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Enhanced Photocurrent Generation in Nanostructured Chromophore/Carbon Nanotube Hybrid Layer-by-Layer Multilayers Akira Baba,*,† Taihei Matsuzawa,† Saengrawee Sriwichai,†,‡ Yasuo Ohdaira,† Kazunari Shinbo,† Keizo Kato,† Sukon Phanichphant,‡ and Futao Kaneko† Center for Transdisciplinary Research, and Graduate School of Science and Technology, Niigata UniVersity, 8050 Ikarashi 2-nocho, Nishi-ku, Niigata 950-2181, Japan, and Department of Chemistry and Center for InnoVation in Chemistry, Faculty of Science, Chiang Mai UniVersity, Chiang Mai 50200, Thailand ReceiVed: April 7, 2010; ReVised Manuscript ReceiVed: July 12, 2010
In this paper, we demonstrate photocurrent generation from nanostructured layer-by-layer (LbL) ultrathin films consisting of chromophores and single-walled carbon nanotubes (SWNT). We fabricated 5,10,15,20tetrakis(1-methyl-4-pyridinio)porphyrin tetr (p-toluenesulfonate) (TMPyP)-SWNT/sodium copper chlorophyllin (SCC)-SWNT LbL film from noncovalently adsorbed composites. SWNT were dissolved in water-soluble cationic TMPyP and anionic SCC, and the resulting solutions were used for electrostatic LbL multilayer fabrication. The solubility of SWNT in water was studied by UV-vis absorption spectroscopy. The composites were highly dispersed owing to the π-π interactions. The fluorescence spectroscopy measurements showed efficient quenching of TMPyP and SCC fluorescence, which was due to the interaction with SWNT. In situ surface plasmon resonance spectroscopy during the LbL multilayer fabrication indicated a stepwise increase in reflectivity, implying the successive formation of nanostructured hybrid ultrathin films. Cyclic voltammetry revealed that the electroactivity of the hybrid film was enhanced by the incorporation of SWNT. The composite LbL film electrode exhibited an enhancement of photocurrent compared to a TMPyP/SCC (no SWNT) film electrode, suggesting efficient charge separation and electron transfer in the system. Introduction Assembling composites of chromophores and carbon nanotubes is a key approach for constructing a wide range of applications, such as photoelectric conversion devices, biosensors, and electron storage devices.1,2 To assemble such composites, it is important to develop a method of solubilizing carbon nanotubes because they have low solubility in most solvents. Many groups have explored the solubilization properties of carbon nanotubes after chemical modifications involving their covalent bonding to organic materials.3,4 Carbon nanotubes with covalently linked porphyrin antennae have been developed as potential supramolecular donor-acceptor complexes for applications such as photovoltaic devices and light-harvesting systems.5 There have also been reports of noncovalent functionalizations of single-walled carbon nanotubes (SWNT) with aromatic compounds such as porphyrin or polyfluorene.6-9 We have previously reported the micro/nanopatterning of SWNTphthalocyanine composites using microcontact printing, dippen nanolithography, and fountain-pen nanolithography.10 The solubilization mechanism involved π-π interactions between the side walls of the SWNT and aromatic compounds. These composites have been studied for possible applications in photochemical solar cells because the chromophore-SWNT composites function as a donor-acceptor system. Recently, photochemical solar cells made from porphyrin-SWNT assemblies prepared by electrochoretic deposition technique have been reported.11 * To whom correspondence should be addressed. E-mail: ababa@ eng.niigata-u.ac.jp. † Niigata University. ‡ Chiang Mai University.
Although fabricating ultrathin films from SWNT-chromophore composites is an important challenge in the development of optoelectronic device applications, there have been few reports on nanostructured assembled ultrathin films, particularly ultrathin films fabricated from noncovalently adsorbed carbon nanotube-chromophore composites. Recently, we reported a convenient method of fabricating SWNT-phthalocyanine hybrid multilayered ultrathin films.12 The technique involves direct adsorption and the electrostatic layer-by-layer (LbL) deposition of SWNT and water-soluble positively and negatively charged phthalocyanine molecules. In this paper, we report an enhancement of the photocurrent generation in the nanostructured choromophores-SWNT ultrathin films by using a self-assembly LbL approach. The LbL self-assembly method, initially reported by Decher, is one of the most convenient techniques for fabricating molecularly controlled ultrathin multilayer films.13 The adsorption process involves the alternate deposition of cationic and anionic species from a solution.14 We used both positively and negatively charged water-soluble porphyrin and chlorophyllin molecules for the solubilization of the SWNT. To investigate the composite ultrathin films properties, we employed surface plasmon spectroscopy, UV-vis spectroscopy, and fluorescence spectroscopy as well as studied the cyclic voltammetric properties. The photocurrent measurements were performed in photoelectrochemical cells in KCl aqueous solution with methyl viologen from chromophore-SWNT composite films on gold electrode as the electron acceptor molecule. Experimental Section Materials. SWNT were purchased from Microphase Co. We obtained 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) and sodium copper chlorophyllin (SCC) from Aldrich (shown in Figure 1). For the function-
10.1021/jp103121m 2010 American Chemical Society Published on Web 08/17/2010
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Figure 1. Schematic drawing of the fabrication of nanostructured 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP)-SWNT/sodium copper chlorophyllin (SCC)-SWNT LbL films.
alization of the gold and glass slide substrates, 3-mercapto-1propanesulfonic sodium and (3-aminopropyl)triethoxysilane (APS), obtained from Aldrich, were used, respectively. Methyl viologen was also obtained from Aldrich. Surface Plasmon Resonance Spectroscopy Measurements. To excite the surface plasmons, we used an attenuated total reflection setup in the Kretschmann configuration, combined with a Teflon cell. A half-cylindrical SF11 prism was used. The Au/glass substrates were clamped against the Teflon cell with an O-ring, which provided a liquid-tight seal. The Teflon cell was then mounted on a 2-axis goniometer for surface plasmon resonance spectroscopy (SPR). Surface plasmons were excited by reflecting p-polarized laser light from the Au-coated base of the prism. The excitation source was a He-Ne laser with λ ) 632.8 nm. To monitor the formation of the LbL film, kinetic measurements were performed with reflectivity changes as a function of time. Angular measurements were taken by monitoring the reflectivity as a function of the incident angle θ0. The gold film thickness (∼47 nm) used in these experiments was chosen for optimum excitation of the surface plasmons. Details of the electrochemical-SPR setup can be found elsewhere.15 Solubilization of Chromophore-SWNT Composites. To solubilize the SWNT with organic dyes, 0.25-mg/mL SWNT and 0.25-mg/mL TMPyP or SCC were mixed in deionized water, then sonicated for several hours. The resulting solution was centrifuged at 2500 rpm for 1 h to remove the aggregates or bundled complexes. The supernatant of the solution was used for the LbL deposition. The solubility of TMPyP-SWNT and SCC-SWNT composites was investigated by UV-vis absorption spectroscopy. Layer-by-Layer Deposition. The LbL deposition of TMPyPSWNT and SCC-SWNT was performed by using the Decher approach.14 The gold surface of the flat solid substrate was functionalized by immersing the slide for 1 h in an ethanol solution of 3-mercapto-1-propanesulfonic sodium salt (10 mg/ mL), followed by rinsing, thus creating an Au/Cr/glass substrate surface with a uniform negative charge. The bare glass substrates were functionalized by APS (0.1% in toluene). The APS layer was charged by immersing the substrate in dilute HCl solution, which was used immediately for preparing the LbL ultrathin film. The Au/Cr/glass substrates or glass substrates with functionalized surfaces were immersed in aqueous solutions of TMPyP-SWNT and SCC-SWNT alternately for 20 min each until the desired number of layers was achieved. Two 2-min rinses with deionized water were performed between depositions. Cyclic Voltammetry. Cyclic voltammetry measurements were performed with a one-compartment, three-electrode cell driven by an HZ-5000 potentiostat (Hokuto Denko Ltd., Japan). In all measurements, the working electrodes consisted of gold
Figure 2. UV-vis absorption properties. (a) TMPyP (0.25 mg/mL) and TMPyP-SWNT (0.25 mg/mL) composites after sonication for 30 min, 3 h, and 5 h. Each composite was centrifuged at 2500 rpm for 1 h in an aqueous solution. (b) SCC (0.25 mg/mL) and SCC-SWNT (0.25 mg/mL) composites after sonication for 30 min, 3 h, and 5 h. Each composite was centrifuged at 2500 rpm for 1 h in aqueous solution.
films (d ≈ 50 nm) that had been vacuum evaporated onto a glass substrate (with an adhesion layer of 2 nm of Cr previously evaporated onto the glass substrate). The counter electrode was a platinum wire and the reference electrode was an Ag/Ag+ nonaqueous electrode. The surface area of the gold electrode was 0.785 cm2. Photocurrent Measurements. Photocurrent measurements were performed in a photoelectrochemical cell controlled by the same system as used for the cyclic voltammetry measurements. The electrolyte was 0.1 M methyl viologen and 5 mM KCl in deionized water. A collimated light from a 350 mW xenon lamp with wavelength from 490 to 740 nm was used for the excitation of the TMPyP-SWNT/SCC-SWNT LbL films on gold electrodes. Results and Discussion Solubilization of SWNT with Water-Soluble Porphyrin. We first examined the solubility of the TMPyP-SWNT and SCC-SWNT composites in aqueous solution by using UV-vis absorption spectroscopy. Figure 2a shows the UV-vis absorption properties of the TMPyP (0.25 mg/mL) and TMPyP-SWNT (0.25 mg/mL) composites after sonication for 30 min, 3 h, and 5 h. Each composite was centrifuged at 2500 rpm for 1 h in an aqueous solution. Figure 2b shows the spectra for the SCC (0.25 mg/mL) and SCC-SWNT composites after sonication/centrifugation. Peaks due to the Soret band (400-450 nm) and the Q-band (500-540 and 570-600 nm for TMPyP and 620-650 nm for SCC) can be clearly observed in the absorption spectra of both composites. After the sonication/centrifugation, an increase in the baseline of the broad absorption band was observed, indicating that both metallic SWNT (400-600 nm
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Figure 4. In situ surface plasmon resonance kinetic reflectivity curve during the deposition of TMPyP-SWNT/SCC-SWNT LbL film.
Figure 3. Photoluminescence properties. (a) TMPyP (0.25 mg/mL) and TMPyP-SWNT (0.25 mg/mL) composites after sonication for 1, 3, and 5 h. Each composite was centrifuged at 2500 rpm for 1 h in aqueous solution. (b) SCC (0.25 mg/mL) and SCC-SWNT (0.25 mg/ mL) composites after sonication for 1, 3, and 5 h. Each composite was centrifuged at 2500 rpm for 1 h in aqueous solution.
region, corresponding to M11) and semiconducting SWNT (600-950 nm, corresponding to S22 transitions) were complexed with both TMPyP and SCC molecules. The baseline gradually increased with increasing sonication time, indicating that the complexization was enhanced by the sonication. On the other hand, adding SWNT to TMPyP or to SCC resulted in decreases in the peak intensity of both the Soret band and the Q-band, accompanied by a red shift of approximately 0.5 nm. These results suggest that an interaction between the SWNT and the TMPyP or SCC was induced. The decrease in the peak intensities is likely due to a decrease in the density of trapped electrons in TMPyP or SCC because the electrons are transferred to the SWNT. Furthermore, we observed isosbestic points at 610 nm for TMPyP-SWNT and at 333, 430, 615, and 643 nm for SCC-SWNT composites. Similar results were recently reported for SWNT-imidazole-porphyrin,16 SWNT-methyl viologen,17 and SWNT-TiO2 composites,18 which acted as donor-acceptor nanohybrid or electron storage systems in the SWNT with Fermi level equilibration. Next, we studied the steady-state fluorescence spectra of TMPyP-SWNT and SCC-SWNT excited at around 400 nm in aqueous solution. Figure 3a shows the fluorescence properties of the TMPyP (0.25 mg/mL) and TMPyP-SWNT (0.25 mg/ mL) composites after sonication for 1, 3, and 5 h. Each composite was centrifuged at 2500 rpm for 1 h in an aqueous solution. Figure 3b shows the spectra for the SCC (0.25 mg/ mL) and SCC-SWNT composites after sonication/centrifugation. Fluorescence peaks can be clearly seen at 470, 557, and 610 nm for TMPyP and at 550 nm for SCC. After they are formed into composites with SWNT, a clear decrease in the peaks is observed for both cases. The quenching was found to be larger for the SCC-SWNT composite than for the TMPyP-SWNT composite when the same amount of SWNT was added and the sonication time was the same. Fluorescence was quenched
by about half in the SCC-SWNT composite formed after sonication for 5 h. Although the TMPyP-SWNT complex showed a larger decrease in absorbance than that of the SCC-SWNT one, the fluorescence quenching of the SCC-SWNT complex was more effective. The reason for this is not clear at this point; however, the dominant quenching process should involve the interaction between TMPyP or SCC and SWNT. In general, energy transfer requires a good overlap between the absorption range of the acceptor and of the light-harvesting chromophore (donor). In this case, one possible explanation for the quenching is that the charge separation between either TMPyP or SCC and SWNT is generated in the donor-acceptor system, as shown in the inset. We investigate this further in a later section on electrochemistry and photocurrent measurement in the multilayer systems. Fabrication of LbL Ultrathin Films. We used SPR spectroscopy to study the in situ adsorption process of the composites. Figure 4 shows the change in reflectivity measured in situ by SPR at a fixed incident angle of 53.5° as multilayers of up to 10 layers were fabricated. The reflectivity change in the first bilayer that can be seen in this figure indicates a large amount of adsorption onto the surface. This result suggests that the TMPyP-SWNT and SCC-SWNT composites were poorly charged and so the amount of deposition on the APS functionalized surface was larger than the deposition on the composites. Abrupt increases in the second and sixth bilayers of TMPyPSWNT were observed. This is probably due to the adsorption of aggregated TMPyP-SWNT composites, which were not dispersed well in the solution and not removed by the centrifugation. However, the decrease in the reflectivity after the rinsing indicates that the adsorbed aggregation was removed during the process; hence, the linear increase in average reflectivity in multilayers of up to 10 bilayers, as indicated by the linear line in the figure, was observed. For each bilayer, the change in reflectivity for the adsorption of TMPyP-SWNT was larger than the change for SCC-SWNT, indicating that a larger amount of SCC-SWNT was deposited. This might be due to the charge balance between the TMPyP and SCC molecules. The decrease in reflectivity during the deposition of SCC-SWNT indicates that some desorption of previously deposited TMPyPSWNT composites occurs during this process. The kinetic data also show that the thickness of each layer can be controlled by changing the immersion and rinsing times for each composite. Since each monolayer of the LbL film consists of chromophoreSWNT composites, the packing density might be markedly affected by the immersion time. We also performed an angular SPR scan and a theoretical fitting to study the thickness of the film. The film thickness for the 10-bilayer film was estimated
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Figure 5. UV-vis spectrum of TMPyP-SWNT/SCC-SWNT LbL film on a glass slide as a function of the number of bilayers (0.5 bilayer indicates 1 layer of TMPyP-SWNT or 1 layer of SCC-SWNT). The inset shows the absorbance of the UV-vis peaks at 410 and 642 nm as a function of the number of bilayers.
to be approximately 5.1 nm on the assumption of a dielectric constant of 1.9 + i0.054. We performed UV-vis spectroscopy to study LbL multilayer formation from the TMPyP-SWNT and SCC-SWNT composites. The results are shown schematically in Figure 1, where 0.5 bilayer indicates a layer of either the TMPyP-SWNT or the SCC-SWNT composite. As can be seen in Figure 5, the change in the absorbance of the first bilayer was larger than the change in the absorbance of the subsequent bilayers; this result agreed with the result of in situ SPR kinetic study. As shown in the inset, beginning with the second bilayer, the UV-vis absorbance exhibited a linear increase with the number of bilayers, suggesting a successive deposition of the film during the assembly of the multilayers. As discussed in the previous section, the UV-vis spectra of the TMPyP-SWNT and SCCSWNT in solution contained Solet peaks at approximately 405 and 422 nm, respectively. By contrast, the Soret absorption band in the multilayered film appeared at 410 nm, almost a superposition of the peaks of the TMPyP and the SCC molecules. The Q-band in the multilayered film was observed at 537 and 646 nm, while in aqueous solution it was observed at 519 nm for SCC and at 628 nm for TMPyP. The amount of red shift is about 18 nm, indicating that the chromophore aggregation originates from dipole-dipole interaction. Cyclic Voltammetry of LbL Ultrathin Films. To study the effect of the SWNT in the composite LbL system, we fabricated three different structures: 10-bilayer TMPyP/SCC films without SWNT, films with 5 TMPyP-SWNT/SCC-SWNT bilayers, and 5-bilayer TMPyP/SCC (50% composite) and 10-bilayer TMPyPSWNT/SCC-SWNT (100% composite) composite films, as shown schematically in the inset of Figure 6. Cyclic voltammetry for these three structures was performed in acetonitrile with a supporting electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a function of scan rate from 20 to 300 mV/s in the potential range of 0 to 0.9 V. As shown in the figure, an oxidation peak appeared at approximately 0.65 V for the LbL films with SWNT, while the TMPyP/SCC film exhibited a low oxidation/reduction current. Since the SWNT do not exhibit a redox peak in this range, the result indicates that SWNT can enhance the electroactivity of TMPyP and SCC multilayer films fabricated by the proposed technique. With increasing scan rate, the peak current gradually shifted toward a higher potential, indicating the kinetic limitations of the system.19 The peak current of the SWNT composite film was larger than that of the film without SWNT. As the film thickness was estimated to be similar for each structure, the enhanced electroactivity in the film should be attributed to the effect of
Figure 6. Cyclic voltammetry properties of three different film structures: 10-bilayer TMPyP/SCC films, films with 5 TMPyP-SWNT/ SCC-SWNT bilayers and 5 TMPyP/SCC bilayers, and 10-bilayer TMPyP-SWNT/SCC-SWNT composites. Performed in acetonitrile containing 0.1 M TBAPF6 at scan rates from 20 to 300 mV/s.
Figure 7. Relationship between anodic peak current and scan rate.
the SWNT. From these peak currents, the scan rate dependence was plotted for each structure and the results are shown in Figure 7. As shown in this figure, all the plots are linear functions of the scan rate, which indicates a surface-controlled process for all structures.20 Furthermore, the slopes obtained from the peak currents indicate that the chromophore-SWNT composite film has a high electron transfer rate.21 Photocurrent Properties of LbL Films. To study the photocurrent performance of the TMPyP-SWNT/SCC-SWNT LbL films, we prepared a photoelectrochemical cell. The
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Figure 8. Photocurrent response upon irradiation by visible light (490-740 nm) to 10 and 20 bilayers of TMPyP-SWNT/SCC-SWNT and TMPyO/SCC (no SWNT) LbL film measured at -0.2 V.
photocurrent measurements were performed in KCl (5 mM) and methyl viologen (100 mM) aqueous solution. Methyl viologen was used as an electron acceptor molecule. Figure 8 shows the photocurrent response of 10 and 20 bilayers of TMPyP-SWNT/ SCC-SWNT LbL film upon irradiation by visible light (490-740 nm), measured at -0.2 V. For comparison, the photocurrent response in 10 and 20 bilayers of TMPyP/SCC LbL film (no SWNT) was also measured under the same conditions (Figure 8). As shown in this figure, upon irradiation by visible light in the Q-band absorption region of chromophores, the cathodic current increased and when the irradiated light was turned off, the current shifted back to almost the initial level, although some fluctuation was observed. The amount of current increase for the TMPyP-SWNT/SCC-SWNT LbL film electrodes was 1.5-2 times more than the increase for the TMPyP/SCC LbL film, indicating that the SWNT is responsible for the enhancement of the photocurrent generation in the chromophore-layered films. These results suggest that the noncovalently adsorbed SWNT with chromophores provide effective charge separation in LbL films upon irradiation of visible light in the Q-band absorption region. Once the excited electrons reach the SWNT from the choromophores, they are effectively transferred to methyl viologen in the solution. A schematic illustration of the photocurrent generation at the chromophores-SWNT electrodes is shown in Figure 9a.22,23 It should be noted that the photocurrent generated in the multilayered system increases as the number of bilayers increases. Since the mechanism of the charge separation from the photoexcited state and the charge transfer to the methyl viologen is responsible for the presence of the SWNT, there should be SWNT pass-way to methyl violegen electrolyte solution in the multilayered system. One possibility is that the TMPyP-SWNT/SCC-SWNT LbL films form a bulk heterolike structure, so that the charge separation and the electron transfer can be generated in the multilayered system. A schematic illustration of this is shown in Figure 9b. This explanation is reasonable because the chromophore-SWNT composite is not a well-ordered film structure.11 Furthermore, it is well-known that LbL films often have interpenetrated
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Figure 9. (a) A schematic illustration of photocurrent generation at the chromophore-SWNT electrodes and (b) SWNT pass-way to electrolyte in bulk heterolike structure of TMPyP-SWNT/SCC-SWNT multilayered LbL film.
structures.24,25 Another possibility is that the electrons are transferred from SWNT to methyl viologen inside the film, since such small molecules can exist inside LbL films in the electrolyte solution. This is reasonable because LbL films usually contain electrolytes in aqueous solution.26 These results indicate that the TMPyP-SWNT/SCC-SWNT LbL multilayered film should have potential for effective photocurrent generation. Conclusions We have demonstrated an enhanced photocurrent generation in TMPyP-SWNT/SCC-SWNT LbL films. The TMPyP-SWNT/ SCC-SWNT LbL films were fabricated with a convenient method that uses noncovalently adsorbed carbon nanotubechromophore composites. SWNT were solubilized with watersoluble charged TMPyP and SCC molecules, which were then used for electrostatic LbL multilayer fabrication. In situ SPR spectroscopy during the LbL multilayer fabrication indicated a stepwise increase in reflectivity, implying the successive formation of nanostructured hybrid ultrathin films. UV-vis spectroscopy revealed the constant deposition of composite films. Cyclic voltammetry showed that the electroactivity of the hybrid film is enhanced by the incorporation of SWNT in the layered film. The slopes obtained from the peak currents for three film structures indicate that the SWNT facilitate electron transfer to the electrode/electrolyte solution. TMPyP-SWNT/SCC-SWNT LbL film electrodes exhibit an enhancement of photocurrent generation as compared with TMPyP/SCC (no SWNT) film electrodes, suggesting efficient charge separation and electron transfer in the system. These results suggest that the noncovalently adsorbed carbon nanotube-chromophore composite is effectively assembled to form nanostructured films, which should provide new opportunities for photoelectric conversion devices. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science and a Grant for the Promotion of
Photocurrent Generation from Nanostructured LbL Films Niigata University Research Projects. Partial funding support from the Center for Innovation in Chemistry (PERCH-CIC), the office of the Higher Education Commission, Ministry of Education is also acknowledged. S.S and S.P would like to thank the office of the Higher Education Commission, Ministry of Education, Thailand for a research scholarship. Supporting Information Available: UV-vis absorption properties of LbL films as a function of the concentration of SWNT. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, S.; Mannsfeld, S. C. B.; LeMieux, M. C.; Lee, H. W.; Bao, Z. Appl. Phys. Lett. 2008, 92, 053306. (2) Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Kam, N. W. S.; Shim, M.; Li, Y. M.; Kim, W.; Utz, P. J.; Dai, H. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4984. (3) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (4) Baskaran, D.; Mays, J. W.; Zhang, X. P.; Bratcher, M. S. J. Am. Chem. Soc. 2005, 127, 6916. (5) Guldi, D. M.; Rahman, G. M. A.; Sgobba, V.; Ehl, C. Chem. Soc. ReV. 2006, 35, 471. (6) Murakami, H.; Nomura, T.; Nakashima, N. Chem. Phys. Lett. 2003, 378, 481. (7) Tomonari, Y.; Murakami, H.; Nakashima, N. Chem.sEur. J. 2006, 12, 4027. (8) Nakashima, N. Sci. Technol. AdV. Mater. 2006, 7, 609. (9) Nish, A.; Hwang, J. Y.; Doig, J.; Nicholas, R. J. Nat. Mater. 2007, 2, 640. (10) Baba, A.; Sato, F.; Fukuda, N.; Ushijima, H.; Yase, K. Nanotechnology 2009, 20, 085301. (11) Hasobe, T.; Fukuzumi, S.; Kamat, P. J. Phys. Chem. B 2006, 110, 25477.
J. Phys. Chem. C, Vol. 114, No. 35, 2010 14721 (12) Baba, A.; Kanetsuna, Y.; Sriwichai, S.; Ohdaira, Y.; Shinbo, K.; Kato, K.; Phanichphant, S.; Kaneko, F. Thin Solid Films 2010, 518, 2200. (13) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (14) (a) Baba, A.; Kaneko, F.; Advincula, R. C. Colloids Surf. A 2000, 173, 39. (b) Baba, A.; Locklin, J.; Xu, R. S.; Advincula, R. J. Phys. Chem. B 2006, 110, 42. (c) Advincula, R.; Park, M. K.; Baba, A.; Kaneko, F. Langmuir 2003, 19, 654. (d) Sriwichai, S.; Baba, A.; Deng, S. X.; Huang, C.; Phanichphant, S.; Advincula, R. C. Langmuir 2008, 24, 9017. (15) (a) Baba, A.; Lu¨bben, J.; Tamada, K.; Knoll, W. Langmuir 2003, 19, 9058. (b) Baba, A.; Sano, Y.; Ohdaira, Y.; Shinbo, K.; Kato, K.; Kaneko, F. IEICE Trans. Electron. 2008, E91C, 1881. (c) Baba, A.; Tian, S. J.; Stefani, F.; Xia, C. J.; Wang, Z. H.; Advincula, R. C.; Johannsmann, D.; Knoll, W. J. Electroanal. Chem. 2004, 562, 95. (d) Baba, A.; Knoll, W. J. Phys. Chem. B 2003, 107, 7733. (16) Chitta, R.; Sandanayaka, A. S. D.; Schumacher, A. L.; D’Souza, L. D.; Araki, Y.; Ito, O.; D’Souza, F. J. Phys. Chem. C 2007, 111, 6947. (17) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Balbinot, D.; Tagmatarchis, N.; Prato, M. Chem. Commun. 2005, 15, 2038. (18) Kongkanand, A.; Kamat, P. V. ACS Nano 2007, 1, 13. (19) Vasantha, V. S.; Chen, S. M. Electrochim. Acta 2005, 51, 347. (20) Baker, R.; Wilkinson, D. P.; Zhang, J. Electrochim. Acta 2008, 53, 6906. (21) Yogeswaran, U.; Chen, S. M. Electrochim. Acta 2007, 52, 5985. (22) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J. Phys. Chem. B 2000, 104, 1253. (23) Guldi, D. M.; Rahman, G. M. A.; Prato, M.; Jux, N.; Qin, S.; Ford, W. Angew. Chem., Int. Ed. 2005, 44, 2015. (24) Decher, G. Science 1997, 277, 1232. (25) Yoo, D.; Shiratori, S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (26) (a) Jiang, G. Q.; Baba, A.; Ikarashi, H.; Xu, R. H.; Locklin, J.; Kashif, K. R.; Shinbo, K.; Kato, K.; Kaneko, F.; Advincula, R. J. Phys. Chem. C. 2007, 111, 18687. (b) Baba, A.; Park, M. K.; Advincula, R. C.; Knoll, W. Langmuir 2002, 18, 4648.
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