Fabrication and Optoelectronic Properties of Novel Films Based on

Sep 20, 2010 - Under illumination, the LBL self-assembled films on ITO showed an effective ...... Guldi , D. M.; Rahman , G. M. A.; Zerbetto , F.; Pra...
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Fabrication and Optoelectronic Properties of Novel Films Based on Functionalized Multiwalled Carbon Nanotubes and (Phthalocyaninato)Ruthenium(II) via Coordination Bonded Layer-by-Layer Self-Assembly Wei Zhao,† Bin Tong,*,† Jianbing Shi,† Yuexiu Pan,† Jinbo Shen,† Junge Zhi,‡ Wai Kin Chan,*,§ and Yuping Dong*,† †

College of Materials Science and Engineering, and ‡College of Science, Beijing Institute of Technology, Beijing 100081, China, and §Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China Received June 1, 2010. Revised Manuscript Received August 9, 2010

4-(2-(4-pyridinyl)Ethynyl)benzenic diazonium salt (PBD) was used to modify multiwalled carbon nanotubes (MWCNTs) by the self-assembly technique. After the decomposition of the diazonium group in PBD under UV irradiation, the PBD monolayer film covalently anchored on multiwalled carbon nanotubes is very stable. The obtained pyridine-modified MWCNTs (Py(Ar)-MWCNTs) have good solubility in common organic solvents. Furthermore, the layer-by-layer (LBL) self-assembled fully conjugated films of Py(Ar)-MWCNTs and (phthalocyaninato)ruthenium(II) (RuPc) were fabricated on the PBD-modified substrates, and characterized using UV-vis absorption spectroscopy, scanning electron microscopy (SEM), and electrochemistry. The UV-vis analysis results indicate that the LBL RuPc/ Py(Ar)-MWCNTs self-assembled multilayer films with axial ligands between the ruthenium atom and pyridine group were successfully fabricated, and the progressive assembly runs regularly with almost equal amounts of deposition in each cycle. A top view SEM image shows a random and homogeneous distribution of Py(Ar)-MWCNTs over the PBDmodified silicon substrate, which indicates well independence between all Py(Ar)-MWCNTs. Moreover, the optoelectronic conversion was also studied by assembling RuPc/Py(Ar)-MWCNTs multilayer films on PBD-modified ITO substrate. Under illumination, the LBL self-assembled films on ITO showed an effective photoinduced charge transfer because of their conjugated structure and the ITO current density changed with the number of bilayer. As the number of bilayers was increased, the photocurrent increases and reaches its maximum value (∼300 nA/cm2) at nine bilayers. These results allow us to design novel materials for applications in optoelectronic devices by using LBL self-assembly techniques.

Introduction Harvesting energy from sunlight using photovoltaic technology is one of the most promising sustainable energy sources. Carbon nanotubes (CNTs) have been regarded as potential materials for highly efficient transportation of charges in optoelectronic and photovoltaic devices because of their energetically low π* orbitals and highly extended conjugation. Moreover, CNTs possess a wide range of direct band gaps that match the solar spectrum,1-3 resulting in photoresponse from the ultraviolet

*Corresponding authors. E-mail: [email protected]; waichan@hkucc. hku.hk; [email protected]. (1) O’Connell, M. J. S.; Bachilo, M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593–596. (2) Palacin, T.; Le Khanh, H.; Jousselme, B.; Jegou, P.; Filoramo, A.; Ehli, C.; Guldi, D. M.; Campidelli, S. J. Am. Chem. Soc. 2009, 131, 15394–15402. (3) Smith, A. M.; Mohs, A. M.; Nie, S. M. Nat. Nanotechnol. 2009, 4, 56–63. (4) Freitag, M.; Martin, Y.; Misewich, J. A.; Martel, R.; Avouris, P. Nano Lett. 2003, 3, 1067–1071. (5) Zhao, Y. L.; Hu, L. B.; Gruener, G.; Stoddart, J. F. J. Am. Chem. Soc. 2008, 130, 16996–17003. (6) Pradhan, B.; Setyowati, K.; Liu, H. Y.; Waldeck, D. H.; Chen, J. Nano Lett. 2008, 8, 1142–1146. (7) Itkis, M. E.; Borondics, F.; Yu, A. P.; Haddon, R. C. Science 2006, 312, 413– 416. (8) Ju, S. Y.; Utz, M.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2009, 131, 6775–6784. (9) Yang, X.; Chen, L. P.; Shuai, Z. G.; Liu, Y. Q.; Zhu, D. B. Adv. Funct. Mater. 2004, 14, 289–295. (10) Fuhrer, M. S.; Kim, B. M.; Durkop, T.; Brintlinger, T. Nano Lett. 2002, 2, 755–759.

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to infrared region.4-7 They also exhibit high charge carrier mobility8-10 and provide good thermal and photo stabilities.11,12 The very low interfacial adhesion and strong π-π interaction, however, lead to poor dispersion and high self-aggregation of carbon nanotubes in the host matrices.13 To disperse CNTs homogeneously for practical applications, different approaches in functionalizing CNTs have been developed, which include noncovalent14-17 and covalent methods.18-22 The formation of (11) Xu, Z. P.; Buehler, M. J. ACS Nano 2009, 3, 2767–2775. (12) Kundu, S.; Nagaiah, T. C.; Xia, W.; Wang, Y. M.; Van Dommele, S.; Bitter, J. H.; Santa, M.; Grundmeier, G.; Bron, M.; Schuhmann, W.; Muhler, M. J. Phys. Chem. C 2009, 113, 14302–14310. (13) Girifalco, L. A.; Hodak, M.; Lee, R. S. Phys. Rev. B 2000, 62, 13104–13110. (14) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105–1136. (15) Yuan, W. Z.; Sun, J. Z.; Liu, J. Z.; Dong, Y. Q.; Li, Z.; Xu, H. P.; Qin, A. J.; Haeussler, M.; Jin, J. K.; Zheng, Q.; Tang, B. Z. J. Phys. Chem. B 2008, 112, 8896– 8905. (16) Yuan, W. Z.; Tang, L.; Zhao, H.; Jin, J. K.; Sun, J. Z.; Qin, A. J.; Xu, H. P.; Liu, J. H.; Yang, F.; Zheng, Q.; Chen, E. Q.; Tang, B. Z. Macromolecules 2009, 42, 52–61. (17) Chen, F. M.; Wang, B.; Chen, Y.; Li, L. J. Nano Lett. 2007, 7, 3013–3017. (18) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y. S.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95–98. (19) Bahr, J. L.; Yang, J. P.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536–6542. (20) Mevellec, V.; Roussel, S.; Tessier, L.; Chancolon, J.; Mayne- L’Hermite, M.; Deniau, G.; Viel, P.; Palacin, S. Chem. Mater. 2007, 19, 6323–6330. (21) Campidelli, S.; Ballesteros, B.; Filoramo, A.; Diaz Diaz, D.; de la Torre, G.; Torres, T.; Rahman, G. M. A.; Ehli, C.; Kiessling, D.; Werner, F.; Sgobba, V.; Guldi, D. M.; Cioffi, C.; Prato, M.; Bourgoin, J. P. J. Am. Chem. Soc. 2008, 130, 11503–11509. (22) Palacin, T.; Khanh, H. L.; Jousselme, B.; Jegou, P.; Filoramo, A.; Ehli, C.; Guldi, D. M.; Campidelli, S. J. Am. Chem. Soc. 2009, 131, 15394–15402.

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covalent bonds on the surface of CNTs could be achieved by surface-carboxylation18 and electrochemical and thermochemical reduction of aryl diazonium compounds.19-22 Among these reactions, diazonium coupling to CNTs has been one of the most promising chemical CNT functionalization routes, because diazonium compounds show an intrinsically high reactivity associated with good stability. Moreover, the resulting covalent linkages formed between CNTs and the functionalizing reagents are highly stable.23,24 When CNTs with excellent charge transport properties are combined with conjugated organic compounds with remarkable optoelectronic properties the resulting hybrid materials exhibit very interesting optical and electronic properties, forming the basis for a new field of fundamental research with numerous potential applications.25-27 Guldi et al.22,23,28 synthesized and characterized single-walled CNTs (SWCNTs) with covalently linked phthalocyanine antennae as potential supramolecular donor-acceptor complexes. Electron transfer from the photoexcited phthalocyanines to the nanotube framework in these phthalocyanine-SWCNTs ensembles was observed in transient absorption experiments, which pave the way to construct novel photovoltaic devices and light-harvesting systems using various phthalocyanine-functionalized carbon nanotubes. Hatton et al.29 constructed model organic photovoltaic cell from organized assemblies of MWCNTs and tetrasulfonate copper phthalocyanine on indium-tin oxide-coated glass electrode. Enhancement in power conversion efficiency was observed, which was dependent on the electronic, optical, and morphological properties of the nanostructure thin film. In fabricating dye-sensitized solar cells (DSSCs), two important factors-the mesogen structure of the inorganic substrate and the organic sensitizer, will significantly affect the conversion efficiency. The surface of metal oxide in DSSCs can be modified with organic molecules functionalized with carboxylic acids,30,31 trichlorosilanes, and trialkoxysilanes.32 However, the charge transport process between the organic dye and metal oxide may be affected because of the nonconjugated anchoring. By using aryl diazonium salts as the anchoring groups, the directional transport of electrons across the dye-metal oxide interface can be facilitated because of the intimate contact between the organic π-system and metal oxide surface.33-35 In most of the DSSCs fabricated, the orientations of organic dye molecules are random. The ability to control the architecture and orientation of organic sensitizing molecules is expected to enhance photovoltaic cell performance by providing directional charge migration with improved charge transport efficiencies. (23) Qin, S. H.; Qin, D. Q.; Ford, W. T.; Zhang, Y. J.; Kotov, N. A. Chem. Mater. 2005, 17, 2131–2135. (24) Flatt, A. K.; Chen, B.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 8918–8919. (25) McGehee, M. D. MRS Bull. 2009, 34, 95–100. (26) Srivastava, S.; Kotov, N. A. Acc. Chem. Res. 2008, 41, 1831–1841. (27) Guldi, D. M.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871–878. (28) Ballesteros, B.; de la Torre, G.; Ehli, C.; Rahman, G. M. A.; Agullo-Rueda, F.; Guldi, D. M.; Torres, T. J. Am. Chem. Soc. 2007, 129, 5061–5068. (29) Hatton, R. A.; Blanchard, N. P.; Stolojan, V.; Miller, A. J.; Silva, S. R. P. Langmuir 2007, 23, 6424–6430. (30) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813–824. (31) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Hahner, G.; Spencer, N. D. Langmuir 2000, 16, 3257–3271. (32) Barness, Y.; Gershevitz, O.; Sekar, M.; Sukenik, C. N. Langmuir 2000, 16, 247–251. (33) Zhao, W.; Tong, B.; Pan, Y. X.; Shen, J. B.; Zhi, J. G.; Shi, J. B.; Dong, Y. P. Langmuir 2009, 25, 11796–11801. (34) Pan, Y. X.; Tong, B.; Shi, J. B.; Zhao, W.; Shen, J. B.; Zhi, J. G.; Dong, Y. P. J. Phys. Chem. C 2010, 104, 8040–8047. (35) Pan, Y. X.; Tong, B.; Zhi, J. G.; Zhao, W.; Shen, J. B.; Shi, J. B.; Dong, Y. P. Acta Chim. Sin. 2009, 67, 2779–2784.

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This is particularly important when more than one layer of dye molecules is deposited on a metal oxide surface. The relationship of molecular orientation and interactions with material properties are vital for rational functional design. Stang and Van der Boom, respectively, reported the fabrication of functional films with a highly ordered molecular structure where the strong metalorganic coordination governs the molecular orientation in the film growth process.36-39 We previously reported the anchoring of conjugated 4-(2-(4pyridinyl)ethynyl)benzenic (PBD) and 4-(2,20 :60 ,200 -terpyrid-40 -yl)benzenic groups onto substrates such as ITO, quartz, and carbon nanotubes by covalent linkage, respectively. The introduction of the terminal pyridine or terpyridine groups allows the subsequent formation of coordination compounds with different metals. By using this approach, multilayer films composed of pyridinecontaining conjugated compounds and metal ions or organometal compounds could be prepared by the layer-by-layer (LBL) selfassembly.33-35 However, the LBL films fabricated from MWCNT covalently modified by (2,20 :60 ,200 -terpyrid-40 -yl)benzenic groups and ruthenium ions [Ru(III)] showed the low photocurrent intensity, which reaches its maximum (∼65 nA/cm2) at six bilayers.34 The observation provides some important insight for the design of improved photocurrent intensity. Phthalocyanines (Pcs) are planar electron-rich aromatic macrocycles that are characterized by their remarkably high extinction coefficients in the red/near-infrared region, an important part of the solar spectrum, and their outstanding photostability and singular physical properties. These features render them exceptional donor/antenna building blocks.28 In this paper, we report the formation of a novel multilayer film composed of MWCNTs and photosensitizing phthalocyanines by the LBL method. Pyridine groups were first introduced on the surface of MWCNTs by the photochemical reaction between 4-(2-(4-pyridinyl)ethynyl)benzenic diazonium salt and pristine MWCNTs under UV irradiation. The resulting pyridine-containing MWCNTs (Py(Ar)-MWCNTs) were deposited on a substrate which has been modified with a monolayer of PBD and photosensitizing (phthalocyaninato)ruthenium(II) (RuPc). The MWCNTs orient parallel to the substrate surface by the formation of N-coordination between PBD and the ruthenium center. The deposition of RuPc and Py(Ar)-MWCNTs was repeated until the desired number of layers was achieved (Scheme 1).

Experimental Section Preparation of Py(Ar)-MWCNTs. The reaction sequence is depicted in Scheme 2. In a typical experiment, ∼8 mg of MWCNTs was sonicated for 10 min in 10 mL of 1,2-dichlorobenzene (ODCB). To this suspension was added a solution of 4-(2-(4-pyridinyl)ethynyl)aniline (1.3 mmol, ∼2 equiv/mol of carbon) in 5 mL of acetonitrile. Bubbled with nitrogen for 10 min, 4.0 mmol of n-butyl nitrite was then quickly injected, and the suspension was stirred at 60 °C for about 12 h. After being cooled to room temperature, the suspension was diluted with 30 mL of dimethylformamide (DMF) and filtered through a 0.2 μm polytetrafluoroethylene (PTFE) membrane. The solid was sonicated in DMF and filtered again, and the process was repeated until the DMF was colorless after sonication. This process (36) Li, S.-S.; Northrop, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. Acc. Chem. Res. 2009, 42, 249–259. (37) Altman, M.; Shukla, A. D.; Zubkov, T.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2006, 128, 7374–7382. (38) Altman, M.; Zenkina, O.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2008, 130, 5040–5041. (39) Motiei, L.; Lahav, M.; Freeman, D.; van der Boom, M. E. J. Am. Chem. Soc. 2009, 131, 3468–3469.

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Zhao et al. Scheme 1. Formation of Py(Ar)-MWCNTs and Multilayer Composed of PBD and RuPc by the LBL Self-Assembly Process

Scheme 2. Functionalization of MWCNTs Coated with Pyridine Moieties by in Situ Generation of the Diazonium Species

removed any unreacted 4-(2-(4-pyridinyl)ethynyl)aniline from the product.

Preparation of RuPc/Py(Ar)-MWCNTs Multilayer Films. The concentrations of RuPc and Py(Ar)-MWCNTs in dimethyl sulfoxide (DMSO) were 0.1 mg/mL. Multilayer films were assembled as following: The substrate modified with PBD was first immersed into a solution of RuPc at 50 °C for 10 min, followed by rinsing with DMSO for 3 min before blow-dried with air. Afterward, it was immersed into a solution of Py(Ar)MWCNTs at 50 °C for 10 min, rinsed with DMSO for 3 min and finally dried by flow air to complete a fabrication cycle. In each cycle, a bilayer of RuPc/Py(Ar)-MWCNTs self-assembled film was deposited on the substrate. 16086 DOI: 10.1021/la1022196

Result and Discussion Characterization of Py(Ar)-MWCNTs. Py(Ar)-MWCNTs were characterized by infrared spectroscopy and thermogravimetric analysis. The FT-IR spectrum of Py(Ar)-MWCNTs (Supporting Information, Figure S1, solid line) shows new C-H stretching bands from the pyridine and phenylene moieties at 3070 and 3030 cm-1, which do not appear in the spectrum of pristine MWCNTs (Supporting Information, Figure S1, dashed line). In addition, other bands due to CtC stretching and carbon skeleton stretch bands of aromatic groups are observed at 2200 and 1600-1500 cm-1, respectively. It clearly shows the functionalization of PBD moieties on the nanotube surface. Because Langmuir 2010, 26(20), 16084–16089

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Figure 1. UV-vis absorption spectra of RuPc/Py(Ar)-MWCNTs multilayer films with the bilayer number of 1 to 9.

Py(Ar)-MWCNTs was prepared according to previous methods described by our group,34 the pyridine-containing modified film of MWCNT surface was probably composed of 1-(4-pyridinyl)-2phenylethynylene’s oligomer (note as “Py(Ar)”) as shown in Scheme 2.34,40,41 The attachment of aryl groups to the substrate surfaces can be assigned to the reaction of an aryl radical formed by a one-electron reduction of the diazonium salt. That is, the multilayer formation has been assigned to the attack of an aryl radical on the grafted layer along an SH homolytic substitution reaction.42,43 The structure of the layer should look somewhat like a substituted polyphenylene. TGA provides quantitative estimation of the degree of the aryl diazonium functionalization of the MWCNTs. One of the remarkable properties of CNTs is their outstanding thermal stability. As can be seen from Supporting Information, Figure S2, the weight loss of MWNTs at 600 °C is about 3%. On the other hand, TGA results revealed that Py(Ar)-MWCNTs start to decompose at about 150 °C, and the weight loss at 560 °C is 18% (Supporting Information, Figure S2), which corresponds to the amount of organic moieties introduced on the MWCNTs surface.44,45 The number of Py(Ar) groups was then estimated as 1 per 68 carbon atoms.21,44 Fabrication of Multilayer Film. With the introduction of the distal pyridine group MWCNTs, the main driving force for the RuPc/Py(Ar)-MWCNTs assembly is attributed to the formation of coordination between ruthenium and pyridyl group. UV-vis spectroscopy was used to monitor the formation of the multilayer by the LBL self-assembly process. After one self-assembly cycle (one bilayer), each side of the quartz substrate adsorbed one layer of RuPc and Py(Ar)-MWCNTs. Figure 1 shows the absorption spectra of the RuPc/Py(Ar)-MWCNTs self-assembled film composed of one to nine bilayers on a PBD-modified quartz substrate. The absorption peaks at 332 and 670 nm are attributed to the B-band and Q-band of RuPc, respectively. The broad absorption band in the entire visible region is due to the absorption of MWCNTs. With an increase in the number of bilayers, the amount of Py(Ar)-MWCNTs deposited increases and the appearance of the quartz substrate darkens because of strong absorption (40) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201–207. (41) Bernard, M.-C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450–3462. (42) Bunnett, J. F.; Wamsert, C. C. J. Am. Chem. Soc. 1966, 88, 5534–5537. (43) Danen, W. C.; Saunders, D. G. J. Am. Chem. Soc. 1969, 91, 5924–5925. (44) Brunetti, F. G.; Herrero, M. A.; Munoz, J. M.; Diaz-Ortiz, A.; Alfonsi, J.; Meneghetti, M.; Prato, M.; Vazquez, E. J. Am. Chem. Soc. 2008, 130, 8094–8100. (45) Yuan, W. Z.; Sun, J. Z.; Dong, Y. Q.; Haussler, M.; Yang, F.; Xu, H. P.; Qin, A. J.; Lam, J. W. Y.; Zheng, Q.; Tang, B. Z. Macromolecules 2006, 39, 8011– 8020.

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Figure 2. Scanning electron micrograph of a five-bilayer RuPc/ Py(Ar)-MWCNTs film on a silicon wafer.

in the visible region.46-48 The inset in Figure 1 shows that the absorbance at 332 and 670 nm increases linearly with the number of bilayers, suggesting that equal amounts of Py(Ar)-MWCNTs and RuPc are deposited at each deposition cycle and the fabrication of multilayers is a stepwise and regular process. The multilayer film may be thought to be a macromolecular network structure because the linkage among layers is the coordination interactions between pyridine in the Py(Ar)-MWCNTs and ruthenium in RuPc, which is very similar to our previous work.34 The surface morphology of the film prepared after five-bilayer RuPc/Py(Ar)-MWCNTs self-assembly cycles was imaged by scanning electron microscopy (SEM). As shown in Figure 2, a top view SEM image shows a random and homogeneous distribution of Py(Ar)-MWCNTs because RuPc is attached with a random orientation due to the radial distribution of pyridine moieties on the MWNT surface, which indicates well independence between all PBD-functionalized MWCNTs although there may be small bundles present. The architectures are highly robust and able to withstand rinsing by different organic solvents and even long time sonication because the network of Py(Ar)MWCNTs is stabilized by the strong coordination interaction between Py(Ar)-MWCNTs and RuPc. Besides, the deposition can be controlled quantitatively by controlling self-assembled bilayers. Such phenomenon was observed in solution-cast films of pristine MWCNTs, which exhibited a nonhomogeneous, densely aggregated distribution on the chip surface, in addition to poor thickness control and poor stability. Electrochemical Characterization. An independent quantitative analysis of the RuPc/Py(Ar)-MWCNTs multilayer on an ITO slide was carried out by cyclic voltammetry. Figure 3 shows the cyclic voltammograms of different RuPc/Py(Ar)-MWCNTs multilayers with a different number of bilayers. The thin films under study were immersed in 0.1 mol/L [NBu4þ][ClO4-] acetonitrile solution. The scan rate was 0.1 V/s over a voltage range of -0.2 to 1.5 V. All the multilayer films displayed chemically irreversible ruthenium(II)/ruthenium(III) oxidation waves at 0.78-0.82 V corresponding to formation of Ru(III), and the reduction wave was not observed. This may be attributed to one-dimensional quantum confinement effects of MWCNTs as semiconductor. (46) Qin, S. H.; Qin, D. Q.; Ford, W. T.; Zhang, Y. J.; Kotov, N. A. Chem. Mater. 2005, 17, 2131–2135. (47) Shen, J. F.; Hu, Y. Z.; Qin, C.; Ye, M. X. Langmuir 2008, 24, 3993–3997. (48) Kim, J. H.; Lee, S. W.; Hammond, P. T.; Horn, Y. S. Chem. Mater. 2009, 21, 2993–3001.

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Figure 3. Cyclic voltammograms of the different bilayer RuPc/ Py(Ar)-MWCNTs films immersed into 0.1 mol/L acetonitrile solution at 0.1 v/s scan rate.

[NBu4þ][ClO4-]

Figure 5. Current density change of bare ITO and self-assembled nine-bilayer RuPc/Py(Ar)-MWCNTs film on ITO.

Figure 6. A fitting plot of photocurrent density vs number of bilayers for RuPc/Py(Ar)-MWCNTs multilayer film. Figure 4. Relationship between the current and potential of oxidation peak versus the number of bilayers.

Nyokong thought that only one peak is observed possibly due to the obscuration of the quinone-like moieties on MWCNT following the attachment of the metal-containing phthalocyanine. At the same time, the covalent bond improves electronic communication between MWCNTs and the phthalocyanine ring,49 so that the electrons migrated quickly along the axial conjugate system of MWCNTs, and back transfer of electrons was inhibited as a result when Py(Ar)-MWCNTs mesogens received electrons from the Ru(II). This demonstrates the potential of using RuPc/ Py(Ar)-MWCNTs as the electron donor-acceptor in photovoltaic cells. However, more detailed mechanism analysis is currently underway in our laboratory. The relationship of current and potential of oxidation peak versus the number of bilayer is shown in Figure 4. The oxidation current increases more significantly with an increase in the number of bilayers from three to nine. This is attributed to the increase in the amount of organic-metal semiconductor RuPc and Py(Ar)-MWCNTs in thicker films. After the current reaches the maximum (13 μA) at nine bilayers, the current, however, start to decrease from ten bilayers and beyond. The saturation and decrease of current might result from increasing internal electrical resistance with increasing film thickness.33-35,50,51 The potentials, however, increase slightly with the increasing of bilayer regardless of the variation of the film thickness, which is related to the decrease of the oxidation complexity (Figure 4). (49) Siswana, M. P.; Ozoemena, K. I.; Nyokong, T. Electrochim. Acta 2006, 52, 114–122. (50) Cao, T. B.; Wei, L. H.; Yang, S. M.; Zhang, M. F.; Huang, C. H.; Cao, W. X. Langmuir 2002, 18, 750–753. (51) Liang, Z. Q.; Dzienis, K. L.; Xu, J.; Wang, Q. Adv. Funct. Mater. 2006, 16, 542–548.

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Photoelectric Conversion Characterization. Steady state photocurrent measurements were performed using a three-electrode electrochemical cell in conditions similar to those previously reported.33-35,50,51 The ITO substrate coated with RuPc/Py(Ar)MWCNTs self-assembled films comprising 3, 6, 9, 12, and 15 bilayers were utilized as working electrodes (0.2 cm2), which were illuminated with a 500 W xenon-mercury lamp. A plot of photocurrent versus irradiated time for nine-bilayers RuPc/ Py(Ar)-MWCNTs films is shown in Figure 5. The RuPc/ Py(Ar)-MWCNTs multilayer films demonstrated a high photocurrent and fast response rate. The experimental results indicate that RuPc/Py(Ar)-MWCNTs film is a promising photosensitizer for DSSCs. The photocurrent was generated as a result of the photoinduced electron transfer along 4-(2-(4-pyridinyl)ethynyl)benzenic group as bridge from RuPc to Py(Ar)-MWCNTs, followed by the migration of electrons Py(Ar)-MWCNTs to the ITO substrate though the Py(Ar)-MWCNTs are horizontal to the surface of the substrate. This is because migration of the electrons occurred at the full-conjugated network based on MWCNTs and RuPc, resulting in a fast abstraction of electrons by the MWCNTs with high electron mobility (Figure 5). Figure 6 shows the plot of photocurrent density for different films with a different number of bilayers. It was found that the photoelectric current increases with the number of bilayers from three to nine at which a maximum is observed (∼300 nA/cm2). Compared with our previous study about the layer-by-layer film based on terpyridine-modified MWCNTs and ruthenium ions,34,35 the photocurrent is much higher with the same number of bilayers. Further increase in the number of bilayers results in the decrease in photocurrent response. The phenomenon is similar to that observed in electrochemical experiment. The result can be attributed to the fact that the film thickness strongly affects the overall energy conversion efficiency. The saturation and Langmuir 2010, 26(20), 16084–16089

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decrease in photocurrent might result from increasing internal electrical resistance and/or higher probability of charge carrier recombination with increasing film thickness.33,34,50-52 More detailed mechanism analysis is currently underway in our laboratory.

Conclusion We have successfully fabricated a series of fully conjugated RuPc/Py(Ar)-MWCNTs multilayer films by the LBL self-assembly method on different PBD-modified substrates. UV-vis absorption spectroscopy results indicate the formation of nitrogen-ruthenium coordination between an axial pyridine ligand and the ruthenium center in RuPc/Py(Ar)-MWCNTs self-assembled multilayer films, and a progressive assembly occurred regularly with almost equal amount of materials deposited in each cycle. Under illumination, the RuPc/Py(Ar)-MWCNTs multilayer films demonstrated a high photocurrent response and fast response rate. When the number of bilayers was increased, the (52) Akatsuka, K.; Ebina, Y.; Muramatsu, M.; Sato, T.; Hester, H.; Kumaresan, D.; Schmehl, R. H.; Sasaki, T.; Haga, M. Langmuir 2007, 23, 6730–6736.

Langmuir 2010, 26(20), 16084–16089

Article

photocurrent increased and reached the maximum (∼300 nA/ cm2) at nine bilayers. The generation of photocurrent is due to the photoinduced charge transfer from RuPc excitons to MWCNTs. The hybrid materials are expected to combine the electrochromic, molecular conductivity, and optoelectronic properties of metal phthalocyanines and the unique charge transport properties of MWCNTs. This work presents us with opportunities for designing new materials of optoelectronic applications. Acknowledgment. The authors are grateful to the National Natural Scientific Foundation of China (Grant No. 50573008, 20634020), The Specialized Research Fund for the Doctoral Program of High Education of China (Grant No. 20050007018, 20091101110031) and Basic Research Foundation of Beijing Institute of Technology for financial support of this work. Supporting Information Available: FT-IR and TGA spectra of Py(Ar)-MWCNTs, and pretreatment of substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la1022196

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