Synthesis of Ru-Clusters on V-SWNTs. The synthesis of Ru-3 on V-SWNTs was carried out using a modified method recently proposed.13,14 A solution of 0.01 M Ru3(CO)12 in 100 mL of THF was stirred for 1 h under inert atmosphere. Then, the V-SWNT powders were dispersed in the solution for 1 h and washed with THF followed by drying in argon for 10 min. For the preparation of Ru-3 on V- SWNTs, PhNO (12 mg, 0.11 mmol) was added to these solutions to (Ru-1 and Ru-2) on VSWNTs for 1 h at room temperature with stirring. Then, the resulting solution was filtered through a poly(tetrafluoroethylene) membrane with a 100 nm pore size. Measurements. The Ru-clusters/V-SWNTs were characterized using an UV-vis absorption spectrometer, a Fourier transform infrared spectrometer (FT-IR), emission spectrometer, a thermogravimetric analyzer (TGA), Raman, and cyclic voltammetry. The UV-vis absorption spectrometer (Varian, CARY 100), emission spectrometer (ISS Inc.), and FT-IR (FT-IR, ATI Mattson, Genesis series FT-IR) experiments were performed at room temperature. Emission spectra were recorded using a photocounting spectrometer. The thermogravimetric analyses were performed with a TGA Q500 (TA Instruments) at 10 °C/ min under N2. The Raman spectra were recorded with a single monochromator Renishaw System 1000 equipped with a cooled CCD detector (-73 °C) and a holographic super-Notch filter. The holographic Notch filter removes the elastic scattering. The samples were excited with the 514 nm Ar source; the spectral resolution was ca. 3 cm-1. The in situ Raman spectra were recorded under a dry air stream (40 mL/min) for the sample in a commercial hot stage (Linkam TS-1500). The sample was heated, and spectra were taken isothermally with a 10 °C/min heating rate. Film thickness was measured with a surface profiler (Tencor p10, Korea). Electrochemical measurements (BAS 100B, Bioanalytical Systems, Inc.) were used using a single compartment with a standard three-electrode glass cell. The reference electrode was Ag/AgCl, and the counter electrode was Pt wire.

Lee et al. I-V curves of the optoelectronic charge transfer device were obtained at various light intensity values with white light with AM 0 and 1.5 filters as a solar simulator in the presence of a water filter (450 W xenon lamp, Oriel Instruments), and the current was measured with a Kiethley 2400 source meter. Electrodes of optoelectronic photoinduced charge transfer devices used the glass substrates. Gold contacts in optoelectronic photoinduced charge transfer devices were prepared using thermal evaporation in vacuum (10-6 Torr) with a mask. The channel is 200 µm long and 1000 µm wide. For performance measurement of the photoelectrochemical cells, the sandwichtype cells were designed with Ru-3/V-SWNTs/ITO and Ptsputtered ITO glass [electrolyte: LiI of 0.1 mol/L and I2 of 0.05 mol/L in CH3CN]. The Ru-3/V-SWNTs/ITO was irradiated by 100 mW/cm2 white light with AM0 and 1.5 filters as a solar simulator in the presence of a water filter (450 W xenon lamp, Oriel Instruments), and the current was measured with a Kiethley 2400 source meter. The action spectra measurements were carried out without bias illumination with respect to a calibrated MellesFriot silicon diode. Action spectra were measured by changing the excitation wavelength (Photon counting spectrometer, ISS Inc., and Kiethley 2400 source meter). Results and Discussion Synthesis and Characterization of a Ru-Cluster on Viologen Modified SWNTs by in Situ Preparation. The VSWNTs were prepared as reported.9 The concentration of viologen in V-SWNTs is 3.72 × 10-4 mol/g. Scheme 1 shows (a) the schematic diagram of the synthesis of Ru-clusters on V-SWNTs and (b) chemical structures of V-SWNTs, Ru-1, Ru2, and Ru-3. In the organometallic functionalization process, the trirutheniumdodecacarbonyl cluster [Ru3(CO)12] easily reacts with the halide group (Br-) of V-SWNTs, to give Ru-1 on V-SWNTs in THF with a color change to dark orange.13,14 The carbonyl groups of anionic Ru-clusters are generally labile and easily dissociated to form Ru-2 clusters with bridging halide. The halides in the metal clusters are easily interconvertible to the terminal position from the bridging position, which opens the coordination site for the next reaction. Furthermore, nitrosobenzene (PhNO) is reacted with Ru-2/V-SWNTs to render formation of Ru-3 on V-SWNTs.13,14 The Ru-1 and Ru-2 were completely formed within 40 min when Br- and Ru3(CO)12 are mixed. These are generated in situ by simply mixing V-SWNTs in THF solution and Ru3(CO)12, and PhNO is then added by the same method. Rapid and quantitative reactions occur in each step, and imido clusters Ru-3 finally form on the V-SWNTs. The preparation of Ru-clusters with carbonyl ligands is effectively monitored by the comparison of the strong and characteristic IR peaks of carbonyl groups with those of authentic compounds in solution (Supporting Information Figure 1). Table 1 shows infrared (IR) peaks of Ru-clusters on VSWNTs. According to refs 13 and 14, IR peaks of the carbonyl group of Ru-1 reflect contributions at 2059 cm-1 (m), 2026 (vs), 2008 (s), 1993 (m), 1973 (sh), 1963 (sh), and 1829 (m) cm-1, and IR peaks of the carbonyl group of Ru-2 show contributions at 2070 (w), 2026 (m), 1991 (vs), 1952 (m), and 1908 (m) cm-1. However, IR peaks of the bridging carbonyl group in the reaction of Ru3(CO)12 with Br- of V-SWNTs are located at 2086 (w, sh), 2070 (vs), 2054 (w), 2032 (vs), 2013 (s), 1993 (vs), 1973 (w, sh), and 1952 (vs) cm-1. These results indicate that the compound from reaction of Ru3(CO)12 with Br- of VSWNTs leads to a mixture of Ru-1 and Ru-2. Occurrence of a

Charge Transfer System with µ3PhN-Ru3 SWNTs

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SCHEME 1: (a) Schematic Diagram of the Synthesis of Ru-Clusters on V-SWNTs, and (b) Chemical Structures of V-SWNTs, Ru-1, Ru-2, and Ru-3

TABLE 1: IR Bands of Carbonyl Groups of Ru-Clusters compd

V(CO), cm-1 (refs 13 and 14)

Ru3(CO)12 2062 (vs), 2029 (m), 2011 (m) Ru-1a 2059 (m), 2026 (vs), 2008 (s), 1993 (m), 1973 (sh), 1963 (sh), 1829 (m) Ru-2b 2070 (w), 2026 (m), 1991 (vs), 1952 (m), 1908 (m) Ru-3c 2070 (m), 2041 (vs), 2018 (s), 1989 (s), 1941 (w, sh), 1907 (w) a

V(CO), cm-1 (found)

2086 (w, sh), 2070 (vs), 2054 (w), 2032 (vs), 2013 (s), 1993 (vs), 1973 (w, sh), 1952 (vs) 2041 (vs), 2018 (s), 1989 (vs), 1941 (w, sh), 1917 (w, sh)

spectrum of Ru-3 on V-SWNTs shows absorption in the 300-450 nm region. This result implies that the band gap of a Ru-cluster is not significantly changed. Figure 1b shows the emission spectra of Ru-3 and Ru-3 on V-SWNTs in THF. The maximum emission peak of Ru-3 appeared at 430 nm. After modification of V-SWNTs by Ruclusters, the emission gets quenched (75%) quite effectively. The emission quenching results show that the Ru-3 clusters in their excited state are able to interact with the V-SWNTs, possibly by a charge transfer process. We expect photoinduced charge transfer from the lowest unoccupied molecular orbital (LUMO) of Ru-3 and V-SWNTs (eqs 1-3).

[Ru3(Br)(CO)11]-. b [Ru3(µ2-Br)(CO)10]-. c [Ru3(µ3-NPh)(Br)(CO)9]-. hν

mixed product can be attributed to the fact that the reaction is immediately and quantitatively reversed to form Ru-1 when cluster Ru-2 is placed under 1 atm of CO.13,14 After the reaction of PhNO, IR peaks of the carbonyl group of Ru-3 show signatures at 2041 (vs), 2018 (s), 1989 (vs), 1941 (w, sh), and 1917 (w, sh) cm-1. These data match very well to that of reference IR peak data of Ru-3, as previously reported.13,14 The free CO2 is liberated during the formation of Ru-3, which is a driving force for the formation of Ru-3. The loading of Ruclusters is about 8 wt % of the SWNTs, which is measured using thermogravimetric analysis (TGA, Supporting Information Figure 2). To obtain information about the structure properties of Ru-3/SWNTs, we took Raman spectra of the V-SWNTs and Ru-3/SWNTs (Supporting Information Figure 3). However, the D-band of the nanotube does not increase/decrease during the building of the Ru-clusters. This result implied that another reaction was not generated at the nanotube side-walls. Excited State Interactions between Ru-Clusters and VSWNTs. Optical performances of Ru3(CO)12, Ru-3, and Ru-3 on V-SWNTs in THF solution were independently measured. Figure 1a shows the UV-vis absorption spectra of Ru3(CO)12, Ru-3, and Ru-3 on V-SWNTs. The ultraviolet and visible light absorption of Ru-clusters is desirable for light harvesting of solar energy. The Ru-clusters have a high extinction coefficient (∼104 dm3 mol-1 cm-1), which is an attractive property for a photoinduced charge transfer system. The UV-vis absorption

Ru-clusters (e in hHOMO) 98 Ru-clusters* (e in hLUMO)

(1) Ru-clusters* (e in hLUMO) + Viologen2+ f Ru-clusters++ Viologen•+ Viologen•+ + SWNTs f Viologen2+ + SWNTs (e)

(2)

(3)

The electron transfer between organometallic clusters and sensitizer materials has been well established in earlier studies.11,12

Figure 1. (a) UV-vis absorption spectra and (b) emission spectra of Ru3(CO)12 (9), Ru-3 (O), and Ru-3/V-SWNTs (b) in THF.

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SCHEME 2: Schematic Diagram of Ru-3/V-SWNT Films: (a) Optoelectronic Charge Transfer Device Structure, (b) Electronic Diagram, and (c) Photoinduced Charge Transfer

Assignment of the photoinduced electron transfer process in Ru-3 and V-SWNTs is consistent with the excited state interaction of Ru-3 observed earlier with a Ru-complex sensitizer.9 Optoelectronic Properties of Triruthenium Cluster Modified SWNTs. Scheme 2 shows the schematic diagram of the optoelectronic photoinduced charge transfer device structure of Ru cluster/V-SWNT films. In this configuration, the channel is 200 µm long and 1000 µm wide smuch longer than that of the SWNTsshence, the network, rather than the contact resistance between Au and the network, is expected to dominate the overall electrical transport. The thickness of Ru-3/V-SWNTs is about 2.4 µm. The concentration of Ru in (Ru-1 and Ru-2) on V-SWNTs was found to be ∼0.24 × 10-8 mol/cm2, while that of Ru-3 on V-SWNTs was found to be ∼0.26 × 10-8 mol/cm2. The surface concentration of Ru-clusters on V-SWNTs was measured by atomic absorption spectroscopy. Figure 2a shows the I-V curves of the SWNT, V-SWNT, and Ru-3/V-SWNT film in the dark. In appreciating the nature of these results, it should be remembered that the data correspond to a network of SWNTs and represent cumulative effects of the transport across individual SWNTs (ballistic transport in the absence of disorder or in the presence of weak disorder) as well as the junctions between SWNTs. In the absence of the functionalizing molecules, the junctions are direct wall to wall contacts between SWNTs, while, in the case of

Figure 2. (a) I-V curves of the SWNT (9), V-SWNT (b), and Ru3/V-SWNT (2) films in the dark and (b) I-V curves of Ru-3/V-SWNTs under various optical intensities of illumination (I ) dark, 25, 50..., and 200 mW/cm2).

SWNTs functionalized with organic molecules of a few angstrom size on their surface, the inter-SWNT contacts are junctions between organic molecules. Based on the existing literature on metal contacted SWNTs, we expect our case to involve p-type conduction.15-17 The linear I-V characteristic for the bare SWNT film reflects its resistive nature, suggesting ohmicity of the inter-SWNT contacts as well as the noninverting nature of the leads. When functionalized with viologen, the characteristic remains linear; however, the current density is seen to increase for the same voltage. The linearity should imply that the intertube electron transfer involves a weak or negligible barrier. An increase in the current density implies effective electron transfers at junctions between organic molecules compared to the wall to wall contact of bare SWNTs, and also an increase in hole carrier density in the SWNTs by electron transfer to viologen molecules. Such a mechanism has been reported in the case of a carbon nanotube and organic compound composite.15-18 One must also consider the possibility of disorder introduced at the molecular anchoring site due to adsorption. Had viologen molecular anchoring on SWNTs caused localized disorder, one would have seen a substantial decrease (and not increase as observed) of the transport current density. Thus, the disorder if present is either absent or is weak and therefore does not modify the ballistic nature of electron transport.15-17 When the V-SWNT system is further functionalized with the organometallic Ru-clusters, the trend is seen to reverse. As seen from Figure 2a, the current density is now seen to decrease (increase in resistivity). Since the organometallic compound loading does not directly connect to the SWNT surface, an increase in the corresponding resistance in this case should represent depletion of hole density on the SWNTs by electron doping from the organometallic Ru-clusters. Given the electronic connectivity of the three systems, the relative line up of the electronic states is a key to the reversal of the electron transfer process in the presence of the organometallic Ruclusters. One must note that this transfer is in the absence of any excitation. One cannot also ignore the possibility that organometallic cluster loading on V-SWNTs can contribute to

Charge Transfer System with µ3PhN-Ru3 SWNTs

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Figure 4. (a) Photocurrent and (b) action spectra of photoelectrochemical cells based on Ru-3/V-SWNTs on ITO.

Figure 3. (a) Photocurrent of optoelectronic photoinduced charge transfer based on Ru-3/V-SWNT composite films (I ) 100 mW/cm2 and VSD ) 3 V), (b) the currents of the devices vs time with different optical intensities (VSD ) 3 V and I from 50 to 200 mW/cm-2), and (c) the band line up of currents vs time with various optical intensities [SWNTs (9), V-SWNTs (b), and Ru-3/V-SWNTs (2)].

an increase in disorder on the SWNT surface and hence a decrease in current density. Figure 2b shows the photoinduced changes in the transport of SWNTs loaded with viologen molecules/organometallic Ruclusters for a range of optical intensities. In this experiment, white light in a Xe lamp was used with air mass (AM) 1.5 conditions. Interestingly, it can be seen that under optical illumination the transport characteristic becomes progressively resistive and begins to show a saturation tendency for an illumination density of ∼125 mW/cm2. We made similar measurements for the SWNTs and V-SWNT films as well to elucidate the importance of different components of this coupled system separately. Temporal Evolution of Photoinduced Change Transfer. Figure 3a shows the temporal evolution of the photoinduced change of the current of Ru-3/V-SWNT composite films (source: xenon lamp, I ) 100 mW/cm2 and VSD ) 3 V). Figure 3b shows the current of Ru-3/V-SWNTs vs time for various optical densities, and the corresponding optical density dependence of response is summarized in Figure 3c. From Figure 3a, it can be seen that, upon switching on the illumination, the current drops rather sharply by about 8% (fast response), followed by a further relatively gradual drop (∼35%) over a time scale of a couple of seconds (slower response). The fast drop may represent initial efficient electron transfer while the slower drop may correspond

to accumulation effects. The initial fast response can benefit switching type charge transfer device operations while the slower but enhanced response may be useful in storage applications where speed is not a criterion. From Figure 3b, it may be noted that the response of unfunctionalized SWNTs is very feeble while that of viologen loaded SWNTs is slightly stronger but still quite weak. Both correspond to a photoinduced decrease in current density or an increase in resistivity. The response of Ru-clusters in loaded V-SWNT systems is clearly seen to be considerably stronger as compared to the other cases.9,10 First, these data clearly show that the optical response in SWNTs emanates only by a charge transfer process and does not represent the standard photoconductivity effect of SWNTs. The fact that the photoresponse of V-SWNTs and Ru-cluster/ V-SWNT systems is in the same direction further suggests the line up of the “excited” electronic states between the Ru-cluster and V-SWNTs. Such a line up facilitates electron transfer directionally toward SWNTs, reducing the corresponding hole density. The band line up sketched in Scheme 2b based on the existing knowledge about the participating components supports this view.9 The electronic state of Ru-3 was measured by UV-vis absorption, emission spectra, and cyclic voltammetry (Figure 1 and Supporting Information Figure 4). Also, a much stronger response observed for the organometallic Ru cluster loaded system emphasizes the beneficial role of organometallic clusters as a coupler of optical density. Given the nature of the excited state interaction between Ruclusters and V-SWNTs involving electron transfer, the Ru-3/ V-SWNT system was applied to the photoelectrochemical cells. A sandwich-type photoelectrochemical cell was assembled with Ru-3/V-SWNTs on ITO and Pt-sputtered ITO substrate (electrolyte: 0.3 M LiI and 30 mM I2 in CH3CN, 0.09 cm2). The Ru-3/V-SWNTs films (1.5 × 2.5 cm2) were illuminated by 100 mW/cm2 white light with AM 0 and 1.5 filters as a solar simulator. After absorption of a photon, the excited electron within the Ru-cluster is transferred to the LUMO level of viologen and/or work function of metallic SWNTs and diffuses through the V-SWNT films to the ITO electrode. The oxidized Ru-cluster is reduced to the original state by the supply of electrons through a liquid electrolyte redox couple. As shown in Figure 4a, the Ru-clusters/V-SWNTs on the ITO system generate photocurrent upon illumination. The photocurrent increases immediately upon illumination and reverts back to its original value when the light is off. The photocurrent for the case of Ru-3/V-SWNTs is 15 nA/cm2, which is higher than that of the Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2/ di(3-aminopropyl)viologen/SWNTs system.9 Figure 4b shows the action spectra of Ru-clusters/V-SWNT composite films. The action spectra can be converted to the quantum yields. The quantum yields (Φ) were determined in terms of the wavelength using photocurrent density, absorbance on the electrode, and input power at the applied potential.19

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Φ ) (i/e)/I(1 - 10-A), I ) Wλ/hc

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(4)

where i is the photocurrent density, e the elementary charge, I the number of photons per unit area and unit time, λ the wavelength of light irradiation, A the absorbance of the adsorbed dyes at λ nm, W the light power irradiated at nm, c the light velocity, and h Planck’s constant. The quantum yield of Ru-3/ V-SWNTs was measured to be 1.3%, which is larger than that of photoelectrochemical cells based on porphyrin self-assembled monolayers (0.1%).20 The action spectrum of Ru-3/V-SWNT films is similar to the UV-vis absorption spectra of Ru-3/VSWNTs, which implies that the photocurrent is generated from Ru-clusters. The maximum photocurrent for Ru-3/V-SWNTs is 15 nA/cm2 (λ300 nm). This result is also generated by photoinduced charge transfer. Conclusions In conclusion, modification of SWNTs is realized by an organometallic click chemistry approach, and the same is examined for optoelectronic photoinduced charge transfer device applications. Following the formation/installation of viologen on SWTNs, Ru3(CO)12 was easily reacted with Br- of viologen, leading to a mixture of Ru-1 and Ru-2. Ru-3 was then synthesized on V-SWNTs by adding PhNO. Upon illumination, the current of linear I-V curves of Ru-3/V-SWNT films is seen to decrease for the same voltage. Furthermore, at the same voltage, currents of optoelectronic devices based on Ru-3/VSWNT films are controlled by optical intensities, which is an optical gate effect. In photoelectrochemical cells, the photocurrents obtained for Ru-3/V-SWNTs is 15 nA/cm2. The quantum yield realized for the case of Ru-3/V-SWNTs is 1.3%. The results presented in this study can thus pave the way for easy design of functional nanocomposites based upon metal clusters and carbon nanotubes. The investigation of the electron lifetime of the Ru-3/V-SWNTs by the time-resolved photoluminescence technique is currently underway. Acknowledgment. This research was supported by Nano R&D program through the National Research Foundation of Korea funded by the Ministry of Education, Science and

Technology (2009-008-3214). S.B.O. would like to acknowledge support of the Department of Information Technology, Government of India. Supporting Information Available: IR spectra, TGA, Raman spectra, and cyclic voltammogram. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425–2427. (2) Cao, J.; Wang, Q.; Dai, H. Nat. Mater. 2005, 4, 745–749. (3) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455–459. (4) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105–1136. (5) Ajayan, P. M. Chem. ReV. 1999, 99, 1787–1800. (6) Umeyama, T.; Imahori, H. Energy EnViron. Sci. 2008, 1, 120– 133. (7) Guldi, D. M.; Aminur Rahman, G. M.; Sgobba, V.; Ehli, C. Chem. Soc. ReV. 2006, 35, 471. (8) Britz, D. A.; Khlobystov, A. N. Chem. Soc. ReV. 2006, 35, 637– 659. (9) Lee, W.; Lee, J.; Lee, S.-H.; Chang, J.; Yi, W.; Han, S.-H. J. Phys. Chem. C 2007, 111, 9110–9115. (10) Aminur Rahman, G. M.; Guldi, D. M.; Campidelli, S.; Prato, M. J. Mater. Chem. 2006, 16, 62–65. (11) Park, J. T.; Cho, J.-J.; Song, H.; Jun, C.-S.; Son, Y.; Kwak, J. Inorg. Chem. 1997, 36, 2698–2699. (12) Cho, Y.-J.; Ahn, T. K.; Song, H.; Kim, K. S.; Lee, C. Y.; Seo, W. S.; Lee, K.; Kim, S. K.; Kim, D. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 2380–2381. (13) Han, S.-H.; Song, J.-S.; Macklin, P. D.; Nguyen, S. T.; Geoffroy, G. L. Organometallics 1989, 8, 2127–2138. (14) Song, J.-S.; Han, S.-H.; Nguyen, S. T.; Geoffroy, G. L. Organometallics 1990, 9, 2386–2395. (15) Allen, B. L.; Kichambare, P. D.; Star, A. AdV. Mater. 2007, 19, 1439–1451. (16) Lee, J.; Lee, W.; Sim, K.; Han, S.-H.; Yi, W. J. Vac. Sci. Technol. B 2008, 26, 847–850. (17) Oh, J.; Roh, S.; Yi, W.; Lee, H.; Yoo, Y. J. Vac. Sci. Technol. B 2004, 22, 1416–1419. (18) Ferrer-Anglada, N.; Kaempgen, M.; Ska´kalova´, V.; Dettlaf-Weglikowska, U.; Roth, S. Diamond Relat. Mater. 2004, 13, 256–260. (19) Lee, W.; Mane, R. S.; Lee, S.-H.; Han, S.-H. Electrochem. Commun. 2007, 9, 1502–1507. (20) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335–5338.

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