[60]-Fullerene and Single-Walled Carbon Nanotube-Based Ultrathin

A step-by-step method was used to prepare homogeneous ultrathin films ...... Paolucci , F., Campidelli , S., Prato , M., Rahman , G. M. A. and Schergn...
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[60]-Fullerene and Single-Walled Carbon Nanotube-Based Ultrathin Films Stepwise Grafted onto a Self-Assembled Monolayer on ITO Qiguan Wang† and Hiroshi Moriyama*,†,‡ †

Research Center for Materials with Integrated Properties, ‡Department of Chemistry, Toho University, Miyama 2-2-1, Funabashi, 274-8510 Japan Received April 18, 2009. Revised Manuscript Received July 8, 2009

A step-by-step method was used to prepare homogeneous ultrathin films composed of [60]-fullerene (C60) and singlewalled carbon nanotubes (SWNTs), grafted onto the functional surface of an alkylsilane self-assembled monolayer (SAM) on an ITO substrate with an ITO-C60-SWNT sequence using amine addition across a double bond in C60 followed by amidation coupling with acid-functionalized SWNTs. Atomic force microscope and scanning electron microscope images of the resulting composite film showed two-component ball-tube microstructures with high-density coverage, where C60 was homogeneously distributed in the SWNT forest. The attachment of SWNTs to the residual amine units in the SAM on the ITO substrate (SAM-ITO) as well as on the C60 sphere results in the C60 molecules in the aggregated clusters being more separately dispersed, which forms a densely packed composite film as a result of the π-π interaction between the C60 buckyballs and the SWNT walls. It was found using ferrocene as an internal redox probe that the oxidative and reductive processes at the film-solution surface were effectively retarded because of obstruction from the densely packed film and the electronic effect of SWNT and C60. In addition, the electrochemical properties of C60 on SAM-ITO plates observed by cyclic voltammetry were significantly modified by chemical anchorage using SWNTs. X-ray photoelectron spectroscopy (XPS) analysis also indicated the successful grafting of C60 and SWNT. The XPS chemical shift of the binding energy showed the presence of electronic interactions between C60, SWNT, and ITO components. Such a uniformly distributed C60-SWNT film may be useful for future research in electrochemical and photoactive nanodevices.

Introduction Carbon nanotubes and fullerenes, composed of fascinating cylindrical and spherical graphitic structures, respectively, have been studied extensively, with the aim of applying them in many fields as a result of their unique physical and chemical characteristics.1,2 Because carbon nanotubes and fullerenes both show photocurrent generation effects,3-5 the combination of these two carbon allotropes, which may give rise to novel structures and properties, has been extensively investigated by different methods, such as encapsulation of C60 by single-walled carbon nanotubes to *Corresponding author. E-mail: [email protected]. (1) (a) Iijima, S. Nature 1991, 354, 56–58. (b) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105–1113. (c) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105–1136. (d) Wu, W.; Zhang, S.; Li, Y.; Li, J. X.; Liu, L. Q.; Qin, Y. J.; Guo, Z. X.; Dai, L. M.; Ye, C.; Zhu, D. B. Macromolecules 2003, 36, 6286–6288. (2) (a) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1996. (b) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537–546. (c) Imahori, H. J. Phys. Chem. B 2004, 108, 6130– 6143. (d) Imahori, H.; Fukuzumi, S. Adv. Funct. Mater. 2004, 14, 525–536. (e) Guldi, D. M. J. Phys. Chem. B 2005, 109, 11432–11441. (f ) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834–2860. (3) Barazzouk, S.; Hotchandani, S.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 17015–17018. (4) Kamat, P. V.; Barazzouk, S.; Thomas, K. G.; Hotchandani, S. J. Phys. Chem. B 2000, 104, 4014–4017. (5) Hotta, H.; Kang, S.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Imahori, H. J. Phys. Chem. B 2005, 109, 5700–5706. (6) (a) Kavan, L.; Dunsch, L.; Kataura, H. Chem. Phys. Lett. 2002, 361, 79–85. (b) Kavan, L.; Dunsch, L.; Kataura, H.; Oshiyama, A.; Otani, M.; Okada, S. J. Phys. Chem. B 2003, 107, 7666–7675. (7) Li, X.; Liu, L.; Qin, Y.; Wu, W.; Guo, Z.; Dai, L.; Zhu, D. Chem. Phys. Lett. 2003, 377, 32–36. (8) (a) Zhang, H.; Fan, L.; Fang, Y.; Yang, S. Chem. Phys. Lett. 2005, 413, 346– 350. (b) Zhang, H.; Fan, L.; Yang, S. Chem.;Eur. J. 2006, 12, 7161–7166. (9) (a) Wu, W.; Zhu, H.; Fan, L.; Yang, S. Chem.;Eur. J. 2008, 14, 5981–5987. (b) Delgado, J. L.; de la Cruz, P.; Urbina, A.; Lopez Navarrete, J. T.; Casado, J.; Langa, F. Carbon 2007, 45, 2250–2252.

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fabricate the so-called peapods,6 covalent linkage of C60 clusters to the surfaces of carbon nanotubes,7-9 and physical attachment of C60 derivatives through π-π interactions with the surfaces of SWNTs.10 Although there are several examples of combined C60SWNT ball-tube materials showing photoelectrochemical behaviors, in which the single-walled carbon nanotubes are treated as electron donors11 or electron-transfer networks,12 the preparation of their composite films, especially those having stable covalent interactions with substrates, has attracted less attention. It is easy to obtain C60 or SWNT films on an optically transparent electrode using electrophoretic deposition; however, for the C60-SWNT two-component film, this technique results in a layer-by-layer structure because of the faster deposition of SWNT than of C60.12 To increase the lifetime of excitons during the photoexcitation, the bundling of SWNT and the clustering of C60 should be decreased. Therefore, designing a more uniformly distributed C60-SWNT film on a solid substrate is required for future research. Encouragingly, an increasing number of different techniques have been developed to prepare ultrathin SWNT or fullerene films besides electrophoretic deposition, including dropdrying from solvent,13,14 the Langmuir-Blodgett technique,15,16 (10) Guldi, D. M.; Menna, E.; Maggini, M.; Marcaccio, M.; Paolucci, D.; Paolucci, F.; Campidelli, S.; Prato, M.; Rahman, G. M. A.; Schergna, S. Chem.; Eur. J. 2006, 12, 3975–3983. (11) D’Souza, F.; Chitta, R.; Sandanayaka, A. S. D.; Subbaiyan, N. K.; D’Souza, L.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2007, 129, 15865–15871. (12) Umeyama, T.; Tezuka, N.; Fujita, M.; Hayashi, S.; Kadota, N.; Matano, Y.; Imahori, H. Chem.;Eur. J. 2008, 14, 4875–4885. (13) Sreekumar, T.; Liu, T.; Kumar, S. Chem. Mater. 2003, 15, 175. (14) Yoshida, Y.; Tanigaki, N.; Yase, K. Thin Solid Films 1996, 281-282, 80–83. (15) Kim, Y.; Minami, N.; Zhu, W.; Kazaoui, S.; Azumi, R.; Matsumoto, M. Jpn. J. Appl. Phys. 2003, 42, 7629–7634. (16) Wang, S.; Leblanc, R. M.; Arias, F.; Echegoyen, L. Langmuir 1997, 13, 1672–1676.

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and self-assembled monolayers (SAMs),17-20 which pave the way to fabricating SWNT- and fullerene-based composite films for use in different types of devices. Compared with other techniques, the method of chemically SAMs showing high stability is considered an especially promising approach. Recently, SWNT SAMs have been prepared on a Au substrate by a surface condensation reaction,17 and also, fullerene C60 molecules as models have been covalently attached to an amine-terminated SAM on a Si(111) substrate.21 In the latter case, the experimental XPS data showed the presence of both primary amine groups -NH2 of the SAM on Si(111) and secondary amine groups -NH- during the C60 attachment, which indicates the low reaction efficiency of this attachment with a considerable number of amine groups left unreacted, supporting the possibility of introducing other functional molecules. More recently, a stable electroactive SWNT monolayer end-capped by a conductive oligomer was prepared in our group that showed potential application in electrochemical nanoarchitectures.22 In view of the intriguing electrochemical properties of C60, introducing C60 in the SWNT film may be a good way to modify SWNTs. From the viewpoint of morphology control, here, we report a more homogeneous SWNT-C60 film covalently grafted onto an optically transparent SAM-ITO surface using a step-by-step method, by integration of the above preparations of C60 SAM and SWNT SAM, in which the SWNTs were anchored to the unreacted amines of the SAM-ITO as well as to C60 aggregates functionalized by amine groups, which is expected to fabricate chemically stable composite films for future research in photoactive systems.

Experimental Section Materials. Single-walled carbon nanotubes, prepared using a high-pressure carbon monoxide (HiPco) chemical vapor deposition process, were obtained from Carbon Nanotechnologies Inc. (Houston, TX). The C60 (99.5þ% purity) was purchased from MER Corp. and further purified by alumina column chromatography. The 3-aminopropyltrimethoxysilane (APTMS) used was purchased from TCI (Japan). N,N0 -Dicyclohexylcarbodiimide (DCC) was purchased from Wako (Japan). 1,2-Ethylenediamine was purified by vacuum distillation prior to use. 1,2-Dichlorobenzene (ODCB) used for electrochemistry measurements was distilled over CaH2. All other chemicals used were of analytical grade and were purchased from Sigma-Aldrich and used without further purification. Measurements. The functional groups on shortened SWNTs were analyzed using an FT-IR spectrometer (JASCO FT/IR4100A, Japan) employing the KBr disk method. The characteristic absorbance measurements of the C60 film on the SAM-ITO plates were carried out using a UV-vis spectrophotometer (JASCO UV550, Japan). The infrared absorption spectra (FTIR-RAS) of sample surfaces were collected by the reflection-absorption method using a JASCO FT/IR-4100 spectrometer equipped with a reflector (RAS PRO-410H, JASCO) and an MCT-Mid detector, using a bare ITO plate as the reference surface. The near-infrared absorption spectra (NIR) of the SWNTs were recorded on a NIRSystems 6500 spectrophotometer (FOSS NIRSystems). For the aqueous SWNT solution, pure water was used as the reference. For the C60-ITO and the SWNT-C60-ITO, a bare ITO substrate was used as the reference. Cyclic voltammetry (CV) of the (17) Diao, P.; Liu, Z. F.; Wu, B.; Nan, X. L.; Zhang, J.; Wei, Z. ChemPhysChem 2002, 3, 898–901. (18) Song, F.; Zhang, S.; Bonifazi, D.; Enger, O.; Diederich, F.; Echegoyen, L. Langmuir 2005, 21, 9246–9250. (19) Fujihara, H.; Nakai, H. Langmuir 2001, 17, 6393–6395. (20) Tsukruk, V. V.; Everson, M. P.; Lander, L. M.; Brittain, W. J. Langmuir 1996, 12, 3905–3911. (21) Zhang, X.; Teplyakov, A. V. Langmuir 2008, 24, 810–820. (22) Wang, Q. G.; Moriyama, H. Bull. Chem. Soc. Jpn. 2009, 82, 743–749.

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Article samples was performed at room temperature using a BAS 100B Electroanalytical System from Tokyo, Japan, with an ALS/ CHI600A electrochemical analyzer attached to a three-electrode cell in which 15 mL of acetonitrile solution (Ag/AgNO3 in a 0.1 M tetrabutylammonium perchlorate (TBAP)/CH3CN solution was used as the reference electrode) containing 0.1 M TBAP was used as a supporting electrolyte. An SWNT-C60-ITO plate was used as the working electrode, and a Pt wire (φ = 0.3 mm) was used as the counter electrode. The system was deoxygenated for 2 h by bubbling nitrogen gas through it prior to the experiment. The morphology examination was performed using a Shimadzu SPM9600 atomic force microscope (AFM) and an Hitachi S-4700 fieldemission microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed on a JEOL JPS-90SX machine equipped with a monochromatic Mg KR X-ray source (1253.6 eV). The instrument was operated with an analyzer chamber pressure maintained below 10-9 Torr. The binding energy was referenced to the C 1s signal of the alkyl chains or alkyl contaminants at 284.6 eV.

Preparation of C60 Film Grafted on the SAM-ITO (C60ITO). The ITO plates (20  20  1.1 mm, sheet resistance =

10 Ω/sq, E.H.C. Co., Ltd.) were cut into 520 mm coupons and ultrasonically cleaned with acetone, ethanol, and a copious amount of deionized (DI) water. To obtain uniformly distributed -OH groups on the ITO surface, the ITO glass samples were immersed in a solution of H2O2/NH4OH/H2O (1:1:5, v/v/v) for a period of 30 min at 80 °C for hydrolysis, after which they were rinsed thoroughly with DI water and dried. The hydrolyzed ITO plates were immersed in a 1% (v/v) solution of APTMS in anhydrous toluene overnight under refluxing for silanization to obtain the SAM, followed by rinsing with dry toluene and water to remove the physically adsorbed silanes from the surface. The SAM-modified ITO glass (SAM-ITO) was then completely dried under vacuum and stored in a nitrogen atmosphere. Grafting of the C60 film occurred via an amine addition reaction across the double bonds in C60.21,23 Typically, the (aminopropyl)silanized SAM-ITO was soaked in 20 mL of dry toluene containing 10 mg of C60 for 2 days under reflux conditions. After being rinsed in dry toluene and acetone and dried under vacuum, the C60-grafted SAM-ITO plate was additionally immediately immersed in 15 mL of 1,2-ethylenediamine at 80 °C for a period of 48 h (Scheme 1) to obtain amine-functionalized C60-ITO. The resulting C60-ITO plate was then removed and thoroughly rinsed using DMF and acetone, then dried at room temperature under vacuum, and stored in a nitrogen atmosphere.

Further Grafting of SWNTs on the C60-Modified SAMITO Surface (SWNT-C60-ITO). To additionally covalently attach SWNTs on the unreacted amines of the SAM-ITO as well as on C60, functionalized shortened SWNTs with carboxylic acid were prepared by suspending the raw SWNTs with the aid of ultrasonic agitation for 6 h in a 3:1 v/v mixture of concentrated sulfuric and nitric acids. After filtration using a 0.1 μm polycarbonate membrane and rinsing with DI water until the pH of the filtrate was 7, the SWNTs with oxidized carbon sites were dried for a period of 3 h at 100 °C in a vacuum oven. FT-IR data (see Supporting Information Figure S1) demonstrated the presence of carboxylic acid groups (νCdO at 1730 cm-1 and νOH at 3400 cm-1) on the surface of the treated SWNTs. Grafting of SWNTs onto the C60-modified SAM-ITO surface was then carried out by immersing the C60-ITO plates functionalized by ethylenediamine in 30 mL of DMF containing 4.0 mg of DCC and 8.0 mg of shortened SWNTs dispersed by an ultrasound bath and kept for 48 h at 80 °C.24 Next, the SWNT-C60-ITO was washed (23) (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193–1194. (b) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945–1947. (c) Ren, S.; Yang, S.; Zhao, Y. Langmuir 2004, 20, 3601–3605. (24) (a) Chukharev, V.; Vuorinen, T.; Efimov, A.; Tkachenko, N. V.; Kimura, M.; Fukuzumi, S.; Imahori, H.; Lemmetyinen, H. Langmuir 2005, 21, 6385–6391. (b) Li, C.; Ren, B.; Zhang, Y.; Cheng, Z.; Liu, X.; Tong, Z. Langmuir 2008, 24, 12911–12918.

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Figure 1. Comparison of the C 1s (a), N 1s (b), and O 1s (c) XPS spectra for bare ITO, C60-ITO, and SWNT-C60-ITO. Scheme 1. Schematic Illustration of the Preparation Route to C60-SWNT Ultrathin Film Grafted on ITO Step-by-Step

thoroughly using DMF and alcohol and dried under vacuum and then stored in a nitrogen atmosphere before characterization.

Results and Discussion XPS Spectra of C60-ITO and SWNT-C60-ITO. XPS provides a direct chemical characterization of the surface layer (2-5 nm depth) and has been applied to investigate the chemical states of relevant elements such as C 1s (a), N 1s (b), and O 1s (c) on bare ITO, C60-ITO, and SWNT-C60-ITO, respectively. In the C 1s spectra shown in Figure 1a, the peak at 285.20 eV was ascribed to the organic carbon contaminants for bare ITO. However, a sharp peak showing increased intensity appeared at a binding energy of 291.00 eV for C60-ITO, attributable to the sp2 carbons of fullerene as well as to the alkylsilane and the ethylenediamine SAM on ITO. In comparison with the reported value of C 1s in bare C60 and methylene group at 284.6 eV,21,25 a binding energy shift of more than 6.0 eV was found in the case of (25) Walters, K. B.; Hirt, D. E. Macromolecules 2007, 40, 4829–4838.

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C60-ITO. Such a higher energy feature corresponding to C 1s just like a hybridized sp3 carbon has typically been observed in C60 derivatives such as fluorinated fullerenes, in which the carbon atoms are directly attached to the more electronegative fluorine atoms.26 Taking into account the electronic effect, where the C60 molecules are electron donating with respect to the surface functionalized SnO2 or In2O3 located in ITO, it is reasonable to consider that the C 1s peak at 291.00 eV results from the delocalized electrons in the C60 grafted onto the ITO layer. The chemical shift of carbons in the methylene groups of alkylsilane and ethylenediamine comes from the similar electronic effect of both ITO components and the grafted C60. However, the C 1s peak at 291.00 eV was no longer visible for C60-ITO further grafted by SWNTs, and only one C 1s peak at 285.10 eV assigned to C-C/C-H was detected. Evidently, the disappearance of the (26) (a) Kawasaki, S.; Aketa, T.; Touhara, H.; Okino, F.; Boltalina, O. V.; Gol’dt, I. V.; Troyanov, S. I.; Taylor, R. J. Phys. Chem. B 1999, 103, 1223–1225. (b) Kepman, A. V.; Sukhoverkhov, V. F.; Tressaud, A.; Labrugere, C.; Durand, E.; Chilingarov, N. S.; Sidorov, L. N. J. Fluorine Chem. 2006, 127, 832–836.

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C 1s peak at 291.00 eV in SWNT-C60-ITO resulted from the shift of the C 1s peak for C60-ITO from 291.00 to 285.10 eV. From the viewpoint of the electronic effect, the shift probably resulted from electron transportation from SWNT to C60,11 which partly compensated the electron loss in C60 as well as in the alkylsilane and ethylenediamine. Moreover, the shoulder peak centered at 289.2 eV corresponds to oxidized carbon of formed amide and residual COOH on functionalized SWNT (Scheme 1), which showed the successful grafting of SWNTs by the amide bonds. Because the N 1s spectrum is another useful approach to monitor the reaction process of C60 attachment to amine SAM on a solid substrate,21 the N 1s spectra of bare ITO, C60-ITO, and SWNT-C60-ITO were examined, as illustrated by Figure 1b. In the spectrum of C60-ITO, the main N 1s XPS peak is located at 406.00 eV, which is attributed to the primary amine group -NH2, resulting from the unreacted amine units on the ITO and the ethylenediamine linked with C60 molecules. The peak located at 401.65 eV is assigned to the secondary amine group -NH-, resulting from the addition reaction of amine with C60. In comparison of the reported XPS feature of a primary amine at 400 eV and a secondary amine at 397 eV,21 the binding energy shifts by about 6.0 and 5.0 eV, respectively. As observed in the C 1s XPS feature, such higher binding energy shifts for N 1s XPS probably resulted from the electron-accepting nature of the surface-functionalized SnO2 or In2O3 in ITO and the similar electronic interactions of nitrogen atoms with the grafted C60. On the other hand, taking into account the silicic acid groups formed from the further hydrolysis of the residual alkoxysilane groups on the SAM-ITO assisted by traces of water in the subsequent steps (see Supporting Information Figure S2), it is reasonable to consider that the attached amine units on C60-ITO were protonated, which also makes the N 1s XPS feature corresponding to amine groups shift to higher binding energy. In fact, such a high binding energy as 406.5 eV for a primary amine group was recently observed in the protonated amine groups of aqueous lysine solution, which results from the additional positive charge around the nitrogen site.27 After grafting by SWNTs (SWNTC60-ITO), the N 1s XPS peaks shifted to lower binding energy. The peak located at 399.85 eV is assigned to the formed amide group -CO-NH- binding directly to the acid-functionalized SWNT.28 Similarly, the binding energy shift of the secondary amine group -NH-C60 from 401.65 to 396.5 eV is because of the electron transportation from SWNT to C60. The high intensity of the amide N 1s XPS peak showed that SWNTs were effectively grafted onto the C60-ITO surface by means of the amidation coupling, which indicates the applicability of this step-by-step grafting strategy. O 1s spectra show the same evolution, as shown in Figure 1c. For the bare ITO, one peak positioned at 530.4 eV corresponds to the metal oxide of SnO2 or In2O3 located in ITO.29 The intensity was significantly increased after attachment by SAM and C60 (C60-ITO), which is attributed to the alkaline oxidative hydrolysis of the ITO surface followed by silanization. It should be noted that the single XPS feature shows that the oxidative reaction of the (aminopropyl)silanized SAM and the grafted C60 at the ITO surface was effectively suppressed during the sample preparation. In the same way as shown in C 1s and N 1s spectra, the peak position shifted to higher binding energy (536.3 eV) (27) Nolting, D.; Aziz, E. F.; Ottosson, N.; Faubel, M.; Hertel, I. V.; Winter, B. J. Am. Chem. Soc. 2007, 129, 14068–14073. (28) Mo, Y.; Bai, M. J. Phys. Chem. C 2008, 112, 11257–11264. (29) Lim, S. F.; Zheng, Y. M.; Zou, S. W.; Chen, J. P. Environ. Sci. Technol. 2008, 42, 2551–2556.

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because of the chemical environmental variation from the surface functionalization of ITO. After grafting by SWNTs (SWNTC60-ITO), the O 1s XPS peak shifted to lower binding energy because of the electron transportation from SWNT to ITO via C60, similar to the case of C 1s or N 1s. As compared with that of bare ITO, the broader XPS spectrum of SWNT-C60-ITO consists of two distinct features. The peak centered at 530.8 eV is assigned to the metal oxide of SnO2 or In2O3 located in ITO and the oxygen in Sn-O-Si, In-O-Si, and Si-O-Si species of the SAM.30 The peak centered at 532.2 eV is characteristic of the formed amide groups and the residual COOH groups on the oxidized functionalized SWNTs,31 which is in agreement with the C 1s XPS analysis for SWNT-C60-ITO. In addition, the FTIR-RAS spectra of SAM ITO, C60-ITO, and SWNT-C60-ITO also demonstrated the step-by-step grafting of C60 and SWNT (see Supporting Information Figure S2). The characteristic absorption peaks corresponding to -NH2, -CH2, and Si-O for the (aminopropyl)silanized SAM; -NH2, N-H, and C60 for the C60-ITO; and N-H, -OH, NH-CdO, COOH, and C60 for the SWNT-C60-ITO implied the successful attachment of C60 and the oxidative functionalized SWNT, which is consistent with the above XPS analysis. UV-vis Spectra of C60-ITO and SWNT-C60-ITO. Figure 2 compares the UV-vis spectra from C60/ODCB solution (a), C60-ITO (b), and SWNT-C60-ITO plates (c) in the range 300-600 nm, where the spectrum of C60-ITO has one strong peak appearing at 320 nm and one broad peak at 400-600 nm, which resembles the characteristic absorption of pure C60 in ODCB, indicating the successful grafting of C60 onto the ITO surfaces. In comparison with that of pure C60, the peak at 320 nm showed a blue shift because of electron transportation from C60 to the ITO layer, and the broad band at 510 nm ascribed to substituted C60 derivatives is caused by breakage of the symmetry and electronic structure of C60 after reaction with ethylenediamine.32 It is interesting to find that the peak maximum of C60 positioned at 320 nm is greatly broadened and shows a red shift of around 10 nm (Figure 2c) after further grafting of SWNTs onto the C60-ITO surface, especially with the difficult definition of the more broadened band at 400-600 nm. This indicates that the packing structures of ethylenediamine-modified C60 clusters may be considerably changed by π-π interactions between C60 assemblies and SWNT sidewalls after the attachment of acid-functionalized SWNTs. NIR Spectra of C60-ITO and SWNT-C60-ITO. Figure 3 shows a comparison of the NIR absorption spectra obtained from the C60-ITO (blue curve) and the SWNT-C60-ITO (red curve), along with that from the oxidized SWNT/water suspension (black curve). The characteristic peaks of the van Hove singularities at 1160, 1193, and 1215 nm in the range 1100-1250 nm for the aqueous SWNT solution demonstrate that the SWNTs are in the semiconducting state,33 which shows that significant electronic perturbation of the SWNTs and disruption of the extended π network occurred upon the oxidative functionalization by the acid mixture. Similar peaks corresponding to the excitonic (30) Kim, Y. H.; Lee, D. K.; Cha, H. G.; Kim, C. W.; Kang, Y. C.; Kang, Y. S. J. Phys. Chem. B 2006, 110, 24923–24928. (31) Utsumi, S.; Honda, H.; Hattori, Y.; Kanoh, H.; Takahashi, K.; Sakai, H.; Abe, M.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. C 2007, 111, 5572– 5575. (32) (a) Geckler, K. E.; Hirsch, A. J. Am. Chem. Soc. 1993, 115, 3850–3851. (b) Murata, Y.; Komatsu, K.; Wan, T. S. M. Tetrahedron Lett. 1996, 37, 7061–7064. (c) Okamura, H.; Murata, Y.; Minoda, M.; Komatsu, K.; Miyamoto, T.; Wan, T. S. M. J. Org. Chem. 1996, 61, 8500–8502. (33) Niyogi, S.; Densmore, C. G.; Doorn, S. K. J. Am. Chem. Soc. 2009, 131, 1144–1153.

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Figure 2. UV-vis absorption spectra of (a) C60/ODCB solution, (b) C60-ITO, and (c) SWNT-C60-ITO.

Figure 3. Near-IR absorption spectra of the oxidized functionalized SWNT/water dispersion with a concentration of 0.1 mg/mL (black curve), C60-ITO (blue curve), and SWNT-C60-ITO (red curve).

transitions for SWNTs of specific chirality indices in the NIR region are seen in the case of SWNT-C60-ITO, which indicates the successful grafting of SWNTs onto ITO. In addition, the red shifts in the NIR spectrum of SWNT-C60-ITO as compared with the SWNT solution result from the π-π interactions between the SWNT bundles and the C60 assemblies attached on ITO, which is consistent with the results from UV-vis spectra. Surface Morphology Analysis. The assembled clusters of C60 on SAM-ITO and the morphology changes induced by subsequent attachment of SWNTs were characterized by AFM. Figure 4a,b shows the images of the C60 that were covalently linked to an (aminopropyl)silanized SAM-ITO surface using addition reactions across the double bonds in C60. A typical spherical grain morphology of C60 is observed, with a maximum diameter of 60 nm, which showed that the C60 molecules are aggregated into small clusters because of their self-assembly. Although the apparent height seems to be 12 nm, in fact, the absolute height of the C60 layer is only ∼5 nm because of the presence of ITO concave holes, which show a depth of 7 nm (denoted as red arrows in Figure 4a,b). Compared with the height value in the range 2-3 nm generally detected by AFM in C60 SAMs,21,34 it is reasonable to consider that this C60 layer (34) Backer, S. A.; Suez, I.; Fresco, Z. M.; Rolandi, M.; Frechet, J. M. J. Langmuir 2007, 23, 2297–2299.

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grafted on the SAM-ITO can be assigned to a semimonolayer film, taking into account the broadening effect of the radius of the tip during AFM scanning. In addition, the physically adsorbed C60 molecules remaining after rinsing can be further linked to the surface of the grafted C60 clusters by ethylenediamine, which is probably another key factor affecting the observed height of the grafted C60 layer. A more detailed investigation is under way. Moreover, the exposed ITO surface (background) indicates a low surface coverage of C60 on the SAM-ITO because of the presence of more unreacted amine groups resulting from the low reaction efficiency at the liquid-solid interface. The calculated value of the surface coverage is ∼3.2  10-14 mol (C60 particle)/cm2. By additional coupling with SWNTs having carboxylic acid functional groups, typically densely packed needlelike protrusions typically are clearly seen on the SAM-ITO surface (Figure 4c), which is a conclusion drawn from the representative image of the chemically linked SWNT layer.17,35 In addition, except for the round domains corresponding to fullerene clustering, the tubelike structures lying flat on the SAM-ITO surface are also found, which are likely to be horizontally bonded functionalized shortened SWNTs (Figure 4d). From the high-resolution image of the needle-ball morphology shown in Figure 4e, it is clearly seen that C60 clusters are mainly surrounded by randomly standing out objects (denoted as red arrows), which suggests that the SWNTs are linked to the external surface of the C60 clusters as well as to the SAM-ITO substrate (Scheme 1). Meanwhile, a higher density coverage value of about 6.5  10-13 mol (C60-SWNT particle)/cm2 at the SAM-ITO surface can be calculated from Figure 4e. The SWNTs linked onto C60ITO showed a dominant length of less than 10 nm judged from the maximum height in Figure 4c, which demonstrates that extensive shortening of the SWNTs has occurred by using this oxidative cutting method, despite the presence of the nanotube with a length of around 200 nm (Figure 4f ). Their average diameter is around 15 nm, implying that most of the functionalized SWNTs are still slightly bundled by chemical adsorption onto the C60 or onto the SAM-ITO surface. Interestingly, the size of fullerene aggregates is significantly reduced after grafting by the functionalized SWNT, which showed a width of less than 30 nm from Figure 4f. In addition, the maximum height (5 nm) is nearly unchanged from Figure 4e as compared with the absolute height of C60-ITO in Figure 4a. Moreover, the reduced size of fullerene aggregates in Figure 4e showed that some C60 molecules in their assemblies were interpenetrated into the SWNT forest, which can be concluded from the blurred difference between them (denoted as blue arrows). Taken together with the UV-vis and the NIR analysis, this suggests that the π-π interactions between C60 buckyballs and SWNT sidewalls make C60 molecules in the clusters more separately dispersed on ITO and form a densely packed ultrathin film with SWNTs. Figure 5 shows the surface images of C60-ITO and SWNTC60-ITO obtained by scanning electron microscopy (SEM). C60 agglomerate particles with diameter sizes ranging from 45 to 100 nm were observed on C60-ITO, as seen in Figure 5a. After attachment of acid-functionalized SWNTs, the diameter of the fullerene clusters decreased to below 35 nm, surrounded by SWNT bundles with a dominant length of 10-20 nm (Figure 5b). Moreover, an SWNT bundle with a length of about 200 nm is also observed, which is consistent with the results from AFM images. As discussed in the above AFM measurements, the π-π interactions between (35) (a) Liu, Z.; Shen, Z.; Zhu, T.; Hou, S.; Ying, L.; Shi, Z.; Gu, Z. Langmuir 2000, 16, 3569–3573. (b) Wu, B.; Zhang, J.; Wei, Z.; Cai, S.; Liu, Z. J. Phys. Chem. B 2001, 105, 5075–5078.

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Figure 4. Typical AFM images of (a, b) C60-ITO and (c, d) SWNT-C60-ITO. (a), (c) are displayed in height 3-D view and (b), (d) are displayed in top view. (e, f ) High-resolution images from (c).

Figure 5. SEM images of (a) C60-ITO and (b) SWNT-C60-ITO.

C60 buckyballs and SWNT sidewalls are responsible for this morphology change. Furthermore, the surface coverage of C60ITO was low because a large number of amine groups on the SAM-ITO were unreacted (Figure 5a), which allows for further grafting of functionalized SWNTs to fabricate composite films with increased surface coverage on the SAM-ITO (Figure 5b). In fact, the surface coverage of SWNT-C60-ITO can be further improved Langmuir 2009, 25(18), 10834–10842

by combination with a layer-by-layer technique employing the simple repetition of this step-by-step grafting method to form multilayers with more densely packed C60-SWNT networks. CV Analysis. To further confirm the stepwise covalent immobilization of the C60 and SWNT layer on the SAM-ITO surface, the electrochemical properties of C60-ITO and SWNTC60-ITO plates were examined by CV in acetonitrile solutions DOI: 10.1021/la9013762

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Figure 6. Cyclic voltammograms of (a) 1 mM C60/ODCB solution, (b) C60-ITO, and (c) SWNT-C60-ITO in CH3CN with 0.1 M TBAP as the supporting electrolyte. Scan rate = 0.05 V/s. Table 1. Redox Potentials of C60 in ODCB, C60-ITO, and SWNT-C60-ITO in CH3CN with 0.1 M TBAP as the Supporting Electrolyte (V vs Ag/AgNO3) sample C60 C60-ITO SWNT-C60-ITO

Eox1 (V)

Eox2 (V)

Eox3 (V)

Ered1 (V)

Ered2 (V)

Ered3(V)

-0.75

-1.13 -1.02 -1.04

-1.58 -1.44 -1.87

-0.90

-1.30 -1.28 -1.29

-1.76 -1.71 -1.92

Figure 7. XPS data for (A) In 3d region and (B) Sn 3d region: (a) bare ITO, (b) C60-ITO, and (c) SWNT-C60-ITO.

with TBAP as the supporting electrolyte, as shown in Figures 6b, c. As a control experiment, the redox properties of pristine C60 were also determined by CV in ODCB, as shown in Figure 6a. The general features of CV curves recorded for C60-ITO and SWNT-C60-ITO reveal two reversible redox couples, corresponding to the second and the third redox waves related to pristine C60.36 In comparison with that of pristine C60 in ODCB, the first redox waves (C60/C60-) are not seen, probably because of the solvent cutoff from residual oxygen and the ultrathin nature of the C60 layer on the SAM-ITO, which is similar to the reported results.8b,23 The numerical values obtained from the CV experiments are summarized in Table 1. From Figure 6a,b, the redox wave values of C60-ITO (C60-/C602- and C602-/C603-, -1.02/-1.28 and -1.44/-1.71 V) are anodically shifted, especially with the oxidation wave shifts, by ca. 110 and 140 mV, respectively, relative to pristine C60 (-1.13 and -1.58 V). This large positive shift, which is known to correlate with the LUMO energy levels,37 suggests that the electrons accepted through the (36) Allemand, P. M.; Koch, A.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. Soc. 1991, 113, 1050–1051. (37) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593–601.

10840 DOI: 10.1021/la9013762

amine addition reaction across the C60 double bonds are probably delocalized to the SnO2 or In2O3 of the ITO layer, as discussed for the C 1s and the O 1s XPS data. However, more negative potentials of the redox wave (-1.04/-1.29 and -1.87/ -1.92 V, Figure 6c) were observed for C60-ITO after additional linking with SWNTs. Such cathodic shifts are due to the electrondonating effect of SWNTs,11 which also suggests the successful linkage of SWNTs with C60 as well as with the SAM-ITO surface. The electronic interactions between ITO, C60, and SWNT were also confirmed by the electronic states for the In 3d and Sn 3d signals studied by XPS, as shown in Figure 7. Both In 3d (Figure 7A) and Sn 3d (Figure 7B) spectra in bare ITO contain a typical doublet with binding energies of 452.20, 444.65, 495.05, and 486.65 eV, which can be assigned as In 3d3/2, In 3d5/2, Sn 3d3/2, and Sn 3d5/2 lines, respectively.38 However, for C60-ITO, higher binding energy features corresponding to In 3d (458.25, 450.55 eV) and Sn 3d (510.27, 492.45 eV) signals were observed, which showed a shift of about 5.0 eV for each doublet of In 3d and (38) Li, J. H.; Fu, H. J.; Fu, L. X.; Hao, J. M. Environ. Sci. Technol. 2006, 40, 6455–6459.

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Scheme 2. Proposed Reaction Scheme and the Structure Change on the ITO Surface Induced by the Alkaline Oxidation, Followed by the Silanization and the Attachment of C60 and SWNT

Figure 8. CV of bare ITO (a), C60-ITO (b), and SWNT-C60ITO (c) in acetonitrile with 3 mM ferrocene as an internal probe in 0.1 M TBAP electrolyte at a 0.5 V/s scan rate.

Sn 3d, in comparison with that in bare ITO. Because of the alkaline oxidation from hydrolysis of ITO by using the H2O2/ NH4OH mixture, followed by APTMS silanization and C60 grafting, it is reasonable to surmise that the In 3d and Sn 3d signals for C60-ITO occurring at higher binding energy result from the variation of valence state and oxygen vacancy, together with a sudden change of In2O3 and SnO2 lattice structure at the interface of ITO induced by the C60 clusters (Scheme 2),39 in spite of the presence of a charge transition from C60 to In2O3 and SnO2. Interestingly, after being further attached by SWNTs, the XPS features of In 3d and Sn 3d are shifted back to the low binding energy side, similar to that of bare ITO, which resulted from the electron-donating character of the SWNTs. This result also showed that SWNTs have a stronger electron-donating nature than C60. Compared with those for C 1s, N 1s, and O 1s XPS signals in Figure 1, the binding energy of In 3d and Sn 3d XPS features for C60-ITO showed a shift in the same way, when further grafted by acid-functionalized SWNTs, which suggests that the presence of an electron transition from SWNT to In2O3 and SnO2 via C60 in terms of the electronic effect induces the shift of related XPS features. (39) (a) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 7809–7817. (b) Hanys, P.; Janeceka, P.; Matolı´ n, V.; Korotcenkov, G.; Nehasil, V. Surf. Sci. 2006, 600, 4233–4238. (c) Masuda, Y.; Kondo, M.; Koumoto, K. Cryst. Growth Des. 2009, 9, 555–561.

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To examine the changes of electrochemical properties on the SAM-ITO surface induced by C60-derived grafted films, CV experiments using ferrocene as an internal redox probe were carried out. The results are shown in Figure 8. The ferrocene oxidation peak potential is ca. 0.48 V with the bare ITO substrate as the working electrode and is shifted to 0.5 and 0.59 V for C60-ITO and SWNT-C60-ITO, respectively. The retardation of ferrocene oxidation resulted from the differently deposited layers covering the ITO, which was composed of siloxane, C60, and SWNT layers, as illustrated by the AFM images in Figure 4. The oxidation potentials of the siloxane, C60, and SWNT layers are reached first; ferrocene is then oxidized at the film-solution interface.40 For C60-ITO, the C60 layer was packed more loosely than SWNT-C60-ITO, aided by the electron-donor effect for SWNT and electronacceptor effect for C60 and SAM-ITO, which leads to the more difficult oxidation of ferrocene on SWNT-C60-ITO than on C60-ITO. Similar blocking for the reduction of the ferrocenium cation on the return sweep is also found, suggesting that the siloxane, C60, and SWNT films are effectively grafted onto the ITO plate step-by-step, to result in somewhat more densely packed films.

Conclusions A C60 layer was successfully grafted onto a silanized SAMITO surface using amine addition across a double bond in C60, which showed a loosely packed spherical morphology with a low coverage because of the low grafting efficiency. To make the C60 clusters more separately distributed, a step-by-step strategy was used. After further tethering by a layer of SWNTs functionalized with carboxylic acid, a homogeneous ultrathin film composed of C60 and SWNT with a high-density coverage was obtained from the AFM images. The blocking for the oxidative and reductive processes at the film-solution surface, related to the packed states of the grafted films from different grafting steps, as well as the electronic effect of C60 and SWNT, was studied using ferrocene as an internal redox probe. The SWNTs covalently linked on the SAM-ITO surface as well as on the C60 clusters make the C60 assemblies more separately distributed, which forms a densely packed composite film on the SAM-ITO as a result of the π-π interaction between the C60 clusters and the SWNT walls. The electrochemistry properties of C60 on SAMITO plates observed by CV were significantly modified by the chemical anchorage of SWNT because of the electron-donating properties of SWNTs. This uniform ultrathin hybrid film of (40) Li, J.; Marks, T. J. Chem. Mater. 2008, 20, 4873–4882.

DOI: 10.1021/la9013762

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C60 and SWNT may open a route for applications in electrochemical and photoelectronic research. Acknowledgment. We are grateful to the Sumitomo Corp. for donating the HiPco of the SWNTs from CNI (Houston, TX). We thank Mr. R. Osada, Chiba Institute of Technology, Japan, for measuring the FE-SEM and XPS data and Mr. A. Kogure, Shimadzu Corp., Japan, for measuring the AFM. We thank Prof. K. Terada, Faculty of Pharmaceutical

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Sciences, Toho University, for measuring the NIR and Mr. M. Fujimoto, JASCO Corp., Japan, for measuring the FTIR-RAS. Supporting Information Available: Detailed information for the FTIR spectrum of the oxidative functionalized SWNTs and the FTIR-RAS spectra of SAM ITO, C60ITO, and SWNT-C60-ITO. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(18), 10834–10842