Langmuir 2000, 16, 6777-6779
6777
Ionic Charge-Selective Electron Transfer at Fullerene-Multilayered Architecture on an Indium-Tin Oxide Surface Sang-Yoon Oh and Sung-Hwan Han* Cleantech Center, Korea Institute of Science and Technology, CheongRyang, P.O. Box 131, Seoul, Korea 130-650 Received February 14, 2000. In Final Form: June 15, 2000 We report the first observation of the ionic charge-selective electron transfer at the fullerene-multilayered indium-tin oxide (ITO) electrode. The surface of the ITO electrode was modified by three layers of selfassembled fullerene/1,12-diaminododecane. The fullerene concentration in each layer was quantitatively measured by Rutherford backscattering spectrometry and UV spectrometry. Electron transfer was not observed from the electrode to the negatively charged Fe(CN)63-, but smooth electron transfer was made to the positively charged Ru(NH3)63+ in an aqueous solution. At the extremely low concentration of Ru(NH3)63+, 2 × 10-6 M, the bare ITO could not detect the ruthenium complex, but the fullerene-multilayered electrode identified the presence of the cationic ruthenium complex, indicating the enrichment of the ruthenium cationic complex in the vicinity of the electrode.
There have been many efforts to investigate fullerene for its interesting characteristics of electronic and optical properties. The formation of fullerene layers has drawn much attention in that it expands the properties and applications of fullerene.1 Langmuir-Blodgett film formation generated physisorbed fullerene layers,2 and selfassembled monolayers formation also provided a good methodology to prepare chemisorbed fullerene layers.3 The construction of highly ordered fullerene-multilayered architectures assisted by the SAM formation is a challenging target in designing an electron transfer system on the electrode surface. Especially the formation of fullerene multilayers on the indium-tin oxide (ITO) surface, which is a transparent conducting oxide, is crucial work to develop photoinduced electron-transfer systems. Herein, we report the first observation of the ionic chargeselective electron transfer at a fullerene-multilayered ITO electrode. * To whom correspondence should be addressed. Tel: +822-9585212. Fax: +822-958-5219. E-mail:
[email protected]. (1) (a) Baum, R. M. Chem. Eng. News 1993, 20, 31-33. (b) Chlistunoff, J.; Cliffel, D.; Bard, A. J. Thin Solid Films 1995, 257, 166-184. (c) Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113-5130. (d) Prato, M. J. Mater. Chem. 1997, 7, 1097-1109. (2) (a) Rikukawa, M.; Furumi, S.; Sanui, K.; Ogata, N. Synth. Met. 1997, 86, 2281-2282. (b) Aktsipetrov, O. A.; Mishina, E. D.; Misuryaev, T. V.; Nikulin, A. A.; Novak, V. R.; Stolle, R.; Rasing, Th. Surf. Sci. 1998, 404, 576-580. (c) Zhang, W.; Shi, Y.; Gan, L.; Huang, C.; Luo, H. Wu, D.; Li, N. J. Phys. Chem. B 1999, 103, 675-681. (3) (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193-1194. (b) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598-11599. (c) Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R. M.; McCauley, J. P., Jr.; Smith, A. B., III.; Blasie, J. K.; Peticolas, L. J.; Bean, J. C. J. Am. Chem. Soc. 1993, 115, 4383-4384. (d) Arias, F.; Godinez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086-6087. (e) Tsukruk, V.; Everson, M. P.; Lander, L. M.; Brittain, W. J. Langmuir 1996, 12, 3905-3911. (f) Lee, H. W.; Jeon, I. C. Synth. Met. 1997, 86, 2297-2298. (g) Dominguez, O.; Echegoyen, L.; Cunha, F.; Tao, N. Langmuir 1998, 14, 821-824.(h) Patolsky, F.; Tao, G.; Katz, E.; Willner, I. J. Electroanal. Chem. 1998, 454, 9-13. (i) Shon, Y. S.; Kelly, K. F.; Halas, N. J.; Lee, T. R. Langmuir 1999, 15, 5329-5332. (j) Feng, W.; Miller, B. Langmuir 1999, 15, 3152-3156. (k) Shon, Y. S.; Kelly, K. F.; Halas, N. J.; Lee, T. R. Langmuir 1999, 15, 5329-5332. (l) Wei, T. X.; Zhai, J.; Ge, J. H.; Gan, L. B.; Huang, C. H.; Luo, G. B.; Ying, L. M.; Liu, T. T.; Zhao, X. S. Appl. Surf. Sci. 1999, 151, 153-158. (m) Imahori, H.; Azuma, T.; Ajavakom, A.; Norieda, H.; Yamada, H.; Sakata, Y. J. Phys. Chem. B 1999, 103, 7233-7237.
Recently, we have reported the preparation of selfassembled monolayers of 1,12-diaminododecane on the ITO surface with quantitative analyses, showing the formation of stable and densely packed 1,12-diaminododecane monolayers.4 The amine functionality on the surface of the diaminododecane-modified ITO was active and reacted with aldehyde to form imine compound and with heteropolyacid to give second layers of heteropolyacid on the ITO surface. In this study, the amine functionality on the SAM/ITO surface was utilized to prepare fullerene layers by an addition reaction.5 Three layers of fullerene were prepared by sequential formations of self-assembled monolayers. The first layer of 1,12-diaminododecane (DD/ITO) was prepared by soaking an ITO (indium-tin oxide)-coated glass in a 5 mM methanol solution of 1,12-diaminododecane at room temperature under an argon atmosphere for 48 h.4 After being rinsed with methanol and benzene, the DD/ITO was dipped in a 0.1 mM benzene solution of fullerene at 50 °C under the argon atmosphere for 24 h. The amine terminal group on the DD/ITO reacted with fullerene in a benzene solution to form the first fullerene monolayers (1C60/ITO ) C60/DD/ITO). The second layer of 1,12-diaminododecane was prepared by the addition reaction of 1,12-diaminododecane in a methanol solution to surface fullerenes of 1C60/ITO under the reaction conditions of 50 °C, 24 h (DD/ 1C60/ITO). The second (2C60/ITO ) C60/DD/C60/DD/ITO) and third layers (3C60/ITO ) C60/DD/C60/DD/C60/DD/ITO) of fullerene were prepared by repeating the preparation method of the first fullerene layers. The successive fullerene layers were quantitatively analyzed by Rutherford backscattering spectrometry (RBS) (Figure 1) and UV spectrometry (Figure 2).6 The RBS experiment enabled us to count the number of carbon atoms on the ITO surface and hence the concentration of (4) Oh, S. Y.; Yun, Y. J.; Kim, D. Y.; Han, S. H. Langmuir 1999, 15, 4690-4692. (5) Wudl, F.; Hirsch, A.; Khemani, K. C.; Suzuki, T.; Allemand, P.M.; Koch, A.; Srdanov, G. Fullerenes: Synthesis, Properties and Chemistry of Large Carbon Clusters; Hammond, G. S., Kuck, V. J., Eds.; American Chemical Society: Washington, DC, 1992; p 161. (6) RBS was performed at the KIST RBS facility with 2 MeV He2+ ion source. UV spectroscopy was performed on a Varian CARY100 spectrophotometer.
10.1021/la0002132 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/22/2000
6778
Langmuir, Vol. 16, No. 17, 2000
Figure 1. Rutherford backscattering carbon peaks on ITO surface: (a) 1C60/ITO; (b) 2C60/ITO; (c) 3C60/ITO.
Figure 2. UV spectra of fullerenes on ITO surface: (a) 1C60/ ITO; (b) 2C60/ITO; (c) 3C60/ITO. Inset is a graph of absorbance at 334 nm vs number of layers (absorption maxima of fullerenes in benzene solution is 334 nm).
1,12-diaminododecane and fullerene. The number of carbon atoms in the DD/ITO was 6.5 × 1015 atoms/cm2 after background correction. After the fullerene monolayers formation, the number of carbon atoms on the ITO surface increased to 1.1 × 1016 atoms/cm2, which was equivalent to 1.2 × 10-10 mol/cm2 fullerene on DD/ITO. As shown in Figure 1, the number of carbon atoms increased systematically: first layer of C60/diaminododecane, 1.1 × 1016 atoms/cm2; second layer of C60/diaminododecane, 0.8 × 1016 atoms/cm2; third layer of C60/ diaminododecane, 1.3 × 1016 atoms/cm2. On the basis of the fullerene concentration of 1C60/ITO, the fullerene concentrations of the second and third layers were measured by UV spectroscopy (Figure 2).7 The UV absorption at 334 nm linearly increased (R2 ) 0.99) and gave fullerene concentrations of individual layers: first layer, 1.2 × 10-10 mol/cm2; second layer, 1.9 × 10-10 mol/ cm2; third layer, 1.9 × 10-10 mol/cm2. It is worthwhile to note that the fullerene concentrations of three layers were similar in the order of magnitude. Especially the second and third layers gave the same fullerene concentration. The theoretical monolayer coverage of fullerenes on a flat surface is ∼1.9 × 10-10 mol/cm2.3a The fullerene concentration calculated by RBS and UV spectroscopy showed that the fullerene was highly packed at individual layers with the rough surface of ITO. Meanwhile, the total number of carbon atoms at the second and third C60/DD layers determined by RBS increased from 0.8 × 1016 to 1.3 × 1016 atoms/cm2, which indicated the presence of extra 1,12-diaminododecane in the layers. It is known that several amines can be added onto a fullerene molecule.5 The part of the extra 1,12-diaminododecane could crosslink fullerene molecules and contribute to the stabilization of the multilayer structure. The fullerene-multilayered ITO electrodes were characterized electrochemically.8 Cyclic voltammetry of a SAM of fullerene on the 1C60/ITO electrode in a 0.1 M Bu4NClO4/CH2Cl2 solution showed two redox waves at E1/2 ) (7) Haiwon, Lee.; Sang-Rae, Park.; Dong-Won, Kim.; Byung-Il, Seo Synth. Met. 1995, 71, 2065-2066. (8) All voltammetric measurements were performed with EG&G PARC M 273A potentiostat, Ag/AgCl reference electrode, and a platinum wire counter electrode. ITO electrode area was 1 cm2.
Letters
Figure 3. (A) Cyclic voltammogram (CV) of a SAM of fullerenes on the 1C60/ITO (C60/1,12-diaminododecane/ITO) electrode in a CH2Cl2 solution: scan rate, 1 V s-1; supporting electrolyte, 0.1 M Bu4NClO4. (B) CVs of 1 × 10-3 M Fe(CN)63- in an aqueous solution at (a) bare ITO, (b) 1C60/ITO, (c) 2C60/ITO, and (d) 3C60/ITO electrodes: scan rate, 0.2 V s-1; supporting electrolyte, 0.1 M KNO3. (C) CVs of 1 × 10-3 M ferrocene in an acetonitrile solution at (a) bare ITO and (b) 3C60/ITO electrode: scan rate, 0.2 V s-1; supporting electrolyte, 0.1 M Bu4NClO4.
-0.6 and -1.2 V (vs Ag/AgCl), Figure 3A. The coverage of fullerene on the ITO surface was ∼1.5 × 10-10 mol/cm2 by the peak integration of the anodic current, which was close to the value derived from the RBS experiment. The redox peaks disappeared as fullerene formed the second and third layers on the ITO surface. Cyclic voltammetry of Fe(CN)63- as a probe molecule was performed to characterize 1C60/ITO, 2C60/ITO, and 3C60/ITO electrodes, Figure 3B.9 Reversible electrontransfer took place at E1/2 ) 0.2 V. It is important to notice that the redox current decreased as the number of fullerene layers increased. The reduction peak current of Fe(CN)63at the bare ITO electrode was 407 µA and successively decreased to 346 and 215 µA at the 1C60/ITO and 2C60/ ITO electrodes, respectively. Finally, the redox current completely disappeared at the 3C60/ITO electrode. The consecutive decreases of redox current indicated that the multilayers of fullerene effectively block electron transfer from the electrode to Fe(CN)63- in solution. Such results also support the premise that the fullerene-multilayered architecture on the ITO surface was effectively constructed by successive additions of the 1,12-diaminododecane/ fullerene layers. The cyclic voltammetry was independent of pH in the range of 3.93-8.33. Cyclic voltammetry of ferrocene, a charge-neutral molecule, was performed at the 3C60/ITO electrode in an acetonitrile solution, and the presence of the surface defects was monitored. As shown in Figure 3C, the shape and intensity of the redox peaks at the 3C60/ITO electrode have changed drastically from those at the bare ITO. The decrease in the redox current of ferrocene at the 3C60/ITO electrode was attributed to the physical hindrance of the mass transfer in the presence of fullerene multilayers. The shape of the sigmoid redox peaks of ferrocene at the 3C60/ITO electrode showed the characteristics of radial diffusion to defect sites whose diffusion layers do not overlap.9a Such radial diffusion indicates that the selfassembled fullerene multilayers were highly packed, and the defect concentration in the multilayers was small. In view of the ferrocene experiments, however, the peak current decrease in the cyclic voltammetry of Fe(CN)63was rather drastic and complete. The perfect blocking of the electron transfer cannot be explained only by the elimination of defect sites and physical blocking of the mass transfer of Fe(CN)63- on the ITO surface. The complete blocking of the electron transfer might have originated from the negative charge of the probe molecule. (9) (a) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 13291340. (b) Gobi, K. V.; Ohsaka, T. J. Electroanal. Chem. 1999, 465, 177186.
Letters
Langmuir, Vol. 16, No. 17, 2000 6779
Figure 5. Cyclic voltammograms of different concentrations of Ru(NH3)63+ in an aqueous solution: (A) 1 × 10-3 M, (B) 2 × 10-5 M, (C) 2 × 10-6 M; dashed line, bare ITO; solid line, 3C60/ ITO electrodes; scan rate, 0.2 V s-1; supporting electrolyte, 0.1 M KNO3.
Figure 4. Cyclic voltammograms of 1 × 10-3 M Fe(CN)63- and 1 × 10-3 M Ru(NH3)63+ in an aqueous solution at (A) bare ITO and (B) 3C60/ITO electrodes: scan rate, 0.2 V s-1; supporting electrolyte, 0.1 M KNO3.
To compare the charge effects of complexes, the positively charged ruthenium complex, Ru(NH3)63+, was tested as another probe molecule, which has a similar molecular size to that of Fe(CN)63-, Figure 4. The hydrated radius of Ru(NH3)63+ at 22 °C is 6.4 Å, and that of Fe(CN)63- is 6.0 Å.9a The cyclic voltammograms of the two complexes at the 3C60/ITO electrode (Figure 4B) were compared with those at the bare ITO (Figure 4A). The redox peaks of Fe(CN)63- and Ru(NH3)63+ appeared at 0.2 and -0.15 V, respectively. The striking difference of the ionic charge selectivity was observed at the 3C60/ITO electrode: redox peaks of Ru(NH3)63+ were almost unchanged while those of Fe(CN)63- completely disappeared. Compared to the cyclic voltammograms of ferrocene at the bare ITO and at the 3C60/ITO (Figure 3C), Ru(NH3)63+ showed the same current density, which gave another exceptional result along with the complete hindrance of the electron transfer to Fe(CN)63-. These experimental results showed that the fullerene-multilayered ITO electrode has a special preference for positively charged molecules and repels negatively charged ones. The cationic charge preference was further investigated by cyclic voltammetry at extremely low concentrations of the ruthenium complex, Figure 5. The cyclic voltammograms at the bare ITO and the 3C60/ITO electrodes were obtained in three different concentrations of Ru(NH3)63+, 1 × 10-3, 2 × 10-5, and 2 × 10-6 M. Under the high concentration of the ruthenium complex, 1 × 10-3 M, the current density at the two electrodes was similar, Figure 5A. The redox current densities at the bare ITO electrode were proportional to the solution concentration of Ru(NH3)63+. The current density at the bare ITO electrode represented the solution concentration of Ru(NH3)63+. Nevertheless, the current density at the 3C60/ITO electrode
became larger than that at the bare ITO as the ruthenium complex concentration decreased. At the concentration of 2 × 10-6 M Ru(NH3)63+, the bare ITO could not detect the ruthenium complex, but the 3C60/ITO electrode could, Figure 5C. The observation of large redox currents at low concentration of Ru(NH3)63+ signified that Ru(NH3)63+ was enriched in the vicinity of the electrode surface. The experimental results imply that positively charged molecules were attracted to the surface of the fullerenemodified electrode and negatively charged molecules were repelled, respectively. Previously we reported the formation of the electrostatic layers of the heteropolyacid (HPA) on the DD/ITO surface.4 The HPA layers were very stable even after washing with 0.1 M HCl. The interaction between the fullerenemultilayered electrode and the cationic ruthenium complex was not as stable as that of HPA and DD/ITO. After the electrode was washed with water, no redox peaks were observed, indicating that the enriched ruthenium complex at the 3C60/ITO electrode surface was easily removed. The multilayers of fullerene and 1,12-diaminododecane were stable even after the sonication and repeated electrochemical experiments within the range of experimental window (+1.0 to -0.8 V vs Ag/AgCl), and they were not damaged upon soaking in a methanol solution for 3 months. In summary, we have designed and constructed a fullerene-multilayered architecture on an ITO surface assisted by SAM formation. The numbers of carbon atoms in each layer were accurately determined by RBS, and the fullerene concentration in each layer was quantitatively analyzed: first layer, 1.2 × 10-10 mol/cm2; second layer, 1.9 × 10-10 mol/cm2; third layer, 1.9 × 10-10 mol/ cm2. The fullerene multilayers showed exceptional ionic charge-selective electron transfer phenomena. Electron transfer was not observed from the electrode to the negatively charged Fe(CN)63-, but smooth electron transfer occurred to the positively charged Ru(NH3)63+ in an aqueous solution. Also, the fullerene-multilayered ITO electrode exhibited a highly improved detection limit of Ru(NH3)63+ complex in solution over the bare ITO. This implied the enrichment of ruthenium complex in the vicinity of the electrode surface. The origin of the ionic charge selectivity is under investigation. LA0002132
